The Helicopter Rotor in Forward Flight

Efficient hover capability is the fundamental characteristic of the helicopter, but without good forward flight performance the ability to hover has little value. During translational flight of the helicopter, the rotor disk is moving edgewise through the air, remaining nearly horizontal, generally with a small forward tilt to provide the propulsive force for the aircraft. A tiltrotor cruises with the rotors tilted to operate as propellers. A compound helicopter reduces the lift and propulsive force required of the rotor. Yet all rotorcraft configurations execute low-speed forward flight with the flapping rotor in edgewise flow, which is the subject of this chapter.

Thus in forward flight the rotor blade sees both a component of the helicopter forward velocity and the velocity due to its own rotation. On the advancing side of the disk the velocity of the blade is increased by the forward speed, whereas on the retreating side the velocity is decreased. For a constant angle-of-attack of the blade, the varying dynamic pressure of the rotor aerodynamic environment in forward flight would tend to produce more lift on the advancing side than on the retreating side; that is, a rolling moment on the rotor. If nothing were done to counter this moment, the helicopter would respond by rolling toward the retreating side of the rotor until equilibrium was achieved, with the rotor moment balanced by the gravitational force acting at the helicopter center-of-gravity. The rotor moment could possibly be so large that an equilibrium roll angle would not be achieved.