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  • Print publication year: 2013
  • Online publication date: May 2013

6 - Forward Flight


The Helicopter Rotor in Forward Flight

Efficient hover capability is the fundamental characteristic of the helicopter, but without good forward flight performance the ability to hover has little value. During translational flight of the helicopter, the rotor disk is moving edgewise through the air, remaining nearly horizontal, generally with a small forward tilt to provide the propulsive force for the aircraft. A tiltrotor cruises with the rotors tilted to operate as propellers. A compound helicopter reduces the lift and propulsive force required of the rotor. Yet all rotorcraft configurations execute low-speed forward flight with the flapping rotor in edgewise flow, which is the subject of this chapter.

Thus in forward flight the rotor blade sees both a component of the helicopter forward velocity and the velocity due to its own rotation. On the advancing side of the disk the velocity of the blade is increased by the forward speed, whereas on the retreating side the velocity is decreased. For a constant angle-of-attack of the blade, the varying dynamic pressure of the rotor aerodynamic environment in forward flight would tend to produce more lift on the advancing side than on the retreating side; that is, a rolling moment on the rotor. If nothing were done to counter this moment, the helicopter would respond by rolling toward the retreating side of the rotor until equilibrium was achieved, with the rotor moment balanced by the gravitational force acting at the helicopter center-of-gravity. The rotor moment could possibly be so large that an equilibrium roll angle would not be achieved.

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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.
Bailey, F.J. Jr., and Gustafson, F.B. “Charts for Estimation of the Characteristics of a Helicopter Rotor in Forward Flight; I – Profile Drag-Lift Ratio for Untwisted Rectangular Blades.” NACA ACR L4H07, August 1944.
Bennett, J.A.J.Rotary-Wing Aircraft.” Aircraft Engineering, 72:133 (March 1940).
Bennett, J.A.J.The Era of the Autogiro.” The Journal of the Royal Aeronautical Society, 65:610 (October 1961).
Castles, W. Jr., and New, N.C. “A Blade-Element Analysis for Lifting Rotors That Is Applicable for Large Inflow and Blade Angles and Any Reasonable Blade Geometry.” NACA TN 2656, July 1952.
Charles, B.D., and Tanner, W.H. “Wind Tunnel Investigation of Semirigid Full-Scale Rotors Operating at High Advance Ratios.” USAAVLABS TR 69-2, January 1969.
de la Cierva, J.The Development of the Autogyro.” The Journal of the Royal Aeronautical Society, 30:181 (January 1926).
de la Cierva, J. “Engineering Theory of the Autogiro; Theory of Stresses on Autogiro Rotor Blades.” 1933.
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.
Datta, A., Yeo, H., and Norman, T.R. “Experimental Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios.” American Helicopter Society 66th Annual Forum, Virginia Beach, VA, May 2011.
Gessow, A. “Equations and Procedures for Numerically Calculating the Aerodynamic Characteristics of Lifting Rotors.” NACA TN 3747, October 1956.
Gessow, A., and Crim, A.D. “An Extension of Lifting Rotor Theory to Cover Operation at Large Angles of Attack and High Inflow Conditions.” NACA TN 2665, April 1952.
Gessow, A., and Crim, A.D. “A Method for Studying the Transient Blade-Flapping Behavior of Lifting Rotors at Extreme Operating Conditions.” NACA TN 3366, January 1955.
Gessow, A., and Crim, A.D. “A Theoretical Estimate of the Effects of Compressibility on the Performance of a Helicopter Rotor in Various Flight Conditions.” NACA TN 3798, October 1956.
Gessow, A., and Tapscott, R.J. “Charts for Estimating Performance of High-Performance Helicopters.” NACA Report 1266, 1956.
Gessow, A., and Tapscott, R.J. “Tables and Charts for Estimating Stall Effects on Lifting-Rotor Characteristics.” NASA TN D-243, May 1960.
Glauert, H. “A General Theory of the Autogyro.” ARC R&M 1111, November 1926.
Glauert, H.The Theory of the Autogyro.” The Journal of the Royal Aeronautical Society, 37:198 (June 1927).
Glauert, H., and Lock, C.N.H. “A Summary of the Experimental and Theoretical Investigations of the Characteristics of an Autogyro.” ARC R&M 1162, April 1928.
Gustafson, F.B. “Charts for Estimation of the Profile Drag-Lift Ratio of a Helicopter Rotor Having Rectangular Blades with -8 deg Twist.” NACA RM L53G20a, October 1953.
Harris, F.D. “Rotary Wing Aerodynamics – Historical Perspective and Important Issues.” American Helicopter Society National Specialists' Meeting on Aerodynamics and Aeroa-coustics, Arlington, TX, February 1987.
Harris, F.D. “Rotor Performance at High Advance Ratio; Theory versus Test.” NASA CR 2008-215370, October 2008.
Johnson, W. “Milestones in Rotorcraft Aeromechanics.” NASA TP 2011-215971, May 2011.
Lock, C.N.H. “Further Development of Autogyro Theory.” ARC R&M 1127, March 1927.
McCloud, J.L. III, Biggers, J.C., and Stroub, R.H. “An Investigation of Full-Scale Helicopter rotors at High Advance Ratios and Advancing Tip Mach Numbers.” NASA TN D-4632, July 1968.
Peters, D.A., and Ormiston, R.A. “Flapping Response Characteristics of Hingeless Rotor Blades by a Generalized Harmonic Balance Method.” NASA TN D-7856, February 1975.
Sissingh, G. “Contribution to the Aerodynamics of Rotating-Wing Aircraft.” NACATM 921, December 1939.
Tanner, W.H. “Charts for Estimating Rotary Wing Performance in Hover and at High Forward Speeds.” NASA CR 114, November 1964a.
Tanner, W.H. “Tables for Estimating Rotary Wing Performance at High Forward Speeds.” NASA CR 115, November 1964b.
Tapscott, R.J., and Gessow, A. “Charts for Estimating Rotor-Blade Flapping Motion of High-Performance Helicopters.” NACA TN 3616, March 1956.
Wheatley, J.B. “An Aerodynamic Analysis of the Autogiro Rotor with a Comparison Between Calculated and Experimental Results.” NACA Report 487, 1934.
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.
Yamauchi, G.K., and Johnson, W. “Trends of Reynolds Number Effects on Two Dimensional Airfoil Characteristics for Helicopter Rotor Analyses.” NASA TM 84363, April 1983.