Why are light aircraft engines so noisy?

Question

I live near an airport where students spend a lot of time flying circuits. During the base leg, pilots can be heard reducing engine revs prior to the final left/right turn to line up with the runway.

I've often wondered why in this day and age light aircraft engines are not made quieter; the technology seems to be stuck in the 1960's.

Can engines be made quieter or is it so prohibitively expensive/bureaucratic to get an engine through certification that aircraft builders just use off the shelf engines?

What's the quietest light pilot trainer out there (and don't say glider, thank you!)?

Answer 1

Engines could be made quieter by the simple solution of adding a muffler (or silencer as it's called in some places). They are available for many models and can be installed, at an astronomical cost. The parts and labor are actually pretty cheap, it's the certification costs that make it sting so badly. It costs a lot of money to develop and certify parts for airplanes. Mufflers aren't mandatory, and because they are so expensive to install most don't because the costs so outweigh the benefits - it costs far less to buy a decent active noise reduction headset instead.

Propellers are a big source of noise, and there are quieter designs for many aircraft, however they have to be certified with a particular engine and model of airplane and those costs add up.

The current certification rules are there to keep things safe, in some cases they can stifle innovation. It's why we still have carbureted engines with manual mixture controls instead of computerized fuel injection and turbonormalization.

As for what the quietest trainer is I don't think there's a definitive answer as I don't know of a source of light airplane noise measurements, and any airplane can be used as a trainer. Of the certified light airplanes out there that are commonly used for primary training the Diamond DA20 Katana is the quietest I can think of, that's because it is much lighter than the more common Cessnas and Pipers and therefore uses a smaller engine and propeller. Smaller engine and propeller generally means less noise. Turbo-diesel powered airplanes like the Diamond DA-40s are quieter because the turbo muffles the exhaust noise.

Answer 2

Prop powered general aviation aircraft are far louder than automobiles. No pilot flies without a headset because the noise makes it otherwise impossible to communicate with the ground or other pilots. Peak levels exceed 100db and the continuous average is around 85db, which is damaging to hearing without protection. The above charts showing arrival and departure noise are for jet powered aircraft, not the light aircraft in the OP's question.

There are two sources of external noise, the prop and the engine exhaust.

Noise from the prop is caused by its tips approaching supersonic speed. Energy used to produce noise is wasted, so you'd think reducing speed would be helpful. Unfortunately, most piston powered aircraft have props fixed directly to their crankshafts for simplicity and reliability. These engines are already turning slowly (compared to car engines) when producing full power, so slowing them even more drops combustion efficiency faster than it is recovered by increased propulsive efficiency due to reduced noise.

Exhaust noise is caused in aircraft the same way it is in autos. However, the typical aircraft engine used in the trainers mentioned by the OP is a four cylinder turning more slowly, so the combustion events required to make high power are two or three times bigger. Your typical econobox car has a 2 liter motor while the aircraft motor here is almost 6 liters. Exhaust noise is consequently two to three times louder, unmuffled, and more difficult to silence because of the lower frequencies produced by the slow engine. Weight is expensive in an aircraft. Mufflers are heavy and reduce power from the even heavier engine, so the minimum acceptable solution is the one used.

The best way to reduce engine noise is to use an electric motor that has no combustion impulses to suppress. The Pipistrel Alpha Electro is a trainer that does just that. The prop is also sized such that the tips stay between mach 0.5-0.6 during operation, keeping prop noise low. At only 60db, it is far quieter than piston powered aircraft.

Alpha Electro

Gearing a prop to allow it to turn slower and better mufflers could substantially reduce noise. These would both be heavy and raise cost not just for acquisition, but also in use as that weight has to be carried everywhere the aircraft goes. The pilot would still need to have hearing protection due to wind noise in the cockpit, which is not addressed as it was not part of the OP's question.

Answer 3

Edited for clarification due to the useful comments below

In general most light airplanes aren't dramatically louder than most cars (see Estimated Airplane Noise Circular on page say, 140 and Car Interior Noises ). What makes a difference though is how humans perceive these noises. If for example a person stands next to an airplane engine and a car engine at the same sound level we'll see that the main problem with the airplane's engine is that most of the time it emits "annoying" noises that cause discomfort. So it is important not to confuse annoying noises with high dBs! Another point to consider is that during approach -as is the OP's case- the noise from the airplane has less obstacles than when the airplane is on the ground resulting in most cases in lower sound attenuation and higher sound levels received.

Can these piston engines become quieter? Sure they can and a muffler is one thing that would help.

Quite many small airplanes don't have mufflers in order to save on costs, space, engine performance, weight and additional maintenance.

But equally, quite many small airplanes have mufflers for additional reasons related to the exhaust temperatures and yet emit excessive amounts of noise. Why? because engine is not the only noise source on the plane.

However from a manufacturer's point of view one should ask if it is necessary to increase design and operational costs to this end. Once the plane takes off there's no real need for it anyway. And if we take it one step further and consider the regulations we see that since the noises are within the acceptable margins there is no real need to enforce suppression and thus manufacturers don't need to add this additional cost and design changes to their planes.

Furthermore airplanes have additional noise sources resulting from their interaction with the air as you can see in the ICAO's diagram below: