Efficient aerodynamic prediction method of contra-rotating propellers in axial flight

An efficient aerodynamic prediction method applicable for contra-rotating propellers in axial flight was developed based on Blade Element Momentum Theory. Firstly, the accurate decomposition of aerodynamic forces and Prandtl tip loss were employed to eliminate errors introduced by the small angle assumptions at high inflow angles. Secondly, based on the helical tip vortex evolution of the contra-rotating system, a wake superposition model was developed, and the interaction pattern was built, which took into account the axial aerodynamic interactions and circumferential aerodynamic interference that the front propeller exerted on the rear propeller. Finally, the aerodynamic model of contra-rotating propellers in axial flight was established based on inflow angle solutions. The method was applied to predict the aerodynamic performance variation with advance ratio for a contra-rotating propeller under different flight speeds. The calculated thrust and power in non-stall conditions were consistent with the measurements, and the whole propulsive efficiency agreed well with measurements. Aerodynamic prediction comparisons of the single propeller, the coaxial rotor and the contra-rotating propeller revealed that the established model can provide more reasonable performance predictions including thrusts, powers and efficiencies for contra-rotating propellers compared with conventional BEMT models, which assume small angles, neglect some of the interactions in contra-rotating system and rely on inflow velocity solutions.