Flight Dynamics Analytically-Derived Mathematical Model of Tiltrotor Aircraft

Abstract

State-of-the-art tilt-rotor flight mechanics models appear to be inadequate in predicting the flight dynamics behavior across the entire flight envelope due to a lack of experimental data suggesting the need for more generic models. This paper presents a middle ground between model flexibility (genericity) and accuracy. For this purpose a generic 7-degrees-of-freedom tilt-rotor flight mechanics model is derived analytically. A novel ordering scheme is employed ensuring a bounded expression simplification error which is not found in existing tilt-rotor models. The research highlights the effects of the tiltable nacelle on the rotor analytical expressions and static flight mechanics. The model is validated against the XV-15 implementation of the NASA GTRS model showing promising overlap. It is concluded that the tiltable nacelle mainly affects the rotor expressions through the velocity experienced by the rotor hub and blade element, and that the nacelle tilt rate and acceleration are considered significant contributors to many rotor expressions. It is also shown that the nacelle tilt rate has an observable effect on handling and performance. Terms unique to tiltable proprotors are shown to exist, most notably the Coriolis effect of the nacelle tilt on the flapping dynamics. Several assumptions common in tilt-rotor modeling are also shown to be grossly violated. The future recommendation is to incorporate more sophisticated wing, control surface, and fuselage models, while the rotor model should include disk tilt and inflow dynamics, and a non-uniform inflow distribution.