Acoustical Modeling of the Heavy Lift Helicopter

The Heavy Lift Helicopter (HLH) program is the first effort, at Boeing/Vertol which used wind tunnel modeling for acoustical design as an inherent part of the aircraft design development. This paper reports on work which has been done on acoustical modeling of the HLH, and the development of the techniques which have been used. The HLH, a tandem rotor helicopter, which weighs approximately 118,000 pounds and has two 90 ft. diameter rotors, is primarily a logistical aircraft. As such, its major tasks are associated with transporting large loads using an external sling. These operations often require that personnel on the ground work in close proximity to the hovering aircraft and places stringent limitations on external noise. The facility used for model testing was the Boeing/ Vertol 20' x 20' low speed wind tunnel. It has a steel walled test section without anechoic treatment and with no special noise attenuation devices at the fan or turning vanes. In order to use this type of facility successfully, several new techniques had to be developed.