Multicopter hardware

Airframe

The airframe or simply the frame is the structures to which you attach the motors. They come in many shapes and forms. The most common configuration is the X and Plus shapes. A quadrotor with X shape would have the forward direction between two of its "legs" and one with a Plus shape would have it in the direction of one of its legs. Many frames have the "legs" spread with the same angle between them but especially for racing drones it is common to use other shapes which you want to achieve different properties in different directions.

Number of motors

The number of motors that the frame can host. More motors means more possible thrust but also more weight, higher complexity, etc. Given that the current increases substantially when you get close to your max thrust with a certain motor/propeller setup it might be worth adding more motors to be able to run them further from their limit and thus reduce the current draw.

Supported propeller sizes

The shortest distance between any two motor axes often defines the size of the largest supported propellers. If this distance is 150mm it would mean that we can have props of size 150mm/(25.4mm/inch)=5inch (5.9 rounded down).

Wheelbase

The wheelbase is the diagonal distance between two motor axes. For X shaped platforms this is simple since the distance between any to opposite motors is the same. For other shapes it is a bit less clear but can be the largest between two motor axes.

Battery

LiPo batteries are the most common type of battery found on multicopters.

Voltage (number of cells)

The voltage of LiPo batteris are typically multiplies of 3.7V. This corresponds to 1 cell (denotes 1S). All of our drones so far use 3S or 4S batteries, i.e. a nominal voltage of 11.1V or 14.8V. When a battery is fully charge each cell typically has a voltage of 4.1-4.2V.

Capacity

How much juice does the battery contain. This is measured in mAh. 1mAh means that the battery can deliver 1mA for 1 hour at the voltage defined by the battery. The larger this number the more energy you are taking with you. If all parameters where kept the same you would want this to be as large as possible. The problem is that size and weight grows as well.

Discharge rate

The rate at which a battery can discharge is measured in number of C. 1C means that the battery can be discharged with a current corresponding to the capacity of the battery, i.e. in 1h. A 1800mAh battery that can discharge at 1C can thus deliver 1800mA or 1.8A. Typically values range from 10C up to 100C. Since motors can easily draw 20A you want to make sure that your battery can deliver what the motor needs (ESC might also provide a bottleneck). 

Charge rate

The rate at which a battery can be charged is defined in the same was as the discharge rate. If you are uncertain charge at 1C, i.e. charge a battery of 3300mAh with 3.3A which will then take 1h if if it is completed drained (which would mean that it is most likely damaged so avoid that). Better batteries can be charged at 3-5C.

Weight

The weight of the battery is important as every gram battery means one more gram to carry when we fly and this means less flight time.

Size

Often overlooked but very important as we want to make sure that the battery fit into the drone.

How these parameters vary together

A store that has a very large set of LiPo batteries can be found here to study typical combinations of these parameters

https://www.stefansliposhop.de/ Links to an external site.

Electronic Speed Control (ESC)

These devices control the speed of the motors. They are connected to the battery and the unit that control the flight of the drone on one end and the motors on the other.

The main parameter is the current rating. Make sure that it can take the amount of current you will draw.

Some ESC will be marked BEC. This deliver 5V out that can be used to power for example a flight controller. The 5V is provided in the middle cable in the smaller 3-pin connector. ESCs that do not give 5V out are marked OPTO. These typically do not have more than 2 cables in the signal cable connecting the flight controller with the ESC. If an OPTO ESC has 3 wires it typically wants 5V input.

Flight controller

The flight controller is the device that acts as the spider in the web on a drone. It control the speed of the motors, it has one or several built in IMUs to estimate the orientation of the drone to stabilise it, etc.

There are many kinds of flight controllers but one of the more popular families of controllers are those that run the PX4 flight stack.

http://px4.io/docs/px4-autopilot/introduction/ Links to an external site.

Motor

The motor makes the propellers spin. The four main parameters for the motor is the size, speed, efficiency and weight.

Size

The size of the motor is typically specified as, for example, 2206 which means that the diameter of the stator is 22mm and that it is 6mm tall. The frame you picked will have mounting holes for motors. Make sure that the motor size is right for the frame. 

Speed

The speed of the motor is given by the so called Kv number which tells how many rpm (revolutions per minute) the motor will per volt applied to it. That is, larger Kv means that the motor will spin faster. Spinning faster will give larger thrust but will also draw more power/current. Larger propellers typically means that you want/need lower Kv on the motor.

Efficiency

You want motors that are efficient so that as much of the power that is given to it generates motion rather than heat. 

Weight

When building small drones for which every gram weight counts, the weight of the motor might also be a parameter.

Choosing a motor

Pick a motor that matches the propellers you want to use. You might need to iterate back and forth between propellers and motor. A motor will typically provide a table like the one below to make this decision easier. On the left you see the battery voltage, followed by the prop type and the throttle. After that you can read off the current and thrust for example. The thrust is specific in g(rams). You want to make sure that you pick ESCs that are powerful enough to handle the current that you will draw depending on the battery and prop you use. An ESC will typically specify the max continuous current and peak current. The continuous should be a lot lower than the max current for most use cases.

Propeller

The propellers generate the thrust to keep the drone in the air.

A tutorial can be found here Links to an external site.

Size and pitch

The two most important parameters for the propellers  specified as the diameter of the circle that it describe when it spins and the pitch. The size is measured in inches (1 inch = 25.4mm). The two sizes are typically combined into one value such as 5030 which means 5 inches diameter and 3 inches climb. The same type of propeller is sometimes denoted 5x3. 

Number of blades

Propellers can have two, three or four blades. Maybe even more but I have never seen it. A 3 blade propeller would typically be denoted, for example, 5030x3 or 5x3x3.

Shape of blades

The shape of the blades can also vary. One of the most common alternative shapes is "bullnose" which means that the blade is flat at the end which means more thrust.

Material

Several different materials are used. ABS plastic is often used in the cheap ones and more expensive ones made of carbon fibre. The carbon fibre props are more stiff and typically thinner. Some of them have so sharp edges that you will cut yourself on them. Not so easy as saying that props made out of X are the best. Motor, ESC, use case, etc also plays in.

Choosing a propeller

Roughly speaking

• you get more thrust with 
  • larger propellers,
  • higher pitch
  • more blades
• the larger props the slower spinning motors you would typically use (lower Kv values)

The following table to copied from https://oscarliang.com/quadcopter-motor-propeller/ Links to an external site. and gives a rough guide for what props size to use for what frame size and then what motor to use for what prop.