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  • Cited by 2
  • Print publication year: 2013
  • Online publication date: May 2013

3 - Hover


Hover is the operating state in which the lifting rotor has no velocity relative to the air, either vertical or horizontal. General vertical flight involves axial flow with respect to the rotor. Vertical flight implies axial symmetry of the rotor flow field, so the velocities and loads on the rotor blades are independent of the azimuth position. Axial symmetry greatly simplifies the dynamics and aerodynamics of the helicopter rotor, as is evident when forward flight is considered. The basic analyses of a rotor in axial flow originated in the 19th century with the design of marine propellers and were later applied to airplane propellers. The principal objectives of the analysis of the hovering rotor are to predict the forces generated and power required by the rotating blades and to design the most efficient rotor.

Momentum Theory

Momentum theory applies the basic conservation laws of fluid mechanics (conservation of mass, momentum, and energy) to the rotor and flow as a whole to estimate the rotor performance. The theory is a global analysis, relating the overall flow velocities to the total rotor thrust and power. Momentum theory was developed for marine propellers by W.J.M. Rankine in 1865 and R.E. Froude in 1885, and extended in 1920 by A. Betz to include the rotation of the slipstream; see Glauert (1935) for the history.

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Bailey, F.J. Jr., “A Simplified Theoretical Method of Determining the Characteristics of a Lifting Rotor in Forward Flight.” NACA Report 716, 1941.
Glauert, H. “Airplane Propellers.” In Aerodynamic Theory, Durand, W.F. (Editor). New York: Julius Springer, 1935.
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Gray, R.B.On the Motion of the Helical Vortex Shed from a Single-Bladed Hovering Model Helicopter Rotor and Its Application to the Calculation of the Spanwise Aerodynamic Loading.” Princeton University, Aeronautical Engineering Department Report No. 313, September 1955.
Gray, R.B.An Aerodynamic Analysis of a Single-Bladed Rotor in Hovering and Low-Speed Forward Flight as Determined from Smoke Studies on the Vorticity Distribution in the Wake.” Princeton University, Aeronautical Engineering Department Report No. 356, September 1956.
Knight, M., and Hefner, R.A.Static Thrust Analysis of the Lifting Airscrew.” NACA TN 626, December 1937.
Kocurek, J.D., and Tangler, J.L.A Prescribed Wake Lifting Surface Hover Performance Analysis.” Journal of the American Helicopter Society, 22:1 (January 1977).
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Landgrebe, A.J.The Wake Geometry of a Hovering Helicopter Rotor and Its Influence on Rotor Performance.” Journal of the American Helicopter Society, 17:4 (October 1972).
Sissingh, G.Contribution to the Aerodynamics of Rotating-Wing Aircraft.” NACA TM 921, December 1939.
Theodorsen, T.The Theory of Propellers.” NACA Reports 775, 776, 777, and 778, 1944.
Wheatley, J.B.An Aerodynamic Analysis of the Autogiro Rotor with a Comparison Between Calculated and Experimental Results.” NACA Report 487, 1934.