Elon Musk had a similar argument against, I think, flying cars on Joe Rogans show. I guess it is one of those things were imagination is greater than reality. Like a lot of space stuff (mostly in regards to distance).

Though I guess high and light applications would disturbe less.

Is Active noise canceling an option?

Active noise canceling is equivalent to beam shaping. It can work in headphones because you can aim the minima at the ear and the maxima away from it, but the best you could achieve by adding speakers to a drone would be selecting where the energy hits. Directing everything to the upper hemisphere would be nice, but that's not how beam shaping works.

On the ground? To an extent, but it would be expensive to deploy and maintain. On the drone? No, you’re adding weight and reducing available power.

I thought iancmceachern asked a really good question. I was curious if it could work even theoretically.

I read the Wikipedia article on Active Noise Control. The third paragraph under Explanation: https://en.wikipedia.org/wiki/Active_noise_control#Explanati... mentions that if it's at the source, it needs to be just as loud as the source (as opposed to being by the ear, in which case even a whisper might be enough).

In addition to being as loud as the source, you need to blast it in all directions. So I guess it's hard/impossible to perfectly hit the same sound waves in all directions. If you ever miss, you'll just be doubling the noise.

Maybe that's why you'll be hard-pressed to find anything in a fixed installation that uses active noise cancellation at the source - even if it's heavy, periodic, and uses a lot of power anyway - such as a home air conditioner. Noise cancellation close to the ear seems much easier for this reason, and maybe at the source it's just impossible.

Not impossible, just impractical unless silence is incredibly important. If you want a quiet helicopter then you do what you can to reduce the noise signiture, then you might consider destructive interference. You wouldn’t use speakers, but that’s not the only way to produce a noise. In fact a clever country might even figure out how to use an arrangement of blades and fans designed to produce noise in such a way that it tends to cancel out the loudest contributions to the signature.

Buuuut... that’s not really viable for anything civilian or even commercial.

Like the Stealth Black Hawk helicopters that were used in the Osama Bin Laden raid!

(There existence is still not officially acknowledged of course, but many pictures exist.)

I was very curious about the technology they used to reduce the noise, as black hawks are very loud.

I believe I read that fan noise is chaotic, and hence impossible to cancel. Don't quote me on that, though.

This right here sounds like overengineering.

What makes a propellor different from a wing in this respect? Is it just because you have several of them next to each other and interfering with each other?

It’s a combination of factors, one of which is that a prop is just out there, unshielded like the fans inside a jet engine (which still make a ton of noise in the case I’d high bypass models). Then there’s cortex shedding, which is mostly occurring along edges and tips. Airplanes also tend to fly at much higher altitudes except on takeoff and landing, or their noise would be an issue too. Finally you’re right that the number and arrangement of blades matters, because each blade produces some turbulence, vortices, etc which then interact with the next blade in sequence. All told the issue is just s high degree of aerodynamic noise. You get that with a wing and any aerodynamic body, and really if you’ve ever seen model jets fly you’ve heard the racket they make.

The major issue is that until drones came around, you didn’t have to worry very often about large number of high performance aircraft operating throughout civilian airspace, at very low altitudes in crowded areas.

A plain wing is way less noisy, though. Construct a glider with the same mass as a typical quadcopter and you’ll be hard pressed to hear any noise from it, despite the fact that the wing is unshielded and has edges to shed vortexes from.

Oh sure, but the glider is moving through the air at a stately pace (generally between 50-95mph), shedding vortices without anything coming behind them to crash into them. A prop operates at much higher speeds, typically between 450-720mph!

Really? I arbitrarily looked up the DJI Phantom 3 and found a max RPM of about 8000 with a prop diameter of 9 inches, which gives a tip speed of about 200 MPH at the most.

That’s still pretty fast, of course. That points to using bigger props as a potential way to cut noise.

You’re right, I was looking at max motor rpm, whoops!

A lot of things: the high rpm motor powering it, the combined area of the blade edge that is higher than the area of the wing edge, and the higher pressure differential between the air behind the propeller compared to the air in front of it. The sound results from this pressure differential collapsing so anything that increases the pressure differential will create more sound.

Both propellers and wings are fundamentally the same thing: airfoils.

However, air passes over a propeller much, much faster than over a wing, and creates shock waves in a way that wings don't.

Does it? The same airfoil needs the same speed to provide a given amount of lift, regardless of whether it’s a wing or a propellor.

The speed of a propeller is low close to the rotor, and sometimes faster than sound at the extremity of the blades. Can’t reduce sound on that.

Propeller efficiency takes a nose dive when the tips go supersonic. Most props therefore do not operate in this range.

Wings don't rotate and thus don't efficiently produce sound waves.