Optimal Energy Utilization for a Solar-Powered Aircraft using Sliding Mode-Based Attitude Control

In this paper an energy optimal dynamic attitude for a solar-powered aircraft is determined and implemented using the finite-time sliding mode approach. A nonlinear constrained optimization technique has been considered to determine the optimal attitude of the aircraft for given a geographical location, solar position, heading direction and other flight conditions for clear sky conditions. The optimization is performed so as to travel in the fastest possible way from one location to another location without utilizing the battery power, provided the solar energy is sufficient for the flight. The gain in the velocity due to the attitude optimization is validated with the test flight results of solar UAV Maraal which demonstrate the efficacy of the approach proposed here. CFD simulation is carried out to estimate aerodynamic forces acting on the aircraft at different sideslip angles. The developed aircraft dynamics incorporates the nonlinearities associated with relatively large aileron and rudder deflections. To obtain the desired attitude, a sliding mode based control scheme is considered. A power rate reaching law is applied to avoid chattering in the controls. Finite-time stability of the closed loop system is discussed and it is shown that the attitude angles reach the desired values in finite time.