Rotorcraft

A rotorcraft is a type of heavier-than-air aircraft. It uses rotary wings to generate lift. These wings spin around a vertical mast called a rotor.

This design allows for unique flight capabilities. Many rotorcraft can perform vertical takeoff and landing. This means they do not need a long runway to fly. They can also hover in one place. This distinguishes them from standard fixed-wing airplanes.

Helicopters are the most well-known type of powered rotorcraft. An engine drives the rotors throughout the entire flight. This power allows the pilot to hover or fly in any direction. They can move forward, backward, or laterally. Because the engine spins the rotor, it creates a torque reaction. This twisting force must be managed by an antitorque device. Common devices include a tail rotor, a fantail, or a NOTAR system.

Some rare helicopters use tip jet propulsion to avoid this torque.

An autogyro uses a different mechanism for lift. It features an unpowered rotor that spins through autorotation. This happens when air flows up through the rotor disk. An engine-powered propeller provides the thrust to move the craft forward.

Early models looked like fixed-wing aircraft with front-mounted engines. Modern autogyros often use a rear-mounted pusher configuration. Juan de la Cierva invented the autogyro in 1920. Later, Etienne Dormoy tested the Buhl A-1 Autogyro with a pusher propeller.

A gyrodyne combines features of both helicopters and autogyros. The engine drives the rotor for takeoff and hovering. For forward flight, it uses propellers mounted on wings. As propeller power increases, the rotor requires less power for thrust. This results in reduced pitch angles and less blade flapping. This method is more efficient than a free-wheeling autogyro. It helps minimize the adverse effects of retreating blade stall at high speeds.

Engineers design rotors with different numbers of blades. Most rotors have between two and six blades on a driveshaft. The number of rotors also varies by design. A monocopter uses a single rotor. A quadcopter uses four rotors, often with two spinning clockwise and two counter-clockwise. Multirotors use more than four blades. These include hexacopters with six rotors and octocopters with eight. These configurations are very popular for unmanned aerial vehicles (UAVs).

There are also complex multi-rotor layouts. Two rotors can be arranged in a tandem configuration. They can be placed side by side, known as transverse. Coaxial rotors consist of one rotor disc above another on concentric shafts. Intermeshing rotors, or synchropters, use two rotors at an acute angle. Their driveshafts are geared together to synchronize the blades. In 1948, the Cierva Air Horse used three rotors. This was done because engineers believed a single rotor could not be strong enough for that size.

Some experimental designs use stopped rotors. These aircraft spin the rotor for vertical flight. They then stop the rotor to use it as a fixed wing for high-speed flight. The Sikorsky S-72 research aircraft tested a four-bladed version called the X-wing. This program was cancelled in 1988 before it ever flew. The Boeing X-50 Dragonfly also tested a two-bladed rotor starting in 2003. However, both prototypes crashed during transition attempts.

New technology continues to explore these transitions. The US Naval Research Laboratory patented a transition method in 2011. An Australian company developed a prototype called the Hybrid RotorWing. This design uses high alpha airflow to provide symmetrical airflow. This allows the craft to transition from fixed to rotary mode mid-flight. Another concept is the tailsitter. In this design, the lifting surfaces act as rotors during takeoff. The craft then tilts over for horizontal flight.