Innovative designs of mufflers for engines of small airplanes have been investigated in an effort to satisfy conflicting demands to maximize the reduction of exhaust noise while minimizing back pressure, size, and weight. Automotive-type muffler designs are not suitable for this purpose because they entail excessive back pressures, sizes, and weights.
The investigation involved identification of key legal and technological issues in the silencing of general-aviation aircraft; development of a baseline theoretical approach to the design and optimization of aircraft engine exhaust systems; and evaluation of conceptual innovative designs of muffler and exhaust systems with dissipative, resistive, and reactive muffler components. The innovative designs call for the use of silicon carbide foam as a broadband absorber material in combination with reactive components; specifically, the designs feature variously sized and shaped expansion chambers with acoustic liners or flow-through baffles.
In experiments, acoustical impedances, resistances to flow, and other relevant properties of absorbers were measured and compared with theoretical predictions to determine such empirical parameters as structural factors. Noise characteristics of commercial aircraft engines were measured and compared with predicted values used in designing mufflers. Then prototypes of 17 different innovative muffler designs were fabricated, and several commercial aircraft engines were tested alternately with stock mufflers and with some of the prototypes to measure acoustic insertion losses as functions of frequency.
It was demonstrated that engine noise dominates overall aircraft noise for engines of less than 250 horsepower (power less than about 190 kW), and that mufflers can be beneficial toward reducing aircraft noise. Final flight weight designs had not been developed at the time of reporting information for this article. However, it was determined that because of the low frequencies of sounds generated by engines of the type in question, dissipative/reactive muffler designs like those investigated will be necessary for satisfying the noise, size, weight, and performance requirements of general aviation. An additional advantage of these muffler designs is that they are also highly suitable for incorporation of catalytic converters to reduce chemical as well as noise pollution.
This work was done by Andrew J. Sherman, Sangvavann Heng, and Edwin P. Stankiewicz of Ultramet for Lewis Research Center.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the category.
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