Drones come in many shapes and sizes, but what we often see as multi-rotors are simple drones with four motors and propellers, also known as quadcopters.
Why is that? Why don’t we often see drones with three, five, six, or many more motors and propellers?
For small drones, four propellers are the right number to provide decent thrust to lift it up and fly, create a balance with a suitable center of gravity, and help maneuver the drone smoothly, as well as hover consistently.
But there’s much more to it, so if you’re looking to really grasp the dynamics of drone flight with four propellers, let’s dig in.
To help, we’ve identified and reviewed the best drone courses for beginners and professionals.
Quadcopters: what do we know about them
Drones, also known as UAVs, are flying devices that can be controlled via a radio transmitter or can fly autonomously.
As you might infer from the name, a quadcopter is made of four motors mounted on individual arms connected to a central frame containing the electronics. Each motor has a propeller to generate the thrust required for the drone to take off.
Due to the unique form of each propeller mounted on the drone’s electric motors, they generate a strong air current in a pusher or puller formation, which is powered by a Lithium (intelligent) battery.
A drone hosts many electronics, from the flight controller and ESC to sensors like IMU, gyroscopes, accelerometer, compass, and GPS module.
Every component is hosted in a frame where the drone arms are connected and well-balanced according to the number of propellers/motors a drone has. In this case, we’re talking about four motors.
This is the sweet spot to generate enough lift, keep the drone well-balanced, and optimize power efficiency.
A drone’s motor can be rapidly accelerated and decelerated to tens of thousands of RPMs (rotations per minute) by the drone’s flight controller.
These quadcopters with four propellers work together to provide the same amount of thrust individually for the drone to take off vertically (VTOL).
Balancing and stability
An even number of motors and propellers on a drone, four in this instance, is the sweet spot for the electronic flight controller and sensors to work all together to provide a well-balanced amount of thrust for the drone to stay stable in the air.
In the case of a drone hovering, each motor receives fluctuating currents from the drone’s flight controller for micro-adjusting while the drone is flying and hovering stably.
Drone sensors such as the inertial measurement units, GPS, and downward vision sensors play a crucial role in providing data in real-time to the flight controller.
If a drone had to have more than four motors to generate lift, this would be more difficult to acquire. If less than four propellers are used, we’re looking at unstable flights and motors over-forcing to generate lift.
Every drone is carefully built and manufactured according to the internal components to be balanced on the frame at the right point where the center of gravity falls.
If something is out of balance, some drone motors and propellers will have to overforce themselves and generate extra lift to improve balance, which is not ideal and may affect a drone’s flight.
Fine control of a drone
Controlling a drone comes from the four-axis radio transmitter joystick input, which sends the signal to the drone via the radio transmission system.
These axes are throttle, yaw, pitch, and roll. See the following image for a mode 2 radio controller.
Because we also have four motors and propellers that respond according to the pilot’s radio input, each motor should act individually or as a pair, resulting in better balance when flying.
Now, let’s put this into practice.
The drone’s controller throttle is on the middle axis, generating more than enough power to exercise liftoff.
You move the “roll” axis to the left. What happens here?
The two right motors exercise extra thrust for the drone to start drifting to the left. Micro-adjustments are also made to the left motors to maintain the same altitude.
What about when acting on the “yaw” axis?
Two opposite motors exercise extra thrust while the two other opposite motors exercise less thrust. This will create an imbalance where the drone will turn in one direction or another, depending on where you want to turn left or right while maintaining the altitude. (So the different direction spinning propellers will readjust their RPM accordingly).
Now, if we had more or less than four motors, there would no longer be perfect harmony between the input of the radio sticks, and the flight controller would have to do additional calculations in relation to every motor for these movements to be performed smoothly.
In this case, we probably wouldn’t have the same perfect stability and fine control of a drone as we have with four-propeller quadcopters.
With great power, we have great thrust.
And that’s somehow true.
But in terms of power efficiency, each drone’s motor pulls a lot of power from the battery, being the main consumer of power in a drone.
The more motors a drone has, the more power it will require for the drone to perform liftoff and fly as intended.
For instance, a 6-propeller drone, also known as a hexacopter, will require 50% more power than a quadcopter because of the extra motors.
In this case, a larger battery is needed on the hexacopter, which adds weight, whereas the motors will need more power to generate extra thrust for the drone to lift off. But it will have extra lift because of the two extra motors we have now.
