How do airplanes fly? | ESL Video

How do airplanes fly?

Quiz by: anakes    

Description: airplanes
Play Video: Keynote (Google I/O '18)
Why do airplanes stay in the air?
What four forces must be balanced?
What is a crucial part of flying?
What part of the wings crash into air molecules more violently?
Low pressure on the top of the plane plus high pressure on the bottom means:
What direction do engines push air?
For the most efficient engine it is good to have:
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.
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