Quadcopter Mechanics

  Flight Mechanics 

   In order to learn how to control a quadcopter, you'll need to learn a bit about quadcopter mechanics - how quadcopters fly by adjusting rotor speeds. Quadcopters have different flight mechanics than conventional aircraft such as airplanes and helicopters. They are Vertical Take Off and Landing (VTOL) aircraft similar to helicopters; however their attitude and flight mechanics are vastly different.

Helicopters use two rotors, one main rotor that provides lift and attitude changes to pitch and roll, and one stabilizer tail rotor. The tail rotor is used for stabilizing the aircraft against torque forces from the main rotor and to control yaw. Quadcopters, and other multi-rotor aircraft rely on four or more rotors that act together to tilt the entire aircraft to perform pitch and roll changes. The motors are controlled by the on board flight controller while analyzing the current attitude using one or more sensors and pilot controls. The steeper of the tilt angle, the faster the aircraft will travel in that direction. This must be balanced with stability as well; too steep of an angle and the aircraft can flip and crash. If the angle isn't large enough, then the aircraft can move too slowly. This attitude change is accomplished by the flight controller commanding the motors with less thrust and the opposite motors with more thrust, which will be discussed in further detail shortly.

Flight Orientations

   Because quadcopters are symmetrical, they can fly in two orientations, plus (+) and x orientation. With plus orientation, the front of the quadcopter is one of the motor booms, while x orientation the front of the quadcopter is two motor booms. This will change the motor out equations slightly as you will see later on in the post.

 

 Axis Representation

   The coordinate system used is the same for each of the plus and x flight orientations. The aircraft flies forward and back along the x axis, left and right along the y axis, ascends and descends along the z axis.
   
   Pitch is a rotation around the y axis, which controls forward/back flight along the x axis. Roll is a rotation around the x axis, which controls left/right flight along the y axis. Yaw is a rotation around the z axis which turns the aircraft left or right changing its heading and direction the aircraft flies in.

Attitude Changes

Plus Flight Orientation	X Flight Orientation

Images used from the Aeroquad open source project (www.aeroquad.com)

     The flight controller changes the aircraft's attitude by commanding the motor thrusts according to the pilot's controls from the transmitter. Quadcopters use a similar principle as helicopters for rotor stabilization and yaw control. Each alternating rotor is clockwise, then counterclockwise. Motor 1 in the pictures above spins clockwise, while motor 2 spins counterclockwise, motor 3 spins clockwise, and motor 4 spins counterclockwise. This counter rotation compensates for torque and gives the quadcopter the ability to control yaw rotation around the z axis. As the motors are spinning in alternating clockwise/counterclockwise rotation, they need proper propellers that can provide thrust below the propeller while in clockwise or counterclockwise rotation. Otherwise, if the propellers were providing thrust in the same direction, two will push air down, while the other two will pull air up. This also has led to another name for these propellers - pushers and pullers which depend on the way they're rotating in order to push air down or pull air up.
In order to execute attitude changes in the Plus Orientation:
Pitch Down (forward flight):

• Motor 1 decreases thrust
• Motor 3 increases thrust

Pitch Up (back flight):

• Motor 1 increases thrust
• Motor 3 decreases thrust

Roll Left (left flight):

• Motor 2 increases thrust
• Motor 4 decreases thrust

Roll Right (right flight):

• Motor 2 decreases thrust
• Motor 4 increases thrust

Yaw Left (turn left):

• Motors 2 and 4 increase thrust
• Motors 1 and 3 decrease thrust

Yaw Right (turn right):

• Motors 2 and 4 decrease thrust
• Motors 1 and 3 increase thrust

In order to execute attitude changes in the X Orientation:
Pitch Down (forward flight):

• Motors 1 and 2 decrease thrust
• Motors 3 and 4 increase thrust

Pitch Up (back flight):

• Motor 1 and 2 increase thrust
• Motor 3 and 4 decrease thrust

Roll Left (left flight):

• Motors 2 and 3 increase thrust
• Motors 1 and 4 decrease thrust

Roll Right (right flight):

• Motors 2 and 3 decreases thrust
• Motors 1 and 4 increases thrust

Yaw Left (turn left):

• Motors 2 and 4 increase thrust
• Motors 1 and 3 decrease thrust

Yaw Right (turn right):

• Motors 2 and 4 decrease thrust
• Motors 1 and 3 increase thrustThe flight controller uses the following motor out equations to control the motors depending on
  the pilot controls:
  Plus Orientation:
  
  • Motor 1 = throttle + pitch + yaw
  • Motor 2 = throttle - roll - yaw
  • Motor 3 = throttle - pitch + yaw
  • Motor 4 = throttle + roll - yaw
  
  X Orientation:
  
• Motor 1 = throttle + pitch + roll - yaw
• Motor 2 = throttle + pitch - roll + yaw
• Motor 3 = throttle - pitch - roll - yaw
• Motor 4 = throttle - pitch + roll + yaw

   Pilot controls change from positive/negative to change direction, when used in these equations the thrust increases/decreases depending on the value read by the receiver. Motor out equations and attitude configurations will differ depending on motor positioning and clockwise/counterclockwise rotating order on different quadcopters.