WEBVTT

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TINA SRIVASTAVA: All right.

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So as we discussed,
we're going to start off

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with the most basic question--
how do airplanes fly?

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It's a very critical question.

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I think everybody should
know the answer to this.

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Going back to the
comic that Minachi

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had with Calvin and Hobbes, and
not knowing how airplanes fly,

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and thinking that it's
magic is not the way

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that any MIT student should be.

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So we're going to cover
how airplanes fly.

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And we're actually
going to go beyond what

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the FAA requires you to know.

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Because frankly, you should
know how airplanes fly.

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So just so that we have a
common vocabulary with which

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to discuss, we're going
to talk a little bit

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about airplane parts.

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So here in my little
airplane, it's kind of a model

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there so you can see that at the
front you have your propeller.

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And so the engine and the
propeller in this little plane

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is up here at the front.

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Who knows what a fuselage is?

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Just shout it out.

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AUDIENCE: The middle part.

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TINA SRIVASTAVA:
The middle part.

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The body.

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It's where the passengers sit.

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Yeah, so that whole middle
part where people sit.

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So if you're thinking
about a big jet engine,

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it's where all the rows of
seats are where everybody sits.

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That tube in the middle
is called the fuselage.

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And the wings stick
out the sides.

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So the middle part
is the fuselage.

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And then one thing
that's interesting

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is the tail actually has
a lot more components.

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People kind of casually
refer to it as the tail.

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But there's a vertical
part that comes up

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in the back of the
vertical part of the tail

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can actually tilt side to side.

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And then you have a
flat horizontal part.

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And that actually has a back
part that can go down and up.

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And so we're going to talk
about what all of these are.

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So the back vertical part
when it goes side to side

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is your rudder.

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The flat part is your elevator
that you can move up and down,

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allows you to
control the airplane.

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We also have, of
course, the wings.

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Sometimes, there are struts
that support the wings.

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So they go from the wing
down to that fuselage.

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And then you have landing gear.

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In this case, you have these
wheels down at the bottom.

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We're also going to talk to you
during this course about sea

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planes.

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So they have slightly
different landing gear.

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But this is a good
place to start.

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The other thing that
we need to talk about

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are just the main four forces
that are on an airplane.

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So they're pretty
straightforward.

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So the force going up is lift.

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And that force is opposed by
the downward force of weight.

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And then when you're moving
the airplane forward,

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that's thrust.

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And it is opposed by drag.

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So what we're
going to talk about

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is that in order for
an airplane to go up,

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the lift has to exceed the drag.

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In order for the
airplane to go forward,

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the thrust has to exceed the--

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excuse me.

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The lift has to
exceed the weight.

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And the thrust has
to exceed the drag.

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So those are the
main four forces

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we're going to be
working with today.

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So now I'm going to spend
a little bit of time

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over here on the blackboard.

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AUDIENCE: Hey video folks, is it
easier to use that blackboard?

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TINA SRIVASTAVA: They
said this blackboard.

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AUDIENCE: This one's better?

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TINA SRIVASTAVA: Yeah.

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AUDIENCE: These are
all chalks of color.

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TINA SRIVASTAVA: Yeah,
fancy-colored chalk.

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All right.

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AUDIENCE: Chalks of color.

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TINA SRIVASTAVA:
Chalks of color.

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All right.

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So I will preface the discussion
about lift with the fact

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that there are a lot of
theories of lift out there,

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some of which are wrong.

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So if you spent some
time googling lift

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before coming here, you
might have actually found

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a couple scenarios that
are completely false.

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So we're going to
focus on what's true,

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but I will cover least one
of those false theories

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to make sure you guys
don't get hung up on that.

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So in order to talk
about it, we're

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going to think
about an airplane.

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And we're going to do a
cross-section of the airplane.

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So if you took a saw,
and you cut off the wing,

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what are you left with?

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And I'll do it this way.

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So if you cut off the wing,
at the front of the wing

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is the leading edge.

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The back of the wing
is the trailing edge.

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If you did it-- if you cut that
off, what does it look like?

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So it looks like this.

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And this shape is
called an airfoil.

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And we'll get into
the specifics later.

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But first we'll just--

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we'll talk about a simple way
to understand how lift works.

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So if this is the wing,
and you have air coming in,

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the air is pushed down by
the shape of this wing.

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So that means is as air flows
by, it gets pushed down.

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Now what is air?

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Air is not nothing.

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Air has molecules.

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It has mass.

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So if you think about
conservation of momentum,

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this is I think the easiest
way to think about lift.

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So conservation of momentum, you
have a bunch of air molecules.

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And those air molecules
are pushed down.

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So you have mass
being pushed down.

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So if mass is being pushed down
for conservation of momentum,

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something must be pushed up.

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And that's the wing.

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So that's the easiest
way to think about it

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that if you're deflecting the
air downward in order to have

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conservation of momentum,
the mass of the wing

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is lifted upwards.

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We're going to break
that down, but I think

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that's a good place to start.

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I'm just going to take
one moment to talk

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about an incorrect
theory of lift.

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So let me emphasize it's wrong.

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One of them is called
equal transit theory.

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Has anyone heard about this?

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Getting a lot of head nods.

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It's wrong.

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Equal transit theory,
which is incorrect,

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says that basically a molecule
of air that's coming over

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that starts over
here at the front

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has to go around the bottom and
meet the tail at the same time

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that a molecule that
goes over the top

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has to meet it at the back.

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There is no physical
principle that says that.

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It is false.

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And in fact, we have
measured that they don't.

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The molecules that
go under the bottom

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of the wing versus
the top of the wing

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don't actually reach the
end at the same time.

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But in this false theory,
equal transit theory,

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they say that you have to reach
the bottom at the same time.

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They also say that there
is more distance basically

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to cover because of the
shape of the airfoil.

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So in order for the
molecules going over

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the top to reach
at the same time

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as the molecules over the
bottom, they have to go faster.

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And so since the
air is moving faster

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over the top and the bottom,
that's what creates lift.

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So that's false.

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And there are many
reasons why it's false,

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the biggest one being that there
is no physical principle that

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says that two molecules starting
at the same time, one going

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over the top and one
going over the bottom,

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reaches the end
at the same time.

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That's just not true.

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And we'll show you some more
diagrams that show in fact

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it doesn't happen,
that molecules don't

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reach at the same time anyway.

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So please despite that being
very widely propagated,

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that is not true.

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And please don't spend
time on that theory.

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So let's focus on what is
true, how does it really work.

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Actually, let me give you one
more reason why that's false.

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The real reason that
equal transit theory

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is trying to tell you
that that generates lift

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is that because of the shape
of the airfoil, the shape

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of the wing, that's why the
distance that it has to travel

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is different over the
top versus the bottom.

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But one reason that's
wrong-- can you pass me

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that paper airplane please?

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Who has built a paper
airplane before?

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I see at least two people
who didn't raise their hand.

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Do we need to do
a class exercise?

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If you have not built
a paper airplane,

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it's really important that you
do just as a general childhood

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experience.

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Here's a paper airplane.

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Thank you, Minachi,
for building it for me.

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If we took this paper airplane
instead of this fancy airplane,

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and we did a cross-section
of this wing,

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what would it look like?

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Yes.

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You demonstrated with your
hands, but shout it out.

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AUDIENCE: It's going to be the
same at the top and the bottom.

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It's just a piece of paper.

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TINA SRIVASTAVA:
Yeah, it's going

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to be the same at the
top and the bottom.

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It's just a piece of paper.

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Exactly.

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It's just like a
little flat rectangle.

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So instead of this fancy
shape that you have here--

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we're going to use
red for wrong--

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it's like a little rectangle.

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That's what a paper airplane's
cross-section of its wing

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looks like.

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Well, surprise.

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Surprise.

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As you said, it's the same
at the top and the bottom.

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So the distance
that a air molecule

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would have to travel over
the top and the bottom

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is identical.

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So really, the equal transit
theory completely falls apart.

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Yet, a paper
airplane still flies.

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So why is that the case?

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Again, remember
the actual reason

00:10:11.560 --> 00:10:15.250
is that if this paper
airplane is inclined,

00:10:15.250 --> 00:10:17.510
it is pushing air down.

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So air that's coming
up is bumping into it

00:10:19.660 --> 00:10:21.110
and being pushed down.

00:10:21.110 --> 00:10:24.340
And therefore, as you
deflect air molecules down,

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conservation of momentum--
the wing is lifted up.

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So now we're going to break
this down in a little bit more

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detail.

00:10:44.750 --> 00:10:48.910
And I'm going to go back
over here to the slides.

00:10:48.910 --> 00:10:52.700
So one thing that's
important, as I said,

00:10:52.700 --> 00:10:56.080
a really detailed
mathematical description

00:10:56.080 --> 00:10:59.350
is not really necessary to
fly a plane or become a pilot.

00:10:59.350 --> 00:11:01.750
The FAA doesn't require
some of this detail.

00:11:01.750 --> 00:11:03.820
But it is important to
know it to the extent

00:11:03.820 --> 00:11:07.240
that it helps you control
the airplane and fly it.

00:11:07.240 --> 00:11:10.190
So here's a good reference
in terms of that.

00:11:10.190 --> 00:11:14.740
But one of the biggest things
is just that for lift, you

00:11:14.740 --> 00:11:19.360
have to increase that
downward momentum of the air.

00:11:19.360 --> 00:11:22.750
And airfoils are--
the shape which

00:11:22.750 --> 00:11:26.200
is called an airfoil
is a type of shape that

00:11:26.200 --> 00:11:31.340
is very efficient at increasing
that downward momentum.

00:11:31.340 --> 00:11:35.510
Now, who knows what
Bernoulli's principle is?

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Who's heard of Bernoulli?

00:11:38.080 --> 00:11:38.580
Good.

00:11:38.580 --> 00:11:40.410
Everyone's heard of Bernoulli.

00:11:40.410 --> 00:11:43.170
Can anyone articulate
Bernoulli's principle?

00:11:46.116 --> 00:11:47.098
Yes.

00:11:47.098 --> 00:11:51.675
AUDIENCE: I think it's like
p plus one half of mv squared

00:11:51.675 --> 00:11:53.300
equals constant the
difference squared.

00:11:53.300 --> 00:11:55.008
So when the pressure
goes down somewhere,

00:11:55.008 --> 00:11:58.805
the speed of the
particle has to go up.

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TINA SRIVASTAVA: Yes.

00:11:59.680 --> 00:12:00.540
Absolutely.

00:12:00.540 --> 00:12:01.380
Absolutely.

00:12:01.380 --> 00:12:05.770
So what Bernoulli
observed was the case

00:12:05.770 --> 00:12:08.380
that when there is a
decrease in pressure,

00:12:08.380 --> 00:12:11.410
there's an increase in velocity.

00:12:11.410 --> 00:12:15.710
That's the core concept
that you have to understand.

00:12:15.710 --> 00:12:20.410
And so when we think about an
airfoil, when we see that--

00:12:20.410 --> 00:12:23.530
and I'll draw another
airfoil for us to talk about.

00:12:29.010 --> 00:12:31.590
When we have air
that's moving very

00:12:31.590 --> 00:12:34.230
fast over the top
of the wing, that

00:12:34.230 --> 00:12:36.360
means an increase
in velocity means

00:12:36.360 --> 00:12:38.440
there's a decrease in pressure.

00:12:38.440 --> 00:12:40.440
So this is the extent
to which you really need

00:12:40.440 --> 00:12:42.750
to know it for the FAA exam.

00:12:42.750 --> 00:12:45.870
So which statement relates
to Bernoulli's principle?

00:12:45.870 --> 00:12:47.580
I'll let you read those answers.

00:13:03.550 --> 00:13:05.260
So is it A, B, or C?

00:13:05.260 --> 00:13:05.950
Shout it out.

00:13:05.950 --> 00:13:06.990
AUDIENCE: C.

00:13:06.990 --> 00:13:08.620
TINA SRIVASTAVA: C. Good job.

00:13:08.620 --> 00:13:10.480
Well done.

00:13:10.480 --> 00:13:13.020
We're going to discuss a
little bit more details though.

00:13:19.640 --> 00:13:23.330
In order for any wing
to generate lift,

00:13:23.330 --> 00:13:25.670
it has to be in a fluid.

00:13:25.670 --> 00:13:30.050
If this airplane was in
space or in a vacuum,

00:13:30.050 --> 00:13:32.630
and there wasn't any
fluid passing by it,

00:13:32.630 --> 00:13:36.020
then there wouldn't be any
molecules to deflect downward.

00:13:36.020 --> 00:13:39.530
And therefore, you
couldn't push the wing up.

00:13:39.530 --> 00:13:41.570
But the fluid doesn't
always have to be air.

00:13:41.570 --> 00:13:44.900
You might see similar
designs underwater

00:13:44.900 --> 00:13:46.970
for underwater drones.

00:13:46.970 --> 00:13:50.405
It just has to be a fluid
that's passing by the object.

00:14:02.180 --> 00:14:08.660
So when you have this airfoil in
a fluid, when the fluid is not

00:14:08.660 --> 00:14:12.530
moving, when it's stationary,
then all of the fluid

00:14:12.530 --> 00:14:15.860
is exerting pressure
on the airfoil.

00:14:15.860 --> 00:14:19.190
So you get all these
little normal forces

00:14:19.190 --> 00:14:20.000
exerting pressure.

00:14:25.940 --> 00:14:28.630
When the fluid is not
moving, and the airfoil

00:14:28.630 --> 00:14:32.360
is stationary in the fluid,
then all of those pressure

00:14:32.360 --> 00:14:36.380
forces, all those normal
forces or forces perpendicular,

00:14:36.380 --> 00:14:39.170
sum to zero because
there's no net force.

00:14:39.170 --> 00:14:40.970
It's just sitting in the fluid.

00:14:40.970 --> 00:14:44.375
But when that fluid is
moving, it generates a force.

