Cowl Exit Streamlining

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Kyle Boatright

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One of the things I'd like to improve on the RV-10 is the cowling exit. One of the real impediments is the nosegear structure, which is a mass of welded tubes that really clogs the exit. Here are two pictures - one from the front which gives a sense of the structure involved and a second photo from aft of the cowl exit showing the structure from that perspective and how it blocks the airflow.

Thoughts on where to get the best bang for my buck (actually time) on improving airflow?
  • Put streamlined (airfoil like) fairings on the tubes that provide side to side triangulation?
  • Create a teardrop fairing around the junction where all the tubes come together?
  • Create one "pan" type fairing which smooths the airflow under the whole assembly?
  • Other ideas?

.Nosegear.jpg Exit2.jpg
 

BJC

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I have some of the same questions for my Glasair fixed tricycle project. For now, I have decided not to try to streamline the tubing / struts. I have made an effort to improve the downward flow from the firewall into the exit. I’ll attach a photo when I find it.


BJC
 

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TFF

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I would think the air would be turbulent at the gear structure. It looks like louvres are on the bottom of the cowl. Are they necessary? I think they would be introducing air in the cowl where you don’t want any extra. Streamlined tubing isn’t going to be worth it. Walling in sides may or may not help. If it keeps air sorted with its pressure differential I can see it might help.
 

Kyle Boatright

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I would think the air would be turbulent at the gear structure. It looks like louvres are on the bottom of the cowl. Are they necessary? I think they would be introducing air in the cowl where you don’t want any extra. Streamlined tubing isn’t going to be worth it. Walling in sides may or may not help. If it keeps air sorted with its pressure differential I can see it might help.

The louvers are Van's solution to marginal cooling on the -10. They are part of the standard kit. Given that there is a pressure differential between the lower cowl and ambient (with the lower cowl having higher pressure), I'm sure the louvers create outflow, not inflow. They are probably a draggy solution, ejecting the air at a 45 degree tangent to the airstream.

It would be nice to gain enough airflow through the normal cowl exit that I could close or partially close the louvers.
 

TFF

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While it would take work, a Cirrus style lower cowl copy makes the most sense. I would think the intakes would be close. It does somewhat what you want. It closes off the gear and the air ejects somewhat like an augmenter except it’s part of the cowl shape. I know you just got done but Show Planes - RV-10 Engine Cowling
 

TFF

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That means there is room for improvement. Making a fairing for the exhaust is nice, but I would have thought it would have been more ejector and got rid of louvers. Starting place for though, not the finish line. It’s very odd to me that the 10 cowl seems to be such a compromise. How hot are the CHTs right now?
 
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Kyle Boatright

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That means there is room for improvement. Making a fairing for the exhaust is nice, but I would have thought it would have been more ejector and got rid of louvers. Starting place for though, not the finish line. It’s very odd to me that the 10 cowl seems to be such a compromise. How hot are the CHTs right now?

Not terrible. On an 85F day, they probably don't see 400 F on the first flight of the day. If I land and get fuel, the CHT's may see 410 on climb out starting with a warm engine. In cruise, keeping them below 400 isn't a problem at 60 or 70%, but I have to manage power in climbs. That said, I'd prefer not to be constrained at all by CHT's.

One thing I became aware of AFTER getting well beyond that point on the -10 is glass/RTV baffle wraps, which do a great job at efficiently pushing every ounce of air through the cylinder fins. If I get really bored one rainy weekend, those might be in my future. But in the meantime, I'm hoping to improve the exit airflow.
 

Monty

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Ejectors/Augmentors usually help ground and climb cooling, but hurt top speed. There are several good NACA papers out there. A fairing and nice rounded edge to a sloped exit ramp, with a variable exit flap (or two on either side) are what you need. Easier said than done...
 

Victor Bravo

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Thoughts on where to get the best bang for my buck (actually time) on improving airflow?
  • Other ideas?

Well... you asked for it :)

The bottom is definitely not the correct place to let the air out of the cowling in my opinion. Not by a longshot. I am convinced that this method was the work of the sales department at aircraft companies, not the engineering department.

In my opinion, the best balance/bargain is to have the air exit out through move-able gills on the side of the cowling just above the wing root and as far rearward (towards the leading edge) as you can get without altering the firewall.

Have a look at the Bearcat and the Sea Fury, two airplanes that require a lot more cooling flow than your RV.

Cooling Flow 9-26-2022.jpg
 

PMD

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Geez, I hate being a spoiler, and I may be a bit off based, but did anyone notice that the tailpipes on the provided link is hard mounted on an engine that is on dynafocals? I also read that there are slip joints and ball joints, so no doubt they are quite aware of the differential movement but this IMHO is a formula for constant wear and maintenance issues.

I also agree that for airframe aerodynamics a pair of side exits for cooling are ideal, but with downflow cooling the locations available would seem to be a bit constrained. Also gets me thinking about the problems associated with a good location and configuration for liquid cooled engines. IMHO the proliferation of CACs, coolant and lube rads on many installations fed by blast tubes but left to find their tortured way out of the cowling via air "leaks" is a clear opportunity to do a better install. Makes one sympathize with upflow cooling on some older twins or the nice cowl flaps on old radials.
 
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Victor Bravo

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I also agree that for airframe aerodynamics a pair of side exits for cooling are ideal, but with downflow cooling the locations available would seem to be a bit constrained.
All of the conventional tractor airplanes that I have ever played around with had enough room for the "downflow cooling air" to turn outbord below the cylinders, and still be plenty far up from the bottom of the cowl.
 

DanH

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Kyle, the -10 is an exit mess for sure. Couple points...

The goal isn't really streamlining. The real goal is increasing exit velocity. If, in theory, you increase exit velocity to something equal to freestream velocity, there is no cooling drag, nor a need for any kind of fairing in the wake of the cowl exit bulge.

Reducing the size of the bulge (the backwards coal shovel on the RV cowl) could be seen as streamlining, i.e. reducing frontal area.

The center exit is never going to have fast flow. There's just too much tubular garbage in the flow path. If you're serious, consider removing the coal shovel, and going back with just enough bulge to fair the nose gear structure. Change the exhaust to outboard duals, along the lines of the Showplanes cowl, the new -119 version of the -14, etc. Fair the tubes inside the cowl with a half round, like an RV-8, maybe a little larger.

The glass work is straightforward. Getting the exhaust made might be a pain.
 

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