How Drones fly? – The complete explanation

Drone-Moving

In a nutshell, drones fly using basic flight principles in complex ways.

To understand how drones fly, we need to understand how propellers work.

There are a few basic physics concepts that will help us to understand this, so before understanding how drones fly, we need to revisit those.

I promise to make it super easy to understand and use as many tools as I have to explain these physics concepts.

Also, I’m going to be using the terms flight and lift a lot, and while these could seem to mean the same, they refer to different things.

Flight is the act of flying, just like birds, airplanes, helicopters, drones, and even paper planes do.

Lift is an element of flight, and in order to understand what makes drones fly, this is the first concept that we need to review.

What is lift?

Lift is defined as the mechanical force that directly opposes the weight of a solid object moving through a fluid, it is also one of the four forces of flight. In this case, the fluid is the air, and the solid will be the propellers of our drone.

When the lift force is greater than the weight, flight is produced. Also, lift is a vertical force, so it will move the drone up or down, but in order to move sideways, thrust is what is needed.

Thrust is another of the four forces of flight and it is what helps the drone move sideways. More on this later.

Something important to know about lift is that it needs three components to exist: a solid, a fluid, and motion.

  • Without a solid, there is nothing to lift.
  • With a fluid missing, the element that produces lift by the interaction with the solid is missing, so the solid won’t be lifted.
  • Without the motion, the interaction will be there, but no force (lift) will be generated to produce flight.

What are the four forces of flight?

Simply explained, there are four forces that are present while an object flies: Lift, weight, thrust, and drag.

  • Lift: The force that takes off the flying object, pushing it up.
  • Weight: The normal weight of any object, caused by gravity and pulling it down.
  • Thrust: The force the flying object produces, pointing in the direction of motion.
  • Drag: The force acting on the flying object, pointing in the opposite direction of motion.

These four forces combined make possible what we call flight.

How flight works?

The easiest way to understand how flight works, is by looking at how airplanes fly, since airplanes are the simplest flying objects on Earth.

Of course, we all can imagine an airplane flies using its wings; however, we need to take a closer look at the mechanics of a wing to understand the basics of flight.

When explaining flight, it is very common to use only a portion of a wing, called airfoil. An airfoil is a cross-sectional part of a wing, seen from the side, used to explain how the four forces of flight interact with aerodynamic shapes.

Here is a cool animation of an airfoil moving through the air. You can see how the air impacts the airfoil and what happens if you change the angle of attack (AoA) of the airfoil, that is, the angle at which the airfoil impacts the air.

Wind effect on Airfoil

We now understand better the definition of flight and how it is produced, yet it may be hard to make the connection with drones and understand how they fly.

But knowing how flight works and with a basic understanding of the four forces of flight, we can now go back to the propellers of our drone.

What is a propeller?

A propeller is a fan-type spinning component that produces thrust.

Phantom Drone Propeller

Most drones (quadcopters) use four propellers that are driven by rotors, one by propeller. These as well are controlled by electronics that take care of everything drones need to fly; we’ll get to this later.

To understand how a propeller produces lift (remember these produce thrust), let’s see one side of the propeller and think of it as a wing.

Airfoil of Propeller

Just like a wing, a cross-sectional part of the propeller has the shape of an airfoil. This means it can produce aerodynamic forces, and everything we already know about the four forces of flight applies here too.

Thus, each side of the propeller is like the wing of an airplane, however, the propeller is a “spinning wing”, and not just that, but the propeller has two sides, so it has two spinning wings.

Drone Propeller Spinning

Now, here is the cool thing…

Remember we said earlier that to produce lift there must be three elements present?

We have the solid, the propeller; the fluid, the air; but motion is missing.

This is why propellers spin. The rotational motion equals the airplane’s wing moving through the runway. Here is where propellers show their efficiency and advantages for many applications, and in some measure, this is part of the success of drones around the world.

Propellers produce lift faster than wings. Also, propellers produce lift and thrust (propulsion) at the same time, while airplanes need propulsion (from an engine) to fly. This is why drones don’t have wings nor engines, as they don’t need them to fly.

How Drones fly the way they do (moving sideways and all that)?

(Most) drones use four propellers arranged in an ‘X’ shape. There are other configurations, but the ‘X’ shape is the most common one.

Also, the propellers on opposite corners of the ‘X’ shape, must always spin in the same direction, be it clockwise (CW) or counter-clockwise (CCW).

Propellers-Spin-Pattern

For instance, the upper right corner and lower left corner propellers may spin in CCW direction, while upper left corner and lower right corner spin in CW direction.

The pairs with fixed spinning directions can change but must always be on opposite corners and keep spinning speeds equal, otherwise the drone won’t be stable while flying.

With a configuration like the one above, a drone can move back and forth, left and right, up and down (with the use of lift) and turn left and right (we’ll get to this later).

Horizontal flight

Getting more into horizontal motion, since a drone is a symmetrical system, the same principle to move right can be applied to any direction. To achieve this, the trick is to rotate the drone on a horizontal axis, so it “points” to the right (since that is where we want it to move).

How Drones Fly

As seen in the image above, and thinking we want to move the drone to the right side, we need to increase the speed of the pair of propellers on the left side. Simultaneously, the pair of propellers on the right side need to spin slower.

This produces two effects:

  1. The drone tilts on its horizontal axis towards the right side, as a result of the difference of speeds (and the amount of thrust) of the propellers.
  2. With the drone “pointing” to the right side, a horizontal component of the lift force appears. This “horizontal lift force” (which in reality is thrust) will move the drone to the right side, only if it’s greater than the drag force (produced by the air resistance mostly).

Important things to have in mind.

