The effect of pre-stressing structural members of an aircraft wing

Abstract

Unstable oil prices, increased competition and changing attitudes towards environmental issues are driving industry to seek ever more efficient aircraft designs. As engine manufacturers reduce fuel consumption, the airframe manufacturers seek to develop designs with lower drag and lighter, stronger structures. For the majority of an aircraft's mission, the wing loads are predominately acting in one direction, thus the idea of pre stressing the wing, in a manner similar to how structural beams in buildings are pre stressed, is developed. Standard and pre-stressed wings are modelled, subjected to realistic loading conditions and compared to determine if any benefits can be obtained. A survey of literature highlights the drive to develop more efficient aircraft. The industry standard method for predicting the aerodynamic loading on a wing is confirmed to be Computational Fluid Dynamics (CPD). The different available CPD models are reviewed and the most appropriate one selected. A CPD model is developed and subjected to a grid independence study. A finite element model is then developed to calculate the resulting structural stresses. Methods for assessing convergence are once again reviewed in literature and applied to the finite element models. Using the wing of the Diamond DA40 aircraft as the basis on which to develop a virtual wing model, typical aerodynamic loading conditions are simulated and the resulting stress state predicted. Pre-stressing is then applied to the wing spars and the resulting stress fields are compared to the original design with no pre-stress applied. It is shown how, through the superimposition of the aerodynamic loads and the pre-stress component, applying a pre-stress to the root of the spars can lead to a 26% reduction in maximum von Mises stress. It is shown how the pre-stressing can reduce the amount of tensile stress to an even greater degree if an increase in overall maximum stress is accepted. The reduction in stress can be used to make lighter structures, whereas the reduction in tensile stress may have benefits relating to fatigue issues and crack propagation. Even though a detailed assessment of aeroelastic phenomena is beyond the scope of this project, the key issues of Divergence, Flight Control Reversal and Flutter are reviewed, highlighting areas where the pre-stress may have an effect. It is also shown how the particular wing being modelled here is not critical with regards to divergence and how pre-stress can be applied without any detrimental effects in this regard.