Longitudinal stability and control of highly flexible solar-powered UAV

The structure of the wing of High Altitude Long Endurance (HALE) Unmanned Aerial Vehicle (UAV) is highly flexible. The aeroelasticity and flight dynamics are highly coupled. This influences the flight safety seriously. In the stage of conceptual design, it is necessary to research the characteristics of flight dynamics and control thoroughly for highly flexible wing. In this paper, the Lagrangian method is employed to model the motion of rigid-wing UAV, the same configuration with static aeroelastic deformation and highly flexible-wing UAV. The longitudinal characteristics of flight dynamics and control are compared and it is found that, while considering the static deformation only, owing to the becoming large of the pitch moment of inertia, the stability derivatives of the pitch axis are reduced and the damp and frequency of short period mode are obviously reduced. The frequency of phugoid changes little but the damping of it is also obviously reduced. It can be found that the longitudinal characteristic roots changed as wing stiffness varies. Particularly, when the stiffness is very small, the short period mode and the first bend mode are coupled and the phugoid is unstable. The pitch rate and the pitch angle feedback gains are smaller than flexible model. Therefore, relatively great adjustment should be made to fulfill the requirement of aeroelastic UAVs. To better reduce the disturbance of angle of attack and normal acceleration, the elevon should be employed to suppress the deformation of wing.