Real-time Simulation of Flexible Aircraft: a Comparison of two Methods

Abstract

As aircraft are becoming more flexible it is becoming important for the real-time flight models used in training and engineering simulators to capture the effects of the increased flexibility on the behaviour of the aircraft. This paper presents a preliminary comparison of two methods for simulation of flexible aircraft; the so called mean-axes method which uses a floating reference frame (not attached to the deforming aircraft) to inertially decouple the rigid body motions and the elastic deformations of the aircraft, and a hybrid quasicoordinate method that uses a local coordinate system fixed to a point on the aircraft. The quasi-coordinate method is derived from an extended version of Lagrange’s equation that allows the use of body frame quasi-velocities. Both models use shape functions to spatially discretize the aircraft and reduce the equations from partial differential equations to ordinary differential equations. The mean-axes method, however, requires the free vibration modes of the aircraft to be used as the shape functions, whereas the quasicoordinate method can use any set of shapes functions that meet the standard requirements. Each method makes its own set of assumptions in deriving the final set of equations. A model of a small business jet was developed using each methodology, and although the two models showed similar behaviour there were significant differences in their response. As the computer codes for each model are very complex and they are in their early stages of implementation, further work is required before these differences in response can be attributed to the differences in the modelling methodology.