It’s like an infinite cycle.
That’s why quadcopters – drones with four motors and four propellers – make the most sense in terms of aerodynamics, fine control, balance and stability, and power efficiency.
However, hexacopters and drones with more than 6 propellers also have unique usage, which we’ll cover later.
Therefore, it is important to remember that quadcopters have an optimal power-to-thrust ratio with four propellers that can be difficult to obtain in any other configuration.
Following a common design
Since drones have become widely available on the market and large companies are investing multi-millions of dollars to develop and improve drone technology, they have all followed a basic drone design – a quadcopter with four propellers.
As we have mentioned before, as it makes the most sense to simply develop drones with four propellers, this was the path most drone companies have followed as a common design.
Of course, some have experimented with two-motor drones, three-motor, and even more, but none of them have been as efficient as the current quadcopters.
When are more than four propellers used?
There have been many cases where technology was invested into developing and researching hexacopters and drones with more than six propellers that could serve unique cases.
Those types of drones are very large in size, sometimes weighing more than 20 kilograms (where standard quadcopters are usually between 250 grams and 1kg).
Four motors and propellers are generally unable to generate enough thrust to lift these types of large drones, and even if they could, they would have to be of exceptional size, disproportionate to the design and the drone’s aerodynamics.
That’s why, even with the greater power requirements, drones with more than four propellers are used to carry larger payloads and, in cinematography, to carry professional-level cinema cameras.
Moreover, with more propellers, it is easier for them to change speed and maneuver smoothly when we’re dealing with very heavy drones.
Normal drones would probably never be able to exert this amount of thrust to generate lift and even keep everything in balance.
If we’re looking at some people-lifting drone designs, we can see that some of them have 6, 8, 12, or even more arms.
But here’s the thing. In many scenarios, we have two motors with two propellers on each arm. One has the role of a pusher while the other is a puller, so they work together to exercise nearly double the amount of thrust per arm with a single motor and propeller.
So, it’s not always true that hexacopters have six propellers mounted on six motors. For the extra thrust required, they can have six arms with 12 motors and propellers.
We have to raise a bit of awareness. Drones are dangerous if not handled well, and in this case, it doesn’t matter if a drone has four, six, or more propellers.
Each drone propeller can spin at speeds of tens of thousands of rotations per minute to generate enough thrust to lift the drone.
These drones’ propellers are like blades. They can do major damage to objects, hurt people, or worse. Drone propellers rotating at these speeds can even be lethal.
Furthermore, as they are built from rigid plastic, these propellers have to be strong enough to literally lift a drone, and the heavier the drone, the more rigid the propellers have to be.
But at times, drone propellers can fail even mid-flight, and the drone will face an imminent crash. That’s something we cannot control other than doing a pre-flight checklist and ensuring the propellers are not bent or dented.
The more propellers a drone has, the higher the chances of a drone propeller failing mid-air just due to the greater total number.
That’s why we can say that, generally, lightweight drones with four propellers are less likely to fail mid-air than heavier drones with more than four propellers.
But this is not always the case.
For instance, the DJI Avata is known to have poor propeller resistance, and many have complained that these propellers started to “explode” mid-air, causing the immediate crash of the Avata. Personally, with this specific drone, I always go with reliable 3rd party propeller options.
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How do drone propellers spin?
In a quadcopter with four propellers, you may think that all of them spin in the same direction. But that’s not correct.
Let’s have a look at the following image, shall we?
This image is simply illustrative, but I hope you get the point.
The front left and front right propellers can spin inwards, and the same has to happen with the back left and back right propellers. This method is called “props in.”
But they can also spin outwards, also termed props out, but that’s not a rule.
However, opposite propellers have to spin in the same direction, the opposite way from the other two propellers. Otherwise, the drone will flip over when trying to take off, and it won’t fly.
With six and multiple propellers, that’s another story.
And see what would be the issue if a drone had three or five propellers?
It would be totally unbalanced when flying, causing the drone to rotate unless, for instance, one propeller spins at double the speed of the other two propellers in a tri-copter. It’s just simpler to create a quadcopter, though.
That’s why it is especially crucial that drone designs have an even number of propellers to generate lift and balance the drone properly.
» MORE: How Long Drone Propellers Last?