Helicopters belong to a class of aircraft known as rotorcraft, the other members of which are autogiros (or gyroplanes) and convertiplanes. Helicopters and autogiros are superficially similar to one another in that both are wholly sustained in flight by the lift generated as a result of the rotation of long thin wings, or rotor blades, in a horizontal plane.
The blades of an autogiro, however, are rotated by the action of air blowing through them, in the manner of a windmill, while those of a helicopter are driven directly by an engine. Autogiros cannot therefore land or takeoff vertically in calm air. Helicopters, on the other hand, can takeoff or land vertically, hover, and fly forward, backward, or sideways irrespective of the wind.
The principles of helicopter flight have been known for centuries. Leonardo da Vinci designed one, and many helicopter models were made by early flight pioneers, such as Sir George Cayley in 1927. The first helicopter capable of carrying a person, built by Paul Cornu in France in 1907, was powered by a 24-horsepower engine, but stability and engineering problems held back further development for decades.
It was not until January 1942 that the world’s first practical helicopter, the VS-316A, was built by a Russian-born U.S. engineer Igor Sikorsky. This machine had the simplest possible con- figuration, the design of which is still used most widely today.
Construction
The main structural element of a helicopter is the fuselage, housing the crew, payload, fuel, and power plant, which until the mid 1950s was a piston engine but is now usually a gas turbine. The output shaft from the engine, turning at several thousand revolutions per minute, is connected to a transmission, which steps down the speed to between 300 and 400 rpm to drive the rotor (the assembly carrying the hub and the attached blades) since the rotor tips must be restricted to subsonic speeds to ensure efficient operation.
The reaction of the rotor spinning in one direction would cause the rest of the helicopter to rotate uncontrollably in the opposite direction. In order to prevent this problem, a secondary rotor, of smaller diameter, is mounted on the rear end of the fuselage and driven by a second shaft from the gear box at such a speed that it exactly neutralizes the turning action of the main rotor.
Each of the blades of the main rotor (modern helicopters have any number from two to seven) is inclined (with its leading edge upward) so that it meets the air at a small angle to the horizontal. This is the pitch angle, analogous to the pitch of a propeller or a screw thread.
Pitch control
When hovering, the combination of rotor speed and the pitch of the blades provides a lift force that is exactly in balance with the weight of the helicopter. In order to climb, the rotor has to generate more lift, and this is achieved not by increasing the speed of the rotor (the rotor speed of a particular helicopter at all times remains virtually constant, irrespective of what the aircraft is doing), but by increasing the pitch of the blades. More lift, however, also means more drag and so extra power is needed from the engine to maintain rotor speed.
The pitch of the blades is controlled from the cockpit by means of the collective pitch lever, so called because it changes the pitch of all the main rotor blades by the same amount. This lever is mounted on the floor and is one of the very few differences between the cockpit of a helicopter and that of a conventional fixed-wing aircraft. Operated by hand, it is moved up to gain height and down to descend.
Since most maneuvers, including climbing and descending, necessitate changes of power, the collective pitch lever has a twist-grip throttle control at the top so that engine power and blade pitch can be controlled and coordinated with one hand.
