Quadcopter Components Introduction

There are sensors connected to a microcontroller to make the decision as to how to control the motors. Depending on how autonomous you want it to be, one or more of these sensors are used in combination.

In this section, I will talk about these essential quadcopter components:

• Frame – The structure that holds all the components together. They need to be designed to be strong but also lightweight.
• Rotors – Brushless DC motors that can provide the necessary thrust to propel the craft. Each rotor needs to be controlled separately by a speed controller.
• Propeller
• Battery – Power Source
• IMU – Sensors
• Microcontroller – The Brain
• RC Transmitter
• Optional

Before we go into explaining how to choose each components, we can take a look some quadcopters that people have built, and the parts they used to get a rough idea. I didn’t build these planes, so I can’t guarantee their performance.

Multicopter Examples Page

Frame

Frame is the structure that holds all the components together. The Frame should be rigid, and be able to minimize the vibrations coming from the motors.

A QuadCopter frame consists of two to three parts which don’t necessarily have to be of the same material:

• The center plate where the electronics are mounted
• Four arms mounted to the center plate
• Four motor brackets connecting the motors to the end of the arms

Most available materials for the frame are:

• Carbon Fiber
• Aluminium
• Wood, such as Plywood or MDF (Medium-density fibreboard)

Carbon fiber is most rigid and vibration absorbent out of the three materials but also the most expensive.

Hollow aluminium square rails is the most popular for the QuadCopters’ arms due to its relatively light weight, rigidness and affordability. However aluminium could suffer from motor vibrations, as the damping effect is not as good as carbon fiber. In cases of severe vibration problem, it could mess up sensor readings.

Wood board such as MDF plates could be cut out for the arms as they are better at absorbing the vibrations than aluminium. Unfortunately the wood is not a very rigid material and can break easily in quadcopter crashes.

Although it is not as important as for the arms which of the three material to use for the center plate, plywood is most commonly seen because of its the light weight, easy to work with and good vibration absorbing features.

As for arm length, the term “motor-to-motor distance” is sometimes used, meaning the distance between the center of one motor to that of another motor of the same arm in the QuadCopter terminology.

The motor to motor distance usually depends on the diameter of the propellers. To make you have enough space between the propellers and they don’t get caught by each other.

Brushless Motors

A little background of Brushless motor. They are a bit similar to normal DC motors in the way that coils and magnets are used to drive the shaft. Though the brushless motors do not have a brush on the shaft which takes care of switching the power direction in the coils, and this is why they are called brushless. Instead the brushless motors have three coils on the inner (center) of the motor, which is fixed to the mounting.

On the outer side it contains a number of magnets mounted to a cylinder that is attached to the rotating shaft. So the coils are fixed which means wires can go directly to them and therefor there is no need for a brush.

Generally brushless motors spin in much higher speed and use less power at the same speed than DC motors. Also brushless motors don’t lose power in the brush-transition like the DC motors do, so it’s more energy efficient.

Brushless motors come in many different varieties, where the size and the current consumption differ. When selecting your brushless motor you should take care of the weight, the size, which kind of propeller you are going to use, so everything matches up with the current consumption. When looking for the brushless motors you should notice the specifications, especially the “Kv-rating“.

The Kv-rating indicates how many RPMs (Revolutions per minute) the motor will do if provided with x-number of volts. The RPMs can be calculated in this way: RPM=Kv*U An easy way to calculate rating of motor you need, check out the online calculator eCalc. It’s an amazing tool that helps you decide what components to purchase depending on the payload that you want to carry.

Make sure you buy the counter-rotating to counteract the torque effect of the props.

I have written a more complete guide on how to choose Motor and propeller.

Propellers

On each of the brushless motors there are mounted a propeller.

You might not have noticed this on the pictures, but the 4 propellers are actually not identical. You will see that the front and the back propellers are tilted to the right, while the left and right propellers are tilted to the left.

Like I mentioned before, 2 rotors rotates in the opposite directions to the other two to avoid body spinning. By making the propeller pairs spin in each direction, but also having opposite tilting, all of them will provide lifting thrust without spinning in the same direction. This makes it possible for the QuadCopter to stabilize the yaw rotation, which is the rotation around itself.

