Forming aluminum for a cooling outlet fairing

[hydroguy2]

Good luck tomorrow Race #71

[Bob Axsom]

After working on this for ~ 30 hours to get it ready for the race tomorrow (please note this is for a immediate need - not a science project) I flew a late afternoon test in calm surface wind and clear sky conditions. The speed was 181.0 kts per the NTPS spreadsheet using the three leg method at 6,000 ft density altitude. That was disapointing but these results are more expected than the successes I experience. The RPM was 2720.

Temperatures were up. CHT 1 through 3 were 366, 375, 373 respectively. CHT on 4 has failed. This is a 20-30 deg. F rise across the board and is the first time I have seen any affect from work on the outlet that runs back several years. This indicates to me that the air mass flowing through the system has been reduced by the bump. The oil temp was also up to 200 which is an extreme rarity im my installation - it is almost always 180.

After the test I inspected the assembly and it was fine. There was no time to take it off and reinstall the light weight triangular flow fences so I flew to Abilene that way and I will race it tomorrow that way.

One the trip down here initially at 6,000 then at 4,000 to reduce the head wing from 60 down to 50 kts with the RPM set at 2450 I was seeing 185 KTAS on the AI slide rule function.

After arrival in Abilene I inspected the assembly again and all is well

Bob Axsom

[Bob Axsom]

Good luck tomorrow Race #71

I know you wish you could be here too. I haven't studied the course yet, I know it is a short ~80 mile triangular course so it will fit into the big show schedule. Shouldn't required a lot of study.

You remember the flat right main I had on the way home from Llano? Well, when I got home Thursday night from my marathon work session I had a message from one of the guys that made it possible for me to get home and to be here in Abilene tonight. He said he is bringing his family to the show tomorrow. I mean, that makes me feel humble - the guy is one the men that folks around here stand up when they want to display what a Texan is.

Bob Axsom

[hydroguy2]

......one of the guys that made it possible for me to get home and to be here in Abilene tonight. He said he is bringing his family to the show tomorrow. I mean, that makes me feel humble - the guy is one the men that folks around here stand up when they want to display what a Texan is.

Bob Axsom

Thanks great! He is probably bragging you up to his family too...."look there's Race #71, you know I helped pit crew on that plane after the last race" ;)

[captainrichhill]

I've been following this thread and I am no way qualified to ADD to it. I was surprised that I didn't see one post after the exit farring failure that mentioned the exhaust annealing the aluminium? I think that is the right term. There are all different characteristics of aluminium based on composition and heat treating. My limited knowledge of heat treating leads me to think that the exhaust heat treated the metal to the point of being brittle and the exhaust pulses made it fail.

Anyone else feel the same? Can a different Aluminium prevent this? Can you use some of the cowl exhaust air as a cooling blanket to keep the temp of the "Hump" down? They do that inside jet engines to keep the case cooler.

Just my inexperienced $.02

Richard:o

[gereed75]

Good Luck Bob!!!!!!!!!!!!!

[Ron Lee]

My issue is that I do not have adequate knowledge of the dynamics of what is being done here to fully understand it. However, is it possible that some sort of Vetterman type system external shell could be integrated with the Coanda bump internals?

[David Paule]

Bob, it looks like the external cover in the last photo extends aft of the peak of the bump fairing. That gives you a diverging duct.

You'd be better off, I think, shortening the cover so that its aft edge is slightly forward of the peak of the bump fairing.

What happens in a subsonic diverging duct is that the same amount of air and exhaust flows past the beginning and the end every second. Since the end is larger, the flow is moving more slowly. You want the flow to go faster.

Dave

[Bob Axsom]

I made cutouts in the development cover. I can use it to try the shorter cover.

Bob Axsom

[Bob Axsom]

I am still a bit numb from lack of sleep but I cut some roses for Jeanine then I stopped by the hangar on the way home. I didn't have the camera with me.

The exhaust residue is in straight line back on the bottom of the fuselage confined by the outboard flow fences outboard is clean on both sides. This indicates to me that separation from the other air surounding the airplane is occurring

The exhaust residue seems unusually thick to me.

The two outboard screws at the aft end of the bump skin are missing - BOTH OF THEM! The two inboard screws (hardware grade sheet metal screws) are inplace and appear to be tight.

the bumpt structure appears fine otherwisw.

I guess I had better take the camera back out there and post some photos. I'll be back in about an hour.

Bob Axsom

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5737.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5738.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5746.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5749.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5751.jpg

Bob Axsom

[Bob Axsom]

Bad data they had an extra mile in the calculation - see below.

Well I'm going to have to get that stuff off of the airplane I guess.

Bob Axsom

[Bob Axsom]

I think I will try the shortened cover suggested by David then no cover at all, just the bump and the fences then I have to come up with the best way to get all of those Cherry rivets out of the fuselage and the mounting angles.

Bob Axsom

[David Paule]

....The two outboard screws at the aft end of the bump skin are missing - BOTH OF THEM! The two inboard screws (hardware grade sheet metal screws) are in place and appear to be tight....

Looks like you've got separated flow aft of the peak of the bumps, and the turbulence and high local loads perpendicular to the bumps that are associated with that.

Dave

[RV6airplanePilot]

Looks like you blew right past that 185 knot barrier.
Congratulations!
Dale
RV6a KHWO

[Bob Axsom]

Looks like you blew right past that 185 knot barrier.
Congratulations!
Dale
RV6a KHWO

Race # Name Aircraft Class "Elapsed Time" "Speed (MPH)" "Speed (KTS)"
Race 35 Mike Smith SX300 Sport 00:18:18.860 308.77 268.31
Race 79 Jim Robinson Glasair III Sport 00:19:18.810 292.80 254.43
Race 390 Jerry Hajek RV-8 RV Gold 00:25:24.130 222.62 193.45
Race 81 Gary Shelley RV-8 RV Blue 00:25:46.280 219.43 190.68
Race 26 Mike Thompson RV-6 RV Blue 00:26:30.500 213.33 185.38
Race 71 Bob Axsom RV-6A RV Blue 00:26:39.130 212.18 184.38
Race 77 Jim Huff Bonanza C-35 FAC4RG 00:28:49.250 196.21 170.50
Race 201 Rebecca Cutri-Kohart Mooney 201 FAC3RG 00:30:18.380 186.59 162.14
Race 03 Howard Hurlbut Bonanza FAC2RG 00:34:42.000 162.97 141.61
Race 55 Team Ely Grumman AA5A FAC5 00:36:29.380 154.97 134.67

These are my speeds not counting the BCAF (I'm going to wait for that one to settle down a bit)

Race Record (RR)# Race Date Class Aircraft Type Pilot Speed Kts Speed MPH
217 AirVenture Cup 2008 7/27/08 RV Blue RV-6A Axsom, Bob & Jeanine 194.56 223.89
782 Rocket 100, 2010 11/20/10 RV Blue RV-6A Axsom, Bob 192.80 221.87
133 Texoma 100, 2008 4/20/08 RV Blue RV-6A Axsom, Bob & Jeanine 188.36 216.80
753 Tennessee Valley Air Race 2010 10/30/10 RV Blue RV-6A Axsom, Bob 187.52 215.83
233 AirCap 200 8/24/08 RV Blue RV-6A Axsom, Bob 185.16 213.08
1213 Hill Country 150, 2012 4/21/12 RV Blue RV-6A Axsom, bob 184.97 212.86
298 Rocket 100 2008 11/23/08 RV Blue RV-6A Axsom, Bob & Jeanine 183.83 211.55
809 Taylor 150, 2011 4/2/11 RV Blue RV-6A Axsom, Bob & Jeanine 183.41 211.07
313 Taylor 100 2009 3/16/09 RV Blue RV-6A Axsom, Bob & Jeanine 182.35 209.85
1201 Taylor 150, 2012 4/14/12 RV Blue RV-6A Axsom, Bob 181.47 208.84
152 BCAF 150, 2008 5/4/08 RV Blue RV-6A Axsom, Bob & Jeanine 181.45 208.81
720 Grace Flight 2010 10/2/10 RV Blue RV-6A Axsom, Bob 181.23 208.54
1176 Texoma 100, 2012 3/31/12 RV Blue RV-6A Axsom, Bob 180.87 208.14
276 Memphis 100 2008 10/19/08 RV Blue RV-6A Axsom, Bob & Jeanine 180.26 207.44
66 AirVenture Cup 2007 7/23/07 RV Blue RV-6A Axsom, Bob & Jeanine 179.99 207.13
641 AirVenture Cup 2010 7/26/10 RV Blue RV-6A Axsom, Bob 179.83 206.94
363 Tennessee Valley Air Race 2009 6/14/09 RV Blue RV-6A Axsom, Bob & Jeanine 178.98 205.97
179 Colorado 150, 2008 6/29/08 RV Blue RV-6A Axsom, Bob & Jeanine 178.63 205.56
539 West Texas 100 6/6/10 RV Blue RV-6A Axsom, Bob 178.04 204.89
102 Memphis 100, 2007 10/28/07 RV Blue RV-6A Axsom, Bob & Jeanine 176.86 203.52
119 Rocket 100, 2007 11/18/07 RV Blue RV-6A Axsom, Bob & Jeanine 176.33 202.92
25 Wichita 300 7/17/07 RV Blue RV-6A Axsom, Bob & Jeanine 169.91 195.53
17 Taylor 100 2007 5/20/07 RV Blue RV-6A Axsom, Bob & Jeanine 168.72 194.16

[NDrv8r]

Bob,
does your latest mod make amy difference on interior noise or vibration on the floor?

[Bob Axsom]

It's really hard to tell because I can't compare them side by side. It is loud but with the headsets and the insulation and uphostery it has never been bad in our airplane.

Bob Axsom

[Bob Axsom]

Looks like you blew right past that 185 knot barrier.
Congratulations!
Dale
RV6a KHWO

The classic example is the 2008 AirVenture Cup. Straight line West to east with no altitude change requirements Jeanine and I did 194.56 kts in that never to be repeated opportunity.

Even in the other races that typically begin and end at the same airport you study the winds aloft for race time the night before and compare them with the race leg directions, speed and distance in climb & descent, cruise speed and you can often come up with a speed that is greater than the airplane's top speed. So the race speeds are nice to compare against the other racers in your class and even more so your own personal best race speed but they are not usable when you are trying to determine top speed. And, top speed is not a constant, little things change and performance decays. The best top speed I have measured using a consistant test method is still 184.4 kts.

Bob Axsom

[hydroguy2]

I'm interested in your flight profile for this race. was it left turns? slow climb to 2800' or did you go higher early and ride the wind. Then hold alt to turn 2, at turn 3 go to Vne to minimum alt and power to the finish? Just wondering? sorry I missed it

[Bob Axsom]

I got the winds at the surface, 3000 and 6000 and interpolated 4 & 5K. There was an unusual wind forecast at 3000 ft roughly 190 at 25 (blowing toward 010) compared with the surface 210 at 10 and 6000's 260 at 15. The course was right turns (it turned out - lots of confusion because of a late change) with legs of 37.4 nm at 304 degrees magnetic, 14.1 at 077 and 29.4 at 144. So after working with it for a while I planned to climb to 3000 on leg 1, descend to 2,300 on leg 2 and stay there on the final leg of this unusually short course. The predicted speed was 210 mph.

On leg 1 I saw speeds in the mid 180's which was disapointing but that was what I got. When I turned on leg 2 I was ready to descend but I saw 195 kts and I knew I wasn't going to get any better than that so I stayed put. Unfortunately this was by far the shortest leg of the race. When I turned on leg 3 I descended to 2300 and locked in there and my speed dropped into the low to mid 170 kt range. My published speed was higher but I think the final speed is going to be 212 mph when they get everything corrected.

Bob Axsom

[hydroguy2]

pesky right turns:D I was figuring left turns. your plan sounds like what I was planning.

[Bob Axsom]

Race # Name Aircraft Class Elapsed Time Speed (MPH) Speed (KTS)
Race 35 Mike Smith SX300 Sport 00:18:18.860 308.77 268.31
Race 79 Jim Robinson Glasair III Sport 00:19:18.810 292.80 254.43
Race 390 Jerry Hajek RV-8 RV Gold 00:25:24.130 222.62 193.45
Race 81 Gary Shelley RV-8 RV Blue 00:25:46.280 219.43 190.68
Race 26 Mike Thompson RV-6 RV Blue 00:26:30.500 213.33 185.38
Race 71 Bob Axsom RV-6A RV Blue 00:26:39.130 212.18 184.38
Race 77 Jim Huff Bonanza C-35 FAC4RG 00:28:49.250 196.21 170.50
Race 201 Rebecca Cutri-Kohart Mooney 201 FAC3RG 00:30:18.380 186.59 162.14
Race 03 Howard Hurlbut Bonanza FAC2RG 00:34:42.000 162.97 141.61
Race 55 Team Ely Grumman AA5A FAC5 00:36:29.380 154.97 134.67

[Bob Axsom]

I pulled the cover off this afternoon to implement the next mod and I photographed what I saw for the record. One rivet head had popped off on the forward side of the bump, see location marked with blue arrow.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5752.jpg

The fwd side of the bumps in both channels are clean except for the stain on the left side where the crankcase vent is directed onto the exhaust pipe.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5754.jpg

In the center line of cover mounting holes the last one has some serious cracking but the others are fine.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5758.jpg

Bob Axsom

[Bob Axsom]

I decided to just go ahead and cut the new cover, I don't expect to use it again as is. The cover now extends back 4 3/4" from the rear of the stock cowl. That stops it a little over an inch forward of the node peak.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5761.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5766.jpg

It looks interesting, like it has potential.

I flew the test shortly before dark this evening and the NTPS spreadsheet clculated TAS was 180.8 kts

The usable CHTs were 360, 373 and 372 for cylinders 1, 2 and 3 respectively.

I removed the cover after the flight and took some more photos to record the condition.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5770.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5771.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5773.jpg

The condition didn't look any different that I could see than it did before the flight. But it took 1 to 2 turns to tighten one of the out board sheetmetal screws the other three sheet metal screws were tight.

The next test will be with the same fins but no cover.

If you think I made the cover too short in this configuration I can rivet an extension on and refly the test.

Bob Axsom

[David Paule]

This was a surprise. If I've got the numbers right, this configuration gave 181.0 kts:
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5736.jpg

and this gave 180.8 kts, slightly slower:
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5761.jpg

Perhaps what it suggests is that the loss of exit velocity due to the original diverging duct, combined with forcing the exhaust to flow parallel to the airplane, causes less speed loss than the speeded-up exit air in the second, but which lets some of the exhaust plume be pointing downward, interfering with the free-stream air.

It kind of looks like the flow is now attached to the aft side of the bump, which it hadn't been before. If this is correct, kind of hard to tell, it's an improvement. On the other hand, how can it be an improvement if it's slower?

The second exit cover is about where I'd have put its trailing edge.

The only suggestions I've got for this configuration would be to add a strip to extend the cover to about the peak of the bump OR to carry it past slightly and have a lip on it that is roughly parallel to the aft face of the bump. I'd favor carrying it just to the peak of the bump rather than past.

Either way, I'd expect some cracking on the parts from the exhaust pulses.

An alternative approach would be to add exhaust pipe curved extensions to get them to lie more parallel to the bottom of the airplane - it's probably time to try this - and then set up the bump and cover to accommodate that, leaving the cover trailing edge about where it is now relative to the bump.

This business of having to think so early in the morning is tough!

Dave

[F1Boss]

This was a surprise. If I've got the numbers right, this configuration gave 181.0 kts:

image snipped

and this gave 180.8 kts, slightly slower:

image snipped

Perhaps what it suggests is that the loss of exit velocity due to the original diverging duct, combined with forcing the exhaust to flow parallel to the airplane, causes less speed loss than the speeded-up exit air in the second, but which lets some of the exhaust plume be pointing downward, interfering with the free-stream air.

