Airplanes stay in the air because of one simple fact:there is no net force on them.And with no net fore,an object at rest stays at rest,and an object in motion stays that way-even if it's in midair 10 km above the earth's surface.Now of course it's not like there aren't forces acting on the airplane;gravity pulls down on the plane itself plus all of the people and baggage inside,and every single air molecule that is shot through the engine or collides with the fuselage or wings pushes on the plane as well.
飞机之所以能飞在空中是因为：它们所受合力为零，合力为零时 静止的物体保持静止状态，运动的物体保持亦保持原态，－－－即使这个物体处在距地面10km高空 也不例外,当然 合力为零不表示飞机不受力的作用,飞机自身以及乘客和行李的总量使其下坠,穿过发动机或者与机身,机翼碰撞的,空气颗粒也会圣飞机产生作用力.
But if all of these forces are balanced-in particular,if the air molecules push the plane UP enough to counteract gravity-then the plane stays up.Getting air molecules to push the plane up is he crucial part of flying,and planes do this by making sure the undersides of the wings crash into more air molecules more violently.than the upper sides of the wings.
When a plane is parked on the ground,air molecules bounce off of the top and bottom of the wings in roughly equal amounts,or with "equal pressure."No lift.But in motion,the curved shape of the wings and their slightly inclined angle means that the bottoms smash into more air molecules that before and smash harder into those molecules,so the pressure on the bottom of the wing goes up.
当一架飞机停于地面,与机翼上下两面相作用的空气颗数量大致相同,也可以说"压力相当"无上升力,但在起飞时 机翼的曲线形状和少许的倾斜角度,意味着 与静止时相比机翼底部会与更多空气颗粒相碰撞,碰撞程度也更剧烈,因此机翼底部所受压力也会增加.
In addition,fewer air molecules now strike the top of the wing and those that do strike it less forcefully,partly because it's being"shielded"by its own forward motion (the way running into the rain keeps your back drier).
And partly because a curving stream of air has lower pressure on the inside of the curve since the molecules get thrown centripetally to the outside.But whatever the reasons,the pressure on the top of the wing goes down.So,low pressure on the top plus high pressure on the bottom,and the plane has lift.
另一部分原因在于,弯曲流动的空气对曲线内侧的作用力较小,因为空气颗粒受向心力作用被甩开了,地论如何 机翼顶部的压力减小了,于是 由于顶部压力小而底部压力大,飞机受到上升力作用.
And if the pressure/force imbalance is big enough,it can lift the plane up into the air against gravity!Now,all this crashing into air molecules to lift the plane also pushes to slow the plane down-which it would,if not for engines.Engines also push air,in this case,backwards either via a propellor,or a jet,or a jet driving a propellor.
当作用力不平衡到一定程度,就可以无视重力托起飞机了,而这些托举起飞机的空气颗粒,也产生了阻碍飞机前进的力,好在发动机的推动力更大,发动机也在推动空气,对飞机而言 是向后推动,无论是螺旋桨发动机 或是喷气式发动机,或者喷气驱动螺旋桨.
For various reasons,it turns out that you want to have a really big propellor driven by a really small jet for the most efficient engine.But even in inefficient engines,the spinning fan blades get their horizontal lift,which we call"thrust,"by moving quickly through air with a curved shape and a slightly inclined angle-they're essentially mini-wings.
And so an airplane is essentially a meta-wing:it flies by moving mini-wings fast enough to push air molecules backwards,which moves the plane forward fast enough that its big wings push air molecules down.Whoa,Wingception.