## Monday, August 30, 2021

### Working principle of a drone or quadcopter

In this article we will discuss the working principle of a drone or quadcopter. How a drone or quadcopter rotates itself without losing its balance? How it move forward or backward? We will discuss each of them one by one. So let us start!

Motion of drone in vertical direction

For lift or motion in vertical direction, drones use rotors. A rotor is like a fan, because they both work in the same way. Spinning blades of rotors push air down. According to Newton’s third law, every action has an equal and opposite reaction, and since the rotor pushes down the air down, the air pushes the rotor up. So this is how the upward and downward force is controlled in any drone.

When we talk about motion in vertical direction, then a drone can do three things – hovering, climbing, or descending. To hover the drone, the net thrust of the four rotors pushing the drone up must be equal to the downward gravitational force which is pulling it down.

For climbing, the thrust or speed of the four rotors is increased so that there is an upward force that is greater than the weight of the drone. For descending of the drone, the rotor thrust or speed is decreased so that the net force is downward.

Forward and Sideway movement of drone/Quadcopters

To fly a drone in forward direction, a forward component of thrust from the rotors is needed. So the question is how will a drone get this forward component of thrust. Well that’s very easy to achieve. If we increase the rotation rate of the rear rotors (let us name them 3 and 4) and decrease the rotational rate of rotors forward rotors (1 and 2), then the drone will move in forward direction. The total thrust force is still equal to the weight of the drone, so the drone will stay at the same vertical level it was before, means it will not climb or descend

Also, since one of the rear rotors is spinning counterclockwise and the other one is rotating in clockwise direction, the increased rotation rates of those rotors will still produce zero angular momentum. In the similar way the front rotors of drone will generate no angular momentum, and hence the drone will not rotate.

In any drone one pair of rotors diagonally opposite to each other move in same direction (let us name them 1 and 3) while the other pair of rotors (2 and 4) moves in another direction. To rotate the drone in any direction, what we do is we decrease the spin of two rotors diagonally opposite to each other (1 and 3) and increase the spin for other two rotors (2 and 4).

Earlier when all the rotors were moving with same rotational speed then the total angular momentum was 0. But now due to the change in rotational speed of the rotors of two opposite pairs, the combined angular momentum of all 4 rotors will be non-zero. But according to conservation of angular momentum, if no external torque is applied on any body, then the total sum of angular momentum should remain constant, which for the case of drone is 0. So to make the sum of angular momentum 0 again, the drone must rotate in opposite direction. That’s it.

Note that since rotational speed of rotors diagonally opposite to each other (1-3 and 2-4) is same, the total force is still equal to the gravitational force, and the drone will continue to hover and the drone can still stay balanced.

There are many instruments that are attached in any drone such as built-in onboard camera (shown below), accelerometer and altimeter. Onboard camera is used to see where the drone is flying without having a direct line of sight to the drone. An accelerometer feeds the drone information about its speed and direction, whereas an altimeter tells the system about its altitude "DJI Phantom 4 quadcopter drone UAV" by Andri Koolme is licensed under CC BY 2.0  