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Tips: Aerodynamics
The following tips and information focus on how to optimize aerodynamics.
Depending on class rules, these suggestions may or may not be valid.
Always check your regulations.
General
Aerodynamics Principles
Aerodynamic
Devices
Aerodynamics
Design Tips
General
Aerodynamic Principals
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Drag
A simple definition of aerodynamics
is the study of the flow of air around and through a vehicle, primarily
if it is in motion. To understand this flow, you can visualize a car
moving through the air. As we all know, it takes some energy to move
the car through the air, and this energy is used to overcome a force
called Drag.
Drag, in vehicle aerodynamics,
is comprised primarily of two forces. Frontal pressure is caused
by the air attempting to flow around the front of the car. As millions
of air molecules approach the front grill of the car, they begin to
compress, and in doing so raise the air pressure in front of the car.
At the same time, the air molecules travelling along the sides of the
car are at atmospheric pressure, a lower pressure compared to the molecules
at the front of the car.
Just like an air tank, if
the valve to the lower pressure atmosphere outside the tank is opened,
the air molecules will naturally flow to the lower pressure area, eventually
equalizing the pressure inside and outside the tank. The same rules
apply to cars. The compressed molecules of air naturally seek a way
out of the high pressure zone in front of the car, and they find it
around the sides, top and bottom of the car. See the diagram below.
Rear vacuum (a non-technical
term, but very descriptive) is caused by the "hole" left in
the air as the car passes through it. To visualize this, imagine a bus
driving down a road. The blocky shape of the bus punches a big hole
in the air, with the air rushing around the body, as mentioned above.
At speeds above a crawl, the space directly behind the bus is "empty"
or like a vacuum. This empty area is a result of the air molecules not
being able to fill the hole as quickly as the bus can make it. The air
molecules attempt to fill in to this area, but the bus is always one
step ahead, and as a result, a continuous vacuum sucks in the opposite
direction of the bus. This inability to fill the hole left by the bus
is technically called Flow detachment. See the diagram below.
Flow detachment applies only
to the "rear vacuum" portion of the drag equation, and it
is really about giving the air molecules time to follow the contours
of a car's bodywork, and to fill the hole left by the vehicle, it's
tires, it's suspension and protrusions (ie. mirrors, roll bars). If
you have witnessed the Le Mans race cars, you will have seen how the
tails of these cars tend to extend well back of the rear wheels, and
narrow when viewed from the side or top. This extra bodywork allows
the air molecules to converge back into the vaccum smoothly along the
body into the hole left by the car's cockpit, and front area, instead
of having to suddenly fill a large empty space.
The reason keeping flow attachment
is so important is that the force created by the vacuum far exceeds
that created by frontal pressure, and this can be attributed to the
Turbulence created by the detachment.
Turbulence generally affects
the "rear vacuum" portion of the drag equation, but if we
look at a protrusion from the race car such as a mirror, we see a compounding
effect. For instance, the air flow detaches from the flat side of the
mirror, which of course faces toward the back of the car. The turbulence
created by this detachment can then affect the air flow to parts of
the car which lie behind the mirror. Intake ducts, for instance, function
best when the air entering them flows smoothly. Therefore, the entire
length of the car really needs to be optimized (within reason) to provide
the least amount of turbulence at high speed. See diagram below (Light
green indicates a vacuum-type area behind mirror):
Lift
(or Downforce)
One term very often heard
in race car circles is Downforce. Downforce is the same as the
lift experienced by airplane wings, only it acts to press down, instead
of lifting up. Every object travelling through air creates either a
lifting or downforce situation. Race cars, of course use things like
inverted wings to force the car down onto the track, increasing traction.
The average street car however tends to create lift. This is because
the car body shape itself generates a low pressure area above itself.
How does a car generate this
low pressure area? According to Bernoulli, the man who defined the basic
rules of fluid dynamics, for a given volume of air, the higher the speed
the air molecules are travelling, the lower the pressure becomes. Likewise,
for a given volume of air, the lower the speed of the air molecules,
the higher the pressure becomes. This of course only applies to air
in motion across a still body, or to a vehicle in motion, moving through
still air.
When we discussed Frontal
Pressure, above, we said that the air pressure was high as the air
rammed into the front grill of the car. What is really happening is
that the air slows down as it approaches the front of the car, and as
a result more molecules are packed into a smaller space. Once the air
Stagnates at the point in front of the car, it seeks a lower
pressure area, such as the sides, top and bottom of the car.
Now, as the air flows over
the hood of the car, it's loses pressure, but when it reaches the windscreen,
it again comes up against a barrier, and briefly reaches a higher pressure.
