A quadrotor, also called a quadrotor helicopter, is an aircraft that is lifted and propelled by four rotors. Quadrotors are classified as rotorcraft, as opposed to fixed-wing aircraft, because their lift is derived from four rotors. They can also be classified as helicopters, though unlike standard helicopter, quadrotors are able to use fixed-pitch blades, whose angle of attack does not vary as the blades rotate. Control of vehicle motion can be achieved by varying the relative speed of each rotor to change the thrust and torque produced by each.

There exist two generations of quadrotor designs. The first generation quadrotors were designed to carry one or more passengers. These vehicles were among the first successful heavier-than-air vertical take off and landing (VTOL) vehicles. However, early prototypes suffered from poor performance, and latter prototypes required too much pilot work load, due to poor stability augmentation.

The more recent generation of quadrotors are commonly designed to be unmanned aerial vehicles (UAVs). These vehicles use an electronic control system and electronic sensors to stabilize the aircraft. With their small size and agile maneuverability, these quadrotors can be flown indoors as well as outdoors.

The advantages of the current generation of quadrotors, versus comparably scale helicopters, are as follows. First, quadrotors do not require mechanical linkages to vary rotor angle of attack as they spin. This simplifies the design of the vehicle, and reduces maintenance time and cost. Second, the use of four rotors allows each individual rotor to have a smaller diameter than the equivalent helicopter rotor, for a given vehicle size, allowing them to store less kinetic energy during flight. This reduces the damage caused should the rotors hit any objects. For small scale UAVs, this makes the vehicles safer to interact with in close proximity. Finally, by enclosing the rotors within a frame, the rotors can be protected during collisions, permitting flights indoors and in obstacle-dense environments, with low risk of damaging the vehicle, its operators, or its surroundings.

Flight Control

Each rotor produces both a thrust and torque about its center of rotation, as well as a drag force opposite to the vehicle's direction of flight. If all rotors are spinning at the same angular velocity, with rotors one and three rotating clockwise and rotors two and four counterclockwise, the net aerodynamic torque, and hence the angular acceleration about the yaw axis is exactly zero, which implies that the yaw stabilizing rotor of conventional helicopters is not needed. Yaw is induced by mismatching the balance in aerodynamic torques (i.e. by offsetting the cumulative thrust commands between the counter-rotating blade pairs).

Angular accelerations about the pitch and roll axes can be caused separately without impacting the yaw axis. Each pair of blades rotating in the same direction controls one axis, either roll or pitch, and increasing thrust for one rotor while decreasing thrust for the other will maintain the torque balance needed for yaw stability and induce a net torque about the roll or pitch axes. This way, fixed rotor blades can be made to maneuver the quad rotor vehicle in all dimensions. Translational acceleration is achieved by maintaining a non-zero pitch or roll angle.


  • Oehmichen No.2, 1920
    Etienne Oehmichen experimented with rotorcraft designs in the 1920s. Among the six designs he tried, his helicopter No.2 had four rotors and eight propellers, all driven by a single engine. The Oehmichen No.2 used a steel-tube frame, with two blade rotors at the ends of the four arms. The angle of these blades could be varied by warping. Five of the propellers, spinning in the horizontal plane, stabilized the machine laterally. Another propeller was mounted at the nose for steering. The remaining pair of propellers were for forward propulsion. The aircraft exhibited a considerable degree of stability and controllability for its time, and made more than a thousand test flights during the middle 1920s. By 1923 it was able to remain airborne for several minutes at a time, and on April 14, 1924 it established the first-ever FAI distance record for helicopters of 360m. Later, it completed the first 1km closed-circuit flight by a rotorcraft.
  • De Bothezat, 1922
    Dr. George de Bothezat and Ivan Jerome developed this aircraft, with six bladed rotors at the end of an X-shaped structure. Two small propellers with variable pitch were used for thrust and yaw control. The vehicle used collective pitch control. It made its first flight in October 1922. About 100 flights were made by the end of 1923. The highest it ever reached was about 5m. Although demonstrating feasibility, it was, underpowered, unresponsive, mechanically complex and susceptible to reliability problems. Pilot workload was too high during hover to attempt lateral motion.
  • Convertawings Model "A"' Quadrotor, 1956
    This unique helicopter was intended to be the prototype for a line of much larger civil and military quadrotor helicopters. The design featured two engines driving four rotors with wings added for additional lift in forward flight. No tailrotor was needed and control was obtained by varying the thrust between rotors. Flown successfully many times in the mid 1950's, this helicopter proved the quadrotor design and it was also the first four-rotor helicopter to demonstrate successful forward flight. Due to a lack of orders for commercial or military versions however, the project was terminated.

Current Programs

Bell Boeing Quad TiltRotor

The Bell Boeing Quad TiltRotor concept takes the fixed quadrotor concept further by combining it with the tiltrotor concept for a proposed C-130 sized military transport.


External links

Open Source Vehicles

Research Projects

Commercially Available Vehicles

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