Introduction to quadcopters

Ronnel Davis
Dec 9, 2017 · 11 min read

Drones! Drones everywhere! They seem to be all the rage these days, with atleast one headline made by drones every other day, whether it is by the FAA bringing out new drone laws, or a drone strike by the US military, or an airport being shut down for an hour or two because of an errant pilot flying his aircraft in sensitive areas. So lets’s talk about drones, namely multirotors — why they’re so widespread and popular now and what the future holds.

What is a multi rotor and why are they so popular?

A multirotor, as the name implies is a craft with multiple engines that drive a propellor each. These are commonly in the form of quadcopters, although there are variants of this basic design in the form of hexacopters with 6 engines, octacopters with 8 engines and tricopters with 3 engines (even though this variant more closely resembles a helicopter with a swiveling tail rotor for yaw and pitch and two engines for thrust and roll.

A quadcopter and an octacopter

The interesting point to be made about multirotors is that they are inherently unstable and requires constant corrections to be made by an onboard computer hundreds or even thousands of times a second to maintain stability. This is in stark contrast to fixed wing aircraft that does not need a computer at all, and in the case of gliders, does not even need an engine. If the onboard computer stops for even a second, or an engine fails or a propellor is damaged, it could prove disastrous to the multirotor.

This is the reason multirotors have risen in popularity recently. In order to keep a multirotor in the air, a powerful computer that was battery efficient and lightweight had to be developed. But now with the advent of smartphones, fast, power-efficient and lightweight chips became mainstream and accessible for other commercial applications and could be used to maintain stability for aircraft. For anyone looking to get started building a quadcopters, a few commercially available flight computers include —

  • Multiwii

Note that this list is in no way exhaustive, and the smart ones out there may even go out and build their own using an arduino or a raspberry pi and a couple other components.

I feel the reason quadcopters have really taken off (pun intended) as compared to the fixed wing aircraft of yore is that they

  • Are easier to fly for beginners

Due to their inherent designs as flying computers, they can take advantage of Moore's law and get cheaper and faster every year. They also can add new capabilities with software and have access to software updates, just like a smartphone. And with the new computational capabilities, they are able to use more sensors for better situational awareness and to make more sense of the environment they are in for more safer, autonomous flying.

How does a multirotor fly?

Be warned that this section involves a bit of physics (but no formulae or anything, promise) required to understand the motion of an aircraft. If you are allergic to science, you may skip this section and go on to the next one.

Most drones have an even number of motors. This is so that they can have an equal number of clockwise and counterclockwise propellors. Now don’t be mistaken that half these props generate lift in one direction and the other half in the other direction. All of them generate lift in the same direction, its just that the blades are mirrored on half of them. This is to cancel out torque generated by a spinning object.

Propellor directions for different multirotor types

You see, according to Newton’s third law of motion, every action has an equal and opposite reaction. So if a motor rotates a propellor clockwise, the motor also generates a torque on the body of the aircraft in a counterclockwise direction. If all four motors in a quadcopter spun in the same direction, it generates a large net torque on the body in the other direction, sending the craft spiralling out of control. This is why half the props spin the other way so that the torques generated by the motors cancel each other out, keeping the drone steady.

An aircraft has three axis of movement, roll, pitch and yaw. This is regardless of aircraft type and apply to fixed wing aircraft, gliders, helicopters, multirotors, even rockets and spacecraft.

Rolling an aircraft

Rolling angles the drone’s body to the left or fight to move the aircraft side to side. While always looking forward, tilt your head to the left of right.

Pitching an aircraft

Pitching angles the drone’s nose up or down to move the aircraft forward or backward. This is the equivalent of looking up or down.

Yawing an aircraft

Yawing changes the direction the drone faces by turning the aircraft to the left and right on the vertical axis. This is the same as turning your head to the left or right.

Now the way a multirotor rolls, pitches and yaws is by carefully controlling the speeds of all its motors. Fo instance, to yaw a quadcopter, motors opposite each other are sped up, resulting in a higher net torque to one side, effectively spinning the aircraft. To pitch an aircraft, motors in the front and at the back of the drone are differentially controlled to be able to go forward and backward. And to roll, motors on each side are differently controlled to go side to side.

How does one get started and build a drone?

Whoa whoa! Now hold on there cowboy! The first thing to ask yourself is do you really want to build one? There are a lot of amazing ready to fly drones out there available to buy including ones from companies like DJI, Parrot, 3DRobotics and several chinese manufacturers who build low cost ones that let you get started in the hobby. This takes away a lot of guesswork and pain from building your own aircraft. It lets you get right into aerial photography or learning to fly or drone racing.

