Cowl Exit Streamlining

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Aesquire

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I'd look at the bottom of the firewall. A 90 degree sharp edge wrecks flow. Is there room for a curved extension to smooth the flow? Even if you lose a few inches of area, you can get gains in cooling and drag.

Since it's behind intakes, a cut in half lengthwise pool noodle speed taped to the front of the firewall right at the bottom lip, is a cheap and temporary test. If you see big gains, duplication in aluminum sheet is easy.
 

PMD

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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.
Except that said space is cluttered to aerodynamic disaster with carb/throttle body, air cleaner, carb heat/alt air, exhaust and muffler(s).
 

Victor Bravo

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Except that said space is cluttered to aerodynamic disaster with carb/throttle body, air cleaner, carb heat/alt air, exhaust and muffler(s).
For right now, forget about the engine stuff inside the cowling. I don't think you're going to redesign the engine itself at this point.

Remember, your airplane has a plenum system, not a duct system. A high pressure / low temperture plenum is filled up by ram air from the front, long before the engine is cooled. The air then moves through the cylinders, picks up the heat, then fills up a low pressure / high temperature plenum. Finally, this plenum is emptied by allowing (or assisting) the warm air to escape the cowling.

A "duct system" is something like a serpentine 6 inch diameter aluminum tube with a 6 inch round oil cooler mounted somewhere between the ends. No enormous changes in cross section or volume before or after the heat exchanger.

For now, I humbly suggest you concentrate on the "low hanging fruit" improvements you can make without tearing the engine and airframe apart. The intakes are already in a fairy decent high pressure area, so concentrate on getting the air out of the cowling efficiently (which is where you began the discussion).

But remember, since I believe your aircraft cowling design uses a plenum underneath the engine, whatever stuff is in that lower cowl plenum is going to be there whether you let the air out at the bottom or the sides. So I strongly suspect that in this case the vast majority of whatever modest improvements you are going to achieve will come from the exit itself.

If you assume everything else will be more or less the same (an aerodynamic disaster inside the cowl, because of all the s**t hanging off of the engine) and look only at the last six inches around and near the air exit, then you can see that the sides of the cowling behind the engine and below the cylinders are much less obstructed than the lower firewall and nosegear mount. The outside of the cowl on the sides will also be in an area of lower pressure than the bottom, which is a big plus as well.

The exception to all of this, of course, is if you create an augmentor system like what Toobuilder has done. This essentially makes the last foot or two of the air exit into a duct with a lot of energy added by the exhaust. My suggestion of a "Venetian Blind mechanism" on the sides is (IMHO) the best that you are likely to do on a stock cowling without going the extra mile to build the augmentor ducts.
 

Aesquire

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Victor, I agree that you could do better with a complete redesign of the lower cowling area. But that's lots of work.

A ramp, or ramps, cutting the firewall shorter, rounding the turn, and tapering down to the belly is a good idea too, but costs foot room. And is a Big redesign.

I'm all about low hanging fruit. Simple wedges on the down stream side of the gear tubing, getting that bottom corner of firewall and belly smoothed out, seems the easy steps.

It would be useful to measure pressure at different points in the lower cowling. A spare altimeter will give you relative numbers, just keep safety in mind, in case your tubing gets loose.

Unfortunately, but changes are far easier in the early stages of design, fairly easy early in the build, and very hard after first flight.
 

Victor Bravo

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Not a complete redesign, just putting a louver/Venetian Blind device in the sides, then closing off the bottom air exit with temporary sheet metal and pop rivets.

If it works, then smooth out and fair in the bottom cowl exit with properly shaped pieces and proper rivets and Bondo and whatever.

If it doesn't work, remove the lower close-out covers and patch the holes where the Venetian Blinds were.
 

rv7charlie

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If you are talking about an Otter seaplane, sure, but augmentor tubes do not have to be tuned for slow speed flight.

Mine are only 4.5 inch diameter (each). plenty of exit velocity on these.
Do you use some form of cowl flap (exit restricter) for cruise flight?

Most a/c don't have any problems at cruise speed with cooling; it's the climb profile high power/low speed (poor ram effect) regime where cooling problems arise. The common issue with augmenters is that with fixed exits and adequate cooling flow in climb made possible with the augmenters, when you start going fast the ram effect is added to the augmenter effect and cooling flow goes up, when cooling needs are going down.

Perhaps the phrasing would have been better if it said that the augmenters hurt efficiency at cruise, instead of top speed.
 

D Hillberg

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Forget what's inside the cowl = It's a plenum area all high pressure low velocity air
The fire wall lower 90 degree angle is what you can change to a rounded exit.
The exit area is what is limited and flow up stream is not an issue. but that sharp edge has got to go...
 

rv7charlie

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louver/Venetian Blind device in the sides
There's info on the CAFE site (if you can still find it) that says the most efficient exit is a 'bluff body'. There's a pic in their pdf showing one, and it's basically the shape of any typical lower cowl exit. Not the bottom location; the shape. Louvers have the same effect as a biplane/triplane/quadraplane/etc wing setup. Lots of intersection drag, skin friction, etc.
 

Toobuilder

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Do you use some form of cowl flap (exit restricter) for cruise flight?

Most a/c don't have any problems at cruise speed with cooling; it's the climb profile high power/low speed (poor ram effect) regime where cooling problems arise. The common issue with augmenters is that with fixed exits and adequate cooling flow in climb made possible with the augmenters, when you start going fast the ram effect is added to the augmenter effect and cooling flow goes up, when cooling needs are going down.

Perhaps the phrasing would have been better if it said that the augmenters hurt efficiency at cruise, instead of top speed.

My whole approach to the augmented exhaust was to remove/reduce the need for a mechanical cowl flap. The reasoning being the higher mass flow out the exhaust in climb would partially offset my loss of ram pressure. And then with the higher Q at cruise offsetting the lower mass flow of the engine (at altitude and lower RPM), I could size the outlet to optimize cruise and "hope" the climb config worked out.

And it did. My inlets are smaller than an RV-8, my outlets are even smaller than the inlets, and I have zero cooling issues in any condition I have tried (and its **** HOT here).
 

rv7charlie

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All I can say is, you're in good company.

edit: Forgot; I can also say that some aero guys worry about the exit flow messing with the efficiency of the wing, since it's disturbed flow over the top of the wing at the root. Perhaps that's why Thorp's exit is up so high on the side of the cowl.
 

Aesquire

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Don't know about the Thorp, Grumman used side exits on the F8F, with exhaust augmentation, and was influenced by the FW-190. ( copied is such a litigious word )
 

rv7charlie

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Another concern about side exits I've seen expressed is the greater risk of getting CO in the cockpit, since the sides of an a/c cockpit are typically harder to seal up than the floor. The SBS RVs, for instance, have their fresh air vents roughly even with the pilot's knees on the sides of the fuselage, directly behind where the cowl exit would be. The T-18's fresh air inlet is actually on the back, center, base of the canopy; it parts your hair in back when open.

I'm not necessarily disagreeing with the idea of side exits; it's in my planB/C/etc list for my rotary install. But we need to operate with all available knowledge when we change a mfgr's design.
 

Aesquire

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If pool noodle is hard to find in fall, the foam pipe insulation is available in different sizes at the local big box hardware store.
 

rv6ejguy

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You'll only really know how successful you are with any approach by flight testing and instrumenting the cooling exits for velocity before and after mods. If you increase exit velocity in cruise and it still cools ok in the climb, you've made it better.

Toobuilder was able to shrink the inlets and exits, almost certainly reducing drag as compared to before

Most other guesses here are just guesses...
 
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