THEORETICAL DETERMINATION OF ROTOR BLADE HARMONIC AIRLOADS
MASSACHUSETTS INST OF TECH CAMBRIDGE AEROELASTIC AND STRUCTURES RESEARCH LAB
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A helicopter rotor in forward flight is subjected to a complex system of loads, both aerodynamic and inertial in origin. Of particular interest are the oscillatory airloads occurring at harmonics of the rotor speed. These loads are the primary source of the blade stresses which establish the fatigue life of the structure and of the oscillatory hub loads which determine the fuselage vibration level. Unlike a sing, the trailing- and shed-vortex system of the blade generates a spiral wake which returns close to the blade under most normal flight conditions. This returning wake critically influences the downwash distribution over the rotor disc and is the primary source of the higher harmonic airloading. The higher harmonic components of the airloading arise primarily from the downwash perpendicular to the plane of the rotor disc generated by this wake. Their analytical determination, therefore, requires some means of computing the downwash, which takes into account the spiral wake geometry, and of determining the unsteady aerodynamic effects associated with the blade passage through this variable velocity field. A better definition of the aerodynamics of a rotor in forward flight will also help in the design of hub and blades for minimum drag at the higher speeds envisaged for the next generation of helicopters.