[Bob Axsom]

I have an RV-6A which I modified to get more speed by adding baffling in the lower cowl. Eventually I gained 4kts but initially there was a 2kt loss due to increased air mass flow (theoretically). I had not installed CHT instrumentation when I built the plane so I did not know what the cylinder head temperatures were. I installed a CHT system and further modified the baffling which resulted in getting the 2kts back and adding another 4kts. The theory is that the the increased air mass flow cooled the engine more but at the same time increased the cooling drag. The latest evolution of that mod reduced the air mass flow which reduced the cooling drag and still produced reasonable CHT values all well under 400 at extreme conditions. It has been stated that reducing the cowl exit cross sectional area will reduce the air mass flow further, raise the CHT levels and increase aircraft speed due to reduced cooling drag. My original thought was to construct some aluminum assemblies to attach to the vertical web cowl support behind the nose gear and change them to tune the best CHT/speed tradeoff. However, as I look at the photos taken from the rear of the cowl outlet it appears that one could construct a shroud to enclose all of the NLG members exposed between the lower cowl baffle and the lower cowl. The question is, would this accellerate the air and increase the air mass flow and raise the cooling drag and slow the plane or would it restrict the flow and provide the desired result?

Your thoughts?
Bob Axsom

 

[pierre smith]

Fairings....

Hi Bob,
I was told that you could go either way.......fair all three NG tubes individually with some left over landing gear material or make a fairing that encloses all three legs. Remember how much drag the round landing gear created before your added fairings? Seems to me it'd be easier to make one fairing.
Regards,

 

[Bob Axsom]

Thinking Time

I went to the hangar and looked at the setup again. I put a platenut on that airfilter bypass door so I can lock it down and I adjusted the prop governer limit screw so I can get more than 2610/2620 RPM while I thought about what I saw. One fairing would clean up the passage but it would reduce the cross sectional area forward of the actual outlet. That could be a perfect combination or it could be too restrictive and drive CHT to an unacceptable level. The faired element abroach would not consume as much area (which could be a good or bad thing). More think time required.

Bob Axsom

 

[Bob Axsom]

Tonight's View

It looks like the individual strut fairing would be the best way to go with this and then one of the struts will have to be a partial job because of the routing of the mixture and throttle cables. Time is so darn short that I may just have to pass before Oshkosh. It is painful once I get the idea in my head but I will probably have to get over it for now.

Bob Axsom

 

[Bob Axsom]

One more day to try

One more day to come up with something. I'm thinking a flat plate across the airbox would be better than the normal cavity. I don't see any quick fix for the NLG braces and strut socket. After tomorrow it will be as fast as it is going to get. The 1966 move Gran Prix just came out on DVD and I received my copy today. That will be part of my personal build up for next weeks big show.

Bob Axsom

 

[Bob Axsom]

Covering the Air Box Recess

Yesterday I moved from contemplating to cutting metal on a cover for the air box recess. I will put the photo thumbnails below. As with many of these "ideas" when you start you can only forsee so far into the final configuration. Several opportunities and implementation ideas make them selves known as you get into the project. I was thinking that I would stack AN960-10 washers on some longer AN3 bolts and use the airbox mounting locations as the cover mounting locations but locating the holes etc. would be a pain. I realized the four small angles with platenuts, mounted on the bottom of the cover plate and inside the air box flange would be a cleaner approach. Also, the new flat plate can form a good base for mounting things like a fairing for the carb and a deflector for the "A" model NLG welded web structure (they must be separated because the air box moves with the engine and the NLG structure is rigid.

Bob Axsom

[/URL][/IMG]

[/URL][/IMG]

[/URL][/IMG]

[/URL][/IMG]

[/URL][/IMG]

 

[penguin]

Bob,

I'm running an O-320 and FP prop in a 6A and have always had higher than desireable CHTs, despite spending some time on the baffling. I always assumed that the cowling exit area is really too small (especially with the nose gear structure and tail pipes in the way), and that restricting it any more would just make the CHTs hotter. I have now ordered some louvers to fit into the bottom of the cowling to increase the exit area. I wonder if blocking off the Van's exit altogether might be advantageous as it would stop the exit air having to fight with the exhausts and nose gear mount? Perhaps a some kind of adjustable blanking plate on the louvers would provide a cowl flap like arrangement to adjust the cooling flow? I'm just thinking out loud here and haven't tried anything - what do you think?

