Stability Analysis of Body-Freedom Flutter in Flying Wing Unmanned Aerial Vehicles

In this paper, the stability margins of a flying wing unmanned aerial vehicle (UAV) undergoing body-freedom flutter (BFF) are investigated. The stability margins are computed using a coupled aeroelastic model comprising finite-element-based structural dynamics with the unsteady vortex lattice method. Two primary modes of BFF are observed, a plunge-dominant mode and a pitch-dominant mode, which are closely related to the structural stiffness and inertial properties. In essence, increasing bending or torsional stiffness increases the tendency of plunge or pitch-dominant mode, respectively. In addition, flutter frequencies of the pitch-dominant mode are higher than the plunge-dominant mode, and the flutter speed is found to be highest near the point of transition between the flutter modes. It is suggested for structural properties of flying wing UAVs to be designed such that the flutter mode is near the transition point for increased stability margins. Additionally, plunge-dominant flutter mode is preferred due to its lower frequency, which will be easier to mitigate. The findings from this study provide insights into the design of flying wings on how stiffness/mass distribution affects the coupling between elastic and rigid-body modes leading to BFF.