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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

• 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...

• 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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