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Gurney Flap on cowl?

E

edhunter

Well Known Member
So before this aviation bug grabbed me I built and ran road race cars. We put gurney flaps on the rear wing to increase the downforce created by the rear wing. The penalty was an increase in drag. Drag increase was proportional to the height of the flap. I’m curious if putting one of these on the cowl trailing edge would increase the pressure differential and increase exit airflow from the cowl. The goal is increased cooling. In researching gurney flaps I found this ….
A search of gurney flaps on this site revealed little. I’ve got an A model so the firewall “bump” seems impractical.
IMG_0338.jpeg
 
I found during my experimentation in the past that it won’t make a measurable difference in temperatures.
Perhaps because the exit flow is parallel to the free stream air, and the turbulated flow from the flap possibly prevents any improvement in outflow volume.
 
There is a NASA paper that documents how Gurney flaps work. In summary, they work by keeping the flow on the upper surface attached all the way to the trailing edge of the airfoil, instead of have the natural separation a few percentage of the chord forward of the trailing edge.
This means two things, Gurney flaps only work on an airfoil section, and they have to be at the trailing edge of the airfoil.

I put a small gurney flap on the lower edge of my Aileron to center the trim. The first time i had it at the trailing edge and it was way too powerful. I removed it and shortened it, but when I reinstalled it it was inset 1/8” from the trailing edge. I ended up extending it spanwise to the same length as before, but this time inset 1/8” and now it is perfect.

So I dont think a gurney flap will work much on the cowl exit. JMHO
 
Well. one has to do what one has to do, but I would call adding a flap on the trailing edge of of the cooling exit of an RV an act of desperation, or at least a patch. To me, it says that something else may be bungled petty bad to need it. At usually well over 100 mph climb speed there is plenty of ram air pressure for cooling. Creating exit suction to make up for other deficiencies is going to be draggy no matter how it is done compared to otherwise good baffling, sufficient intake size, etc. Further, cooling only increases by about the square root of pressure, so the increased cooling from the suction will be modest.

The Gurney flap should actually be called the Cumulo flap, since I invented it inadvertently in the '60s. Phil Hill and Dan Gurney tried to patent it, but as I knew, it was well trod ground already by NACA and others. NACA dangled different size, angles and shapes of plates from near wing trailing edges in wind tunnels and knew just about everything that could be known about plate form flaps.
 
Trailing edge fences were used on helicopter horizontal stabilizers in the early 60’s, well before Dan Guerney used them.
 
Cumulo said:
Well. one has to do what one has to do, but I would call adding a flap on the trailing edge of of the cooling exit of an RV an act of desperation, or at least a patch. To me, it says that something else may be bungled petty bad to need it. At usually well over 100 mph climb speed there is plenty of ram air pressure for cooling. Creating exit suction to make up for other deficiencies is going to be draggy no matter how it is done compared to otherwise good baffling, sufficient intake size, etc. Further, cooling only increases by about the square root of pressure, so the increased cooling from the suction will be modest.

The Gurney flap should actually be called the Cumulo flap, since I invented it inadvertently in the '60s. Phil Hill and Dan Gurney tried to patent it, but as I knew, it was well trod ground already by NACA and others. NACA dangled different size, angles and shapes of plates from near wing trailing edges in wind tunnels and knew just about everything that could be known about plate form flaps.
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Thank you for your contribution to humanity. These little steps result in big steps later on.

When I was racing (crew chief) in the early 90's, the CART crew would change the size of the Gurney flap to change the downforce. I found the NASA paper that used a water flow tank to determine the proper size for the least drag. I showed it to the racers, but they believed Gurney rather than the science. My driver/owner insisted a main wing and flap configuration on the rear wing was the best because landing airplanes did that. So I drove him to the takeoff end of the runway and he realized too that a main wing /flap configuration may not be the best. Turns out neither of us were correct, because I set up the car so that the rear wing sucked the air out from the underside. (I never told him)
 
To increase cooling you really have to increase the mass flow through the cowling. The problem is to understand which part is choking the air. Assuming the baffling is good and you are not leaking past the intake ramps often increasing the exit area will increase the flow. I have found of 2 x RV6 that a set of louvres dropped the temperatures significantly (CHTs 30 to 40F, oil by 10F). Some have found smoothing the flow from the cylinders to the outlet is beneficial, perhaps an RV8 style wrap on the lower lip of the fuselage? A models limit the flow more due to the iron work to support the nosewheel.
 
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