Stability derivatives of a flapping wing-body system*

Flapping flight is often aerodynamically unstable. While usually undesirable in human-designed aircraft, birds and bats exploit this instability to achieve agile flight. However, despite its importance to animal flight performance, the stability of flapping-wing systems has not received the same degree of attention as lift and thrust production. Here, we characterize the pitch stability of a flapping-wing robotic testbed in a wind tunnel and report on the cycle-averaged lift, drag, and pitching moment as a function of angle of attack, wind speed, and flapping frequency. Our data show a positive slope of cycle-averaged pitching moment versus angle of attack across a range of Strouhal numbers, confirming that the flapping-wing testbed is statically unstable in pitch. We also show that our experiments are consistent with results from unsteady vortex lattice method simulations. We plan to expand our investigation to yaw and roll stability as well as the effects of the upstroke-to-downstroke ratio on static stability.