A muffler is a device used to reduce the noise emitted by the exhaust of an internal combustion engine. Mufflers are installed within the exhaust system of an engine to reduce the loudness of sound pressure by the engine through the process of acoustic quieting, wherein vibrations are dampened to prevent them from creating significant noise. Mufflers are found in all automobiles, but did you know that they are also in some aircraft? While not common on older aircraft, many new, small, piston aircraft have mufflers. This blog will cover the use of mufflers in aircraft as well as new technologies being used to achieve the same thing.
There are a few reasons why very few aircraft feature mufflers. For one, the majority of noise from small aircraft is caused by the propellers ripping through the air. Shrouding the propellers could diminish some of this noise, but doing so also decreases performance. Second, weight is at a premium in small aircraft. Their cargo space is already limited, and adding a muffler means you will have to leave something else behind. External mufflers can also increase drag. Third, a muffler can cause back pressure in the exhaust, thereby diminishing performance. Back pressure is a resistance or force opposing the desired flow of fluid through pipes, causing friction loss and pressure drop. Finally, small aircraft vibrate a lot. These vibrations could be detrimental to exhaust joints and cause your system to come loose. As standard mufflers are not always suitable for aircraft, aircraft manufacturers have had to investigate new and innovative means of reducing exhaust noise.
With the aim of reducing exhaust noise while minimizing back pressure, size, and weight, a great deal of time and resources has been put into the development of new muffler designs for use on small aircraft. As the type of muffler used on automobiles are not suitable for aircraft due to their excessive weight, size, and back pressure, mufflers for aviation have had to be greatly improved and tailored to the unique requirements of flight. This development and investigation has resulted in the identification of many regulatory and technological issues in the use of mufflers on general aviation aircraft, namely the development of a basic theoretical approach to the design and optimization of aircraft engine exhaust systems and evaluation of conceptual designs of muffler and exhaust systems with dissociative resistive, and reactive muffler components.
Rather than traditional mufflers, these new designs call for the use of silicon carbide foam as an absorber material used in combination with various reactive components. Specifically, these designs consist of many differently sized and shaped expansion chambers with acoustic liners or flow through baffles. Experiments measured the acoustical impedances, resistances to flow, and other relevant properties of absorbers to determine perimeters like structural factors and others. Additionally, noise characteristics of commercial aircraft engines were measured and compared to predicted values used in designing mufflers. Following this, prototypes of 17 different new muffler designs were built and tested against stock mufflers to determine the acoustic insertion losses and analyze the system’s function.
These studies have shown that engine noise is the dominant overall noise for engines of less than 250 horsepower, and also determined that mufflers can be beneficial in reducing aircraft engine noise. While final flight weight designs are still in development, it has been determined that, due to the low frequencies of sounds generated by aircraft engines, dissipative and reactive muffler designs like those being studied will be required to satisfy the noise, size, weight, and performance requirements of general aviation. Furthermore, mufflers with this design have also proven to be highly suitable for incorporation of catalytic converters to reduce both chemical and noise pollution.
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