How to build drone propulsion

How to build drone propulsion

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We previously talked about How to build a drone, we will now talk more about propulsion system.

We will detail each part of the propulsion system, what caracteristics you must understand to choose the best possible combination.

Please note that these infos are valid for various drones : tricopter, quadcopter, hexacopter, …

The first thing you must think about is what do you need :

  • What will you carry with your drone (Camera, FPV system, gimbal, …) and so what will be the payload
  • What size do you need for the frame and also what kind of propellers it can handle.

Motors

To choose the right motor, you need to consider various parameters.

The first one is KV, it is the main unit to understand about motors. KV is the motor velocity constant, and is measured in RPM per volt. This means that a 900KV motor with 3S battery (11.1v) will achieve approximatively 10 000 rotations per minute (900*11.1).

For a mid size multirotor, we generally use motors between 800kv and 1500kv with a 3S battery.

 

Generally speaking, the higher the motor’s rotation speed is ,the less driving torque he has. and vice versa. A motor with a low KV,will then support larger propellers than a motor with a higher KV.
However the KV is not the only one important factor.In fact the motor has other important characteristics such as his maximum rated continuous power(expressed in AMPS or VOLTS).For an identical KV you will find motors with different sizes and various powers. When you choose your motor you must decide at the same time what will be the feeding voltage and verify what type of propellers you will then use in order to stay within the operating limits.

An other parameter is that all the motors do not have the same efficiency.The quality of the bearings,of the magnets, of the windings and also the number of magnetic poles play an essential role.The size of the propellers and the voltage you will choose to feed them are also important.

When you choose your motors it is important to choose motors which are supplied with supplier’s consistent operating datas.

Tools such as eCalc will allow you to simulate your configuration taking into account the operating datas of your motors (with a more or less accurate precision…..).

Some examples of configurations that I use :

250 class

DYS BE1806 2300KV

(with 5″ propellers)

450 class

SunnySky X2212 980KV

(with 10″ propellers)

650 class

Sunnysky V2216 800KV

(with 13″ propellers)

ESC

esc-afro

This part is very important as it will govern the reactivity of the motor.For multi-rotors you will have to use special ESC which will be able to operate with an updating rate of approx. 450hz.For this you have two options:

-Either buy ESC certified for multi-rotors(generally update their firmware SimonK, Maytech,Afro…..)

-Or buy ESC which are compatible and update them yourself with the firmware SimonK(it is necessary to open the ESC and buy a special cable if you do not want to spend hours on it.

Your ESC must support an amperage which is at least equal to the maximum amperage of your motor. Generally you go for a slightly higher ESC so that it is not subject to overheat.If your motor absorbs 14A max you will rather go for a 20A ESC in order to keep some margin(the weight difference being marginal).

BEC or not ?

The BEC is a 5 V output which allows to feed other 5 V equipments( generally radio, flight controller).A lot of ESC are equipped with a BEC but some are not(for example ESC type OPTO are not). In order to manage the feeding of your 5 V equipment, you can choose either ESC with integrated BEC or use a dedicated BEC (called UBEC).

Some examples of configurations that I use :

Large quadricoptère (~2Kg) : ESC Emax SimonK 25A

Medium quadricoptère  (~1,2Kg) : ESC Emax SimonK 20A

Mini quadricoptère (~500g) : ESC Emax SimonK 12A

Battery

batterie-2200mah

For a multi-rotor we will use a LiPo type battery (Lithium Polymère), 3 essential informations :

– The capacity expressed in mah (ex: 2200mah)

– The voltage expressed in S ( cells’ quantity) : each cell giving 3.7V (on a LiPo) (1S = 3.7V; 2S = 7.4V; 3S = 11.1V, 4S = 14.8v, …)

– The discharge rate expressed in C : 1C represents a current equal to 1 time the capacity of thebattery. Our 2200mah in 30C battery must be able to deliver a maximum amperage of 2200mah*30 = 66A
You will have then to choose a battery with a sufficient capacity (mah) to obtain an acceptable flight time (without exploding the weight), a consistent voltage with your motors and your ESC (for example 3S), and a sufficient discharge rate to feed yor motors (example: 4motors 15A max, battery 2200mah 30 C = 66A).It is generally recommended to keep some margin to avoid to stress out the maximum capacity of the battery, considering also that other equipments are also consuming some energy( radio, flight controller……).
A lot of medium size machines are fed in 3S as this voltage is consistent for most of the cases and it allows to feed a number of equipments directly foreseen for this voltage( camera, video transmitter, leds,..).
If we want to have more flying range, to lift more weight, we will be inclined to increase the voltage of the battery and decrease the KV of the motors.

 

Some examples of configurations that I use :

Large quadricoptère (~2Kg) : LiPo battery 3S 30C 5200mah

Medium quadricoptère  (~1,2Kg) : LiPo battery 3S 30C 2200mah

Mini quadricoptère (~500g) : LiPo battery 3S 25C 1300mah

Propellers :

hélices carbone

There are differents types of propellers with various materials and two main characteristics which define them:
– The size: expressed in Inch it is the length of the propeller.The more the length is the more lift offered by the propeller : (for a quadricoptère it generally varies between 8″ et 13″).
– The pitch: it is the angle of the propeller ( it generally varies between  4 and 6). The lower the pitch is the more traction at low speed offered by the propeller.However its maximum rotation speed will be more limited than a propeller with a high pitch.

Configurations with large propellers  (with then a lower rotation speed) will offer a more stable flight but with less reactivity compared with those with small propellers which will be much more dynamic.

Please note also that on a multi-rotor , all the propellers are not rotating in the same direction.You have then CW (clockwise) and CCW (counter clockwise) propellers.

Finally I hope that I haven’t forgotten anything , please do not hesitate to contact me if you have questions , I will then update or bring some corrections( in case of mistakes).

Some examples of configurations that I use :

Large quadricoptère (~2Kg) : 13″ propeller

Medium quadricoptère (~1,2Kg) : 10″ propeller

Mini quadricoptère (~500g) : 5″ propeller

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