How does a Quadcopter Work?

Quadcopters, drones – no matter what you want to call them these little devices are both fascinating and a lot of fun to play with! And for those of you with a do-it-yourself nature building a quadcopter can be an awesome project.

But how exactly does a quadcopter work? How does it fly and what components do you need to put together to make one of your own? Follow along and I’ll solve that mystery for you.

How Quadcopters Fly

In this article I’ll be focusing on quadcopters but a lot of what I explain here applies to other flying devices with more than four propellers. So if you’re building an octocopter a lot of what you read here will also be applicable.

Before I explain how a quadcopter or any multi-rotator works there are some important concepts that we need to define – pitch, yaw and roll. These terms refer to the three dimensions that an aircraft in flight is free to move it.

• Pitch – Pitch refers to the nose of the aircraft going up or down. You could think of it as climbing or diving
• Yaw – Yaw on the other hand refers to the nose of the aircraft turning left or right. You could simply think of this as turning.
• Roll – To understand roll think of an axis running from the front to the back of the aircraft. When an aircraft rolls it is turning on this access. You can also think of roll as a tilt.

 

Quadcopters make use of 4 Motors. Two of these motor spin clockwise while the other two spin counterclockwise. Motors on the same axis spin in the same direction, as illustrated here.

Altitude is a term that you’re probably familiar with – it simply describes going up or down. A quadcopter can either hover or adjust its altitude by applying equal thrust to all four rotors.

 

To adjust its Yaw, or make it turn left or right, the quadcopter applies more thrust to one set of motors. For example a quadcopter may apply more thrust to the two motors that spin clockwise to make a turn.

Pitch and roll on the other hand are adjusted by apply more thrust on one rotor and less to the other opposing rotor. For example the quadcopter can adjust its pitch by applying more thrust to the clockwise spinning motor in the front and less thrust to the clockwise spinning motor directly opposite in the back.

 

One thing that makes quadcopters easy to fly is that you don’t actually have to adjust the motor speeds manually as this is where the controller and electronics come in. I’ll discuss the electronics in a moment but first let’s examine the motors.

Motors

Without motors your quadcopter isn’t flying anywhere and it is vital that you select the correct ones. Unfortunately motor specifications can sometimes be confusing.

Most quadcopters use brushless DC motors although the motors in cheaper quadcopters and micro-copters often do have brushes.

Brushless motors consist of a rotor with a permanent magnet and a number of electromagnets surrounding it, these are also called poles. Brushless motors can have anywhere from 2 to 14 poles. The greater the number of poles the more precisely the motor can be controlled.

Brushless motors have three wires, these wires control the motors three phrases. By changing the connections on these wires you can have the motor spin clockwise or counterclockwise.

One of the key specifications of a quadcopter motor is its KV rating. KV does not stand for kilovolts, instead it is a rating of the number of revolutions per minute (RPM) that the motor will turn when a 1-volt potential difference is applied to the motor with no load. KV can also be thought of as RPM/volt. Typical KV ratings for quadcopters are between 500 and 2500. A low KV motor can swing a big propeller and thus create a larger amount of thrust.

As a general rule of thumb quadcopters that weigh more than 1 kilogram will use a motor in the range of 700 to 900 KV. Copters that weigh between a half and 1 kilogram use motors rated between 900 and 1300 KV. Craft under 500 grams make use of motors between 1300 and 2200 KV.

The maximum current rating of the motor is another important factor. You will need to know this value, measured in amperes, before choosing your electronic speed controller (ESC) and battery.

The thrust rating of a motor is specified in conjunction with a propeller size. This is an important specification that requires knowing the total weight of your quadcopter, keep in mind that this weight includes the weight of the motors themselves.

The basic rule of thumb is that the motor and propeller combination should be able to generate twice the weight of the craft in thrust. So with a quadcopter having four Motors this means that each individual motor should be able to provide thrust equal to half the weight of the entire quadcopter. In other words a 1 kilogram quadcopter requires four motors, each capable of at least half a kilogram of thrust.

Another important motor specification is the diameter of the shaft. This is important as it determines what propellers you will be able to match with the motor.

You also want to look at the mounting screw pattern and motor mounts to ensure that your motor can be successfully mounted to the frame of your quadcopter.

Propellers

Diameter and pitch are the two basic specifications of a propeller. Propellers are generally sold in sets with 2 clockwise and 2 counterclockwise propellers .

Propellers come in a variety of diameters and pitches and can be made of plastic, carbon fiber or even wood. Small propellers with diameters under 8 inches are used for racing and acrobatics along with high KV Motors. Propellers over 8 inches diameter are used with low KV motors in designs meant to lift heavy payloads such as video cameras.

Pttch is generally defined as “travel distance per rotation”. The higher pitch means a slower rotation which will increase your vehicle speed but will also use more power. You can actually extend the flight time of your quadcopter by simply substituting lower pitch propellers. Lower pitch propellers can generate more torque and will require your motor to consume less current.

