Helicopter (II)

In a helicopter, the total lift and thrust forces generated by the rotor are exerted perpendicular to its plane of rotation. When a helicopter hovers in a windless condition, the plane of rotation of the rotor is parallel to the ground, and the sum of the weight and drag forces are exactly balanced by the sum of the thrust and lift forces. In vertical flight, the components of weight and drag are combined in a single vector that is directed straight down; the components of lift and thrust are combined in a single vector that is directed straight up. To achieve forward flight in a helicopter, the plane of rotation of the rotor is tipped forward. (It should be understood that the helicopter's rotor mast does not tip but rather the individual rotor blades within the plane of rotation have their pitch angle varied.) For sideward flight, the plane of the rotation of the rotor is tilted in the direction desired. For rearward flight, the plane of the rotation of the rotor is tilted rearward.

Because the rotor is powered, there is an equal and opposite torque reaction, which tends to rotate the fuselage in a direction opposite to the rotor. This torque is offset by the tail rotor located at the end of the fuselage. The pilot controls the thrust of the tail rotor by means of foot pedals, neutralizing torque as required.

There are other forces acting upon a helicopter not found in a conventional aircraft. These include the gyroscopic precession effect of the rotor – that is, the dissymmetry of lift created by the forward movement of the helicopter, resulting in the advancing blade having more lift and the retreating blade less. This occurs because the advancing blade has a combined speed of the blade velocity and the speed of the helicopter in forward flight, while the retreating blade has the difference between the blade velocity and the speed of the helicopter. This difference in speed causes a difference in lift—the advancing blade is moving faster and hence is generating more lift. If uncontrolled, this would result in the helicopter rolling. However, the difference in lift is compensated for by the blade flapping and by cyclic feathering (changing the angle of pitch). Because the blades are attached to a rotor hub by horizontal flapping hinges, which permit their movement in a vertical plane, the advancing blade flaps up, decreasing its angle of attack, while the retreating blade flaps down, increasing its angle of attack. This combination of effects equalizes the lift. (Blades also are attached to the hub by a vertical hinge, which permits each blade to move back and forth in the plane of rotation. The vertical hinge dampens out vibration and absorbs the effect of acceleration or deceleration.) In addition, in forward flight, the position of the cyclic pitch control causes a similar effect, contributing to the equalization of lift.

Other forces acting upon helicopters include coning, the upward bending effect on blades caused by centrifugal force; Coriolis effect, the acceleration or deceleration of the blades caused by the flapping movement bringing them closer to (acceleration) or farther away from (deceleration) the axis of rotation; and drift, the tendency of the tail rotor thrust to move the helicopter in hover.

Essential vocabulary:

1. thrust force – сила тяги
2. to exert – оказывать, действовать
3. plane of rotation – плоскость вращения
4. to hover - реять
5. drag force – тормозная сила
6. to tip - наклоняться
7. mast - мачта
8. to tilt – наклонять
9. pitch angle – угол наклона
10. torque – вращающий момент
11. offset – офсет, смещение, сдвиг
12. gyroscopic precession effect – эффект гироскопической прецессии
13. retreat - отступление
14. rolling – прокрутка, прокатывание
15. flapping hinge – маховик (шарнир), закрылок
16. hub – узел, втулка
17. feathering – вращение (по принципу пера), резать крылом воздух
18. to permit – позволять, разрешать
19. to attach – прикреплять
20. to increase – увеличивать, повышать
21. to decrease – снижать, уменьшать
22. to equalize - выравнивать
23. to dampen out – демпфировать, тормозить
24. to absorb – поглощать, впитывать
25. to contribute – содействовать
26. coning – конус
27. bending – изгиб
28. centrifugal force – центробежная сила
29. acceleration – ускорение
30. drift – парение, дрейф

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