The lower pressure area above the hood of the car creates a small lifting
force that acts upon the area of the hood (Sort of like trying to suck
the hood off the car). The higher pressure area in front of the windscreen
creates a small (or not so small) downforce. This is akin to pressing
down on the windshield.
Where most road cars get
into trouble is the fact that there is a large surface area on top of
the car's roof. As the higher pressure air in front of the wind screen
travels over the windscreen, it accellerates, causing the pressure to
drop. This lower pressure literally lifts on the car's roof as the air
passes over it. Worse still, once the air makes it's way to the rear
window, the notch created by the window dropping down to the trunk leaves
a vacuum, or low pressure space that the air is not able to fill properly.
The flow is said to detach and the resulting lower pressure creates
lift that then acts upon the surface area of the trunk. This can be
seen in old 1950's racing sedans, where the driver would feel the car
becoming "light" in the rear when travelling at high speeds.
See the diagram below.
Not to be forgotten, the
underside of the car is also responsible for creating lift or downforce.
If a car's front end is lower than the rear end, then the widening gap
between the underside and the road creates a vacuum, or low pressure
area, and therefore "suction" that equates to downforce. The
lower front of the car effectively restricts the air flow under the
car. See the diagram below.
So, as you can see, the airflow
over a car is filled with high and low pressure areas, the sum of which
indicate that the car body either naturally creates lift or downforce.
Drag Coefficient
The shape of a car, as the
aerodynamic theory above suggests, is largely responsible for how much
drag the car has. Ideally, the car body should:
- Have a small grill, to
minimize frontal pressure.
- Have minimal ground clearance
below the grill, to minimize air flow under the car.
- Have a steeply raked windshield
to avoid pressure build up in front.
- Have a "Fastback"
style rear window and deck, to permit the air flow to stay attached.
- Have a converging "Tail"
to keep the air flow attached.
- Have a slightly raked underside,
to create low pressure under the car, in concert with the fact that
the minimal ground clearance mentioned above allows even less air flow
under the car.
If it sounds like we've just
described a sports car, you're right. In truth though, to be ideal,
a car body would be shaped like a tear drop, as even the best sports
cars experience some flow detachment. However, tear drop shapes are
not condusive to the area where a car operates, and that is close to
the ground. Airplanes don't have this limitation, and therefore teardrop
shapes work.
What all these "ideal"
attributes stack up to is called the Drag coefficient (Cd). The
best road cars today manage a Cd of about 0.28. Formula 1 cars, with
their wings and open wheels (a massive drag component) manage a minimum
of about 0.75.
If we consider that a flat
plate has a Cd of about 1.0, an F1 car really seems inefficient, but
what an F1 car lacks in aerodynamic drag efficiency, it makes up for
in downforce and horsepower.
Frontal
Area
Drag coefficient, by itself
is only useful in determining how "Slippery" a vehicle is.
To understand the full picture, we need to take into account the frontal
area of the vehicle. One of those new aerodynamic semi-trailer trucks
may have a relatively low Cd, but when looked at directly from the front
of the truck, you realize just how big the Frontal Area really
is.
It is by combining the Cd
with the Frontal area that we arrive at the actual drag induced by the
vehicle.
Scoops
Scoops, or positive pressure
intakes, are useful when high volume air flow is desireable and almost
every type of race car makes use of these devices. They work on the
principle that the air flow compresses inside an "air box",
when subjected to a constant flow of air. The air box has an opening
that permits an adequate volume of air to enter, and the expanding air
box itself slows the air flow to increase the pressure inside the box.
See the diagram below:
NACA
Ducts
NACA ducts are useful when
air needs to be drawn into an area which isn't exposed to the direct
air flow the scoop has access to. Quite often you will see NACA ducts
along the sides of a car. The NACA duct takes advantage of the Boundary
layer, a layer of slow moving air that "clings" to the
bodywork of the car, especially where the bodywork flattens, or does
not accellerate or decellerate the air flow. Areas like the roof and
side body panels are good examples. The longer the roof or body panels,
the thicker the layer becomes (a source of drag that grows as the layer
thickens too).
Anyway, the NACA duct scavenges
this slower moving area by means of a specially shaped intake. The intake
shape, shown below, drops in toward the inside of the bodywork, and
this draws the slow moving air into the opening at the end of the NACA
duct. Vorticies are also generated by the "walls" of the duct
shape, aiding in the scavenging. The shape and depth change of the duct
are critical for proper operation.
Typical uses for NACA ducts
include engine air intakes and cooling.
