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note: this question is old (2016) but so far remains unanswered. Perhaps innovations have improved on the noise even since this was first posted.

The light weight quadcopter and hexacopter UAVs that can cary say 2kg to 20kg payloads for "delivery" produce enough noise that a steady stream of pizza and book deliveries might not be so welcome in many populated areas.

Are there any candidate technologies that can significantly reduce the noise heard by people on the ground? One possibility is of course to fly higher, but that may not always be possible or desirable, and I don't think they'll be deploying pizzas and books with little parachutes from 500 or 1000 foot heights.

Better propeller design? Cleverly ducted/muffled thrust? Anything?

I'm sure possible techniques will interfere with thrust, so anything that has an impact of less than 50% would count here. Batteries will continue to get better over time, but I can't imagine much more innovation can be done with propellers.

I'm not looking for guesses, but it doesn't have to be proven on this scale either. My feeling is that there really isn't anything besides distance that can help very much.

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    $\begingroup$ I'm not understanding the parachute point. Couldn't you fly in high, then drop vertically under reduced power? You'd need a quick burst of power just above the ground to decelerate, but the subsequent takeoff would be unloaded and hopefully less noisy. $\endgroup$
    – MSalters
    Jul 4, 2016 at 10:56
  • $\begingroup$ @MSalters that's a good idea. If it's really quick, then the guy next door won't have time to run outside and shoot at it. In the mean time I'd like to understand if there is "anything besides distance that can help" reduce ground noise. $\endgroup$
    – uhoh
    Jul 4, 2016 at 14:52

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A quiet fan is a large fan.

(This should be understood in relation to either mechanical power or lift. Of course, a multi kilowatt industrial turbine is way noisier than the fan cooling your computer.)

A single large and slower propeller (or two coaxial) instead of 4 or 6 small ones will do wonders, provided one can get good enough automatic pitch control. A better energy efficiency will be a bonus.

We just invented the helicopter.

Edit:

There is a fundamental difference in how rotors of quadcopters and helicopters work.

The quadcopter has fixed blades and variable speed. To vary the speed of an electric motor is easy, the whole thing is simple. But: we need at least 3 rotors (and things get easy with at least 4) in order to control the vector of the lift. Rotors are small, they rotate at high speed. Results:

  • blades have some compromise form, they are not optimal at any speed
  • blades have some compromise angle of attack, again not optimal in any condition
  • blades are small for the load they lift (no place for bigger ones).

These three things mean a constant partial stall for some part of the blade. A lot of energy is lost to sound and vortices.

On the other hand, on a heli we have:

  • single rotor with big blades, running at more or less constant speed, optimized for lift, drag and noise at that speed
  • the angle of attack (pitch) of the blades is precisely controlled at every revolution. That's how both the lift amount and the lift vector are controlled (keywords: group and cyclic control).

The reason why small UAV are not usually made as helicopters is the pitch control. The mechanism is complex and maintenance-heavy, its control algorithm is complex as well.

On the other hand, the technology constantly improves and we COULD get something workable in the near future.

On the good side, helicopters powered by internal combustion have the limitation of the engine speed not being able to change much and not being able to change quick. A variable pitch AND a variable speed of the electric motor may offer additional means of control and efficiency improvements.

In regard to comments that helicopters are noisy as well: The lightest one is probably ~500kg, there are ones that are tens of tons. This is the main reason why they are noisier than a quadcopter.

I don't want to even imagine the sound of a 4-rotor 1000kg copter.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Farhan ♦
    Aug 29, 2021 at 0:28

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