Iterative Blade Element Momentum Theory for a Coaxial Rotor

This paper presents a new blade element momentum theory (BEMT) for a coaxial rotor in hover. The new BEMT solves the upper and lower rotor induced velocities iteratively so that the mutual rotor-to-rotor interaction is accounted for. The upper rotor induced velocity is affected by the lower rotor thrust and induced velocity, while the lower rotor induced velocity is affected by the upper rotor thrust and induced velocity. Two empirical constants are included in each rotor calculation. This new BEMT provides the performance of each rotor as a function of the rotor separation distance. The new BEMT is validated with measurement data for two coaxial rotor experiments. The first experiment validates the thrust to power coefficients at a given separation distance. The second experiment validates each rotor's figure of merit, thrust, power, interference loss factors, etc., as a function of the rotor separation distance. It is shown that the BEMT captures the trends and magnitudes of the performance as a function of the rotor separation distance compared to the measurement data. Detailed radial distributions of aerodynamic properties are also presented at several separation distances.