VAF Forums

VAF Forums (
-   RV General Discussion/News (
-   -   Forming aluminum for a cooling outlet fairing (

Ron Lee 03-17-2012 01:30 PM

This does need to solely benefit racers. If I could increase my cruise speed 10 knots or more AND get better cooling, that would be swell.

Bob Axsom 03-17-2012 01:46 PM

I know what you mean Ron
The development of the a better design than the truncated cowl outlet could benefit anyone that chose to use it.

Bob Axsom

Mike S 03-18-2012 09:32 AM

Drag reduction
This whole discussion about fairings and shape ETC is all in search of the Holy Grail of speed and efficiency through drag reduction.

The link below is a good source to drag reduction information, that is well worth reading.

While the aircraft detailed here are not RV's, the concepts put forth are universal and I suspect a couple of Bobs in the mix may be interested:D

gereed75 03-18-2012 07:20 PM

Just to keep this thread going with additional food for thought, see the attached

This a shot of the outlet fairing on Jim Smith's RV-6. Look familiar Larry?? Not sure if the two are related. Jim's airplane has several aero tweaks, including wing tips designed by Paul Lipps. Jim claimed speed gains similar to what we have discussed with this mod.

And here is one way that the Man Himself -B. Rutan (not our man himself) chose to fair his exhaust on Catbird.

Photos from Osh 2011.

Still working on some ideas, but not ready to post yet. Fly on!!

gereed75 03-19-2012 06:49 PM

Check this Coanda blown diffuser

Bob Axsom 03-21-2012 11:51 PM

My current thoughts
I like to make my speed mods so they can be backed out if they don't work. This mod has many features and it seems to me that this one can be implemented in increments. Here are my thoughts while I take a break from the photo print, book building work.

1 - A long central keel fin/separator of 0.090 attached to the cowl support (it's an A model) and at the rear on the bottom of the fuselage - alignment is critical - test but expect nothing.

2 - Two shorter 0.032 vertically tapered side flow fences extending STRAIGHT back from the sides of the cooling air outlet - test and expect some small increase in speed.

3 - Cover the space across the flow fences and center fin/separator with a screw attached plate spanning the bottom of the outlet similar to what Gary and Dan have with their single piece fairings - test and see what I get.

Subsequently the cover plate in 3 can be removed to experiment with a coanda "bump" against the fuselage starting up near the firewall and with the shape and size of the cover itself. Exhaust shroud/free stream air deflectors/suctioun creators seem likely to be tried. From Gary's tests I think there is something to be gained here and the beauty of it is the flexibility to experiment.

Bob Axsom

gereed75 03-22-2012 11:02 AM

Sounds like a very reasoned approach. I think you will find your best results will come from 2).

This is a simpler, more versatile fabrication than the Vetterman 3 / Jim Smith exit fairing. I am sure they work well, but it is a nothing or all committment.

Good luck

David Paule 03-22-2012 01:25 PM

We know that the leading edge of the wing has a relatively strong suction. That might contribute to a sideward movement of the exit air as it tries to jgo there. If so, it suggests that the side flow fences ought to extend some distance aft of the wing's leading edge.

Also, if the air flow path inside the cowl is constrained with the various flow control devices and fences, don't forget to allow for sufficient cooling of the accessories at the rear of the engine.


Bob Axsom 03-22-2012 05:33 PM

That doesn't seem to be a problem

Originally Posted by David Paule (Post 642273)

Also, if the air flow path inside the cowl is constrained with the various flow control devices and fences, don't forget to allow for sufficient cooling of the accessories at the rear of the engine.


I thing it stays fairly cool in zone 3 with it being isolated from the engine heated air but on trips in cruise configuration I have two blast tubes out of zone 1 (before engine heating) putting air in there. Often I have flown to fairly distant cities with a fuel stop required to get there in race configuration with the cover plates on. Fayetteville, Arkansas to Mitchell, South Dakota, then race hard to Oshkosh and return home for example. I have not measured it but I suspect it is cooler in there than with the stock setup - I know it is a lot cleaner.

Bob Axsom

Bob Axsom 03-25-2012 12:20 AM

Getin' Into It
I made up a rectangular center separator fin And worked on fitting it in tonight. The problem with some of these mods is the amount of disassembly that has to be done to do some small thing like add a platenut. Tomorrow I'll have to pull the cowl off and dissassemble the console to get to the areas where I need to add platenuts. The pressure to get ready for the race at Sherman, Texas next Saturday is not going to allow me to do this the way I thought. I really want to get relative speed numbers for the three major components but I just don't have time to dissassemble, modify, reassemble, test, disassemble, modify, reassemble, test, dissassemble, modify, reassemble, test. The penalty is I will just get one gross number.

Bob Axsom

All times are GMT -6. The time now is 07:40 AM.