It is understandable jet engines couldn't have mufflers, but even long before the jet age, airplane piston engines never had mufflers the way cars do. Is there any specific reason?
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4$\begingroup$ I don't have numbers for it but mufflers reduce engine power. $\endgroup$– ratchet freakAug 30, 2017 at 8:45
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7$\begingroup$ @ymb1 Certainly related, probably not a duplicate; that question asks why are they so noisy (to which the answer, in part, may very well be "because they lack mufflers", as per the top-voted answer there); this asks why no mufflers, to which the answer cannot be "because they lack mufflers" because that would be circular reasoning. $\endgroup$– userAug 30, 2017 at 10:57
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5$\begingroup$ Faulty question. Most do have mufflers. Exceptions being very old ones, such as warbirds with radial engines. $\endgroup$– Brian KnoblauchAug 30, 2017 at 12:42
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9$\begingroup$ They reduce engine power, add weight, have limited life before they burn out, and add new risk factors like fuel leaking onto a hot muffler. Apart from the need to meet noise regulations (which is a political requirement, not a technical one), those are four reasons no good aircraft designer would even consider using them - they totally fail the "simplify and add more lightness" aircraft design principle. $\endgroup$– alephzeroAug 30, 2017 at 14:50
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4$\begingroup$ Every modern GA aircraft that I am aware of has a muffler. Single engine planes use a muffler shroud to heat air for the defrost and cabin heat. The muffler isn’t nearly as large or effective as the ones on cars but a large part of the noise from an airplane is from the prop, so having a larger muffler would not eliminate the noise. $\endgroup$– JScarryAug 30, 2017 at 17:48
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5 Answers
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Today, most GA aircraft in Europe do have mufflers so they can pass the ever more stringent noise requirements. Here is one mounted to a Cessna 172 (picture source):
Historically, mufflers were considered as a disadvantage because they reduce engine performance by increasing back pressure. By partially blocking the exhaust flow, the muffler increases cylinder pressure on the last upstroke of the cycle (the exhaust stroke) so it faces more resistance against moving up. Also, more of the burnt gasses remain in the cylinder so less fresh fuel-air mixture can be ingested in the next cycle. At most, in aircraft with an open cockpit a tube was installed to duct the exhaust gasses away from the pilot. Below is a picture of a Mercedes D.IIIa in a Fokker D VII (source):
However, night flying would make aircraft easily detectable by their exhaust flames, so in WW II tubes were added to the exhaust stack in order to avoid optical detection. Below is a picture of an Avro Lancaster with Bristol Hercules engines (source):
Our much better understanding of piston engines now allows to design mufflers which do not cause much power loss and in case of a tuned resonance muffler you even get more power at the design speed because a reflected pressure wave will arrive from the muffler at the exhaust port just when the valves close and will increase cylinder pressure. However, given the low RPM speeds of our old-fashioned aviation piston engines, those mufflers need to be large and the equally old airframe designs have no space for them. So in aviation the mufflers are still causing a power loss.
answered Aug 30, 2017 at 9:40
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$\begingroup$ The most well-known tuned exhaust system is made by Power Flow for general aviation aircraft. powerflowsystems.com $\endgroup$– PugzMay 19, 2018 at 14:31
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$\begingroup$ @Pugz: Just another snake oil vendor … $\endgroup$ May 19, 2018 at 16:29
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$\begingroup$ @Pugz: From their FAQ: 'We have accomplished flight-testing and found that drag caused by the tailpipe is overcome by thrust coming out of it.' Yes, their exhaust seems to be better, but being larger, it causes more drag. This supports what I said in the answer above. $\endgroup$ May 19, 2018 at 21:36
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Even in the US planes designed in the 40s and 50s, the most common planes have mufflers.
Mooney's do. 
And the ever popular Cessna 172
answered Aug 30, 2017 at 9:53
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The direct answer to the question you asked is that there's usually much more separation between an operating aircraft and almost anybody else who might hear it, so noise simply doesn't matter nearly as much as it does with something like a car that frequently operates within, say, a meter of other cars and pedestrians, and often only a few tens of meters from houses, offices, hospitals, etc.
On the other hand, I do feel obliged to point out that contrary to popular belief, an exhaust system can reduce noise levels without causing loss of power--in fact, a properly designed system can actually increase power in many cases, especially some that are quite applicable to most aircraft.
Just for example, a simple tuned expansion chamber typically looks roughly like two cones with their large ends welded together. It helps increase power output fairly simply.
When the engine's exhaust value opens, exhaust starts to flow out of the exhaust port (being pushed fairly quickly by the piston). That means we a have a stream of fairly dense exhaust moving away from the exhaust port at fairly high speed.
When the exhaust port closes, that exhaust gas keeps moving away from the port, leaving a partial vacuum behind it.
In an "open pipe" exhaust system, atmospheric air flows into fill the vacuum very quickly, so by the time the exhaust port opens again, the exhaust gas has to "push" the air out of the way to flow out of the port.
With a tuned exhaust system, we have a pipe (of some sort) near the exhaust port to keep air from the atmosphere from just filling the vacuum next to the port. The dual cone expansion chamber does more than that though: it's designed so the dense exhaust gas bounces off the far end of the chamber, and back toward the port. It gets back to the port end of the chamber, and bounces off it again so it's now moving away from the exhaust port.