It kind of looks like the flow is now attached to the aft side of the bump, which it hadn't been before. If this is correct, kind of hard to tell, it's an improvement. On the other hand, how can it be an improvement if it's slower?

The second exit cover is about where I'd have put its trailing edge.

The only suggestions I've got for this configuration would be to add a strip to extend the cover to about the peak of the bump OR to carry it past slightly and have a lip on it that is roughly parallel to the aft face of the bump. I'd favor carrying it just to the peak of the bump rather than past.

Either way, I'd expect some cracking on the parts from the exhaust pulses.

An alternative approach would be to add exhaust pipe curved extensions to get them to lie more parallel to the bottom of the airplane - it's probably time to try this - and then set up the bump and cover to accommodate that, leaving the cover trailing edge about where it is now relative to the bump.

This business of having to think so early in the morning is tough!

Dave

Hey Bob:

Time to straighten the exh outlets a bit, and second extend the cover to the peak of the bump. Third, the tipped lip is a good addition, in a separate experiment. You might be able to forego the exh angle change, and simply fab a slightly longer cover as your next experiment.

You are developing an exhaust augmenter which will work at both low speed (climb) and high speed (cruise). Previous attempts at such have not been successful, but yours is different from any others I have seen.

The B25 blows the exhaust along the surface of the outside of what you would call the outlet duct cover (looks a lot like cowl flaps if you ask me) and it may be this exhaust action that produces the suction in cruise....and eliminates and exhaust pulsing on a 'speed bump' as you have installed here. The exh pulses do beat up the cowl flap hinges, as these are part of the preflight check. The cover (cowl flap) and the top of the bump curve are level with each other (with the cowl flaps closed), and there is a slight gap between the top of the bump and the aft edge of the cowl flap, with a reverse bump (valley) aft the bump. I will try to get pics of that setup to you so you can give it a try too. Sure looks like someone is using the coanda principle there...using external mixing of the high velocity exh gasses to accelerate the slow moving engine cooling air?

http://i947.photobucket.com/albums/ad314/f1boss/B25/th_B25engine.jpg (http://s947.photobucket.com/albums/ad314/f1boss/B25/?action=view&current=B25engine.jpg)

The cowling sections with the stack exh have 'reverse scoops' covering each stack.

Post WW2 mods to the B25 have converted some of the individual stacks to a collector to increase carb heat capability. I am not referring to the collector exhaust in this discussion.

No sense in trying to re-invent the wheel!

Carry on!
Mark

[Bob Axsom]

We had thunderstorms here this morning but it is clearing up so I am going to get these new planned tests completed and decide what to do next. They all sound worth giving a shot.

I am going to the airport shortly and fly the no cover configuration with the existing extended bump.

Then I think I will replace the heavy fins with the light triangular flow fences.

It might be worth flying a naked test between the two because the conversion process takes me to that state (it's complicated involving long magnetic screwdriver and non-stainless steel screws to attach all three items to the center mount with no additional fasteners).

After that is all done the past the bump cover extension with a conforming to the aft bump angle lip seems logical then cut the aft part with the lip off fao another test.

Mark when we were checking in at Dyess in Base Ops Friday, the crew for the B-25 Devil Dog was checking in as well. We flew the race as the first event on Saturday morning and I continued the flight on home but Mike returned to base and might have some stories to share with you about their performance.

Bob Axsom

[Bob Axsom]

I went to the airport and flew the test as is with no wipe down but I potographed it after the flight which may be compared with the post flight photo from yesterday. Here are the two I took after the flight today:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5775.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5776.jpg

The speed was 182.1 kts so a little faster. The center mount bracket is still there hanging in the wind so if I were to replace these three vertical units with the much cleaner triangular flow fences it should be even faster.

I have rethought my test sequence. I'm going to do the two with the modified cover while the hard cover mount structure is still there then the naked and the triangles with the bumps.

Bob Axsom

[johnny stick]

Bob,
This a great thread. Could you summarize the baseline and different options you have tried and the resulting speed change?

Thanks
Johnny Stick

[Bob Axsom]

I feel near the end but there are a couple more things I want to try.

Bob Axsom

[johnny stick]

Bob,
I understand your reluctance to post preliminary data, especially since it was very hard and expensive to obtain. I am just curious how much extra speed you have obtained. Must be significant. Would you be willing to label the data something like: baseline, option 1, option 2, etc? without details of what the different options mean or the order they were tested? I am not a racer, I do not have an RV flying in the near future, but I really enjoy your true grass roots research. I can't help but think your work has yielded a wealth of data just by having different data than expected, even if the speed has not increased significantly . Thanks for all you do.

[Bob Axsom]

These are post flight photos.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5780.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5782.jpg

I completed this mod at around 7:30 pm and I had to really rush to get a test in and be on the ground before dark. Even in the rushed state I think I followed the test method. The speed was 179.6 kts but it has been suggested that these last minute before dark tests might provide distorted results because of sinking air. I don't really want to waste any more time on this but if the weather is good when I get out there, I'll refuel and do it over.

Tuesday: A 30 percent chance of showers and thunderstorms before 1pm. Cloudy, then gradually becoming mostly sunny, with a high near 82. Breezy, with a south wind 10 to 15 mph increasing to between 20 and 25 mph. Winds could gust as high as 35 mph. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Bob Axsom

[Bob Axsom]

Last nights configuration was slow again today 177.9 kts.

Then I removed the cover and the three heavy fins and installed the three light triangular fins with the long tail bumps.

Latest configuration before the test flight:
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5791.jpg

Same configuration after the flight:
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5796.jpg

It was faster but still slow 179.1

It is time to study what has been revealed and refine the direction.

Bob Axsom

[Bob Axsom]

I see NOTHING!

Bob Axsom

[hydroguy2]

A noble quest, none the less

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0003.jpg

At first I said:

I went through the anotated NTSP spreadsheets and attached notes I collected during this effort and I saw no mod that showed up consistently in the higher speed results. I still think there is something to be gained by working with the cooling air outlet as others have been successful in doing - I just haven't found it yet. The cover extending the outlet parallel with the bottom of the fuselage to get the cooling air aligned with the flight path before releasing it; the bump in search of the Coanda effect to turn the air back up parrallel with the bottom of the fuselage; the flow fences to minimize the turbulent air recombination aft of the outlet; the confinement of the exhaust into chambers with the cooling air outlet for a pumping action to accellerate the air flow through the engine cooling system; and efforts to best position the trailing edge of the outlet cover with the "bump" for some improved performance have revealed little in this gross search. I think it is there but it does not appear to be a very big performance enhancement and it will take finer work than I have done so far to identify it.

I feel that the use of the "bump in the outlet path was demonstrated to restrict the cooling air mass flow in these tests and the immediate jump to a 2" bump, trying no other height, may be too limited an unfocused to learn much other than that. I was disappointed that I did not see an aircraft speed increase resulting from the reduced cooling air mass flow indicated by the CHT increase.

The test configuration on 4-19-12 with the two chambers formed by the fins and the cover and no bump has possibilities that I plan to explore. I'm thinking of putting that back on with a new cover with a ground adjustable closure ramp on the back and test fly with changes to the opening on successive flights.

The more I look at the matrix the more interesting observations I make. More thought required.


Then I started high lighting rows and columns and I noticed and increase and decrease in speed related to the restriction in the outlet path. The correlation is not obvious in absolute speed numbers but there is some error margin in the test method, the test sequence is not perfect and the test configurations are not exhaustive.

Note these examples:

1) The two thin wall triangular fin tests on 4/12/12 and 5/1/12 - the cunfiguration with no bump in the flow path is faster.

2) The two thick wall tapered trailing edge fin configuration tests on 4/18/12 and 4/26/12 is an imperfect comparrison because the 2" equal tail bump was destroyed in the race at Llano on 4/21/12. However, if you consider that both have a 2" bump restricting the flow and the one with the long cover is slower than the one with the cutout (back pressure relieved?) cover. That seems consistent with the thought. One might say that the test with the short cover on 4/29/12 should be faster than the identical configuration except for the long cover tested on 4/26/12 and I say 0.2 kt difference is too close to call with the error margin in this test method and the rounding in the NTPS spreadsheet calculations.

3) The first five tests have a conflict with this idea. The first two test speeds were too high to fit this relationship but if you look at the other three the interrelationship is clear. plugs in the inlet slowest, plugs and cover removed = faster, install vented cover = slower.


This is where it gets tight, I have flown the stock configuration outlet with a few more race preps at 184.4 kts and I want something better than that. It seems like a good idea at some point to fly the plane without any outlet mods installed to establish a comparable baseline then target small improved flow mods to attempt to exceed that new baseline. At this time, on our airplane, it seems to me that the cover is a speed limiting device - could be wrong. My inclination right now is favoring the flow fences and the bump in some form.

Bob Axsom

[Bob Axsom]

On 4-18-12 I tested the short tail bump using the development cover with exhaust cutouts and got 181.7 kts. Then I replaced it with the new replacement long cover with the equal length tail bump configuration still in place.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5711.jpg

I quicky flew the night before just to verifly it was OK then went to Llano, TX for the race. This is the configuration that I found shattered after the race but during that race I caught, passed and beat the SARL record holding FAC1RG Bonanza S35 which had beaten me the last couple of times. I'm thinking I should fabricate duplicates out of Stainless Steel and test that configuration. I'm thinking the long tail on the current bump "may" not be as good as the more curved surface. It makes sense to me that if the Coanda effect exists removing the curve would remove the effect.

I still have the old development cover with the cutouts - maybe I should test with it and see if I can duplicate the 181.7 speed.

Bob Axsom

[kentb]

You have inspired me to do some experimenting on my own this summer.
For now I am working on some ideas to do on my 9A

I was looking at your last picture and noticed that the outside of your cowl and exit tunnel aren't smooth in the corner. You might gain some here by smoothing this. Green area
Also (forgive my drawing) have you tried extending the cover back and up towards the fuse? Yellow areaPlease be advised that I have no skills as an aeronautical engineer.:D

http://img193.imageshack.us/img193/199/bobiw.png (http://imageshack.us/photo/my-images/193/bobiw.png/)

Kent

[Bob Axsom]

It is fun when it works.

I plan to fare those corners in for sure, they look pretty crude don't they? I have thhought about that yellow area and a few have mentioned it so it is worth looking at.

Good luck with your experiments.

Bob Axsom

[ccrawford]

Hi Bob, awesome work here.

Did you happen to record your engine temperatures vs OAT during your test runs? Be curious to know if you are getting improved cooling which might allow you to close up the holes at the front of the cowl.

[Bob Axsom]

I had to do that to get the density altitude but my temperatures are always low as a by-product of all the lower cowl baffling I developed in there several years ago. These mods made my temperatures go up to more normal levels. I don't think this would help in that way. I could be wrong, depending on how you work it I guess but for me the temperatures went up 20-30 degrees F with these mods.

Bob Axsom

[johnny stick]

Bob,
Awesome work. I think this thread will be referred to in the future for a long time. There is a lot of important data here. I think repeating the baseline test is a very good idea. I know I have learned a lot through your experimentation. Thank you for all you have done.

[Bob Axsom]

I ordered some 0.016" stainless steel sheet from Aircraft Spruce to support one thought to replicate the bumps that were fast, untested but shattered in the race at Llano, TX.

I am also giving a lot of thought to reducing the height of the bumps but this requires a lot of thought because of the mounting. The idea behind this is the restriction with the existing configuraton has been shown th raise the CHTs AND slow the airplane down. I had similar experience when trying additional an bulkhead/baffle in the lower cowl to block leaks forward around the propshaft between the cowl and spinner and separately, lower flow directing baffles to turn the air toward the outlet. Both of those extensive efforts reduced the space for the cooling air mass flow and I believe, restricted the cooling air mass flow. The airplane slowed down instead of speeding up as planned. In each case the speed came back when the new baffles were removed. I think I am seeing a similar effect on speed from the outlet restriction without reducing the lower cowl volume.

Test results that I have given a lot of thought to were the tests with no cover and no bump and the corresponding no cover with bump tests using both flow fence configurations (4 test flights).

big fences no cover no bump = 178.4 kts
big fences no cover 2" long tail bump = 182.1 kts
small triangular fences no bump = 181.5 kts
small triangular fences 2" long tail bump = 179.1

Long ago I established that the error margin could be as great as plus or minus 2 kts but you can't discard everything you don't immediately understand as normal variations associated with the test method and not the item under test. I studied the photographs and I think the small triangular flow fences taper down too close to the peak of the bumps to provide an effective boundary and the effect of the bump is lost. Here are four comparable photographs:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5791.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5793.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5732.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5728.jpg

They are not perfectly comparable but you can see the difference I think. They give me a reason for the opposite reaction to the installation of the bump. It would be interesting to see the small triangular fin configuration performance with smaller height bumps.

There is an existing configuration that I haven't flown with the 2" long tail bump that is the covered case with exhaust cutouts. I think I will fly it and collect that data first then start some tests with different bumps. Any change of the pumps or flow fences is an awkward and time consuming task because of the RTV.

Back to work.

Bob Axsom

[Bob Axsom]

Took too much time converting to get the test in today but I have a good feeling about this one:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5799.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5800.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5801.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5802.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5803.jpg

Bob Axsom

[Bob Axsom]

Tested this afternoon 179.8. A little more than 1.5 kts slower than the same configuration with the short tail 2" bump. Aircraft Spruce shipped the Stainless Steel for a possible replacement . The previous aluminum short tail bump shattered in flight. The vent tube oil flow appears straight (no swirl etc.).

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5808.jpg




Bob Axsom

[hydroguy2]

You might want to run a test with all of it removed, just to be sure other things haven't happened to influence the results.

[Bob Axsom]

A better sanity check would be to remove the cover and fly a speed test.

It is starting to get interesting now.

Bob Axsom

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/T14.jpg

If the weather is good today I think I will pull the cover and re-fly test 10.

Bob Axsom

[Kevin Horton]

Bob,

Have you ever tested the exact same configuration multiple times on different days to see how much scatter there is in the results?

I'm betting you would find a kt or two of scatter, which means that perhaps you need more than one test point on one flight to decide whether a change has given a small speed increase or not.

Why not fly a couple more tests on different days with your current configuration?

[johnny stick]

Bob, thanks for presenting your results to date in a chart form, this is good stuff.

[Bob Axsom]

Bob,

Have you ever tested the exact same configuration multiple times on different days to see how much scatter there is in the results?

I'm betting you would find a kt or two of scatter, which means that perhaps you need more than one test point on one flight to decide whether a change has given a small speed increase or not.

Why not fly a couple more tests on different days with your current configuration?

Any credible test program would need that. I want to try to see what I can get out of this strictly for the speed of this airplane so I am motivated a little differently. Test 14 in this series was a little disappointing - I am starting to visualize a combination of forces at play and I thought the relief of the cutouts in the cover would have a greater positive effect than I saw.

http://i446.photobucket.com/albums/qq186/raxsom/T14.jpg

With the test rsults sorted by speed I think I have a better direction for the next step:

http://i446.photobucket.com/albums/qq186/raxsom/T14speed.jpg

Here are some things I think are true from the test results so far:

The bump is better than no bump
Restriction in the outlet path can slow the plane down

There are a lot more things that I suspect but I do not know so I will just stew on those as I proceed. I am going to go back to test configuration #10 and re-fly that. I can rationalize that it is the best configuration tested so far just by direct observation but even with a test method error margin of 2kts it is still well within the top group tested.

Test configuration 7 and the related long cover configuration that was not tested but appeared to perform well but was shattered are also on my mind. I am not convinced that the long tail bump is better than the symetrical bump and I am not convinced that there is nothing to be gained from some form of cover - yet.

