Page 42 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

Chapter 2: Quadcopter Flight Dynamics 21

the elevator changes the pitch attitude in a normal aircraft. The quadcopter change in pitch
happens by altering the speed of both motors forward of the CG. Reducing the speed should
pitch the quadcopter forward, and it will proceed in a forward direction. However, it is not
simply a matter of changing motor speeds because the altitude at which the quadcopter is
operating should not change as a result of the pitch command. All the motor speeds must
change, both to maintain altitude and to effect a pitch rotation. The following set of equations
should help to clarify all of the quadcopter operations. Figure 2.10, which is a modified
version of Figure 2.3, shows all the motors with matching equation and rotation identifiers.

MP1 = Motor speed for the left front motor
MP2 = Motor speed for the right front motor
MP3 = Motor speed for the left rear motor
MP4 = Motor speed for the right rear motor
T = Throttle setting

Straight up or down vertical flight:
MP1 = MP2 = MP3 = MP4 = T

Pitch change in a hover state:

The motor speeds for MP1 and MP2 must be changed in order to pitch the
quadcopter about the lateral axis. However, only changing these two motor
speeds will upset the altitude that is established. Therefore, the flight-control
board computes an offset speed that it subtracts from both forward motors, while
it adds the same offset to both rear motors, thus allowing for a pitch change
but not changing the overall throttle setting. This ensures that the quadcopter
does not change altitude.

Figure 2.10 Quadcopter diagram with motor identifiers.