The Physics of Drones

Eliza Mahoney
Mr. Gray
Honors Physics Per. G
23 May 2017
Blog Post #7

Flying

How do drones fly? Drones use rotors for propulsion and control in their flight. A rotor can be thought of a fan, with spinning blades to push the air down. As the rotor pushes down on the air, the air pushes up on the rotor, according to Newton's third law. This is the basic idea behind the lift of the drone, which is dependent on the control of the upward and downward force. The faster the spin of the rotor, the faster the climb of the drone.

The three options a drone has in the vertical plane are to descend, hover or climb. To hover, the net force of the four rotors pushing up must be equal to the gravitational force pushing it down. This means the net force must be zero. To make the drone climb, or to increase the thrust (speed), the nonzero upward force must be greater than the weight. Otherwise known as the net force being positive. After it starts to climb, the thrust can be decreased a small amount, but now there are three forces acting on the drone: weight, thrust and air drag. So, the thrust still has to be greater than the other forces to continue the climb. Descending requires the opposite: the thrust to be decreased so it is less than the other forces, and the net force is downward, or negative. 

Turning (Rotating)

There are two sets of rotors on a drone, and with the two sets rotating in opposite directions, the angular momentum is zero. Angular momentum is very similar to linear momentum. It is calculated by multiplying the angular velocity by the moment of inertia. So, the angular momentum all depends on how fast the rotors are spinning. 

If there is no torque on the drone, then the angular momentum must remain constant. Consider one set of rotors to have a positive angular momentum and the other set to have a negative angular momentum. In a normal descent, hover, or climb, they all add up to zero. To rotate, the spin of rotors 1 and 3 must be decreased and the spin of rotors 2 and 4 must be increased. This way, the total angular momentum no longer adds up to zero but no thrust is lost and the drone remains hovering. This way, the total force remains equal to the gravitational force but the drone is rotating. Flipping the increasing of spin to rotors 1 and 3 and decreasing 2 and 4 will rotate the drone the opposite way.

Who would have thought that a drone combines many different aspects of physics in its flight? Newton's laws, torque, angular momentum, and net force are all components of keeping a drone in the air. Think about this next time you fly a drone!