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# The forces on an airplane

How do airplanes fly? It's not magic. Learn about the forces that help (and hinder) airplane flight. Created by MIT+K12.

Video transcript

So this is an airplane here. OK, so you probably
already knew that. If you've flown in one, or
maybe just seen them fly. But even if you've seen
them or been in one, do you know how they work? Is it magic? [CHANTS GIBBERISH] Are there invisible fairies
that hold the plane aloft? All right, men. We've got a busy morning and
lots of flights to carry. Or is it science? Well, you can guess that the
answer is indeed science. That's ridiculous. What? So to discuss how
an airplane flies, we first have to
talk about the forces on an airplane
which push it around in all sorts of
different directions. Now we're going to
focus on airplanes today because they're awesome,
but most of these forces apply to any other vehicle. The first force acts on all
these vehicles-- really, it acts on everything. It's the weight force,
which points down towards the center of Earth. Weights is equal to the
mass of the airplane-- m right here-- times the
acceleration due to gravity. Here on Earth, g is equal to
9.81 meters per second squared. Now that's only for Earth. The acceleration due
to gravity really depends on the mass of the
planet that your are on. The larger the planet,
the higher the gravity. So 9.81 meters per second
squared here on Earth. The moon, however--
it's smaller than Earth. So the acceleration due
to gravity is only 1/6 that on Earth-- 1.6
meters per second squared. This is why astronauts can
bounce high on the moon, but not on Earth. This isn't nearly as much fun. Obviously, there has
to be another force opposing the weight and
pushing the airplane up. This force is called lift. Lift operates perpendicular to
the airplane's wings, which are right here in this side view. Now, if these are
our only two forces, our aircraft will be
able to go up and down, but it won't go anywhere. So we have to have a force that
pushes the airplane forward, and this is called thrust. All vehicles have
thrust, otherwise they wouldn't go anywhere
like our airplane. Why didn't you buy
a car with thrust? I'm sorry. We can at least
roll down the hill. On an aircraft, this thrust
is produced by engines. There are two main
types of engines. We have propellers, like
this little guy right here. And jet engines,
like our first model. Whatever the type
of engines, they all work by the same principle. Let's draw a little side
view of an engine here. The engines accelerate air
out the back this direction. And by Newton's
third law, there's an equal and opposite
reaction, and that's the thrust force pushing
the aircraft forward. This is really the same thing
that happens when you blow up a balloon and you let it go. The air come out the back and
the balloon moves forward. We have a force that
opposes the thrust. It's called drag. It points opposite the
direction of flight. The major type of
drag is pressure drag, which is the force caused by the
air smacking into the airplane. So we try to minimize
this type of drag by making the airplane as
aerodynamic as possible. That means that it
has smooth lines in the air flows nice and
cleanly over the front here. You can feel the pressure
drag when you stick your hand out the window of a moving car. Uh, honey, honey. Your hand-- your hand, please. When your hand this
horizontal, it's aerodynamic and you really don't
feel a lot of drag. But if you slowly turn
your hand vertical, you really feel the
drag increasing. So these are our four
forces on the airplane, but perhaps you're thinking-- So this really cool
and everything, but how do we increase and
decrease the airplanes lift to move up and down? That's a great question. Let's look at the
equation for the magnitude of lift per unit wing area. We'll call that L. L
equals 1/2 times rho times cl times v squared. That simple. OK, OK, I'll tell you what
each of these things mean. So rho-- it's not a P.
It's the Greek letter, rho. Rho is the density
of the air, which is a measure of
the number of air molecules in a certain volume. Density of the air varies
with altitude and temperature, so you go higher up. There, the air is thinner,
and so the density is lower. If we want to simplify
things, we generally use the standard density, which
is 1.2754 kilograms per meters cubed. v here is the speed
of the aircraft, or how fast it's traveling. And cl is something called
the coefficient of lift. It's a number that gives
us some information about the shape of the
aircraft's wings-- these things right here. The coefficient of lift changes
with the angle of attack. Angle of what? Aircraft can pitch up and down,
and even if they're pitched up, they're still traveling in a
horizontal direction like that. Now the angle formed here by the
horizontal direction of travel and the direction of
the aircraft's nose is called the angle
of attack, and we denote that with the
Greek letter alpha. So we can make a little
plot here of that. We're going to put coefficient
of lift up on the y-axis, and the angle of attack
down on the x-axis. So as the airplane
starts to pitch up-- if I can get a little
hand here-- thank you. As the aircraft starts to pitch
up, the coefficient of lift increases. This is a good thing
because we have more lift. As we continue to
increase, we eventually reach a point where
we keep pitching up but the lift starts decreasing. This is something called stall,
and it's not a good thing. So we generally avoid try
to pitching up this much. There's a similar equation for
the drag per unit wing area, D. D equals 1/2 rho. Not cl-- that wouldn't
make any sense. cd, as you can guess, is the
coefficient of drag times the velocity squared. The coefficient
of drag is-- it's another number that tells us
something about the wings, and it also varies with
the angle of attack. So as the angle of
attack increases-- oh, thank you-- the coefficient
of drag increases as well. Thank you very much. This is because as the
aircraft is pitching up, there is more wing area
perpendicular to the flow. Now, this reminds
me of something that we talked about earlier. Exactly. This is very similar to
whenever you hold your hand out the window of a car. And so, that's pretty
much everything you need know about
how an aircraft flies. So the next time you're on an
airplane or you just see one, you can really know exactly
what it is that's keeping it up in the air. Nope. No, it's not them either. Ah, there you are. Now you got it.