For now I'll re-fly test configuration #10 which I hope will not disappoint me. If test #10 results are validated I can focus on optimizing that configuration for a while.

Bob Axsom

[johnny stick]

the difference between runs 4 and 6 are significant and probably outside of the margin of error. I think here is a clue. By looking at the data, maybe the long rectangular fences and/or long cover is bringing the cowling air too far into the high pressure area from the wings. I am also wondering if the outside edges are seeing up flow from the wings or a slow down of the air on the underside due to influence of the wings. The air in the outlet area may not be going straight back along the fuse, but rather have an outward component; maybe the outside fences are catching this and that is why the rectangular are slower. maybe maybe maybe??

[johnny stick]

Why not try flow fences along the fuselage left and right sides just ahead of the leading edge maybe extending down the wing a little bit? Isn't this similar to the triangle fins but slightly more outboard?

[Bob Axsom]

I just got things cleaned up here at home and I heading to the airport to reconfigure to test #10 and see if that high number holds up in a re-fly.

Bob Axsom

[Bob Axsom]

I flew test case 10 again and the speed dropped off to 179.3 kts TAS. Then I reinstalled the vented cover and reflew test case #14 as suggested by Kevin Horton. It did not hold up either - 178.3 kts TAS. The hierachy is the same but the separation is reduced:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0003-1.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0004.jpg

More thought required.

Bob Axsom

[Bob Axsom]

1 - Tests 15 and 16 show in the current form the uncovered version is still faster than the covered version even though there was a 2.8 kt drop in speed from the earlier uncovered test 10 and only a 1.5 kt drop from the repeated covered test 14.

2 - Tests 8 through 16 do not clearly demonstrate that the long tail bump is faster that the equal tail bump. A replacement for the operationally destroyed equal tail bump needs to be made and tested further.

3 - The results from test 13 are are very little different from those of test 15 (-0.2 kt) so the percieved ineffectiveness of the small triangular fins when compared with the results of test 10 may not be valid even though it is still slower.

4 - Tests 10 and 15 were flown with the aluminum angle and all the open platenuts for mounting the cover exposed to the open air. If new fins were made for this "no cover" version without cover mounting provisions would it be measurably faster?

5 - Test 13 seems to indicate that the outboard flow fences have to be taller than the bump by some amount for the bumps to be effective.

6 - Is the center fin necessary or does it add drag with no benefit?

7 - All tests have been with a 2" high bump, would a shorter bump be better?

First, I think I will answer the questions in thoughts 4 and 6. This could be done by making similar shaped fins from 0.032 aluminum, assemble, fly a test then mark and cut away the exposed portion of the center fin and fly another test. Gross differences in performance should be revealed. I may be able to do that today.

Bob Axsom

[kentb]

This most likely will not help you, but....
In looking at your speed sorted chart and dividing into good speeds and not so good speeds (I chose 180 as the break point). The only thing that can be eliminated is the "extended short cover". All other parts of the configuration appear both above and below the dividing line.
I believe that what you are seeing is the changes interact with each other and it will be hard to deduce the correct parts to use.
The only only hope will be to continue to try all the combination that you can think of and see which you like the best (fastest of course).

Good luck. I love you efforts and reports.

Kent

[RVbySDI]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0001-3.jpg

With the test rsults sorted by speed I think I have a better direction for the next step:

http://i446.photobucket.com/albums/qq186/raxsom/IMG-18.jpgBob,
I am getting an error message on your images stating they have been moved. Any thing you can look into?

[Bob Axsom]

It has been apparent to me over the last few days that I am not dealing with a single variable even when I change only one variable. I agree with your observations and conclusion.

All I got done yesterday was pull the parts off to replace the three vertical elements. Will try to do more today. I am trying to force myself to exercise properly for my health. I have been working until 3 am many nights until I was too tired to exercise so yesterday I switched priorities and you can imagine the result.

I also deleted some of the obsolete tables from photobucket and learned not to do that again. It effects the thread here in a couple of ways. The images are linked and not really resident here so when they are deleted from there they are deleted from here also. There is usually a notice posted here so there is some awareness. However, I found one image that was not in this thread before so at least under some conditions the link can be associated with the wrong image. In the case I saw it was a different subject all together (a magazine cover) but some different image re-links could go undetected and provide incorrect information. I will not do that again. Well it's afternoon and I haven't exercised so I had better get at it or I will not get the change for retest done today either.

Bob Axsom

[kentb]

It has been apparent to me over the last few days that I am not dealing with a single variable even when I change only one variable. I agree with your observations and conclusion.

All I got done yesterday was pull the parts off to replace the three vertical elements. Will try to do more today. I am trying to force myself to exercise properly for my health. I have been working until 3 am many nights until I was too tired to exercise so yesterday I switched priorities and you can imagine the result.

I also deleted some of the obsolete tables from photobucket and learned not to do that again. It effects the thread here in a couple of ways. The images are linked and not really resident here so when they are deleted from there they are deleted from here also. There is usually a notice posted here so there is some awareness. However, I found one image that was not in this thread before so at least under some conditions the link can be associated with the wrong image. In the case I saw it was a different subject all together (a magazine cover) but some different image re-links could go undetected and provide incorrect information. I will not do that again. Well it's afternoon and I haven't exercised so I had better get at it or I will not get the change for retest done today either.

Bob Axsom

And I don't be the flight testing. We all need to take care of our bodies, which means our minds too. A rested and health test pilot will always collect the best information.
Rest, exercise and have a good meal. Than go back to doing flight testing.:)

Kent

[Bob Axsom]

Bob,
I am getting an error message on your images stating they have been moved. Any thing you can look into?

I have all of the images and can supply them to you. I guess I owe it to everyone to go through the thread and put the images back in or insert the updated table. I have the excel files but in order to get them presented here properly I have to print then, scan them, move them to the desk top, import them to iphoto, edit them by cropping to the proper size (the size of the table), upload to to photobucket, copy and paste as an image here. Maybe I will just do that for each table configuration that was in the thread - it is kind of important to the continuity of it all. That will be tonight's task. Jeanine always said I have OCD.

Bob Axsom

[LarryT]

Bob,

I truly applaud all the work you are doing. We will all benefit.

While I know part of the enjoyment of this excercise is getting to fly the plane, you've burned a lot of AvGas flying straight and level very carefully.

Do you think it would help to tuft the airframe and video it in flight? There are enough interactive variables that a person more skilled than I (i.e., you) might spot trends.

During testing your speed differentials have been on the order of 1 - 2%, which are pretty difficult to attribute solely to airframe modifications since variations in temperature, pressure and humidity are wider than that.

Larry Tompkins
544WB -6A
W52 Battle Ground WA

[Bob Axsom]

We all have our ways and many work. My way is to go for the objective directly. When the outlet configuration allows the plane to go faster the plane will be faster regardless of the way it came to be. I just can't use the camera and yarn approach to in hopes that when the tufts align with the flight path that is proof that all of the physical variables of the outlet are optimized. It just can't be true that there is only one configuration that will yield straight after outlet flow and that configuration is also the fastest configuration. The speed is ALL I care about.

I guess I am committed to go back and try to fix my problem with the posted images now - that pains me.

Bob Axsom

[Bob Axsom]

I went through all 31 pages and could only find post 299 on page 30 and post 302 on page 31 in this obvious category.

Bob Axsom

[rvmills]

Bob,

Been watching this each night as I toil away at the other end of my cowling. Haven't posted in this thread in a bit, but lots of interesting things going on.

I'm glad you are re-testing various configurations and doing multiple comparative tests. With the numbers so close, its hard to tell whether results are in the scatter and noise, or if you are seeing real delta v. That's the hardest thing to determine in our speed mod testing, eh!

I queried a while back on how committed you are to the long fences, because I have the forms for Gary Reed's original outlet mod design (this one):
https://lh4.googleusercontent.com/-T5cGqG6xN_g/T2Q_AhZx4BI/AAAAAAAAAbw/LPffy44sQtE/s400/gerrys%25201.jpg

The feeling I got from the last time we corresponded on it was that the large fences are a bit too permanent to swap out for something like Gary's design. However, if you can make the swap more easily than I understood it before, with Gary's permission, I can send you his templates to work with and send back. (I won't have the time to work exit shape this season (well, if I want to fly this year!). I also want to test the new Ram Air set-up first, which (as you know) I've been working to blend into my current cowl exit shape. Putting glass on the form tomorrow...here's the current look:
https://lh5.googleusercontent.com/-E41vkZEd3NQ/T6jEQLHoWNI/AAAAAAAAAcg/5dNz1rPdUVA/s400/pre-glass%2520cowl%25201%2520sm.JPG

Between the ram-to-cowl blend and the spinner gap fill I've got a pretty complex layup ahead of me (old spinner was 12" and had a 1.5" wide backplate...new Van's spinner is 13" and has the narrow backplate...result, BIG gap to fill).

To keep this on topic, after I test and fly this configuration for this season, I want to go back and mod the exit. As you can see, my tunnel diverges laterally...my guess is that is not a good thing. Looks cool, but probably not conducive to acceleration and alignment of the exit air. I'd like to change the shape to make the tunnel sides parallel to the longitudinal axis, and decrease the cavernous volume it currently has. With the new ram air having a round shape, it's started me thinking about a 6 into 1 exhaust, and keeping that round shape moving aft to the exit, much like Dan's smallest exit. Different to a degree because I have no exit ramp like the 8, but along those lines. If I stay with 6 into 2, I'd have to figure out how to decrease the cross section and deal with the two pipes. Channeling flow aft of the firewall would be step 2 for me (my thinking at this point anyhoo.)

That last part is the other suggestion I thought I'd make to you, though its a departure from your current path. Ken and Dan have had success by reducing the volume of the exit tunnel (Dan with a variable insert, Ken with a smaller cross-section exit and a cowl flap). But both those guys seem to have pinched the flow forward of the firewall (at least to a degree).

Have you given any thought to reducing the size of the cowl tunnel? I know the NG leg slot makes it harder, but that might be where some more acceleration potential of exit air is hiding.

Just some thoughts...really admire your rigor in all of this, and the multiple tests will be interesting to watch for data. I feel like I should throw some Avgas $$ your way, since you are testing a bunch of stuff for all of us! :)

Good luck on the testing ahead!

Cheers,
Bob

[Bob Axsom]

Yes I have thought of pinching the flow forward of the firewall and made several mods and tested only to find they slowed the plane down. the only thing that worked was the curved surface extending from the bottom rear of the engine to the bottom of the fuselage, side wedge planes that reduce the width of the outlet flow from full cowl to the ~14" wide outlet opening, and the horizontal plates closing off the the lower cowl outboard of the baffle and valve covers just below the cowl split line. This combination yielded the largest single speed gain of any mod I have made to the airplane - 4kts. Nothing else including something that looked like a rectangular venturi was anything but negative (many aluminum and rubber assemblies offered to the god of speed to no avail). I am completely against any further work inside the cowl. I think I have very close to the best possible combination in there.

I do not feel the same way about the exit itself and I am a long way from exhausting the possibilities. The problem is I cannot conceive an idea with supporting rational and immediately test it. Each variation has to be fabricated, materials have to be purchased and shipped, implementation details like mounting have to be worked out, the work has to be done, the test flown using exactly the same test method as previous tests, the raw data post processed, the results studied, compared with other test results and recorded for the evolving effort. Judgement, aversion to wasted effort and laziness do effect my progress - imagination is not a problem. If I merely copy mindlessly there would be little motivation for me to do the work - think of a prospector and a ditch digger. This little paragraph describes some realities I have to deal with in my mental, physical, time and life process.

Reducing the exit cross section in the outlet flow path - This has been accomplished by the two 2" bumps between the flow fences on the bottom of the fuselage and the cover. The evidence is provided by the increased cylinder head temperatures. Is the 2" height too much? I suspect it is. This was an arbitrary size triggered as a starting point by Mark Frederick's comment about cutting the size in half earlier in this thread. The installation of these bumps is quite complex and that is a hinderance to finding the right size. Since this may be turning into a long term effort I may have to stop and come up with a different installation method. The RTV is also a significant problem. It has been pointed out that it is not structural but you wouldn't think so if you had to clean it off. I have some ideas.

Turning the outlet flow with the shape of the bump - This may be the most important part of the whole experiment. Most of the testing has been with the one 2" high long tail shape and the most successful configuration is without a cover. I suspect that the long tail eliminates the turning force too early in the flow and the tested and raced once equal tailed bump may be better. I need to test it more.

The sun is coming up and I need to get work done today and I need more sleep so I will stop this post.

Bob Axsom

[rvmills]

Hi back atcha Bob! All that you said with respect to experimenting and testing makes sense. I've considered how to work with my current exit shape to see if I can find improvements, and still may do some of the followng:

1. Because my exit bottom is large and raked forward (the aft edge of the cowl tunnel is not vertical...the lower edge is forward of the firewall, giving me about 85 sq in of exit...cavernous), Tom Martin told me to first extend the skin (with sheet AL) back to the pipes and see what the result was. Easy first step.

2. Then I could start filling in the cavern from the bottom up to see if acceleraton of the air and more speed results (foam or whatever...gotta think on that, as I don't want to create a fire hazard).

3. Then I could add fences and/or a Vetterman-like after-body to check the results.

All that sounds good, but my lower cowl tunnel is so low, that the more I think about it, the more I think I need to just bite the bullet and reduce the area by cutting the aft cowl and re-shaping it. Dan's switch-able set-up is attractive, because it allows one to go bigger if you find cooling inadequate.

Recently a smart aero guy whispered in my ear that reducing the stock -6 exit to about 1/2 the normal area, and adding a cowl flap for climb may be as effective as all the fence/afterbody work.

Not sure how one would accomplish this is a -6A (with the nose strut and slot), but what I was getting at above and am referring to here is that you might find more benefit from taking the plunge and reducing the outer cowl tunnel dimensions, instead of just filling the inside. Inside shape will remain important, but reducing the outer shape will give you convergence and reduced frontal area all at once. Its a large commitment though...and one I'm wrestling with hard right now!

Just more thoughts...looking forward to your continued work Bob! Having fun following along, and scheming with you!

Cheers,
Bob

[F1Boss]

If the nose strut is in the way for proper outlet placement, or function, why not move the outlet?

Statement overheard at Reno:
"I've never seen an idea so good that I couldn't copy it!"

Look where Lancair put the outlets on the go-fast Legacy. There is science behind that placement (on the bottom, near the outer corner) - might as well give it a shot! Might take some exh system re-configuring? Won't work on the skinny tandem airframes...

Using Dan's idea of a replaceable outlet section, it would be easy to experiment. Are ya listenin', Super Bob?

Carry on!
Mark

[gereed75]

Bob, I see now how your non-standard exit changes from the standard.

By all means, forward my prototype to where ever it might do the most good (or harm!?!?). I remain convinced that just streamlining the exit air has significant benefit, both for cooling efficiency and drag reduction.

Right now I am kinda freezing my design. I have some ideas about where to go, but I am wondering about the benefit at this point. It seems that I am stuck on a plateau.

Multiple 3 leg NTPS speed runs without configuration changes on different days with the same method yield results +/- 1.5 knots.

I recently closed my elevator ends with no apparent speed increase (maybe a decrease). I kind of wonder if 180 - 181 KTAS at 6000 DA corrected for temps is as fast as my stock IO 360 can make my particular -6 go?????

Waiting for my first SARL opportunity (Indy) to see what happens.

[NDrv8r]

I previously posted a video on tuft testing using a cowl flap. The testing revealed poor airflow across the flap when closed.

