[Bob Axsom]

These photos were taken in 2006 before I sawed off the turned down exhaust pipes and I was trying a lower forward baffle to improve on the 4 kt speed gain I got from the previous lower cowl baffles. It did not work out to provide an increase in speed. Recent comments by Scott McDaniels has got me looking at this again with a different objective. Originally I was trying to turn the air and avoid the topo of the FAB and some of the NLG structure. The idea this time will be to cut off the long baffle and use it to seal the front end to prevent front end blowout around the spinner. This will do something similar to the horizontal baffles along the valve covers and sill of the lower cowl - restrict the possible flow directions and focus the flow out the cooling air outlet. As many others are doing, I will begin the annual condition inspection in a few days and I plan to incorporate the mod while everything is apart. I will report the results but the full benefit will not be known until the plane is converted back to race configuration in March.

Bob Axsom

 

[N434TM]

Interesting for sure. Looking forward to seeing the results.

Thanks
Tom

 

[Bob Axsom]

Preliminary Results

This is an awkward mod to get just right. I marked the edge of the lower cowl inline with the mounting screw holes then removed the cowl from the airplane. I taped together file folders and taped it into the lower cowl in line with the marks at the edges and marked its contact with the inside of the cowl. I trimmed an fit the resulting template until it fit and then marked 0.5" back from the lower edge for clearance and cut it down to that size. Then I went to my lower cowl front baffle reject pile from earlier experiments and put together the four pieces that had already been fitted to the front end of the cowl under the inlet ramps and around the alternator, crankcase, starter and solenoid. I found that the configuration of the previous baffle was curved in the center and I had to modify the center splice plate to flatted out the assembly. After I had a reasonable flat assembly I taped the template onto it and traced the lower edge onto the assembly with a Sharpie pen and cut the baffle assembly to size.

Now I had the basic assembly for the new lower cowl front end baffle. I riveted red rubber sealing strip material to the lower newly cut and deburred edge with an aluminum backing (clamping?) strip. I was entered in a race at Taylor, Texas on 3-15-09 so there was a deadline and I would not have time to test it for compatibility and performance - I would just have to put it in and rely on the previous installation flight testing for safety just accept what I got for performance. At the end of the daylight on the day before leaving for Taylor I had it attached to the lower side of the cooling air inlet ramps and positioned at an angle that would seal it against the lower cowl with two angle struts between the baffle assembly and clamps on the forward cross over exhaust pipe. There was a problem.

When I tried to install the lower cowl there was very high resistance - I got it on by myself but it was very difficult. When I put on the upper cowl it would not go back to align with the hinge on the skin above the firewall. It was time to go home for dinned at this point and I knew I had to take the cowl off and either remove the new baffle or modify the installation to make it compatible.

During dinner I decided I would cut back the struts 0.05" to increase the slant back angle and give it one more try before before pulling the baffle from the plane.

When I removed the cowl I saw a small impression in the internal cowl thermal blanket inlign with a place on the right side part of the baffle that extnded down a little beyond the left part. I drilled out a couple of rubber seal holding rivets for access, cut away some of the metal in the offending area and riveted the seal back in place. I thought about not cutting off the struts but I had no time to do more than one installation so I made them 0.5" shorter as planned and put everything back together. It went together very nicely as it usually does so I knew the hard interference was eliminated. Visibility into the lowed cowl is very limited but by lifting the horizontal red rubbed seal outside the valve covers on cylinders 1 and 2 and using a flashlight, I could see the seal was OK on the left side but the rubber on the right side was pulled back from the cowl by the shortened strut. I inserted a long bladed screwdriver through my small viewing port and pushed the baffle from the back side. It went forward a little but not enough to seal against the cowl. That was the way it was going to be for the race.

The next day I flew from Fayetteville, Arkansas to Taylor, Texas and everything was fine with what appeared to be higher than normal true airspeed and lower cylinder head temperatures. Just casual observation - not a test.

In the race the airspeed was approximately 182 kts INDICATED, which is more than I normally see. I believe it is faster but I have to increase the length of the right side positioning strut and perform a real test flight to get the data.

Bob Axsom

 

[chaskuss]

Another way to skin that cat

Bob
I've heard of other side by side RV owners placing large, spaced chunks of foam into the lower rear sections of the rear cowl. Once they got the shape right, they glassed over the foam. Same idea of trying to direct the flow of the air to the exit as cleanly as possible.
Charlie Kuss

 

[Bob Axsom]

I found that that intuitively obvious mod doesn't work

I worked on that for some time and if you look at the the original photographs you will see an edge on curved aluminum panel that curves from the rear of the engine to the bottom of the fuselage and two wedge baffles that direct the outgoing air to the width of the cowl cooling air outlet. That slowed the plane down. When I added the horizontal baffle to eliminate the airflow path from the lower cowl up past the valve covers and into the zone behind the curved baffle in front of the firewall my speed jumped by 3 kts over my previous high baseline. This latest mod is to address the air path out of the front of the lower cowl - some of which flows back into the inlets. I am ready for a new test with the lengthened right positioning strut which now provides a good seal. My test requirements are fairly rigid and I must be able to fly VFR at 6000 ft density altitude. It may take a few days before the weather is right but I will report the results shortly thereafter.