Well if I haven’t persuaded you out of your lifelong dream to build and fly your own aircraft, then congratulations! We move right on to the next step which is deciding what kind of a drone do you want to make. Right now there are a several different types of drones that cater to different mission types. For examples there are general purpose drones that do a bit of everything but not anything in particular. There are racing drones which are fast and agile, there are photography drones that are big and stable and fly for a while. There are long distance drones for pilots who enjoy flight and to stay up in the air as long as possible. And then there are highly mission specific drones like the ones for agriculture, search and rescue, surveillance, deliveries and such.

But to get started, I would like to talk about the three kinds of drones I’ve personally been involved in creating which involves a general purpose drone, a racing quad and an aerial photography platform.

For all the quads, the general blueprint of things you would have to buy are -

The frame

This is the chasis of the aircraft and gives structural support and rigidity to the entire craft and provides a points to mount the rest of the components. Most frames have a number in their name that tells you the diagnonal length of the aircraft. So for example, an F450 has a diagonal lenth of 450mm. Similarly, an F550 has a diagonal length of 550mm.

The Tarot690 — a 690mm hexacopter frame

The powertrain

This includes the brushless motors, the electronic speed controllers (ESC’s), the propellors and the power distribution board.

  1. When it comes to motors, they have a kv-rating that stands for revolutions per volt. Generally, higher kv motors are used for smaller race quads and use up more power and spin smaller props. For example, a 2200kv motor would spin propellors that are 3–5 inches while a 920kv motor can spin propellors that are 10–12 inches across.
The powertrain for a hexacopter

The battery

This is the power source and the current defacto is Lithium polymer or Li-Po batteries. A lipo battery would have three numbers to identify it —

  1. The number of cells it contains — LiPo batteries are made up of multiple cells that each provide 3.7 volts. Connecting multiple cells lets you provide more voltage. So a two cell would provide 7.4 volts, a 3 cell would provide 11.1 volts and so on. You would need a 3 cell or 4 cell depending on your application. This is usually represented as a number followed by an S, like a 3S or a 4S for a 3 cell or 4 cell respectively.
LiPo batteries of all shapes and sizes!

The flight computer

This is the brains of the entire operation, and they come in all shapes, sizes and price points. I would highly recommend that you don’t skimp out on this one and go for the most expensive one you can afford. Cheaper ones would just be dangerous, fail easily, damage other components or even make your entire craft fly away. The pain is not worth the few dollars you can spend to get a higher quality flight computer.

A very high end DJI flight computer

The transmitter and receiver

This is the part of the quadcopter that lets you talk to the flight computer. They come in all shapes and sizes and the biggest differentiating factors are the number of channels they provide. Higher end versions have a larger number of channels which you can use for mode switching, gimbal control, GPS Return to Home and failsafe activation. The also have a variety of interfaces including PPM, PWM, S-Bus, D-Bus and so on. It is best to check which interfaces your flight computer supports before buying one.

The FlySky 10 channel transmitter and 6 channel receiver

I would say that the general purpose drone would be the cheapest to build and easiest to fly among the three, and would be ideal to get into the hobby. I would personally recommend the DJI F450 frame and kit, and to use the NAZA flight computer. It is a bit on the pricey side but it saves you a lot of headache and pain in the long run, trust me. For this you would want a frame between 400 and 600 mm, motors between 900kv and 1400kv, a prop size between 8–10 inches, a 2200mah 3 cell or 4 cell battery with a C rating above 30.

The DJI F450

You could also build a racing drone, this would consist of a frame less than 300mm, motors above 1800kv, a prop size of less than 6 inches and a 2200mah 4 cell battery with a C rating above 40.

An FPV racing drone

My personal favorite is a aerial photography platform, this is basically a general purpose drone but with a gimbal to allow for cameras to be fitted. I would also recommend a frame that would keep the propellors out of the cameras view. I would recommend a TBS discovery frame witch uses a dead cat style frame to keep the arms apart, giving the camera a clear field of view. You may also opt for a larger frame, a larger battery, lower kv motors and larger props to allow for more stability and longer flight times.

The TBS discovery

Overall, I have found the drone building and flying hobby to be very rewarding, with loads of helpful people on the internet, generous and helpful clubs and groups that are ready to help you with information and parts in any way they can. It lets you learn a lot about electronics, aircraft design, aeronautics, control theory and software programming. I have to warn you that this is not without it’s downsides, with injuries, property damage, and the threat of breaking parts and burning components lurking around every corner. You will spend a lot of days crying about a flyaway drone, a fried flight computer, a broken prop and a burnt motor. But much more often will be the exhilarating thrill of building and flying an aircraft with your own two hands and to see it lift off into the sky, to take the human dream of flight and to make it come alive. Cheers and safe flying!

The quad that inspired this post and taught me a lot about drones

Ronnel Davis

Written by

I am a 21 year old computer science graduate from NIT Calicut with an affinity for iOS and web development.