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

14 - Noise


Helicopter Rotor Noise

The helicopter is the quietest VTOL aircraft, but its noise level can still be high enough to compromise its utility unless specific attention is given to designing for low noise. As the restrictions on aircraft noise increase, the rotor noise becomes an increasingly important factor in helicopter design. The complex aerodynamics of rotors lead to a number of significant noise mechanisms. Helicopter rotor noise tends to be concentrated at harmonics of the blade passage frequency NΩ, because of the periodic nature of the rotor as seen in the non-rotating frame. There is sound radiated because the mean thrust and drag forces rotate with the blades and because of the higher harmonic loading as well. The spectral lines are broadened at the higher harmonics because of the random character of the rotor flow, particularly variations in the wake-induced loads. The acoustic pressure signal is basically periodic in time (the period is 2π/NΩ), with sharp impulses due to localized aerodynamic phenomena such as compressibility effects and vortex-induced loads. Figure 14.1 illustrates the spectrum of rotor-generated sound. The contributions to helicopter rotor noise can be classified as vortex or broadband noise, rotational noise, and impulsive noise or blade slap. Although the distinction between these types of rotor noise is not as sharp as was once thought, the classification remains useful for purposes of exposition. Cox (1973), Burton, Schlinker, and Shenoy (1985), and Brentner and Farassat (1994, 2003) have presented summaries of helicopter rotor noise mechanisms and analysis.

