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  • So, this

  • 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

  • nopeno it's not them either

  • ah... there you go now you got it

So, this

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B1 airplane aircraft drag coefficient lift angle

The Forces on an Airplane

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    李應振 posted on 2013/02/02
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