MODELLING THE EFFECTS OF BLADE TORSIONAL FLEXIBILITY IN HELICOPTER STABILITY AND CONTROL

Various aspects of the coupled flap-torsion motion of an articulated rotor blade are discussed. The rotor blade is assumed to be rigid in the flap-wise direction with torsional flexibility concentrated at the root of the blade. This analytical model was developed to model the response of a servo-flap controlled rotor. The servo-flap rotor is torsionally soft and as a result is very sensitive to aerodynamic modelling assumptions. The influence of modelling assumptions on the flutter boundaries of a rotor blade in hover and the effects of torsional flexibility on flapping behavior of conventional rotors are discussed. It is shown that prediction of the flap-torsion dynamics of torsionally soft rotors is very sensitive to a number of small parameters. The formulation of the aerodynamic forces and moments presented here indicates that torsionally soft rotors have very low levels of damping even for the case of an aerodynamically and inertially balanced rotor blade.