Altitude Control of a Tethered Multi-Rotor Autogyro in 2-D Using Pitch Actuation via Differential Rotor Braking

For tethered multi-rotor autogyros to be viable energy efficient unmanned aerial vehicles (UAVs), control analysis and stability investigation of autorotative flight are vital. In this paper, a simplified model-based altitude control technique is presented which is effective in the presence of both uniform and variable wind profile. A two-rotor autogyro, tethered to the ground and constrained to move in the 2D plane of the wind direction, is adopted for the study. The reduction to 2D simplifies the system and helps focus on the feasibility of altitude control and pitch modulation by exclusively using differential braking, which is a novel concept. In this arrangement, control inputs are the braking torques in each of the two rotors. The assumption is that with another two rotors in the lateral direction the roll and yaw motion of the system can be controlled when extended to 3D. The aerodynamics and tether modeling are based on Blade Element Momentum (BEM) method and catenary mechanics respectively. The characteristics of the equilibria of the tethered multi-rotor autogyro are investigated. For the aforementioned set-up, the differential rotor braking input is designed based on a proportional feedback law, and is effective in controlling the autogyro's altitude with the help of restoring effect provided by the tether tension.