Dynamics and Stability Analysis of a Tethered Unmanned Rotorcraft

Naval aviators use tethers to assist rotorcraft during ship board landing because of the ability to increase robustness against wind turbulence and to center the rotorcraft on the landing spot. Autonomous tethered landing with unmanned rotorcraft and automatic winch device for applying the tether force requires the knowledge of how the tether force impacts the wind affected dynamics of the rotorcraft. To this day, the influences of applied tether force and wind velocity on robustness and stability of the rotorcraft have been little explored. In this paper, we present an analysis of the dynamics and stability of a nonlinear model of a small-scale tethered rotorcraft. We develop a simplified model of a rotorcraft's longitudinal dynamics and vary model parameters including tether force, trim conditions, and the horizontal wind in order to study the interdependence of those parameters and their impact on the model's equilibrium points and stability. This is a preliminary step towards design of an automatic control of an unmanned rotorcraft capable of autonomous tethered landing and development of tether force control laws for the winch device.