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Say you like to draw.
Specifically, say you
like to draw sports cars.
What about the
spoilers in the back?
Do they impact the
car's performance?
Or are they there just for show?
OK, say we have two cars now.
One has a spoiler,
and one does not.
If they were racing under the
same conditions, same engines,
and same route, the one with
the spoiler would actually win.
What if we add a third
car with an airfoil?
Which one do you think
will win the race now?
Well, in order to understand
the differences that
makes one car faster
than the other,
let's take a closer look at
the main forces, lift and drag,
that are acting on the car.
Lift and drag are forces
created as air flows
around different shapes.
Let's look at a cross section
of an airfoil, or wing,
to clearly show these effects.
Air flows around
the wings smoothly,
except at the tail end, where
there is some swirling air.
The smooth, steady air
is called laminar flow,
while the swirling air
is called turbulent flow.
Laminar flow consists of
smooth, parallel lines,
while turbulent flow
has chaotic swirls.
Pressure is defined as
force divided by area.
However, since we
are most concerned
about the forces
of lift and drag,
it's easier to
write the equation
as force equals
pressure times area.
Pressure differences between
two surfaces of an object
creates a force going
from high to low pressure
because the pressures
want to equalize.
Let's identify the areas
of high and low pressure.
The air travels
faster above the wing,
creating an area
of low pressure.
Conversely, slower air
on the bottom of the wing
creates a high pressure area.
The air flow above the airfoil
separates toward the back
and creates turbulence,
impacting lift and drag.
Now that the high and low
pressure areas are labeled,
it is easy to identify the
forces of lift and drag.
The high pressure area below
the wing and low pressure
area above it creates
a force called lift.
The high pressure area
at the front of the wing
and a low pressure
area at the back
creates a force called drag.
Drag pushes the wing
backwards, while lift
pushes the wing upward.
If we take this same
airfoil and tilt it upwards,
it creates greater
pressure differences
and changes the area of the
wing that the pressure acts on.
If you remember the
equation we talked
about earlier, both the area and
the pressure affect the force.
This is why it is important
to determine the best
angle for the
airfoil to generate
the optimal amount of lift
and drag for your application.
Airfoils on cars use
the same principle,
but are flipped upside down.
And drag force is still
pushing the wing back,
but the high and low pressure
areas have been switched.
Thus, a down force
is created instead.
Minimizing lift on a
vehicle is important,
because it causes the
tires to lose traction.
Less traction causes
bad handling and slower
accelerations.
Airfoils use a down force to
push the back of the car down.
Let's take a closer look at
these effects on sports cars.
Say air is flowing
from right to left.
The basic airflow around the
car would look like this.
Air approaches the
front of the car,
travelling in laminar flow.
As it goes around
the car's body,
the air separates at the back of
the car, and parts of the flow
become turbulent and swirl.
The flow in the back is
an area of low pressure,
while the shaded regions on
the front and bottom of the car
represent areas
of high pressure.
The pressure difference
between the front and back
creates drag.
And the pressure difference
between the top and bottom
creates lift.
Now let's look at the
car with the spoiler.
The airflow around
it is similar,
but with key changes in
pressure differences.
When the air hits the
spoiler, pressure builds up,
causing a decrease in lift.
The flow is then deflected
upward, which increases drag.
However, the
advantage of less lift
is greater than the
disadvantage of more drag,
so the spoiler will improve
the car's performance.
Instead of using
spoilers, we can
use an airfoil that decreases
lift without increasing drag.
Air flows around the car.
But this time a localized
pressure difference
at the airfoil
creates a down force
that doesn't block the
airflow like the spoiler.
Lift is decreased without
significantly increasing drag.
In order to test
these principles,
we built a small wind tunnel.
Wind tunnels are usually large
and have a controlled current
of air or smoke
moving through them.
Here's the set up.
At one end is the nozzle,
where the smoke is introduced.
Next to lies the test section,
where the object of study
is placed.
The fan in the back pulls
the smoke out the other side.
When performing the
experiments we used lights
to illuminate the test section.
It is important to redirect
the light in the wind tunnel
to create a good contrast
between the smoke
and background.
In this empty wind tunnel
you can easily see the flow.
From the nozzle to
the test section,
you can see the
smoke accelerating
and flowing smoothly
through to the other side.
Now let's get down
to the fun stuff.
We chose a 124th model
Porsche Carrera to test,
because it's a high performance
sports vehicle that I
would like to own someday.
Take a look at the streamlines.
The smoke flows
smoothly over the front
of this gorgeous machine, but
eventually becomes turbulent
at the back of the car.
Slow motion really captures the
path of the turbulent airflow.
Let's see that again.
Nice.
Next we try looking
at the effect
of a spoiler on
the Carrera and how
it blocks the flow over
the rear of the car
to create a high pressure zone.
Air flows over
the top of the car
and eventually hits the spoiler.
After hitting the
spoiler, the air
is deflected in two directions.
Some of it swirls
backwards, while the rest
is redirected upwards.
The redirected air leads
to a lower pressure
zone behind the car,
which can increase drag.
However, the deflected air
that creates the high pressure
zone over the trunk
reduces the effect of lift
with a down force.
In other words, your car will
be able to accelerate faster
and handle better.
Next we have an airfoil
that demonstrates
the effects of
pressure difference
and their relation
to lift and drag.
The smoke sticks to
the airfoil until it
separates near the tail.
The area under the
flow separation
is lower pressure because there
are only small amounts of air
there, which causes
the air above it
to expand and fill the
space, creating turbulence.
The slower airstream,
moving under the airfoil,
creates a higher pressure zone.
We can see a cooler visual
when the airfoil is tilted,
and the effects of the air
flow become more pronounced.
The flow over the
top of the airfoil
expands into a bigger area,
creating more turbulence
and low pressure zones, while
the bottom of the airfoil
blocks a greater
area of the air flow,
leading to high pressures.
Drag and lift
change dramatically
compared to the airfoil,
with the low angle of attack.
You can experiment with
the effects of lift
and drag by sticking your
hand out a car window
and adjusting the angle.
Now let's get back
to the sports cars.
Here we have a
Lamborghini Diablo
for your viewing pleasure.
It features an airfoil
on the back that
acts like an upside down wing.
This creates down force without
significantly impacting drag,
because it allows the
air to flow around it
instead of blocking the
flow like a spoiler.
In slow motion you can easily
see the flow separating
when it hits the airfoil, and
that the turbulence occurs
further away from
the rear of the car,
compared to the Carrera.
Back to the drawing board.
Let's review what we've learned
about tricking out sports cars.
We found that an
airfoil provides
the best solution to decrease
lift and drag on your car.
It looks cool, and will give
you better handling, faster
acceleration, and
higher top speeds.
Now we can clearly see that
if these three cars were
to race along the same track
under the same conditions,
the car with the airfoil
would win, because
of superior aerodynamics.
The one with the spoiler
would place second.
Fun fact-- at 130
kilometers an hour,
F1 cars can generate
more down force
than the weight of the car,
allowing them to theoretically
drive upside down in a tunnel.
Do not try that at home.
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