Iterative BEMT analysis extended to model coaxial rotor aerodynamic performance in hover

Abstract

This paper addresses the aerodynamic modelling of coaxial contra-rotating unmanned aerial vehicle (UAV) rotor blades in hover using a novel adapted blade element momentum theory (BEMT) model. The investigation incorporates numerical, computational, and experimental methods. The traditional BEMT approach is extended to iteratively solve for the axial velocities of both upper and lower rotors, accounting for mutual rotor-to-rotor interaction, rotor axial separation distance, and tip loss effects. Wake contraction is evaluated using a prescribed wake model. Computational fluid dynamics (CFD) simulations were implemented to conduct two-dimensional (2D) axisymmetric studies to validate the wake contraction model. Results from the new coaxial BEMT model were then compared to three-dimensional (3D) CFD results for a rotor pair. The BEMT predictions show strong alignment with CFD, accurately capturing both the wake contraction radius and the radial distributions of aerodynamic loads. The developed BEMT coaxial model was also validated against literature data published for different blade designs, showing strong agreement. Additionally, the BEMT results were compared with experimental measurements across various rotational speeds and showed good agreement. The study indicates that the developed coaxial BEMT model is effective in capturing the trends and magnitude of the performance of coaxial contra-rotating rotor blades at much lower set-up and computational costs than higher fidelity CFD calculations.