Trajectory optimization of an autonomous dynamic soaring UAV

This paper has a single goal of finding admissible state trajectories for an unmanned aerial vehicle while utilizing dynamic soaring technique in a linear wind gradient. Loiter pattern trajectory of the small UAV is optimized by employing three non-linear optimal control problems. The whole atmospheric altitude is divided into three regions(below 100m, 100m–115m and above 115m) and each are assigned with carefully chosen wind velocities. The resultant optimized trajectories reveal the possibility of applying dynamic soaring to UAVs which take off with a particular initial velocity from any of the three altitude regions. Performance measures adopted are, maximizing specific energy and specific energy rate extracted by the vehicle, and minimizing the control effort delivered. A 3-DOF point mass model system dynamics of UAV is studied, and control variables are identified as bank angle and lift co-efficient. The state trajectories, euler angle variations, aerodynamic performances are analyzed by changing controls. A suitable wind profile is chosen. The state equations, controls, terminal conditions, objective function, and initial trajectory(initial conditions) are then incorporated into a MATLAB dynamic optimization routine using numerical methods and optimized with the help of genetic algorithm in order to produce converged optimal dynamic soaring trajectories. The results of this study could have a direct impact on the application of UAVs, both military and civilian, as a thrust-off situation could help in saving of fuel and thereby prolong duration of flight, extension of range, and endurance.

I. Introduction

Unmanned Aerial Vehicles [16] have capabilities that are humanly impossible. Hence improving their performance is an important area of research, be it civilian or military. If by some means we could extract energy from atmosphere then the vehicle can save fuel and time and thereby improve the endurance. This could change the total performance of a UAV. A soaring UAV (SUAV) is a design concept that utilize the weather conditions to the advantage of the aircraft. Soaring can be defined as the sustained flight performed by birds, gliders, and aircrafts for longer periods, without applying the power, using the wind gradient to generate sufficient lift to compensate for the drag and maintain themselves in air. Like the very concept of flying, this technique is also seen, studied and mimicked by observing birds whose size and wing structure allow it to fly(travel and loiter) using the wind gradient, without flapping wings. Soaring is of different types depending upon whether an updraft, a thermal, a vertical gust or a horizontal shear is being utilized to sustain flight. Updrafts are produced when wind blows over hills and mountain ridges resulting in slope soaring and rising columns of warm air called thermals resulting in thermal soaring. Dynamic soaring is another kind of soaring technique. What makes dynamic soaring different from other static soaring techniques is that energy is absorbed from air masses with varying velocities at different horizontal layers of atmosphere and not from the rising air to maintain flight. But only certain kinds of sea birds like Albatross could perform this [1]. The gliders and UAVs tend to adopt it which explains the sudden rush of ongoing researches in this field. This work is meant to gather knowledge on the extraction of energy from wind gradient by means of which an unmanned aerial vehicle could overcome its drag without the help of thrust factor while it is airborne.