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The calculations here are fairly easy. You multiply the current voltage of the motor with its RPM/V, multiply that with 60 and 2*π to get the revolutions per minute of the rotors. You do that with all rotors and then calculate the thrust and torque of the copter:
Thrust
The thrust of the copter itself is only one way: upwards (at least the upwards of the copter). The actual acceleration is later calculated and considers the angles of the copter and other things.
The calculation is fairly easy. You simply square the RPMs of all motors, add them together and multiply them with the lift constant/coefficient of the rotors.
Torque
Now it gets a little bit more interesting. Once again we square our RPMs. Then we divide our calculations into our three axes: The Roll (rotation around x-axis), pitch (rotation around y-axis) and yaw (rotation around z-axis). Yaw is pretty straightforward: Every Propeller creates a counterforce if it turns. So if a propeller turns clockwise, it creates a force counterclockwise on the arm it is attached on. Since our copter is a rigid body, the rotation translates to the whole body. So if all our rotors would turn clockwise, we would create four forces to turn our copter counterclockwise. WE DON'T WANT THAT! So we turn two of the rotors (the one in the front left and the one in the back right) counterclockwise. Now if we need to turn the copter clockwise, we can rotate these two rotors faster than the other two and vice versa. Now we only have to consider the drag of the rotors and we get the torque around the z-axis.
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