Stability Analysis of Tailsitters in Vertical Takeoff and Landing Flights

摘要

The stability of tailsitters is severely impacted by ground friction/reaction forces when the landing gear contacts the ground. This paper analyzes the stability of tailsitters during taking off from and landing on the ground. A distributed propulsion tailsitter with four electrically driven propellers is studied and modeled. The pitch loop of this tailsitter can be stabilized by both aerodynamic effectors immersed in the propeller slipstream and differential thrust of the propellers installed on upper and lower wing sides. The stability criteria and stability margin of tailsitters in the landing-gear/contact-ground flight stage are defined. The open-loop stability of tailsitters on high-friction and low-friction grounds is analyzed. The closed-loop stability of tailsitters with thrust-vectoring control and trailing-edge elevators control are discussed separately. On high-friction ground or uneven terrain, the tailsitter pivots about the ground contact point instead of the center of gravity. It is shown that the ground friction/reaction forces induced by the trailing-edge elevators generate a reversed pitching moment, which makes the pitch loop of the tailsitter unstable. A Proportional-Derivative controller with thrust vectoring as an effector is given and proved to guarantee the pitch loop stability. The influences of the thrust increase rate are also discussed. The experimental results of a tailsitter unmanned air vehicle verify the conclusions of this paper.