Impedance tailored fluidic pitch links for passive hub vibration control and improved rotor efficiency

Replacing rigid pitch links on rotorcraft with fluidic pitch links changes the blade torsional impedance. At low frequency, the blade must have high impedance to pass through the pilot's collective and cyclic commands to control the aircraft. At higher frequencies, however, the pitch link impedance can be tuned to change the blade pitching response to higher-harmonic loads. Active blade control to produce higher-harmonic pitch motions has been shown to reduce hub loads and increase rotor efficiency. This paper investigates whether fluidic pitch links can passively provide these benefits. An analytical model of a fluidic pitch link is derived and incorporated into a rotor aeroelastic simulation for a rotor similar to that of the UH-60. Eighty-one simulations with varied fluidic pitch link parameters demonstrate that their impedance can be tailored to reduce rotor power and all six hub forces and moments. While no impedance was found that simultaneously reduced all components, the results include cases with reductions in the lateral 4/rev hub force of up to 91% and 4/rev hub pitching moment of up to 67%, and main-rotor power of up to 5%.