Low Drag Cabin Air Exhaust - Whats Optimum?

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Toobuilder

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I know a certain record setting RV-4 builder/racer that found speed by sealing up the elevator pushrod at the aft fuselage bulkhead. It seems that cabin vent air was working to the back of the fuselage and exhausting at the rudder and elevator hinge line - creating measurable airframe drag (about 2 knots IIRC). Got me to thinking about the extreme drag sensitivity the sailplane mafia has. The cabin air entry on a sailplane is generally right on the stagnation point on the nose - a low drag and high performing spot - but what comes in must also go out. What do the competition sailplane guys use for the exhaust? I assume this has been studied and perfected in that camp.
 

rv7charlie

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Don't know the answer to that, but the slider RVs in stock config have horrible leaks out the sides at the canopy tracks. Like holding a piece of plywood edge to the wind out both sides of the fuselage.
 

aivian

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I've seen a few configurations in sailplanes, though never a comprehensive analysis of alternatives or trades on ventilation approaches. Here are the approaches I've seen.
  • Exhaust through the tail area using the rudder horn fairings, a dedicated vent, or natural leakage. Some of these work well and some...not so much. I haven't seen any other exits on current Schempp or Schleicher aircraft so I suspect that they are exhausting through the rudder horns.
  • Through gaps in the underside of the wing root (somewhat unusual, I'm only familiar with it as this is how my Libelle is configured). I don't find it very effective and suspect that it is not particularly low drag.
  • A dedicated vent on the turtledeck, sometimes paired with an internal duct. This is used by Jonkers and available as a retrofit for a number of older sailplanes.
I can't find a great depiction of the entirety of that last system, but often this exhaust is connected to a duct which surrounds the headrest and then converges to the exhaust. The Jonkers folks have done a lot of analysis suggesting that this is a low drag approach, it is mentioned here, albeit not in great detail. I have talked to several people who have installed the turtledeck vent, comments are generally very positive but are more about how quiet and effective the ventilation is than about drag. I believe at least one HP-24 has a vent like this so BobK can probably provide more insight.

It may be worth contacting the Idaflieg as they may have done some tests on ventilation configurations. I haven't seen any papers about this in Technical Soaring, but their test reports don't always get written up for TS.
 

plncraze

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The late Peter Masak in his "Performance Enhancement..." passed on the suggestion from others who had done it, saying you dump your exhaust behind the gear doors or tail skid since flow is already disturbed. In a power plane be careful of exhaust coming back up the tail cone into the cabin
 

BJC

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Similar to having to run a Mercedes’ heater to help cool the engine when stuck in Florida summertime traffic, to go fast in an airplane requires that the pilot’s cooling system be shut down.

Some are totally dedicated to speed; some prefer just a little less speed with some comfort.


BJC
 

BJC

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Too:

No first hand experience, but I did research the subject some years ago. What stuck with me: Seal everything to prevent all uncontrolled leakage of air into and out of the airplane. That includes such things as aileron and flap actuator systems as well as the previously mentioned tailcone. I know of examples of exhaust soot being deposited in the the aft fuselage due to air inleakage through the rudder actuator rod. Canopy slider tracks need to be sealed.

Intake the ventilation air at a stagnation point. Alternatively, use a properly sized and positioned submerged NACA inlet. Dump the air away from potential exhaust intake, use a streamlined entry path to a reverse scoop. Upper fuselage, aft of the cockpit seemed to work well for some designs. Do not install the inane “reverse NACA inlet.”

I know that you already knew all that, but it felt good to post it.


BJC
 

Victor Bravo

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I believe it was specific to the glider. The Ventus and Discus sailplanes used the same fuselage, and the word had come down (from smarter people than I) that the best way to exit the air was to drill a1.5 inch hole in the lower fuselage just behind the gear doors. Then you glued in a 6 inch piece of 1.5 inch PVC pipe, such that it created an internal duct that ejected the air at a 45 degree angle out of the bottom of the fuselage, and then sand the edge of the tube flush with the lower skin. So we did this on my Ventus, and hoped for the best in the upcoming Nationals. Unfortunately we never had time to do much of a side by side comparison, this glider burned up in a hangar fire a month or so later. I do recall it provided better ventilation in the cockpit.
 
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