Bob Axsom

 

[pierre smith]

Outstanding speed gains, Bob...

.....and you might also consider reducing the gap between your prop blades and spinner...the gap appears considerable and probably costs you.

Regards,

 

[DanH]

Go for it Bob!

Reducing flow through the spinner gap is a good thing. A previous thread discussed methods of sealing in the spinner gap and propshaft area, but your baffle approach has merit too. As you've experienced, the Lycoming architecture makes front perimeter baffling difficult; ring gear, fat shaft, starter, and alternator are all in the way. Much easier to do with a late model Continental. This is a Mooney Acclaim S install:



Lower plenum air has no possible flow path to the spinner gap. Upper plenum pressure doesn't leak much because (not shown) the annular propshaft gap has been reduced to a dead minimum and there is a nice stagnation point on each side of the spinner inboard of the inlets. There is likely still some small outflow in the low pressure area at the top of the spinner gap.

Very similar scheme with the Columbia (now Cessna) 400. The Acclaim and 400 run together as the the fastest production piston singles. The advertised numbers are of course at high altitudes, but they seem to have very good performance at RV flight levels too.....in particular considering airframe size.

 

[Bob Axsom]

Spinner and Mooney baffle comments

I have seen Paul Lipps comments about closing the holes between the prop blades and the spinner and the thought is sitting there in my brain field but it hasn't started to grow yet. It is a difficult dynamic and assemetric problem. Hanging something on my prop blades that are already severely challenged for efficiency (Peak .855 at 165 KTAS drops to .835 at 200 KTAS) is not that exciting to me. Add to that the fact that the spinner backplate is right at the high pitch rotational limit and doesn't provide room for the hole filler to travel with the blade inside the spinner cavity anyway. I don't feel motivated to tackle the problems yet but after the next three ideas I want to try - its possible.

I see in the Mooney cooling system some things that have evolved in my RV-6A. The baffle across the front does axactly what I am doing with my front baffle. I notice also that they turn the side baffles outward in the drawing at least. That is similar to the extra horizontal side baffles I added outboard of the standard side baffles to prevent backflow past the valve covers into the zone 3 cavity I have between the engine and the firewall. Since Mooney doesn't isolate that area I don't see what benefit it gives them but it is interesting to see it. We are certainly consistent in blocking the backflow out of the front of the cowl from the lower cowl.

Bob Axsom

 

[Bob Axsom]

Possible Mooney side baffle design purpose

As we did our walk in the park this morning I thought about the turned out side baffles in the Mooney design. They would function as a passive pressure control (relief) system that would keep the air moving in the same direction through a very wide range of altitudes and speeds.

Bob Axsom

 

[Norman CYYJ]

Bob could you use a brush type seal on the prop/spinner to reduce the drag?

 

[hevansrv7a]

Prop efficiency

I have seen Paul Lipps comments about closing the holes between the prop blades and the spinner and the thought is sitting there in my brain field but it hasn't started to grow yet. It is a difficult dynamic and assemetric problem. Hanging something on my prop blades that are already severely challenged for efficiency (Peak .855 at 165 KTAS drops to .835 at 200 KTAS) is not that exciting to me. ...
Bob Axsom

Bob, I'd love to know how your were able to measure prop efficiency!

 

[Bob Axsom]

Brush seals and efficiency

I don't trust the effect of brush seals so it is a personal decision thing here. I know a solid sealed barrier works so that was my choice. The physical implementation of a brush barrier seems awkward to me in addition to my distrust of its efficiency.

The prop efficiency is read off of a chart supplied to me in confidence by Hartzell. The same chart shows the 7496 bladed 74" prop peaks at .879 at 185 KTAS and only decays to .878 at 200 KTAS. The 7497 peaks at .857 at 170 KTAS then drops to .843 at 200 KTAS. I have no idea what the numbers are for the new composite Hartzell.

Bob Axsom

 

[DanH]

<<the turned out side baffles in the Mooney design. They would function as a passive pressure control (relief) system that would keep the air moving in the same direction through a very wide range of altitudes and speeds.>>

Naaa. They don't do anything but seal the upper plenum to the cowl, just like the standard seals on a typical Lycoming installation. Some Continental installations have horizontal side baffles, but again, no mystery about their purpose.

 

[Bob Axsom]

The Results are in ...

