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Sunday, November 28, 2021

How do drones really fly?

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A flying quadcopter drone with an attached camera.
Dmitry Kalinovsky / Shutterstock.com

Multirotor drones they are now common 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 is simple!

How helicopters fly

A blue helicopter displayed on a white background.
Photos SS / Shutterstock.com

We will start with something completely different: helicopters. It might seem like an odd detour, but knowing a little about how helicopters fly will make understanding drone flight a lot easier.

A typical helicopter has a main rotor and a tail rotor. There are other designs, but they all work to control the same forces. This is a very Basic explanation of how helicopters fly, but adequate to our objective when it comes to understanding the flight of drones.

The helicopter has a main rotor that generates a downward thrust, lifting the craft into the air. The problem is that when the rotor rotates in one direction, it exerts a force on the helicopter body (thank you Newton!) And therefore both the rotor and the helicopter body would rotate, just in opposite directions.

Obviously, this is not a good way to fly, which is why helicopters have tail rotors. This rotor generates a horizontal thrust to counteract the torque of the main rotor.

Jacob Lund / Shutterstock.com

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.

A Russian Kamov Ka-52 helicopter.
Andrey Kryuchenko / Shutterstock.com

You’re probably also familiar with the US military. CH-47 Chinook, which has two massive counter-rotating main rotors that neutralize each other’s torque while providing massive lift capacity.

A US Army CH-47 Chinook helicopter.
SpaceKris / Shutterstock.com

What does this have to do with your quadcopter? Everything!

Multi-rotor drones and the problem of torque

If you look at the basic layout of the quadcopter, you will notice that all four rotors are arranged in an X pattern. Two attachments rotate clockwise and the other two rotate counterclockwise. Specifically, the front supports rotate in opposite directions to each other and the same goes for the rear supports. As such, accessories facing each other rotate diagonally in the same direction.

The end result of this arrangement is that if all accessories rotate at the same speed, the drone must remain perfectly still with the nose fixed in place.

Use of 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 torque cancellation principle to maneuver. If you deliberately slowed down some engines and revved others, the imbalance would cause the entire ship to spin.

Similarly, if you rev ​​both rear engines, the rear of the drone would lift up tilting the entire ship 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 reduce the speed of one rotor, you also reduce its thrust and another rotor has to accelerate to compensate. Otherwise, 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, causing unwanted movement.

The only reason a quadcopter or other multirotor craft can fly is because of complex real-time troubleshooting 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 flight control systems on board calculate exactly how fast each motor must turn the rotors to achieve this.

A drone running through the air.
Harry Powell / Shutterstock.com

From the pilot’s perspective, the control inputs are the same as for any aircraft. First, we have the yaw, where the drone rotates around its vertical axis. Second, we have the pitch, where the nose of the drone tilts up or down, making it fly forward or backward. Finally, we have 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 drone movements are a combination of these movements. For example, flying diagonally is a mix of pitching and rolling at the controls. The flight controller on board does all the complicated work of figuring out how to translate a command to, for example. Tilt the nose down at specific engine speeds.

Collective vs. Fixed Pitch Rotors

There is one last important aspect of how multirotor drones fly, and it has to do with 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 cuts through the air never changes.

Propellers of a drone.
marekuliasz / Shutterstock.com

Going back to helicopters for a moment, the main rotor is typically a “collective pitch” design. Here, a complex set of links can alter the angle at which the rotors attack.

Helicopter rotor blades viewed from below.
Anupong Nantha / Shutterstock.com

If the pitch is zero (the rotor blades are flat), no thrust is generated, no matter how fast the rotor is turning. As positive pitch (thrust down) increases, the helicopter begins to lift. Most importantly, the rotors can be moved at a negative pitching position. Here, the rotor pushes upward, so the craft can descend faster than the simple pull of gravity.

Negative pitch means that, theoretically, the helicopter can fly upside down but most large-scale helicopters are too big and heavy to do this in practice. Model helicopters have no such limitation. This has led to the rise of “3D” RC helicopter flying and Mind-blowing performances from expert pilots..

With a fixed pitch rotor, the only way to increase thrust is to increase the rotor speed, unlike in a helicopter where the rotor speed can remain constant while the pitch varies. This means that the drone has to constantly speed up or slow down its rotors, it cannot fly in any attitude within 3D space, and it cannot descend faster than free fall.

Why don’t we have mass launch drones? There have been attempts like the Stingray 500 3D Quadcopter, but the complexity and cost of such a design limit it to specialized applications.

Easy to fly, not easy to fly

Multi-rotor drones like DJI Mini 2 is it so marvels of computer engineering and technology. They can only fly due to the convergence of various sciences and technologies, all so you can get some amazing clips on vacation. Now the next time you take your drone out for a spin, you’ll have a new respect for what your little one can do.

A technological marvel

DJI Mini 2 drone

This compact and lightweight drone has a solid camera and a great price.

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