Structural modelling for the flutter analysis of the S200 aircraft

Abstract

Aeroelasticity studies the interactions between inertial, elastic and aerodynamic forces acting on a structure. One of the most problematic aeroelastic instability for an aircraft is the flutter, a dynamic phenomenon encountered at relatively high airspeeds. Above the flutter speed, the lifting surfaces experience self-excited oscillations, generally leading to structural failure. Before being declared airworthy, any aircraft must go through a flutter analysis to certify that its flight envelope is clear of flutter, i.e. that the aircraft flutter speed is greater than its maximum design speed.

This master thesis focused on developing a simplified structural model of the S200 wing, a two-seater aircraft developed by the Belgian company Sonaca Aircraft. By combining the structural model with an aerodynamic model developed in Loic Camberlin's master thesis, a flutter analysis will be performed using a new aeroelastic modelling approach developed by the Aeroelasticity Research Group of ULiège. This approach consists in a modal frequency domain implementation of the Unsteady Vortex Lattice Method. As the S200 has already been certified for flutter, the objective was to apply this method on a practical case.

A modal analysis was conducted on the simplified structural model considering an empty wing with controls fixed. The results were then compared to an actual Ground Vibration Test performed during the S200 flutter analysis. It was concluded that the structural model behaviour was representative of reality, despite the important simplifications included in the developed model. A flutter analysis was then performed on the developed aeroelastic model. The results were firstly compared to a widely used method for such computations called the p-k method. They matched perfectly and the magnitude of the flutter speed seemed relevant considering a wing with controls fixed. However, the computed flutter speed exceeded the validity range of the incompressible flow assumption made to build the aerodynamic model. The results thus could not be fully trusted.

That being said, the results of the p-k method were quite consistent with the actual S200 flutter analysis. This suggested that the new aeroelastic modelling approach could provide relevant results for the studied case. Yet, in order to make it appropriate in practice, the model complexity should be increased by accounting for moveable control surfaces cases for instance.