I just returned from a 10 hour trip, and having the flap closed makes a definite 4kt gain over open. the cyl temps are in the 325 range, the ot 195, ff 8.5gph, TAS 169, somtimes 170kt at 10000.
Makes me wonder how it will work with a smoothing after body ala Veterman in conjunction with the movable flap.

I want to focus on the magic 7 degree number blending back into the bottom fuselage. The idea is close off excess exit area, smooth the flow over the cowl flap between the stacks, increase the flow around the exhaust pipes to better match exterior flow.

On a related note, Dan, Kevin and others that have done cowl pressure tests. what kind of pressures have you noted in the upper plenum? At 180Kt I am seeing 14.7in H2O. At 180 kt the pitot pressure should be around 21.5 in h2o. I have the low side of the sensor tied to my aircraft static port and the high side to a static port (1/8 tube with .040 holes drilled on 4 sides) at the fuel distribution block. Should there be this great of a loss?
If I understand, if there were perfect pressure recovery I should get the same pressure as a pitot tube???

I also have a sensor on the engine mount tube. The upper plenum pressure stays constant and the lower plenum pressure increases from 2 to 4 when closing the cowl flap.

[Sig600]

Ahhh! Bob, what have you done!?! :o:D

[hydroguy2]

....... of wonder if 180 - 181 KTAS at 6000 DA corrected for temps is as fast as my stock IO 360 can make my particular -6 go?????........

Kind of where I am. I don't have the time or gas money to run the meticulous testing that Racer Bob has been doing for us all. I would really like to make some significant modifications to my lower cowl and even try a new 4 to1 exhaust....but this ain't happening any time soon. I have built a after fairing for the cowl exit, but afraid to drill the fuselage to mount it. I've seen 192-193kts TAS at 6600' DA. but usually it's nearer to 189. I know(in comparison to others) I'm slippery for a -7, since I cruise at 175kts burning 8.5 or so. 9gph is 178-180kt TAS.

At this point, my plane is faster than me!:o I should be able to run down that #71, but still trying to figure out the methodology.

Get me faster.......then we'll work on fine tuning Aurora is my next step.

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5820.jpg

This was a dogged task to make these fins to duplicate the profile of the heavy ones without the cover mount hardware but they are finally done and ready to test. These are made of 0.032 2024-0 in place of the 2024T3 0.090 originals. I was ready to fly this evening but the ceiling wouldn't allow me to go to test altitude. Tomorrow is supposed to be sunny.

Once this test is done (basically to see how much the cover mounting hardware affected speed) I think I will try a couple of changes on the fin profiles.

Bob Axsom

[vlittle]

...Recently a smart aero guy whispered in my ear that reducing the stock -6 exit to about 1/2 the normal area, and adding a cowl flap for climb may be as effective as all the fence/afterbody work...

Bingo. Reducing the exit area and the frontal area of the cowl is the secret to success, however....

As pressure in the lower cowl increases due to the reduced outlet area, air will tend to leak in other areas. Around the prop spinner is one, and any gaps in the hinges/platenuts/camlocks as well. Tightly sealing the cowling and making all of the cooling air exit the bottom will be the challenge. A good way to test this is to tape the cowling joints all around and measure cht and airspeed.

Around the spinner, Tom Martin's method seems to work... but his results actually reduced airspeed due to increased mass airflow... so the reduction in outlet area is necessary to regain speed.

Bottom line... reduce cowl exit area, seal the cowl air leaks and reduce cowl frontal area. 100% money-back guarantee to increase speed.

[DanH]

...what kind of pressures have you noted in the upper plenum? At 180Kt I am seeing 14.7in H2O. At 180 kt the pitot pressure should be around 21.5 in h2o.

Dynamic pressure of 21.5" H2O at 180 knots on a standard day means you would be flying just slightly below sea level. You're probably not piloting a 180 knot submarine ;). What is your altitude when you're measuring 14.7"?

I have the low side of the sensor tied to my aircraft static port and the high side to a static port (1/8 tube with .040 holes drilled on 4 sides) at the fuel distribution block.

That will work.

Should there be this great of a loss? If I understand, if there were perfect pressure recovery I should get the same pressure as a pitot tube?

I think you'll find your max possible Q to be quite a lot less when you factor in altitude (multiply by density ratio)...so your 'loss" will be less than you think now. That said, it's not really a loss. Upper plenum pressure will never equal max Q. Remember, the airspeed pitot is a closed end system, while the cooling flow is open ended. The cooling system does not bring the air to a full stop, thus it does not convert all the available kinetic energy to increased static pressure.

[1Oldguy]

I beleive you are on the right track reducing the air exit size. That is for sure if you have the cowling set up for fuel injection with a stock vans intake scoop. On my IO320 I used the intake scoop for a carburated engine which has nearly a quarter or one third less exit area. I was able to make this work with my Bexdix injection system by thinning my fiberglass air filter housing by an inch or so. All fit fine. Then added my gen two Vetterman fairing and life was good and faster also. Just remember there is a fine line between the sweet spot (size/area) and temperture control. We all try to fine tune our planes but in the end they are all just a mixture of compromises.:)

[rvmills]

OK, here goes the mutha of all multi-quotes! :eek:

If the nose strut is in the way for proper outlet placement, or function, why not move the outlet?

Statement overheard at Reno:
"I've never seen an idea so good that I couldn't copy it!"

Look where Lancair put the outlets on the go-fast Legacy. There is science behind that placement (on the bottom, near the outer corner) - might as well give it a shot! Might take some exh system re-configuring? Won't work on the skinny tandem airframes...

Using Dan's idea of a replaceable outlet section, it would be easy to experiment. Are ya listenin', Super Bob?

Carry on!
Mark

A. Hey man, stop listening to my conversations at Reno! :p
B. I'll buy the beer and provide a shady spot under my cowling for you to show me your idea (so I can steal in!)...right after we crawl under Lee's Legacy at PRS...deal?
C. Kinda leanin' towards 6 into 1 and a single opening...two openings seems counterproductive...but if they are small, and the air is well channeled...hmmm...and I'm always listenin'! ;)

Bob, I see now how your non-standard exit changes from the standard.

By all means, forward my prototype to where ever it might do the most good (or harm!?!?). I remain convinced that just streamlining the exit air has significant benefit, both for cooling efficiency and drag reduction.

Right now I am kinda freezing my design. I have some ideas about where to go, but I am wondering about the benefit at this point. It seems that I am stuck on a plateau.

Multiple 3 leg NTPS speed runs without configuration changes on different days with the same method yield results +/- 1.5 knots.

I recently closed my elevator ends with no apparent speed increase (maybe a decrease). I kind of wonder if 180 - 181 KTAS at 6000 DA corrected for temps is as fast as my stock IO 360 can make my particular -6 go?????

Waiting for my first SARL opportunity (Indy) to see what happens.

Gary, I've played with the forms, and I really need to mod the cowl to make it work. If/when I go there, I may not get back to stock. Still interested to see if you have recorded speed gains from the firewall radius alone and the exit mod alone? Are those experiments reversible and testable one at a time and back to ground zero?

I previously posted a video on tuft testing using a cowl flap. The testing revealed poor airflow across the flap when closed.

Makes me wonder how it will work with a smoothing after body ala Veterman in conjunction with the movable flap.

I want to focus on the magic 7 degree number blending back into the bottom fuselage. The idea is close off excess exit area, smooth the flow over the cowl flap between the stacks, increase the flow around the exhaust pipes to better match exterior flow.

Larry, can you repost the link to the video? Any pics of the cowl flap set-up? Thanks!

Then added my gen two Vetterman fairing and life was good and faster also.

1Og, any pics of the V gen 2 afterbody? Apologies if its already in this thread... :o

Bingo. Reducing the exit area and the frontal area of the cowl is the secret to success, however....

As pressure in the lower cowl increases due to the reduced outlet area, air will tend to leak in other areas. Around the prop spinner is one, and any gaps in the hinges/platenuts/camlocks as well. Tightly sealing the cowling and making all of the cooling air exit the bottom will be the challenge. A good way to test this is to tape the cowling joints all around and measure cht and airspeed.

Around the spinner, Tom Martin's method seems to work... but his results actually reduced airspeed due to increased mass airflow... so the reduction in outlet area is necessary to regain speed.

Bottom line... reduce cowl exit area, seal the cowl air leaks and reduce cowl frontal area. 100% money-back guarantee to increase speed.

All good info Vern! I did not know Tom lost speed when he closed down the cowl on the prop hub. I knew he no longer had to open his cowl flap for climb, so it makes sense. Glad you mentioned it here, as that would have been disappointing to seal there and slow down...until the light bulb came on. Will plan those mods together, in sequence. I discovered that with my new Van's spinner and my extended hub, I can't use Tom's method, as the forward edge of the cowl won't close on the round part of the hub. Will be using Dan's method instead.

Hey Dan, have you measured the effect (temps and speed) of the baffle material closures you use at the hub? I know you built it that way, but given Vern's comments, I wonder if there is an optimum blend of sealed prop hub and exit area...perhaps its just that the seals allow you to go smaller and smaller, and gain speed via mass airflow reduction, until you reach your EGT CHT comfort limits. Thoughts?

Get me faster.......then we'll work on fine tuning Aurora is my next step.

Do both Brian! Keep working on the airplane, then remember, "slow is smooth, and smooth is fast". Lead the turns, don't over-g and bleed, and get on and stay on the line. Oh, and always guess right on the winds! :p

Ahhh! Bob, what have you done!?! :o:D

Just trying to fly my baby again before your deployment is up...at this rate...

An HRII, a J-3 and an RV-7 have kept me going, but things are going together versus coming apart now...I sat in the plane for the first time in 5 months to replace the 396 and made airplane noises again...getting closer!

Once this test is done (basically to see how much the cover mounting hardware affected speed) I think I will try a couple of changes on the fin profiles.

Bob Axsom

Bob, what's this test looking at? Thinner guage fences? Hardware changes...anything else? With long .032 fences in there, I'd be thinking about vibes too. Wanna borrow my Countour camera and take some video down there? I can send it with Gary's forms if you like.

I asked Gary about the separate gains from his firewall radius and his afterbody fairing. I think the sum was 3-4 knots (Gary?). The radius is reported to be a very positive thing, so the afterbody may be incremental, versus a silver bullet. And going long on the fences may be hurting the overall effect. We should research the effect of underbody strakes and fences on speed. They usually add stability, not speed. Not sure at what length or what station your flow fences become draggy. Alignment becomes more and more critical as length increases. If they are adding a yaw input anywhere, it will be draggy.

Are you going to stop by Reno and PRS on your way to the Euphrata race? Would like to see all this in person...and you can join Mark and I under the Legacy! :D

Cheers,
Bob

[Bob Axsom]

The test in a few hours is using 0.032 of place of the original 0.090 partly because that is what I had on hand. There is a little more to it than thinner material. That configuration was flown in test case number 10 and provided the fastest speed in this series of tests 182.1 kts. Then I repeated it in test case 15 and the speed was only 179.3 kts. I have always said that I thought the error margin of the test method I use is +2 so the the difference may be just that. Both flights had the 2" high bump with a long tail (you can see it in the photo) and the three vertical members (flow fences, fins, separator ...). The vertical members had a lot of stuff at the lower end that is used for mounting the cover (cover is not installed in this configuration). A ~10 long piece of 3/4"x3/4"x1/16" aluminum angle and 19 #8 floating platenuts. This test case has the same fin profile on all three parts but none of the cover mounting hardware. It should be faster but the difference may be lost in the test method error margin. It was agonizing to build the parts for such a seemingly trivial test but it is done now and it will be good to see the results in the table. Another previous test to keep your eyes on with respect to the upcoming test is test case 13 which had the same bumps, no cover and 0.032 fins of the straight triangular configuration. Its speed was 179.1. By the way these triangular fins were also used with no cover and no bump in test case 6 and yielded a speed of 181.5 kts.

I have to use my judgement in evaluating these results and call things meaningful as I see them and I may get it wrong but even then it keeps me from getting into a mental fog with no direction. "Do something even if it's wrong" applies here.

I have several ideas about what is wrong with my test configurations so far and I just received a shipment of material from Aircraft Spruce today to use in addressing the problems.

A difference to observe in test case 13 fin and bump height relationship and those same relationships in test cases 10, 15 and 17 (after sun rise) is, at the bump peak in 13 the small triangular fins extend just slightly beyond the bump and may not isolate the cooling air outlet flow allowing interaction with the surrounding air at the worst possible time. If you try to visualize it I think you will see what I am talking about. It is possible that test 17 will yield a new high in this series because the bump will be working the Coanda magic and the cover mounting hardware will be gone.

The bump shape. I have only tried two and one had a catastrophic failure observed at the fuel pump after completing the very successful race at Llano. I thought some advice in this thread was good and I took the bump profile in that direction - the long drawn out shape after the node. From a streamlining perspective it seemed like a good idea but the Coanda effect depends on the curved surface continuing to curve in the direction you want the air to go. When I flatten the surface out the air flow loses interest and goes its own way. I think I need to work back toward my original shape but the 0.016" 2024 T3 seemed too weak for the task. This may be misleading though because I observed evidence of high frequency trailing edge oscillations with excursions of 2 to 3 inches on the side of the center fin. It could have shattered from fatigue. The long bump is made from 0.032" 2024 0 aluminum and the trailing edge is secured with screws so it is much stronger, less responsive and mechanically secured against oscillation. These features could be applied to the old shape or at least something in that direction.

I am seriously considering stopping at the PRS event on the way to Washington. Is the Legacy you are talking about race #5?

Well I'm going back to bed for a little while.

Bob Axsom

[rvmills]

Good luck in the tests Bob! I think Lee's Legacy is Race 5, but there are several Legacy entrants...all fast! Talked to Rick Vandam (Sport Class Sec) yesterday, and there are 10 Sport Class Rookies coming to PRS (a few of them are RVs...couple 8's and a 3), and a total of 18 Sport Class Racers will be at PRS...more than any other class! Hope to see you there...come hang out with us!

Cheers,
Bob

[Bob Axsom]

The speed was 179.1 kts. For Test case #18 I removed the center fin. Letting the RTV cure. Will fly shortly.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5828.jpg

Bob Axsom

[Bob Axsom]

Test #18 achieved 177.8 kts - pretty bad and I had time left so I flew the triangle again and got higher speeds on every leg and ended up with 180.2 Using Kent's 180 good/bad split logic this one is still barely in the game. I will update the tables and think about what to do next.

http://i446.photobucket.com/albums/qq186/raxsom/IMG-18.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0001-3.jpg

Bob Axsom

[Bob Axsom]

I'll drop out for a while and get back to you later when I have something on the other side. I ordered some more aluminum tonight/this morning and I am going to construct a removable base that I can use for more rapid change of experiments and avoid the use of RTV.

Bob Axsom

[Kevin Horton]

Test #18 achieved 177.8 kts - pretty bad and I had time left so I flew the triangle again and got higher speeds on every leg and ended up with 180.2 Using Kent's 180 good/bad split logic this one is still barely in the game. I will update the tables and think about what to do next.

http://i446.photobucket.com/albums/qq186/raxsom/IMG-18.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0001-3.jpg

Bob Axsom

Bob - The fact that you can fly the same configuration twice and get speeds that are 2-3 kt different suggests your current test technique produces results that are only accurate to 3 kt or more. The changes your are making to the aircraft probably have an effect on performance that is smaller than the accuracy of your test. It is useless to try to compare results from single data points given the demonstrated test to test variability. If you wish to determine whether any given mod helps or hurts you'll need to get a test protocol that yeilds higher accuracy.

Given the level of accuracy you need to measure, you need to fly several tests on each configuration and average the results. Given that there can be small effects from rising and descending air mass, you should do these tests on several different days.

Do you have the ability to record engine and flight data for later processing? What test protocol do you use? It might be possible to tweak the test protocol somewhat to reduce the test to test variablity.

