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There have been, it seems, a proliferation of quad-copters commercially available. Amazon seriously tried to use them for deliveries. (Search Drone on their website.) The NFL uses one for cameras.

There are dual bladed copters (helicopters!) triple, and quadruple bladed vehicles available. However, does any of these designs have inherit advantages?

Is one inherently (or often) more stable? Are any of these designs more energetically efficient than the others? Does one scale more robustly with load?

It's easy to imagine loose mechanical analogies for the behavior (stability) of one or two bladed vehicles - I know what a stool is (for example.) However, the closest I got to seriously studying fluid flow was Engineering Mechanics II.

I actually want to understand what's happening. Can the quadcopter use weaker blades, since it has four working together? It seems power consumed would rise faster than efficiency...

Thanks in advance. I may or may not understand derivations, but it would be cool to see results that describe what's actually happening! I'd love to build one on a hobby level.

I'll be appreciative of as much detail as you can throw at me - I have a few days off and others might like it too!

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  • $\begingroup$ a quadcopter is less stable than a helicopter due to the fact that there are more spiny things and it requires more thrust and has more inertia. It's only advantage is it's much simpler to make. $\endgroup$
    – tox123
    Jan 18 '15 at 23:17
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    $\begingroup$ Think complexity. With a single rotor you have to have a complex mechanism like rotor disk and tail rotor. With a quad copter, you can just use differential speeds on each rotor to control. $\endgroup$
    – BowlOfRed
    Jan 18 '15 at 23:21
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    $\begingroup$ This article looks helpful. As others said, less stable and less efficient, but has the advantage of "mechanical simplicity". $\endgroup$
    – Hypnosifl
    Jan 18 '15 at 23:54
  • $\begingroup$ For efficiency, the longer and thinner the blades, the better. Smaller rotors are less efficient, thus less flying duration. If you have an odd number of lifting rotors, you have net torque, so you need a tail rotor. Planes with lifting rotors can hover, but planes with wings have to fly in circles if they want to stay in one place. Put all those considerations together. $\endgroup$ Jan 20 '15 at 2:40
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If you just give something one rotor it will spin out of control. On a helicopter there must be either a small second rotor or some kind of control surface that uses the wind generated by the rotor in order to counter the torque. It also needs to be able to increase or decrease it, for if you do want to spin the helicopter, and it needs to be able to generate torque on the other axes in order to stabilize it. You only need one rotor, which probably makes it cheaper for big ones, but it's complicated.

If you have four rotors, it's much simpler. You have two rotating clockwise and two counterclockwise. That way, there's normally no torque. If you want to turn clockwise, you slow the clockwise ones and speed up the counterclockwise ones. Reverse that to turn counterclockwise. If you want to lean to one side, slow the rotor on that side and speed up the one on the opposite side. Since those are both spinning in the same direction, it doesn't add any torque.

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