Detection of In-Flight Propeller-Induced Structure-Borne Noise

A potentially important source of structure-bor ne interior noise transmission in advanced turboprop aircraft is the impingement of the propeller wake/vortex on downstream aerodynamic surfaces. It can be safely assumed that this potential source of interior noise may well adversely affect achievable interior noise levels unless noise control measures are conscientiously incorporated into the aircraft design. Through the use of a laboratory- based test apparatus, techniques were developed to estimate the level of in-flight structure-borne noise transmis- sion from combined frequency response function ground testing and in-flight structural response measurements. All phases of the procedure were simulated in the laboratory and the expected level of accuracy of the procedure is addressed. A MAJOR focus of interior noise control for twin-engine propeller aircraft has been the reduction of the propeller direct airborne component via improved sidewall treat- ments.1'4 A majority of the research effort in this area has been aimed at the advanced high-speed turboprop aircraft wherein the direct airborne propeller-generated noise is quite intense and appears to be most critical for the success of this new generation aircraft.5 Continued efforts to develop light- weight sidewall treatments are presently being pursued6 and, coupled with the development of improved high-speed, low- noise propeller designs,7 the direct airborne sidewall transmis- sion problem may well be resolved in the near future. However, the potential for other sources and transmission paths governing the interior noise levels is quite high, based on most recent discoveries and historical data. Historically speaking, interior noise levels of propeller- driven aircraft are much higher than the acceptable levels of present-day turbofan aircraft, even after apparently ample ap- plication of noise control measures. Engine vibration-induced, structure-borne interior noise transmission has been shown to be equal to or greater than the direct airborne noise transmis- sion levels in a single-engine, propeller-driven aircraft.8 The potential for engine vibration as a source of structure-bor ne interior noise in twin-engine aircraft has not been thoroughly investigated; nevertheless, adequate procedures for engine vibration isolation system evaluation have been developed.9"10 A potentially more important source of structure-bor ne in- terior noise transmission is provided by the interaction of the propeller wake and aircraft wing structure. The wing surface downstream of the propeller may experience significant aerodynamically induced, fluctuating pressures due to the pro- peller wake, especially from the tip vortex.11 Extensive ground tests of a Twin Otter aircraft revealed that the propeller wake and tip vortex interaction with the wing surface was the major source of interior noise for the aircraft at 50% or greater engine torque.12 The interior noise spectra were dominated by contributions at the propeller blade passage frequency and its harmonics. The expected levels of propeller wake/vortex-induced, structure-borne noise transmission in an advanced turboprop aircraft is not known, nor can it be determined with present- day technology. It can only be safely assumed that this source