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Model Studies of Helicopter Rotor Flow Patterns

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Final technical rept.

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The report discusses an experimental program undertaken in a water tunnel wherein the tip vortex patterns were made visible through air bubble injection at the rotor tips. The model was a scaled version of the 48-foot- diameter Bell UH-1D rotor. The rotor configuration was tested as a standard 2- bladed unit and also as a 1- and 3-bladed unit. Correlation was established by comparing full-scale vertical lift values at hover conditions with scaled model values, where the model was mounted above the tunnel floor at a distance equivalent to out of ground effects for the full-scale vehicle. Employing solely a Prandtl-Glauert correction factor produced excellent lift value agreement between the model and the full-scale vehicle. Different aspects of the tip vortex patterns were visible from 2 to 6 diameters downstream thus the technique appears promising for both near and far field studies. High speed movies and stop and time exposure photographs were used to document the variation in vortex patterns. Discrete tip vortex patterns are created for forward flight regimes in the general ranges of hover to 10 knots, 10 to 50 knots, and above 50 knots. Tip vortices shed from one blade do intersect with following blades under certain conditions, thus verifying a limitation in the commonly employed mathematical models, where this limitation has been hypothesized by certain other investigators.

Subject Categories:

  • Aerodynamics
  • Helicopters
  • Fluid Mechanics

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