is an airplane here

ok, so you probably already knew that

if you've flown in one or maybe you've just seen them fly

but even if you've seen them or been in one

do you know how they work? Is it magic?

wingardium leviosa!

Are there invisible fairies that hold the plane aloft?

Alright men we've got a busy morning and lots of flights to carry

or is it science?

Well, you guessed it the answer is indeed: science

That's ridiculous!

What?

so to discuss how airplane flies we first have to talk about the forces on

an airplane

which pushed around and all sorts of different directions

Now we're gonna focus on airplanes today

because they're awesome

most of these courses apply to any other vehicle

The first force acts on all these vehicles

really it acts on everything

it's the weight force, which points down

towards the center

of Earth

Weight

is equal to the mass of the airplane m, right here, times acceleration due

to gravity. Here on earth

g is equal to 9.81

meters per second squared

now that's only for earth

the acceleration due to gravity really depends on the mass of the planet

that your on

the larger the planet the higher the gravity

so, 9.81 meters per second squared here on earth. On the moon

however is smaller than earth so the acceleration due to gravity is only

one sixth

that on earth - one point six meters per second squared

this is why astronauts can bounce high on the moon

but not on earth

this isn't nearly as much fun

obviously there has to be another force opposing the weight and pushing the

airplane up

this force

is called lift

Lift operates perpendicular

to the airplane's wings, which are

right here in the side view

now

if these are only two forces our aircraft will be able to go up and down

but it won't go anywhere so we have to have a force that pushes the airplane forward

and this

is called thrust

all vehicles have thrust otherwise

they wouldn't go anywhere like our airplane

Why didn't you buy a car with thrust?

I'm sorry. We can at least roll down the hill

On the aircraft this thrust is produced by engines

There are two main types of engines

we have propellers like this little guy right here

and jet engines, like our first model

Whatever the type of engines, they all work by the same principle

So we draw a little

side view of an engine here

The engines excelerate air

out the back

this direction

and by newton's

third law

there's an equal and

opposite reaction and that's the thrust force

pushing the aircraft forward

this is really the same thing that happens when you blow up a balloon and

you let it go. The air comes out the back and the balloon

moves forward

We have a force that opposes the

thrust - it's called drag and it points opposite

the direction of flight

The major type of drag is pressure drag which is the force caused by the air

smacking into the airplane

so we try to minimize this type of drag by making the airplane as

aerodynamic as possible

that means that has smooth lines

in the air flows nice and cleanly the over the front here

you can feel the pressure drag when you stick your hand out the window moving car

uh...

honey your hand

you hand please

when your hand is horizontal it's aerodynamic and you really don't feel a

lot of drag. But if you slowly turn your hand vertical

you can really feel the drag increasing

So these are four forces on the airplane

But perhaps you're thinking:

so this really cool and everything

but how do we increase and decrease the airplane's lift to move up and down

that's a great question

Well, let's look at the equation for the magnitude of lift per unit wing

area. We'll call that L

L equals

one-half

times rho times C_L times v squared

it's that simple

okay okay okay I'll tell you what each of these means

so rho, it's not a "p", it's the greek letter rho

rho is the density of the air which is a measure of the number of air

molecules in a certain volume

density of the air varies with altitude and temperatures

so as you go higher up

there the air is thinner and so the density is lower

if we want to simplify things we generally use the standard density

which is 1.2754

kilograms

per meters cubed

v, here, is the speed of the aircraft or how fast it's traveling

and C_L is something called the coefficient of lift

it's a number of that gives us some information about the shape of the

aircraft's wings

these things right here

the coefficient of lift changes with the angle of attack. Angle of what?

Aircraft can pitch up

and down. And even if their pitched up

they're still travelling

in a horizontal direction like that

now

the angle formed here by the horizontal direction of travel

and the direction of the aircraft's nose

is called the angle of attack. And we denote that with the greek letter

alpha

so you can make a little

plot here of that. We're gonna put coefficient of lift

up on the y axis and the angle of attack

down on the x axis

so as the airplane starts to pitch up

If I can get a hand here Thank you.

as the aircraft starts to pitch up

the coefficient of lift increases

this is a good thing because

we have more lift

as we continue to increase we eventually reach a point

where we keep pitching up but

the left starts decreasing

this is something called

stall

and it's

it's not a good thing

so hot we generally avoid pitching up this much

there's a similar equation for the drag per unit wing area: D

D equals 1/2 rho

not C_L - that really wouldn't make any sense

C_D

as you can guess it's the coefficient of drag

times the velocity squared

the coefficient of drag is - it's another number that tells us something about the

wings

and it also varies

with the angle of attack, so as the angle of attack increases (oh, thank you)

the coefficient of drag increases as well. Thank you very much

this

is because

as the aircraft is pitching up

there is more wing area perpendicular

to the flow

Now, this reminds me of something that we talked about earlier

exactly this is very similar to whenever you hold your hand

out the window of a car

and so

that's

pretty much everything you need to know about how an aircraft flies

so the next time your on an airplane or

you just see one

you can really know exactly what it is that's keeping it up in the air

nope… no it's not them either

ah... there you go now you got it