6 ms·

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 t

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.

mikeash 8y ago
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.
- toufiqbarhamov 8y ago
  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!
  - mikeash 8y ago
    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.
    - toufiqbarhamov 8y ago
      You’re right, I was looking at max motor rpm, whoops!

mikeash 8y ago

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.

toufiqbarhamov 8y ago
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!
- mikeash 8y ago
  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.
  - toufiqbarhamov 8y ago
    You’re right, I was looking at max motor rpm, whoops!