00:15:01.390 --> 00:15:04.280
So that's the force it
generates generally when

00:15:04.280 --> 00:15:06.280
the fluid is moving forward.

00:15:06.280 --> 00:15:08.220
And a force is a vector.

00:15:08.220 --> 00:15:11.540
So it has direction
as well as magnitude.

00:15:11.540 --> 00:15:15.740
So there is a vertical component
and a horizontal component

00:15:15.740 --> 00:15:17.030
to that.

00:15:17.030 --> 00:15:21.970
So we call the vertical
component the lift.

00:15:21.970 --> 00:15:25.020
Does anyone know what we call
the horizontal component?

00:15:25.020 --> 00:15:25.720
AUDIENCE: Drag.

00:15:25.720 --> 00:15:26.860
TINA SRIVASTAVA: Drag.

00:15:26.860 --> 00:15:27.370
Good job.

00:15:33.090 --> 00:15:39.670
Now, here's a dumb question.

00:15:39.670 --> 00:15:43.005
What part of the
aircraft generates lift?

00:15:47.460 --> 00:15:47.960
Yes.

00:15:47.960 --> 00:15:48.980
AUDIENCE: The whole aircraft.

00:15:48.980 --> 00:15:50.480
TINA SRIVASTAVA:
The whole aircraft.

00:15:50.480 --> 00:15:51.170
Good job.

00:15:51.170 --> 00:15:55.070
So a lot of people might
be under the misimpression

00:15:55.070 --> 00:15:58.460
that it's only the wings
that are generating the lift.

00:15:58.460 --> 00:16:01.970
Well, actually, the whole
aircraft is generating lift.

00:16:01.970 --> 00:16:03.620
And it's not just aircraft.

00:16:03.620 --> 00:16:06.740
Any objects that are
moving through fluid

00:16:06.740 --> 00:16:09.740
have this phenomenon.

00:16:09.740 --> 00:16:11.760
And sometimes, it's
not a good thing.

00:16:11.760 --> 00:16:13.820
So what is this a picture of?

00:16:17.180 --> 00:16:17.930
A race car.

00:16:17.930 --> 00:16:20.070
Come on, guys.

00:16:20.070 --> 00:16:24.050
I know we're in an
airplane class but--

00:16:24.050 --> 00:16:26.480
who can tell me what is
that thing sticking up

00:16:26.480 --> 00:16:28.210
at the back of the race car?

00:16:28.210 --> 00:16:29.105
AUDIENCE: A spoiler.

00:16:29.105 --> 00:16:30.230
TINA SRIVASTAVA: A spoiler.

00:16:30.230 --> 00:16:32.937
What's a spoiler?

00:16:32.937 --> 00:16:34.270
AUDIENCE: It spoils the airflow.

00:16:34.270 --> 00:16:36.280
TINA SRIVASTAVA: It
spoils the airflow.

00:16:36.280 --> 00:16:39.640
So when a race car is
driving on a race track,

00:16:39.640 --> 00:16:43.570
and it's going through the
air-- the fluid is air--

00:16:43.570 --> 00:16:48.820
actually just the race car
itself is generating lift.

00:16:48.820 --> 00:16:52.930
And that lift can cause the
race car to kind of lift

00:16:52.930 --> 00:16:58.188
upward and not be as much
in traction with the ground.

00:16:58.188 --> 00:17:00.730
And when you're a race car, and
you want to go really, really

00:17:00.730 --> 00:17:02.770
fast, you want to have
very good traction

00:17:02.770 --> 00:17:04.410
with your wheels
against the ground

00:17:04.410 --> 00:17:06.079
so you can go as
fast as you can.

00:17:06.079 --> 00:17:08.380
So the reason that you
have a spoiler at the back

00:17:08.380 --> 00:17:12.130
is actually to counteract the
lift that's being generated

00:17:12.130 --> 00:17:13.450
by the race car.

00:17:13.450 --> 00:17:17.050
So it's not just airplanes
and wings that generate lift,

00:17:17.050 --> 00:17:19.390
but really anything
moving through a fluid

00:17:19.390 --> 00:17:20.790
can generate lift.

00:17:26.783 --> 00:17:28.950
So we're going to talk a
little bit about equations.

00:17:28.950 --> 00:17:30.840
Don't get scared here.

00:17:30.840 --> 00:17:34.860
We'll just dive into
it step by step.

00:17:34.860 --> 00:17:38.400
So we have f equals ma.

00:17:38.400 --> 00:17:40.970
Hopefully, this is
not the first time

00:17:40.970 --> 00:17:43.900
you're hearing
about that equation.

00:17:43.900 --> 00:17:48.807
So can somebody just shout
out what is acceleration?

00:17:48.807 --> 00:17:50.890
AUDIENCE: Change in velocity
with respect to time.

00:17:50.890 --> 00:17:52.807
TINA SRIVASTAVA: Change
in velocity over time.

00:17:52.807 --> 00:17:53.440
Very good.

00:17:53.440 --> 00:17:57.000
So velocity again
is also a vector.

00:17:57.000 --> 00:18:02.090
So velocity being a vector has
both a magnitude and direction.

00:18:02.090 --> 00:18:04.120
So you can change
the velocity either

00:18:04.120 --> 00:18:07.600
by changing the magnitude
or the direction.

00:18:07.600 --> 00:18:10.210
In the case of an
airfoil, we're changing

00:18:10.210 --> 00:18:13.090
the direction of the air.

00:18:13.090 --> 00:18:14.205
So the air has velocity.

00:18:14.205 --> 00:18:14.830
It's coming in.

00:18:14.830 --> 00:18:17.110
We're changing the
direction of that air.

00:18:17.110 --> 00:18:19.240
And that's generating the lift.

00:18:19.240 --> 00:18:23.230
So because we changed the
direction of the velocity,

00:18:23.230 --> 00:18:24.340
that creates a force.

00:18:24.340 --> 00:18:25.780
That's our force f.

00:18:25.780 --> 00:18:27.460
So f is actually
here representing

00:18:27.460 --> 00:18:30.610
the rate of change of
momentum of pushing those air

00:18:30.610 --> 00:18:33.310
molecules down and
generating a force,

00:18:33.310 --> 00:18:36.650
creating the airfoil
to be lifted up.

00:18:36.650 --> 00:18:40.610
So that's why we discussed
again that equal transit

00:18:40.610 --> 00:18:41.450
theory is false.

00:18:41.450 --> 00:18:46.790
Because even an paper airplane
with a completely flat

00:18:46.790 --> 00:18:48.830
cross-section of
its wing, as long

00:18:48.830 --> 00:18:51.440
as it's inclined upward
such that the air is

00:18:51.440 --> 00:18:55.360
being pushed down will fly.

00:18:55.360 --> 00:18:57.430
So here's another question.

00:18:57.430 --> 00:19:02.740
Which moves faster--
the wing through the air

00:19:02.740 --> 00:19:04.390
or the air past the wing?

00:19:14.040 --> 00:19:15.800
Wow, you're very quiet.

00:19:15.800 --> 00:19:16.800
Which moves faster?

00:19:16.800 --> 00:19:17.300
Yes.

00:19:17.300 --> 00:19:18.440
AUDIENCE: The air over the wing.

00:19:18.440 --> 00:19:20.065
TINA SRIVASTAVA: The
air over the wing.

00:19:20.065 --> 00:19:22.910
We have one for the air over
the wing is moving faster

00:19:22.910 --> 00:19:24.470
than the wing through the air.

00:19:24.470 --> 00:19:25.760
Anyone else?

00:19:25.760 --> 00:19:26.672
Yes.

00:19:26.672 --> 00:19:28.150
AUDIENCE: Depends on where on
the wing you're talking about.

00:19:28.150 --> 00:19:30.025
TINA SRIVASTAVA: Depends
on where on the wing

00:19:30.025 --> 00:19:31.240
you're talking about.

00:19:31.240 --> 00:19:32.000
Yes.

00:19:32.000 --> 00:19:33.458
AUDIENCE: Because
if you define air

00:19:33.458 --> 00:19:35.590
to be the air that's
immediately next to the--

00:19:35.590 --> 00:19:39.340
that is in contact with
the wing or the general air

00:19:39.340 --> 00:19:41.470
as in the air space.

00:19:41.470 --> 00:19:42.542
TINA SRIVASTAVA: Yes.

00:19:42.542 --> 00:19:45.290
AUDIENCE: If it's the air
that's in contact with the wing,

00:19:45.290 --> 00:19:46.623
they're going at the same speed.

00:19:46.623 --> 00:19:48.415
TINA SRIVASTAVA: So it
depends on which air

00:19:48.415 --> 00:19:49.570
you're talking about.

00:19:49.570 --> 00:19:51.100
True.

00:19:51.100 --> 00:19:55.430
Actually, what we're discussing
is about frame of reference.

00:19:55.430 --> 00:19:58.630
So depending on your
frame of reference,

00:19:58.630 --> 00:20:01.480
if your frame of
reference is the airfoil,

00:20:01.480 --> 00:20:04.690
you can take it to be that
the airfoil is stationary.

00:20:04.690 --> 00:20:08.710
And you see the wing to
be stationary and the air

00:20:08.710 --> 00:20:10.180
to be moving past you.

00:20:10.180 --> 00:20:12.490
If your frame of
reference is out here,

00:20:12.490 --> 00:20:17.500
you might see the air to be
stationary and the airplane

00:20:17.500 --> 00:20:19.340
to be moving through it.

00:20:19.340 --> 00:20:21.370
So depending on what your
frame of reference is,

00:20:21.370 --> 00:20:25.210
you can actually have
the identical result.

00:20:25.210 --> 00:20:27.580
So the answer is actually
that it's the same.

00:20:27.580 --> 00:20:29.750
So depending on your
frame of reference,

00:20:29.750 --> 00:20:32.350
it's exactly the same the
speed of the air moving

00:20:32.350 --> 00:20:37.100
past the airfoil versus the
airfoil moving through the air.

00:20:37.100 --> 00:20:41.500
And the reason-- so does anyone
want to dive more into that?

00:20:41.500 --> 00:20:44.720
Are you guys familiar with this
concept of frame of reference?

00:20:44.720 --> 00:20:45.220
Yes.

00:20:45.220 --> 00:20:47.420
A lot of head nodding.

00:20:47.420 --> 00:20:48.010
Great.

00:20:48.010 --> 00:20:52.270
So the reason that's significant
is that as we learn about lift

00:20:52.270 --> 00:20:56.050
and as we study
this, we actually

00:20:56.050 --> 00:20:59.410
could create a whole bunch
of different airfoils,

00:20:59.410 --> 00:21:02.800
and then build airplanes, and
then fly them through the air,

00:21:02.800 --> 00:21:03.880
and measure them.

00:21:03.880 --> 00:21:06.560
But that's very expensive.

00:21:06.560 --> 00:21:14.740
So instead, what we do is we
basically take the airfoil.

00:21:14.740 --> 00:21:17.380
And we put it on a
stick, and then we

00:21:17.380 --> 00:21:20.880
put it inside a wind tunnel.

00:21:20.880 --> 00:21:23.620
Has anyone been
in a wind tunnel?

00:21:23.620 --> 00:21:26.130
Got a couple people.

00:21:26.130 --> 00:21:29.600
Hey, we saw that like over 60
of you guys were aero-astro.

00:21:29.600 --> 00:21:31.980
You need to go to your
Wright brothers wind tunnel.

00:21:31.980 --> 00:21:34.980
It's being upgraded
actually right now

00:21:34.980 --> 00:21:37.260
over in your building 33.

00:21:37.260 --> 00:21:40.060
So because it's
exactly identical,

00:21:40.060 --> 00:21:43.290
the air moving past the
airfoil or the airfoil

00:21:43.290 --> 00:21:46.140
moving through the
air, it's a lot cheaper

00:21:46.140 --> 00:21:49.260
to put the airfoil on a
stick in a wind tunnel,

00:21:49.260 --> 00:21:53.040
and then shoot air past it,
and then do your measurements

00:21:53.040 --> 00:21:56.520
rather than continuing to take
off airplanes and fly them

00:21:56.520 --> 00:21:57.870
through the air.

00:21:57.870 --> 00:22:00.920
So we're going to be talking
about that a little bit.

00:22:04.940 --> 00:22:10.870
So the question is, what
factors affect lift?

00:22:10.870 --> 00:22:13.570
So there are a lot of
things that affect lift.

00:22:13.570 --> 00:22:15.860
So one has to do with
the object itself.

00:22:15.860 --> 00:22:19.570
So I was talking about
the shape of the airfoil.

00:22:19.570 --> 00:22:21.340
So we talked about
a different shape,

00:22:21.340 --> 00:22:23.770
which is just a
flat piece of paper

00:22:23.770 --> 00:22:25.660
or a rectangle as a shape.

00:22:25.660 --> 00:22:27.760
You can have a
more slender shape.

00:22:27.760 --> 00:22:29.800
And the way that
you modify the shape

00:22:29.800 --> 00:22:32.650
can significantly
impact your lift.

00:22:32.650 --> 00:22:35.770
So for example, one
of the modifications

00:22:35.770 --> 00:22:38.470
can be back here at the end.

00:22:38.470 --> 00:22:42.870
If you made your
airfoil longer like this

00:22:42.870 --> 00:22:45.340
and point even
farther down, then it

00:22:45.340 --> 00:22:48.410
would push the air in a
slightly different way.

00:22:48.410 --> 00:22:51.340
So that would affect the
lift that that airfoil

00:22:51.340 --> 00:22:52.000
could generate.

00:22:52.000 --> 00:22:57.350
It would also affect the
drag that it induces.

00:22:57.350 --> 00:23:01.117
Another aspect is just
the size of the wing

00:23:01.117 --> 00:23:02.200
and the shape of the wing.

00:23:02.200 --> 00:23:03.850
So we see a lot of
different kinds.

00:23:03.850 --> 00:23:09.150
So this is a big
rectangular wing.

00:23:09.150 --> 00:23:12.440
In a jet, you might
see a swept wing.

