How can propeller blades be made silent?

Question

In particular drone propeller blades. I read that the noise we hear is the propeller compressing the air at the edge of it to produce a sonic boom. Is that true and is there a way to negate that?

fred_dot_u · Accepted Answer

Operating the unmanned aerial system in a vacuum will remove almost all of the noise.
Unfortunately, the unmanned aerial system will not fly in a vacuum, because air is required. The compression of air is part of the mechanics of flight, along with the rarefaction of air.
In the world of aerodynamics, a high speed propeller will generate more noise than a low speed propeller. A large diameter propeller will move more air than a smaller diameter propeller and can operate at a slower RPM.
I was once an ultralight pilot, operating an early design with a 36" (0.9 m) diameter propeller. It was necessary to operate at a screaming RPM in order to acquire very little thrust.
After upgrading the system to a 52" (1.3 m) prop, along with a belt drive reduction, the amount of power available at a much lower engine and prop speed was increased substantially.
A later aircraft I owned used a 60" (1.5 m) propeller and a greater reduction gear ratio, for even more power, and extremely low noise level. Operating at 200' (60 m), I could engage in shouting conversations with people on the ground.
For multi-rotor unmanned aerial systems, there is a necessary compromise between weight, power and noise. In the case of my ultralight aircraft, there was a weight increase, but it was offset by the increased power.
I'm surprised that more designs are not available for large diameter multi-rotor systems.
You can find examples of twin rotor helicopters, either coaxial or linear (Chinook style) that have huge rotors and could be considered quieter than multi-rotor unmanned aerial systems with small propellers.
As a last point, it is the tips of the smaller blades that are traveling at speeds in excess of the speed of sound that creates a sonic compression. Even conventional aircraft are able to operate propellers at such levels, although it is discouraged, as mechanical failures can result. Doing so in wet weather will also erode the tips of the propeller.

niels nielsen · Answer

in a subsonic air propeller blade, there are two sources of noise: first, the "hiss" of air moving past the blades and then blade passage noise which is a sound wave that sweeps around with the blade as it revolves, producing a sound pulse with each blade passage. It has a characteristic frequency of (prop revolutions per second) x (number of blades) and yields a buzzing hum that rises and falls in pitch as the prop speeds up or slows down.
The hiss is minimized by running the prop at slower speeds, which requires adding diameter or more blades to maintain the flow rate. the blade passage noise is minimized by widening the blades to the point where the leading and trailing edges of the adjacent blades begin to overlap.
Another trick to minimize blade passage noise is to cut a sawtooth pattern into the trailing edge of each blade right at the blade tip, which breaks up the sound wave pattern and muffles the hum- while increasing the hiss, however.

How can propeller blades be made silent?

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