Forward Flight

The prediction of rotor characteristics in forward flight presents many problems. The unbalance in the velocity at corresponding locations on the advancing and retreating blades, as discussed earlier, gives rise to aero­dynamic flapping which effects a change in the angle of attack of a blade section as it transverses the azimuth. The cyclic pitch control also varies the blade pitch angle around the azimuth. Compressibility and stall effects vary appreciably around the azimuth. The bound circulation of the blade changes periodically, leaving a varying trailing vortex sheet in its wake. The heli­copter rotor blade offers a crowning example of unsteady aerodynamic phenomena.

In spite of these difficulties, the helicopter aerodynamicist is able to pre­dict with reasonable satisfaction the power required by a helicopter in forward flight. There is a limited range of low speeds in which the calcula­tions are doubtful because of the induced power, and at high forward speeds the calculations are questionable because of retreating blade stall and com­pressibility effects. A method, presented later, has been found to give results of compressibility and retreating blade stall that are in agreement with test data.

Before investigating in detail the aerodynamic forces on a rotor blade in forward flight we can gain considerable insight into the power required in a study of its separate sources.