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.

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Fixed-pitch coaxial, contrarotating rotors are often used in multirotor aircraft in applications ranging from payload delivery to urban air mobility. This paper examines minimizing net power at a given thrust for the coaxial pair using both an analytical approach based on blade element momentum theory and experimental results conducted in a hover test stand, as well as experimental results in hover for an X8 configuration multirotor. Appropriate distribution of thrust (realized via differential rotor speeds) to the upper and lower rotors results in approximately 5% savings in total power. Under this condition the thrust distribution is approximately 60% by the upper rotor and 40% by the lower rotor (which occurs when the lower rotor shaft speed is roughly 10% higher than the upper rotor shaft speed). Similar ratios were obtained in both the analytical solution and in experiment, at various total thrust levels. Hover flight tests of the X8 multirotor also showed approximately 5% reduction in hover power at this RPM setting.