Today was a pretty nice day in northwest Arkansas but I had a commitment to go to a movie (The Reader - excellent by the way) with my wife so I wasn't sure how the time would work out. I got everything buttoned up at 5 pm took off under a high overcast. 6,000 ft pressure altitude temp was 9 C so I flew the triangle at 5,400 ft. per the US Air Race Handicap Procedure. I ran wide open throttle and leaned for best speed (~1300F Cyl 4 EGT). The RPM was maxed at 2,720-2,730. I neglected to record the manifold pressure on this flight but it is typically 25.5 - 25.8. The oil temperature was higher than usual at ~200. The oil pressure was ~85. CHTs were 1=292, 2=344, 3=341, 4=317. EGTs were 1=1259, 2=1351, 3=1325, 4=1303. I flew three tracks on headings of 360, 120 and 240 respectively using autopilot and altitude hold after trimming for level flight. The speeds recorder at 20 second intervals were:

360 = 185, 185, 185, 185, 186
120 = 179, 180, 179, 179, 179
240 = 175, 176, 176, 176, 175

I plugged 185.2, 179.2 and 175.6 into the NTPS spreadsheet and it told me that the wind was out of 202.2 degrees at 5.6 kts and the true airspeed was 180.0 kts which is 2.6 kts slower than the previous high of 182.6 kts. My plan now is to remove the baffle and refly the test to verify that the mod is ineffective in increasing the airplane speed. Of course there is an error margin in this test method but my experience has been that true improvements do not result in a decrease in speed using this method if the airplane was identical in all other ways except the mod. With the possible exception of the nav antenna the plane was race ready and everything was the same for both tests. Regardless of the validity of that statement the next test should clarify the effect of the new baffle on speed.

Bob Axsom

 

[Bob Axsom]

Idle thought

Early in my cooling air drag reduction effort I had a similar experience to that of the last test flight. I added the three baffle panels at the rear of the engine and tests showed the plane was ~2kts slower. A knowledgable Lancair modifier told me that the improvement in the flow path could have increased the air mass flow through the system and thus slowed the plane. I made another mod by adding the two horizontal baffles just below the top of the lower cowl from the rear of the engine to the front and there was a dramatic increase in speed. Here is the thought. I have two inlet plugs that I developed earlier that proved effective in increasing the CHTs with no speed improvement. It might be informative to reinstall those plugs and fly another test before pulling the front baffle. I'll have a few days to think on that before I can get around to flying again.

Bob Axsom

 

[DanH]

<<horizontal side baffles I added outboard of the standard side baffles to prevent backflow past the valve covers into the zone 3 cavity I have between the engine and the firewall>>

Bob, I've been pondering the above. Generally air moves from high pressure to low pressure. Why do you think air was moving into "zone 3"?

<<A knowledgeable Lancair modifier told me that the improvement in the flow path could have increased the air mass flow through the system and thus slowed the plane. I made another mod by adding the two horizontal baffles just below the top of the lower cowl from the rear of the engine to the front and there was a dramatic increase in speed.>>

I tend to agree with the Lancair modifier. Try this theory; mass flow increased because the rear baffle wall and its two wings greatly improved the approach to the cowl outlet. Without it, relatively slow air is expected to suddenly re-accelerate right at the exit opening. Not a good plan in any case, and the firewall edge in a standard cowl exit makes the situation worse. Exit air is expected to make a right angle turn and accelerate; guaranteed separation. The plume makes the effective outlet area smaller.

Your curved rear wall plus wings removed that firewall edge problem, making the exit in effect larger and increasing mass flow. The airplane got slower.

You then added the horizontal baffles along the lower cylinder line. I suggest those horizontal baffles don't block flow into zone 3, but rather from zone three. The pressure source is leakage past your standard upper baffle seals. The new horizontal baffle required upper plenum pressure to leak past two sets of seals. Total mass flow was reduced because of reduced leakage, and the airplane got faster.

Comparing three pressure measurements should tell the tale; upper plenum, zone 3, and exit plenum just under the horizontal baffle.

Really admire your work. Keep swinging the bat!

 

[Vlad]

Carb heat arm

Bob on your first two pictures you have an elegant solution to old problem of carb heat arm wear. Regular cable and a bearing. Could you please post closer picture of the lever. Sorry for deviating let's go back to cooling drag...

 

[Bob Axsom]

Here's another photo and description

Bob on your first two pictures you have an elegant solution to old problem of carb heat arm wear. Regular cable and a bearing. Could you please post closer picture of the lever. Sorry for deviating let's go back to cooling drag...

This is not a good image but It may help (at the time I was focusing on adding a plate to eliminate the pocket on top of the filter air box - did nothing for speed). The manufactured part is two pieces of small aluminum angle riveted together. There is a #10 platenut riveted to the underside of the aft end and an AN3 bolt and an #10 washer (AN960) are used to attach the control wire to the assembly. At the front end a rod end bearing with a threaded stud in the ball is used to connect the assembly to the carb heat lever.

Bob Axsom

 

[Vlad]

Thank you Bob. That's how I will do it.

 

[Bob Axsom]

<<horizontal side baffles I added outboard of the standard side baffles to prevent backflow past the valve covers into the zone 3 cavity I have between the engine and the firewall>>

Bob, I've been pondering the above. Generally air moves from high pressure to low pressure. Why do you think air was moving into "zone 3"?

