Helicopter

I

Introduction

Helicopter, heavier-than-air craft that derives its lift not from fixed wings like those of conventional aeroplanes, but from a power-driven rotor or rotors, revolving on a vertical axis above the fuselage. Helicopters can rise or descend vertically, hover, and move forwards, backwards, or sideways. The helicopter was the first type of heavier-than-air craft capable of vertical flight. It differs from the autogiro, another form of rotating-wing aircraft, in that its rotor provides lift, propulsion, and most of the flight control.

The rotor of a helicopter usually has two or more blades radiating symmetrically from a central hub. The rotor is driven by an engine, usually in the fuselage, through gears, which reduce the speed of rotation to less than the speed of the engine. An important feature of helicopter design is the development of devices to counteract the torque, or reaction force, developed when the rotation of the rotor in one direction tends to turn the fuselage in the opposite direction. The commonest form of antitorque device is a small propeller, similar to an aeroplane propeller, mounted at the tail of the helicopter on a lateral axis, in such a position as to push the tail to one side. Other types of helicopters use paired main rotors turning in opposite directions, which automatically neutralize each other's torque. In some helicopters the paired rotors are mounted one above the other on a single axis; and in others they are placed on struts on either side of the fuselage or at the front and back of the fuselage. Experimental helicopters have used small jet engines (see Jet Propulsion) mounted on the tips of the rotor blades to provide power and to eliminate torque.

When a helicopter is rising vertically from the ground or descending vertically, the lift on all the rotor blades is the same, because they are all moving through the air at the same speed. When the craft is moving forwards (or in any horizontal direction), however, the lift on some blades is greater than that on others. During each cycle the speed of the blades through the air varies, depending on whether the direction of the rotation is the same as or opposite to that of the motion of the helicopter. The airspeed at a particular point on a blade is equal to its speed of rotation at that point plus the forward speed of the helicopter during one half of the cycle and minus the forward speed during the other half. Hence, if the blades were fixed in a horizontal position, the amount of lift provided by each blade would vary during the cycle because lift increases with airspeed, and the helicopter would tilt to one side. To avoid this type of instability, most single-rotor helicopters have flapping blades. The blades are hinged close to the hub in such a manner that each blade rises when moving at greater airspeed to reduce the lift and drops when moving at lower airspeed to increase the lift. Thus, the effect of the variation of airspeed is nullified.

A helicopter can be flown in any direction by tilting the rotor in the desired direction. Tilting the rotor changes its lift from purely vertical to a combination of vertical and horizontal. To turn a helicopter, the rotor is first tilted in the direction of the turn, and then the thrust of the tail propeller is altered to turn the fuselage in the desired direction. Ascent and descent in helicopters are controlled by increasing or decreasing the speed of the rotor, the pitch of the rotor blades, or both. In the event of a power failure, the rotor of a helicopter is disengaged and will autorotate like the rotor of an autogiro, maintaining enough lift to permit the craft to descend slowly enough to avoid a catastrophic impact.

II

Uses

The helicopter has two principal advantages over conventional aircraft: the ability to fly slowly or hover; and the ability to take off and land in a restricted space. Airports for helicopters are called heliports. One of the most important non-military uses of the helicopter is in searching for and rescuing lost people, particularly in the sea and in mountainous regions. A helicopter can provide rescue for a person from a life raft, a mountain ledge, or other hazardous places. If the land space is large enough, the helicopter can alight and take a person aboard. If the area is too small for a landing, a rope ladder can be lowered from the hovering helicopter, or the person being rescued can be lifted with a winch, cable, and harness. A helicopter can provide quick, safe transport to hospitals and other such institutions. The craft can also operate from the decks of small vessels at sea and can land and take off from a rooftop in the centre of a congested city. Because a helicopter can hover and fly as slowly as desired, it provides an efficient means for the inspection of pipelines and power lines from the air. They are especially valuable for the supply of offshore oil and gas drilling rigs.

