Flutter prediction for unmanned long endurance aircraft using virtual structural model and experimental modes

Aeroelastic stability is an important consideration in the design and certification processes of modern, flexible aircraft. This includes demonstration of freedom from flutter at all combinations of airspeed and altitude within the flight envelope. This paper presents the analysis and clearance of flutter characteristics for an unmanned medium altitude long endurance aircraft by two distinct methods. The traditional method based on a theoretical Finite Element (FE) model was adopted prior to the conduct of Ground Vibration Tests (GVT) where as the more recent method utilized a Virtual Structural Model (VSM) built on the experimental modal parameters that were obtained from the GVT. The Direct Matrix Abstraction Program (DMAP) feature of MSC NASTRAN was used to create the VSM whose nodes correspond to the GVT accelerometer locations, and to insert the GVT modes into the model. In both approaches the unsteady airloads were generated using the Doublet Lattice Method (DLM). The GVT/VSM based analyses closely mirrored the original FE model analysis and confirmed the existence of sufficient flutter margins, thereby enabling the flight certification of the aircraft. The VSM route to flutter analysis is outlined in this paper as an alternative to using a GVT-tuned FE model. This is beneficial (i) if no FE model is available as it happens with bought out aircraft, or (ii) if the configurations of interest are too few to justify the effort of FE model tuning.