A Methodology for Rotorcraft Brownout Mitigation Using Rotor Design Optimization

The paper describes a methodology for the design of helicopter rotors with improved brownout char-acteristics. A brownout metric is introduced, based on the density of the sediment cloud in the field of view of the pilot. The metric becomes the objective function of a procedure in which blade chord, rotor radius, and blade twist rate are optimized to minimize brownout in a representative landing ma-neuver. The analysis includes a free-vortex wake model and simulates the dynamics of the sediment particles immersed in the rotor downwash. The results show that the optimized configurations have im-proved visibility within 2–3 rotor radii from the pilot, as desired, because the rotor generates increased groundwash that pushes the dust cloud away from the pilot. The clouds, however, do not necessarily become smaller or less dense further away from the pilot’s field of view. Simple polynomial expres-sions are provided that quantify brownout mitigation in terms of rotor design parameters, within the assumptions used in the derivation, and can be used in preliminary design applications. Nomenclature B(X) Objective function of the mitigation procedure, Eq. (6) c Blade chord CT Rotor thrust coefficient, CT = T/ ρpiΩ2R4 DL Disk loading, DL = T/A g(X) Constraint equations of the mitigation procedure, Eq. (7) ks Wake ferquency Nb Number of blades Nr Number of rotors R Rotor radius S Unit vector in the direction from the candidate new design to the current design T Main rotor thrust Vtip Blade tip speed, Vtip = ΩR w Average slipstream flow velocity X Vector of design variables y Intermediate variable used in constraint formulation, y = 1/cR3 φ,θ,ψ Helicopter roll, pitch, and yaw angle