Pete

 

[Bob Axsom]

That's it for This Year

This is as far as I got with the mod involving the air box. Tomorrow I will do a little final trimming and sealing then reinstall the cowl. You notice all of the draggy looking NLG structure behind the airbox... that will have to wait until next year's race. I do plan on riveting an air deflector on the the new fairing at the trailing edge to avoid the web welded between the struts right behind the airbox. It is obvious that a shroud could be mounted on the new airbox fairing to completely enclose the carburetor. It will be interesting to develop this to the best configuration I can come up with. One thing I learned very forcefully in developing the airflow mod in the lower cowl is, if you clean up the airflow path the result can be the air flows more efficiently - more air mass flows - the CHTs get lower than allowed for normal operation - AND !!!COOLING DRAG INCREASES! If you cut down the air mass flowing through the cowl the CHTs will increase (to what they were before perhaps) and the speed will increase above what you had before the mod to improve cooling air flow was implemented. I may be back in that situation after this latest mod. S-o-o-o-o, I plan to put a couple of mounting holes in the bottom center vertical cowl brace so I can bolt on units to reduce the cowl outlet cross sectional area and throttle the air mass flow if necessary.

Jeanine and I will be flying our little RV-6A Blue Bird against an RV-6, and RV-7 and five RV-8s including John Huft's past winner. Anything except last in class will be a victory for us.

Bob Axsom

[/URL][/IMG]

[/URL][/IMG]

 

[Bob Axsom]

I Honestly Don't Know the answer

As I get into this a little more I am learning more with each alteration and I am getting inputs from others that have been through this learning process before. I don't believe the exit area is too smallGeorge AKA GMCJETPILOT has provided some good information under a thread called "cooling drag" I believe and in some others as well. What I found was the baffling I added in the lower cowl allowed more air mass to flow throught the system which improves the cooling with the heat transfer to the larger quantity of air but the larger quantity of air increases the cooling drag and slows the plane down. Since the CHTs are now lower because of the improved cooling you can reduce the size of the inlet or outlet to control the air quantity to the desired level determined by CHTs and increase the speed of the aircraft over the initial speed. It is a bit of work and not without risk but this specific process is working for me. I would not add louvers but that's just my need for speed talking.

Bob Axsom

Bob,

I'm running an O-320 and FP prop in a 6A and have always had higher than desireable CHTs, despite spending some time on the baffling. I always assumed that the cowling exit area is really too small (especially with the nose gear structure and tail pipes in the way), and that restricting it any more would just make the CHTs hotter. I have now ordered some louvers to fit into the bottom of the cowling to increase the exit area. I wonder if blocking off the Van's exit altogether might be advantageous as it would stop the exit air having to fight with the exhausts and nose gear mount? Perhaps a some kind of adjustable blanking plate on the louvers would provide a cowl flap like arrangement to adjust the cooling flow? I'm just thinking out loud here and haven't tried anything - what do you think?

Pete

 

[Bob Axsom]

NLG Inverted Pyramid Cap

I made a cap for the inverted pyramid welded into the nose gear strut support but I do not have time to test it or the airbox cap separately. I am finishing the deflector that goes on the rear of the airbox cap and should close up everything tomorrow. The deflector looks like a small spoiler from a toy car.

Bob Axsom

[/URL][/IMG]

[/URL][/IMG]

 

[RV8RIVETER]

DON'T NEED LOUVERS

Bob,

I'm running an O-320 and FP prop in a 6A and have always had higher than desireable CHTs, despite spending some time on the baffling. I always assumed that the cowling exit area is really too small (especially with the nose gear structure and tail pipes in the way), and that restricting it any more would just make the CHTs hotter. I have now ordered some louvers to fit into the bottom of the cowling to increase the exit area. I wonder if blocking off the Van's exit altogether might be advantageous as it would stop the exit air having to fight with the exhausts and nose gear mount? Perhaps a some kind of adjustable blanking plate on the louvers would provide a cowl flap like arrangement to adjust the cooling flow? I'm just thinking out loud here and haven't tried anything - what do you think?

