Development of an Aeroelastic Stability Boundary for a Rotor in Autorotation

摘要

The results obtained with the aid of the model demonstrate the interesting, and unique, characteristics of the autorotative regime - with instabilities possible in bending and torsion, but also in rotorspeed. Coupled rotor speed/flap/twist oscillations (flutter and divergence) occur if the torsional stiffness of the blade is lower than a critical value, or if the blade centre of mass is significantly aft of the blade twisting axis, as is the case in helicopter pitch-flap flutter. The instability shown here, however, is specific to the autogyro, or autorotating rotor, as it is coupled with rotorspeed, and so differs from both helicopter rotor flutter and fixed-wing flutter. The coupling with rotorspeed allows a combined flutter and divergence instability, where the rotor begins to flutter in rotorspeed, teeter angle and torsional twist and, once the rotorspeed had dropped below a critical value, then moves into divergence in flap and rotorspeed. It was found that the aeroelastic behaviour of a rotor in au-torotation is significantly affected by the strong coupling of blade bending stiffness and teeter angle with rotorspeed, and the strong coupling between blade aeroelastic twist and rotor torque.