lamyukman999 發問於 教育及參考書教學 · 1 十年前

# Why aeroplane can fly???(Use English, please)

Why aeroplane can fly???(Use English, please)

### 1 個解答

• John
Lv 7
1 十年前
最愛解答

How is it that aeroplanes fly, although they are obviously heavier than air? In order to fly, aeroplanes need a strong enough force to counteract the force of their own weight: a lift.

Physics distinguishes two types of lift: static and dynamic. In contrast to static lift for 'stationary' objects like hot-air balloons, with dynamic lift one must set the flying object or the air surrounding it in motion. Scientists research problems of dynamic lift above all with aid of aerodynamics, the laws of motion of gaseous matter applied to flying bodies surrounded by air.

Dynamic lift

Dynamic lift can be demonstrated simply with aid of a small experiment: If one holds a piece of paper that is stiffened on one side a little at an angle in a flow of air, the onrushing air pushes the paper upwards: The paper experiences dynamic lift.

An aeroplane starts to climb if the lift is greater than the force of the weight of the aeroplane. The lift is closely connected with a certain pressure distribution on the lifting surface of the wings of the aeroplane. On top of the wing there must be an uplift pressure and underneath it an overpressure. The difference is created by the air flowing over the wings. As the oncoming air separates it stands still on the front side of the wing and will normally rejoin at the rear of the wing. Above the stagnation point the divided stream flows through a narrower cross-section than beneath the wing. Because the stagnation point is located underneath the wing, more air is transported over the wing profile than emerges from the same cross-section in front of the wing. Therefore the air particles flow faster over the wing than underneath it. According to Bernoulli's Law a vacuum forms where the speed is greater, creating an upward suction.

Laminar, turbulent und vorticity flows on the wing profile

REMARK: We could have this image split into three.END

The pressure on the under side of the wing profile becomes greater. Both forces enable the wings and with them the airplane to be simultaneously 'sucked' and 'pushed' upwards. (See picture) The lift can be increased in three ways: with greater wing surface, by flying faster and or with a greater angle of incidence. At higher speeds the pressure difference on the wings increases and with it the lift. Large wing surfaces and a more pronounced curvature of the wing profile also increase the lift. At the same time the resistance every flying object encounters in the air increases and slows it down. An aeroplane wing must be constructed so that at the point of maximum lift air resistance is at a minimum.