how can a four-propeller drone spin in midair, parallel to the ground plane, when all of its propellers are pointing straight down?
In: Physics
I used to have drones. They move by fixed blades (different from helicopters) with different rotating speeds. And the 4 blades atleast on the quadricopter drone rotate this way:
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-> <- This way they counter each other and the drone stays completely stable.
Each propellor as it spins imparts a torque, or twisting moment to the drone. Same as on a helicopter. Except a helicopter has either a tail rotor that pushes sideways to keep it from spinning, or a contra-rotating second propellor (err blades, whatever) that go in the opposite direction to counteract the spin.
On a drone, the twisting of each prop is cancelled out by the twist from the others. But by varying the speed of one or more prop in relation to the others, you can use the torque differential to spin the drone around.
Actually each corner or “pod” on a drone typically has two contra-rotating props to cancel the torque of one another, but the idea for the whole system is the same. By varying the relative speeds of different props you can muck with the balance of torque between them and twist the drone about.
A simple explanation:
Sit in an office chair, put your arms out. Then quickly twist so your arms go clockwise. The base of your body spin counterclockwise.
In a helicopter the blades spin one way, and the body will try to spin the other. This is what the tail rotor prevents.
There are some helicopters that don’t use a trail rotor. The Chinook has two rotors, the second one spins the opposite direction to balance it out.
In a drone, two rotors spin right, the others spin left. If they do so properly the drone doesn’t rotate.
By changing the rotation rate, slowing one rotor down you can unbalance it and the craft will spin a bit.
You can tip the craft in any direction by changing the speeds a bit to. Two in front spin slower, craft tips forward. It doesn’t rotate since those two are spinning opposite directions and are working in sync.
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