9 - Aeroelastic and aeromechanical behaviour

Abstract

The effect on the motion of the blade of coupling between its degrees of freedom can be ignored for performance and stability and control aspects but blade and rotor system instabilities due to coupling must be considered. There are important instabilities involving coupling of rotor blade and airframe motion. Rotor blade instabilities include main rotor pitch-lag instability, main rotor pitch-flap flutter, main rotor stall flutter, main rotor blade weaving, tail rotor pitch-flap instability, main and tail rotor flap-lag instability, and tail rotor pitch-flap-lag instability. Pitch-lag instability is experienced as a limit cycle oscillation of the rotor blades. The type of coupling normally encountered in main rotor pitch-flap flutter type is due to adverse offsets of the chord-wise center of gravity of the blade section from the blade-feathering axis. Main rotor stall flutter is based on negative damping in pitch given to aerodynamic pitching moment hysteresis. Main rotor blade weaving involves the same basic coupling mechanism as the main rotor pitch-flap flutter problem. This type of instability depends on the existence of some form of coupling between blade pitching and flapping motions. In tail rotor pitch-flap instability, the flapping and lagging motions are coupled. There is coalescence of the flap, lag, and torsion mode frequencies in tail rotor pitch-flap-lag instability. This chapter closes with a discussion on coupled rotor blade and airframe instabilities, which include ground resonance and air resonance.