Pete

Pete

IMHO. The inlet and exit area of the -6 is more than sufficient for cooling, especially a 320. You can install louvers if you like, but you should really take a close look at your cowling/baffles. You could be loosing alot of pressure past the baffle seals or have the air going back out the other inlet.

Do you have any pictures of your installation? It might make it easier to to stear you in a particular direction.

 

[Bob Axsom]

Final Configuration of Mod Before Test

This is what I ended up with.

Bob Axsom

[/URL][/IMG]

[/URL][/IMG]

[/URL][/IMG]

 

[Bob Axsom]

Test Results - Inconclusive

The test setup was flawed for the airbox and landing gear inverted pyramid cover by a change to the prop governer limit setting and weather. I changed the gov setting to allow 2700 RPM and the air was not calm. At 6000 ft the OAT was 23C so I flew the test at 4,000ft per the U.S. Air Race Handicap procedure for 6,000 ft. density altitude. Engine RPM was 2680 as opposed to 2610 on the previous tests. On the three headings requiring 5 consecutive GPS speeds 20 sec. apart within one kt I got:

360 = 166,176,[168,168,168,168,169]
120 = 171,175,175,172,175,175,173,176,[174,174,175,174.174]
240 = [172, 172, 171, 171, 171]

for an average of 171.3kts

This is three kts worse the the speed before this latest mod but it was hot and rough. After the test I turned to 360 again and reduced the RPM to 2510 and recorded fives speeds before the heat determined it was time to go home. They were 168, 170, 174,169,177. The CHTs at the end of the 2680 RPM run were 1 = 321, 2 = 369, 3= 368 and 4 = 341. After the short run at 2510 the CHTs were 1= 305, 2 = 356, 3 = 369, 4 = 341 (all F).

On the climb out from the airport the ball was all the way to the right side of the inclinometer even with full right rudder and the rate of climb was around 1,000 FPM which is less the I normally see. In level flight the ball was centered. My initial thought is deflector at the rear of the airbox cover is a problem and I am considering pulling it but not the cover before leaving for Dayton Friday.

Bob Axsom

 

[Bob Axsom]

Deflector/Spoiler removed

After today's test I decided to removed the small deflector I included on the trailing edge of the air box cover. The holes have been closed the cover reinstalled and the cowl reinstalled. I will fly it again in the morning to see what I can see. Unless there is some major set back this will be the race configuration - no more time to tune.

Bob Axsom

 

[Bob Axsom]

New Speed Test Run Without Deflector/Spoiler

It is hot again today in Arkansas but the surface winds were calm and I made another speed test run after getting everything back together with the deflector removed from the right side trailing edge of the air box cover. At 6,000 ft the OAT was 26C so I made the speed test run at 3,700ft under the watchful eye of Razorback Approach. On the climb out I found that I could nearly center the ball which was drasticly different than yesterday. This told me that the deflector did make a difference and that the air in this zone of the cowl is a force to be dealt with. Perhaps more than usual because of the previous mod which isolated some of the cowl volume from the cooling air flow path and provided direction of air flow to the exit area. Using the U.S. Air Race handicap proceedure again I got these GPS readings (5 consecutive speeds 20 sec. apart with no more than 1 kt difference are required in this procedure):

360 = 175, 175, 175, 173, 172, 174, 174, 175, 173, [175, 174, 175, 175, 175]
120 = 177, [179, 180, 179, 179, 180]
240 = 161, 162, [164, 163, 164, 163, 164]

for an average speed of 172.6

The test was run at 2620 RPM and at the end of the run the CHTs were:

#1 = 318, #2 = 375, #3 = 372, #4 = 343 (all F)

That is an increase of 1.3 kts over the configuration with the deflector/spoiler but 1.7 kts slower than the configuration without the airbox cover. Soooooo, I'm going back out the the airport and remove the air box cover and the NLG inverted pyramid cover for the race. This seems like a bad mod on the surface but I think only the deflector was bad the rest helps the airflow I believe but ironically the efficiency allows more air mass to flow in the system and this added cooling drag slows the plane down. After we get back from Oshkosh I will put the airbox cover back on to gain what I percieve to be an improvement in internal airflow and work with inlet and/or outlet restrictions to control volume.