[Bob Axsom]

Bob - The fact that you can fly the same configuration twice and get speeds that are 2-3 kt different suggests your current test technique produces results that are only accurate to 3 kt or more. The changes your are making to the aircraft probably have an effect on performance that is smaller than the accuracy of your test. It is useless to try to compare results from single data points given the demonstrated test to test variability. If you wish to determine whether any given mod helps or hurts you'll need to get a test protocol that yeilds higher accuracy.

Given the level of accuracy you need to measure, you need to fly several tests on each configuration and average the results. Given that there can be small effects from rising and descending air mass, you should do these tests on several different days.

Do you have the ability to record engine and flight data for later processing? What test protocol do you use? It might be possible to tweak the test protocol somewhat to reduce the test to test variablity.

All good observations.

Aircraft Spruce got a little richer last night as I try to improve the test bed. I bought enough 2024 T3 0.063" and 0.090" alclad for four base plates that span the the outboard flow fence mounts I have riveted to the bottom of the fuselage with Cherry rivets. One thing that has been a hinderance is not wanting to deteriorate the fuselage in the process of attaching of test articles. This change will free me from that problem/worry. I will have a easily removable and replaceable false fuselage bottom a little less than 1" below the fuselage directly behind the outlet.

Bob Axsom

[kentb]

Bob, I notice that you don't have hinge pin in the middle of your exit air like I do on mine. Do you not use hinge to attach the lower cowl? If you do, how do you secure the ends of the pins.
I have been think on how to move mine so they are not in the exit air. I am sure that they don't effect the air much, but they will be in the way when I start doing my experimenting.

Kent

[Bob Axsom]

There was so much stress on the lower cowl to fuselage interface that I had to remove the hinges from that location - all the others are OK. Rivet heads were popping off on the fuselage side of the hinges. At first I went up in rivet diameter then I replaced the hinges with solid plates. They have platenuts for the cowl side. When I had hinges I had a small loop in the end of the hinge pins and after fully inserting them I safety wired them to the engine mount. I made up a little hinge pin puller tool out of the soft hinge pin material (that I had replaced with hard hinge pin material) to reach in and grab the little loop (after cutting the safety wire from the inside) and pull them out. I still use that puller for the other hinge pins - it's just a finger loop and a hook.

Bob Axsom

[jrovey]

All good info Vern! I did not know Tom lost speed when he closed down the cowl on the prop hub. I knew he no longer had to open his cowl flap for climb, so it makes sense. Glad you mentioned it here, as that would have been disappointing to seal there and slow down...until the light bulb came on. Will plan those mods together, in sequence. I discovered that with my new Van's spinner and my extended hub, I can't use Tom's method, as the forward edge of the cowl won't close on the round part of the hub. Will be using Dan's method instead.

Hey Dan, have you measured the effect (temps and speed) of the baffle material closures you use at the hub? I know you built it that way, but given Vern's comments, I wonder if there is an optimum blend of sealed prop hub and exit area...perhaps its just that the seals allow you to go smaller and smaller, and gain speed via mass airflow reduction, until you reach your EGT CHT comfort limits. Thoughts?



I added the baffle material like Dan's before the Texoma race. Prior to this mod my small oil leaks under the front of the engine migrated forward. After adding the prop seal the oil moves rearward. Makes sense. I also noticed my CHT's increased about 15 degrees which does not agree with Tom's result. Did not see an obvious speed change but I did not run the triangle course either. It appears a significant amount of air from the lower cowling was exiting between the cowl/spinner gap on my RV8.

Bob Mills - It will be interesting to see how you decide to modify your exhaust. I chose the 4-1 and extended the collector with the intention of reducing the cowl outlet. Still trying to decide if I go the bluff body route or not. Not enough time for me to try lots of options so the result is no action.

On a side note I would like to see the T-6 class at Reno go away and see a 4 cylinder RV only class added. RV's are not competitive in any Reno class (not talking about Super RV's here).

[Bob Axsom]

I'm a little under the weather at the moment but I pulled the last mod this evening. I have to remove a lot of Dow 736 red RTV still but then I think I will fly a test with no fins or covers or bumps before I start working with the test bed.

Bob Axsom

[Bob Axsom]

I flew the test twice today and the speeds were all over the place on each leg. I never came close to meeting the USAR test method requirement of five 20 second interval recordings within 1 kt. Regardless, I recorded a large number of speeds and divided by the total to get the inputs for the NTPS spread sheet then I repeated it. The input average numbers look right but the results are disappointing. 178.8 kts and 177.6 kts respectively. Test fuel cost $71.50. The RPM was 2720, the EGT on CYL #4 was 1300, the CHTs were 357, 369, 365 and 342(?) but the power didn't seem as robust as it had been in the past. For what its worth here are the updated charts:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0003-2.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_0002-4.jpg

Bob Axsom

[rvmills]

I added the baffle material like Dan's before the Texoma race. Prior to this mod my small oil leaks under the front of the engine migrated forward. After adding the prop seal the oil moves rearward. Makes sense. I also noticed my CHT's increased about 15 degrees which does not agree with Tom's result. Did not see an obvious speed change but I did not run the triangle course either. It appears a significant amount of air from the lower cowling was exiting between the cowl/spinner gap on my RV8.

Bob Mills - It will be interesting to see how you decide to modify your exhaust. I chose the 4-1 and extended the collector with the intention of reducing the cowl outlet. Still trying to decide if I go the bluff body route or not. Not enough time for me to try lots of options so the result is no action.

On a side note I would like to see the T-6 class at Reno go away and see a 4 cylinder RV only class added. RV's are not competitive in any Reno class (not talking about Super RV's here).

Jason,

On the day of the test for the prop seal baffle, was the OAT hotter, or did you do that test with a heat-soaked engine (second, third flight on a hot day)? All that could raise oil temps and CHT and give an apples to oranges comparison. Kevin is spot on on the multiple test profile. When I'm really in the test mode, I try to do them on multiple days and, if its a reversible mod, do them in opposite order (A first on one day, B first on the next...hoping for similar conditions...which can be the hard part).

As far as exhaust goes...I need to do more research, and talk to Larry. There is a 6 into 1 Rocket exhaust out there too, but I've never seen it on a Rocket (hmmm). Before I start messing with exhaust and the outlet, I want to get the current configuration flying (new baffles, plenum and ram air), get some baseline data, including instrumenting up per Dan's pressure differential set up (need to get smarter on that too...looks pretty simple, but just need to work through the set up).

Dan, if you're still reading this thread, have you measured an impact to performance from the measuring devices? (Might be a dumb question, but I'm good at those! ;))

Jason, as far as Reno goes...with a few more RVs coming to PRS this year (and where is Race 391, hmmmm?), if we get enough RVs there in September (any September), the Sport Bronze could almost become a de-facto RV/Rocket class. And if there are a couple mini-races within the heats (6 bangers up front, 4 bangers a bit back), its still good racing and good entertainment (for the pilot and the crowd). From what I've seen, starting a new class at Reno is very difficult...politically with RARA, scheduling-wise with the Air Boss...and logistically, you really gotta have your stuff in one sock to play there as a new class. Also, good luck getting rid of the T-6s...good group, cool airplanes, and crowd-pleasing NOISE! :D

All in fun!

Off to sand-fill!

Cheers,
Bob

[jrovey]

On the day of the test for the prop seal baffle, was the OAT hotter, or did you do that test with a heat-soaked engine (second, third flight on a hot day)? All that could raise oil temps and CHT and give an apples to oranges comparison. Kevin is spot on on the multiple test profile. When I'm really in the test mode, I try to do them on multiple days and, if its a reversible mod, do them in opposite order (A first on one day, B first on the next...hoping for similar conditions...which can be the hard part).


My observation is about 15 deg CHT rise over many days at many different ambient temps. Just seems to run a little hotter which is fine with me. I was kindof on the cool side and now in cruise and 75% power the CHT spread is around 360-370. When in an aggressive climb the CHT's rise faster now than before. I know this is not good data numbers but I really don't have an interest performing back to back comparisons on the front prop seal. It seemed like a good thing to do and am leaving it on regardless. Because my CHT's are a little higher I assume mass airflow through the engine is a little lower and speed would be slightly higher but cannot confirm any of that.

Thanks for the Reno comment. Even though the T-6 guys are nice and make lots of noise at 230 mph there is very little if any interesting stuff going on in that class.

If I ever have $48,000 laying around then I will get the Superior XP-408 engine and come to Reno. It is a 230 hp parallel valve so I assume 50 more hp for little weight penalty. My stock XP-360 180 hp just isn't enough to bother.

Jason

[Bob Axsom]

No wonder the engine guys are going out of business. I've looked at my data and I think for this race season I'm going to have to find the best outlet configuration I can and go with it. The last two test I ran yesterday I know are invalid (20 & 21). The wide velocity swings (>10 kts, the worst I have seen) from the up and down drafts over the Ozarks on altitude hold was unavoidable. I can go out there at dawn tomorrow and fly it before the sun heats everything up and gets the air moving around. I am leaning toward the configuration in test 7. I know how silly this sounds but it just looked right. I bought a sheet of 0.016 stainless steel to make the bumps out of instead of the 0.016 2024-T3 aluminum that had shattered by the end of the race at Llano, TX. I may make the bump lower and I may make the cutouts larger before I go the the test bed configuration. The methodical repetitive testing is all well and good but speaking from the perspective of a mature retired guy on fixed income you would like throw away some less promising options before wasting a lot of time and money on them.

Bob Axsom

[DanH]

Dan, if you're still reading this thread, have you measured an impact to performance from the measuring devices?

No attempt to do so.....any impact due to measurement would probably be within the data scatter.

Here's the stuff currently living in the cowl exit flow, pitot-static on the left and shielded temperature probe on the right. In theory the two devices probably slow exit velocity a little. I wouldn't expect any change due to the piccolo tubes above and below the cylinders.

http://i50.tinypic.com/ff1vk8.jpg

All that could raise oil temps and CHT and give an apples to oranges comparison.

Yep. Oil and cylinder head temperatures alone are poor indicators; too many other factors make them rise or fall. If you want the same CHT or oil temp with less mass flow you must improve heat transfer efficiency, i.e. make less air carry more heat. Consider using a different figure of merit for development:

change in air temperature / media delta = heat transfer efficiency

or (for example)

(exit air temperature - OAT) / (CHT - OAT)

Here is a real example, data from a flight to Jackson MS and back last summer. The outbound leg was in the early AM, and being a business trip I was in a hurry to get there. So max cruise with rich-of-peak mixture, and thus elevated CHT despite low OAT. The return leg in the afternoon was more relaxed, running LOP with lower CHT despite higher OAT.

(exit 234 - OAT 64) / (CHT 324 - OAT 64) = 0.65

(exit 232 - OAT 86) / (CHT 307 - OAT 86) = 0.66

Efficiency is similar, despite very different CHT and OAT conditions.

Let's suppose you made some clever modification which caused the air to pick up more heat......say exit temp was now 250F instead of 232F:

(exit 250 - OAT 86) / (CHT 307 - OAT 86) = .74

A few notes:

Exit air temperature in this case was measured immediately below #1 cylinder, not at the cowl exit. I wanted separate cylinder and oil cooler data. Temperature (thus efficiency) measured at the cowl exit will be lower.

Most of us have fixed cowl exit areas, so mass flow is increased as we go faster. Exit temperature will drop given the same CHT and OAT, but more air. So, you must fly the same TAS for direct comparison of cooling efficiency mods.

[kentb]

Dan, I mounted to temp probes in my cowl exit air. One at the center and one under the exhaust pipe (about 1.5 inches from the pipe). I was surprised that the probe under the pipe was only 2 degrees higher then the one in the middle (this is an RV9A).

OAT 60
exit air 140
cht 340
(140 - 60) / (340 - 60) = 28.5 :o

Does my plane suck or what?


By the way what is the exit air speed on your plane?

Kent

[DanH]

OAT 60
exit air 140
cht 340
(140 - 60) / (340 - 60) = 28.5 :o
Does my plane suck or what?

Remember, my example efficiency of .65~.66 was based on outlet air from a cylinder head only....meaning very hot surfaces compared to an oil cooler. The temperature of mixed cylinder outlet air and oil cooler outlet air will lower, so efficiency measured at the cowl outlet will be lower. Note this mixed air method does not result in a strictly accurate efficiency value; the equation input was CHT, not some average of CHT and oil cooler fin temperature. There's no handy way to do that. No big deal....we're looking for a figure of merit, something relatively easy to measure, exactly as you have done.

Back to your 0.28 value; poor, but not a lot less than values in our test database, assuming you took the temperature readings at high TAS. The test partners have agreed to not discuss actual values until we're satisfied with the work, so please be patient....all in due course.

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5838.jpg

Worked from midnight until a little after 4 am on this new approach. There is a race in Terrell, TX Saturday.

Bob Axsom

[Bob Axsom]

Interestingly, you can see the pattern of the original "bump" in the RTV residue on the side of the center vane - it was 2" high. All the work last night was spent developing an effective interface with the front end of this bump and the existing lower cowl baffle I have extending from the back of the engine to the bottom of the firewall. I was able to use an existing platenut, add another one and feed the leading edge under the trailing edge of some existing sheet metal in the complex existing baffle. I didn't want to rush into the rest of the structure without some thought so I came home for a little sleep. I want to avoid RTV this time. I have an idea for the side support and anxiety is keeping me from resting so I guess it is time to get back to work.

Bob Axsom

[gereed75]

I think that this approach intuitively looks great. It integrates the airflow from your previous work.

Good luck for positive results and some well earned rest!!

[rvmills]

I think that this approach intuitively looks great. It integrates the airflow from your previous work.

Good luck for positive results and some well earned rest!!

+1...Good luck on the test Bob!

Cheers,
Bob

[Bob Axsom]

Today went well. The hardest part of some of these mods is the implementation details while preserving the quality of the airplane. It is so frustrating to do rough work to test a concept with things that are crude and it is a double edged sword ... Ah well. I've been fighting some infection with 300 Mg Clindamycin for the last week and today is the first one that I feel like I am good. Consequently I am better organized and patient with this one and it is going to look good and be a good installation even if it doesn't work and that makes me feel good.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5842.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5846.jpg

Bob Axsom

[Bob Axsom]

It is amazing how little I got done but I'm using platenuts and screws To put this version together. I only had 8 single ear platenuts in my stash so ordered the total requirement (12) in regular, miniature and floating for 36 to cover any complications with next day air delivery. The next race is Saturday and this is Wednesday (well technically it's Thursday). Parts $24 shipping $35 There is no way everything is going to come together before the race. I'm still carrying the bug gut load from Texoma but washing is going to have to wait again. What I have together so far looks great.

Bob Axsom

[Bob Axsom]

The support structure is done to a first cut stage but it needs more time to finish than I have before the race in Terrell, TX tomorrow. Going to have to put the plane back together, fly there, run the race to stay in the point hunt and come home to finish it before the race at Carbondale on 6-9-12.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5854.jpg

Bob Axsom

[Bob Axsom]

The right panel (bump?) skin is fitted and trial installed except for the trailing edge. I should work that out tomorrow.

Bob Axsom

[Bob Axsom]

Obviously I did the right thing not trying to cram this in before the race at Terrell, TX last Saturday. I will have it done before the Big Muddy race at Carbondale, Illinois a week from Saturday. As far as the bumps are concerned they are essentially done except for securing the tail end. I will have to use some RTV which I did not want to do but I have some side gaps so I got to plug'm. I was successful in making the panels and support beams removable with a screw driver - it's more experimenter friendly that way - and no additional holes were added to the airframe.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5903.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5905.jpg

The little stainless steel part is one of two that I will finish tomorrow to secure the trailing edge of the panels.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5907.jpg

They are unique for each panel to match the interface. The relief cutout in the trailing edges provides a path for the outlet air from cowl zone 3 - oil cooler, blast tubes (when not blocked for racing), etc.

