Fluid-Mechanics Mechanisms in the Stall Process of Airfoils for Helicopters

Abstract

The stalling of helicopter rotor blades occurs on the retreating side of the rotor disk. The lower velocity on the retreating side (typically 0.3 to 0.5 Mach number) is compensated for by higher lift coefficients, which require higher angles of attack. When stall occurs, the blade dynamic and elastic properties become important in determining the subsequent changes in angle of attack. The aerodynamics of dynamic stall govern the aeroelastic response and can lead to a dynamic response known as stall flutter. In describing the fluid mechanics involved in dynamic stall, the blade motions must be taken into account. The three basic structural modes that are excited by dynamic stall are the blade torsional mode, the bending (normal to the chord) mode, and the flapping mode. The excitation of these three modes is illustrated in Fig. 1 from Crimi’s aeroelastic analysis of a two-dimensional section of a helicopter blade in [5.83] and [5.84]. The three basic structural modes of motion, at three different frequencies, contribute to the angle of attack. The mixture of modes causes the sequence of stall and unstall occurrences to be at irregular time intervals. The structural response most important to the angle of attack is the pitching-angle displacement. The sharp spikes in the aerodynamic pitching moment act as a series of impulses to cause pitching oscillations.