Transient loads control of a variable speed rotor during lagwise resonance crossing

Varying rotor speed during operation is a potential way to improve rotorcraft performance. The transient aeroelastic response of a stiff in-plane rotor system undergoing variable speed operation in forward flight is considered. During crossing of the fundamental lag mode near 2/rev, high transient lag bending moments are observed. The flapping amplitude and duration of the resonance crossing event have a strong influence on the peak lagwise root bending moment. Embedded chordwise fluidlastic dampers are explored as a way to reduce the peak bending moments. Determination of the fluidlastic damper properties is based on the analysis of a two degree-offreedom blade-damper system. Parametric studies show that tuning port area ratios, loss factors, and device mass can be modified to enhance damper performance, and control the stroke. Results indicate that more than 6% critical lag damping can be provided around the resonance rotor speed, and that approximately 65% peak-to-peak moment reduction can be achieved with those devices. The stroke of the damper is limited to less than 2.5% blade chord length in the worst-case scenarios. Embedded chordwise fluidlastic dampers can successfully control the lagwise transient loads during the lagwise resonance crossing for the variable speed stiff in-plane rotors studied.