Pitch-flap stability of an articulated rotor with fluidic pitch links

In order to reduce vibration, researchers have been exploring alternatives to conventional rigid pitch links. One viable passive vibration reduction device is the fluidic pitch link. By replacing rigid pitch links on rotorcraft with fluidic pitch links, changes can be made to the blade torsional impedance. At high frequencies, the pitch link impedance can be tuned to change the blade pitching response to higher harmonic loads. Although all have not been demonstrated simultaneously, fluidic pitch links have been shown to be able to reduce rotor power and all six hub forces and moments. While it has been shown to reduce vibration, its impact on stability has yet to be explored. This paper will focus on exploring the aeroelastic stability of a helicopter with fluidic pitch links. Each rotor is articulated and is modeled by rigid pitch and rigid flap degrees of freedom. Quasi-steady aerodynamics are used for the lift and moment terms in the aeroelastic model. The control system stiffness is modeled as an axial spring. The fluidic pitch links have two degrees of freedom: axial displacement of the piston, which is directly related to pitch, and volume of fluid entering the inertia track. A positive impact on aeroelastic stability from several fluidic pitch link designs are shown for hover. The positive stability margins that are found for the fluidic pitch links in hover are marginally affected by the periodic terms that appear in forward flight. The fluidic pitch links are shown to help stabilize the pitch mode and enable substantially larger aft center of gravity offsets to be used in rotor design.