What is flapping?
, as the aircraft speed increases, the average air velocity increases over the advancing rotor blade but decreases over the retreating blade. In order to keep the blade from creating unequal lift and causing a rolling moment, a teetering hinge allows the advancing blade to flap up, decreasing its angle of attack while the retreating blade flaps down and increases its angle of attack. At a certain point, if flapping gets too high, the angle of attack on the retreating blade can exceed the stall angle, and the blade will experience “retreating blade stall.” In the SR/C aircraft, the rotor is unloaded with a wing and propeller, so that the rotor does not need to produce much lift. The pitch on the retreating blade does not need to be very high to produce the required lift, so the blade does not stall.
