4 ms·

Can you explain the physics behind why it's more efficient to have larger rotors than multiple rotors?

by eric-hu 14y ago

jackpirate 14y ago
Two reasons. First, surface area. For an equivalent surface area, one rotor results in a much smaller craft than 4 rotors. Second, precision machining. It is much easier to balance 1 set of rotors weighing hundreds of pounds than it would be to balance 4 of them. I'm guessing your typical quadcopter is too small right now for this to be a real concern.
- Ao7bei3s 14y ago
  Balancing props is a very real concern, but for another reason than efficiency: unbalanced props cause more vibration and lead to reduced image/video quality due to the rolling shutter effect (image looks wobbly). That being said, many people don't bother with propeller balancing. Size of the aircraft is a very important point, but there are other reasons besides the two you mentioned. Generally, larger motors are more efficient than smaller ones. Larger, slower, props are more efficient than smaller, faster, ones, because Thrust ~ Rotor Speed^2 but Power ~ Rotor Speed ^ 3. Why? I don't recall, sorry. I'm more interested in the software. (If you do figure it out though, let me know.)
  - sopooneo 14y ago
    Because KE is proportional to v^2, and then you get another factor of v since the volume (and thus mass) of air through the rotors per unit time is proportional to how fast the air is moving. At least that's my guess. And if I'm right, it's also why, to a first approximation, the power generated by a windmill is proportional to the cube of the wind speed.
D_Alex 14y ago
To first approximation, the power needed to generate a given thrust is inversely proporional to the rotor diameter. See here; http://en.wikipedia.org/wiki/Momentum_theory http://en.wikipedia.org/wiki/Momentum_theory
mapt 14y ago
"The primary reason a helicopter becomes less efficient as it shrinks is that the Reynolds number decreases as the scale decreases. The lower Reynolds number, a measure of the viscosity of air relative to the other forces acting on it, means the air appears thicker to whatever is passing through it, so the drag increases disproportionately compared to the lift." - http://www.ae.utexas.edu/faculty/422-dr-jayant-sirohis-research-scales-up-the-efficiency-of-miniature-aerial-vehicles http://www.ae.utexas.edu/faculty/422-dr-jayant-sirohis-resea... It's a scale-dependent effect of air.

Can you explain the physics behind why it's more efficient to have larger rotors than multiple rotors?

by eric-hu 14y ago

Can you explain the physics behind why it's more efficient to have larger rotors than multiple rotors?

Two reasons. First, surface area. For an equivalent surface area, one rotor results in a much smaller craft than 4 rotors. Second, precision machining. It is much easier to balance 1 set of rotors weighing hundreds of pounds than it would be to balance 4 of them. I'm guessing your typical quadcopter is too small right now for this to be a real concern.

Ao7bei3s 14y ago
Balancing props is a very real concern, but for another reason than efficiency: unbalanced props cause more vibration and lead to reduced image/video quality due to the rolling shutter effect (image looks wobbly). That being said, many people don't bother with propeller balancing. Size of the aircraft is a very important point, but there are other reasons besides the two you mentioned. Generally, larger motors are more efficient than smaller ones. Larger, slower, props are more efficient than smaller, faster, ones, because Thrust ~ Rotor Speed^2 but Power ~ Rotor Speed ^ 3. Why? I don't recall, sorry. I'm more interested in the software. (If you do figure it out though, let me know.)
- sopooneo 14y ago
  Because KE is proportional to v^2, and then you get another factor of v since the volume (and thus mass) of air through the rotors per unit time is proportional to how fast the air is moving. At least that's my guess. And if I'm right, it's also why, to a first approximation, the power generated by a windmill is proportional to the cube of the wind speed.

D_Alex 14y ago

To first approximation, the power needed to generate a given thrust is inversely proporional to the rotor diameter. See here; http://en.wikipedia.org/wiki/Momentum_theory http://en.wikipedia.org/wiki/Momentum_theory

mapt 14y ago

"The primary reason a helicopter becomes less efficient as it shrinks is that the Reynolds number decreases as the scale decreases. The lower Reynolds number, a measure of the viscosity of air relative to the other forces acting on it, means the air appears thicker to whatever is passing through it, so the drag increases disproportionately compared to the lift." - http://www.ae.utexas.edu/faculty/422-dr-jayant-sirohis-research-scales-up-the-efficiency-of-miniature-aerial-vehicles http://www.ae.utexas.edu/faculty/422-dr-jayant-sirohis-resea... It's a scale-dependent effect of air.