A Static Aeroelastic Analysis of a Flexible Wing Mini Unmanned Aerial Vehicle

The static aeroelastic behavior of the Nighthawk mini unmanned aerial vehicle is examined using acombined experimental and computational approach. Three wings are examined. In order of increasingstiffness they are: a flexible wing, a stiff wing, and a fictitious rigid wing with zero deflection. Photogrammetryis used during wind tunnel testing to measure the average deflected shape of the flexible and stiff wingsduring flight. The independent variables during wind tunnel tests are angle of attack (ranging from -5.1othrough 13.4o) and velocity, which is 20 mph, 30 mph, and 40 mph. Roll angle and yaw angle are controlvariables, held constant at 0o. The measured deflection of each wing is used to adjust the wing shape forcomputational fluid dynamics analysis. Solutions are obtained for the flexible, stiff, and undeflected (or rigid)wings using a steady-state viscous flow solver with a Spalart-Allmaras turbulence model. The flexible andstiff wings experience two forms of deformation during flight. They bend upward along the span increasingthe dihedral, and the leading edge twists downward (wing washout). The amplitude of deflection is greatestfor the flexible wing. As a result, the flexible wing is more stable, but also exhibits worse static aerodynamicperformance. The rigid wing has the greatest lift (CL max=1.29) and the highest lift-to-drag ratio (L/Dmax =10.2).Stall occurs first near the root for all three wings. None of the wings stall at the tip in the range of angles ofattack tested. A separation bubble forms under the wing at angles of attack less than 8o. This separationdecreases the overall lift. It is most prominent on the flexible wing.

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