Prop‐Rotor Stability at High Advance Ratios

An analytical method for determining the stability of wing‐mounted prop‐rotor pylon systems in fully converted airplane configuration has been developed and is described. Application of this method to an existing tilt‐rotor VTOL configuration, the Bell XV‐3, and to subsequently tested scale models, has indicated that the method simulates the instabilities arising from the coupling of prop‐rotor flapping motions to pylon motions. It has been found that the principal destabilizing factor in these pylon motions is the inplane force generated by blade flapping at high advance ratios. The stability of the prop‐rotor pylon system is decreased with decrease in pylon mounting stiffness and damping and with increase in speed and pitch‐flap coupling (delta‐three). Stability of the system may generally be increased by Proper controls coupling, such as lagging swashplate coupled to pylon displacements.