Aeroelastic evaluation of a flexible high aspect ratio wing UAV: Numerical simulation and experimental flight validation

Abstract

Aeroelastic phenomena occur when aerodynamic and structural loads interact in an unstable manner. This is a major concern in the design of modern high aspect ratio wing (HARW) aircraft, as these structures are more prone to deflection during flight and thus affecting the flight performance. Studying these effects in full scale aircraft is prohibitively costly and risky. Scaled flight test demonstrators with flexible wings offer a low cost and risk solution to evaluate the aeroelastic response of HARW aircraft. In this work, the sizing, design and testing of an in-house developed flexible-wing unmanned air vehicle (UAV) is presented. First, the rigid-wing aircraft platform and its performance characteristics are presented, followed by the wing modifications to make it flexible. The wing flexibility was designed such that coupling of flight-dynamic modes and elastic modes is observed in-flight. A full numerical characterization of the aircraft was first performed, including structural, aerodynamic and aeroelastic computational models. Next, ground vibration and static loading tests were performed to validate the structural models. Flight tests were conducted to obtain data on aircraft dynamics, and strain measurements along the wing spar. An indirect shape reconstruction method based on strains was employed to extract wing displacements from measured strain data. Flight test data was then compared against simulated aeroelastic results. It was observed that for the HARW flexible wing aircraft, a degree of coalescence between the aircraft short period mode and first out-of-plane bending mode of the wing was present. By using different excitation functions applied to the control surfaces, this coalescence was recorded during flight tests and good agreement with the simulated results was observed. The results obtained are an important step in the study of fluid-control-structure interaction issues in HARW aircraft.