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How Do Quadcopters Work?

How Do Quadcopters Work?

Ever wondered how do quadcopters work? The basics are really very simple! In this article we explain the physics of quadcopter flight and the role each of its components play in making it happen.

Physics of Quadcopter Flight

The Thrust and Weight Forces

Each of the rotors (propellers) of a quadcopter drone generates an amount of thrust proportional to the speed at which the respective rotors are rotating. Yes, the design of the propeller also affects the amount of thrust generated, but since we cannot influence that in flight, it is considered separately.

Together, the rotors provide sufficient total thrust (TT) to support the weight when the vehicle is in the hover.

Total Thrust (T) is equal and opposite to Weight (W) when the quadcopter is in the hover

FIG 1: Total Thrust (TT) is equal and opposite to Weight (W) when the quadcopter is in the hover

By increasing or decreasing the TT, the vehicle will ascend or descend since weight remains constant (in this demonstrative example).

The quadcopter flight controller can accelerate the RPM of the rotors on one side, causing the vehicle to roll towards the opposite side.

FIG 2: Accelerated RPM on one side results in the quadcopter tilting towards the side with the lesser thrust

The total thrust vector tilts in as well, and the horizontal component of the vector “drags” the vehicle into that direction.

FIG 3: The thrust components tilts, and the horizontal component R2 causes lateral displacement

Pitching and Rolling

The same principle applies for whichever direction you want your device to move towards, whether its left, right, forward, backwards or skew. And, as an added bonus, your drone calculates automatically which rotors need speed adjustment, and how much, for any given set of circumstances and required movement.

Yawing

There is an interesting catch to all of this, though. If the rotors would all spin in the same direction, the body of the drone would want to react by rotating in the opposite direction.

FIG 4a: Propellers spinning in the same direction results in a rotational moment in the opposite direction

For this reason, the direction of rotation of the opposite propellers are designed to be opposite as well, so that the rotational moment can be cancelled out.

FIG 4b: The rotational moment of the vehicle is cancelled by placing clockwise and counter-clockwise rotating motors opposite each other

More about Propellers

This opposite rotation of propellers has an important implication. It means that the propeller pairs would be so constructed that they are mirror images of each other. This explains why your newly purchased drone might have come with two pairs of spare propellers.

FIG 5: Quadcopters are fitted with equal numbers of clockwise and counter clockwise rotating propellers

The discussion above also implies that we can control the yaw by varying the speed of one of the propellers. However, this also means we have to take extreme care when replacing a damaged propeller, and make sure that the right propeller is fitted on the right motor!

Mentioned before, a rotating propeller blade produces thrust used to overcome weight to make your pretty quadcopter fly. This thrust is proportionally equivalent to the RPM, so, the higher the RPM, the faster your drone can climb. But, also, the higher the RPM, the more drag is produced, a force which opposes the rotational speed of the propeller.

There is another factor to consider. The thrust produced is also a function of the size of the propeller. If we would compare motors running at the same RPM with different size propellers fitted, the thrust produced would progressively increase with bigger propellers, but only up to a certain point. With large propellers beyond this critical value, the motor would start spinning slower due to a drastic increase in drag. The thrust produced would reduce exponentially.

FIG 6: Some manufacturers sell motor/ propeller kits to ensure an optimum match
FIG 7: Carbon fibre propellers are more expensive than PVC, but generally much more efficient, such as http://www.gobrushless.com/testing/thrust_calculator.php– have fun!

Other Components of a Typical Quadcopter

We know for now that the thrust needed to ascend a quadcopter and
to keep it in the air is produced by the propellers (sometimes referred to as
rotors). The propellers are driven by brushless motors these days, which offer some benefits above older design motors: less wear and tear, no sparks, and (since the technology is well developed and mature) low cost.

FIG 8: Schematic impression of the components of a Quadcopter

Brushless Motors

Brushless motors have two main parts:  the rotor, fitted with magnets and rotate, and the stator, fitted with electromagnets and remains static.

What does the numbers on brushless motors mean? Apart from some
manufacturer specific numbers that will often appear, you might find a
four-character code e.g. 2216. This is the dimensions of the stator or the
casing- 22mm (diameter) x 16mm (height) in our example.

There is also a Kv rating- 850Kv means that the rotor rotates 850 revolutions per minute (RPM) for an application of 1V under no-load conditions. You might additionally come across something like 12N 22P, indicating the motor is fitted with 12 electromagnets, and 22 permanent magnets.

FIG 8: These Crazepony 4pcs EMAX RS2205 2300KV Brushless Motor 2CW 2CCW for QAV250 QAV300 FPV Racing Quadcopter""” target=”_blank” rel=”noreferrer noopener” aria-label=”Crazepony EMAX RS2205 2300KV 12N14P brushless motor (opens in a new tab)”>Crazepony EMAX RS2205 2300KV 12N14P brushless motors spin at exceptional speed and are ideal for racing drones

Before buying, check on the spec sheet what the shaft size is. It does
not help having a motor with a 3.0mm shaft and the propeller has a 4.0mm hole through the hub!

Connecting a motor directly to a charged battery does not help
much. It would start spinning at maximum speed, which is of little use to the drone. The means by which its rotational speed is regulated is by connecting an Electronic Speed Controller (ESC). Think of an ESC as a rheostat which allow more or less current to flow through.

Electronic Speed Controller (ESC)

There are two critical considerations when selecting an ESC. A
bigger motor might require more power than what the ESC can supply, causing the ESC to heat up and even burn out in severe cases. It is therefore necessary to consider the Amp rating of the ESC and ensure that it is adequate for the demands of the motor. For example, taken from a spec sheet:

  • Steady current: 30A
  • Work current: 30A, max to 40A (in 10s)
FIG 9: ESC’s with higher Amp ratings are often more expensive.
FIG 10: This Asael Peña on Unsplash

This Post Has 4 Comments
  1. Hi Arie, very interesting and informative post of how the quadcopters work. For sure I already wondered about it and you explained it very well, there are so many technical details I didn`t know about. I`m curious, how long can fly some smaller quadcopters without charging batteries?I`m sure quadcopters have a bright future and their ways for using are still at the begining. I like your article, thanks for sharing with us!

    1. Thanks for the compliment Luke. The smaller ones last for a mere few minutes whilst the bigger ones typically last for 30 minutes or more. Generally, the advertised battery charge endurance is being exaggerated by the manufacturers- so be wary when you buy. Happy hovering mate!

  2. What an informative and helpful tips on materials for building Quadcopters. Quadcopters or drones, no matter what we want to call them these little devices are both fascinating and a lot of fun to play with! I can categorically say, for those of you with a do-it-yourself nature building a quadcopter can be an awesome project. Because have take it up as a project also during my undergraduate days and it’s fun. So there you have it, the basic elements of a quadcopter. I totally agree that putting all these pieces of materials listed in the article together, you can design a quad or multi-rotor of your own. I support the part that bigger motor might require more power than what the ESC can supply, causing the ESC to heat up and even burn out in severe cases. It is therefore necessary to consider the Amp rating of the ESC and ensure that it is adequate for the demands of the motor. Awesome tips.

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