Helicopter

A helicopter is a rotorcraft that generates both lift and thrust from one or more horizontally spinning rotors. This design enables vertical takeoff and landing, hovering, and movement in any direction—forward, backward, or sideways. Because of these capabilities, helicopters can operate in crowded or remote locations where fixed-wing aircraft, as well as many short takeoff and landing (STOL) or short takeoff and vertical landing (STOVL) designs, require a runway.

The first practical, fully controllable helicopter was the Focke-Wulf Fw 61, introduced in 1936. In 1942, the Sikorsky R-4 became the first helicopter to enter full-scale production. Between 1939 and 1943, Igor Sikorsky developed the VS-300 through four iterations, establishing the foundation for modern helicopters with a single main rotor and a single tail rotor.

While many early designs used multiple main rotors, the most common configuration today is a single main rotor paired with a vertical anti-torque tail rotor (a unicopter, distinct from the single-blade monocopter). However, twin-rotor helicopters (bicopters) with tandem or transverse rotors are still used for their greater payload capacity compared to monorotor designs. Coaxial-rotor, tiltrotor, and compound helicopters also remain in operation. Four-rotor helicopters (quadcopters) were first built in France in 1907, and along with other multicopters, have been developed mainly for specialized uses such as commercial drones, driven by the rapid growth of drone racing and aerial photography in the early 21st century, as well as for military roles like artillery spotting, aerial bombing, and kamikaze drones.

The English word "helicopter" comes from the French *hélicoptère*, coined by Gustave Ponton d'Amécourt in 1861. It combines the Ancient Greek roots *helix* (ἕλιξ, meaning "helix, spiral, whirl, convolution") and *pteron* (πτερόν, meaning "wing"). English speakers often mistakenly break the word into "heli-" and "-copter," leading to terms like "helipad" and "quadcopter." Common nicknames include "chopper," "copter," "heli," and "whirlybird." In the U.S. military, "helo" (pronounced ) is a frequent slang term.

A helicopter is a rotorcraft whose lift and thrust come from one or more horizontally spinning rotors. This differs from an autogyro (or gyroplane) and gyrodyne, which use a free-spinning rotor for all or part of flight and rely on a separate thrust system to move forward, so airflow spins the rotor to generate lift. A compound helicopter also has a separate thrust system but continues to power the rotor throughout normal flight. U.S. federal regulations define a helicopter as a rotorcraft that depends primarily on its engine-driven rotors for horizontal motion.

The rotor system—or simply rotor—is the rotating part that generates lift. Main rotors are mounted horizontally to provide vertical lift, while a tail rotor is mounted vertically to produce horizontal thrust that counteracts torque from the main rotors. A rotor consists of a mast, hub, and blades. The mast is a cylindrical metal shaft extending upward from the transmission, with the hub at its top as the attachment point for the blades. Main rotor systems are classified by how blades attach and move relative to the hub: hingeless, fully articulated, and teetering, though some modern systems combine these.

Most helicopters have a single main rotor, but the torque from its aerodynamic drag must be countered. Igor Sikorsky’s VS-300 used a smaller tail rotor for this purpose, which pushes or pulls against the tail to offset torque, typically mounted at the end of a tail boom. Other anti-torque systems include the ducted fan (Fenestron or FANTAIL) and NOTAR, which uses the Coandă effect on the tail boom to generate anti-torque similar to how a wing produces lift.

Using two or more horizontal rotors turning in opposite directions is another way to counteract torque without a tail rotor. This allows power normally diverted to the tail rotor to be applied fully to the main rotors, improving efficiency and lifting capacity. Common counter-rotating configurations include: tandem rotors (one behind the other), transverse rotors (mounted at the ends of wings or outriggers, now used on tiltrotors and some early helicopters), coaxial rotors (one above the other on the same axis), intermeshing rotors (mounted close together at an angle so they intermesh without colliding, known as a synchropter), and multirotors (three or more rotors, with specific terms like tricopter, quadcopter, hexacopter, and octocopter for three, four, six, and eight rotors respectively; quadcopter is the most common).