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gofour
Unregistered
User
(8/26/02 11:06:16 pm |
Aerodynamics
Part 1
Aerodynamics as I define it is simply the science of air movement
and its effects. I must first make it clear that I am not an
aerodynamic expert and my understanding of this science is by no
means comprehensive. I do however understand enough of the basics to
make an attempt at an explanation as it relates to the race car
(I'll do my best and I'm sure the experts may find some technical
flaws). In order to keep this as brief as possible I will try to
minimize the amount of pure science, engineering terms, and various
subtleties. I will also limit my comments to what I consider to be
the two most important elements of aerodynamics in racing: Drag and
Downforce. Each of these elements warrants independent discussion
and therefore I will be breaking this explanation into two parts. I
understand that the hot topic of the day is downforce but I will be
saving that for part two. In part one I will be discussing drag and
I believe that you will understand why as we go along.
Part
1: Drag
Drag was the first aerodynamic concept to be
addressed in stock car racing. As a matter of fact this was the only
concept that racers concerned themselves with until recent years.
Drag was the enemy and racers worked hard to minimize it as much as
possible. It is still an extremely important element but with the
ever expanding use of windtunnel testing, aerodynamic engineers, and
overall emphasis on the sciences in racing - teams have come to
understand that it not a stand alone concept (many other elements
are at work and must be balanced against one another).
For
the techie's out there we will begin by making a few assumptions in
order to keep this reasonable. I will be assuming that we are
discussing a modern full bodied racing stock car (Winston Cup or
similar), there is no substantial wind, the car surface is smooth
and the straighaways are longer than the turns.
Drag is a
complicated concept but for our purposes I will only focus on the
major elements. I find it helps to have a mental image of things as
you read along. First imagine the car as moving through a sea of
air. Since air is invisible to the eye it may help to think of the
car as moving through water (completely submerged like a submarine)
or through smoke. Keep in mind that air has mass (weight) and is
actually rather heavy. Anyway think of the car as always being
completely surrounded by and pressed on by air at all
times.
Some things are obvious. For example everyone
understands that a car is pushing its way through air as it moves.
This air is also pushing back on the car. (This is expressed in
physics as "every force is accompanied by an equal and opposite
force"). Remember the air is pushing on the car everywhere - front,
sides, top, bottom, etc.. When the car is at rest the air is pushing
everywhere on the car equally but when the car is moving in a
straight line ("forcing" its way through the air) the air is pushing
back against the frontal surfaces (front valence and windshield) the
greatest. When this happens the air is said to be in a state of HIGH
PRESSURE.
When you think high pressure think of the air from
a hair dryer blowing on the front.
By comparison when the car
is moving it is "pulling" the air that was pushing on it from behind
when it was sitting still. This reduces the air pressure behind the
car. When this happens the air is said to be in a state of LOW
PRESSURE.
When you think low pressure think of a vacuum
cleaner sucking from behind.
There you have it the two major
elements of drag.
Another important thing to know about drag
is that it increases exponentially with speed. As a matter of fact
everytime you double the speed you square the amount of drag. In
other words as the car moves twice as fast - the drag becomes 4
times larger.
Drag is also affected a great deal by the size
and shape of the object moving through the air. I have copied a few
shapes and their drag
coefficients:
At
first glance it may be difficult to understand why these shapes give
the results they do. When you look at each think about the surface
area in the front of each shape. Remember the arrows refer to the
direction of air flow (the object would be moving in the opposite
direction). The smaller the front surface area the less air to be
"pushed" (less high pressure). Also notice the rear of each object.
The shape that allows the air to "come back together" sooner will
leave less of a wake behind the object (less low pressure).
Therefore the best combination of frontal surface area and rear air
recombination will have the lowest drag.
Imagine the last
time you stirred something in the kitchen. Fudge is a good example
(if anyone would like to make some to try this experiment out at
home be sure to let me know and I'll e-mail my address to you so you
can send me some of the finished product).
The spoon you are
stirring with pushes the fudge in front of it creating a high
pressure area and leaves a wake behind it creating a low pressure
area. The larger the spoon the more drag, the smaller the spoon the
less drag. If you turn the spoon around backwards thus improving the
aerodynamic shape the easier it is to stir (less high pressure area
and less low pressure area). If you stir twice as fast the fudge
will be four times harder to stir. As the fudge gets thicker it gets
harder to stir (much like air that is more humid creates more drag).
Anyway you get the idea - but don't forget to send me the
fudge!
By now I'm sure you are starting to realize that drag
is much more important at the larger faster tracks than the smaller
slower tracks and that little changes (like all these cars that are
an 1/8" too low at post race inspection) can have a huge
effect.
Teams used to focus solely on building cars that
would have the lowest possible drag while still passing inspection.
This became hugely popular in 1960 when Junior Johnson was driving a
Chevy in practice for a race. He couldn't keep up with the Pontiacs
that were practicing. He got close to the back of one of the
Pontiacs and to his amazement he could stay right with the other car
if he stayed close. Teams also began to understand that that two
cars together were faster than one alone because the the lead car
was not affected by the low pressure area behind it and the car
behind was not affected by high pressure area in front. A few times
the rear window of the lead car was even sucked out by the car
behind. Aerodynamics became big. Low drag was king.
It goes
without saying that the less of the engines horsepower that is
consumed overcoming drag the higher the speeds but could there be
more to this aerodynamics puzzle?
Next installment -
Aerodynamics Part 2: Downforce - in a few days (we will look a bit
more at drag then too). Oh and don't forget that
fudge!
Later,
gofour
Edited by: ZNAKOMI
at: 8/27/02 7:16:55 am
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bobbyfan55
Registered User
Posts: 29
(8/27/02 9:05:01 am)
|
Re: Aerodynamics
Part 1
Gofour,
Thanks! I'm printing this out to save with the
others.