I probably didn't explain the situation well. I informally identify the three zones in my cowl as 1 = the upper plenum where the inlet air first arrives (not a hard plenum by the way), 2 = the lower plenum or chamber under the engine and finally 3 = the area behind the engine and lower cowl baffling in front of the firewall. In race configuration I have plates over the blast tube ports and heater port in the rear of the upper plenum so the only opening into zone 3 from zone 1 is through the oil cooler and baffle seal leaks. I believe the baffle seal leaks are minimal because when I put in the blast tube and heater port cover plates the speed of the airplane increases. If there were gross leaks past the baffle seal strips I do not believe there would be a noticeable change in speed when the port cover plates were installed. Before I added the horizontal baffles zone 2 had a gross leakage path to zone 3 up around the valve covers past the upper plenum over the baffling below and aft of the engine. Below the fuselage the curved center lower cowl baffle is mounted with screws and is held off below the fuselage by four AN960-10 washers giving a pressure vent of 1/4" x ~7" to dump the oil cooler air (and the heater air when not used in non-race configuration). Experimentally increasing the opening this by 1/16" (another washer added to each mounting point) decreased the speed slightly so it is not static in there. We agree that the pressure difference in the two zones will cause the flow direction to be from high to low I am told by my thermal engineering friend that the velocity of the air through the oil cooler is very low. I don't personally know because I have no way of measuring it. Regardless, when I added the horizontal baffles below the sill of the lower cowl it completely isolated zones 2 and 3 and there was a dramatic jump in speed. My thought was that the the air coming through the cylinder fins was confined to the area under the engine, reduced the flow options and increased the preasure. An increase in pressure in the lower plenum would reduce the differential between the upper and lower plenum and reduce the air flow mass and velocity through the cooling system. The change in cooling air mass flow I believe is responsible for the increase in airplane speed. I think I said 3 kts earlier in this thread but in reality it was 4 kts over the original baseline and 6 kts above what it had dropped to when the rear baffling in the lower cowl was added before the horizontal baffle was developed. Without special instrumentation I will never know exactly why it is faster but then as long as it is faster it is good enough in the short run. 'Tis a puzzlement!

Bob Axsom

 

[DanH]

<< the only opening into zone 3 from zone 1 is through the oil cooler and baffle seal leaks>>

Ahhhh. I assumed your oil cooler outlet was ducted to the zone 2 plenum, and knew nothing about the 1/4" x 7" slot exit.

<<Before I added the horizontal baffles zone 2 had a gross leakage path to zone 3 up around the valve covers past the upper plenum over the baffling below and aft of the engine>>

Better ask; how did you determine the flow was from 2 into 3, and not 3 into 2?

......Below the fuselage the curved center lower cowl baffle is mounted with screws and is held off below the fuselage by four AN960-10 washers giving a pressure vent of 1/4" x ~7" to dump the oil cooler air .......Experimentally increasing the opening this by 1/16" (another washer added to each mounting point) decreased the speed slightly so it is not static in there.>>

Interesting. Not static is no surprise. Was this experiment before or after adding the horizontal seals to isolate 2 and 3?

 

[Bob Axsom]

......Below the fuselage the curved center lower cowl baffle is mounted with screws and is held off below the fuselage by four AN960-10 washers giving a pressure vent of 1/4" x ~7" to dump the oil cooler air .......Experimentally increasing the opening this by 1/16" (another washer added to each mounting point) decreased the speed slightly so it is not static in there.>>

Interesting. Not static is no surprise. Was this experiment before or after adding the horizontal seals to isolate 2 and 3?

It was after the horizontal seals were added. My assumption is the vent size is controlling the pressure in zone 3 and effectively limiting the air flow through the oil cooler. The change was small but I don't want to give up anything. I have thought about reducing the size but so far I have been content - the oil temperature could be higher though. In my last test the oil temperature was 200 and it is usually no higher than 180 - that observation is nagging at my brain a little bit. Everything is connected and oil temperature is a parameter of interest in cooling drag experiments. I have rationalized explanations but for now I will just monitor it and when I can get some free time from my three page spring chore activities, reinstall the old experimental inlet plugs fly a test and decide what to do from there.

Bob Axsom

 

[Bob Axsom]

Test with inlet plugs installed

Some time ago I developed som inlet plugs for our RV-6A and the results were not impressive. I decided I would reinstall them and fly a test with the plugs and lower forward baffle in place. Before I reinstalled them I cut away a fiberglass web I had applied across the depression in the leading edge between the spinner and the inlet to test the effect on speed - it was bad - and sanded a smooth recession back in this area.

Here are the numbers for today's test.

20 second interval speeds for each track:
360 - 192, 191, 192, 192, 192
120 - 174, 174, 174, 175, 175
240 - 178, 178, 179, 179, 178

I plugged 191.8, 174.4 and 178.4 into the National Test Pilot School spread sheet and learned the wind was 10.4 kts out of 166.8 deg. True airspeed is 181.7 kts

EGT: 1=1320F, 2=1391F, 3=1364F, 4=1330F
CHT: 1=305F, 2=376F, 3=379, 4=342F

Temp at 6000ft pressure alt. 11C
Test flown at 5,100 ft (6,000 density altitude)
MAP 24.7 (the previous test was probably in this range as well)
Oil temp = 190
Oil Pres = 80
Throttle = wide open
RPM = 2720
Mixture = rich of peak for best speed

As observed earlier this is better than the 180.0 kt test with the new forward baffle and without inlet plugs.

