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I want to build a quadcopter but don't have any idea about motor and rotor size selection. Is there a formula or equation to use? What are other things that need to be considered? Trial and error method would be waste of time and money.

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The most important thing to remember here is the Purpose. For what purpose do you want to use the Quadcopter?

That being said , let’s start with the very basic .

Required Thrust = ( Weight x 2 ) / 4

In order to choose a motor you need to determine how much weight you are planning to take, and then work out the thrust required to lift the quadcopter .Also , when purchasing motors you need to consider Watt’s and Efficiency as well apart from Motor KV and Thrust.The higher the Efficiency, the better it is.The manufacturer/seller of Motors will provide the specifications about the Motor. This will help you to find out the thrust,power, rpm etc of the motor.

http://www.hobbyking.com/hobbyking/store/__28675__D2830_11_1000kv_Brushless_Motor_UK_Warehouse_.html?strSearch=1000kv

Quadcopters have 2 clockwise(CW) and 2 counter-clockwise(CCW) propellers and it has two flight configurations. X config and + config. In X config two arms will face front side while in + config only one arm will face front side. If you are making Quadcopter with plus config you can tune your quadcopter easily by single axis tuning method .

Single Axis PID Tuning A Quadcopter

Increased propeller pitch and length generally draws more current. Also the pitch can be defined as the travel distance of one single prop rotation. In other words, higher pitch means slower rotation, but will increase your vehicle speed which in turn will use more power.If your intention is to do acrobatics, you will need torque propellers which provide more acceleration and it puts less pressure on the power system. Lower pitch propellers will also improve stability.A higher pitch propeller moves greater amount of air, which could create turbulence and cause the aircraft to wobble during hovering. If you notice this with your quadcopter, try to choosing a lower pitched propeller.

http://blog.oscarliang.net/how-to-choose-motor-and-propeller-for-quadcopter/

You may also choose to use this below Quadcopter Calculator. You can dial in everything from motors to props to ESC.

http://www.ecalc.ch/indexcalc.htm

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  • $\begingroup$ thanx for such detailed information..... i will check out the links........I understood why division by four in the formula...it's because it's a quadcopter...so equal distribution among 4 motors..but why weight*2? $\endgroup$ – MANU Aug 25 '14 at 13:32
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    $\begingroup$ @IRON-MAN: You need excess lift in order to accelerate upwards. $\endgroup$ – Peter Kämpf Aug 25 '14 at 13:46
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    $\begingroup$ At thrust=weight you can just hover and land, you cannot climb. Weight*2 means you can hover at 50% max thrust (since in that case thrust*0.5=weight) $\endgroup$ – slebetman Jan 13 '15 at 7:13
  • $\begingroup$ If i want to use 3 motors then? $\endgroup$ – user14571 Apr 20 '16 at 1:52
  • $\begingroup$ The need for thrust greater than weight is not just to be able to climb, but also for attitude control authority. Three rotors instead of 4 will not provide enough degrees of freedom to maintain attitude control, unless you can also tilt at least one of the rotors or use cyclic. $\endgroup$ – pericynthion Dec 6 '17 at 18:40

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