Dot Physics: The Crazy Science of Drone Flight

Drones.

They're everywhere now.

And maybe somewhat annoying.

I bet you didn't know how a quadcopter works.

First let's start off with a hovering drone.

Two rotors rotate clockwise

and two counterclockwise.

By rotating in opposite directions,

this makes a total anchored momentum zero

so that the drone as a whole will stay stationary.

To hover, all motors push up the same amount.

Now, what if you wanna accelerate the drone forward?

In this case you need to tilt the drone forward

so that there is a component of thrust pushing forward.

To do this, decrease the thrust in the front two motors

while increasing the thrust in the back two.

Voila, forward momentum.

Do something similar to accelerate back or sideways.

But how do you turn?

This is the fun part.

It has to do with the conservation of angular momentum,

which is the measure of the object's rotational motion.

If I decrease the rotation of the two clockwise motors,

the drone would have to rotate clockwise

in order to keep the total angular momentum at zero.

Actually, I need to also increase the rotation

of the counterclockwise motors

to keep the drone from falling.

But technically it's not just angular momentum,

there's also a drag torque on the props.

Where torque is like a rotational force

that changes the angular momentum.

In the end,

a drone can be controlled by just varying the speed

in these four motors.

No complicated mechanical parts,

but of course it's still complicated enough

that you need a computer to translate

your control inputs into manipulations of their motors.

So, that's the physics of how to annoy someone

with a flying machine.