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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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    $\begingroup$ bigger, slower spinning propellers will do the trick. But this means bigger UAVs, and much more weight for caging the props. If you leave the props uncaged, the damage risk goes up, both to persons and the props themselves. $\endgroup$ – Peter Kämpf Jul 3 '16 at 6:12
  • $\begingroup$ Thanks @PeterKämpf - I'm new to the "world of propellers." I wonder if there is a handy place on the web where I can get a feeling for how thrust (and motor torque) roughly scale with rotational speed and diameter - either some approximate, nominal equations or even plots? $\endgroup$ – uhoh Jul 3 '16 at 8:00
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    $\begingroup$ This site would be a good starting point, look here for example. Google is your friend (or Bing, or Yahoo, or whatever you prefer). $\endgroup$ – Peter Kämpf Jul 3 '16 at 8:10
  • $\begingroup$ 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 '16 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 '16 at 14:52

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