Quadcopter

A quadcopter, also known as a quadrocopter or quadrotor, is a rotorcraft with four rotors. While experimental quadrotor helicopters and convertiplanes existed for decades, the design remained a novelty until the rise of modern drones. The small size and low inertia of these unmanned aerial vehicles allow for a very simple flight control system, making the small quadrotor practical for the first time.

Torque

Each rotor generates lift and torque around its center, plus drag opposing the direction of flight. Typically, two rotors spin clockwise and two spin counterclockwise.

Design principles

Flight is controlled by independently adjusting each rotor’s speed, which changes its lift and torque. Pitch and roll are managed by shifting the net center of thrust, while yaw is controlled by altering the net torque. Unlike conventional helicopters, quadcopters usually lack cyclic pitch control, where blade angles change dynamically as they rotate.

In early aviation, quadrotors (then called helicopters) were seen as a solution to vertical flight problems. Counter-rotation eliminates torque-related control issues and the inefficiency of a tail rotor, which provides no lift.

Short blades are also easier to build. Several manned designs appeared in the 1920s and 1930s, and were among the first successful heavier-than-air vertical takeoff and landing vehicles. However, early prototypes performed poorly, and later ones required excessive pilot effort due to weak stability augmentation and limited control.

If all four rotors spin at the same speed—two clockwise, two counterclockwise—the net yaw torque is zero, eliminating the need for a tail rotor. Yaw is produced by unbalancing the aerodynamic torques, such as by offsetting thrust commands between counter-rotating pairs. Quadcopters are subject to normal rotorcraft aerodynamics, including the vortex ring state.

The main mechanical parts are a frame, four rotors (fixed- or variable-pitch), and motors. For best performance and simplest control, the motors and propellers are equally spaced. A quadcopter can also use a coaxial rotor configuration, where each arm has two motors spinning in opposite directions—one facing up, one down—for more power and stability at lower weight.

Autonomous flight

The quadcopter configuration is relatively easy to program for autonomous flight, enabling experiments with complex swarming behavior based on basic sensing between drones.