Perhaps I should re-fly this but I think I need to remove both the plugs and the new baffle and validate the 182.6 kt speed achieved earlier. I'll sleep on it.


Bob Axsom

 

[Kevin Horton]

Bob,

I am very impressed by all the work you are doing to improve the speed of your -6A, and congratulate you on the great results you have achieved. I offer the following advice, for you to use or discard at your discretion. No hard feelings if you ignore me

My gut feel is that you are making too many decisions on the value of a mod after just one flight's worth of data. There can be various reasons why the speed achieved on any given flight might be a knot or two different from the true performance of that configuration. Most of the mods you are making would only yield less than a knot increase per mod. If you want to fully evaluate the effect of each mod, you really should freeze the configuration, and average the performance measured on several flights. As it sits now, you may be discarding mods that are worthwhile, and keeping some clangers.

Also, the method you are using to account for non-standard conditions doesn't stand up to close scrutiny. So, even with the same configuration, if you tested on days with substantially different temperatures, selecting a test altitude using your technique, you would get different speeds. If you are interested, I can work with you on a way to correct test results to standard weight, altitude and temperature. I hadn't planned on broaching this subject until I had more data from my aircraft to validate this technique, but it'll be months before I can do any more flying.

 

[Bob Axsom]

Thanks Kevin

your approach is similar to another fellow I highly respect - and I drive him nuts too (John Huft). I have to thank you both for turning me on to the NTPS spread sheet - it certainly is better that the usually underestimating average method.

I know the case may be made for exhaustive testing but I have to settle for "good enough". These little things I am playing with now are not expected to yield much if anything - if a configuration isn't clearly superior I'll probably throw it out and look elsewhere. The test I flew today yielding 181.7 kt is so close to the previous high that I think it is worth another flight test (not cheap in terms of time or money) as well as another flight in the no forward baffle and not inlet plug configuration to semi-validate the 182.6 kt top speed. I know it is hard to accept that a test program can be conducted this way but for a one man show I am satisfied that it works to determine relative merit and it keeps the program moving. I really don't care about the absolute accuracy of the numbers even though I present them as though I do. Noise level differences just aren't worth bothering with - that's just to me of course.

When I get out on the race course I want to try to get better speeds than my competitors and improve my performance. On March 15, 2009 I got beat handily by three RV-8s but I know who they are and I have my spirit fired up to pick them off. If I don't make it I don't really think it will be because I didn't test rigorously enough.

Get well Kevin and shoot me a jab anytime you see that I have blown it - again.

Bob Axsom

 

[bigginsking]

Give your results a reality check

Bob:
Kudos to you for having the patience for this.

Your post says:
"wind was 104 kts out of 166.8 deg"
That seems like alot considering only 45 degrees away(.772 in trig)your 120 heading is telling showing 174 ish knots.

Something is wrong, Are you using magnetic heading or GPS track? Having the wrong heading makes for ugly results. I've used the same spreadsheet and the column heading asks for track not heading.

Bill Judge
N84WJ

 

[Bob Axsom]

I corrected the wind speed

I have not checked the spreadsheet results but they seem very close - usually just a bit higher that the three track average. I accidently left out the decimal in the wind speed in my post - it should have been 10.4 not 104. The flight angles are GPS track angles as required by the spreadsheet. The spreadsheet says the wind speeds are in knots and the direction is calculated magnetic. The wind data is derived from the GPS track angles and GPS ground speeds speeds that I enter. I have used it for several tests but I have never checked the wind results against anything since the one number I am looking for is the calculated true airspeed. Sometime it would be interesting to look at that.

Bob Axsom

 

[N941WR]

Bob,

I've enjoyed watching you make improvements on your RV and every time I wonder what I could apply to my -9.

It looks to me like you are running a CS prop. How do these changes impact your prop? Can you get to a point where the prop can no longer adjust for the increased speed?

 

[Bob Axsom]

Bob Axsom

The changes I have made that work for me are all drag reduction mods but as the charts from Hartzell show me propeller efficiency does decay after peaking at some speed peculiar to the specific prop. Propellers are so expensive that I just can't experiment in that area but I know personally of several builder/pilots that have gone to the expense of buying different props and trying them. I have a half inch thick folder and a couple of books on propellers but I don't have real knowledge of propeller performance that would allow prediction - my experience is just too limited in that area. A confidential 8,000 ft propeller performance chart I received from Hartzell shows my 7666 bladed 72" prop efficiency peaks at 165 kts True Air Speed with just over 0.855 efficiency and since I am up around 182 kts now the chart shows the propellor efficiency has dropped to approximately 0.85. At 200 KTAS it is down to a little over 0.836 (where the 7496 bladed Hartzell is up around 0.878). In our speed range I believe as you gain more speed there will be some loss of prop efficiency but drag reductions will still prevail in airplane speed changes (increases).

If you get a chance to watch the video attached to the "RVs are entering the Taylor 100" thread listen to the second plane at the start and at the finish. It is Bruce Hammer's Race#91 Glasair TD I. He has a high pitch fixed pitch prop and it is really turning. He and his Brother Steve who also races a Glasair TD I (Race #73) are always in the 250 MPH group. They are both so fast that I think they enter different classes in the AirVenture Cup Race so they can both win.

