Multirotor drones are now commonplace and advanced enough that anyone can fly them, yet most people probably don’t understand how they stay in the air. Understanding basic drone flight physics can make you into a better drone pilot. It’s simple!
How Helicopters Fly
We’ll start off with something completely different: helicopters. It might seem like a strange detour, but knowing a bit about how helicopters fly will make understanding drone flight much easier.
A typical helicopter has a main rotor and a tail rotor. Other designs do exist, but they all work to control the same forces. This is a very basic explanation of how helicopters fly, but appropriate to our goal when it comes to understanding drone flight.
The helicopter has a main rotor that generates thrust in a downwards direction, lifting the craft into the air. The problem is that as the rotor turns in one direction, it exerts a force on the helicopter body (thanks Newton!) and therefore both the rotor and helicopter body would spin, just in opposite directions.
This is obviously not a great way to fly, which is why helicopters have tail rotors. This rotor puts out horizontal thrust to counteract the torque from the main rotor.
There are tailless helicopters with other anti-torque systems, such as the Russian Kamov Ka-52, which uses two main rotors spinning in opposite directions, known as a coaxial arrangement.
You’re probably also familiar with the US Army CH-47 Chinook, which has two massive counter-rotating main rotors that neutralize each other’s torque while also providing massive lift capacity.
What does this have to do with your quadcopter? Everything!
Multirotor Drones and the Torque Problem
If we look at the basic quadcopter’s layout, you’ll notice that the four rotors are arranged in an X pattern. Two props turn in a clockwise direction and the other two in a counter-clockwise direction. Specifically, the front props spin in opposite directions to each other and the same is true of the rear props. As such, props that are across from each other diagonally spin 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 stay perfectly still with its nose fixed in place.
Using Torque and Thrust to Maneuver
If you don’t want to keep the drone’s nose fixed in one position, you can use this torque-canceling principle to maneuver. If you purposefully slowed down some motors and sped up others, the imbalance would cause the whole craft to turn.
Likewise, if you sped up the two rear motors, the back of the drone would lift up 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 a rotor down, you also reduce its thrust and another rotor has to speed up to compensate for it. If not, the total thrust would decrease and the drone would lose altitude. However, if you increase the thrust of a rotor it causes the drone to tilt more, which causes unwanted movement.
The only reason a quadcopter or other multirotor craft can fly is thanks to 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 work out exactly what speed each motor should spin the rotors to achieve it.
From the pilot’s perspective, the control inputs are the same as for any aircraft. First, we have yaw, where the drone turns around its vertical axis. Second, we have pitch, where the drone’s nose pitches up or down, making it fly forward or backward. Finally, we have roll, where the drone moves side-to-side. Of course, you also have control over the amount of thrust, which changes the altitude of the drone.
All 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 onboard flight controller does all the complicated work of figuring out how to translate a command to, for example. pitch the nose down into specific motor speeds.
Collective vs. Fixed Pitch Rotors
There’s one last important aspect of how multirotor drones fly, and that has to do with the rotors themselves. Almost all drones that you can buy today use “fixed pitch” rotors. This means that the angle at which the rotor blade slices into the air never changes.
Going back to helicopters for a moment, the main rotor is typically a “collective pitch” design. Here, a complex set of linkages can alter the angle at which the rotors attack.
If the pitch is zero (the rotor blades are flat) then no thrust is generated, no matter how fast the rotor is spinning. As positive pitch (throwing thrust downward) is increased, the helicopter begins to lift. Most importantly, the rotors can be moved into a negative pitch position. Here, the rotor is thrusting upwards, so the craft can descend faster than the mere pull of gravity.
Negative pitch means that, theoretically, the helicopter can fly upside down but most full-scale helicopters are too big and heavy to do this practically. Scale model helicopters have no such limitation. This has lead to the rise of “3D” RC helicopter flight and mind-bending performances by skilled pilots.
With a fixed-pitch rotor, the only way to increase thrust is to increase rotor speed, unlike a helicopter where the rotor speed can remain constant while pitch varies. This means the drone has to constantly speed up or slow down its rotors, can’t fly in any attitude within 3D space, and can’t descend faster than freefall.
Why don’t we have collective-pitch drones? There have been attempts such as the Stingray 500 3D Quadcopter, but the complexity and cost of such a design limit it to specialist applications.
Easy To Fly, Doesn’t Fly Easily
Multirotor drones like the DJI Mini 2 are marvels of engineering and computer technology. They can only fly because of a convergence of various sciences and technologies, all so you can get a few awesome clips on vacation. Now, the next time you take your drone out for a spin you’ll have a new respect for what the little guy can do.
A Technological Marvel
DJI Mini 2 Drone
This lightweight, compact drone has a solid camera and a great price.