Effect Of Blade Design Parameters On Helicopter Stall Boundaries

An experimental and analytical study has determined the effects of blade structural design parameters on stall-related operating boundaries of helicopter main rotors. Emphasis was placed on the effects of blade properties on the buildup of vibratory control loads in stall. Two-dimensional oscillating airfoil tests and a series of blade aeroelastic response calculations were completed for a typical low twist rotor blade with symmetric airfoil section. It was found that vibratory torsional moments were reduced by reducing torsional stiffness or torsional inertia and moving the blade aerodynamic center axis aft of the coincident elastic and center-of-gravity axis. In addition, increasing blade twist was found to reduce the control load stall speed at high CT/σ, but increase the rate of buildup of control loads with airspeed. Analytic results were generalized to a set of design recommendations aimed at reducing vibratory control loads and expanding flight envelopes of typical rotors.