00:23:12.440 --> 00:23:14.250
There are different
types of shapes.

00:23:14.250 --> 00:23:16.050
And then there's
also just the area.

00:23:16.050 --> 00:23:17.850
So regardless of whether--

00:23:17.850 --> 00:23:19.230
if this is your--

00:23:19.230 --> 00:23:22.170
if you're looking
down at an airplane--

00:23:22.170 --> 00:23:25.830
so this is kind of
the broad, flat wings,

00:23:25.830 --> 00:23:28.650
or you could have very
thin, skinny wings

00:23:28.650 --> 00:23:30.870
that you might see on a glider.

00:23:30.870 --> 00:23:35.760
Regardless, there is a
surface area of the wing.

00:23:35.760 --> 00:23:40.230
That area also impacts
the lift quite a bit.

00:23:40.230 --> 00:23:45.460
And the aspect ratio as we
just discussed in the shape

00:23:45.460 --> 00:23:47.050
can affect lift.

00:23:47.050 --> 00:23:49.480
The other thing other
than the object itself,

00:23:49.480 --> 00:23:52.210
other than the
wing itself, motion

00:23:52.210 --> 00:23:56.140
can affect lift, so the
velocity of the air.

00:23:56.140 --> 00:23:58.960
And the very
importantly is what's

00:23:58.960 --> 00:24:01.150
called the angle of attack.

00:24:01.150 --> 00:24:05.960
So it's the angle with which
this airfoil has to the air.

00:24:05.960 --> 00:24:11.670
So if you had one airfoil
that was pointed up

00:24:11.670 --> 00:24:17.840
like this versus one, the same
one but it was not tilted up,

00:24:17.840 --> 00:24:22.460
this airfoil would be having a
higher angle of attack or angle

00:24:22.460 --> 00:24:24.950
to the wind than this one.

00:24:24.950 --> 00:24:27.800
Now, this might seem like
a very fancy description,

00:24:27.800 --> 00:24:31.340
but who has been in a car
driving down the highway,

00:24:31.340 --> 00:24:34.220
and you stuck your
hand out outside?

00:24:34.220 --> 00:24:35.970
And if you tilt your
hand up a little bit,

00:24:35.970 --> 00:24:38.720
you'll see that the wind
kind of pushes your hand up.

00:24:38.720 --> 00:24:42.110
And if you tilt it down,
your hand pushes up.

00:24:42.110 --> 00:24:44.510
And you kind of glide
your hand out the window.

00:24:44.510 --> 00:24:47.310
So I'm getting a
lot of head nods.

00:24:47.310 --> 00:24:49.460
So that's really all
that angle of attack

00:24:49.460 --> 00:24:52.910
is talking about that if
you angle your hand up,

00:24:52.910 --> 00:24:55.040
it gets pushed up a lot more.

00:24:55.040 --> 00:24:57.110
If you angle it down,
it gets pushed down.

00:24:57.110 --> 00:24:59.820
That's the angle of attack.

00:24:59.820 --> 00:25:02.040
And we're going to define
it more specifically when

00:25:02.040 --> 00:25:04.530
we talk about the
terms associated

00:25:04.530 --> 00:25:06.300
with an airfoil in
the shape, but it's

00:25:06.300 --> 00:25:11.070
good to get the
general concept first.

00:25:11.070 --> 00:25:13.710
And then another
factor affecting lift

00:25:13.710 --> 00:25:17.190
is the air, the fluid
that it's in, so

00:25:17.190 --> 00:25:20.970
the actual mass of the
airflow coming around you.

00:25:20.970 --> 00:25:22.980
So there are a lot
of aspects to that.

00:25:22.980 --> 00:25:24.670
We talked about whether
you're in water,

00:25:24.670 --> 00:25:28.690
whether you're in air, or
the density of the air.

00:25:28.690 --> 00:25:32.100
Another component of that
air is the viscosity.

00:25:32.100 --> 00:25:34.100
Does anyone know
what viscosity is?

00:25:38.490 --> 00:25:39.345
Yes.

00:25:39.345 --> 00:25:40.560
AUDIENCE: Resistance to flow.

00:25:40.560 --> 00:25:42.870
TINA SRIVASTAVA:
Resistance to flow.

00:25:42.870 --> 00:25:44.550
The way I like to
think about it is

00:25:44.550 --> 00:25:49.695
if you've ever baked brownies,
and you have your mixing bowl

00:25:49.695 --> 00:25:51.960
and your spatula in
there, and if you just

00:25:51.960 --> 00:25:54.900
have the water and
the oil and eggs,

00:25:54.900 --> 00:25:58.170
and you're mixing it around,
you can mix pretty quickly.

00:25:58.170 --> 00:26:01.270
And it doesn't stick to
the spatula that much.

00:26:01.270 --> 00:26:03.780
But if you were mixing
molasses or once you

00:26:03.780 --> 00:26:08.310
get all that brownie batter in
there, it's harder to do it.

00:26:08.310 --> 00:26:10.140
And it sticks to the spatula.

00:26:10.140 --> 00:26:12.000
So that's what we're
talking about when

00:26:12.000 --> 00:26:13.350
we're talking about viscosity.

00:26:13.350 --> 00:26:15.420
So it's the tendency
for these molecules

00:26:15.420 --> 00:26:19.440
to stick to each other and
to stick to the object that's

00:26:19.440 --> 00:26:20.970
moving through them.

00:26:20.970 --> 00:26:23.910
So with the case of the
airfoil, we're talking about--

00:26:23.910 --> 00:26:28.140
and we were discussing this
just a moment ago about which

00:26:28.140 --> 00:26:29.490
air were we talking about.

00:26:29.490 --> 00:26:31.680
So some air that
might be very close

00:26:31.680 --> 00:26:34.890
might kind of stick
to that airfoil

00:26:34.890 --> 00:26:38.340
or stick to the wing versus
just moving smoothly past it.

00:26:38.340 --> 00:26:40.680
So viscosity has a big impact.

00:26:40.680 --> 00:26:43.920
And then compressibility
also affects lift.

00:26:43.920 --> 00:26:46.340
So the compressibility
of the air--

00:26:46.340 --> 00:26:47.530
did I turn off my mic?

00:26:51.750 --> 00:26:54.900
So certain types of
fluids are compressible.

00:26:54.900 --> 00:26:57.540
So you could take
a balloon of air.

00:26:57.540 --> 00:26:59.970
And you can move it
into a cold environment

00:26:59.970 --> 00:27:02.520
and have it shrink or
in a hot environment

00:27:02.520 --> 00:27:05.550
and have it expand while
having the same amount of mass

00:27:05.550 --> 00:27:06.390
inside the balloon.

00:27:06.390 --> 00:27:07.900
So I'm getting a
lot of head nods.

00:27:07.900 --> 00:27:10.950
So that just shows the
compressibility of the air,

00:27:10.950 --> 00:27:13.320
whereas some types of
fluids are not compressible.

00:27:13.320 --> 00:27:14.640
They're incompressible.

00:27:14.640 --> 00:27:18.080
And they affect lift
in a different way.

00:27:18.080 --> 00:27:19.880
So although I've told
you all these things

00:27:19.880 --> 00:27:23.240
that affects lift, one
thing I will admit to you is

00:27:23.240 --> 00:27:27.980
that calculating lift is
difficult. It's very difficult.

00:27:27.980 --> 00:27:31.700
In fact, we don't really
know how to do it properly.

00:27:31.700 --> 00:27:34.820
This is a snapshot
from Wikipedia

00:27:34.820 --> 00:27:37.340
of all the different
theories of lift.

00:27:37.340 --> 00:27:39.020
So there are a lot
of different ways

00:27:39.020 --> 00:27:42.770
that people go about
trying to calculate lift.

00:27:42.770 --> 00:27:46.440
And it turns out that
it's very hard to do.

00:27:46.440 --> 00:27:53.130
So one that you see up
there is Navier-Stokes.

00:27:53.130 --> 00:27:55.950
So Navier-Stokes is
a set of equations

00:27:55.950 --> 00:28:00.720
that does a really good
job of predicting lift.

00:28:00.720 --> 00:28:05.180
And it really takes into
account a lot of things.

00:28:05.180 --> 00:28:07.880
It takes into account
conservation of energy,

00:28:07.880 --> 00:28:10.760
conservation of mass,
conservation of momentum,

00:28:10.760 --> 00:28:15.320
viscosity, even a lot of things
like thermal conductivity

00:28:15.320 --> 00:28:17.390
and a whole bunch
of considerations.

00:28:17.390 --> 00:28:23.660
But the problem is that solving
those equations is very hard.

00:28:23.660 --> 00:28:27.560
We try to use supercomputers to
estimate every little aspect.

00:28:27.560 --> 00:28:29.310
And it's very difficult to do.

00:28:29.310 --> 00:28:30.950
And we're not
really able to solve

00:28:30.950 --> 00:28:34.010
those equations to determine
precisely what the lift is

00:28:34.010 --> 00:28:36.820
going to be.

00:28:36.820 --> 00:28:38.930
Let me talk about some
of the limitations

00:28:38.930 --> 00:28:42.510
that we have in solving
these equations.

00:28:42.510 --> 00:28:46.190
So first of all, it
has to do with how

00:28:46.190 --> 00:28:48.950
the air flows over the wing.

00:28:48.950 --> 00:28:54.230
If the air is moving very
smoothly past the airfoil,

00:28:54.230 --> 00:28:56.240
then it's very easy to come up--

00:28:56.240 --> 00:28:58.850
not easy, but it's
easier to approximate.

00:28:58.850 --> 00:29:03.260
We can predict what a particular
air molecule is going to do.

00:29:03.260 --> 00:29:05.720
But as you see there, when
it starts spinning around

00:29:05.720 --> 00:29:07.570
and becoming turbulent--

00:29:07.570 --> 00:29:11.900
so if you start seeing a
particular air molecule that's

00:29:11.900 --> 00:29:15.320
moving around, and becoming
turbulent, so not doing

00:29:15.320 --> 00:29:18.590
laminar flow but turbulent,
and moving around, and bumping

00:29:18.590 --> 00:29:21.050
into other air molecules,
then predicting

00:29:21.050 --> 00:29:23.600
what that molecule does and
what all the molecules do around

00:29:23.600 --> 00:29:25.710
it become very, very
difficult. In fact,

00:29:25.710 --> 00:29:28.260
we have a very hard
time doing that.

00:29:28.260 --> 00:29:32.718
And so instead, we
basically assume

00:29:32.718 --> 00:29:33.760
that that doesn't happen.

00:29:36.340 --> 00:29:40.870
And we impose some
limitations or conditions

00:29:40.870 --> 00:29:43.480
on the airflow which
are not actually true

00:29:43.480 --> 00:29:46.180
but help us with
approximating lift.

00:29:46.180 --> 00:29:48.400
So one of those is
the Kutta condition

00:29:48.400 --> 00:29:52.390
that you see at the bottom left,
which is this smooth flow off.

00:29:52.390 --> 00:29:55.510
So basically, you say that
none of this turbulence

00:29:55.510 --> 00:29:57.440
is happening.

00:29:57.440 --> 00:30:01.220
And the air moves
very cleanly off.

00:30:01.220 --> 00:30:04.370
And you also have a couple
other specific requirements

00:30:04.370 --> 00:30:07.220
such as that no air
molecule from the top

00:30:07.220 --> 00:30:10.640
comes over to the bottom, and
no air molecule from the bottom

00:30:10.640 --> 00:30:12.250
goes around to the top.

00:30:12.250 --> 00:30:16.430
And you just assume that
they move smoothly off.

00:30:16.430 --> 00:30:18.200
And so that Kutta
condition is actually

00:30:18.200 --> 00:30:23.210
very helpful in
approximating lift.

00:30:23.210 --> 00:30:26.810
We also make other assumptions
that there's no viscosity

00:30:26.810 --> 00:30:29.390
or that the fluid
is not compressible.

00:30:29.390 --> 00:30:32.660
Sometimes, these
assumptions are appropriate.

00:30:32.660 --> 00:30:35.560
And sometimes, they're not.

00:30:35.560 --> 00:30:39.520
Another thing that's really
critical about our ability

00:30:39.520 --> 00:30:42.610
to estimate lift is that as
I've been talking to you here

00:30:42.610 --> 00:30:46.150
on the blackboard, I have
talked about a cross-section,

00:30:46.150 --> 00:30:46.910
that you just--

00:30:46.910 --> 00:30:47.920
you cut off the wing.

00:30:47.920 --> 00:30:50.215
And you're only looking
at one cross-section.

00:30:50.215 --> 00:30:52.090
So since we're talking
about a cross-section,

00:30:52.090 --> 00:30:55.640
we're talking in
two-dimensional space.

00:30:55.640 --> 00:30:57.460
Well, we can actually
do a pretty good job

00:30:57.460 --> 00:31:01.120
of estimating lift in a
two-dimensional environment.

00:31:01.120 --> 00:31:04.900
But the fact of the matter is
wings are not two dimensional.

00:31:04.900 --> 00:31:06.790
And the wing comes
out into the classroom

00:31:06.790 --> 00:31:09.170
and back into the blackboard.

00:31:09.170 --> 00:31:12.100
And to estimate actually
how all these air flows

00:31:12.100 --> 00:31:15.370
work at the edge of the
wing is very difficult.

00:31:15.370 --> 00:31:19.150
Has anyone heard
about tip vortices?

00:31:19.150 --> 00:31:21.520
Couple head nods.

00:31:21.520 --> 00:31:24.490
So we have a picture
there that shows a jet

00:31:24.490 --> 00:31:26.860
to just show a
little bit about what

00:31:26.860 --> 00:31:32.980
the air does when it comes off
the edge, the end of the wing.

00:31:32.980 --> 00:31:36.640
We're going to talk about
tip vortices a little bit.

00:31:36.640 --> 00:31:40.210
But the problem is that
it no longer is adhering

00:31:40.210 --> 00:31:42.100
to all of our conditions.

