On the controllability of fixed-wing perching

The ability of birds to perch robustly and effectively is a powerful demonstration of the capabilities of nature´s control systems. Their apparent robustness to gust disturbances is particularly remarkable because when the airspeed approaches zero just before acquiring a perch, the influence of aerodynamic forces, and therefore potentially the control authority, is severely compromised. In this paper we present a simplified closed-form model for a fixed-wing aircraft which closely agrees with experimental indoor perching data. We then carefully examine the LTV controllability along an optimized perching trajectory for three different actuation scenarios - a glider (no powerplant), a fixed propeller, and a propeller with thrust vectoring. The results reveal that while all three vehicles are LTV controllable along the trajectory, the additional actuators allow the perch to be more easily acquired with less control surface deflections. However, in all three cases, disturbances experienced just before reaching the perch cannot be effectively rejected.