The propellers come in different diameters and pitches (tilting). You would have to decide which one to use according to your frame size, and when that decision is made you should chose your motors according to that. Some of the standard propeller sizes used for QuadCopters are:

• EPP1045 10 diameter and 4.5 pitch  this is the most popular one, good for mid-sized quads
• APC 1047 10 diameter and 4.7 pitch  much similar to the one above
• EPP0845  8 diameter and 4.5 pitch  regularly used in smaller quads
• EPP1245  12 diameter and 4.5 pitch  used for larger quads which requires lot of thrust
• EPP0938  9 diameter and 3.8 pitch  used in smaller quads

Aerodynamics is just way too complex for non-academic hobbyists. It’s even unlikely we can explain all that theory stuff in a few words. But in general when selecting propellers you can always follow these rules:

1. The larger diameter and pitch the more thrust the propeller can generate. It also requires more power to drive it, but it will be able to lift more weight.
2. When using high RPM (Revolutions per minute) motors you should go for the smaller or mid-sized propellers. When using low RPM motors you should go for the larger propellers as you can run into troubles with the small ones not being able to lift the quad at low speed.

To learn about what effects the type of material have, on flight performance, check out this post.

Analysis of Propeller Pitch, Diameter, and RPM

Pitch VS Diameter: the diameter basically means area while pitch means effective area. So with the same diameter, larger pitch propeller would generate more thrust and lift more weight but also use more power.

A higher RPM of the propeller will give you more speed and maneuverability, but it is limited in the amount of weight it will be able to lift for any given power. Also, the power drawn (and rotating power required) by the motor increases as the effective area of the propeller increases, so a bigger diameter or higher pitch one will draw more power at the same RPM, but will also produce much more thrust, and it will be able to lift more weight.

In choosing a balanced motor and propeller combination, you have to figure out what you want your quadcopter to do. If you want to fly around stably with heavy subject like a camera, you would probably use a motor that manages less revolutions but can provide more torque and a longer or higher pitched propeller (which uses more torque to move more air in order to create lift).

ESC – Electronic Speed Controller

The brushless motors are multi-phased, normally 3 phases, so direct supply of DC power will not turn the motors on. Thats where the Electronic Speed Controllers (ESC) comes into play. The ESC generating three high frequency signals with different but controllable phases continually to keep the motor turning. The ESC is also able to source a lot of current as the motors can draw a lot of power.

The ESC is an inexpensive motor controller board that has a battery input and a three phase output for the motor. Each ESC is controlled independently by a PPM signal (similar to PWM). The frequency of the signals also vary a lot, but for a Quadcopter it is recommended the controller should support high enough frequency signal, so the motor speeds can be adjusted quick enough for optimal stability (i.e. at least 200 Hz or even better 300 Hz PPM signal). ESC can also be controlled through I2C but these controllers are much more expensive.

When selecting a suitable ESC, the most important factor is the source current. You should always choose an ESC with at least 10 A or more in sourcing current as what your motor will require. Second most important factor is the programming facilities, which means in some ESC you are allowed to use different signals frequency range other than only between 1 ms to 2 ms range, but you could change it to whatever you need. This is especially useful for custom controller board.

Battery

As for the power source of the quadcopter, I would recommend LiPo Battery because firstly it is light, and secondly its current ratings meet our requirement. NiMH is also possible. They are cheaper, but it’s also a lot heavier than LiPo Battery.

Battery Voltage

LiPo battery can be found in a single cell (3.7V) to in a pack of over 10 cells connected in series (37V). A popular choice of battery for a QuadCopter is the 3SP1 batteries which means three cells connected in series as one parallel, which should give us 11.1V.

Battery Capacity

As for the battery capacity, you need to do some calculations on:

• How much power your motors will draw?
• Decide how long flight time you want?
• How much influence the battery weight should have on the total weight?

A good rule of thumb is that you with four EPP1045 propellers and four Kv=1000 rated motor will get the number of minutes of full throttle flight time as the same number of amp-hours in your battery capacity. This means that if you have a 4000mAh battery, you will get around 4 minutes of full throttle flight time though with a 1KG total weight you will get around 16 minutes of hover.