On Mon, 4 Jan 1999, Mike Bultemeier wrote:
> >
> > At what speed do ground effect type cars start to make real down force?
> > Does anybody have an answer?
And Josh2 replied:
> I'm going out on a limb here, I'm a rocket scientist, not an
> aerodynamics engineer. But I seem to recall that ground effect for an
> aircraft is a function of wing area vs. distance from the ground.
>
> That being true (I hope), then one can probably assume that ground
> effect downforce can be achieved at almost any speed given the properly
> sized wing/ground-effect area. So for ground effect to work at very low
> speed, a very large wing are would be needed.
>
> Then again, you probably couldn't fit through the cones at that point...
>
> I dunno, hopefully someone with considerable more aerodynamics
> experience will speak up and enlighten us all. Oh well...
Keep in mind that aerodynamic lift (downforce is just inverse lift)
increases with the square of air velocity, so small increases in speed
have greater and greater effects. All those venturi tunnels artfully
carved into the bottoms of high-end sports and open-wheel cars are
designed to increase the local air velocity under the car at relatively
low road speeds, producing lower air pressure under the car -- downforce.
Really well-designed systems can get downforce as low as 30-40 mph.
On the cars most of us can afford, the problem is less enhancing downforce
than it is reducing lift. In profile, cars *do* resemble an airfoil --
fairly flat on the bottom and a long curve defining the upper surface.
Not a good enough airfoil to make it fly, but good enough to generate
low-pressure spots on the rear upper surfaces and high-pressure spots
underneath the front wheels, even at highway speeds -- say, 70-80 mph .
This can make high-speed handling a bit uncertain, especially if you're
blasting up the autobahn (or across the Nevada desert....) at 100+.
Before racing car speeds mandated that real wings (mounted upside down)
to generate downforce, sports and racing cars used spoilers to
"spoil" the lift over the rear wheels caused by those low-pressure
zones. There are some drag benefits as well. Ditto for front spoilers
(air dams) under the bumper -- they prevent that high-pressure lift bubble
from forming under the front wheels. Now you find spoilers on Volvo
station wagons!
In fact, if you look at the latest crop of supercars -- the McLaren F1 in
particular, you won't see any wings. Controling local airflow through
venturi tunnels generates downforce and spoils lift at speeds too low for
wings to be effective. The only reason the LeMans versions sprouted wings
was that the rules prohibit venturi tunnels and require a flat bottom.
Not real smart IMO -- losing critical small bits of body work change the
aerodynamic characteristics of the car in a hurry at speed, as happened to
the GT1 Porsche that flipped at Road Atlanta.
But the bottom line? At our cornering speeds, it's tough to generate
downforce with anything but a huge wing (or a sucker car). But we
probably see some lift reduction on some of the longer straights, and
that's just as good.
Jamie
'92 Prelude Si
ex-wind tunnel pilot
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