Ever wondered how do quadcopters work? The basics are really very simple! In this article we explain the physics of quadcopter flight and the role each of its components play in making it happen.
Physics of Quadcopter Flight
The Thrust and Weight Forces
Each of the rotors (propellers) of a quadcopter drone generates an amount of thrust proportional to the speed at which the respective rotors are rotating. Yes, the design of the propeller also affects the amount of thrust generated, but since we cannot influence that in flight, it is considered separately.
Together, the rotors provide sufficient total thrust (TT) to support the weight when the vehicle is in the hover.
By increasing or decreasing the TT, the vehicle will ascend or descend since weight remains constant (in this demonstrative example).
The quadcopter flight controller can accelerate the RPM of the rotors on one side, causing the vehicle to roll towards the opposite side.
The total thrust vector tilts in as well, and the horizontal component of the vector “drags” the vehicle into that direction.
Pitching and Rolling
The same principle applies for whichever direction you want your device to move towards, whether its left, right, forward, backwards or skew. And, as an added bonus, your drone calculates automatically which rotors need speed adjustment, and how much, for any given set of circumstances and required movement.
There is an interesting catch to all of this, though. If the rotors would all spin in the same direction, the body of the drone would want to react by rotating in the opposite direction.
For this reason, the direction of rotation of the opposite propellers are designed to be opposite as well, so that the rotational moment can be cancelled out.
More about Propellers
This opposite rotation of propellers has an important implication. It means that the propeller pairs would be so constructed that they are mirror images of each other. This explains why your newly purchased drone might have come with two pairs of spare propellers.
The discussion above also implies that we can control the yaw by varying the speed of one of the propellers. However, this also means we have to take extreme care when replacing a damaged propeller, and make sure that the right propeller is fitted on the right motor!
Mentioned before, a rotating propeller blade produces thrust used to overcome weight to make your pretty quadcopter fly. This thrust is proportionally equivalent to the RPM, so, the higher the RPM, the faster your drone can climb. But, also, the higher the RPM, the more drag is produced, a force which opposes the rotational speed of the propeller.
There is another factor to consider. The thrust produced is also a function of the size of the propeller. If we would compare motors running at the same RPM with different size propellers fitted, the thrust produced would progressively increase with bigger propellers, but only up to a certain point. With large propellers beyond this critical value, the motor would start spinning slower due to a drastic increase in drag. The thrust produced would reduce exponentially.