Joe wrote:
This is an interesting web site, it doesn't list the Berlina or GTV
but it list the '71 Spider at .380cd and a '90 Spider at .410cd -I
guess that big rubber spoiler was just there for "looks"!
Or--perhaps to trade off a bit of drag against less uplift at
speed--which is generally what spoilers are intended to do.
With that reduction in lift comes a reduction in induced drag. A car
with an effective spoiler will have a lower drag coefficient than the
same car without the spoiler. If an aerodynamic device creates
downforce at the expense of additional induced drag then it's a wing,
not a spoiler.
Spoilers "spoil" or disrupt the flow over a surface to reduce the amount of
lift made--without regard to their affect on drag. A spoiler's sole purpose
is to create a region of "separated" flow downstream. The separated flow
region has low momentum--low velocity--and that means that the pressure
inside of that region will generally plateau, or remain constant. Without
the spoiler, the flow in that region would maintain higher velocity and
hence lower pressure, generating lift, if it's an upper surface. Spoilers
found on the back of cars, like the spoilers on Spiders, rarely do anything
at all. On the Spider, the spoiler is operating in a huge region of
separated and recirculating flow--with the top up or down. It has no clean
flow going over it. It's strictly ornamental. On a car like a NASCAR, the
bottom of the car is rather clean and allows a significant flow to pass
between it and the ground. Looking at the car from the side, you'll see
that the front end is low, with a small gap between the chin and the road.
The back end is higher, with a significantly larger gap to the road. In
cross-section, it's a crude, upside down airfoil, and generates
ground-effect downforce. The spoiler helps to increase the "camber" or
upward concavity of the whole body. Picture an airplane wing with a plain
flap turned upside down. And so in that application, the spoiler helps to
increase the ground effect downforce that the body makes. But it's also not
operating as a spoiler. It's operating as a flap. True spoilers are found
on jet wings. They're located inboard, toward the fuselage, and are used to
both control the airplane in roll and to reduce the lift that the wing
generates, when needed. Even in that application, the spoiler, when
deployed, increases drag--dramatically. The reduction in lift does reduce
induced drag. But the creation of large regions of separated flow more than
makes up for it. Separation consumes huge amounts of energy. On cars, it's
responsible for about 95 percent of the drag and it's where 95 percent of
the fuel goes. Generally, "chin spoilers" on cars aren't spoilers at all.
Instead, they operate like the Kruger flap on the leading edge of a 727's
wing, providing a solid landing spot for the leading stagnation point. In
the process, they reduce the separated flow over the leading edge of the
hood, and reduce drag. On a race car, they also help to reduce separation
of the flow as it enters the gap between the car and the ground, smoothing
the flow under the car and aiding in making the most ground-effect downforce
possible. Yes, even cars that *aren't* designed as ground-effect cars can
generate ground-effect downforce.
Anyway, the spoiler on the Spider is just for looks, like the little wings
on the Honda CB900F and CBX, and the whale tale on the Porsche 930. Well,
actually on the 930 it served a purpose. It provided a space to mount the
intercooler.
Rich Wagner
Montrose, CO, USA
'82 GTV6
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