Helicopter rotor hubs are geometrically complex components that experience a wide rage of aerodynamic behaviors ! and flow physics. This includes strong unsteadiness, large amounts of separation, laminar-turbulent transition, and interactional aerodynamic behaviors (Ref. ). At high forward flight speeds (high advance ratios), the parasitic drag of the hub accounts for O(30%) of the total power required to fly (Ref. ). A common method for characterizing this contribution is the hub drag factor, K_(fe), which correlates the flat-plate area of the hub with the helicopter gross weight and functions as a technology factor (Ref. ). In a recent assessment of needs for future vertical lift systems, Ormiston suggested that the hub drag factor needs to be reduced from the current state of the art of K_(fe) = 0.5 down to a value 0.2 (Ref. ).
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