Speed Mod Idea Float #2

[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]

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]

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]

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 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]

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

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[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]

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

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[Bob Axsom]

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

Originally Posted by 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]

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

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