00:31:42.100 --> 00:31:44.110
Now, we don't have smooth flow.

00:31:44.110 --> 00:31:45.640
We definitely have
turbulent flow.

00:31:45.640 --> 00:31:46.760
We have spinning flow.

00:31:46.760 --> 00:31:49.060
And we have air molecules
hitting other air molecules.

00:31:49.060 --> 00:31:51.640
And it becomes extremely
difficult for us

00:31:51.640 --> 00:31:54.520
to model all of
those air molecules.

00:31:54.520 --> 00:31:55.520
We really can't do it.

00:31:55.520 --> 00:31:58.510
So going from two dimensions
to three dimensions

00:31:58.510 --> 00:32:00.760
is really a limitation
of a lot of the equations

00:32:00.760 --> 00:32:04.300
that we have to
approximate lift.

00:32:04.300 --> 00:32:06.640
So what do we do?

00:32:06.640 --> 00:32:10.720
Well, first of all, we go back
to our two-dimensional surface.

00:32:10.720 --> 00:32:14.530
And we talked about all
of these normal forces,

00:32:14.530 --> 00:32:16.900
so when you have all
the fluid going past,

00:32:16.900 --> 00:32:19.240
and it has pressure,
and it's supplying

00:32:19.240 --> 00:32:23.570
all these forces perpendicular
to the airfoil all around.

00:32:23.570 --> 00:32:25.178
So how do you approximate lift?

00:32:25.178 --> 00:32:26.470
Well, you say, oh, that's fine.

00:32:26.470 --> 00:32:30.430
You just sum all
those forces around.

00:32:30.430 --> 00:32:33.850
Well, that's great if you know
what all of those forces are,

00:32:33.850 --> 00:32:37.370
but it's not great if you don't
know what all of them are.

00:32:37.370 --> 00:32:40.862
So what is the
solution that we--

00:32:40.862 --> 00:32:42.250
what we do?

00:32:42.250 --> 00:32:44.450
Basically, we
calculate what we can,

00:32:44.450 --> 00:32:47.600
and then we measure the
rest experimentally.

00:32:47.600 --> 00:32:50.740
So in this equation of lift,
for example, so we have

00:32:50.740 --> 00:32:52.480
L is for lift.

00:32:52.480 --> 00:32:56.830
Some of the other terms
that you have there--

00:32:56.830 --> 00:32:59.240
rho is the one that
looks like a p.

00:32:59.240 --> 00:33:02.590
So rho is talking
about the air density.

00:33:02.590 --> 00:33:04.060
You have velocity.

00:33:04.060 --> 00:33:08.060
And A is the wing
area we talked about.

00:33:08.060 --> 00:33:12.910
And then we have this fancy
little symbol there C sub

00:33:12.910 --> 00:33:15.670
L or the coefficient of lift.

00:33:15.670 --> 00:33:18.400
And basically, we
say that I don't

00:33:18.400 --> 00:33:21.490
know how to come up
with characterizing

00:33:21.490 --> 00:33:25.270
all those complications
about viscosity

00:33:25.270 --> 00:33:30.610
and some of the effects like
that have to do with turbulence

00:33:30.610 --> 00:33:34.000
and shock waves, Mach
number, Reynolds number, all

00:33:34.000 --> 00:33:35.180
these types of things.

00:33:35.180 --> 00:33:37.840
And so we say we'll
measure what we can,

00:33:37.840 --> 00:33:40.460
and then we'll-- or we'll
calculate what we can,

00:33:40.460 --> 00:33:43.420
and then we'll actually, in a
wind tunnel where we put this

00:33:43.420 --> 00:33:46.990
guy on a stick, we'll actually
measure the coefficient

00:33:46.990 --> 00:33:47.800
of lift.

00:33:47.800 --> 00:33:50.170
And that's how we
really calculate lift

00:33:50.170 --> 00:33:52.900
these days is using
a lot of measurement

00:33:52.900 --> 00:33:55.570
to inform what's actually
happening because it's just

00:33:55.570 --> 00:33:57.960
very complicated.

00:33:57.960 --> 00:34:01.140
AUDIENCE: Tina, that
velocity is squared, right?

00:34:01.140 --> 00:34:04.800
So if you go twice as fast, you
get four times as much lift.

00:34:04.800 --> 00:34:06.550
TINA SRIVASTAVA: That
is the relationship.

00:34:06.550 --> 00:34:08.469
Absolutely.

00:34:08.469 --> 00:34:11.199
And the other thing
that's really important

00:34:11.199 --> 00:34:13.750
is that that coefficient
of lift is measured

00:34:13.750 --> 00:34:15.969
for a given angle of attack.

00:34:15.969 --> 00:34:18.100
So we talked a little
bit about angle of attack

00:34:18.100 --> 00:34:20.150
with your hand
outside the window.

00:34:20.150 --> 00:34:24.170
So let's get into defining it
a little bit more in detail.

00:34:24.170 --> 00:34:26.800
So in order to describe it,
I have to come up with a few

00:34:26.800 --> 00:34:29.920
more terms that have
to do with the airfoil.

00:34:29.920 --> 00:34:32.050
So we talked about
the very front

00:34:32.050 --> 00:34:35.679
of the airfoil or
the front of the wing

00:34:35.679 --> 00:34:40.300
is called the leading
edge, and then the back

00:34:40.300 --> 00:34:41.469
is the trailing edge.

00:34:45.409 --> 00:34:47.840
And we talked about the
trailing edge a little bit

00:34:47.840 --> 00:34:50.090
when we were talking
about the Kutta condition

00:34:50.090 --> 00:34:53.719
that no air molecule-- we're
assuming no air molecule

00:34:53.719 --> 00:34:58.310
can cross the trailing
edge to the other side.

00:34:58.310 --> 00:35:00.510
So then the camber is in there.

00:35:00.510 --> 00:35:03.260
So that's just talking
about really representing

00:35:03.260 --> 00:35:08.450
the curvature of that
airfoil and then a chord line

00:35:08.450 --> 00:35:12.410
that goes in between so you
can measure how that is.

00:35:12.410 --> 00:35:20.727
So try and do your
little zoom in fanciness

00:35:20.727 --> 00:35:21.560
that you were doing.

00:35:21.560 --> 00:35:23.852
AUDIENCE: I think I set--
yeah, maybe it went to sleep.

00:35:29.958 --> 00:35:31.440
Giving up?

00:35:31.440 --> 00:35:33.330
TINA SRIVASTAVA: I'll
just point at it.

00:35:33.330 --> 00:35:35.370
So this is the chord
line of the wing.

00:35:35.370 --> 00:35:37.620
So you can see that
this is a full airplane.

00:35:37.620 --> 00:35:39.580
The airfoil is right here.

00:35:39.580 --> 00:35:41.730
And you see this chord
line going from the back

00:35:41.730 --> 00:35:42.300
to the front.

00:35:48.631 --> 00:35:51.330
Is somebody trying to
come in the door there?

00:35:54.510 --> 00:35:55.040
Great.

00:35:55.040 --> 00:35:57.940
And then we'll talk about
some of these terms.

00:35:57.940 --> 00:36:00.140
Basically, the most important
thing to think about

00:36:00.140 --> 00:36:02.280
is the angle of attack.

00:36:02.280 --> 00:36:06.080
Thank you for
checking on the door.

00:36:06.080 --> 00:36:08.870
So talking about how we
can control the lift,

00:36:08.870 --> 00:36:10.370
so some of the
things we can do have

00:36:10.370 --> 00:36:11.720
to do with the aircraft design.

00:36:11.720 --> 00:36:13.940
So we can build an airfoil.

00:36:13.940 --> 00:36:18.860
And we can talk about how curved
that airfoil is, the curvature

00:36:18.860 --> 00:36:20.600
on the top, how curved it is.

00:36:20.600 --> 00:36:23.090
We can design the wing area.

00:36:23.090 --> 00:36:26.330
When we're flying, we
can control the airspeed.

00:36:26.330 --> 00:36:28.460
And then the angle of
attack is something

00:36:28.460 --> 00:36:30.950
that you can control when
you're in the airplane

00:36:30.950 --> 00:36:33.320
by pitching down or pitching up.

00:36:33.320 --> 00:36:36.290
And we'll describe pitching
and how you control

00:36:36.290 --> 00:36:38.790
an airplane in more detail.

00:36:38.790 --> 00:36:41.030
Another thing that's
relevant is flaps.

00:36:41.030 --> 00:36:43.850
So I talked about in
this drawing right

00:36:43.850 --> 00:36:48.260
here where I added this white
part of the trailing edge that

00:36:48.260 --> 00:36:51.680
moves down, that really is
kind of similar to flaps.

00:36:51.680 --> 00:36:54.920
So when your flaps are up,
they're sort of in line

00:36:54.920 --> 00:36:56.090
with the rest of the wing.

00:36:56.090 --> 00:36:57.950
But when your flaps
are down, it's

00:36:57.950 --> 00:37:00.530
the effective thing
like pushing--

00:37:00.530 --> 00:37:04.250
pulling a piece of your
trailing edge downward,

00:37:04.250 --> 00:37:08.510
which causes again more of that
air to be deflected downward.

00:37:08.510 --> 00:37:11.600
So it increases your
drag, but it also

00:37:11.600 --> 00:37:14.060
increases your lift because
you're deflecting more air

00:37:14.060 --> 00:37:15.770
molecules down.

00:37:15.770 --> 00:37:18.530
And then we also talked
about spoilers, for example,

00:37:18.530 --> 00:37:20.960
as something that
can, like on a car,

00:37:20.960 --> 00:37:26.190
that can actually disrupt the
lift by disrupting the airflow.

00:37:26.190 --> 00:37:29.790
And when we talked about
the four forces of flight,

00:37:29.790 --> 00:37:31.890
if you're doing steady
flight, you're not climbing

00:37:31.890 --> 00:37:34.350
or you're descending, but
you're just flying straight,

00:37:34.350 --> 00:37:37.350
that means that your
lift and your weight

00:37:37.350 --> 00:37:41.710
basically cancel each other out.

00:37:41.710 --> 00:37:43.870
If your lift is greater
than your weight,

00:37:43.870 --> 00:37:45.220
then you can climb.

00:37:45.220 --> 00:37:47.110
And if your weight is
greater than your lift,

00:37:47.110 --> 00:37:48.112
then you descend.

00:37:48.112 --> 00:37:49.570
But if you're just
flying straight,

00:37:49.570 --> 00:37:54.680
you're in an equilibrium where
those two forces cancel out.

00:37:54.680 --> 00:37:55.180
So good.

00:37:55.180 --> 00:37:58.440
I have a more detailed
diagram of angle of attack.

00:37:58.440 --> 00:38:01.360
So you can see here
the chord line.

00:38:01.360 --> 00:38:05.080
You can also see the
relative wind and same things

00:38:05.080 --> 00:38:06.130
that I drew here--

00:38:06.130 --> 00:38:10.120
the lift and the drag and then
that resultant force vector.

00:38:16.420 --> 00:38:20.070
So you can actually
control the angle of attack

00:38:20.070 --> 00:38:22.190
in a number of ways.

00:38:22.190 --> 00:38:27.250
One of the ways that we
talked about is pitching down.

00:38:27.250 --> 00:38:31.460
So pushing your yoke
forward causes the airplane

00:38:31.460 --> 00:38:33.100
to pitch down.

00:38:33.100 --> 00:38:36.340
And it does that by
changing the elevator

00:38:36.340 --> 00:38:37.540
at the back of the airplane.

00:38:37.540 --> 00:38:39.430
We'll describe that
in more detail.

00:38:39.430 --> 00:38:42.440
But the other things that can
affect the angle of attack,

00:38:42.440 --> 00:38:45.050
you can actually affect
before you even take off.

00:38:45.050 --> 00:38:47.410
So it has to do with
your aircraft weight,

00:38:47.410 --> 00:38:50.350
for example, and the
center of gravity,

00:38:50.350 --> 00:38:53.210
as well as your airspeed
when you're flying.

00:38:53.210 --> 00:38:57.320
So here are a couple of
diagrams that show you

00:38:57.320 --> 00:39:02.210
how the lift changes with the
effective angle of attack,

00:39:02.210 --> 00:39:04.500
and then there is a
critical angle of attack.

00:39:04.500 --> 00:39:07.860
So that's when you can
keep climbing for a while.

00:39:07.860 --> 00:39:11.240
But if you get too
steep, what happens?

00:39:11.240 --> 00:39:13.197
Who knows what happens
when you go to steep?

00:39:13.197 --> 00:39:14.030
AUDIENCE: You stall.

00:39:14.030 --> 00:39:15.155
TINA SRIVASTAVA: You stall.

00:39:15.155 --> 00:39:15.900
That's right.

00:39:15.900 --> 00:39:19.620
So the air can't really
effectively go over the wing.

00:39:19.620 --> 00:39:21.420
And it starts separating.

00:39:21.420 --> 00:39:23.220
And so you're no
longer effectively

00:39:23.220 --> 00:39:25.020
pushing the air down.

00:39:25.020 --> 00:39:28.520
And you lose the lift
that you were generating.

00:39:28.520 --> 00:39:30.920
And one thing I also
want to point out here

00:39:30.920 --> 00:39:37.130
in these diagrams is you see
with these little colored lines

00:39:37.130 --> 00:39:39.440
the air that's coming in.

00:39:39.440 --> 00:39:40.370
And it's going out.

00:39:40.370 --> 00:39:43.280
And you can see
that in this case,

00:39:43.280 --> 00:39:46.190
the blue lines are
showing that the air that

00:39:46.190 --> 00:39:50.810
went over the top of the airfoil
went faster and actually got

00:39:50.810 --> 00:39:54.630
to the back faster than the
air that went from the bottom.

00:39:54.630 --> 00:39:59.870
So again, please don't fall
for the equal transit theory.

00:39:59.870 --> 00:40:01.010
So practice question.

