Helicopter Transmission Noise Mechanisms, Analysis Methods, and Noise Reduction Techniques

This paper describes how, in spite of advances made in developing quieter gearing systems and in soundproofing fuselage interiors, helicopter noise levels are still too high, particularly in military rotorcraft, impeding communications between pilots and mission controllers. The paper describes how there is still a need for research and development in quieting noise sources, transmission paths, and fuselage interiors. Fluctuating gear meshing sources may be reduced by optimizing tooth profiles to maximize gear contact ratios and minimize geometric transmission errors. Also, transmission structural components must be designed to avoid coincidence of any system resonance frequencies with gear mesh frequencies. Finally robust bearing and housing designs will reduce the misalignment (and increased noise) induced by strong static loads encountered during flight. Noise transmission paths may be treated by passive acoustic tailoring of the transmission housing and transmission support structure. Composite structures made of interwoven filaments surrounded by resin may be designed to have a nearly limitless range of material stiffnesses in various directions. Also, viscoelastomers may be embedded within the housing structure at locations in regions of high strain to absorb vibrational energy. There are challenges associated with developing viscoelastomers that have good damping performance at the high operating temperatures encountered in rotorcraft transmissions, however. The propagation of vibrational energy from the housing mountings to the fuselage may be minimized through advanced isolation mounting concepts. Periodically-layered metallic and elastomeric isolators are potential attenuators of dynamic stresses at high frequencies. The impedance difference between layers is the attenuation mechanism, in which an incident wave is scattered and essentially split into a reflected and refracted wave. The layers consist of two alternating materials. Theory predicts high frequency “stop bands” within which vibratory energy is attenuated. An axisymmetric approximation method has been used at Penn State University to predict high-frequency layered isolator behavior in compression. The existence of high-frequency stop bands has been corroborated with experiment. Standard cabin noise control techniques (passive control via blankets and/or damping treatments) may be augmented with active control methods in the future. However, although active control procedures have shown promise, they must be extended to encompass the many mid-high frequency gear meshing tones typical of helicopter transmissions. Also, the size, cost, and power requirements of such systems need to be minimized.


  • English

Media Info

  • Media Type: CD-ROM
  • Features: Figures; Photos; References;
  • Pagination: pp 874-883
  • Monograph Title: Noise-Con 04. The 2004 National Conference on Noise Control Engineering

Subject/Index Terms

Filing Info

  • Accession Number: 01054384
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: Jul 26 2007 1:43PM