exhaust exit fairing dimensions

[glenn654]

We've all read about how efficient and low drag the NACA scoops are.

Has anyone tried to use small scoops in a reversed position in the high pressure areas Dan highlighted, I would think one on either side of the cooling exit would be the better place... they may be better that louvers.

Glenn Wilkinson

[Marc Bourget]

Few build a "proper" NACA scoop. They're efficient as intakes in high pressure locations, usually where the cross-sectional area is growing.

Multiple exits, unless well positioned and well designed, usually add add vortex generators. (not absolute, just trying to prompt thinking!)

Peter Garrison designed a cowl for the T-18 where tufting the side exits demonstrated an almost perfect "after cone". Visualize a five foot "hemi-teardrop" with the last 18" cut off. The air "filled in" for the missing tail of the tear drop.

mjb

[Marc Bourget]

Dan,

Another point, supported by CR-3405 is that you can oversize the inlet with little or no additional drag IF it's designed correctly. As I mentioned in another post proper exit design gets you an efficient "after body" or after cone.

Maybe it's not the size but how you design it? :^)

I sent you a PM did you see it? I'd like to backchannel some questions.

mjb

[DanH]

Originally Posted by Marc Bourget

Another point, supported by CR-3405 is that you can oversize the inlet with little or no additional drag IF it's designed correctly.

Well, I've said that more than a few times......

As I mentioned in another post proper exit design gets you an efficient "after body" or after cone.

I really think the RV-8's slot exit is quite good, after eliminating all the cowl exit chute hanging below the belly skin line.

I sent you a PM did you see it?

No PM here.

[glenn654]

I really enjoy threads of this type...I learn something every time.

For example, I had never considered that air loss through the gear leg holes in the cowl would be significant but now I know better.

Glenn Wilkinson

[rvmills]

Lots of great info, for sure!

Dan, thanks for the amplification on the cowl flap's (variable-size exit's) effect on oil cooler performance. Very interesting. Perhaps related, as I don't have a ducted oil cooler, but after shrinking my exit, the impact on oil temp was also more marked than the impact on CHT, though both are higher during slower operations than before. However, both still move with speed changes, whereas it sounds like your CHTs are fairly steady. Still thinking through this effect.

Tom, great description of the corner seals...I'm all over that! Any chance you can post a pic? I think I get it, but you know what a picture is worth! As for louvers, I completely agree. My thought with them is to make them interchangeable with flat plates, so for anything but racing, cooling is improved...and racing performance is maintained with little impact from cowl flap internal parts clutter...kind of a KISS "cowl flap" version. Then again, it's kinda like a ground adjustable prop...it leaves a lot to be desired in terms of flexibility. A variable exit also maintains cruise performance...and heck, every flight has some low-speed elements that could use additional cooling.

Testing appears to be on the horizon...good lead on the custom louvers Pooner.

Neat info!!

Cheers,
Bob

[rv969wf]

Couple of videos to watch.

https://www.youtube.com/watch?v=dtIwhaSiu_4 7,500 ft. WOT

https://www.youtube.com/watch?v=2zwiMMBEMH8 3,000 ft. WOT

This last summer I was at a flying event in Natchez, MS and I did some low altitude WOT testing at 800 ft. turning the engine 2,680 rpm and picked up a significant speed gain over WOT at my home airport of 3,000 ft and the manifold pressure was the highest that I'd ever seen it, 31.4" All temps are great, many times I have a hard time to keep cylinder temps up. All I know is that the cooling mods that I've done seem to be working, I know some of the mods are not perfect and could use some attention, the oil cooler exit is in a terrible place but I just don't have any room to make it better. I just go fly it and don't mess with anything anymore other than doing an annual and oil changes, if it works leave it alone and go fly.