What we're shooting for is that the partial vacuum behind the dense exhaust gas is just outside the engine's exhaust port just as the exhaust valve opens. When this happens, that partial vacuum is actually helping to "suck" the next "pack" of exhaust gas out of the combustion chamber.
As to this helping reduce noise: the noise from the exhaust is basically because of the "packs" of compressed exhaust gas followed by rarified air immediately following. With the tuned chamber, we're letting the next pack of compressed exhaust "fill" in that partial vacuum. As a result, the gas flowing out of the exhaust has a much more consistent pressure.
As to why this is particularly applicable to aircraft: mostly because in an aircraft the engine's speed is much more predictable most of the time. Unlike something like a car, most aircraft are basically designed around a cruising speed, and spend the vast majority of their time operating within a fairly narrow range of engine RPM, so it's fairly easy to design an expansion chamber for that speed, with minimal consideration for other speeds.
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3$\begingroup$ What you describe sounds like a basic muffer used on two-cycle racing bikes and some high performance chainsaws: 2-cycle muffler. I can see the "two cones put together " shape. $\endgroup$– JPhi1618Aug 30, 2017 at 19:14
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1$\begingroup$ @JPhi1618: Yes, that's one common application of this design. It's not restricted to two-stroke engines (though for a four-stroke engine, the time between valve openings roughly doubles, so the chamber has to be ~twice as long for the same RPM). $\endgroup$ Aug 30, 2017 at 19:18
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$\begingroup$ "there's usually much more separation between an operating aircraft and almost anybody else who might hear it, so noise simply doesn't matter nearly as much as it does with something like a car". My parents live about a mile from an airport. Piston-engined planes flying hundreds of feet above their house make much more noise than cars driving down the street. $\endgroup$ Aug 30, 2017 at 19:20
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2$\begingroup$ @DavidRicherby: Much of that noise is caused by the propeller. $\endgroup$ Aug 30, 2017 at 19:51
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6$\begingroup$ @PeterKämpf OK, but the answer claims that noise from planes doesn't matter much because planes are far away. I'm pointing out that this argument doesn't hold much water. That part of the argument doesn't depend on what part of the plane is making the noise. $\endgroup$ Aug 30, 2017 at 19:59
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Propellers often run at a high enough RPM that the tips of the blades exceed the speed of sound. Ever notice that a prop-plane is extremely loud at take off, but very quiet on landing? Supersonic blade tips made mini sonic-booms.
Our ear hears an 'angry snarl' sound. Not the engine, not all the time. This is why a 5-bladed prop is somewhat quieter than a two bladed prop, each running with the same net thrust. The two bladed prop needs higher RPM, this pushes the tips past the sound barrier. A 5-bladed prop runs at lower RPM, less sonic booms.
OK, the point is this: If your propeller is capable of making noise that greatly exceeds engine noise, why silence the engine? Yes, a prop-plane at straight and level flight is not as quiet as a car, but the thing is literally miles away from the nearest unprotected ear, so less need for sound suppression of the engine. Here in Seattle, where it seems everyone has a sea-plane, regulations over blade-count are coming into effect. 2 blades? Nope, too much noise at take-off. 3? OK, better. Still loud at high RPM's. 5 or 7? Yay! Never need to spin it fast enough for supersonic operation. Some newer props have many oddly curved blades, and run very quietly. Quiet propeller = efficient propeller.
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$\begingroup$ For a prop plane to be "literally miles away from the nearest unprotected ear" it has to be at an altitude of at least 12,000ft over any area with any kind of population density. (I.e., pretty much the whole of a place like England.) $\endgroup$ Aug 31, 2017 at 18:18
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5$\begingroup$ I dispute the claim that propellers are often run with supersonic tips. Specific models of aircraft have been noteworthy for doing so, but it is not common. $\endgroup$ Aug 31, 2017 at 22:38
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$\begingroup$ Some quick back-of-the-envelope calculation shows that supersonic tips are at least plausible. The speed of sound is around 300 m/s, or 18000 m/minute. The speed of the tip depends on the diameter of the prop, and the speed it rotates. If the prop rotates at 2400 RPM, 18000 m/s translates to 7.5 m/revolution. So a prop at 2400 RPM would go supersonic if the radius (= length of a blade) is around 2.4 meters (or around 8 feet). Of course, smaller props will go supersonic when they rotate faster. $\endgroup$ Sep 1, 2017 at 3:52
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2$\begingroup$ @KevinKeane And how many planes have 16ft-diameter props? Nobody disputes that there are some planes with supersonic prop tips (e.g, the Tu-95 "Bear" which, btw, has 18ft props) but, as Erin says, that's not the norm. $\endgroup$ Sep 1, 2017 at 12:20
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$\begingroup$ Re "...iterally miles away from the nearest unprotected ear...": You must have some awfully long extensions on your shoes to allow your feet to reach the rudder pedals :-) I suspect the real reason is the same as why many motorcycles, snowmobiles, and the like don't have effective mufflers, and some people replace their car's (semi)-effective muffler with a louder one: the noise, and the annoyance it causes other people, gives them a feeling of power. $\endgroup$– jamesqfMay 5, 2018 at 18:40
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Plane engines are designed to run close to WOT (wide open throttle) in a narrow range RPM for most of the flight.
Matching the pipe length to the RPM is called exhaust tuning, which improves the performance and to my understanding reduces the noise.
This means tuning the exhaust of each cylinder should be simple, basically cut to the right length and left open would be best.
(Source) Example exhaust tuning.
