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A primary design goal with a coaxial rotor is to minimize the combined sources of losses on the upper and lower rotors that have their source in aerodynamic interference. To this end, parametric studies were conducted using a free-vortex wake method to study the aerodynamic interference effects of changing interrotor spacing, blade twist rates, and blade planform on the interdependent loads produced on the upper and lower rotors, respectively. A formal, multistep optimization process was then conducted by coupling the aerodynamic method to an optimization approach based on the method of feasible directions, the goal being to expeditiously find the individual blade geometries that would give the highest levels of efficiency from the coaxial as a system. Because of the inherent aerodynamic differences between the upper and lower rotors of a coaxial, it is shown that the best performing coaxial rotors may require the use of different blade shapes on each rotor, but substantially different blade designs may also achieve similar values of aerodynamic efficiency. It is also shown that the nonconvexity of the design problem for a coaxial rotor may limit the usefulness of formal optimization methods, and extensive parametric studies may still be required in the process of design.

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Results from a Lagrangian-Lagrangian-based brownout simulation were compared to photogrammetric measurements of the evolving dust clouds generated by a helicopter during low altitude taxi-pass and approach-to-touchdown maneuvers. An inviscid-viscous method was used to represent the carrier phase from a combination of the rotor flow field (modeled as an inviscid flow using a free-vortex wake) and a semiempirical boundary layer flow model at the ground. The mobilization and initial uplift of dust particles were represented using semiempirical models for the effects of shear, unsteady pressures, and bombardment mechanisms, the subsequent particle motion being solved for by using a subset of the Basset-Boussinesq-Oseen equations. Although not all the measurements required for validation were available, the results showed good agreement to the measured evolution of the dust clouds. The results showed a significant sensitivity to the weight of the helicopter, as well as the initial flight path that it followed toward the ground. Small ambient winds were also found to affect both the initial development and the subsequent movement of the dust clouds. A particle quantity analysis showed that the majority of the smaller sized particles were uplifted by direct entrainment and bombardment ejection mechanisms, which when acting together rapidly increased the net quantity of suspended dust.

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