I plan to fly the first test using the cover with the exhaust cutouts. I haven't decided yet but I am thinking of enlarging the cutouts to match the exhaust pattern deposited on the inside during previous tests.

I hope for a knot out of this but expect nothing.

Bob Axsom

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5910.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5911.jpg
The mod development work is done. Should be able to test tomorrow.

Bob Axsom

[Bob Axsom]

When I used to drive up the 405 to LAX this was a common visual treat:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5950.jpg

So why on this day was it in Fayetteville, Arkansas? Walmart shareholder's meeting perhaps ...

Anyway, as you can see 6,000 ft density altitude VFR for speed testing wasn't possible. Maybe tomorrow. I did enlarge the cutouts in the cover.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5983.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_5984.jpg

Bob Axsom

[Bob Axsom]

After flight photo (no shattered panels)

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5985.jpg

Results in test order:

http://i446.photobucket.com/albums/qq186/raxsom/T1to23.jpg

Sorted by speed and ranked:

http://i446.photobucket.com/albums/qq186/raxsom/rankedT1to23.jpg

These are encouraging. I will probably fly this configuration at Carbondale, IL in the Big Muddy Air Race since it is fairly fast and I have so much cleanup to do to get ready. But all the fence and cover configurations need to be tested with these bumps. The two test results were only 0.6 kts apart even though it was bumpy and I could not meet the USAR Handicap procedure requirement of no deviation of more than 1 kt in a series of five samples 20 seconds apart. At 180.6 and 180.0 kts They rank 6th and 9th in the series of 23 tests. My gut feel is this configuration of bump is worth more study.

Bob Axsom

P.S. The Goodyear blimp is still at Drake Field - they must be giving VIP Walmart shareholders rides. I asked if the Goodyear blimp is the one I used to drive by on the 405 on my way to LAX and the pilot responded "No that one is still there, this one is based at Akron, Ohio."

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5913.jpg

BA

[David Paule]

Just to compare the exit areas to what my 1955 Cessna 180 has, here are some photos. The 180 has cowl flaps which are VERY effective both for cooling and for speed.

Here's the inlet area. Pretty large, eh? Also notice the diverging duct to the large air filter. Somewhat spoiled by the edge of the lower lip, though.

http://i46.tinypic.com/2czq9u8.jpg

Here's the cowl flaps open:

http://i48.tinypic.com/30vznux.jpg

And again from the side:

http://i46.tinypic.com/swtuu8.jpg

Here's the cowl flaps closed. Note the relatively small exit area and the position and length of the exhaust pipe:

http://i48.tinypic.com/kdljjd.jpg

Here's what they look like inside the cowl. Note that the sides are effective even when the flaps are closed, to channel the air into the exit:

http://i49.tinypic.com/351sgo9.jpg

As you can see from the cracks and doublers, there's a lot of vibration down there. For reference, this airplane has about 3,600 hours TT.

Dave

[Bob Axsom]

I have thought of cowl flaps of course and have discussed them with the likes of Tom Martin but I don't want to go there yet. The NLG structure and FAB requirements have to be considered. One option that might work is to retain the side view profile for ground operation and climb the pull the trailing edge up toward the fuselage to reduce the opening to limit the cooling air mass flow. I have approached that configuration with the long unvented cover and the two inch bump. The temperatures went up as I would expect but also under cylinder #2 the aluminized cover for the heat barrier was destroyed by heat from the exhaust I just patched that last night with a new piece of heat barrier - this is all that expensive $79 a package stuff from Aircraft Spruce. It is really good stuff but even it has its limits and I have to be careful. I don't know exactly what outlet configuration I need to optimize this for speed and safety - I am trying not to be too conservative but at the same time paying attention to what the airplane is telling me. The exhaust pipes are very close to the new forward shifted bumps that may cause me some problems as well - I may have to re-rig that whole tail pipe suspension system and that requires a lot of intricate work with my lower cowl baffling going right through it. I sound defensive I'm sure but I really appreciate your input and the real world example of what works for you on that airplane.

Bob Axsom

[Bob Axsom]

I removed the cover and reran the test twice with the new bump which is a continuation of the baffle between the engine and the bottom of the firewall.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_5997.jpg

The speeds of the two tests were 181.8 and 181.7 kts respectively and both are faster than the two tests of the same bump with a cover with large exhaust cutouts.

All tests in the order conducted:
http://i446.photobucket.com/albums/qq186/raxsom/IMG-23.jpg

All tests sorted by speed and ranked fastest to slowest:
http://i446.photobucket.com/albums/qq186/raxsom/IMG_0001-7.jpg

Bob Axsom

[johnny stick]

Bob,
It looks like you have acheived success. Good job and great perserverance. A few more tweaks and you could be well over the 185 mark.

[Bob Axsom]

When I flew the test yesterday three normal race prep items were not done so the results could have been a fraction of a knot higher. The other parameters I noted: RPM 2720, CHT 1=362, 2=373, 3=366, 4=inop, EGT 4=1299.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0002-6.jpg

At least superficially it appears that the new bump is the best so far and that covers are worse than no covers. The flow fence info is not as clear. The best speeds so far are the thick flow fences with the vertically tapered aft end with all the cover mount hardware attached.

Bob Axsom

[Bob Axsom]

This has got to be better:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6012.jpg

Hope to test it tomorrow.

Bob Axsom

[Bob Axsom]

Pulling the plane out for the test flight I heard a strange noise from the Right MLG. Looking closely the inboard side of the rear half of the subfairing was rubbing the tire and a large chunk was missing.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6021.jpg

I removed the subfairings from both main landing gear and flew the test but even though I previously stated I couldn't measure any benefit from them, it is not a directly comparable test. The speeds were slower at 180.8 and 179.9 respectively. I am going to have to go back to the modeling clay stage to get this rebuilt for the race in Washington on the 16th. I'm going to have to go without them in Carbondale, IL this Saturday (6-9-12). It will be a while before this test is ready to repeat.

[Bob Axsom]

The speed was good in the Big Muddy Air Race at Carbondale on 6-9-12 at just under 211 mph without the subfairings on the MLG, not tape, tiedown rings left in by oversight and not quite perfect flying so I think this outlet configuration is going to be a winner. I just got back from the hangar where I applied the first layer of fiberglass on the fairing, clay and tire. I use 24 hour cure EZ Poxy resin so this will take three or four days to complete through paint. and I have to fly to Ephrata, Washington for the Great Northwest Air Race III on Saturday so there is a good chance the test will be after I get home - we will see.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6063.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6065.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6066.jpg

Round mold tire in route from Aircraft Spruce.

Bob Axsom

[Bob Axsom]

The new mod has still not been tested. I flew from northwest Arkansas to Ephrata, Washington, then flew in the Great Northwest Air Race III and returned home. It needs the oil changed and a modification to the panel on the left side. The race speed at Ephrata was 216+ mph which is my best so far this year but that is with my best effort to work the wind and doesn't necessarily mean anything with respect to the top speed of the airplane.

Texoma 208.14
Taylor 208.84
Hill Country 212.86
BCAF 212.18
Terrell 206.28
Big Muddy 210.98
GNAR III 216.05

Yesterday the oil and filter arrived from Aircraft Spruce and I removed the cowl to start the oil change and mod examination. The left panel made of 0.016" 2024-T3 has been cut by the trailing edge of the exhaust pipe. The possibility of failure of the panel at this point was not unexpected because of the closeness of the exhaust pipe and the thinness of the aluminum panel. The cut is a very straight line conforming to the edge of the exhaust pipe approximately 3/4" long and 1/32" wide. The operating time was a little over 20 hours. Closer examination revealed cracks in the panel starting at the cut and progressing forward toward the front of the plane in an irregular pattern. The nature of the cracks appears to me to be exactly the same as the failed panels observed after the completion of the Hill Country race but they are isolated to the area directly aligned with the side of the exhaust pipe. No cracks extend from the rear of the cut in the panel. I may replace the panel with one made of stainless steel or I may put in a bump strip of stainless steel or I may re-rig the exhaust pipe suspension (not an easy job with all the baffling I have in the lower cowl).

Bob Axsom

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6073.jpg

There are cracks on on both sides of the exhaust pipe cut. I plan to replace the easily replaceable panel with one made of 0.016 stainless steel and currently I'm thinking an extension link in the exhaust suspension.

Interesting critter on my rear window (it is about 6" long):

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6067.jpg

Bob Axsom

[whiskeypapa]

That's a Walking Stick. One of my favorite insects!

[Bob Axsom]

Observed a crack in the FAB mounting plate when I removed the cowl for the oil change.http://i446.photobucket.com/albums/qq186/raxsom/IMG_6074.jpg

When I took that off I noticed a cracked rivet head by one of the platenut installations on the FAB mounting flange. When I tried to drill the rivet the whole installation fell off in my hand.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6075.jpg

As I worked that salvage with a ring doubler I noticed a tiny crack about 1/4" long at each end of the FAB metal top at each end of the slight bend about 4" back from the front.

My fix for all of this was to drill out all of the rivets holding the fiberglass bowl to the top plate install a ring doubler on the inside with a spacer plate where the piece had broken out. Stop drill the cracks, debur the holes and add doublers across the end of the bend area.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6082.jpg

I also have to replace a vacuum hose (ordered Saturday) so the long awaited test of the new "bump" with the triangular fins probably will not happen before Thursday

Bob Axsom

[hydroguy2]

All in a days work. good catch to find it before complete failure.

I wish I had found my loose exhaust hanger BEFORE failure. I'm waiting for the return parts to be here...maybe tomorrow. But I did fix some other stuff the popped up.

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6084.jpg

This panel is made of 0.016" stainless steel to replace the 0.016 aluminum panel that was damaged by the exhaust pipe. The final fitting and trimming has been completed.

Bob Axsom

[David-aviator]

Mornin' Bob,

Good to see another senior citizen at work all hours of the day and night. :)

It's gettin' so hot around here, sure its the same story where you live, the only time to get anything done is early in the day. We're expecting 108F this afternoon.

I enjoy following this thread.

Exhaust systems need attention. Long time ago, I had a failure with a pusher that took out the prop, that was an interesting day.

[Bob Axsom]

I actually flew two tests in this configuration earlier but without the MLG subfairings on June 7, 2012 the speeds were 180.8 kts and 179.9 kts. Today I had the sub fairings on the MLG and the speed for the one test I was able to get in before dark was 181.2. The surface winds were calm and according to the NTPS spread sheet they were only 7.4 kts at 6,000 dalt. I made the three leg run just before dark and I had to get back fast to be legal with no lights but I think the results are good. The first two legs met the USAR handicap procedure requirements of no variation of more than one kt in 5 consecutive 20 sec. interval readings but the last leg did not:

000 - 188, 189, 189, 188, 189
120 - 177, 177, 178, 177, 177
240 - 177, 179, 178, 176, 178

NTPS KTAS = 181.2, Wind = 178.2 deg. 7.4 kts

Normally I would fly until it stabilized with nice calm conditions like I experienced tonight but I was out of daylight and I need some numbers so I took what I got.

This doesn't seem like a bad configuration but there are some things to look at and think about before trying something different.

I'm down to a can of corn, sauerkraut and crescent rolls so I have to take a break and go to my neighborhood Walmart Super Center and get some groceries while I think about this. If I go in there after midnight there is some subtle hostility from the shelf restocking crews so I try to avoid doing that. More later.

Bob Axsom

P.S. It was around 100F here today and the highest CHT (#2) was 384.

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0001-10.jpg

I am working on a summary of the testing so far and in the process I made up these combination tables. There are 144 configurations of the three main variables in my testing so far. The tables show the combinations, the ones that have been tested and the number of times they have been tested. Just interim information.

Bob Axsom

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/IMG-27.jpg
http://i446.photobucket.com/albums/qq186/raxsom/IMG_0001-11.jpg

The numbers in the cells are the relative speeds in the 28 tests. The current firewall baffle continuation bump matrix represents 7 tests. 5 are in the "No cover" column and all would be in the top 10 test speeds if the MLG sub fairings had been installed in 2 of the tests where they were removed for repair.

The number 1 speed in the 2" long tail bump test I consider invalid because a repeat test of the same configuration yielded the #18 speed.

I think the matrices for no bump and 2" long tail bump show they are inferior.

That leaves the 2" equal tail bump that had the #3 test speed and performed well in the race at Llano, TX but was found shattered after the race.

All of this leads me to believe that the firewall baffle continuation bump is the right way to go but it needs to be higher - perhaps 2". Tests of similar configurations show an increase of CHT which I relate to reduced cooling air mass flow.

More work to do.

Bob Axsom

[Bob Axsom]

The upcoming race demands preparation that supersedes development so I have to pick a race configuration and hold off of the bump change for now. The experience I have with the thin triangular fences/fins is that they are slower that the thicker ones that taper on the aft portion only and this is true even with all of the open platenuts hanging in the breeze. So I am going to switch to those. I have the 0.090 material at the hangar and I could duplicate the thick fins without the platenuts if all goes well. I have not tested this bump with the long cover - perhaps I could squeeze that in before I go. I don't expect it to be faster but for completeness I would like to do it. I added a layer of fiberglass (4th) to the repaired RMLG subfairing yesterday and tonight I put on a coat of epoxy resin and microballoon fill. I still need to sand that and paint it so whatever the testing I do will not happen until Monday or Tuesday.

Bob Axsom

[hydroguy2]

when is your planned arrival at Three Forks? I'll come meet you.

I got my exhaust back and installed yesterday, finishing up my intake boot rework and making some injection adjustments tomorrow. I hope to test everything monday night or tuesday morning. then finish race prep thursday.

[Bob Axsom]

Thanks Brian. I would guess 2 to 3 pm local on Thursday but I haven't planned the flight yet - just a guess based on the flight to Ephrata earlier.

Bob Axsom

[hydroguy2]

I'm pretty sure you'll be the first one here is you come in on Thursday. I'll let Cody know and make sure the courtesy car is available for you.

BTW, got Aurora put back together tonight, made some AFP injection adjustments and the new intake boot is much better. Probably won't see any speed change, but I'll do a test tomorrow, maybe prerun the course, too.

oh and then this view from our porch a few minutes ago. Fire started an hour ago in the fooothills west of turn 7, burning towards turn 8 pushed by 40mph winds. rain may be coming behind it though
http://i92.photobucket.com/albums/l40/Hydroguy2/photobucket-14703-1341189248428.jpg

[Bob Axsom]

After a few days to apply another layer of fiberglass the the right MLG subfairing, fill it and paint it I finally got back to the outlet fairing work. As an elementary school student one of my comic book heros was Batman. In the mid 60s a popular song that Jeanine and I danced to was the Batman theme (listen to the Ventures version to get an idea). So, when I made this latest cover mod I had a feeling that destiny was being played out before my eyes.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6095.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6096.jpg

The thought was, at the angle of attack of the airplane in flight any squared off surface perpendicular to the flight path is going to have a broad turbulent flow coming off of it and if these kinds of surfaces were brought to points like a "V" there would be less drag.

At 19:53 CDT I was ready to test the batman cover. Both runs were near identical:

180.5 kts
180.8 kts

Good but no cigar. The no cover tests with the thick fins with the platenuts hanging in the breeze are still the fastest followed closely by the thin fins and no cover which in turn is followed closely by the Batman cover. Tomorrow is the last day before going to Montana for the Big Sky Air Race on Saturday. I guess I have no choice but to remove the Batman cover and go to Montana with the thick fins and no cover. I would like to make up a set of thick fins without the platenuts hanging in the breeze and test that but I'm out of time.