Bob Axsom

 

[N916K]

Bob

Thanks for all the hard work you've done on the speed mods.

Good luck in the race.

Cam

 

[Bob Axsom]

Thanks CAM

I'm glad you found the information interesting. It certainly has been a good experience for me. I feel like we have made a step forward in spite of the temporary back steps. The thread on cooling drag in the Traditional Engine catagory appears to have some very knowledgable contributors - I have been very interested in what they have to say.

As for the race we leave for Dayton on Friday morning. I have everything back together in the fastest currently known configuration for our airplane (174+ kts)+. I've checked the removable drag items and I'm making a little aluminum fairing to put on the tail tiedown ring and that completes the speed mods. Tonight we (my wife and I) are going to create the flight plans and tomorrow I'm going to clean up the plane and put the numbers "71" on both sides of the vertical stabilizer & rudder and the underside of the left wing. It is an exciting time. Maybe I can wax it in Dayton - time is short.

Bob Axsom

 

[Bob Axsom]

Last test today

It was so hot here today that I simply could not get a good test run speeds were varying over 10 kts at the 20 second intervals. GPS in the SL-60 would not come up presumably due to the heat so I flew along getting the numbers off of my old GPS 90. I had the Comant Nav antenna legs and wing tie down rings removed but I saw no gain in speed (well I guess not anyway - just not a good test time). I reinstalled the NAV antenna legs and tested the system. I could not get glideslope or VOR indications. Removed and reinstalled and everything came back - not a good idea to do that - I'm flying with the antenna installed.

Bob Axsom

 

[glenn654]

speed mod...new wing tips

Has anyone tried Vans sheared wing tips on a -4? Any performance gain?

Glenn
N654RV @ OKZ

 

[penguin]

Pete

IMHO. The inlet and exit area of the -6 is more than sufficient for cooling, especially a 320. You can install louvers if you like, but you should really take a close look at your cowling/baffles. You could be loosing alot of pressure past the baffle seals or have the air going back out the other inlet.

Do you have any pictures of your installation? It might make it easier to to stear you in a particular direction.

I don't have any recent pictures, but I have a standard set up with baffles and black fabric on top. When I had the motor rebuilt I spent a fair amount of time to ensure leaks were minimized. I have also filled in the inborad side of the upper cowl ramps (which helped lower temps and increased speed slightly). A couple of well respected local builder have fitted louvers (to 2 6s and a 7) and seen good results. My oil temps are also slightly high (205 to 210) which I think also points to lack of flow out. I'm tempted to temporarily put a small lip on the cowl exit to try to pull more air through, to see if that helps. I also like the idea of "fairing" the nose gear support tubes.

Good luck to Bob in the race to Osh.

Pete

 

[pierre smith]

Sheared tips........

Hi Glenn,
We put sheared tips on our 6A with the flush nav lights and strobes. Much cleaner plus it adds a couple square feet of wing area. I get 204 TAS at 8000 feet WOT solo (2700 RPM),
Regards,

 

[RV8RIVETER]

Pete

There were quite a few -6's in the Austin chapter, I don't know one that had louvers. The only one I personally know of is Don Christianson's -8 and he has an angle valve. It has been mentioned, quoting the NASA report I believe, that 1/3 of the air blows by the baffle seals on most factory airplanes, so since Vans modeled these, I would have to think something similar is going on here. If you think about, those flexible seals really are not design to hold pressure. Since you already have bafles, I would recommend you think about installing a simple aluminium dog house plenum, I don't think that would be much harder to fabricate than installing louvers. Of course, in the end, it is your plane so whatever course you decide I wish you success.

Yes, good luck BOB!