Like conventional aircraft, the helicopter can be flown by instruments at night or in bad weather. It has the further advantage of greater safety because of its manoeuvrability and controllable speed. Helicopters are used successfully for fire patrols in forest areas, for dusting crops with insecticides, for aerial prospecting, and for aerial planting of seed for reforestation and erosion control; they are also employed for passenger transport and, in some large cities, for postal service, sometimes even bringing the mail directly from the airport to the roof of the post office. Helicopters range in size from the single-passenger type to large, multiengined craft carrying 50 or more passengers.

Military helicopters are used in similar applications, and also for combat and in antisubmarine roles. Special helicopters have been designed for the lifting of heavy equipment. These so-called flying cranes have been used to place power-transmission pylons and pipelines in inaccessible areas and to salvage military equipment in war zones. Economically, the principal limitations of the helicopter are its low maximum forward speed of about 320 km/h (200 mph), its mechanical complexity, and the accompanying high cost per passenger-kilometre. Commercially operated helicopters are therefore currently limited to flight distances of 160 km (100 mi) or less.

III

History

It is reported that the ancient Chinese had a hand-spun toy, sometimes called a “flying top”, that rose as it revolved rapidly. It is probable, however, that the first person to envisage the possibility of a helicopter powerful enough to lift a human being and actually to experiment with models of his designs was the 15th-century Italian artist, engineer, and architect Leonardo da Vinci, who made drawings showing an aircraft with a helical rotor in about 1500. Leonardo planned to use muscular power to turn the rotor, although such power would never have been sufficient to operate a helicopter of this type successfully.

Among experimenters who worked in the first quarter of the 20th century were the Frenchmen Maurice Leger, Louis Charles Bréguet, Étienne Oehmichen, and Paul Cornu; the Hungarian-American Theodore von Kármán; Raoul Pescara in Spain; Jacob Christian Ellehammer in Denmark; Igor I. Sikorsky in Russia; and Emile Berliner and his son Henry in the United States. The Russian-born George DeBothezat and his co-worker Ivan Jerome developed their four-rotored craft for the US Army Air Service. Corradino d'Ascanio in Italy, Oscar von Asboth of Hungary, and others attacked the many problems of vertical lift. The Berliner helicopter was probably the first craft to make a controlled flight supported by powered rotors. The distance was only about 90 m (100 yd), and the altitude about 4.6 m (15 ft), but the helicopter was being manoeuvred at the will of the pilot, Henry Berliner. The invention of the hinged, flapping rotor blade by the Spaniard Juan de la Cierva for his autogiros made possible the development of practical helicopters.

The first truly successful helicopter was a twin-rotor machine designed by the German engineer Heinrich Focke, which was flown in 1936. In 1939 the aeronautical engineer Igor Sikorsky, by then a naturalized American, flew a practical single-rotor craft, the VS-300. Its successor, the XR-4, made the first cross-country flight, from Stratford, Connecticut, to near Dayton, Ohio, a distance of about 1,225 km (760 mi), on May 13 to 17, 1942.

In 1967 two Sikorsky HH-3 helicopters made the first transatlantic flight, from New York to Paris, using aerial refuelling. The experimental Lockheed AH-56A and Piasecki Pathfinder-3 were the first models to attain speeds in excess of 400 km/h (250 mph). Approximately 2,000 helicopters were used by US forces in the Vietnam War to evacuate the wounded, to transport personnel and cargo, to observe enemy activities, and to draw ground fire.

The economic use of the helicopter for commercial transport over flight distances up to about 400 km (250 mi) has not yet been proved. In this field, the helicopter has to compete with vertical take-off and landing (VTOL) and short take-off and landing (STOL) aircraft. Some advanced helicopter designs have supplementary stub wings and forward-directed propulsion in addition to a main and a tail rotor. If successful, these compound helicopters would be able to carry passengers at cruising speeds up to about 425 km/h (265 mph).