Baeder, J.D.Euler Solutions to Nonlinear Acoustics of Non-Lifting Rotor Blades.” American Helicopter Society and Royal Aeronautical Society International Technical Specialists Meeting: Rotorcraft Acoustics and Rotor Fluid Dynamics, Valley Forge, PA, October 1991.
Batchelor, G.K.An Introduction to Fluid Dynamics. London: Cambridge University Press, 1967.
Boxwell, D.A., and Schmitz, F.H.Full-Scale Measurements of Blade-Vortex Interaction Noise.” Journal of the American Helicopter Society, 27:4 (October 1982).
Boxwell, D.A., Yu, Y.H., and Schmitz, F.H.Hovering Impulsive Noise – Some Measured and Calculated Results.” Vertica, 3:1 (1979).
Brentner, K.S.Prediction of Helicopter Rotor Discrete Frequency Noise.” NASA TM 87721, October 1986.
Brentner, K.S.An Efficient and Robust Method for Predicting Helicopter Rotor High-Speed Impulsive NoiseJournal of Sound and Vibration, 203:1 (1997a).
Brentner, K.S.Numerical Algorithms for Acoustic Integrals with Examples for Rotor Noise Prediction.” AIAA Journal, 35:4 (April 1997b).
Brentner, K.S., Burley, C.L., and Marcolini, M.A.Sensitivity of Acoustic Predictions to Variation of Input Parameters.” Journal of the American Helicopter Society, 39:3 (July 1994).
Brentner, K.S., and Farassat, F.Helicopter Noise Prediction: The Current Status and Future Direction.” Journal of Sound and Vibration, 170:1 (1994).
Brentner, K.S., and Farassat, F.Analytical Comparison of the Acoustic Analogy and Kirchhoff Formulation for Moving Surfaces.” AIAA Journal, 36:8 (August 1998).
Brentner, K.S., and Farassat, F.Modeling Aerodynamically Generated Sound of Helicopter Rotors.” Progress in Aerospace Sciences, 39:2–3 (February 2003).
Burley, C.L., and Brooks, T.F.Rotor Broadband Noise Prediction with Comparison to Model Data.” Journal of the American Helicopter Society, 49:1 (January 2004).
Burton, D.E., Schlinker, R.H., and Shenoy, R.The Status of Analytical Helicopter Noise Prediction Methods.” NASA CR 172606, August 1985.
Cox, C.R.Aerodynamic Sources of Rotor Noise.” Journal of the American Helicopter Society, 18:1 (January 1973).
Cox, C.R.Helicopter Noise Certification Experience and Compliance Cost Reductions.” Nineteenth European Rotorcraft Forum, Cernobbio, Italy, September 1993.
Cox, C.R., and Lynn, R.R.A Study of theOrigin and Means of Reducing Helicopter Noise.” TCREC TR 62-73, November 1962.
Curle, N.The Influence of Solid Boundaries upon Aerodynamic Sound.” Proceedings of the Royal Society of London, Series A, 231:1187 (September 1955).
Davidson, I.M., and Hargest, T.J.Helicopter Noise.” The Journal of the Royal Aeronautical Society, 69:653 (May 1965).
Deming, A.F.Noise from Propellers with Symmetrical Sections at Zero Blade Angle.” NACA TN 605, July 1937.
Deming, A.F.Noise from Propellers with Symmetrical Sections at Zero Blade Angle, II.” NACA TN 679, December 1938.
Deming, A.F.Propeller Rotation Noise due to Torque and Thrust.” NACATN 747, January 1940.
di Francescantonio, P.A New Boundary Integral Formulation for the Prediction of Sound Radiation.” Journal of Sound and Vibration, 202:4 (1997).
Farassat, F.Theory of Noise Generation from Moving Bodies with an Application to Helicopter Rotors.” NASA TR R-451, December 1975.
Farassat, F.Linear Acoustic Formulas for Calculation of Rotating Blade Noise.” AIAA Journal, 19:9 (September 1981).
Farassat, F., and Brentner, K.S.The Use and Abuses of the Acoustic Analogy in Helicopter Noise Prediction.” Journal of the American Helicopter Society, 33:1 (January 1988).
Farassat, F., and Succi, G.P.A Review of Propeller Discrete Frequency Noise Prediction Technology with Emphasis on Two Current Methods for Time Domain Calculations.” Journal of Sound and Vibration, 71:3 (1980).
Ffowcs Williams, J.E., and Hawkings, D.L.Sound Generation by Turbulence and Surfaces in Arbitrary Motion.” Philosophical Transactions of the Royal Society of London, 264A:1151 (May 1969).
Garrick, I.E., and Watkins, C.E.A Theoretical Study of the Effect of Forward Speed on the Free-Space Sound-Pressure Field Around Propellers.” NACA Report 1198, 1954.
Gmelin, B.L., Heller, H., Mercker, E., Philippe, J.-J., Preisser, J.S., and Yu, Y.H.The HART Programme: A Quadrilateral Cooperative Research Effort.” American Helicopter Society 51st Annual Forum, Ft.Worth, TX, May 1995.
Gutin, L.On the Sound Field of a Rotating Propeller.” NACA TM 1195, October 1948.
Hubbard, H.H., and Maglieri, D.J.Noise Characteristics of Helicopter Rotors at Tip Speeds up to 900 Feet Per Second.” Journal of the Acoustical Society of America, 32:9 (September 1960).
Hubbard, H.H., and Regier, A.A.Free-Space Oscillating Pressures Near the Tips of Rotating Propellers.” NACA Report 996, 1950.
Leverton, J.W., and Pollard, J.S.A Comparison of the Overall and Broadband Noise Characteristics of Full-Scale and Model Helicopter Rotors.” Journal of Sound and Vibration, 30:2 (1973).
Lighthill, M.J.On Sound Generated Aerodynamically. I. General Theory.” Proceedings of the Royal Society of London, Series A, 211:1107 (March 1952).
Loewy, R.G., and Sutton, L.R.A Theory for Predicting the Rotational Noise of Lifting Rotors in Forward Flight, Including a Comparison with Experiment.” Journal of Sound and Vibration, 4:3 (1966).
Lowson, M.V.The Sound Field for Singularities in Motion.” Proceedings of the Royal Society of London, Series A, 286:1407 (August 1965).
Lowson, M.V., and Ollerhead, J.B.A Theoretical Study of Helicopter Rotor Noise.” Journal of Sound and Vibration, 9:2 (1969).
Marze, H.J.Helicopter External Noise: ICAO Standards and Operational Regulations.” Eighth European Rotorcraft Forum, Aix-en-Provence, France, September 1982.
Ollerhead, J.B., and Lowson, M.V.Problems of Helicopter Noise Estimation and Reduction.” AIAA Paper No. 69-195, February 1969.
Schlegel, R.G., and Bausch, W.E.Helicopter Rotor Rotational Noise Prediction and Correlation.” USAAVLABS TR 70-1, November 1970.
Schlegel, R., King, R., and Mull, H.Helicopter Rotor Noise Generation and Propagation.” USAAVLABS TR 66-4, October 1966.
Schmitz, F.H., and Yu, Y.H.Transonic Rotor Noise – Theoretical and Experimental Comparisons.” Vertica, 5:1 (1981).
Stuckey, T.J., and Goddard, J.O.Investigation and Prediction of Helicopter Rotor Noise. Part I. Wessex Whirl Tower Results.” Journal of Sound and Vibration, 5:1 (1967).
United States Government, “Part 36 – Noise Standards, Aircraft Type and Airworthiness Certification.” Code of Federal Regulations, Title 14, Aeronautics and Space, 2012.
Watkins, C.E., and Durling, B.J.A Method for Calculation of Free-Space Sound Pressures Near a Propeller in Flight Including Consideration of the Chordwise Blade Loading.” NACA TN 3809, November 1956.
Widnall, S.E.A Correlation of Vortex Noise Data from Helicopter Main Rotors.” Journal of Aircraft, 6:3 (May-June 1969).
Yu, Y.H., Caradonna, F.X., and Schmitz, F.H.The Influence of the Transonic Flow Field on High-Speed Helicopter Impulsive Noise.” Fourth European Rotorcraft and Powered Lift Aircraft Forum, Stresa, Italy, September 1978.
Yudin, E.Y.On the Vortex Sound from Rotating Rods.” NACA TM 1136, March 1947.