Bob Axsom

 

[Bob Axsom]

Two Test Flights 3-29-09

We had a nice clear day here in Northwest Arkansas so I was able to fly two test flights. The first was with no change (lower cowl front baffle and cooling air inlet restrictor plugs installed) and the second was with the cooling air inlet plugs removed. I filled the tanks before and after each flight. It was not an ideal day in so far as winds are concerned but I have to work with what I have when I can fly. West of Fayetteville, Arkansas's Drake field the Ozark hills and mountains cause some irregular flight conditions requiring some manipulation of data (out of line highs and lows were discarded and more than 1 kt differential was allowed) was necessary. Both flights were flown at 6,000ft density altitude, wide open throttle, leaned for best speed.

Parameter/Flight 1/Flight 2
Engine start/14:37/15:56
Temperature at 6,000ft/+6C/+7C
Press Alt flown/5,700ft/5,600ft
RPM/2720/2710
MAP/24.2/24.2
Oil temp & press not recorded
Track 360-1/164 kts/173 kts
Track 360-2/165 kts/172 kts
Track 360-3/168 kts/174 kts
Track 360-4/166 kts/174 kts
Track 360-5/167 kts/172 kts
Track 120-1/202 kts/199 kts
Track 120-2/204 kts/198 kts
Track 120-3/206 kts/200 kts
Track 120-4/203 kts/198 kts
Track 120-5/202 kts/200 kts
Track 240-1/170 kts/168 kts
Track 240-2/169 kts/169 kts
Track 240-3/171 kts/170 kts
Track 240-4/171 kts/172 kts
Track 240-5/169 kts/171 kts
NTPS CTAS /180.5 kts/181.7 kts
Wind speed /23.0 kts/17.3 kts
Wind Dir Mag/306.2/297.6
CHT 1/287 F/295 F
CHT 2/359 F/340 F
CHT 3/363 F/333 F
CHT 4/325 F/304 F
EGT 1/1199 F/1244 F
EGT 2/1314 F/1348 F
EGT 3/1319 F/1289 F
EGT 4/1280 F/1268 F
Eng run time/45 min/35 min
Fuel burn/6.473 g/6.096 g
Fuel cost/$23.56/$22.19

These tests duplicate the configurations flown in the two previous tests but the results are reversed. In this case the "plugs in" speed was 1.2 kts slower than the "plugs out" speed and the previous "plugs in" test speed (3-25-09). The "plugs out" speed was 1.7 kts faster than the previous "plugs out" speed (3-20-09). My conclusion once again is that the main effect of the cooling air inlet plugs is to increase the CHTs and they have not real effect on speed. This saves me having to carry the plugs and install them before races. I want to conduct one more test and that will be done after the new lower cowl front baffle is removed. One side of me hopes the 182.6 kt previous high speed water mark is reached but if that happens there will remain the question of how the addition of a baffle in the lower cowl at the front end lowers the speed of the airplane. The restriction of cooling air inlet reduced the air mass flow through the system as is indicated by the CHT rise so it can be argued that the decrease in airplane speed is not due to increased air flow efficiency and thus air mass flowing through the system cause by the fwd baffle blocking the air back flow out of the front of the cowl. Oh well, one more test without the baffle needs to be flown to validate the effect of the baffle.

Bob Axsom

 

[Joel]

Bob, I noticed in all your runs that your #1 cylinder is about 70 degrees colder than #3. Shortly after I joined this site, I read a post where someone else had a similar problem. He riveted a 1" piece of angle stock across the rear of the #1 inlet ramp adjacent to the cylinder. This reduced the air flowing through #1 and increased it through #3, which balanced the cylinder temp's to within 40 degrees across all cylinders, reduced his oil temp by about 20 degrees, AND increased his airspeed, IIRC, around 5mph.

In looking at the inlets on your engine, I don't see that. I can't find that article at this time, and the numbers may be slightly off, but I was struck by how many factors were improved by such a simple change.

Hope this helps, Joe

 

[kiwipete]

Within 3 degree's

Joe

If you play around long enough with the baffles (ramps plus washer between #3 and baffle)you can get CHT's very close, In the cruise mine normally run with 2-3 degree split between all cylinders. Yesterday it was quite cool reading were:
1) 306
2) 306
3) 306
4) 308

Peter

 

[Bob Axsom]

Low Temp on Cylinder #1 and a Serious Omission

I have known about this situation since I added the probes to the cylinders of course but I just never gave it any speed change priority. An actually observed speed change of 5 mph is so great that I have to think about it more seriously. A blocking/deflecting plate in front of cylinder #1 with size altered to "tune in the temperature" is a common device that I am going to have try. I can use the screws that are used to mount the bottom baffle to also mount the deflector. Thanks for the input. I will report the results after I do this.

After I made the post on the last two flights I realized that all of the tests were conducted with the same configuration less the specific modification under test but this is not true of the previous high speed watermark 182.6 kt test. In that test the assembly access holes in the elevator and rudder were carefully taped whereas the current lower cowl fwd baffle tests have been run with them wide open. This taping was shown to increase the speed by something on the order of 1 kt.

