Airplanes stay in the air because of one simple fact: there is no net force on them.
And with no net force, 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.
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 the 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 than before (and smash harder into those molecules),
so the pressure on the bottom of the wing goes up.
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.
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.
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.