The diameter of the propeller refers to its size. Increasing the diameter increases the propeller efficiency but will also consume more current from the motor.

Matching the motor to the propeller is also a factor for stability, this can be important when carrying a camera. For a large quadcopter to carry big payloads large propellers and low KV motors tend to work best.

Electronic Speed Controllers (ESC)

The device that controls the Brushless DC motors is called an Electronic Speed Controller or ESC. You will need one of these for each motor.

The ESC will have three sets of wires. There are three heavy-gauge wires that connect to the three wires on your brushless motor. Two other heavy-gauge wires connect to your power distribution board, this supplies voltage to the ESC and motors. There will also be three smaller wires that connect to your flight controller.

The ESC must be rated to handle the maximum current that your motor consumes. It is always wise to choose an ESC that is rated higher than the motor’s maximum current.

Flight Controllers

The Flight Controller is the brains of your quadcopter. This is the device that controls the speed of your motors by sending signals to your ESCs.

Advanced flight controllers has more features and are coupled with sensors such as gyros, accelerometers, sonar, GPS and magnetometers. Simple flight controllers contain only gyros and these are quite sufficient for most beginners.

The flight controller will accept the signals from the onboard radio receiver so that you can control your quadcopter remotely. It then does its magic and sends signals to the ESC’s to control the motor speeds.

Flight controllers are actually small computers and as such can be programmed and updated by connecting them to your computer. Despite their complexity flight controllers are actually not that expensive.

Radio Transmitters & Receivers

Your quadcopter will have an on-board radio receiver and you will control the device with a handheld radio transmitter. Both the receiver and transmitter are specified by the number of channels that they support and by the frequency upon which they operate.

Each channel in the radio system controls a single function or device in the quadcopter. At minimum you will need for channels to control a quad, although most people start with at least six. Extra channels can be used to control lighting or a gimbal motor to position a camera.

The four basic channels are used as follows – one channel for throttle, one channel for turning right and left, one channel for pitching forward and backwards and one for rolling left and right – in other words throttle, yaw, pitch and roll. A channel can also be used to switch between different flying modes.

If you plan to use several quadcopters you can buy one transmitter and several compatible receivers, obviously this will only let you fly one copter at a time. This is a good strategy as it allows you to afford a good quality transmitter, receivers are generally fairly inexpensive.

Transmitter-receiver combinations work in two different modes, these modes essentially determine the control configuration on the transmitter.

The mode 1 configuration has the elevator control on the left joystick and the throttle control on the right joystick. Mode 2 reverses these controls and this is the most popular configuration. If you’re a beginner it’s generally advised to set your equipment up to work in mode 2.

Batteries

Without a battery your quadcopter isn’t going anywhere. The selection of the battery is one of the most important decisions you will make when designing and building your quadcopter.

Larger batteries will have larger capacities and will therefore allow for a longer flight time. However as the capacity of the battery goes up so does its weight, and adding weight to your quadcopter will reduce the flight time as more current will be required from your motors to lift the payload. Bear in mind that a heavier quadcopter is also less agile.

The principal battery specification is its current capacity, which is rated in milliamps per hour or “mAh”. A 2200 mAh battery can supply 2.2 amperes of current for 1 hour.

Another specification of the battery is its voltage. Batteries consist of a number of cells, each cell of a common LiPo battery is capable of delivering 3.7 volts. Thus battery voltages are commonly specified as multiples of 3.7 volts – for example 3.7 volts, 7.4 volts, 11.1 volts and 14.8 volts are common. If you do choose a higher voltage battery make certain that both your ESC and motors are capable of supporting that voltage.

Frame

Finally we come to the frame, this holds your quadcopter together. Frames can be made of a number of materials such as wood, plastic or carbon fiber. Carbon fiber can be very strong for its weight but can also be brittle, a factor that should be considered as undoubtedly you will crash your quadcopter at one point.

The frame size of a quadcopter is specified in millimeters, this is the distance between the motors on your quad. Common sizes are 180, 250 and 400 millimeters. Racing quadcopters are usually the 250 millimeter variety.

When choosing a frame for your quad make certain that it has enough space and clearance to mount things like your ESC and flight controller, as well as any additional items such as cameras that you wish to carry.

Some hobbyists also build their own frames, often out of wood. Wood is actually a good choice as it’s easy to work with and it is fairly expensive. It’s also easy to modify to attach accessories to your quadcopter. Wood however can be subject to warping and this will cause your quadcopter to fly erratically.

You can even make a frame or parts of a frame with a 3D printer.

Conclusion

So there you have it, the basic elements of a quadcopter. By putting all these pieces together you can design a quad or multi-rotor of your own. Keep watching this site as I will be examining these individual components in greater detail in other articles as well as illustrating full quadcopter builds.

Stay tuned!