Investigation of Structural Coupling Problems in Tiltrotor Aircraft

The paper investigates structural coupling problems in tiltrotor aircraft. A detailed tiltrotor model, representative of the Bell XV-15, has been built. The airframe model has been modified with a thinner wing to better reveal structural coupling proneness. A linearized FCS has been introduced to analyze the overall stability on an extended frequency band, ranging from the flight mechanics up to the aeroelastic modes. In addition to the FCS, biomechanical models of the pilot, acting on the power-lever and on the center stick, are included in feedback loop. Overall stability analyses demonstrate that the FCS improves handling qualities although several structural coupling mechanisms arise, in combination with the involuntary pilot's response, reducing flutter clearance. A modified version of the XV-15, using differential collective pitch for yaw control in airplane mode, has been also investigated. This configuration reduces costs and weights although the FCS destabilizes the antisymmetric wing chord mode at low speed flight, severely limiting the flight envelope. Means of prevention, based on notch filters, are implemented and discussed.