00:40:19.410 --> 00:40:20.950
A, B, or C?

00:40:20.950 --> 00:40:21.870
AUDIENCE: A.

00:40:21.870 --> 00:40:24.420
TINA SRIVASTAVA: A. Good.

00:40:24.420 --> 00:40:27.417
So the angle of attack
is defined there.

00:40:27.417 --> 00:40:29.250
And one thing that I
would like to point out

00:40:29.250 --> 00:40:31.800
is that this is also the
case for a propeller.

00:40:31.800 --> 00:40:35.880
So your propeller also
looks a lot like an airfoil

00:40:35.880 --> 00:40:39.720
or like a wing that's
sideways and spinning around.

00:40:39.720 --> 00:40:42.840
And so also the angle of
attack for a propeller

00:40:42.840 --> 00:40:44.670
is defined basically
the same way

00:40:44.670 --> 00:40:46.770
is the angle between
the propeller's chord

00:40:46.770 --> 00:40:50.280
line and the relative wind.

00:40:50.280 --> 00:40:52.770
So let's define the
center of pressure.

00:40:52.770 --> 00:40:56.880
So it's basically
the point on the wing

00:40:56.880 --> 00:40:59.850
where the lift is centered.

00:40:59.850 --> 00:41:04.650
And so that can actually move
as you can see in this figure.

00:41:04.650 --> 00:41:08.280
Based on the angle of attack,
the center of pressure

00:41:08.280 --> 00:41:11.560
can act in a different location.

00:41:11.560 --> 00:41:14.380
And that's really important
to understand also

00:41:14.380 --> 00:41:17.110
that it's not that the
lift is always coming right

00:41:17.110 --> 00:41:17.655
at the front.

00:41:17.655 --> 00:41:19.780
Depending on where you are,
it might be pulling you

00:41:19.780 --> 00:41:21.230
in different directions.

00:41:21.230 --> 00:41:23.110
And that can affect
the maneuverability

00:41:23.110 --> 00:41:23.860
of your aircraft.

00:41:23.860 --> 00:41:26.837
And we'll get into
that in more detail.

00:41:26.837 --> 00:41:28.420
So we talked a little
bit about flaps,

00:41:28.420 --> 00:41:31.570
that flaps actually
can increase the lift

00:41:31.570 --> 00:41:33.130
that you're able to produce.

00:41:33.130 --> 00:41:37.160
But it's a trade-off because
it also increases the drag.

00:41:37.160 --> 00:41:43.000
So when in the course of
the flight, takeoff, cruise,

00:41:43.000 --> 00:41:46.950
or landing, when
do you use flaps?

00:41:46.950 --> 00:41:48.650
Does anyone know?

00:41:48.650 --> 00:41:50.178
AUDIENCE: Takeoff and landing.

00:41:50.178 --> 00:41:51.720
TINA SRIVASTAVA:
Takeoff and landing.

00:41:51.720 --> 00:41:52.920
Landing.

00:41:52.920 --> 00:41:57.240
Yeah, the reason that you,
especially on landing--

00:41:57.240 --> 00:41:59.370
many times people use
flaps on takeoff as well.

00:41:59.370 --> 00:42:01.185
But the reason is
just that you like

00:42:01.185 --> 00:42:03.060
to have your aircraft
configured that in case

00:42:03.060 --> 00:42:05.370
you didn't take off, you
can land without making

00:42:05.370 --> 00:42:08.310
a lot of dramatic changes.

00:42:08.310 --> 00:42:10.530
The reason that you
do that is basically

00:42:10.530 --> 00:42:16.930
that by increasing your lift
but also increasing the drag,

00:42:16.930 --> 00:42:20.460
drag affects how fast
you're moving forward.

00:42:20.460 --> 00:42:24.550
And so you can actually
have the airspeed

00:42:24.550 --> 00:42:28.830
be higher with the
ground speed being lower.

00:42:28.830 --> 00:42:33.380
What it does is it allows you to
go very slow without stalling.

00:42:33.380 --> 00:42:36.180
And so that really helps
you land an airplane.

00:42:36.180 --> 00:42:38.490
So basically, it
allows you to come

00:42:38.490 --> 00:42:41.730
in at a kind of
steeper angle to land,

00:42:41.730 --> 00:42:45.660
maintaining the airspeed that
you need in order to do that.

00:42:45.660 --> 00:42:49.230
And you'll notice that there
are different flap settings.

00:42:49.230 --> 00:42:53.340
So you can either have flaps
at 10 degrees, 20 degrees, 30

00:42:53.340 --> 00:42:53.980
degrees.

00:42:53.980 --> 00:42:55.440
We'll discuss that
in more detail.

00:42:55.440 --> 00:42:57.065
And Phillip will talk
about it in terms

00:42:57.065 --> 00:43:00.240
of performance I think as well.

00:43:00.240 --> 00:43:03.930
Thrust-- so we talked about
that forward force thrust.

00:43:03.930 --> 00:43:06.990
In this type of an aircraft,
a single engine propeller

00:43:06.990 --> 00:43:09.270
aircraft, it's the
propeller that's

00:43:09.270 --> 00:43:13.390
rotating that is really
producing the thrust.

00:43:13.390 --> 00:43:16.350
And it's really, as I
said, the propeller blades

00:43:16.350 --> 00:43:17.970
are kind of like
an airplane wing--

00:43:17.970 --> 00:43:19.428
it's a good way to
think about it--

00:43:19.428 --> 00:43:23.310
that are just spinning round
and round and generating lift.

00:43:23.310 --> 00:43:26.460
But in this case, it's
moving air molecules front to

00:43:26.460 --> 00:43:28.380
behind your airplane.

00:43:28.380 --> 00:43:32.160
And then although this is
also just a force, instead

00:43:32.160 --> 00:43:33.900
of talking about it
in pounds, we usually

00:43:33.900 --> 00:43:36.090
talk about the
horsepower required

00:43:36.090 --> 00:43:39.470
to drive the propeller.

00:43:39.470 --> 00:43:41.150
So let me also talk about drag.

00:43:41.150 --> 00:43:43.700
So there are a couple
different types of drag.

00:43:43.700 --> 00:43:47.870
So one drag is just what's
called parasitic drag

00:43:47.870 --> 00:43:48.900
or parasite drag.

00:43:48.900 --> 00:43:52.280
It's basically when the aircraft
is moving through the air

00:43:52.280 --> 00:43:55.760
that you get some kind
of resistance to that.

00:43:55.760 --> 00:44:00.840
That's parasitic drag, whereas
this drag is induced drag,

00:44:00.840 --> 00:44:04.840
which is the drag that's created
by the lift, so this backwards

00:44:04.840 --> 00:44:07.760
D. And so you can
see in this figure

00:44:07.760 --> 00:44:11.270
that the total drag is a
sum of that induced drag

00:44:11.270 --> 00:44:12.890
and the parasite drag.

00:44:16.740 --> 00:44:19.870
AUDIENCE: Do we also call
the induced drag just lift

00:44:19.870 --> 00:44:21.540
in an unwanted direction?

00:44:21.540 --> 00:44:24.130
TINA SRIVASTAVA: Lift in
an unwanted direction.

00:44:24.130 --> 00:44:28.390
Sure, whatever can have
you associate induced drag

00:44:28.390 --> 00:44:29.140
with lift.

00:44:29.140 --> 00:44:31.210
That's the drag created by lift.

00:44:34.460 --> 00:44:37.460
Ground effect-- does anyone
know a ground effect is?

00:44:40.010 --> 00:44:42.620
Only a couple of you.

00:44:42.620 --> 00:44:45.450
So let's talk about
it a little bit.

00:44:45.450 --> 00:44:48.590
So basically, when you're very
close to the ground within one

00:44:48.590 --> 00:44:51.260
wing span of the
ground, you actually

00:44:51.260 --> 00:44:55.310
have some of the airflow
going on with your airplane is

00:44:55.310 --> 00:44:56.990
blocked by the ground.

00:44:56.990 --> 00:45:00.580
And so your induced
drag decreases.

00:45:00.580 --> 00:45:02.210
Now, with the induced
drag decreases,

00:45:02.210 --> 00:45:04.700
it's actually the case
that your airplane

00:45:04.700 --> 00:45:10.010
can become airborne at a lower
speed than it's supposed to.

00:45:10.010 --> 00:45:12.740
So what you might
notice is that when

00:45:12.740 --> 00:45:14.780
you're on the
runway taking off--

00:45:14.780 --> 00:45:17.300
this is probably the first
part of your flights.

00:45:17.300 --> 00:45:19.730
After you did your
pre-flight, your engine runup,

00:45:19.730 --> 00:45:21.500
you pulled out onto the runway.

00:45:21.500 --> 00:45:24.050
And you'll have
determined in advance

00:45:24.050 --> 00:45:28.190
what is the air speed at
which you should rotate.

00:45:28.190 --> 00:45:29.540
Now, that's really important.

00:45:29.540 --> 00:45:35.720
With a Cessna 172, for
example, it's around 55 knots.

00:45:35.720 --> 00:45:38.390
And you want to look at
your airspeed indicator.

00:45:38.390 --> 00:45:40.550
Because if you
just feel yourself,

00:45:40.550 --> 00:45:44.600
you might notice that
much lower, like 40 knots,

00:45:44.600 --> 00:45:46.760
that the plane has
already taken off.

00:45:46.760 --> 00:45:48.050
You're already floating.

00:45:48.050 --> 00:45:49.030
You're flying.

00:45:49.030 --> 00:45:50.990
And you might be very
excited about that.

00:45:50.990 --> 00:45:55.910
And you might want to just pull
back on your yoke to take off.

00:45:55.910 --> 00:45:59.450
Well, you won't be
able to sustain flight.

00:45:59.450 --> 00:46:02.810
And so this is what why ground
effect is really important

00:46:02.810 --> 00:46:05.150
is that you can kind of
float over the ground

00:46:05.150 --> 00:46:06.800
because you're so
close to the ground

00:46:06.800 --> 00:46:10.913
that the ground is blocking
some of the effects of the air.

00:46:10.913 --> 00:46:12.830
And so what you want to
do is really make sure

00:46:12.830 --> 00:46:15.968
that you continue your
ground roll, continue.

00:46:15.968 --> 00:46:17.510
Even if you're a
little bit airborne,

00:46:17.510 --> 00:46:20.360
stay close to the ground
until your airspeed comes up

00:46:20.360 --> 00:46:23.930
to that rotate speed, so
in this case, 55 knots,

00:46:23.930 --> 00:46:29.060
and then you pull back
on your yoke to take off.

00:46:29.060 --> 00:46:32.045
So again, so when does
ground effect happen?

00:46:32.045 --> 00:46:33.420
When you're close
to the ground--

00:46:33.420 --> 00:46:37.940
when you're within one
wing span of the ground

00:46:37.940 --> 00:46:40.850
So let's talk a little
bit about stability.

00:46:40.850 --> 00:46:44.420
And we'll start by just talking
about the three axes of flight.

00:46:44.420 --> 00:46:49.790
So there is a
longitudinal axis, which

00:46:49.790 --> 00:46:54.130
is basically from the nose
to the tail of your airplane.

00:46:54.130 --> 00:46:56.180
And there's a
lateral axis, which

00:46:56.180 --> 00:47:00.560
is from wingtip to wingtip and
then vertical going straight

00:47:00.560 --> 00:47:01.880
through the plane.

00:47:01.880 --> 00:47:04.310
So you have the ability
to control all three

00:47:04.310 --> 00:47:06.470
of those axes.

00:47:06.470 --> 00:47:10.100
So the elevator, which I keep
talking about is like your yoke

00:47:10.100 --> 00:47:12.530
where you push it forward
or you pull it back,

00:47:12.530 --> 00:47:14.960
that allows you to
pitch the airplane.

00:47:14.960 --> 00:47:17.930
So pitch nose up,
pitch nose down--

00:47:17.930 --> 00:47:21.680
that's you controlling the
back part of this tail,

00:47:21.680 --> 00:47:25.730
the elevator, which
allows you to have

00:47:25.730 --> 00:47:29.240
motion in this direction,
so pitch nose down.

00:47:29.240 --> 00:47:30.620
So you might hear that a lot.

00:47:30.620 --> 00:47:32.358
In case you're getting
close to stalling

00:47:32.358 --> 00:47:34.400
because your angle of
attack is getting too high,

00:47:34.400 --> 00:47:39.010
they might say, nose
down or pitch nose down.

00:47:39.010 --> 00:47:41.650
You also have
ailerons, which are out

00:47:41.650 --> 00:47:44.410
on the side of your wings.

00:47:44.410 --> 00:47:47.290
And those ailerons
control the roll.

00:47:47.290 --> 00:47:50.230
So that's rolling along
the longitudinal axis.

00:47:53.520 --> 00:47:57.170
And then your rudder, which
is at the back of the tail,

00:47:57.170 --> 00:47:59.180
the vertical part of the tail--

00:47:59.180 --> 00:48:00.650
that controls yaw.

00:48:00.650 --> 00:48:03.980
So this is called yaw,
this type of motion.

00:48:03.980 --> 00:48:07.220
So when you're turning, you
actually kind of do a roll

00:48:07.220 --> 00:48:10.520
and yaw usually to enact a turn.

00:48:13.880 --> 00:48:16.340
There are some cases
where you actually

00:48:16.340 --> 00:48:21.680
want to have adverse
yaw or you actually--

00:48:21.680 --> 00:48:23.960
adverse yaw means
basically you're

00:48:23.960 --> 00:48:27.170
using the yaw direction in maybe
the opposite direction at which

00:48:27.170 --> 00:48:31.040
you're trying to turn with
the roll or other angles

00:48:31.040 --> 00:48:32.120
of your plane.

00:48:32.120 --> 00:48:37.630
And so this just talks
about an adverse yaw

00:48:37.630 --> 00:48:42.040
is when you're not turning the
rudder in the same direction

00:48:42.040 --> 00:48:44.620
that you're using your aileron.

