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14 - Noise

Published online by Cambridge University Press:  05 May 2013

Wayne Johnson
Wayne Johnson
Affiliation:
Aeromechanics Branch of NASA Ames Research Center
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Summary

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.

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Rotorcraft Aeromechanics , pp. 493 - 544
Publisher: Cambridge University Press
Print publication year: 2013

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  • Noise
  • Wayne Johnson
  • Book: Rotorcraft Aeromechanics
  • Online publication: 05 May 2013
  • Chapter DOI: https://doi.org/10.1017/CBO9781139235655.015
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  • Noise
  • Wayne Johnson
  • Book: Rotorcraft Aeromechanics
  • Online publication: 05 May 2013
  • Chapter DOI: https://doi.org/10.1017/CBO9781139235655.015
Available formats
×

Save book to Google Drive

To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.

  • Noise
  • Wayne Johnson
  • Book: Rotorcraft Aeromechanics
  • Online publication: 05 May 2013
  • Chapter DOI: https://doi.org/10.1017/CBO9781139235655.015
Available formats
×