Bob Axsom

[Veetail88]

Might not be faster Bob, but I'm betting your radar signature will be lower! :D

[Bob Axsom]

I enlarged the batman cutouts by cutting a triangular section out of the two inboard edges and rounding the fwd end of the cutout.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6128.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6129.jpg

I made two trips around the area in normal three leg 6,000 ft dalt test fashion. The speeds were 182.6 kts and 181.5 kts respectively for an average of 182.05 kts. These rank as the fastest and the 7th fastest of the 32 test flights in this series. The average of the two test flights today is 1/2 of a tenth of a kt slower than the previous high speed in this test series.

Bon Axsom

[David Paule]

I wonder if a more parabolic shape to the cut-out, instead of straight cuts, might have a small advantage?

Dave

[Bob Axsom]

It's a thought. I suspect the improvement in speed has to do with the exhaust plume impinging on the cover based on experimental observations. If that is true the focus of each cutout should be centered on the exhaust pipe outlets (which is what I have been instinctively moving toward without considering a parabolic shape). The interaction of the cooling air flow and the exhaust flow has always been a question.

Good observation thanks. I think I will make a pattern of the current cutouts using file folder stock the recut the development cover following a more parabolic line and give it a test flight.

Bob Axsom

[David Paule]

Also, what about adding exhaust extensions? I'm thinking that they could extend the current tube another eight inches or a foot, then make a gentle bend so that they end up parallel to the airflow. That would give you some exhaust separation from the skin.

You'd no doubt need some bracing for that, both to keep the extension under control and to keep from putting too high loads on the upper parts of the pipes. And it could well be that the trade-off between the drag of all that might not be made up by the added thrust and cleaned-up exhaust.

It would separate the effects of the exhaust from the cooling air exit to some degree, but it might be to the detriment of both. As you can tell, I'm not at all sure whether this would offer an advantage or not.

This is sort of the opposite of a jet-augmented exhaust, which might be preferable if it could practically be done.

Dave

[Bob Axsom]

Also, what about adding exhaust extensions? I'm thinking that they could extend the current tube another eight inches or a foot, then make a gentle bend so that they end up parallel to the airflow. That would give you some exhaust separation from the skin.

You'd no doubt need some bracing for that, both to keep the extension under control and to keep from putting too high loads on the upper parts of the pipes. And it could well be that the trade-off between the drag of all that might not be made up by the added thrust and cleaned-up exhaust.

It would separate the effects of the exhaust from the cooling air exit to some degree, but it might be to the detriment of both. As you can tell, I'm not at all sure whether this would offer an advantage or not.

This is sort of the opposite of a jet-augmented exhaust, which might be preferable if it could practically be done.

Dave

I sawed off the exhaust to get the direction I have now and got a speed gain I see no reason to go back I think I'm guiding it in the right direction as far as these pipes are concerned. On the positive side I recut the cover outlet - not technically a parabola but in that vicinity as opposed to a blunted triangle.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6130.jpg

I spent so much time trying to coordinate things on the computer today that I will have to wait until tomorrow to test fly the change but it is ready to go:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6137.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6138.jpg

Bob Axsom

[David Paule]

Dang! After a career in aerospace development, that's the quickest I've seen one of my suggestions implemented in hardware. Usually it takes a couple days or more. Usually more.

I'm impressed. Fly safe!

Dave

[Bob Axsom]

I flew the test this morning when every thing was calm and the results were good. The first test was 184 kts and the second was 182 kts for an average of 183 kts. Now that I know what I'm looking at, it seems to me that the right side cutout of the cover at the forward outboard edge could be relieved a little more and I might pick up another fraction of a knot. Unfortunately I am starting to run into AVC launch schedule pressure. I am thinking about tracing the current cutout outline then cutting it once again and making one more test flight but first, breakfast!

Bob Axsom

[johnny stick]

Bob,
Seems you have almost hit the magic 185kts.Wow what a great accomplishment. What's the next mod series, wing root fairings?

[Bob Axsom]

Next I have to convert this development patch work cover and outboard fins into a single piece and retain the center fin as is - hopefully in the next few days.

http://i446.photobucket.com/albums/qq186/raxsom/IMG-29.jpg

http://i446.photobucket.com/albums/qq186/raxsom/IMG_0001-14.jpg

Bob Axsom

[Bob Axsom]

The development cover and outboard fences unrolled to pattern flatness.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6147.jpg

I had some 0.040" 6061 O bare so that is the first (hopefully the last) single piece copy:

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6149.jpg

I fit well:http://i446.photobucket.com/albums/qq186/raxsom/IMG_6155.jpghttp://i446.photobucket.com/albums/qq186/raxsom/IMG_6156.jpg

Bob Axsom

[Dennis Fox]

Have you ever thought about getting rid of those draggy com antennas?
as Bert once said, (anything hanging out is drag)
Hold a old antenna or a welding rod at arms length out your car window at 75mph and notice the pull on your hand, DUA !! now imagine that at 180k.

[jrovey]

Have you ever thought about getting rid of those draggy com antennas?
as Bert once said, (anything hanging out is drag)
Hold a old antenna or a welding rod at arms length out your car window at 75mph and notice the pull on your hand, DUA !! now imagine that at 180k.

I am currently working on eliminating the COM antenna. I don't want to put an Archer antenna in a wingtip. My idea is to create a COM antenna/Pitot tube combination. Every time I look at my simple 1/4" pipe pitot tube I think "why can't that also be my antenna"? I have searched on the internet about this idea for a GA airplane and have not come up with anything. Someone else must have thought of this idea before.

I have been reading about handheld tranciever antenna design from HAM radio sources since they are a shorter 1/4 wave antenna than a standard whip. The VSWR/field strength meter I ordered will show up next week and I will start experimenting. I expect the performance will be less than the bent whip currently on the belly of my RV8 and not sure what the result will be but looking forward to the learning process.

[Bob Axsom]

Have you ever thought about getting rid of those draggy com antennas?
as Bert once said, (anything hanging out is drag)
Hold a old antenna or a welding rod at arms length out your car window at 75mph and notice the pull on your hand, DUA !! now imagine that at 180k.

I think about EVERYTHING that touches the air - inside and out - I can't imagine a racer that doesn't. A long time ago I was a guy that worked on com equipment in the Air Force and I draw the line at compromising my com system for anything. During a race I have to make turn calls, finish calls and that's all - pretty trivial stuff - but the plane is my transportation, it's not just a racer.

I live in Arkansas but last Saturday I was in a race in Three Forks, Montana. When I was coming home I was very tired. I had only slept for about three hours because I had to prepare for the race, then I flew the race, waited for the disappointing results and finally it was time to head for home. Storms were over Colorado in the Denver area and I knew from talking to a briefer in Sheridan, Wyoming that they were moving north. Coming south from Muddy Mountain VOR I could see it and I'm flying VFR but on Victor airways. I left the airway to the east, flying to light and lowering terrain, looking for an out. I know it's on me to find the way out and I am too tired to play super pilot, drag out my IFR charts and try to work my way into the system while flying the plane avoiding the storm, etc. Regardless of what CFIs and other perfect pilots do, I Fly the plane and work my way through the options that I personally have control over first. At one point I saw the path to Cheyenne was VFR but a low ceiling. It was not night time but under the clouds it was pretty dark. I was in the 20-30 mile range from the airport and according to the ATIS it was still VFR there. I committed to flying there and I called the tower on the way in. I gave my location and all the usual stuff but I also emphasized that I was not familiar with the area. He said do you see the large tank painted red and white ? I confirmed that I did and he said the airport is not far behind that. As I got in closer at about 500 ft AGL getting squeezed between the land and the clouds I saw the airport and reported it. He cleared me to land on runway 31 and I told him that I had a runway insight to my right but I could not confirm that it was 31. He said no that is a different runway, whatever number it was. So I continued on until another one showed up on the right and we confirmed that it was 31 and I landed and taxied off. I had no Idea where the FBO was because I had not planned to land there but it was a port in a storm and I really didn't care if it was a little awkward. I called ground told the operator I need to taxi to the FBO, I would be staying the night and I was unfamiliar with the airport. He gave me taxi instructions and it was all over but tucking the Blue Bird away and getting a room for the night. Drag could not have been further from my mind. I do unscrew the nav antenna elements during a race but my previous testing of drag effect was not measurable - still if they are off it is less drag. I am sure the bent whips are even less draggy. It's a personal choice but so far I don't mess with my optimized VHF com antennas.

Bob Axsom

[logansc]

Bob: For what it's worth, the SL-40 you sold me works better (has more range and is clearer) on my Rocket transmitting and receiving through a cheap(er) Bob Archer antenna in my right wingtip than does my Garmin 530W transceiver operating through a more expensive exterior Comant whip antenna located on the belly of my airplane.

Based on that (admittedly only a single data point), I would say that you don't necessarily have to give up any communications performance to gain an improvement in drag by dropping your external antenna.

Just a thought...


Lee...

[Alan Carroll]

Has anyone thought about mounting an antenna under the canopy, above the passenger's seat? You'd have to build a support structure of some sort and perhaps a ground plane. It would also have to be easily removable to return the airplane to non-racing configuration.

I agree about the drag being pretty low; I remove one of my two com antennas for racing but can't measure a difference in speed.

[gereed75]

Bob, I've been following the whole thread. Glad you are finally beginning to see some good results.

The current configuration is looking a lot like the one I fashioned basically on intuition. I have always though that getting the outlet curves smooth is a benefit by minimizing sharp discontinuities that "trip" the airflow into something more turbulent than it needs to be.

Gettin' smooth now!

Have you seen any consistent effect on CHT's.

Continued good results!!

[Bob Axsom]

The CHTs range from around 350 for #1 to 373 for #2 now and before they were 320 to 350. It indicates to me that the air mass flowing through the system has been reduced some.

Bob Axsom

[flyvans.com]

Has anyone thought about mounting an antenna under the canopy, above the passenger's seat? You'd have to build a support structure of some sort and perhaps a ground plane. It would also have to be easily removable to return the airplane to non-racing configuration.

I agree about the drag being pretty low; I remove one of my two com antennas for racing but can't measure a difference in speed.

We mounted an ELT antenna inside of the tipup rollbar on the passenger side as COM2. Fine for short range stuff, ground or formation communication. Not useable for inflight/ATC, too limited in range / field of view.

Sounds like the Bob Archer used for COM as well would be a better way to go.... Still possible issues with field of view however.

[Bob Axsom]

When Jeanine and I built our RV-6A 1996-2004 our intention was to race in whatever cross country air races that came up. Usually one per year until SARL was created, now we have over 20 per year (www.sportairrace.org). With that focus during the original build, I contacted the manufacturer of the best looking "in the wingtip" antennas. When I told him that I fly IFR he told me that the antennas were intended for sport flying applications and he would not recommend them for serious IFR flying. That was a NO GO decision driver for me.

I could make cover plates for all of the external antennas, two communication, transponder and marker beacon (I already take off the navigation elements by unscrewing them), remove them for racing on site and reinstall them before flying home. The 12" stock wing tips have been replaced by a 3" streamlined tips of my own design and the performance of the com antennas placed in them for turn and finish line calls would be further compromised. I would have to route a coax to at least one wing tip and change the RF I/O connection to the race only coax someway (change connections, coax switch, etc.) disable the transponder (you don't want transmission attempts into an unloaded output, maybe some FAA problems there as well - depending on race location perhaps). Still, if I run out of more promising and less compromising opportunities the passive drag of external antennas will be kept in mind - I look at them and wonder each time I wash the belly.

The speed differential between out RV-6A and the current RV Blue race leaders is from 3 to 9 kts and I think I am going to have to deal with the thrust member of the speed equation to overcome that. Two of them will be in the AirVenture Cup Race next Sunday and we will see what we have accomplished so far. I am hoping this engine cooling air outlet mod cuts into the margin.

I completed a trial "final installation" of the new one piece cover this morning at 05:40 and there are some problems.

http://i446.photobucket.com/albums/qq186/raxsom/IMG_6157.jpg

The 0.040" 6061 0 material I had available and used, is very soft. It is just a little harder to form than sheet lead so it shapes nicely but its softness causes mounting problems. I knew this from the 0.032" 2024-0 material used in the development cover. I planned to drill large mounting holes and use #8 hardware with dimpled washers to provide more distribution of the fastener head compression but I went too small at 1/4" dia and the washers are not all flat because of imperfect hole alignment. I plan on removing it today, increasing the hole size to 3/8" like I do on fiberglass and plastic parts installed with dimple washers and see if that works. If that is not satisfactory I will go to harder material for a new cover and cure the problem that way. Since I plan to leave for Mitchell, South Dakota for the race Friday morning there is some schedule pressure so I guess I had better eat, get cleaned up and head back to the hangar.

Bob Axsom

[Bob Axsom]

The one piece cover is reinstalled and the dimpled washers are sitting the way the should. I would have liked to see if there was any effect on speed from the cleanup but I'm out of time. I've got a lot of prep work to do before flying to Mitchell, SD Friday fot the 2012 AirVenture Cup race - one secret weapon - so no more effort on the cooling air outlet fairing. I may test it later just to get a new baseline but I think this one is done.

The secret weapon, well it is a strategy thing - tell you later Alan.

Bob Axsom


P.S. I'm calling this configuration speed 183Kts speed which is 1.4 kts less that I got one time in testing a while back so it is hard to claim an improvement in performance. I did two tests in this series (20 and 21) with no fences, no covers and no bumps which yielded 177.6 and 178. 8 knots respectively for an average of 178.2 KTAS which would indicate a 4.8 kt speed gain but the three long angle mounting rails were riveted to the fuselage already and were exposed to the air in tests 20 and 21. S-o-o-o-o, conservatively I'll take the pre-mod baseline speed as the average of the 184.4 and the 182.2 or 181.3 kts. That still yields a gain of 1.7 kts. I'm happy with that. 04:30 and I'm on my way to the hangar to do race prep.

[Alan Carroll]

The secret weapon, well it is a strategy thing - tell you later Alan.



I look forward to it - we're all going to need some some new tricks to catch Brent!

[hydroguy2]

I look forward to it - we're all going to need some some new tricks to catch Brent!

isn't that the truth

[160kt]

I have spent many an hour trying to locate the source study for the sugggested optimal NACA exit ramp slope to be 7 Degrees. I can't find it. Since it came up in this discussion I thought I'd ask here.

Does anyone here know the source and have a link to it?

Thanks in advance,
Ned



Some very thought provoking discussion! I know Bob is gearing up for the Hill Country Air Race tonight, but this will give him some great post-race reading material...to bounce his results off of!

Comments below:



Paeser postulated the best angle for exit air was at 10 degrees to the free stream...not far from the NACA 7 degrees...though it may be a slightly different concept at work (not sure).

There may be some acceleration of the air on the first half of the bumps, but I would be concerned about separation on the aft side (as you and others have mentioned). There may be some coanda effect, but I doubt laminar flow can hack that angle, and it sort of has a venturi-like look to it. Only down side is the back half of the venturi would expand and slow the exit air...opposite the goal. (more on that below).

Also, I think slowing the freestream air locally would be counterproductive...I know you're talking about makiing the flows more compatible, but it would seem slowing anything in this area would be drag-producing. Just a comment for discussion...this is all facinating stuff! :)



Dan, I think there is probably some area reduction...at least compared to Bob's previous configuration, without the bump. I do see your point about re-filling those cutouts (though I'm concerned about blocking the exhaust directly and causing odd flow patterns downstream), and I also wondered about exhaust stains on the bump. Looks like it is there, but the swirl pattern may indicate separation or turbulent flow.