I talk "racing" a lot with a guy that used to crew for
Andy Petree when Petree was driving, and it's fun to be able to
impress him now and then with a little tech knowledge on my part.
So, you're a fudge man! Yummmmmm, now that you've got me in
the mood for chocolate, one of the world's greatest foods!
Ruth
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4the4
Registered
User
Posts: 588
(8/27/02 12:20:27
pm)
|
Re: Aerodynamics
Part 1
Gofour great post! You should be on that nascar tech show.
this
is great unlike some other boards we learn stuff here from people
that know racing
Zank Our board could build a car that would
woop up on a skinner board car !
4the4 |
ZNAKOMI
Jackman
Posts: 1666
(8/27/02 4:59:16 pm)
 |
Re: Aerodynamics
Part 1
(4the4, I replied in a new thread)
|
gofour
Rookie
(8/31/02
11:04:53 am) |
Banner
link
ZNAKOMI,
I do not know if you are counting on me to link
these threads to the banner but I don't know how. Is this something
you will be doing for me?
Thanks,
gofour
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Japanfan
Pit Crew
Posts: 2
(8/31/02 7:31:21 pm)
|
I have a
Question
What effect does a cars side by side have? I have heard that a car
will take the air off the side and they have problems.
Also
the getting close to the car in front has a problem of aero push.
Are you going to cover that in the downforce topic.
Again thanks
your topic inters me greatly and are very informative.
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4faninTexas
Pit Crew
Posts: 79
(9/1/02 3:19:53 am) |
Wow!
Great explanation gofour! I've been going back and reading some of
your explanations of the different topics and they are quite good.
Keep it up man.
Japanfan,
Gofour may have to correct me,
but I believe the reason running side by side slows two cars down is
actually two-fold. First, they are not running nose-to-tail
(drafting) and thus they are not splitting the air between the two
of them. Each car is pushing all the air by itself. Therefore, cars
that are lined up in the draft are going to run faster. Secondly,
when two cars are running side by side the air off of the front of
one car packs the front wheel well of the other car full of air. By
forcing all that air into the wheel well, the air does not "come
back together" around the sides of the car the way it was designed
to do in the wind tunnel. The air goes into the wheel well and
creates a great deal of drag. I hope that makes sense. I might not
be as good at explaining all this stuff as Gofour is.
Randy
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Japanfan
Pit Crew
Posts: 3
(9/1/02 8:06:07 am |
Let me reword my
question
I understand the air problem down a back or front stretch but I
have heard of a problem in turns. Like taking air of the spoiler and
removing the air to lean on.
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ZNAKOMI
Jackman
Posts: 1673
(9/1/02 10:02:30 am) |
With ya
gofour
I'll link it up right now, been rebuilding the computer (almost
done), so I'm a couple projects behind.
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4faninTexas
Pit Crew
Posts: 80
(9/3/02 3:27:20 pm) |
Japanfan
In the turns, the cars experience the same aero forces but with
different effects. When you are driving down the interstate and you
pass an 18-wheeler, you really don't feel anything different until
you get to the front of the truck. As you go by the front, the air
off of the truck pushes your car away from it. Once you're back in
"clean air" your car handles like normal. The same thing happens in
racing except all the forces are magnified because the speeds are
much higher. When a car passes another car, the air can push them
apart or pull them together. Let's say Mike is passing a car in a
turn at Talladega for example. He's on the inside and he's trying to
pass him in the corner. The air doens't flow around Mike's car the
same as it does when he's by himself. Remember gofour's diagram of
how the air moves around objects? Well, instead of Mike's car
feeling the normal effects of air moving past the right side of his
car, there is a low pressure area created between the two cars. The
air flows around the two cars instead of between them. This low
pressure area causes Mike's car to want to drift up into the car he
is passing. (Usually, this "sucking" sensation only occurs when the
cars get really close together). The air the would normally flow
around the right side of Mike's car (the air he would be "leaning
on") isn't there and instead his car wants to drift up into the car
he's passing. I hope that makes sense.
As far as taking the air
off of someone's spoiler, that one is allot easier. When a car is
running by itself the air flows over the front, over the top, down
the rear window, and over the spoiler. The spoiler is angled up so
when the air flows over it, it creates downforce on the rear of the
car. BUT, when another car closes rigtht up on the back of another
car, the air doesn't flow the same. Instead of flowing all the way
down the back window and over the spoilerof the front car, the air
flows straight over the top of the car onto the top of the car
behind. Thus, the air never touches the front car's spoiler and
therefore, never causes any downforce. When the downforce is
removed, the front car gets very loose and wants to spin out. This
is why when you see one car close right up on the back of another
one in a turn, the lead car usually drifts up the race track. He's
drifting up because if he tries to continue turning like he has
been, the rear end will spin toward the wall. This is called
"chasing" the car up the track. I hope that is as clear as I
intended it to be.
Randy
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Japanfan
Pit Crew
Posts: 4
(9/3/02 8:27:13 pm) |
Thanks clear as
a bell
OK now I understand the air leaning on. They really are not leaning
on air as much as the air pressure changes when cars are side by
side. I did not think about that aspect of the air flow.
I
knew about the car behind. Many drivers have claimed to be hit form
behind when in reality all that happen was the air was taken off the
spoiler.
I find the dynamics of air flow over cars
intriguing. I found great information comparing CART NASCAR and F1
cars. NASCAR and F1 both do not permit channels under the cars to
help with downforce, where as CART does.
I wander if that
maybe a partial answer to the dreaded resistor plate.
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MMM
Fans