Flutter margin with non-linearities: real-time prediction of flutter onset speed

The flight envelope states the conditions where an aircraft is safe to fly and those conditions must be carefully determined for every configuration. That is why, even today, the numerical tools have become more and more precise, flight test is the only reliable method to define the flight envelope, especially in the conditions where the numerical model reaches a great degree of complexity (non-linearities). However, flight test is expensive and different procedures must arise in order to match reliability (safety) and cost (flight hours). This dilemma is even worse when free-flutter conditions must be provided for a new aircraft or a new external configuration, because the envelope expansion is done by increasing the flight test hours. The present article shows a procedure to predict the flutter speed based on real-time tuning of a quasi non-linear aeroelastic model. A two-dimensional non-linear (freeplay) aeroeslastic model is implemented in MatLab/Simulink with incompressible aerodynamic conditions. A comparison with real compressible conditions is provided. Once the numerical validation is accomplished, a parametric aeroelastic model is built in order to describe the proposed procedure and contribute to reduce the number of flight hours needed to expand the flutter envelope.