Last year I adopted the taping mod with gaffer's tape and used it for the latter part of the SARL race season. It was very tedious to get installed properly but once I got it on it held up very well with no maintenance required. When I did the annual condition inspection I had to remove the tape to perform the inspection. I used up most of my available time before the Taylor 100 race this month (March 15, 2009) getting the lower cowl forward baffle installed and I was not able to do as good an installation job on the tape or the race numbers (contact paper) - I never even had time to wash the plane before these installations. Both the hole closure tape and the race numbers were partially blown off during that race (182+ kts race average around a 100+ nautical mile course with 5 very sharp turns). I attribute this to the contaminated airplane surface and imperfect application. Anyway, it is clear that I have to correct this omission for a test to compare the forward baffle speed against the previous configuration that yielded 182.6 kts. I don't think I need to reinstall the cooling air inlet plugs - I don't see any speed gain there with two out of three "plugs in" and "plugs out" comparative tests in the airplane's history. They do raise the CHTs in almost all cases - cylinder 1 sometimes doesn't follow the rule.

I have a little conflict in addressing the taping for a proper test comparative of the baffle with the previous high speed configuration. I have a modification in mind that will close the holes and allow for inspections without the use of tape but it requires a lot of careful work and time to implement. There are two ways to conduct a quick comparative test - one is to carefully clean the surface and reapply the tape and the other is to remove the lower cowl forward baffle and run the test with the tail holes open and accept a lower benchmark speed as the "no forward baffle" baseline. Tests are expensive in terms of time and money. Removing the lower cowl forward baffle would only require one test but the desire to see higher speeds in every flight fights my will to keep it simple and clean. I will deal with it and report the results. Clearly, the idea of a lower cowl forward baffle to increase speed is not dead yet.

Bob Axsom

 

[N941WR]

Bob,

I'm using aluminum HVAC tape on the front of my cylinders. Someday I'll get around to putting aluminum angle up there.

One recommendation I made to a builder based in Anchorage was to put plate nuts on the bottom of the ramps so he could make different sets of deflectors, one for the winter and one for the summer. That would make tuning the deflectors much easier as well as simplify changing them out for the different flying seasons.

Just a thought.

 

[Bob Axsom]

Test fly without baffle

The skies are cloudy today (3-30-09) but I think I can get a flight in. I have decided to pull the lower cowl forward baffle for the next flight.

Bob Axsom

 

[F1Boss]

temps not quite right

CHT 1/287 F/295 F
CHT 2/359 F/340 F
CHT 3/363 F/333 F
CHT 4/325 F/304 F

Hey Bob:

These temps are substantially colder than what would be required for max available power from a given fuel flow -- I think you'll have better luck if you can get 'em all over 350F at least, using 380F as your 'in cruise' target; be careful to stay below 400F for longevity reasons. In the case of CHTs, colder is not always better!

Mod idea: I have seen some fellas build a tight seal around the prop shaft, using 1/2" thick foam (as is used to make can coozies) as the seal. This is a different method of stopping outflow around the spinner, which is what your fwd baffle does if I read things correctly. Might put this on your list of future projects.

Data mining: Kevin has a good point in that a single days' test flying might not give you genuine data points -- there is a definite difference in TAS if you fly on a day when (all else being equal) the local air mass is descending (you would be flying in a slow climb = slower), vs stable (level flight) or ascending (flying in a slight descent). Look at the local altimeter setting trend - if memory serves, a change from low to high pressure indicates a descending air mass; a rapid change rate would indicate a higher rate of descent. So, do all your testing when the pressure is moving from high to low, and you'll get the desired results: more speed!

You are doing good work -- keep it up!

Carry on!
Mark

 

[DanH]

<<I have seen some fellas build a tight seal around the prop shaft, using 1/2" thick foam (as is used to make can coozies) as the seal.>>

Mark, was that seal cowl-to-shaft, or cowl-to-spinner backplate?

 

[Bob Axsom]

A new thought for me

CHT 1/287 F/295 F
CHT 2/359 F/340 F
CHT 3/363 F/333 F
CHT 4/325 F/304 F

Hey Bob:

These temps are substantially colder than what would be required for max available power from a given fuel flow -- I think you'll have better luck if you can get 'em all over 350F at least, using 380F as your 'in cruise' target; be careful to stay below 400F for longevity reasons. In the case of CHTs, colder is not always better!

Mod idea: I have seen some fellas build a tight seal around the prop shaft, using 1/2" thick foam (as is used to make can coozies) as the seal. This is a different method of stopping outflow around the spinner, which is what your fwd baffle does if I read things correctly. Might put this on your list of future projects.

Data mining: Kevin has a good point in that a single days' test flying might not give you genuine data points -- there is a definite difference in TAS if you fly on a day when (all else being equal) the local air mass is descending (you would be flying in a slow climb = slower), vs stable (level flight) or ascending (flying in a slight descent). Look at the local altimeter setting trend - if memory serves, a change from low to high pressure indicates a descending air mass; a rapid change rate would indicate a higher rate of descent. So, do all your testing when the pressure is moving from high to low, and you'll get the desired results: more speed!

