Optimal rotation speed combination of contra-rotating rotors for multicopters

Contra-rotating rotors are widely adopted for multicopters due to the advantage of compactness and redundancy. Unlike traditional contra-rotating propellers or rotors with fixed rotation speeds, contra-rotating rotors of multicopters are driven by individual electrical motors, allowing for independent rotation speeds. This enables the optimization of aerodynamic performance and noise emission for a given take-off weight by adjusting the rotation speed combination of the two rotors. The Lattice-Boltzmann solver PowerFLOW was used to simulate the contra-rotating system, which was then validated by experimental results. The results show that the optimal speed combination for maximum power loading is achieved by setting the upper rotor to 4200 rpm and the lower rotor to 6450 rpm. For noise emission, the optimal combination is 5700 rpm and 5820 rpm for the upper and lower rotor, respectively. This study reveals that the optimal aerodynamic performance is achieved through a reduction in thrust loss at the middle radial range of the lower rotor. The noise emission reduction is due to the decrease in both broadband and tonal noise.