Alternate Strategies for Optimal UAV Thermaling

Abstract

This paper investigates optimal glider trajectories for gaining altitude from thermal updraft columns (“thermaling”). Thermal updraft columns provide a natural energy resource that can be beneficial for enabling long- term autonomous soaring objectives. In this paper, computational optimal control is used to investigate possible alternatives to the common strategy of circularly orbiting the thermal to harvest potential energy. The Legendre– Gauss–Lobatto pseudospectral method is applied to a simulated glider in multiple scenarios over a range of thermal sizes, thermal intensities, and glider start conditions. Results for the Gedeon thermal model, which simulates dropoff in wind intensity with distance from the thermal core and additionally incorporates external downdrafts, are considered. These are then compared in performance to centered circular orbiting as well as two other alternate heuristic strategies suggested by the optimal control results: off-centered circular orbiting and “figure eights”. Results suggest benefits to using alternatives to centered circular orbiting when encountering small-diameter thermal updrafts. Results also support the utility of centered circular orbiting for medium- to large-diameter thermals while providing new insights into the optimal parameters of the circular orbit.