Rotor blade harmonic air loading

Abstract

The determination of the air loads acting on rotor blades in forward flight presents an interesting and challenging problem in applied aerodynamics. Of particular importance for design purposes are the oscillatory components of this loading occurring at harmonics of the rotor speed. Unlike a wing, the trailing and shed vortex system of the blade generates a spiral wake that returns close to the blade. Because of its close proximity to the blade, the wake cannot be considered as rigid. Also, since the resulting loads are highly time-dependent, unsteady aerodynamic effects become important. An analytical treatment for the limiting case of hovering flight results in a simple closed-form solution that demonstrates that the oscillatory lift under normal operating conditions can be less than 50% of its quasi-static value. It is shown that the air loads in forward flight depend primarily on the equivalent lift deficiency and the down wash generated by the steady-state blade vortex system. It is also shown that, at low speeds, the rotor wake can be drawn up into the rotor leading edge and that this is probably the primary cause of transition roughness and noise encountered under certain flight conditions.