  • Since we increased the speed of two propellers but decreased the speed of the other two, the weight and lift forces stay in balance, which means the drone only rotates on its axis but does not move up or down.
  • All four propellers are still spinning in pairs in opposite directions; thus, no angular momentum will be produced (the drone won’t turn left or right).
  • If drag force is still greater than the horizontal lift force (thrust), a slight increase on the speed of all four propellers will be enough to move the drone horizontally.

As mentioned earlier, this process is the same to move the drone in any horizontal direction.

What about turning the drone left or right?

Drone Turning

Drones can also rotate without moving sideways. When I say rotate, I mean turning left or right, kind of what you do with your neck.

In order to do this, drones need to use something called angular momentum, which is nothing more than the term used in physics to describe rotational motion.

Again, the trick is in the speed of the spinning propellers.

For instance, turning the drone to its left side (CCW), requires the pair of propellers spinning CW to slow down, and the pair spinning CCW to speed up.

Thus, the resulting angular momentum will favor the CCW rotation and thus the drone will turn left.

Notice lift and weight forces are in balance, since two propellers slowed down but two more sped up, so the drone won’t move up or down.

Also, the propellers that changed speed are on an ‘X’ shape, which prevents the drone to lean to any side, thus avoiding horizontal motion.

The same analysis we did here is true if we want to turn the drone to the right side.

How drones change the speed of propellers?

So far, we’ve only spoken about how drones fly in terms of the mechanic forces that are produced by the propellers, but in order for those propellers to work the way they do, we also have to explore the electronics of the drone.

like I anticipated earlier, propellers are driven by motors, which at the same time are controlled by electronics inside the body of the drone.

Going deeper into the electronics of a drone

Let me briefly describe these components to have a better understanding of how drones fly.

Drone motors

Drone Motor

These are very interesting devices which in essence are electromagnets. Each propeller uses one motor and the main function of these is to make the propeller spin in the required direction and speed.

Brushed and Brushless drone motors

There are brushed and brushless motors for drones. The difference between them is in how they are built, which results in a difference in how they work and perform.

Brushless motors are the industry preferred ones because of their efficiency and lifetime. They cost a bit more, but the difference is not that noticeable these days, and you can find these even in drones under 200 or 300 dollars.

On the other hand, brushed motors are simpler in construction and strictly DC motors. The brushes on these tend to wear and with time they lose power and efficiency. Lifetime is also an issue for brushed motors. These are the cheapest option, but today are used only in entry level drones.

Brushed vs Brushless Motors

Electronic Speed Controller (ESC for short)

In the case of brushless motors, an electronic speed controller or ESC for short, is required. The speed of these motors is controlled by the voltage they get. The more voltage, the faster the motors spin. Brushed motors don’t require an ESC.

ESC

An ESC will deliver the amount of voltage required to change the speed of a motor (and ultimately the propellers), depending on what the pilot wants the drone to do.

Thus, ESCs are responsible for the changes of speed that produce all the drone movements that we explored before.

Each motor requires an ESC, and that is how motors’ speed can be controlled individually.  This is especially helpful in advanced drones with obstacle avoidance sensors.

When an obstacle is detected, the drone may require to slightly decrease or increase the speed of only one motor to avoid an obstacle.

However, ESCs are controlled as well by another device, the flight controller.

The flight controller

This is the “brain” of the drone. The flight controller is responsible for taking decisions about everything the drone has.

Drone Flight Controller

You can think of the flight controller as the central unit that has inputs and outputs, taking decisions all the time to ensure a stable flight, take off, landing, and overall operation.

The flight controller receives inputs from sensors on the drone, the receiver (which we’ll discuss later), GPS module and the gyro. Other electronics may be present, but these are the basic ones.

With these inputs, the flight controller processes all data and gives instructions to each part of the drone. These instructions are called, outputs.

The most common outputs of the flight controller go to the ESCs (and ultimately the motors), the camera, the gimbal of the camera, and video transmitter.

The transmitter/receiver

Transmitter and Receiver

You may know that a drone works using a remote control. Well, that remote control also has a transmitter, which is the component on the remote control that transmits, via radio frequency, the instructions (such as moving to the right or left) to the drone.The drone has to have a something to receive that signal and interpret it for the flight controller. This something is called: receiver.

The receiver is the device, inside the drone, that receives the radio frequency signal from the transmitter, which is inside the remote control, and decodes the signal to pass it on to the flight controller.

This is how the flight controller knows the instructions from the pilot and determines what to do, based on the input from the receiver and all the other we described here.

The GPS Module

Just as the name describes, the GPS module helps locate the drone using a GPS. This is important for some advanced drones in order to be able to return to home, or follow a pre-programmed path, take pictures, and then return to home.

Drone GPS Module

The GPS also helps the drone when a sudden gust of wind hits it but does not crash it. In this case, the drone knows its position thanks to the GPS module.

The Gyro

Drone Gyro

This is another electronic device that helps the drone stabilize flight. If you are familiar with gyroscopes, you know these are devices that are used to maintaining and measuring orientation and angular velocities.In physics it is often represented as a spinning disc in which the axis of rotation can change to any orientation by itself.

For drones, gyroscopes or gyros are electronic devices that do the exact same thing, help to measure and maintain orientation and angular velocity.

Last words on how drones fly

As you can see, to understand how drones fly, a lot of concepts are needed. Also, I tried to make this writing as clear and comprehensive as possible.

We didn’t touch anything in terms of math or equations because the intention of this post was to explain this as simple and as useful as possible.

Yet, should you need more information on the equations related to flight, let me know in the comments section and I can help with that as well.

Also, let me know what you think of this post, did I miss something or is there something you’d like to see here? Post a comment with your thoughts as I’ll be thrilled to learn about you.

Last but not least, and if you enjoyed this post, do me a favor and share it on your social channels so that more people know about this resource.

Till the next one!

Leave a Comment