Multirotor drones are now commonplace and advanced enough that anyone can fly them, but most people probably don’t understand how they stay in the air. Understanding the basic physics of drone flight can make you a better drone pilot. It’s simple!
How helicopters fly
We’re going to start with something completely different: helicopters. It might seem like a strange detour, but knowing a little bit how helicopters fly will make it much easier to understand the flight of drones.
A typical helicopter has a main rotor and a tail rotor. Other designs exist, but they all work to control the same forces. It’s a very basic explanation of how helicopters fly, but appropriate for our purpose when it comes to understanding drone flight.
The helicopter has a main rotor that generates downward thrust, lifting the craft into the air. The problem is, when the rotor turns in one direction, it exerts a force on the body of the helicopter (thanks Newton!)
This is obviously not a great way to fly, which is why helicopters have tail rotors. This rotor produces horizontal thrust to counter the torque of the main rotor.
There are tailless helicopters with other anti-torque systems, such as the Russian Kamov Ka-52, which uses two main rotors rotating in opposite directions, known as a coaxial arrangement.
You’re probably also familiar with the US Army’s CH-47 Chinook, which has two huge counter-rotating main rotors that neutralize each other’s torque while providing massive lift capacity.
What does this have to do with your quadcopter? All!
Multirotor drones and torque problem
If we look at the basic quadcopter layout, you will notice that all four rotors are arranged in an X. Two props rotate clockwise and the other two rotate counterclockwise. Specifically, the front attachments rotate in opposite directions to each other and the same goes for the rear attachments. As such, props that face each other turn diagonally in the same direction.
The end result of this arrangement is that if all the props are spinning at the same speed, the drone should remain perfectly still with its nose fixed in place.
Using torque and thrust to maneuver
If you don’t want to keep the nose of the drone fixed in one position, you can use this principle of torque cancellation to maneuver. If you were to deliberately slow down some engines and speed up others, the imbalance would spin the whole machine.
Likewise, if you accelerated the two rear motors, the rear of the drone would lift, tilting the whole craft forward. This is true for a pair of rotors, so you can tilt the craft in any cardinal direction.
There are problems with this approach! For example, if you slow down a rotor, you also reduce its thrust and another rotor must speed up to compensate for it. Otherwise, the total thrust would decrease and the drone would lose altitude. However, if you increase the thrust of a rotor, the drone tilts more, causing unwanted movements.
The only reason a quadcopter or other multirotor craft can fly is the complex real-time problem solving performed by the hardware that controls it. In other words, when you tell the drone to move in a particular direction in 3D space, the onboard flight control systems determine exactly how fast each motor has to spin the rotors to achieve it.
From the pilot’s point of view, the control inputs are the same as for any aircraft. First, we have the yaw, where the drone spins around its vertical axis. Second, we have pitch, where the nose of the drone tilts up or down, causing it to fly forward or backward. Finally, we have the roll, where the drone moves from side to side. Of course, you also have control over the amount of thrust, which changes the altitude of the drone.
All the movements of the drone are a combination of these movements. For example, flying diagonally is a mixture of pitch and roll on the controls. The on-board flight controller does all the complicated work of figuring out how to translate a command, for example. nose down at specific engine speeds.
Collective or fixed-pitch rotors
There is one last important aspect of multirotor drone flight, and that concerns the rotors themselves. Almost all of the drones you can buy today use “fixed-pitch” rotors. This means that the angle at which the rotor blade slices through the air never changes.
Getting back to helicopters for a moment, the main rotor is generally a “collective pitch” design. Here, a complex set of links can change the angle of attack of the rotors.
If the pitch is zero (the rotor blades are flat), no thrust is generated, regardless of the rotational speed of the rotor. As the positive pitch (downward thrust) increases, the helicopter begins to lift. More importantly, the rotors can be moved in a negative step position. Here, the rotor pushes upwards, so that the craft can descend faster than the mere force of gravity.
A negative pitch means that, theoretically, the helicopter can fly upside down, but most full-scale helicopters are too big and heavy to do so in practice. Model helicopters have no such limitation. This led to the rise of “3D” RC helicopter flight and the mind-blowing performance of skilled pilots.
With a fixed pitch rotor, the only way to increase the thrust is to increase the rotor speed, unlike a helicopter where the rotor speed can remain constant while the pitch varies. This means that the drone must constantly speed up or slow down its rotors, cannot fly in any attitude in 3D space, and cannot descend faster than free fall.
Why don’t we have collective pitch drones? There have been attempts such as the 3D Stingray 500 quadcopter, but the complexity and cost of such a design limits it to specialized applications.
Easy to fly, does not fly easily
Multirotor drones like the DJI Mini 2 are marvels of engineering and computer technology. They can only fly thanks to the convergence of various sciences and technologies, all so that you can get some awesome clips on vacation. Now the next time you take your drone out for a ride, you’ll have a new respect for what the little guy can do.
A technological marvel
DJI Mini 2 Drone
This lightweight and compact drone has a sturdy camera and a great price.