On the feasibility of the Rayleigh cycle for dynamic soaring trajectories

Abstract

Dynamic soaring is a flight technique used by albatrosses and other birds to cover large distances without the expenditure of energy, which is extracted from the available wind conditions, as brightly perceived five centuries ago by Leonardo da Vinci. Closed dynamic soaring trajectories use spatial variations of wind speed to travel, in principle, indefinitely over a prescribed area. The application of the concept of closed dynamic soaring trajectories to aerial vehicles, such as UAVs, may provide a solution to improve the endurance in certain missions. The main limitation of dynamic soaring is its dependence on the wind characteristics. More than one century ago, Lord Rayleigh proposed a very simple model, based on the repeated crossing of a step wind profile, presently known as Rayleigh cycle, that provides a clear explanation of the physical phenomenon. The present paper studies the feasibility of closed, single-loop, energy-neutral trajectories for a broad set of wind and vehicle conditions. Through the use of trajectory optimization methods, it was possible to see how the shape of the wind profile, the initial flight conditions and the vehicle constraints influence the required wind strength to perform dynamic soaring trajectories and consequently their feasibility. It was possible to conclude that there are optimal values for the initial airspeed and initial height of the vehicle, that minimize the required wind strength. In addition, it was seen how the structural and aerodynamic constraints of the vehicle affect dynamic soaring at high and low airspeeds respectively. Finally, some new trajectories that can be performed in conditions of excess wind are proposed. The purpose is to maximize the time spent aloft and the path length while maintaining the concept of single-loop, energy-neutral trajectories, making them especially useful for aerial vehicles surveillance applications.

Opportunistic soaring by birds suggests new opportunities for atmospheric energy harvesting by flying robots

“…Hence, because the wind is expected to be slower closer to a surface, the general principle of flying windward in a wind of increasing speed and leeward in a wind of decreasing speed can be restated more specifically for gradient soaring as climbing windward and descending leeward [ 100 ]. An alternating windward rise and leeward fall is therefore diagnostic of gradient soaring [ 18 , 108 , 110 , 113 , 114 ], usually involving an undulating flight path whose sinuosity is expected to vary according to how the overall travel direction relates to the overall wind direction [ 106 , 107 , 109 ]. This phasing of the horizontal and vertical components of flight can be used to identify dynamic soaring even in cases where the details of the wind field may be unknown [ 19 ].…”

“…In the light of these constraints, morphing-wing designs analogous to those of birds have been suggested as a solution for optimized energy extraction [ 15 , 138 ]. Open circuits are ideal for long-distance travel, but the literature describing these is currently confined to simulation studies [ 15 , 114 , 160 , 161 ]. Real-world implementation of open circuits is currently constrained by inadequate sensor measurements, limited computation, and the challenge of estimating a dynamic wind field, but would enable atmospherically aware sUAS to progress along a line of buildings, hedge line, or waves in a maritime setting, while extracting energy to stay aloft.…”