Noise Transmission and Control for a Light Twin-Engine Aircraft

Abstract

One of the dominant source-path combinations for cabin noise in light twin-engine aircraft is propeller noise being transmitted through the fuselage sidewall. This source-path was investigated and candidate sidewall add- on treatments were installed and tested using both an external sound source and the propeller in ground static engine runs. Results indicate that adding either mass or stiffness to the fuselage skin would improve sidewall attenuation and that the honeycomb stiffness treatment provided more improvement at most frequencies than an equal amount of added mass. It is proposed that double-wall construction in conjunction with skin stiffening should provide a good weight-efficient combination for the aircraft studied. NE of the principal source-path combinations of cabin noise in light, twin-engine aircraft is propeller noise transmitted through the fuselage sidewall. Improved methods of controlling this cabin noise are needed to provide a comfortable passenger environment, while at the same time controlling aircraft weight and fuel consumption. Lighter weight noise control methods are needed to replace traditional approaches which have relied largely on relatively heavy damping and mass treatments. A number of approaches have been investigated for reducing cabin noise for this type of aircraft. Flight tests indicated interior noise can be reduced about 3.5 dB(A) by a reduction of engine rpm in an aircraft with variable pitch propellers.1 Design of propeller configurations is being in- vestigated as a means of reducing the noise generated at the source.2 Theoretical prediction methods for sidewall noise transmission have been developed to aid the search for noise- resistant sidewall structures. Theoretical analysis of interior noise transmission has included mechanical analogy models, rigid-stiffener/flexible-panel models, and more complex flexible-stiffener/flexible-panel models. 35 The analyses have been compared with laboratory test data for verification and have been used to examine a number of candidate noise control treatments including variations of skin thickness, stiffener stiffness, and structural damping, and addition of damping, mass, and honeycomb panel stiffening. Previous work has not included evaluation of candidate noise control treatments in an experimental situation using an actual aircraft. Such studies are needed to evaluate and compare candidate treatments, and to guide further development of noise control treatments. The purpose of this paper is to describe an experimental program of evaluation of three noise control treatments. The work is focused on added stiffness in the form of honeycomb panels. Also, two mass treatments are included for comparison. The tests were carried out using a light twin-engine aircraft (Fig. 1). Can- didate treatments were developed using the aircraft with a horn noise source in the laboratory. The performance of the stiffness treatment was verified using ground static runs of the aircraft engines. The laboratory portion of this investigation is described in Ref. 6.