I don't have any recent pictures, but I have a standard set up with baffles and black fabric on top. When I had the motor rebuilt I spent a fair amount of time to ensure leaks were minimized. I have also filled in the inborad side of the upper cowl ramps (which helped lower temps and increased speed slightly). A couple of well respected local builder have fitted louvers (to 2 6s and a 7) and seen good results. My oil temps are also slightly high (205 to 210) which I think also points to lack of flow out. I'm tempted to temporarily put a small lip on the cowl exit to try to pull more air through, to see if that helps. I also like the idea of "fairing" the nose gear support tubes.

Good luck to Bob in the race to Osh.

Pete

 

[gmcjetpilot]

More than that, 55%

There were quite a few -6's in the Austin chapter, I don't know one that had louvers. The only one I personally know of is Don Christianson's -8 and he has an angle valve. It has been mentioned, quoting the NASA report I believe, that 1/3 of the air blows by the baffle seals on most factory airplanes, so since Vans modeled these, Yes, good luck BOB!

Actually it is 55% on the particular plane they tested, so it varies from plane to plane. I have seen beautiful soft seals that where a work of art and looked very tight and others that looked like a pasta strainer. They NASA report went on to say with the "Dog House" the same cooling was obtained with the 38% less cooling air. Again for the airplane that was tested, which had a similar "soft seal" set up as our Van's standard baffle kit.

The bottom line is a solid sealed "Dog House" produces less leakage. Less "by-pass" leakage of air, that is air that is not doing cooling, the less air we need, the smaller the inlets are needed and the less cooling drag. Also air that leaks into the lower part of the cowl, the low pressure or expansion area and exit, the less pressure differential across the engine cylinders and again less cooling efficency. Even the BEST of a soft seal installation will leak more than a dog house.

 

[gmcjetpilot]

No big deal

Has anyone tried Vans sheared wing tips on a -4? Any performance gain? Glennc N654RV @ OKZ

Glenn the performance gain would be Nil to small in my opinion and from what I recall from what Van's conclusion was. In fact I recall Van's conclusion was there was little change in performance, but the new tip shape provide an easier way to mount NAV / Ldg lights in the leading edge in or on the wing tip, so fan switched to the new sheared tip for practical purpose more than performance.

If there is a gain it is measured in fractions of a MPH or may be one.

Remember when Horner tips came out? Well I am to young to remember but owned a Piper with aftermarket Horner wing tips. Horner tips are like the original type vans wing tip, flat on top and angled bottom. They replaced the more symmetrical stock Piper wing tips. The theory was it gave more speed or lower stall or some such thing. Now Horner tips are not popular and we going back more to what Piper originally had 40 years ago?

The way I look at Van's "sheared tip" is it is a "wing let" laying down. Yes longer span and more area do things for you but not enough to change unless you do it for the looks. I do like the shared tip look better.

Actually Van's seared wing tips could be better and there is an after market wing tip that looks identical but is better. It is a secreat and I am not saying. Well may be I'll tell.

Anyway aerodynamic fashions come and go. You will get PhD Aerodynamisist argue about the pros and cons of winglets. One Chief Boeing aerodynamisist told me the best winglet was one that was laying flat, i.e., more wing span. The B-747 added 12 feet more span and also the winglet. At some point you run out of room to park planes. The B-777 does not have wing lets. Hummm

So wing tip's on wings is a bit of black art and not really a quantum leap in performance. In a large jet saving 1% is a lot due to massive fuel flow.

People incorrectly put wing-lets on something and think it's all good. It is not always. In fact ANA or JAL airlines took their 747-400 wing-lets off because of the way they flew, short legs. The weight was not paying it's way onto the plane for the mission they flew, albeit a non-standard one for a B747 which is suited more for 14 hour flights, not 2 hour ones.

So unless you have a desire to spend money and put new wing tips on, I would spend my time flying and the money you save on not buying a wing tip on gas. The difference will be small.

 

[Bob Axsom]

Please Disreguard the #1 CHT Data

When returning from Oshkosh I could not ignore the low temperature and variations of the #1 CHT anymore. After getting home I removed the cowl and found one of the thermocouple wires in the extender cable (supplied complete by EI as part of the C4 product (4 cylinder CHT system)) was broken. I consider all of the previously reported Cylinder #1 CHT data suspect. The other three temperatures track the operational modes and are very stable. Electronics International is sending me some replacement terminals.

Bob Axsom