Banktoturn (You model guys never use the rudder!):
Here's a reference right on the thread. Been too long for me to give a quick text reference, sorry. Maybe Jim has one?
JimCasey
h. Sharp trailing edges allow less force to be transmitted to the control surfce by the alternating vortex sheets as they depart the control surface. When ONE sharp trailing edge is impractical, two sharp trailing edges can be used (Squared trailing edge). Rounded trailing edges are worst by this theory, both in terms of flutter and control response.
And in spite of Tall Paul's implications, it works at Mach 0.035 too. But you'd probably have to get up to 0.1 Mach (60 mph) before you started noticing much difference in air drag.
It is, of course, speed dependent. At lower air speeds the boundary layer will follow a steeper taper, as the air speed increases we reach the point where boundary layer separation is almost guaranteed. This was the case with the X-15.
A later Mach 3.5+ example that did not use the blunt trailing edges was the A-11/YF-12/SR-71 series airplanes. This series was originally designed as high speed high altitude fighter/interceptors, capable of acceptable maneuverability at sub-Mach speeds as well as the high speed cruise. And they had to take off from the ground under their own power. Even so, while the public name was "Blackbird" (on the '71 planes) the private name was "Sled." It was a hell of a ride. How about 30,000 ft/min climb at 1400 mph?
But back to the flat/blunt aileron and flutter resistance. By having the parallel upper and lower surfaces the aileron reverses the taper of the airfoil, tending to compress the air flowing from the wing, and in doing so the boundary layer is more firmly attached to the surface. Then at the blunt trailing edge there is a sudden separation, but it is away from the control surface, and with any speed on, the turbulence generated wont affect the control surface at all. When the surface is deflected by control command input the air stream force itself resists the deflection, and that force prevents the onset of flutter.
Put the churn on the surface with flutter.
. Fly a whille, then you'll have butter!
Bill.