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CARLA: Hi, everyone.

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Good morning.

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My name is Carla.

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I'm going to be talking about
a very fundamental concept

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in aerodynamics.

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But before I start, have you
all gone in planes before?

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Yeah?

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OK.

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Have you wondered, how does
the pilot land the plane?

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A little?

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All right.

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I'm going to explain
two concepts that

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will help you understand how
you actually land the plane.

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And these concepts are the
concepts of lift and angle

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of attack.

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So I want you to picture a
plane going through the skies.

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And in real life, the planes
don't fly perfectly horizontal.

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They actually fly at an angle.

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And we're going to draw a
little plane going at an angle.

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And this angle between the
horizontal and the plane--

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it's going to be known as
alpha, the angle of attack.

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And it's usually
about 10 degrees.

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So that's why it seems
that you're going uphill

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when you walk across a plane.

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Now, there are two forces
acting on the plane.

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You're going to have gravity--
the weight of the plane,

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pulling the weight down.

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And you're going to also
have a force upwards that's

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keeping the plane afloat.

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And that force is called lift,
which we're going to note by L.

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Lift is generated as air
rushes through the wings.

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The wings are
designed in such a way

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that as air goes
through them, they're

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going to generate
a force upwards.

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So that's what keeps
the plane uplift.

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Now, there's a
relationship between lift

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and the angle of attack.

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And I'm going to illustrate
it in the following plot.

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So we're going to
have-- on the x-axis I'm

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going to have the angle
of attack, in degrees.

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And then for the
y-axis, I'm going

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to have the lift,
which is a force

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and therefore has
units of Newtons.

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Now, it is known that at
small angles of attack,

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if you increase the angle
of attack a little bit,

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you're going to get more lift.

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So if you increase
it a little bit,

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you're going to get more lift.

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So you're going to
get an upwards trend

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in the first part of the plot.

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Now, it might be that you have
a very high angle of attack--

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that the plane's going
actually really steep.

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The flow is going to be
quite adverse to the wings

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and they're going to
stop generating lift.

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So if you make the
angle even steeper,

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you're going to
get even less lift.

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So that means that in
this part of the plot,

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at very high angles
of attack, you're

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going to get even less lift
as you increase the angle.

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Yes there's a question.

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STUDENT 1: By extreme
angles, do you

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mean like, greater that 45
degrees or like 10 degrees?

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CARLA: Greater than 15
actually-- greater than 15.

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So you have these two regimes.

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And there's this mode of
transition in between.

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And the maximum
occurs at 15 degrees.

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And this condition at which
the plane starts to lose lift

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is known as a stall.

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So when an airplane starts
to lose lift, it stalls.

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Now airplanes usually
fly about 10 degrees.

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You want the angle
of attack to be high,

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so that you get a
lot of lift, but not

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so high that you will stall.

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So to land the plane, what
does a pilot actually do?

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Here approaches the runways at
10 degrees, the normal angle.

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And when he's about
to hit the ground,

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he changes the angle of attack
to 15 degrees and stalls.

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So they lose a lot of lift and
they actually crash the plane,

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gently to the ground.

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All right.

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Are there any questions?

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Yes.

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STUDENT 2: Do the
aerodynamics of the plane

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affect the stall point?

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CARLA: They do.

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The stall point
depends on the velocity

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of the plane-- which
the plane's going at,

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and the shape of the wings.

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But it's usually around 15
degrees for any sort of plane.

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There are planes-- the one
that actually can fly vertical,

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those ones have other
sorts of control.

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The winds are stalling but
they have fancy motors that

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can hold that angle.

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There was another question.

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No?

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Yes.

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Yes, what's the question?

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STUDENT 3: Are the wings
also angled with respect

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to the plane?

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CARLA: Sorry.

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Could you be a
little louder please?

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Could you repeat
the question please?

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STUDENT 3: Are the wings
also angled with respect

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to the plane?

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CARLA: Yes, yes, yes.

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So the question is if the wings
have an angle with respect

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to the plane.

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And yes they do.

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The plane moves
something like this,

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and the wings are
at a small angle

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of 2 to 3 degrees with respect
to the horizontal of the plane.

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I'm exaggerating it, but
it is at a small angle.

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But then the cross-section
actually changes.

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And if you look at the
airplane from the front,

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the wings are actually doing
something like this here.

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So there's a number of angles
going on with the plane.