Minimum loss load, twist and chord distributions for coaxial helicopters in Hover
We present an approach for determining the optimal (minimum power) torque-balanced coaxial hovering rotor using Blade Element Momentum Theory including swirl. We quantify the effects of the swirl component of induced velocity on performance, optimal induced wash distribution, and optimal blade twist and chord. The optimization accounts for the presence of a finite number of blades using the Prandtl tip loss factor, the effect of profile drag using experimentally or computationally determined drag polars, and the mutual interference between the two rotors using an empirically determined influence coefficient method. We show that including the swirl component of induced wash decreases the optimal figure of merit and has a larger impact at higher disk loadings, as expected. However, at the disk loadings typically found on helicopters, the effect of swirl is relatively small, particularly compared to other physical effects such as mutual interference or tip losses. Additionally, accounting for swirl affects the optimal rotor design near the root of the blade, at locations that would often be part of the root cutout of a realistic rotor.
