From RC Helicopter Wiki
A quadcopter, quadrotor or quad is a type of radio controlled and/or GPS controlled electric helicopter. They are also produced in full scale.
Quadcopters generally consist of a central hub from which four radial arms extend at ninety-degree intervals. The ends of each arm are equipped with an upward- or downward-facing outrunner motor driving standard and reverse rotation model airplane propellers or quadcopter-specific propellers.
The result is an extremely stable yet nimble model aircraft with high payload capability. This makes the quadcopter an excellent base for aerial photography, FPV flight or as an autonomous flying robot/unmanned aerial vehicle.
Each rotor produces both thrust and torque, with rotors one and three rotating clockwise and rotors two and four counterclockwise. This makes the net aerodynamic torque and angular acceleration about the yaw axis exactly zero in much the same manner as a coaxial helicopter. Directional flight is induced by mismatching the balance in aerodynamic torques, i.e., by offsetting the equal thrust of the motors at hover. This eliminates the need for linkages of any kind in a model quadcopter.
While the lack of linkages makes a model quadcopter among the simplest of flying models from a mechanical standpoint, the electronics necessary to fly a quadcopter are often extremely sophisticated. Many rely on an inertial measurement unit combining a gyroscope with an accelerometer, barometer and magnetometer. This may be tied into a standard R/C system, a programmable GPS system, a first-person-view system or a combination of any or all of the above.
The Blade mQX, the world's first hobby grade R/C micro quadcopter introduced in January 2012, relies on AS3X technology introduced by Horizon Hobby in October 2011. AS3X is a three-axis gyroscopic stabilization system which automatically stabilizes micro models which in turn causes them to perform like far larger models, even in wind. The motherboard of the mQX allows it to be flown in "X" configuration (two motors fore, two aft) or "plus sign" configuration (one motor both fore and aft; one motor per side).
Similar to the quadcopter is the Y-copter. In this configuration, there are three arms radiating at 120-degree intervals with one or possibly two motors per arm. A hexcopter has six radials at 60-degree intervals.
Perhaps the best-known quadcopters are manufactured by Taiwanese model manufacturer Gaui.
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