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Attack of the Drones: Quadcopters

Issue No 33, 26 June 2023

By: Anthony O. Ives

Quadcopters, known by most people as drones have become increasingly popular since 2015. Quadcopters are especially popular with the general public who normally would not have an interest in aviation or model aircraft, as they are electronically stablised and hence you do not need to learn to fly them as with traditional model aircraft. People who fly traditional model aircraft often say that the drone was the worst invention as due to the ease of use and their popularity they have become nusiance and brought about a lot of additional rules that now apply to traditional model aircraft as well. However, quadcopters do have their advantages with their simple design they have made aerial photography available to most people and can perform other aerial tasks at a much lower cost.

A quadcopter as its name suggests uses four rotors driven by four electric motors with each individual motor controlled by its own individual speed controller [1]. Four rotors allows control of roll, pitch and yaw in the same way as helicopter [2]. Here is description for each control:

For pitch: By increasing the thrust of the front two rotors and reducing it on the back two rotors and vice versa will cause the quadcopter to pitch back and pitch forward. This is known as differential longitudinal thrust.
For roll: By increasing the thrust of the left two rotors and reducing it on the right two rotors and vice versa will cause the quadcopter to roll clockwise and roll anti-clockwise. This is known as differential lateral thrust.
For yaw: By increasing power on the front right and back left rotors and reducing it on the front left and back right rotors and vice versa will cause the quadcopter to yaw clockwise and anti-clockwise without losing height. This assumes that the rotors on the front right and back left rotate in same direction but opposite to the remaining rotors in order to keep the quadcopter from spinning in stationary hover. This is also a form of differential thrust which is specific to quadcopters, it will be referred to as differential yaw thrust in this article.
For vertical position: Increasing and reducing thrust equally on all four rotors will cause the quadcopter to climb or descend respectively. This is known as collective thrust.

The rotors on a typical quadcopter are fixed pitch which keeps everything simple so therefore in addition to all the usual radio equipment needed for any radio controlled aircraft [3] you will need a electronic mixing unit and possibly an electronic stabilising unit.

The electronic mixing unit will decide which rotors should produce more or less thrust in order for the quadcopter to pitch, roll or yaw based on the command it recieves from the radio commands. In a simple setup for example the elevator servo would be pluged in to the elevator channel so the aircraft would pitch up or down depending on the movement of the elevator servo. However, with quadcopters things are more complicated because you need to increase and decrease the thrust of different rotors at the same time hence the need for a mixing unit. Some transmitters allow you to program mixing so the mixing could done by the transmitter and there would be no need for an onboard mixing unit.

Some form of electronic stabilising unit is needed for a quadcopter as with flying RC helicopters as it extremely hard to fly helicopters without them. However, there is different stabilising systems, some as with most RC helicopters just make the helicopter easier to fly but you would still need to learn to fly. Other stabilising systems allow anyone to fly a quadcopter which is the type that is used in most modern quadcopters. But without a stabilising system quadcopters are just as difficult to fly as a helicopter although the principle will be simliar to a helicopter. Quadcopters are popular for the simplicity of their design combined with the use of a stabilising system.

Quadcopters can be completely automatically controlled and guided using satelite navigation to carry out a wide range of operations with the most popular being photography. Quadcopters can generally be known as a multicopter, quadcopters are more common because its more straightforward to understand with rotors control pitch, roll and yaw. But a multicopter can have any number of rotors above four, including six and eight however, four is simpler hence why that number of rotors are more commonly used.

Each quadcopter rotor can be sized in the same way as the propeller was in reference [4], except instead of drag using the total weight of the quadcopter divided by the number of rotors which in this case is four. The picture below also shows a typical basic commerical quadcopter:

Picture of a Basic Quadcopter

There may be some benefits to using more than four rotors depending on how the rotor and motor combination is sized however, generally an even number of rotors is used as it keeps everything simple and symmetrical. With the recent interest in eVTOLs (electric Vertical Take Off and Landing) aircraft, some designs have looked very simliar to the small radio controlled multicopters. However, as a multicopter becomes larger its looses it simplicity and becomes more complex, this is probably the reason why the single rotor helicopter has always dominated. Other types of helicopters do exist such as tandem rotor, contra-rotating rotor and intermeshing rotor but there designs are in a smaller number however, maybe different designs will be more beneficent for eVTOLs only time will tell.

Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:

Mathematics
Helicopters
VTOL UAVs (RC Helicopters)
Sailing and Sailboat Design

If there is one or more of these topics that you are specifically interested in please also let me know in your comments this will help me to write blog articles that are more helpful.

References:

[1] http://www.eiteog.com/EiteogBLOG/No21EiteogBlogMotors.html

[2] Helicopter Theory, Wayne Johnson, 1980, Dover Publications

[3] http://www.eiteog.com/EiteogBLOG/No14EiteogBlogRC.html

[4] http://www.eiteog.com/EiteogBLOG/No14EiteogBlogRC.html

Disclaimer: Eiteog makes every effort to provide information which is as accurate as possible. Eiteog will not be responsible for any liability, loss or risk incurred as a result of the use and application of information on its website or in its products. None of the information on Eiteog's website or in its products supersedes any information contained in documents or procedures issued by relevant aviation authorities, manufacturers, flight schools or the operators of aircraft, UAVs.