My pea-brained effort at a similar concept to what your last paragraph says is pictured below (a mod of a picture I did earlier):
https://lh4.googleusercontent.com/-lYow4StBF7Y/T5EIySPtW3I/AAAAAAAAAcU/7n8nSbxUw5I/s800/Exit%2520mods2sm.jpg

From discussions with Paul Lipps, the coanda radius at the base of the firewall might do the lion's share of redirecting and attaching the flow exiting the cowl. Then the continuation of that curve...a softened version of Bob's current bump, as others have suggested) might maintain laminar flow and continue that coanda effect further aft. Opposite that upper bump, on the lower surface, another bump that forms the front half of a venturi could add to the area reduction and exit flow acceleration. Tuning of the system would mean trying to direct the flow aft without reopening of the venturi prior to the end of the ramp (I think)...which I don't depict well in this picture. I do agree with you that decreasing the down-angle of the pipes might increase Bob's current coanda effect, and might allow the aft part of the bumps in my picture to be more aligned with the belly and the freestream.

My kluge of a bunch of stolen ideas, somewhat applied to what Bob is currently working...all FWIW! ;)

And great work Bob...as always! Good luck in the race!! :D

Cheers,
Bob

[BillL]

I have remembered the 7 deg angle since college, but never asked for reference. I searched the nasa database ( http://ntrs.nasa.gov ) and found this report: Search for this title, "NACA-TN-2888". I cannot add the link as it will yield an error. I just scanned the report, but it appears to show experimental development of pressure recovery for a 2-d diffuser. The efficiency appears to peak right at 7 degrees. ref figure 15. The report is 85 pages long so only 30 min was spent evaluating. Happy reading.

[rzbill]

I was the one that mentioned the 7 degree angle much earlier in this thread.
It is for INLETS, not exits, but the principle I was discussing at the time was the same. My original post was concerning expansion or channels with increasing volume. Depending on conditions, there is a maximum expansion rate that can be done before the air separates from the structure. Previous experimentation by NACA can help.

A poster asked for a ref document. Using the search method above, look for NACA-RM-A7I30. That is the study I based my commentary on.

[160kt]

Bill and Bill,
Thank you very much for your responses. I've read both those papers before but neither are specifically defining the design of NACA exit ramps. I remain quite curious. I would like to find the study explaining and defining the design. I've seen posters quoting the 7 degree slope and that the NACA papers say a straight side shape for exits is better than the curved shape formulated for inlets. I've spent literally days reading the NACA studies to no avail. One fellow Mark Langford http://www.n56ml.com/nacaducts/index.html experimentor made his exit ramps based on the "earlier" NACA studies and later posted that the "later" NACA studies indicated that the inlet design is not optimum for the outlet. When I ask him for references he vaguely suggested to look in an old Kitplanes. I eventually found the article he remembered and the author did respond to me with about 40 references none of which had the info on the exits. I'm wondering if it's some kind of urban legend perpetuated by the Internet....
The author was Todd Parker and his article was posted in Sport Aviation March 2006

So if anyone knows where the exit ramp design development for the NACA ducts I sure would appreciate the info
Thanks
Nec

[Bob Axsom]

What is your understanding of the 7 deg. optimum outlet? If you understand the concept it would be possible to begin an experiment and test process based on that.

Bob Axsom

[DanH]

A 7 degree straight-sided exit ramp inset into the belly? Sounds familiar.

http://i50.tinypic.com/14dg3ft.jpg

[rzbill]

I'm wondering if it's some kind of urban legend perpetuated by the Internet....
The author was Todd Parker and his article was posted in Sport Aviation March 2006

So if anyone knows where the exit ramp design development for the NACA ducts I sure would appreciate the info
Thanks
Nec

There is plenty of exit geometry testing by NACA. I cannot remember seeing an instance of them testing a reverse of the curved sidewall inlet. Maybe they did, I just did not see it. It is not a sidewall shape I would automatically try for an exit, due to my belief that the sidewall geometry is directional for generation of edge vortices spilling into the inlet. I don't see that working in reverse. There are some BASICS that can be drawn from theory and testing. A converging ramp, such as the RV8 exit floor has a large range of functional angles because it is in a positive pressure gradient (my assumption by inspection, not data) which keeps the flow attached. The backside of Bob Axsoms coanda bump was an expansion area with negative pressure gradient. The exhaust/oil pattern showed separation, a small piece of data. The NACA INLET is designed to minimize separation in an expanding environment. The 7 degree data is VALID for this narrow application (expansion) as suggested. I had additionally suggested convergence of the cowl at 7 degrees (trying to minimise separation on the cowl exterior and matching flows to reduce the disruption at the rear edge of the cowl) in order to reduce the volume expansion past Bobs coanda bump inside the cowl. As Mills said, that could be counterproductive. I'm not sure. The 7 degree common aero knowledge is hardly an urban legend

EDIT REF: "It is empirically observed that the best divergence angle for a two dimensional subsonic diffuser of a given area ratio is about 7 degrees" Hill, Peterson - MECHANICS AND THERMODYNAMICS OF PROPULSION. Addison Wesley Pub 1965.

[160kt]

Page 3 of Dave Anders notes
http://sacrvators.com/Aircraft%20Efficiency%20N230A.pdf

3) shape - NACA studies
- straight sided rectangular shallow angle converging ramp - ramp as parallel to free stream flow as possible at end
- width to depth ratio of ramp
a) 7 to 1 for cooling systems
b) 1 to 1 for exhausts (augmenter

Evidently Anders knows about the NACA studies?

The RV8 outlet and the Grumman Tiger/Cheetah outlets have straight sided exit ramps. The Grummans have some of the ramp covered. It would be great to know how to design it.

If there is a study that NACA did then I'm thinking a whole lot of experimentation has already been done. Why repeat that? I want to experiment with the benefit of the science already concluded guiding my efforts

[160kt]

I'm curious why RV's have the "exit ramp" out in the slip stream. The Grummans picked up quite a bit of speed when the "exit ramp" was placed internally. The Grumman Traveler exit ramp looks like an RV 6 until 1975 when they placed it "inside" the cowl. It picked up 6 knts. Bob Axsom have you got room to move all that "stuff" inside the lower cowl skin and make the skin even with the fuselage skin, not hanging below?

Dave Anders believed whatever NACA data he had enough to put the ramp inside the cowl AND to get the ramp slope down to 7:1 he modified the bottom of the fuselage to continue the ramp way back.......

Dan Horton picked up more speed the more he removed the hanging down part......

[DanH]

The RV8's exit slot (after trimming the cowl) is about 7 to 1.

Version 4 of my own cowl exit will again reduce fixed area by extending the exit 4" aft of the firewall. However, it will also incorporate some variable exit area for low airspeed use.

[160kt]

Correctly assessing or quantifying improvements I have found to be very difficult. I discovered it is extremely hard to get repeatable results with flight testing for a particular mod. So I understand Bob Axsom's efforts in this thread. He is an extremely hard worker. And I don't want to take away from all he is doing and has done. But here are so many variables it makes it really hard to determine if what's done really is what causes the speed increase. For example, in this thread you one time mention that CHT's went up a little. To me this says cooling air went down. The logic flows that the stuff put at the exit may have caused lower cooling airflow which in turn may be contributing to the speed increase. So how much speed increase is do to that versus the effect of the changes with the air exit? Well that's the whole problem. And until you properly instrument the cowl you just won't know whether all your work actually increased the speed by itself or it just affected another variable that actually caused the speed increase.

[Bob Axsom]

I worked in aerospace for 50 years from the lowest level to the project office and I recognize wisdom were I see it I think and you guys have it. I just flew down to Abilene, TX yesterday and worked most of the night with changing winds aloft forecasts and race course changes including one added turn that was published after most people went to bed with the wrong latitude value. What a great group - We worked together and and got by what could have been an awkward situation. I woke at 00:50 CDT this morning and saw the e-mail announcing the added turn and started working it into my race flight plan and altitude strategy (winds). I developed a crude Excel spread sheet last year for helping me plan what altitude I will fly in each leg. The race is a real race not one of those compromise things with handicaps or fuel burn estimates, etc. The fastest wins and that tends to stimulate speed modification action. MANY speed modifications do not work even though the experimenter's intuition says it has to work. I had 3.4 hours sleep last night (yes I maintain a sleep log) and a very stressful day including some serious scud run challenges through the Ozarks to get home here in northwest Arkansas so bear with my lack of clarity - I'm tired but I'm trying.

When you try to exploit a concept I believe you must work with it until you succeed or you are convinced that it will not work and have some rational thoughts about why it failed. If you learn something from the failure it was a good experiment. The cooling drag concern is popular with the technically elite so it must be worth looking at and trying to reduce it must be a good thing. I have a significant stack of aluminum, rubber and stainless steel from my failures. Not what I have read or been taught in the classroom but what I have fabricated installed and tested. As I have gone through this growing and satisfying experience I have accepted intellectual input on test methodology from some of our best contributors to this forum and from lessons learned when I used to be a useful member of the aerospace community but in every case I control my experiments and I do not bow to technical arrogance and I judge the results. If you don't do that you will be defeated early in you life as an experimenter.

My initial thought on cooling drag was the cooling air should have easy passage through the cowl to the fixed outlet. I made a curved aluminum panel from the bottom of the engine to the bottom of the firewall so the air would follow the curved surface to bottom of the fuselage. THIS IS NOT A SIMPLE THING TO CREATE! All of the things that transition from the airframe to the engine have be accommodated by this thing you are constructing from many pieces and it has to be serviceable and the penetrating wires, tubes engine mounts, etc. have to be accessible for maintenance. OK ... ? Anyway, I did that and as I was doing it I said to myself, if I just let the air follow this cowl wide curved panel to the bottom of the firewall/fuselage the air outboard of the outlet goes into those lower corners and gets trapped there or at least does not have a equal shot at the outlet. So, I decided to make vertical panels that slant from the inner side walls of the lower cowl inboard at an angle determined by the rearmost location of cylinders 3 and 4 (of course they are staggered so the angles are different) to the width of the outlet. When all of this work was done and tested I found the the top speed of the airplane had dropped below my base line speed of 170.67kts by 2 knots. I made many modifications and nothing worked until I sealed the lower cowl with two additional horizontal baffles below the cowl split line preventing a back flow path from the lower cowl up past the valve covers and back to the area between the engine and the firewall. When those baffles were installed the speed jumped up 6 kts for a 4 kt gain over the baseline for my specific airplane.

I have made many changes since that was achieved but NONE did anything but lower the speed and when the change was removed the speed came back. I believe that a near optimum combination of upper plenum and lower cowl configuration has been achieved for my specific airplane with significant additions of aluminum and rubber to my sacrifices to the God of Speed. Some new idea may arise in this area that would make the plane faster but I personally have tried enough that I do not believe they will.

After some time I thought the outlet configuration was not effective based on tuft testing by others that showed back flow. A fellow named Geer, from Pennsylvania I believe, offered in this forum and experiment that he had tried with the outlet that he reported increased the speed of his plane. I did not want to just copy his shroud design and gain limited knowledge - basically GO/NOGO. I saw several areas to work on but I'm getting tired and I suspect you have long since stopped reading so I will jump ahead to the final configuration.

The bumps are now stainless steel panels that are attached to the curved baffle from the engine to the bottom of the fuselage so now the panel extends from the engine past the bottom firewall/fuselage interface and gently curves back to the fuselage with a narrow gap at the trailing edge to pass the air that comes through the oil cooler, blast tubes and heater dump from zone 3. There is a center fin that keeps the exhaust pipe outputs separated, outer vertical panels to keep the the cooling air and the ambient air separated and a horizontal panel that does not let the outlet air travel vertically downward into the ambient air under the fuselage without starting to travel aft toward the tail. The vertical and horizontal panels evolved through many trials and are now actually one piece. This all made sense to me but the real breakthrough came when someone in this forum suggested that I should try parabolic like cutouts between the the horizontal panel and the two outboard panels. After I did that the speed jumped over 1.7 kts. The cutout is not unlike the plume pattern of the exhaust on earlier versions of this outlet shroud. The original wide coverage of the bottom of the fuselage with oil and exhaust products is now defined by straight bands the width of the outlet all the way to the tail - no evidence of turbulent flow at all. Refinements in the implementation can be made but I do not believe there is anymore speed there. Wow that was tiring - now the system will probably say I used too many words in a post.

If you don't try to test because you say it's is too difficult to get consistent results that is too bad. I wish you would try harder to control the testing otherwise you will always be stuck with hand me down knowledge which is good but not the best kind. You have to work at it to really know the facts on the best level.

Bob Axsom

[hydroguy2]

Plenty of us are still reading Bob. I've seen the improvements you've made and continue to make. You are a true Experimenter. Keep up the good work

[rockwoodrv9]

Im another one who reads every post you make Bob. I am not building a race plane, but I sure enjoy how you keep searching for that one more knot! I would love to see pictures of your modifications, but since you have spent so much effort making them, they are your "advantage" in your racing!

Keep it up Bob, many of us here and one upstairs, are surely following your progress and racing successes.

[humptybump]

Thanks Bob. I will likely never get the racing bug but your test methodology has been incredibly valuable for me. I too read your technical posts carefully and completely.

[Jerry Cochran]

Hi Bob,

I too read every one of your posts even tho I can't fly right now because of a panel overhaul. This question has prob been asked before but a quick search gave no joy.

How do you assure accuracy of one speed test to another since you're looking for a knot or two at a time? I've tried to repeat testing using the same altitude/rpm/MAP even on the same flight using the 3 or 4 heading GPS method and have not been consistent to within a knot.

What am I doing wrong?

Thanks, no hurry with a reply...

Jerry

[Bob Axsom]

http://i446.photobucket.com/albums/qq186/raxsom/USARHP_zps6af057bc.jpg

The flight procedure I use is the 2005 USAR handicap procedure image above. Once at wide open throttle, max RPM, leaned for max power and trimmed for hands off level flight at 6000 ft density altitude for EVERY TEST OF SPEED I engage the autopilot heading and altitude hold and start recording GPS ground speed for the 360 deg. GPS ground track until I have 5 consecutive 20 second interval recordings that do not vary by more than one GPS ground speed knot then I turn 120 degrees and repeat for the next leg until I have completed 3 legs then I return to base with my data and plug the valid leg average GPS ground speed for the five recordings for each leg and the GPS ground track for each leg into the National Test Pilot School (NTPS) Excel spreadsheet that was provided in this forum. Embedded formulas compute the wind experienced during the test and the true air speed of the airplane.

http://i446.photobucket.com/albums/qq186/raxsom/NTPS_zps3c35bcd0.jpg

Originally I used the handicap procedure alone bur Kevin Horton convinced me that that procedure would always error on the side of lower speed. I verified that in actual use and have used it with the NTPS excel file ever since. John Huft also supported the use of the NTPS spreadsheet and recommended multiple runs before accepting a result as accurate. I verified again that the suggestion was correct for refining the estimate but there are practical time and money limits to my resources so I now fly two triangles, one right after the other, and average the two estimates. The estimates are always very close but this approach dampens out the variations.

I now fly my tests under a MOA east of the Fayetteville, Arkansas's Drake Field and 6,000 ft density altitude is often around 4,000 ft MSL and ~2000 ft AGL over the Ozarks there. Wind and thermals during tests in non calm conditions occasionally cause a speed excursion outside the acceptable range of the USARHP and I compromise my standard by recording the observation and continue with data collection until I have a five good consistent recordings around the one anomaly and strike it from the list when I get back on the ground.

A few more more important lessons learned:

Try to minimise variations in the aircraft under test that are not the item of interest in the test (Fuel load, aerodynamic fairing installations, tape, vent position, etc.).

Always record the date of the test on the raw data collection sheet.

Always record what is being tested on the raw data collection sheet

Always print the NTPS spreadsheet for the test and staple it to the raw data sheet for the test.

Always File the paperwork.

I hope that helps.

Bob Axsom