Skip to main content Accessibility help
  • Print publication year: 2013
  • Online publication date: May 2013

5 - Forward Flight Wake


During translational motion of the helicopter, when the rotor is nearly horizontal, the rotor blades see a component of the forward velocity as well as the velocity due to their own rotation (Figure 5.1). In forward flight the rotor does not have axisymmetry as in hover and vertical flight; rather, the aerodynamic environment varies periodically as the blade rotates with respect to the direction of flight. The advancing blade has a velocity relative to the air higher than the rotational velocity, whereas the retreating blade has a lower velocity relative to the air. This lateral asymmetry has a major influence on the rotor and its analysis in forward flight. Thus the rotor blade loading and motion are periodic with a fundamental frequency equal to the rotor speed Ω. The analysis is more complicated than for hover because of the dependence of the loads and motion on the azimuth angle.

As a consequence of the axisymmetry, the analysis of the hovering rotor primarily involves a consideration of the aerodynamics. In forward flight, however, the lateral asymmetry in the basic aerodynamic environment produces a periodicmotion of the blade, which in turn influences the aerodynamic forces. The analysis in forward flight must therefore consider the blade dynamics as well as the aerodynamics. This chapter covers a number of aerodynamic topics that are familiar from the analysis of the rotor in vertical flight. In particular, we are concerned with the momentum theory treatment of the induced velocity and power in forward flight. Then the rotor blade motion and its behavior in forward flight are considered in Chapter 6.

Related content

Powered by UNSILO
Ashley, H., and Landahl, M.Aerodynamics of Wings and Bodies. Reading, MA: Addison-Wesley Publishing Company, Inc., 1965.
Cassier, A., Weneckers, R., and Pouradier, J.-M. “Aerodynamic Development of the Tiger Helicopter.” American Helicopter Society 50th Annual Forum, Washington, DC, May 1994.
Castles, W. Jr., and De Leeuw, J.H.The Normal Component of the Induced Velocity in the Vicinity of a Lifting Rotor and Some Examples of Its Application.” NACA Report 1184, 1954.
Cerbe, T., Reichert, G., and Curtiss, H.C. Jr. “Influence of Ground Effect on Helicopter Takeoff and Landing Performance.” Fourteenth European Rotorcraft Forum, Milan, Italy, September 1988.
Cheeseman, I.C., and Bennett, W.E.The Effect of the Ground on a Helicopter Rotor in Forward Flight.” ARC R&M 3021, September 1955.
Coleman, R.P., Feingold, A.M., and Stempin, C.W.Evaluation of the Induced-Velocity Field of an Idealized Helicopter Rotor.” NACA ARR L5E10, June 1945.
Cook, C.V.A Review of Tail Rotor Design and Performance.” Vertica, 2:3/4 (1978).
Curtiss, H.C. Jr., Erdman, W., and Sun, M.Ground Effect Aerodynamics.” Vertica, 11:1/2 (1987).
Curtiss, H.C. Jr., Sun, M., Putman, W.F., and Hanker, E.J. Jr.Rotor Aerodynamics in Ground Effect at Low Advance Ratios.” Journal of the American Helicopter Society, 29:1 (January 1984).
Drees, J.M.A Theory of Airflow Through Rotors and Its Application to Some Helicopter Problems.” Journal of the Helicopter Association of Great Britain, 3:2 (July-August-September 1949).
Empey, R.W., and Ormiston, R.A. “Tail-Rotor Thrust on a 5.5-Foot Helicopter Model in Ground Effect.” American Helicopter Society 30th Annual National Forum, Washington, DC, May 1974.
Glauert, H.A General Theory of the Autogyro.” ARC R&M 1111, November 1926.
Glauert, H.Elements of Aerofoil and Airscrew Theory. Cambridge: Cambridge University Press, 1947.
Hamel, D., and Humpert, A. “Eurocopter EC135 Initial Flight Test Results.” Twentieth European Rotorcraft Forum, Amsterdam, Netherlands, October 1994.
Hansen, K.C. “Handling Qualities Design and Development of the CH-53E, UH-60A, and S-76.” Royal Aeronautical Society International Conference on Helicopter Handling Qualities and Control, London, UK, November 1988.
Heyson, H.H.A Note on the Mean Value of Induced Velocity for a Helicopter Rotor.” NASA TN D-240, May 1960.
Huber, H., and Masue, T. “Flight Characteristics Design and Development of the MBB/KHI BK 117 Helicopter.” Seventh European Rotorcraft and Powered Lift Aircraft Forum, Garmisch-Partenkirchen, Germany, 1981.
Johnson, W.Model for Vortex Ring State Influence on Rotorcraft Flight Dynamics.” NASA TP 2005-213477, December 2005.
Leverton, J.W.Reduction of Helicopter Noise by Use of a Quiet Tail Rotor.” Vertica, 6:1 (1982).
Lynn, R.R., Robinson, F.D., Batra, N.N., and Duhon, J.M.Tail Rotor Design. Part I: Aerodynamics.” Journal of the American Helicopter Society, 15:4 (October 1970).
Mazzucchelli, C., and Wilson, F.T. “The Achievement of Aerodynamic Goals on the EH101 Project Through the “Single Site” Concept.” Seventeenth European Rotorcraft Forum, Berlin, Germany, September 1991.
Nathan, N.D., and Green, R.B. “Wind Tunnel Investigation of Flow around a Rotor in Ground Effect.” American Helicopter Society Specialists' Conference on Aeromechanics, San Francisco, CA, January 2010.
Prouty, R.W.A Second Approximation to the Induced Drag of a Rotor in Forward Flight.” Journal of the American Helicopter Society, 21:3 (July 1976).
Roesch, P., and Vuillet, A.New Designs for Improved Aerodynamic Stability on Recent Aerospatiale Helicopters.” Vertica, 6:3 (1982).
Sheridan, P.F.Interactional Aerodynamics of the Single Rotor Helicopter Configuration.” USARTL TR 78-23A, September 1978.
Sheridan, P.F., and Wiesner, W. “Aerodynamics of Helicopter Flight Near the Ground.” American Helicopter Society 33rd Annual National Forum, Washington, DC, May 1977.
Stepniewski, W.Z.Introduction to Helicopter Aerodynamics. Morton, PA: Rotorcraft Publishing Committee, 1955.
White, F., and Blake, B.B. “Improved Method of Predicting Helicopter Control Response and Gust Sensitivity.” American Helicopter Society 35th Annual Forum, Washington, DC., May 1979.