Here are some considerations relative to exhaust systems on the perceived noise in the cockpit. Our planes typically have broad areas of composite or sheet metal panels in the region under the cockpit. These panels can become excited into resonance by the pulsations and harmonics in the engine exhaust. The speed of sound in the exhaust is considerably higher in the hot gases in the pipe. As a result, when the sound wave traveling down the pipe reaches the abrupt, cut-off end, it is reflected back in-phase toward all of the exhaust valves connected to that pipe - two, four, or six, - depending on the number of cylinders and the number of pipes. There will be resonant frequencies at those individual pipe lengths going back and forth between the end of the pipe and the exhaust valve or piston. Because the exhaust pulse is not sinusoidal but has a fairly sharp rise time, it will be rich in harmonics that will also seek their various resonances. But if the exhaust pipe is cut off at an angle at its output, two things will occur; one, there will not be a definite sharply-defined length for the resonance to occur, and second, the sound wave will undergo refraction at the slant surface, causing the noise to be bent away from the pipe. From the first effect, the noise energy spectrum will be spread over a wider range, resulting in less energy at any particular frequency and thus lower noise amplitude. From the second effect, if the exhaust pipe is pointed back at just a slight angle to the fuselage, the gas flow will be to rear, giving, if not thrust, at least less drag. And by having the slanted cut on the bottom and slightly toward the outside, the noise will be directed down and out to the side, away from the bottom of the plane's broad bottom, keeping cockpit noise levels lower. Some exhaust systems try to achieve the same noise-reduction effect by turning the exhaust pipe sharply downward at the outlet; this can have several deleterious results. The downward exhaust pipe sticking out into the free-stream causes drag, and the downward gas flow interferes with the smooth rearward flow coming off the bottom of the cowling, generating turbulence which also increases drag. Depending on the orientation of the pipe aperture to the freestream, it can also cause a low pressure at the outlet which may result in more crossflow across the cylinder from intake to exhaust, resulting in fuel mixture in the exhaust which ignites and causes backfiring, often falsely attributed to incorrect timing from a magneto or an EI.
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