00:48:44.620 --> 00:48:47.650
And so this is where you
talk about coordinated flight

00:48:47.650 --> 00:48:49.210
or uncoordinated flight.

00:48:49.210 --> 00:48:53.530
When you're actually in an
airplane, the rudder or the yaw

00:48:53.530 --> 00:48:54.800
is controlled by your feet.

00:48:54.800 --> 00:48:58.930
So you have feet pedals
that control the rudder.

00:48:58.930 --> 00:49:01.810
And the yoke that
you're holding onto

00:49:01.810 --> 00:49:04.690
or a joystick that you're
holding onto front and back

00:49:04.690 --> 00:49:07.210
controls the pitch.

00:49:07.210 --> 00:49:10.540
And then turning it like in
the steering wheel of a car

00:49:10.540 --> 00:49:12.700
is only controlling the roll.

00:49:12.700 --> 00:49:14.680
So you actually
also use your feet

00:49:14.680 --> 00:49:17.350
for that third
direction of the yaw.

00:49:19.900 --> 00:49:24.250
So just talking about
stability in general,

00:49:24.250 --> 00:49:26.770
this isn't going to dive
into a whole diffy q

00:49:26.770 --> 00:49:28.090
discussion or anything.

00:49:28.090 --> 00:49:31.390
But just in general,
something that's stable--

00:49:31.390 --> 00:49:33.490
so it's just talking
about like a little bowl

00:49:33.490 --> 00:49:36.790
if you have a ball in a bowl,
even if the ball gets jostled

00:49:36.790 --> 00:49:39.880
around, it'll return
to the center point.

00:49:39.880 --> 00:49:41.590
Unstable would be the opposite.

00:49:41.590 --> 00:49:44.500
So if you have a convex
surface, then if the ball moves

00:49:44.500 --> 00:49:46.210
even just a little
bit, it'll really

00:49:46.210 --> 00:49:48.410
get moved out of control.

00:49:48.410 --> 00:49:51.670
So the reason that
we talk about this

00:49:51.670 --> 00:49:54.990
is basically when you're
flying in an airplane,

00:49:54.990 --> 00:49:56.840
and you're talking
about stable aircraft,

00:49:56.840 --> 00:50:02.050
for example, the reason I really
love flying a Cessna 172, even

00:50:02.050 --> 00:50:04.210
though it's kind of
the training airplane,

00:50:04.210 --> 00:50:07.280
is that the-- as people
call it, it flies itself.

00:50:07.280 --> 00:50:10.000
So if you notice the plane's
doing something weird

00:50:10.000 --> 00:50:12.130
and turning, almost the
best thing you can do

00:50:12.130 --> 00:50:13.810
is just let go.

00:50:13.810 --> 00:50:16.540
And the controls will
normalize, and then the plane

00:50:16.540 --> 00:50:19.870
will fly straight and level,
which is really great.

00:50:19.870 --> 00:50:21.640
There are other
types of aircraft

00:50:21.640 --> 00:50:23.780
that are inherently unstable.

00:50:23.780 --> 00:50:26.320
So we have Minachi
and Oxsana over here

00:50:26.320 --> 00:50:29.140
who do aerobatic flights.

00:50:29.140 --> 00:50:31.425
And Mark will be talking
about that tomorrow.

00:50:31.425 --> 00:50:32.800
So that's where
you actually want

00:50:32.800 --> 00:50:35.470
an airplane that's not
so stable so that you

00:50:35.470 --> 00:50:39.220
can cause it to do all kinds
of crazy maneuvers and turns

00:50:39.220 --> 00:50:41.320
and twists very easily.

00:50:41.320 --> 00:50:43.900
You pretty much can't
get a Cessna to do that.

00:50:43.900 --> 00:50:46.000
It really wants to fly
straight and level.

00:50:51.440 --> 00:50:53.660
So then there are
also other aspects

00:50:53.660 --> 00:50:56.810
that can affect stability,
such as your center of gravity,

00:50:56.810 --> 00:50:58.790
so how you load the airplane.

00:50:58.790 --> 00:51:01.010
We'll have a specific
lecture that just

00:51:01.010 --> 00:51:03.110
talks about weight and balance.

00:51:03.110 --> 00:51:06.560
But one thing to keep in
mind is that as people

00:51:06.560 --> 00:51:08.660
sit in your airplane
or as you put bags

00:51:08.660 --> 00:51:11.820
in the baggage compartment,
you're loading the airplane.

00:51:11.820 --> 00:51:15.590
And so if you have too
much weight aft of the CG

00:51:15.590 --> 00:51:17.840
or behind the center
of gravity, you

00:51:17.840 --> 00:51:19.310
can cause the plane
to basically go

00:51:19.310 --> 00:51:22.730
like this, which isn't very
helpful when you're flying.

00:51:22.730 --> 00:51:25.070
If you have things a
little too forward,

00:51:25.070 --> 00:51:26.960
it actually pushes
the nose down.

00:51:26.960 --> 00:51:29.300
In general, the nose
down is a little bit

00:51:29.300 --> 00:51:33.080
more stable from the
perspective of lift

00:51:33.080 --> 00:51:34.412
and getting air to fly over.

00:51:34.412 --> 00:51:35.870
You don't want
something that keeps

00:51:35.870 --> 00:51:39.910
trying to stall whenever
you let go of it.

00:51:39.910 --> 00:51:43.690
And then similarly, you can
talk about the stability

00:51:43.690 --> 00:51:47.900
in the lateral direction
in the roll direction.

00:51:47.900 --> 00:51:50.980
And some of these things
like swept-back wings

00:51:50.980 --> 00:51:54.580
like you see on a jet can
affect that type of stability.

00:51:57.920 --> 00:51:59.750
And then finally,
there's stability

00:51:59.750 --> 00:52:02.840
about the vertical axes.

00:52:02.840 --> 00:52:05.990
Generally, this is going to
be kind of fixed for the given

00:52:05.990 --> 00:52:07.680
aircraft that you're in.

00:52:07.680 --> 00:52:10.990
But you can affect it as
you design an aircraft.

00:52:10.990 --> 00:52:13.890
So we started talking
about stall already.

00:52:13.890 --> 00:52:17.760
So when you have
your angle of attack

00:52:17.760 --> 00:52:21.420
past its so-called
critical angle of attack,

00:52:21.420 --> 00:52:25.020
it can cause the air to
basically no longer be

00:52:25.020 --> 00:52:28.620
able to flow over the top and
no longer be able to effectively

00:52:28.620 --> 00:52:30.480
deflect air down.

00:52:30.480 --> 00:52:33.000
And so the air
kind of separates.

00:52:33.000 --> 00:52:34.710
And you can stall.

00:52:34.710 --> 00:52:36.960
So it's really important to
know that you can actually

00:52:36.960 --> 00:52:40.020
stall at any airspeed.

00:52:40.020 --> 00:52:42.720
Even with full
power, you can stall.

00:52:42.720 --> 00:52:44.220
In fact, one of the
maneuvers you'll

00:52:44.220 --> 00:52:46.680
have to do in order to
get your pilot's license

00:52:46.680 --> 00:52:49.560
is a power on stall.

00:52:49.560 --> 00:52:52.500
So you can stall both
where your engine is idle,

00:52:52.500 --> 00:52:55.650
like you're coming in for a
landing, and you get too steep,

00:52:55.650 --> 00:52:58.080
but you can also
stall with full power.

00:52:58.080 --> 00:53:02.280
And you just made your
angle go too steep.

00:53:02.280 --> 00:53:06.570
So it's really affecting that
critical angle of attack.

00:53:06.570 --> 00:53:09.390
And again, once you have that
angle of attack too steep,

00:53:09.390 --> 00:53:12.958
then there's a very significant
loss of lift, which is not good

00:53:12.958 --> 00:53:14.250
when you're flying an airplane.

00:53:17.870 --> 00:53:20.870
So when can you stall?

00:53:20.870 --> 00:53:25.460
At any airspeed and
any power setting,

00:53:25.460 --> 00:53:29.840
and it's really based
on the angle of attack.

00:53:29.840 --> 00:53:31.952
So if you-- yes, go ahead.

00:53:31.952 --> 00:53:34.160
AUDIENCE: So what happens
after the end of the graph?

00:53:34.160 --> 00:53:35.670
Does it just plunge zero?

00:53:35.670 --> 00:53:37.840
Is it not like any solution?

00:53:37.840 --> 00:53:39.237
Like why does it stall?

00:53:39.237 --> 00:53:40.820
TINA SRIVASTAVA:
Yeah, basically, it's

00:53:40.820 --> 00:53:42.510
not generating any lift.

00:53:42.510 --> 00:53:43.010
Right.

00:53:43.010 --> 00:53:46.280
You can see this like
with a paper airplane.

00:53:46.280 --> 00:53:49.760
Sometimes, if you-- it kind
of stops and kind of crashes.

00:53:49.760 --> 00:53:52.790
We'll see how Minachi's
paper airplane does here.

00:53:55.740 --> 00:53:56.740
Well, that one--

00:53:56.740 --> 00:54:00.190
I definitely had a low angle of
attack, so it flew very well.

00:54:00.190 --> 00:54:01.780
Let's see if I can
get it to stall

00:54:01.780 --> 00:54:03.970
or if it's too stable
of an airplane.

00:54:07.270 --> 00:54:09.420
That one-- basically,
after it stalled,

00:54:09.420 --> 00:54:12.180
it basically went nose
down, which is good.

00:54:12.180 --> 00:54:14.530
It has a little extra
paper folding at the front

00:54:14.530 --> 00:54:16.520
so that the nose will go down.

00:54:16.520 --> 00:54:18.700
But it's really bad basically.

00:54:18.700 --> 00:54:22.880
If you stall, it
can go that way.

00:54:22.880 --> 00:54:25.700
The other thing that can
happen after you stall a lot

00:54:25.700 --> 00:54:28.600
usually is you can
enter a spin, which

00:54:28.600 --> 00:54:30.190
is actually the next case.

00:54:30.190 --> 00:54:35.170
So this is when you're
uncoordinated in your stall.

00:54:35.170 --> 00:54:36.880
So what I mean by uncoordinated?

00:54:36.880 --> 00:54:39.500
So that's what I was just
talking about before,

00:54:39.500 --> 00:54:42.280
where your roll and
your yaw are not

00:54:42.280 --> 00:54:44.450
going in the same direction.

00:54:44.450 --> 00:54:46.990
And here you can have
a situation where

00:54:46.990 --> 00:54:48.820
both of the wings have stalled.

00:54:48.820 --> 00:54:51.880
So the airflow has separated
over both of the wings.

00:54:51.880 --> 00:54:54.940
But one may be more
stalled than the other.

00:54:54.940 --> 00:54:59.770
And it causes the airplane to
have a very, very hazardous

00:54:59.770 --> 00:55:02.740
condition or an
intentional condition

00:55:02.740 --> 00:55:04.540
if you're Oxsana over
there, and you're

00:55:04.540 --> 00:55:08.130
trying to spin your airplane
to do a fancy trick.

00:55:08.130 --> 00:55:11.330
This is very dangerous
close to the ground.

00:55:11.330 --> 00:55:13.670
As you'll hear, you
only intentionally

00:55:13.670 --> 00:55:15.230
do this in certain
types of aircraft

00:55:15.230 --> 00:55:17.510
when you're wearing
parachutes in certain airspace

00:55:17.510 --> 00:55:19.520
when you're very high
above the ground.

00:55:19.520 --> 00:55:21.480
You don't want to do this.

00:55:21.480 --> 00:55:23.960
And in fact, if you're just
getting your private pilot's

00:55:23.960 --> 00:55:27.890
license or your PPL,
you're not going

00:55:27.890 --> 00:55:30.830
to practice a spin because
it's pretty dangerous thing

00:55:30.830 --> 00:55:32.420
to do in many aircraft.

00:55:32.420 --> 00:55:34.280
But you do have
to learn about it

00:55:34.280 --> 00:55:36.590
and make sure you
don't get into a spin.

00:55:39.430 --> 00:55:43.165
So let's talk a little bit
about maneuvering flight.

00:55:43.165 --> 00:55:44.540
So basically, that
means when you

00:55:44.540 --> 00:55:46.360
were flying straight
and level, that's

00:55:46.360 --> 00:55:48.580
kind of when you're at
an equilibrium where

00:55:48.580 --> 00:55:50.710
your lift and your weight
kind of cancel out.

00:55:50.710 --> 00:55:53.080
And the plane's just
going straight and level

00:55:53.080 --> 00:55:54.520
at the same altitude.

00:55:54.520 --> 00:55:58.720
But climbing is when your lift
temporarily exceeds the weight

00:55:58.720 --> 00:56:00.670
so you can actually climb.

00:56:00.670 --> 00:56:03.320
So once you are
in a steady climb,

00:56:03.320 --> 00:56:06.350
then you can actually still have
your forces be in equilibrium.

00:56:06.350 --> 00:56:08.680
So remember f equals ma.

00:56:08.680 --> 00:56:12.130
So a is acceleration, which
is a change in velocity.

00:56:12.130 --> 00:56:13.840
So if you're not
changing your velocity,

00:56:13.840 --> 00:56:16.070
and you're just
in a steady climb,

00:56:16.070 --> 00:56:18.341
then you're also
not accelerating.

00:56:21.500 --> 00:56:24.400
Now, this is a little
bit complicated,

00:56:24.400 --> 00:56:26.840
so I will say this is
a little bit tricky.

00:56:26.840 --> 00:56:33.770
There is a tendency for
these airplanes to turn left.

00:56:33.770 --> 00:56:35.620
And there are actually
multiple things

00:56:35.620 --> 00:56:39.460
that contribute to this
left-turning tendency.

00:56:39.460 --> 00:56:41.620
And when you're in
an airplane flying,

00:56:41.620 --> 00:56:45.350
you might hear your
instructor say right rudder.

