Dynamic Soaring Kinetic Energy Reference Frames

Abstract

Dynamic soaring in the boundary layer of a vertical variation of horizontal wind offers unmanned aerial vehicle gliders the ability to greatly extend mission endurance. By flying periodic trajectories within these gradients of horizontal winds, a dynamic soaring bird or glider uses the locally accelerating wind to its advantage, extracting sufficient energy from the environment during each trajectory cycle to overcome losses due to induced and parasitic drag. The dynamic soaring literature currently offers two competing definitions of aircraft kinetic energy, pointing to opposing views regarding the source of dynamic soaring energy exchange between the wind and the aircraft. A series of heuristics is developed to logically establish the wind-fixed/wind-aligned reference frame (that is, airspeed) as the appropriate choice for scalar kinetic energy computation while not imposing any such limitation on the usage of arbitrary reference frames for dynamic soaring equations of motion. Furthermore, while acknowledging that other kinetic energy reference frames are valid from the standpoint of physics, only wind-fixed frame kinetic energy tracks useful energy transfer from the vertical gradient of horizontal winds, with no attribution of kinetic energy from downwind turns.