Experimental Model of Coaxial Rotors Performance Degradation Induced by Wake Effect

Coaxial rotors are widely used for large-sized multi-rotors to achieve the efficiency of spatial layout. However, as the wake effect of the upper rotor affects to the lower rotor, performance degradation of the lower rotor is common. In this paper, a model of performance degradation for the coaxial rotor configuration is proposed based on experiment results on a given combination of lower and upper rotor pulse width modulation (PWM) signals. For the performance reduction model, loss of actuator effectiveness with respect to the given PWM signal of the lower motor is estimated comparing with the single rotor configuration. For the coaxial rotor experimental data, the PWM percent of the upper rotor is fixed with a constant value in each scenario, while the lower rotor angular velocity is changed with a specific interval in the coaxial rotor configuration. Also, the PWM percent of the upper rotor is changed with another specific interval for each of the scenarios. In each scenario, thrust, torque and angular velocity of the rotor are measured. For comparison, these same measurements were taken for the single rotor configuration with the same PWM percent interval of the upper rotor in the coaxially configured rotor. By comparing the coaxial rotor and single rotor configuration, thrust and torque efficiency degradation with respect to the PWM signal combination of the upper and lower rotors is derived. To evaluate the performance reduction of thrust and torque, the loss of actuator effectiveness with the function of the upper and lower rotor PWM signal is derived with the ratio between the single rotor and coaxial rotor configuration in the same PWM percent of the upper rotor. This can be applied to precise multi-rotor modeling for simulation or controller design.