00:56:45.350 --> 00:56:47.410
And it is really
to counteract some

00:56:47.410 --> 00:56:49.510
of these left-turning
tendencies.

00:56:49.510 --> 00:56:53.110
So we're going to break them
down and talk about them.

00:56:53.110 --> 00:56:55.180
But this can be a
very in-depth subject,

00:56:55.180 --> 00:56:58.360
so I will definitely
refer to the PHAK, which

00:56:58.360 --> 00:57:01.240
is the Pilot Handbook of
Aeronautical Knowledge.

00:57:01.240 --> 00:57:04.640
Chapter 5 goes
into all of these.

00:57:04.640 --> 00:57:07.070
So the first one is torque.

00:57:07.070 --> 00:57:10.160
So basically, the thing is
when you're-- if you're sitting

00:57:10.160 --> 00:57:12.470
in the airplane, and
you're looking forward

00:57:12.470 --> 00:57:17.180
at your propeller, most
US engines actually have

00:57:17.180 --> 00:57:19.910
the propeller
rotating clockwise.

00:57:19.910 --> 00:57:22.910
So and you can see that
arrow that says action.

00:57:22.910 --> 00:57:25.430
That's the propeller
rotating clockwise.

00:57:25.430 --> 00:57:30.060
And so because of Newton,
we know for every action,

00:57:30.060 --> 00:57:31.910
there's an equal and
opposite reaction.

00:57:31.910 --> 00:57:34.190
So because the propeller
is turning to the right,

00:57:34.190 --> 00:57:37.070
the whole airplane is
trying to roll to the left.

00:57:37.070 --> 00:57:41.290
So that is the first
left-turning tendency.

00:57:41.290 --> 00:57:43.660
Before we move to the next
one, are there any questions

00:57:43.660 --> 00:57:45.405
on this left-turning tendency?

00:57:50.170 --> 00:57:51.960
Great.

00:57:51.960 --> 00:57:54.840
So the next one
is p-factor, which

00:57:54.840 --> 00:57:57.280
is an asymmetrical thrust.

00:57:57.280 --> 00:58:00.810
This happens when the airplane
has a high angle of attack,

00:58:00.810 --> 00:58:04.800
so either when it's climbing
or in this condition called

00:58:04.800 --> 00:58:08.340
slow flight, which is where it's
kind of an uncomfortable thing.

00:58:08.340 --> 00:58:10.590
You have to do this in
your flight training.

00:58:10.590 --> 00:58:13.620
So basically, you have your
power setting pretty high,

00:58:13.620 --> 00:58:16.470
but you've kind of
pitched the airplane up.

00:58:16.470 --> 00:58:21.000
And so you're not getting as
much airflow over your control

00:58:21.000 --> 00:58:23.280
surfaces like your
ailerons and your elevator.

00:58:23.280 --> 00:58:25.798
So they call your
controls mushy.

00:58:25.798 --> 00:58:27.840
So it's hard to kind of
coordinate your airplane.

00:58:27.840 --> 00:58:30.210
But you kind of sit
in that environment

00:58:30.210 --> 00:58:32.460
to basically understand how
it's difficult to control

00:58:32.460 --> 00:58:33.940
the airplane in
that environment.

00:58:33.940 --> 00:58:36.960
So if you're pitched up, and
you have a-- so you have a high

00:58:36.960 --> 00:58:39.210
angle of attack, and you're
either climbing or in slow

00:58:39.210 --> 00:58:43.650
flight, you have this
tendency where the--

00:58:43.650 --> 00:58:46.140
because you're
angled to the wind,

00:58:46.140 --> 00:58:49.170
the right propeller blade,
which is descending,

00:58:49.170 --> 00:58:52.450
is kind of cutting into
the air as it's coming in.

00:58:52.450 --> 00:58:54.660
So it's actually
generating more thrust,

00:58:54.660 --> 00:58:58.460
whereas the ascending
left propeller blade,

00:58:58.460 --> 00:59:01.410
so the propeller blade that's
going up on the left side

00:59:01.410 --> 00:59:04.230
is kind of coming away from
the wind that's coming at it.

00:59:04.230 --> 00:59:07.020
And so it's not
generating as much thrust

00:59:07.020 --> 00:59:08.820
as the right propeller blade.

00:59:08.820 --> 00:59:11.400
So that causes the
center of thrust

00:59:11.400 --> 00:59:13.650
to move towards the right.

00:59:13.650 --> 00:59:19.695
And that creates a little bit of
a yaw tendency of the airplane.

00:59:19.695 --> 00:59:20.570
Does that make sense?

00:59:24.240 --> 00:59:24.740
Great.

00:59:24.740 --> 00:59:25.630
Lot of head nods.

00:59:25.630 --> 00:59:28.300
P-factor was one that
both Phillip and I

00:59:28.300 --> 00:59:30.760
spent quite a bit of time
getting our heads around.

00:59:30.760 --> 00:59:32.760
And Professor Hansman
helped us out there.

00:59:35.550 --> 00:59:38.890
So another one is called
the corkscrew effect.

00:59:38.890 --> 00:59:41.550
Sometimes, it's
called slipstream

00:59:41.550 --> 00:59:43.830
or spiraling slipstream.

00:59:43.830 --> 00:59:45.990
It basically has
to do with the fact

00:59:45.990 --> 00:59:50.070
that that propeller remember is
just kind of like a wing that's

00:59:50.070 --> 00:59:51.300
spinning around.

00:59:51.300 --> 00:59:57.630
And so it's basically
pushing the air back.

00:59:57.630 --> 01:00:00.060
And since the propeller
is spinning around,

01:00:00.060 --> 01:00:02.850
that air that's coming
back from the propeller

01:00:02.850 --> 01:00:05.760
is spinning around the airplane.

01:00:05.760 --> 01:00:09.330
And as it spins around, when
it comes up to the back,

01:00:09.330 --> 01:00:12.750
it pushes on the vertical
stabilizer, that tail piece,

01:00:12.750 --> 01:00:18.590
and causes the plane
also to do a left yaw.

01:00:18.590 --> 01:00:21.730
Does that make sense?

01:00:21.730 --> 01:00:24.120
Some good head nods.

01:00:24.120 --> 01:00:24.620
Yes.

01:00:24.620 --> 01:00:28.390
AUDIENCE: Why doesn't it
also cause it to roll?

01:00:28.390 --> 01:00:30.730
TINA SRIVASTAVA: Why doesn't
it also cause it to roll

01:00:30.730 --> 01:00:32.550
was the question.

01:00:32.550 --> 01:00:36.040
And it could, especially
if it's hitting the wing.

01:00:36.040 --> 01:00:37.480
But in general,
what we've seen is

01:00:37.480 --> 01:00:39.400
that it can depend
on whether you're

01:00:39.400 --> 01:00:40.720
in a high wing or low wing.

01:00:40.720 --> 01:00:44.610
But the biggest thing that
it sort of hits is here.

01:00:44.610 --> 01:00:46.570
Now, in general, when
you get to a left yaw,

01:00:46.570 --> 01:00:48.850
you sort of kind of roll.

01:00:48.850 --> 01:00:52.310
These are connected angles.

01:00:52.310 --> 01:00:54.890
But I think just
what we've observed

01:00:54.890 --> 01:00:57.110
is primarily that
the air, when it hits

01:00:57.110 --> 01:01:00.050
the vertical stabilizer, is
the biggest surface that's

01:01:00.050 --> 01:01:02.660
kind of pushing it and
the angle that it's at.

01:01:02.660 --> 01:01:04.520
So if you sum it
all together, yes.

01:01:04.520 --> 01:01:06.990
I'm actually quite confident
you'll get some roll,

01:01:06.990 --> 01:01:09.620
but the biggest thing that
you notice is the yaw.

01:01:13.580 --> 01:01:16.310
So let's see if we
understood p-factor

01:01:16.310 --> 01:01:17.480
as well as we think we did.

01:01:36.450 --> 01:01:38.266
A, B, or C?

01:01:38.266 --> 01:01:38.766
AUDIENCE: A.

01:01:38.766 --> 01:01:40.450
TINA SRIVASTAVA: A. Good job.

01:01:40.450 --> 01:01:43.540
I actually have my little hint
there that the B is actually

01:01:43.540 --> 01:01:47.560
talking about torque, which
is a different left-turning

01:01:47.560 --> 01:01:48.507
tendency.

01:01:48.507 --> 01:01:50.590
And then finally, we're
going to talk a little bit

01:01:50.590 --> 01:01:54.180
about gyroscopic precession.

01:01:54.180 --> 01:01:56.110
It's a little bit
complicated if you're not

01:01:56.110 --> 01:01:57.760
familiar with the gyroscope.

01:01:57.760 --> 01:01:59.980
But when Phillip
talks to you about all

01:01:59.980 --> 01:02:01.690
the different controls
in your airplane,

01:02:01.690 --> 01:02:03.760
you'll have to learn
about gyroscopes all over

01:02:03.760 --> 01:02:04.840
again in a little bit.

01:02:04.840 --> 01:02:08.890
But in general, what do you
need to know about a gyroscope?

01:02:08.890 --> 01:02:10.030
What is a gyroscope?

01:02:10.030 --> 01:02:12.460
A gyroscope is
something you can hold.

01:02:12.460 --> 01:02:13.090
It's spinning.

01:02:13.090 --> 01:02:14.920
You can play with them.

01:02:14.920 --> 01:02:17.800
What they allow you to do
is have rigidity in space.

01:02:17.800 --> 01:02:20.860
And they also have this
concept of precession.

01:02:20.860 --> 01:02:25.240
And precession is basically
that the resultant action

01:02:25.240 --> 01:02:28.870
of a spinning rotor when a
deflecting force is applied

01:02:28.870 --> 01:02:32.990
happens 90 degrees
ahead of that rotation.

01:02:32.990 --> 01:02:38.210
And so because of that,
you can consider that the--

01:02:38.210 --> 01:02:41.510
you have the propeller
spinning, and that causes

01:02:41.510 --> 01:02:43.550
this gyroscopic precession.

01:02:43.550 --> 01:02:46.040
And that basically
causes 90 degrees out

01:02:46.040 --> 01:02:49.330
of that sink is this
force which causes

01:02:49.330 --> 01:02:53.765
a yawing movement, a pitching
and a yawing in this case.

01:02:58.810 --> 01:03:03.110
Once we talk more about
gyroscopes and how they work,

01:03:03.110 --> 01:03:05.260
you'll also learn
different flight controls

01:03:05.260 --> 01:03:09.160
that you look at in the plane,
leverage these gyroscopes.

01:03:09.160 --> 01:03:10.990
And we'll come back
and circle back

01:03:10.990 --> 01:03:14.440
to making sure we understand the
key fundamentals of gyroscopes.

01:03:14.440 --> 01:03:14.940
Yes.

01:03:14.940 --> 01:03:17.107
AUDIENCE: So why is p-factor
a left-turning tendency

01:03:17.107 --> 01:03:18.653
and not a pitch up tendency?

01:03:18.653 --> 01:03:19.570
TINA SRIVASTAVA: Sure.

01:03:19.570 --> 01:03:20.778
So let's go back to p-factor.

01:03:27.480 --> 01:03:30.020
So what we're talking
about is the difference

01:03:30.020 --> 01:03:31.850
in the center of thrust.

01:03:31.850 --> 01:03:35.300
So the thrust, when
you're straight and level,

01:03:35.300 --> 01:03:37.370
the thrust is just forward.

01:03:37.370 --> 01:03:41.900
But what we're seeing is
that when the right blade,

01:03:41.900 --> 01:03:44.310
because when you're in
a high angle of attack,

01:03:44.310 --> 01:03:46.730
the right blade is
generating more thrust

01:03:46.730 --> 01:03:47.630
than the left blade.

01:03:47.630 --> 01:03:51.540
So the center of thrust
is slightly to the right.

01:03:51.540 --> 01:03:54.920
So that is why because it's to
the right and not up or down.

01:03:54.920 --> 01:03:57.050
Up or down would cause
a pitch up or down.

01:03:57.050 --> 01:03:59.000
But since it's to
the right, that's

01:03:59.000 --> 01:04:01.322
why it's causing the yaw action.

01:04:01.322 --> 01:04:04.094
AUDIENCE: So it is not
90 degrees ahead because

01:04:04.094 --> 01:04:05.020
of precession.

01:04:05.020 --> 01:04:07.910
TINA SRIVASTAVA: So
precession is separate.

01:04:07.910 --> 01:04:11.660
It is generating its own
factors and dynamics.

01:04:11.660 --> 01:04:14.300
So both of these things are
acting at the same time.

01:04:14.300 --> 01:04:17.360
So precession does
in fact affect pitch

01:04:17.360 --> 01:04:19.280
just like you
correctly recognized.

01:04:19.280 --> 01:04:22.010
But this is an
additional factor that's

01:04:22.010 --> 01:04:24.020
happening is that since
the center of thrust

01:04:24.020 --> 01:04:28.310
is actually moved to the
right, it's causing the yawing.

01:04:28.310 --> 01:04:29.820
Did that answer your question?

01:04:29.820 --> 01:04:31.200
AUDIENCE: No, but that's OK.

01:04:31.200 --> 01:04:33.210
TINA SRIVASTAVA: You want
to chime in, Phillip?

01:04:33.210 --> 01:04:35.740
AUDIENCE: It's an external
force, as opposed to generating

01:04:35.740 --> 01:04:37.758
by the propeller.

01:04:37.758 --> 01:04:39.550
PHILLIP GREENSPUN: It's
a little bit tough.

01:04:39.550 --> 01:04:43.360
I think, yeah, we should
table it and refer you

01:04:43.360 --> 01:04:46.300
to that physics book See How
It Flies, which has some of it.

01:04:46.300 --> 01:04:48.970
But the one thing I
would add on p-factor

01:04:48.970 --> 01:04:52.540
is another thing
to keep in mind is

01:04:52.540 --> 01:04:56.560
whether the propeller is
advancing or retreating

01:04:56.560 --> 01:04:57.820
into the wind.