Spoilers
Spoilers are used primarily
on sedan-type race cars. They act like barriers to air flow, in order
to build up higher air pressure in front of the spoiler. This is useful,
because as mentioned previously, a sedan car tends to become "Light"
in the rear end as the low pressure area above the trunk lifts the rear
end of the car. See the diagram below:
Front air dams are also a
form of spoiler, only their purpose is to restrict the air flow from
going under the car.
Wings
Probably the most popular
form of aerodynamic aid is the wing. Wings perform very efficiently,
generating lots of downforce for a small penalty in drag. Spoiler are
not nearly as efficient, but because of their practicality and simplicity,
spoilers are used a lot on sedans.
The wing works by differentiating
pressure on the top and bottom surface of the wing. As mentioned previously,
the higher the speed of a given volume of air, the lower the pressure
of that air, and vice-versa. What a wing does is make the air passing
under it travel a larger distance than the air passing over it (in race
car applications). Because air molecules approaching the leading edge
of the wing are forced to separate, some going over the top of the wing,
and some going under the bottom, they are forced to travel differing
distances in order to "Meet up" again at the trailing edge
of the wing. This is part of Bernoulli's theory.
What happens is that the
lower pressure area under the wing allows the higher pressure area above
the wing to "push" down on the wing, and hence the car it's
mounted to. See the diagram below:
Wings, by their design require
that there be no obstruction between the bottom of the wing and the
road surface, for them to be most effective. So mounting a wing above
a trunk lid limits the effectiveness.
- Cover Open wheels.
Open wheels create
a great deal of drag and air flow turbulence, similar to the diagram
of the mirror above. Full covering bodywork is probably the best solution,
if legal by regulations, but if partial bodywork is permitted, placing
a converging fairing behind the wheel provides maximum benefit.
- Minimize Frontal Area.
It's no coincidence that Formula 1 cars are very narrow. It is
usually much easier to reduce FA (frontal area) than the Cd (Drag
coefficient), and top speed and accelleration will be that much better.
- Converge Bodywork Slowly.
Bodywork which quickly converges or is simply truncated, forces the
air flow into turbulence, and generates a great deal of drag. As mentioned
above, it also can affect aerodynamic devices and bodywork further
behind on the car body.
- Use Spoilers. Spoilers
are widely used on sedan type cars such as NASCAR stock cars. These
aerodynamic aids produce downforce by creating a "dam" at
the rear lip of the trunk. This dam works in a similar fashion to
the windshield, only it creates higher pressure in the area above
the trunk.
- Use Wings. Wings
are the inverted version of what you find on aircraft. They work very
efficiently, and in less aggressive forms generate more downforce
than drag, so they are loved in many racing circles. Wings are not
generally seen in concert with spoilers, as they both occupy similar
locations, and defeat each other's purpose.
- Use Front Air Dams.
Air dams at the front of the car restrict the flow of air reaching
the underside of the car. This creates a lower pressure area under
the car, effectively providing downforce.
- Use Aerodynamics to
Assist Car Operation. Using car bodywork to direct airflow into
sidepods, for instance, permits more efficient (ie. smaller FA) sidepods.
Quite often, with some for-thought, you can gain an advantage over
a competitor by these small dual purpose techniques.
Another useful technique
is to use the natural high and low pressure areas created by the
bodywork to perform functions. For instance, Mercedes, back in the
1950s placed radiator outlets in the low pressure zone behind the
driver. The air inlet pressure which fed the radiator became less
critical, as the low pressure outlet area literally sucked air through
the radiator.
A useful high pressure
area is in front of the car, and to make full use of this area,
the nose of the car is often slanted downward. This allows the higher
air pressure to push down on the nose of the car, increasing grip.
It also has the advantage of permitting greater driver visibility.
- Keep Protrusions Away
From The Bodywork. The smooth airflow achieved by proper bodywork
design can be messed up quite easily if a protrusion such as a mirror
is too close to it. Many people will design very aerodynamic mounts
for the mirror, but will fail to place the mirror itself far enough
from the bodywork.
- Rake
the chassis. The chassis, as mentioned in the aerodynamics theory
section above, is capable of being slightly lower to the ground in
the front than in the rear. The lower "Nose" of the car
reduces the volume of air able to pass under the car, and the higher
"Tail" of the car creates a vacuum effect which lowers the
air pressure.
- Cover
Exposed Wishbones. Exposed
wishbones (on open wheel cars) are usually made from circular steel
tube, to save cost. However, these circular tubes generate turbulence.
It would be much better to use oval tubing, or a tube fairing that
creates an oval shape over top of the round tubing. See diagram below:
Check Back Later For More
Aerodynamics Tips.
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