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A
front hood air duct:
the aerodynamic mechanism for creating downforce
At
the rear, downforce is easily obtained using a wing-type
spoiler. At the front, though, adding too big an
aerodynamic device can negatively affect minimum
ground clearance and/or the approach angle. Increased
aerodynamic resistance resulting in reduced acceleration
is also another example of the many problems associated
with obtaining appropriate downforce in a road-going
car.
The solution we chose was to design the underbody
of the car as flat as possible to encourage smooth
airflow under the car, maintaining airflow speed
to create downforce. This method not only provides
for adequate ground clearance and approach angle
but also does not unduly increase the forward-protruding
surface of the body. However, this led to a new
problem: how to extract the airflow through the
front radiator that had previously been channeled
underneath the car?
Taking advantage of the car's mid-ship layout, an
air duct was added in the front hood to provide
the necessary extraction route. Longitudinal fins
were also added to the outer left and right sides
of the front under-cover tray to prevent the air
passing under the car from entering the front wheel
wells. Similarly, spats have been added to both
sides of the air ducts to channel air passing through
the ducts away from the wheel wells. The opening
ratio under the front bumper has also been reduced
to limit as much as possible the actual amount of
air flowing through. All these innovations result
in a smoother airflow both under the body and through
the front hood, achieving the desired downforce.
No large aerodynamic appendage was required, helping
maintain the original NSX's overall design and ensure
a relatively low aerodynamic drag. Downforce was
thus achieved without sacrificing top speed.
Wind tunnel tests have shown that when the car is
at an angle relative to wind direction, the longitudinal
fins of the front under-cover tray function in the
same way as the chin spoiler, effectively reducing
body lift and improving transient characteristics. |
Wind
tunnel testing
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View
of the underbody
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Cd
(Drag coefficient): 0.32
Cl (Lift coefficient/overall): -0.100
Clf (Lift coefficient/ front): -0.040
Clr (Lift coefficient/rear): -0.060
Compared
to the original NSX-R, this translates into an increase
in vertical force acting on the front tires of 36.2kgf,
and of 25.0kgf on the rear tires (test results measured
at 180 km/h in both cases).
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Lift
reduction measures (effect measured piece-by-piece)
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Extensive
circuit testing to determine the optimum
equilibrium between downforce and front-to-rear
balance
The air duct in the front hood could have been made
simply by cutting an opening and trimming the edges
with plastic. But because we wanted to maintain
the beauty of the original design including the
simplicity of line worthy of a car cut for speed
like the NSX-R, and to reach the weight reduction
target we had set for ourselves, we chose carbon
fiber instead. The rear spoiler is similarly a single
piece of carbon fiber designed to achieve the required
downforce while maintaining a low drag coefficient
in a simple shape embodying functionality and beauty.
Both parts are formed using an autoclave, a method
more often seen in aircraft manufacture. Multiple
layers of pre-pregs made of resin-impregnated carbon
fibers are cured in a high-pressure oven to form
the parts. The front hood is made of carbon Aramid
fibers for added resistance to tearing. In the event
of an accident, it is designed not to shatter into
small pieces. During the laminating process, fibers
are offset by 45 degrees, with each layer above
and below being symmetrically angled to provide
equal strength in all directions.
Nine to ten hours are required to complete the laminating
process of each single part. After lamination, the
whole lay-up is wrapped in a baking film, and a
vacuum is applied to consolidate the laminate prior
to curing for 2-3 hours in the autoclave at a pressure
of two to three atmospheres. Once in the autoclave,
it takes one hour to bring the part to temperature,
while some five hours are required for the cooling
down process. Air released from the resin when liquefying
at high temperature is carefully bled off to form
a strong CFRP (Carbon-Fiber Reinforced Plastic).
The front hood's outer skin is formed separately
from the inner frame before being glued together.
Glue thickness is strictly maintained at less than
0.5mm. The resultant strength is superior to that
of the base materials. The rear spoiler is a hollow,
one-piece molding made using a proprietary process
developed in cooperation with a parts supplier.
Durability, a matter not normally emphasized in
aerodynamic carbon fiber parts manufacture, has
been pursued to the utmost. In all aspects of the
product, durability on par with steel is achieved.
The painting process has also been the object of
painstaking attention, especially regarding the
undercoating, with both parts undergoing a "5 coat/5
bake" process. For the front hood in particular,
paint has been applied so as to let the roughness
of the carbon fiber surface show through ever so
slightly.
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In
order to endow the new NSX-R with outstanding high-speed
performance, we turned our attention to aerodynamics
and their effect on high-speed cornering power,
braking, turn-in, and other aspects affecting vehicle
controllability. This led us to a new technical
approach called "aerodynamically-induced stability".
In addition to increasing high-speed cornering power,
we have also striven to improve vehicle control
quality - the ease with which the driver can control
the car, and thus exploit its full potential. This
enabled the chassis to be tuned for reduced understeer
at low to medium speeds. The resultant improved
handling at both low and high speeds endows the
New NSX-R with outstanding speed on all types of
circuits.
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The
third advantage of downforce is that it helps reduce
body roll as well as body pitch. This in turn helps
reduce sudden variations in vertical forces applied
to the tires at the limit, increasing vehicle stability
in the wake of driver input. Vehicle behavior is
also more linear near the limit of adhesion, contributing
to increased driver control. In other words, creating
downforce to press the vehicle onto the road as
speed increases not only contributes to increased
absolute cornering speed and thus absolute dynamic
performance, but also significantly improves vehicle
control quality as measured by response to driver
inputs and vehicle stability at the limit. These
are the fundamentals behind downforce and aerodynamic
stability as a means to improved high-speed vehicle
handling.
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Downforce
balance front to rear (Straight-line driving at
constant speed)
By
creating a downforce with the same front-to-rear
balance as vehicle weight, changes in steering characteristics
from low to high speeds remain well under control.
At higher speeds this translates into a more linear
response. More precise control of the vehicle helps
the driver delve further into the car?s potential.
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Testing
at Honda's proving grounds in Takasu, Hokkaido |
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