01:04:57.820 --> 01:05:00.610
So if you think about it,
when the airplane is level,

01:05:00.610 --> 01:05:08.230
the propeller is not moving
relative to the oncoming wind.

01:05:08.230 --> 01:05:12.640
But if you tilt the airplane
up, as the propeller goes down,

01:05:12.640 --> 01:05:15.010
it's actually
advancing into the wind

01:05:15.010 --> 01:05:18.010
and getting a little bit of
an efficiency boost that way.

01:05:18.010 --> 01:05:19.642
Whereas when it's
coming up, it's

01:05:19.642 --> 01:05:21.850
going from the front of the
airplane towards the back

01:05:21.850 --> 01:05:22.550
of the airplane.

01:05:22.550 --> 01:05:24.365
So it's retreating

01:05:24.365 --> 01:05:26.490
TINA SRIVASTAVA: Yeah, so
what Phillip's describing

01:05:26.490 --> 01:05:29.913
is why the right propeller
blade is generating more thrust

01:05:29.913 --> 01:05:31.830
than the left propeller
blade, which is what's

01:05:31.830 --> 01:05:33.690
moving the center of thrust.

01:05:33.690 --> 01:05:36.570
So I think the real thing
to answer your question

01:05:36.570 --> 01:05:39.060
is that there's more
than one effect happening

01:05:39.060 --> 01:05:40.560
simultaneously.

01:05:40.560 --> 01:05:41.180
Yeah, so the--

01:05:41.180 --> 01:05:42.847
PHILLIP GREENSPUN:
I'm not sure that you

01:05:42.847 --> 01:05:46.110
get gyroscopic precession
from that action

01:05:46.110 --> 01:05:50.760
here because it's generating
lift by pushing air.

01:05:50.760 --> 01:05:52.710
I'm not sure that
all the thrust really

01:05:52.710 --> 01:05:57.290
has to go through,
for p-factor at least,

01:05:57.290 --> 01:06:01.760
through the center of
the spinning propeller.

01:06:01.760 --> 01:06:06.770
Also, I know in helicopters,
the physics 101 answer

01:06:06.770 --> 01:06:07.730
is 90 degrees.

01:06:07.730 --> 01:06:11.975
But the real answer for
engineering it is 72 degrees.

01:06:11.975 --> 01:06:13.100
So it does get complicated.

01:06:13.100 --> 01:06:16.760
Fortunately, it's beyond the
scope of what the FAA tests you

01:06:16.760 --> 01:06:19.100
on because they
themselves, I'm sure,

01:06:19.100 --> 01:06:20.405
don't understand it fully.

01:06:20.405 --> 01:06:21.322
TINA SRIVASTAVA: Yeah.

01:06:21.322 --> 01:06:23.930
How about we come back after
we've talked about gyroscopes

01:06:23.930 --> 01:06:27.380
in excruciating detail and then
we have a set of terminology

01:06:27.380 --> 01:06:31.130
to talk about, let's come
back to discussing that more.

01:06:31.130 --> 01:06:32.035
Thanks.

01:06:32.035 --> 01:06:34.160
AUDIENCE: I have a simple
question for you maybe.

01:06:34.160 --> 01:06:35.410
TINA SRIVASTAVA: Yes.

01:06:35.410 --> 01:06:36.130
AUDIENCE: Just to
help me remember,

01:06:36.130 --> 01:06:37.312
why is it called p-factor?

01:06:37.312 --> 01:06:38.810
What is the p for?

01:06:38.810 --> 01:06:40.850
TINA SRIVASTAVA:
Power or propeller.

01:06:40.850 --> 01:06:46.852
So the p is referring
to that propeller,

01:06:46.852 --> 01:06:48.310
right propeller
more than the left.

01:06:48.310 --> 01:06:52.310
It also usually happens when
you're at a higher power.

01:06:52.310 --> 01:06:54.560
So some flight
instructors like you

01:06:54.560 --> 01:06:58.260
to think about when you have
higher power in the airplane,

01:06:58.260 --> 01:06:59.990
you need to put on
more right rudder

01:06:59.990 --> 01:07:02.904
to counteract that
left-turning tendency.

01:07:02.904 --> 01:07:04.779
PHILLIP GREENSPUN: We're
a little bit behind.

01:07:04.779 --> 01:07:07.140
Should we take our bathroom
break now and then--

01:07:07.140 --> 01:07:08.890
TINA SRIVASTAVA: I'm
actually almost done,

01:07:08.890 --> 01:07:11.480
so I think we can finish there.

01:07:11.480 --> 01:07:15.320
So one thing is to talk-- so
we talked about with climbing

01:07:15.320 --> 01:07:17.840
flight, f equals ma.

01:07:17.840 --> 01:07:20.480
So once you're done
changing the velocity,

01:07:20.480 --> 01:07:22.490
and you don't have a
change in velocity,

01:07:22.490 --> 01:07:23.990
your forces are in equilibrium.

01:07:23.990 --> 01:07:27.710
So the same is the case
with a descending flight.

01:07:27.710 --> 01:07:30.878
When you're actually
turning, your forces

01:07:30.878 --> 01:07:32.420
are not in equilibrium
because you're

01:07:32.420 --> 01:07:35.340
having this change in velocity.

01:07:35.340 --> 01:07:40.880
And so you actually have a
number of changes happening.

01:07:40.880 --> 01:07:44.150
And it's basically considered
accelerated flight,

01:07:44.150 --> 01:07:46.940
which is same as when you're
driving if you're turning.

01:07:46.940 --> 01:07:49.040
So when you're flying,
when you're doing a turn,

01:07:49.040 --> 01:07:51.440
you're accelerating because
you're constantly changing

01:07:51.440 --> 01:07:54.230
the direction of your velocity.

01:07:54.230 --> 01:07:56.750
You also have load
factor, which we'll

01:07:56.750 --> 01:07:59.840
get into in more detail when
we talk about performance

01:07:59.840 --> 01:08:02.960
of an aircraft and how the
load affects your performance.

01:08:02.960 --> 01:08:05.180
But another thing
to think about back

01:08:05.180 --> 01:08:08.330
when we were talking about that
zero gravity flight and a plane

01:08:08.330 --> 01:08:11.218
flying in a parabolic
trajectory or a roller

01:08:11.218 --> 01:08:12.510
coaster when you're at the top.

01:08:12.510 --> 01:08:14.677
But when you're at the
bottom of the roller coaster,

01:08:14.677 --> 01:08:17.660
you really feel like
you're being pressed down

01:08:17.660 --> 01:08:19.520
into your seat.

01:08:19.520 --> 01:08:22.819
In fact, when we run
that zero gravity flight,

01:08:22.819 --> 01:08:25.529
although at the top, we had
30 seconds of weightlessness

01:08:25.529 --> 01:08:27.080
so we could do our
experiments, when

01:08:27.080 --> 01:08:29.040
you go to the bottom
of the parabola,

01:08:29.040 --> 01:08:32.660
you basically get 2G or
twice what you normally feel.

01:08:32.660 --> 01:08:35.630
And so you have to kind of
lay down and let that happen

01:08:35.630 --> 01:08:37.020
before you come up again.

01:08:37.020 --> 01:08:38.960
And so when you think
about load factor,

01:08:38.960 --> 01:08:40.819
just think about you
being at the bottom

01:08:40.819 --> 01:08:43.010
of your roller coaster
and really feeling

01:08:43.010 --> 01:08:46.640
a kind of twice
that force on you.

01:08:46.640 --> 01:08:48.710
And then just to
kind of end, we want

01:08:48.710 --> 01:08:52.100
to talk about most of the time
we're talking about the planes

01:08:52.100 --> 01:08:54.710
that you'd be flying, but
another type of aircraft

01:08:54.710 --> 01:08:58.729
altogether is a blended
wing body aircraft.

01:08:58.729 --> 01:09:01.680
So just like this is
one example of that.

01:09:01.680 --> 01:09:04.130
So what it means is
that that fuselage

01:09:04.130 --> 01:09:06.770
or that kind of tube in
the middle that you sit in

01:09:06.770 --> 01:09:11.210
is blended into the wings
so that the whole body is

01:09:11.210 --> 01:09:14.000
generating more lift
because the whole surface is

01:09:14.000 --> 01:09:15.500
kind of designed that way.

01:09:15.500 --> 01:09:17.060
It's really kind of cool.

01:09:17.060 --> 01:09:19.310
And from an aerodynamic
perspective,

01:09:19.310 --> 01:09:22.430
it's got a much better
lift to drag ratio

01:09:22.430 --> 01:09:24.590
because the whole
thing is really

01:09:24.590 --> 01:09:27.800
deflecting the air molecules
downward and generating

01:09:27.800 --> 01:09:29.180
that lift.

01:09:29.180 --> 01:09:31.460
So I just asked kind
of a thought question.

01:09:31.460 --> 01:09:35.330
If this is so much better, it's
more efficient of an aircraft

01:09:35.330 --> 01:09:39.529
and aerodynamically has much
better properties, why do you--

01:09:39.529 --> 01:09:41.569
it actually-- we've
also found that it's

01:09:41.569 --> 01:09:43.279
better in terms
of fuel efficiency

01:09:43.279 --> 01:09:45.560
because it has less
drag and more lift.

01:09:45.560 --> 01:09:49.819
Why do you think that
JetBlue and American Airlines

01:09:49.819 --> 01:09:53.136
don't fly aircraft
that look like this?

01:09:53.136 --> 01:09:55.800
AUDIENCE: They don't have routes
with a thousand passengers.

01:09:55.800 --> 01:09:56.480
TINA SRIVASTAVA: They
don't have routes

01:09:56.480 --> 01:09:57.605
with a thousand passengers?

01:09:57.605 --> 01:10:02.250
Well, you could make a smaller
blended wing body aircraft.

01:10:02.250 --> 01:10:02.910
Yes.

01:10:02.910 --> 01:10:04.590
AUDIENCE: Passengers
like windows.

01:10:04.590 --> 01:10:06.298
TINA SRIVASTAVA:
Passengers like windows.

01:10:06.298 --> 01:10:07.270
That's actually a big--

01:10:07.270 --> 01:10:09.990
it's a big reason truthfully.

01:10:09.990 --> 01:10:10.770
Yes.

01:10:10.770 --> 01:10:13.145
AUDIENCE: It's very different
from what's currently made,

01:10:13.145 --> 01:10:15.250
so the development would
be expensive and risky.

01:10:15.250 --> 01:10:16.500
TINA SRIVASTAVA: So it's
very different from what's

01:10:16.500 --> 01:10:17.160
currently made.

01:10:17.160 --> 01:10:18.660
And then you said
so the development

01:10:18.660 --> 01:10:19.830
would be very risky.

01:10:19.830 --> 01:10:22.110
Actually, I think it's more
than just the development.

01:10:22.110 --> 01:10:24.690
Because it's different
from what's currently made,

01:10:24.690 --> 01:10:28.860
the entire infrastructure
supports the current format

01:10:28.860 --> 01:10:30.990
of an airplane with
a tube and wings.

01:10:30.990 --> 01:10:34.140
So we're talking about
airports, jet bridges,

01:10:34.140 --> 01:10:38.185
the way that people load
food carts onto a plane,

01:10:38.185 --> 01:10:39.810
the way that passengers
get on and off,

01:10:39.810 --> 01:10:41.430
the fact the
passengers don't have

01:10:41.430 --> 01:10:43.950
as many windows on
this type of aircraft.

01:10:43.950 --> 01:10:45.900
It's unfortunately that
whole infrastructure

01:10:45.900 --> 01:10:49.860
that surrounds it that is a
big contributing factor to why,

01:10:49.860 --> 01:10:52.470
even though there's
a better design, why

01:10:52.470 --> 01:10:53.770
we don't move towards that.

01:10:53.770 --> 01:10:55.950
So this was a big,
big thing for me

01:10:55.950 --> 01:10:59.730
when I was an undergrad
at MIT aero-astro.

01:10:59.730 --> 01:11:02.970
I'm thinking I'm going to design
the next best amazing airplane.

01:11:02.970 --> 01:11:06.240
But even if you do design the
next best amazing airplane,

01:11:06.240 --> 01:11:08.370
it may not be widely
deployed because

01:11:08.370 --> 01:11:11.370
of these other infrastructure
aspects, which really got me

01:11:11.370 --> 01:11:13.090
into systems engineering.

01:11:13.090 --> 01:11:15.450
But enough with that
thought exercise.

01:11:15.450 --> 01:11:18.690
For time, we'll just summarize
what did we learn today.

01:11:18.690 --> 01:11:21.270
So we talked about how does
an airplane generate lift?

01:11:21.270 --> 01:11:23.940
And we talked about different
factors that affect lift.

01:11:23.940 --> 01:11:26.890
We also discussed that lift
is very hard to calculate.

01:11:26.890 --> 01:11:30.600
And so we experimentally
measure a lot of aspects of it.

01:11:30.600 --> 01:11:34.260
And we discussed the different
forces on an airplane--

01:11:34.260 --> 01:11:37.470
stability, and kind of this
left-turning tendencies,

01:11:37.470 --> 01:11:40.860
and some of the different
aircraft configurations.

01:11:40.860 --> 01:11:42.443
So are there any
questions about that?

01:11:42.443 --> 01:11:43.902
PHILLIP GREENSPUN:
Yeah, Tina, what

01:11:43.902 --> 01:11:45.750
do you think about
let's do questions,

01:11:45.750 --> 01:11:47.510
let's take a bathroom
break, and then--

01:11:47.510 --> 01:11:48.880
TINA SRIVASTAVA: Yeah, so
you can think about it.

01:11:48.880 --> 01:11:49.530
PHILLIP GREENSPUN: People
with questions talk.

01:11:49.530 --> 01:11:50.947
I'm going to call
the pizza people

01:11:50.947 --> 01:11:53.150
and give them my credit card.