Aeroelastic stability of trailing-edge flap helicopter rotors

This paper investigates the aeroelastic stability of a helicopter rotor blade with trailing edge flaps. The coupled blade/flap/actuator equations were linearized by using small perturbation motions about a steady trimmed solution. Stability is then determined from an eigenanalysis of these homogeneous equations using the Floquet method. The baseline correlation of stability calculations without trailing-edge flaps is carried out with wind tunnel test data for a typical five-bladed bearingless rotor system. Good agreement is seen for both hover and forward flight conditions. Stability calculations for a rotor with trailing-edge flaps are conducted to examine the effect of flap aerodynamic balance and flap mass balance. The effects of various key design variables such as flap overhang length, flap CG offset, rotor control system stiffness, blade torsional stiffness, actuator stiffness, and trailing-edge flap size and location on the aeroelastic stability characteristics of a trailing-edge flap rotor system are also examined in different flight conditions. Excessive flap aerodynamic over-balancing and mass unbalancing are shown to cause instabilities of the trailing-edge flap blade system.