Movement of a multi-rotor

Multi-rotors are a popular platform, with many applications for many remotely piloted aircraft (RPA). Multi-rotors do not rely on complex mechanical linkages or actuating control surfaces to get their movement. Instead, they rely on fixed-pitch rotors and use variation in motor speed for vehicle control.

When embarking on any project, especially one as complex as multirotor construction and piloting, it is useful to have an understanding of the theoretical underpinnings involved. So, when building and flying multirotor, it is valuable to have at least a basic understanding of the physics of multirotor flight. Here we will focus on the physics involved in manoeuvring a quadcopter, which involves adjusting the balance of forces acting on the craft. The principles discussed can be easily extended to a copter with six or eight rotors.

  1. Movement of a multirotor aircraft moves about three axes: the Vertical axis which runs vertically through the aircraft, the Lateral axis which runs across the aircraft, and the Longitudinal axis which runs along the length. Rotation about these axes cause three different movements, they may be familiar — they are called Pitch, Roll and Yaw.
Figure 1.1: Movement and Axes of a quadcopter is described by roll, pitch and yaw.

  • Pitch tilts the multirotor forwards or backwards causing it to move forward or backwards.
  • Roll tilts the multirotor side to side causing it to move sideways left or right.
  • Yaw rotates the multirotor clockwise or anticlockwise changing the direction it is facing (bearing) as it stays level to the ground.
    The roll, pitch and yaw angles are controlled by the pilot. A fourth input is also required to fly a multirotor: the throttle. The throttle is used to vary the altitude by changing the overall thrust of the multirotor.
    The operator controls these movements using two gimbal sticks on the transmitter. The most natural transmitter stick control configuration is what is called Mode 2 as shown in Figure 1.2. Forward, Backwards, Left and Right movements are controlled by the right-hand stick. While yaw and altitude are on the left-hand stick.
Figure 1.2: Mode 2 Transmitter Stick Controls

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