Experimental Evaluation and Flight Simulation of Coaxial-Rotor Vehicles in Icing Clouds

Abstract

Coaxial rotor configurations are common in multirotor unmanned aircraft systems (UAV) and are used in some manned helicopters. In the case of small-to-medium-sized unmanned rotorcraft, there is a lack of experimental data on icing and a lack of analysis of vehicle performance and flight characteristics in icing conditions. This paper (a) describes experimental methodology and results of a coaxial rotor system with 0.46-m-diameter rotors operating in hover in an icing cloud, (b) develops an empirical model relating the change in torque and thrust as a function of rotor speed and ice accretion rate based on experimental data, and (c) applies this model to a simulation of UAV in icing conditions. Experimental results showed linear growth in required torque and linear reduction in thrust at a given rotor speed as ice accretes, with the rate of change of torque and thrust dependent on shaft speed and temperature. A significant difference in ice accretion rate was observed between the upper and lower rotors, with the lower rotor showing a lower rate of ice accretion. Simulation of UAV flight in icing conditions showed rapid loss of control and a loss of ability to maintain hover, with loss of sustained flight occurring within 40 s of the onset of icing. Simulation of flight with an ice-shedding event (which results in a step change in thrust and torque) results in a perturbation in pitch and roll, leading to significant lateral acceleration in addition to loss of altitude.