[h=2] TUTORIAL - How to build Multirotor from scratch9（2/2)[/h]
A tutorial From: http://www.multicopterclub.com/

PROPELLER
We always neglect this plastic piece. Just because it's cheap? Who knows!! But in multirotor application contribution of prop is remarkable. Specifications of prop are easy to understand and they are dialmeter and pitch. Type of prop is important as well but we will see effect of diameter and pitch on flight of multirotor. Generally we see prop with the specification of
7x3.5
8x4.5
9x5
10x3.8
10x4.5
10x6
11x4.7
12x3.8
First value is diameter of prop and second value is pitch. Both are in inches.
Diameter: Virtual circle that prop generates.
Pitch: Amount of travel per revolution.
As we see above our motor runs at 6660 RPM at NO LOAD. But when you mount prop on it, RPM will be reduced. Here we will take example of two props 10x3.8 and 10x6. When you mount 10 inch diameter prop RPM of motor will be reduced to 3600 RPM (Revolutions Per Minute).
60 Revolutions Per Second.
Our 1st prop has 3.8 inch pitch. Means per revolution it will travel 3.8inch. So
60 x 3.8 = 228 Inch/Sec = 5.7 m/sec
For 2nd prop, it has 6 inch pitch.
60 x 6 = 360 Inch/Sec = 9.1 m/sec
So we can say if we have 10x3.8 prop our quad will climb in the air at 5.7 m/sec, while with 10x6 prop climb rate will be incread to 9.1 m/sec.
Bigger dia prop can produce more thrust.
So which prop is best for our multirotor?
Generally you will get suggested prop value in motor specification, so you should go with it and buy 1-2 pair extra. But what if prop value is not given. You will see kind of table with different props, Volts, Amp, Thrust and Efficiency. Here you will have to try trial and error method. But it doesn't mean you swing 13x3.8 prop on 1700kV motor.
Lower kV motor can deal with bigger prop. With increasing kV value size of prop will be decreased. So you will have to keep this in mind. For multirotor you should go with low pitch prop if you need more stability and less vibrations. How to balance prop? We will see in next part.

ESC(Electronic Speed Controller)
ESC supplies power from battery but not constant, it varies according to input signal. ESC also has BEC(Battery Eliminated Circuit). BEC is nothing but 5V output from ESC that can power up receiver, servomotor(for camera gimbal) and FC. But how to select ESC for our multirotor? Well it's really simple. You only need to keep in mind that Ampere rating of ESC should be higher than max amp rating of motor. For example the motor we selected draws maximum 15Amp so your ESC rating should be higher than 15amp. Say 18-20Amp. Still confused? Here is simple forumla. Thanks to Sai sir(rcforall) of RCI.
ESC = 1.2-1.5 x max amp rating of motor
1.2-1.5 x 15 = 18-22.5A
So you can select ESC between range of 18A to 22A. But please note it's note thumb rule. You can go with either 18-20A or 25A (it might be bit over powered). Whichever ESC you choose make sure it has programing facility(throttle range, battery mode etc). Go with quality ESCs check user reviews and buy 1 spare.

BATTERY
This beautiful monster eats quality food. If you don't feed them they refuse to fly. So how to select battery for your multirotor?
For that maximum Amp rating of motor comes first. If you remember our motor draws max 15amp. We are working on quadcopter and it has 4 motors, so all 4 motors will draw
4 x 15amp = 60Amp.
Now let me explain few specifications of battery. You will see them on battery and website as well.
mAh
C discharge rating
2S, 3S or 4S.

mAh:
1000mA = 1A
You can compare mAh rating of battery with petrol tank of your vehicle. Big tank more petrol you can fill and more you can drive. The same way more mAh rating gives you more flight time.
C Discharge Rating:
Maximum current(A) at which battery can be discharged at particular time.
1S, 2S, 3S & 4S
1S = 1 cell of 3.7V
2S = 2 cells in parallel x 3.7V = 7.4V
3S = 3 cells x 3.7V = 11.1V
4S = 4 cells x 3.7V = 14.8V
4S means more power than 3S. If you remember we discussed that
Watt = V x I
Here we are increasing value of Volt so watt will be increased. Our motor draws max 15A. So watt value for 3S and 4S will be
At 3S battery 11.1 x 15 = 166.5 Watt
At 4S battery 14.8 x 15 = 222 Watt
but make sure your motor and esc are capable of handling 4S.
Generally you should go with 3S battery only.
Now as we know we need atleast 60A current. So here we will take example of two batteries.
2200mAh 25C
2200mAh 40C
Which battery is good for our multirotor? Let's see
2200mAh = 2.2A
2.2 x 25C = 55A
2.2 x 40C = 88A
We need atleast 60A so 2200mAh 40C is good for us. Go ahead and get it.

I hope you have gathered enough confident to build your own multirotor from scratch. Next part will be even more interesting. So stay tuned.