The Effects of Nonlinear Bending Stiffness on the Flutter Speed and Pitch Motion of UAV with High Aspect Ratio

摘要

A nonlinear finite element incorporated with the Von Karman strains is used to simulate the pitching, bending and vertical motions of an UAV subjected to air flow circulation around the high aspect ratio wing. The Hopf bifurcation is used to determine the Eigen value at flutter speed for each of vibration amplitude. The nonlinear bending stiffness will decrease the flutter speed, and shift the energy from the vertical and bending modes to the pitching mode at flutter speed. The increased pitching amplitude will shift the resultant of lift force outboard of wing and instantly reincrease the wing tip deflection, which accounts for the persistent high dihedral during the continuous turbulent flow period. The increase of the vibration amplitude per unit height of dihedral becomes more pronounced for higher vibration amplitude. As pitch mode become more dominated for higher nonlinear bending stiffness, a growing dihedral will result in a divergent pitching motion.