When choosing the motors, it should be good to have some idea on how and what the copter will be used for. Are you lifting something heavier to the air or should the aircraft be quick to react to the smallest command sent by the controller?
Multicopters use brushless electric motors. Therefore, motors do not contain carbon brushes that can be found in many other electric devices that utilize electric motors. In brushless motor the rotation is achieved by electrifying the coils inside the motor perfectly timed by the speed control. This technique reduces the amount of moving (wearing) parts into minimum and enables the motor to achieve better efficiency.
Motor's "power" can be described using the KV-number. This value tells you approximately how many cycles the motor rotates around its axis in a minute per one volt of input voltage. Multicopters usually use motors with the range of 700kv - 1400kv. Motors having a lower KV are usually physically larger, so they are able to produce greater torque to the propeller. Higher KV number ensures a higher engine speed, which is useful if the copter is used to fly more acrobatically.
The 750kv engine is a good choice for example flying with the GoPro, because the engine has enough torque to rotate a larger propeller, which in turn produces more lift (the helicopter is able to pick up more load).
The 1100kv is a good average, and which I use in my copter. GoPro still flies smoothly but it’s not worth to try to lift any more than it. This would stress the motors more and the flight time would not be that great.
These figures, however, are mainly rough guides. The final amount of lift that the copter will produce or the speed it will achieve are affected by many other things than just the motor. Motor choice is one of the trickiest things that you have to make while building a copter and you should follow the RC chats and forums and try to see what would work best in your case.
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