A study on coaxial rotor aerodynamics

Abstract

The blade element momentum theory and fundamental aerodynamic performance of coaxial rotor are discussed. The simple momentum theory and blade element theory were used to calculate some aerodynamic parameters such as thrust, torque and power of coaxial rotor. The blade element momentum theory (BEMT) is used to determine the distribution of local airflow into the upper and lower rotors at the torque balanced condition. The upper rotor inflow ratio is similar to that of single rotor, whereas the lower rotor inflow ration is splitted into 2 areas: inner and outer area affected by sliptream from upper rotor. All derivations are assumed that the inner part of the lower rotor operates in the vena contracta of the upper rotor with fully developed slipstream velocity there. Prandtl tip loss is also derived for area of interchange of lower rotor. The thrust and power coefficients, inflow ratio, figure of merit of coaxial rotor calculated by theory well agree with experimental data. The results from the BEMT were validated using a free-vortex wake calculation. Finally, a linear blade twist is considered for maximizing the figure of merit of the rotor. Results that were obtained for an optimum coaxial rotor were discussed.