Application of Nonlinear Geometrical Effects to the Analysis of Structural Loads using Fast Evaluation Methods

摘要

New materials used in commercial aircraft design allow higher flexibility in wing design and hence larger deformations for the analysis of structural loads. Models developed for the purpose of loads analysis frequently take the effect of structural deformation into account as boundary condition for the aerodynamic model but leave the aerodynamic grid undeformed. This approach was discussed in previous papers, e.g. in the scope of the VARLOADS environment, and is justified by the assumption of small deformations. It allows a linearization of the splines, which couple external forces with with rigid body motion and the structural flexibility. It further allows a linear and constant integration matrix. Using these features it is possible to design a very efficient and fast implementation of the aircraft model, which is important for optimization and a wide coverage of the flight envelope. In this paper we include the effect of nonlinear geometrical deformation to the evaluation of the equations of motion. A solution is desired which does not destroy the local linearization of the system but which allows to relinearize for large deformations and hence keeps the high efficiency of the model. In the VARLOADS environment this can be realized in a particularly elegant way using the modular application of grids, splines and spline rules. Only on the structural side second order effects need to be taken into account explicitly. The resulting model will be applied to a pull-up manoeuvre for a generic commercial passenger jet aircraft. It turns out that the effect on the rigid body motion and its resulting change in the angle of attack acting on the aerodynamic model is significant. The influence of the geometrical nonlinearities on the integration matrix is large as well. However, the two effects balance well such that, even though the overall effect on the aircraft is significant, the structural loads are little affected over a large part of the airframe. A discussion will be given on the applications and limitations of this approach.