You are doing good work -- keep it up!

Carry on!
Mark

Mark this input is highly valued as is that from Joel and others. That is a new task for me to work on. I got the lower cowl forward baffle out today but ran out of test flight time. When I get this done I should know whether the lower cowl forward baffle is worthwhile.

Bob Axsom

 

[Bob Axsom]

And the answer is ...

I got up at 5:23 this morning dressed in the dark and drove to the airport to complete the final test to determine whether a baffle in the front end of the lower cowl is of any value for increasing the speed of the airplane. Yesterday I removed the baffle under test and closed everything back up - all that I had to do was pull the plane out of the hangar and fly the test. When I got to the airport it was very dark and no one else was present. I opened the hangar door and got back in my 1986 Dodge Power Ram compact pickup truck to stay warm while I waited for the sun to come up. At 7 o'clock I could see the sky and it was covered with a low broken layer so I closed the hangar door and went home for breakfast of tea, oat bran and toast with no butter with no test results. As the day went on the sky cleared to that beautiful blue that we all know and love. The day had started and my next oportunity to fly the test would come after getting cleaned up and dressed again, completing our daily walk in the park and buying our weekly groceries - retired life may not be exciting but it is satisfying to me.

At 14:57 I was back in the airplane turning the key to start the action. Once again the flight conditions were far from perfect. The surface wind was from 260 degrees at 19 gusting to 23 with runway 34 active the altimeter was 29.82. The altimeter setting for the two tests flown the day before yesterday were 29.94 and 29.92 respectively so an opinion could be had that the trend is downward indicating a rising air mass and each test, all things being equal, would produce a higher speed (reference Mark Frederick's - F1 and EVO Rocket Man - earlier post in this thread) than if the air mass was vertically static. After takeoff I turned to 090 and climbed to the east. The initial rate of climb was unusually high, 2000 ft/min as opposed to my usual 1,500 ft/min which is consistent with the rising air theory - just an observation.

At 6,000 ft pressure altitude the OAT was was 0.0 C so the 6,000 ft density altitude test was flown at 6,300 ft with the altimeter set at 29.92 per the www.us-airrace.org handicap procedure as approved by ATC.

Recorded data:

MAP 23.7
Oil Temp 160
Oil Press 85
CHT 1=294, 2=329, 3=331, 4=310
EGT 1=1306, 2=1409, 3=1391, 4=1354
360 Track speeds 175, 177, 176, 175, 172
120 Track speeds 208, 209, 209, 211, 211
240 Track speeds 162, 159, 161, 160, 159
Engine run time 43 minutes
Fuel burn 6.852 gallons
Fuel cost $24.94

I plugged the average speed for each track into the NTPS spreadsheet and it revealed to me the True Air Speed for this test was 183.3 kts and the wind was 283.2 degrees at 27.6 kts. This is not only faster than any of the tests with the new test baffle installed in the front of the lower cowl, it is 0.7 kt faster than any previous test.

From all of this testing I conclude adding a baffle to the front of the lower cowl, in an effort to eliminate blowback of the cooling air out of the front of the cowl, does not result in a faster airplane. Here is a point that should not be overlooked - adding the baffle produced a change in speed (slower) and that is significant. If one knew why the change occurred perhaps there is a concurrent modification that would result in a speed gain - like the horizontal baffles and the rear lower cowl baffles that worked for a net gain of 4 kts after the rear baffles alone had reduced the speed by 2 kts (6kt difference). For now the lower cowl forward baffle components have joined their cousins on the shelf of offerings to the God of Speed (ref. quote of New Zealand's Burt Munro in the movie "The World's Fastest Indian").

I guess I can think about that deflector in front of Cylinder 1 now.

Bob Axsom

 

[F1Boss]

Fwd cowl seal

<<I have seen some fellas build a tight seal around the prop shaft, using 1/2" thick foam (as is used to make can coozies) as the seal.>>

Mark, was that seal cowl-to-shaft, or cowl-to-spinner backplate?

That would be cowl-to-shaft, tho a cowl-to-spinner type might work well too, if you can get the seal to rub on a smooth area of the backplate. If the seal is a bit tight, it would seem to wear itself to a near zero clearance fit. I'm thinking that a bit of 'Rockblocker' tape around the shaft (or on the surface of the backplate) would keep the seal from wearing on the sfc?

We made the molds for the 550 cowl with the ability to fit the front of the cowl within 1" of the shaft, with the idea of fitting this particular seal to the ship. I have not done this yet (too dang many projects going), so I don't have any data to publish. Due to the movement of this engine installation, I might look harder at your suggestion of a cowl-to-backplate seal, which seems like it would tolerate engine movements easier.

This same type pf seal could be used to seal the aileron/flap and aileron/tip gaps...this was a mod I saw on a Navion, with the seal material captured in an aluminum channel.

As I've hear said before, regarding aircraft speed mods: There is no idea so good that I cannot copy it!

Carry on!
Mark