Effect of Thrust on Bending-Torsion Flutter of Wings

The effect of thrust on the flutter of a high-aspect-ratio wing is investigated. The wing is represented by a beam using a nonlinear mixed finite element method. Aerodynamic forces are calculated using a finite-state, two-dimensional unsteady aerodynamic model. The effect of thrust is modeled as a follower force of prescribed magnitude. Without the thrust force, the wing is shown to become unstable for freestream airspeeds greater than the flutter speed. On the other hand, in the absence of aerodynamic forces, the wing becomes unstable for values of the thrust in excess of a critical magnitude of the force. When both effects are present, the airspeed at which the instability occurs depends on the thrust magnitude. For validation, an analytical solution for the en vacuo case (accounting only for the effect of thrust) was developed and shown to closely match results from the numerical method. Parametric studies show that the predicted stability boundaries are very sensitive to the ratio of bending stiffness to torsional stiffness. Indeed, the effect of thrust can be either stabilizing or destabilizing, depending on the value of this parameter. An assessment whether or not the magnitude of thrust needed to influence the flutter speed is practical is made for one configuration.