ORIGINAL: GRH
Hi Dick,
I'm not surprised to see you here ruffling feathers.
The funny thing is that Dick provides truth in small portions that are for some reason hard for many to swallow. I only wish I could imitate your brevity of thought so I'll try:
Thin flat plates (no camber) with a sharp disturbance at the leading edge are the best low Reynolds number sections for aerobatics because they possess a linear response in angle of attack over a larger Reynolds number range (speed). Thickness simply causes aerodynamic problems on small models...if you don't believe me build a hand launch glider with a thick section.
Dick would be proud...No theory here...just measured data.
banktoturn..personally I despise rounded leading edges on true low Re sections (60,000 and below) because you don't get full transition which is what makes these things really work. It's not like you're going to support much of a pressure distribution on a section like this and for the foamies that's not the point...make it linear and get your CLmax from a source that capable of providing it...the propeller.
As an aerodynamicist it's sacrilege to make such statements but unfortunately it's true. Dick's theory for years has been pretty much don't ask much out of your wing and you won't be disappointed. When I first met Dick I was against his philosophy because I thought that we could do better...in the end I realized that his airplanes flew good because they were linear, not because CLmax or L/D is high. Aerobatics doesn't benefit from these sort of things...that's when I shifted my focus to Stability and Control.
Airfoil design for aerobatics is what I call "piddling about a point"...fun to do but pretty much worthless. My design criteria is to make the airfoil have as linear a lift curve (especially around +/-2 degrees) as possible over as wide a Re range as possibly. Then you have to start talking to you Structures guy because he's bloke who demands that the airfoil be thick.
It's funny because when you start working with models and using with the square-cube law of scaling you quickly realize that the aerodynamics simply can't keep up.
Once again I've said too much...
George Hicks
Misguided Aerodynamicist and Theory Monger
BTW, Dick are you going to Joe Nall for the Team JR meeting?
George,
You seem to start out by lumping thinness and flatness together again. A section does not need to be flat (i.e. have constant thickness) in order to be thin. Nor does it have to be flat to have a 'disturbance' at or near the leading edge.
I'm not sure what you mean by 'full transition'. If you mean transition to turbulence, that can certainly be arranged, even with a rounded leading edge. Certainly, a thin wing can't generate the kind of CLmax that a thick one can, but if one is building very light planes, a high CLmax shouldn't be needed. If you rely on thrust rather than lift, though, I suppose it makes more sense to call those flat things fins than wings.
Your point about not asking too much of the wing is well taken. I made the same observation, and it certainly makes it more likely that a flat plate will be satisfactory. I'm not exactly sure what you mean by linear, but I doubt that the lift curve for a flat plate is much closer to linear than that of a thin conventional section.
The structures guy would sure prefer a thick section. However, once he concedes that it must be thin, he doesn't much care whether it is flat or has a more conventional section. The strength comes from the thickness, not the section shape.
banktoturn