Helicopter (I)

Helicopter is an aircraft with one or more power-driven horizontal propellers or rotors that enable it to take off and land vertically, to move in any direction, or to remain stationary in the air. Other vertical-flight craft include autogiros, convertiplanes, and V/STOL aircraft of a number of configurations.

The idea of taking off vertically, making the transition to horizontal flight to the destination, and landing vertically has been for centuries the dream of inventors. It is the most logical form of flight, dispensing as it does with large landing fields located far from city centres and the inevitable intervening modes of travel—automobile, subway, bus—that flight in conventional aircraft usually requires. But vertical flight is also the most demanding challenge in flying, requiring more sophistication in structure, power, and control than conventional fixed-wing aircraft. These difficulties, solved over time by determined engineers and inventors, made the progress of vertical flight seem slow compared to that of conventional flight, for the first useful helicopters did not appear until the early 1940s.

Helicopters (Principles of flight and operation)

Unlike fixed-wing aircraft, the helicopter's main airfoil is the rotating blade assembly (rotor) mounted atop its fuselage on a hinged shaft (mast) connected with the vehicle's engine and flight controls. In comparison to airplanes, the tail of a helicopter is somewhat elongated and the rudder smaller; the tail is fitted with a small antitorque rotor (tail rotor). The landing gear sometimes consists of a pair of skids rather than wheel assemblies.

The fact that the helicopter obtains its lifting power by means of a rotating airfoil (the rotor) greatly complicates the factors affecting its flight, for not only does the rotor turn but it also moves up and down in a flapping motion and is affected by the horizontal or vertical movement of the helicopter itself. Unlike the usual aircraft airfoils, helicopter rotor airfoils are usually symmetrical. The chord line of a rotor, like the chord line of a wing, is an imaginary line drawn from the leading edge to the trailing edge of the airfoil.

The relative wind is the direction of the wind in relation to the airfoil. In an airplane, the flight path of the wing is fixed in relation to its forward flight; in a helicopter, the flight path of the rotor advances forward (to the helicopter's nose) and then rearward (to the helicopter's tail) in the process of its circular movement. Relative wind is always considered to be in parallel and opposite direction to the flight path. In considering helicopter flight, the relative wind can be affected by the rotation of the blades, the horizontal movement of the helicopter, the flapping of the rotor blades, and wind speed and direction. In flight, the relative wind is a combination of the rotation of the rotor blade and the movement of the helicopter.

Like a propeller, the rotor has a pitch angle, which is the angle between the horizontal plane of rotation of the rotor disc and the chord line of the airfoil. The pilot uses the collective and cyclic pitch control to vary this pitch angle. In a fixed-wing aircraft, the angle of attack (the angle of the wing in relation to the relative wind) is important in determining lift. The same is true in a helicopter, where the angle of attack is the angle at which the relative wind meets the chord line of the rotor blade.

Angle of attack and pitch angle are two distinct conditions. Varying the pitch angle of a rotor blade changes its angle of attack and hence its lift. A higher pitch angle (up to the point of stall) will increase lift; a lower pitch angle will decrease it. Individual blades of a rotor have their pitch angles adjusted individually.

Rotor speed also controls lift—the higher the revolutions per minute (rpm), the higher the lift. However, the pilot will generally attempt to maintain a constant rotor rpm and will change the lift force by varying the angle of attack.

As with fixed-wing aircraft, air density (the result of air temperature, humidity, and pressure) affects helicopter performance. The higher the density, the more lift will be generated; the lower the density, the less lift will be generated. Just as in fixed-wing aircraft, a change in lift also results in a change in drag. When lift is increased by enlarging the angle of pitch and thus the angle of attack, drag will increase and slow down the rotor rpm. Additional power will then be required to sustain a desired rpm. Thus, while a helicopter is affected like a conventional aircraft by the forces of lift, thrust, weight, and drag, its mode of flight induces additional effects.

Essential vocabulary:

1. to take off - взлетать
2. stationary - неподвижный
3. autogiro - автожир
4. transition - переход
5. destination – место назначения
6. to dispense - распределять
7. inevitable - неизбежный
8. intervening modes – появляющиеся способы, методы
9. to require - требовать
10. sophistication - сложность
11. to determine – определять, устанавливать
12. airfoil – аэродинамический профиль
13. blade - лопасть
14. to mount – прикреплять, устанавливать
15. mast - мачта
16. to advance - продвигаться
17. chord line – прямая линия
18. trailing edge – ребро, край
19. to affect - влиять
20. to flap – развеваться, махать
21. pitch - шаг
22. angle - угол
23. humidity - влажность
24. performance – летные данные
25. to sustain – поддерживать, выдерживать, нести
26. thrust – тяга, толчок
27. drag - торможение
28. to induce - порождать
29. to generate - производить

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