How many of you have done any wind tunnel
testing lately? Not many (or any) I suspect. Other than the NASCAR guys
and maybe a few well-funded Funny Car teams, I’m willing to bet that few
if any of you have had the opportunity to evaluate the aerodynamics of
your race car in a wind tunnel.
I learned about wind tunnels for race cars in the mid ‘60’s, when I was
working for Craig Breedlove. You may recall that Craig held the world Land
Speed Record several times and was the first man to have officially held
the LSR over 400, 500 and 600 MPH on the Salt Flats. We had a ¼ scale
model built of the Spirit Of America and did extensive wind tunnel testing
in Lockheed’s wind tunnel. Interesting results. What we learned was that
almost everything we learned was basically flawed.
First, picture a wind tunnel in your mind. Basically a huge open-ended
chamber with a big fan on one end. Now picture an airplane model suspended
in the center of the chamber with air flowing over, under and around the
airframe. This is exactly what a wind tunnel is designed for and it will
give valid results regarding aerodynamic loads, air velocity, coefficient
of drag, lift, roll, yaw, pitch, etc.
Now, picture this same wind tunnel with a model of a race car instead of
an airplane. Same thing, right? Not hardly. The airplane is in “free air”
over, under and all around the fuselage. A car, on the other hand, needs
to be on a “roadway”. Herein lies the problem. In a wind tunnel, the model
stands still and the air is moving. In real life, the airplane is moving
and the air is standing still. No big deal when testing an airplane, but
what about testing a car?
Here are the dynamics of a speeding car. The car is moving and the air AND
the ground is standing still. In a wind tunnel, the car AND the ground is
standing still and the air is moving. Big difference.
The air does funny things near the earths surface. There is a phenomena
known as “ground effect” that is related to extremely fast ground
transportation. With the Spirit Of America LSR jet car, the way that our
model behaved in the “open air” was quite different from how it responded
within the parameters of the “ground effect” area. This area, roughly
between the earth’s surface (the roadway) and the “open air” is
approximately 48” high, or almost the entire height of our jet car (less
the rear stabilizer wing). Most race cars fall inside of this area too. I
don’t know how the NASCAR engineers deal with this issue but it is an
important force to be reckoned with.
It took the SOA crew a long time to come to terms with the “ground effect”
problems and how to deal with it. One big issue was getting “high
pressure” air into the air ducts, where to mount the ducts, and how big
(or small) to make the openings. Another issue was creating adequate down
force (negative lift), without having so much down force that it might
blow the tires out. The wind tunnel didn’t help us much in either of these
areas. In fact, at 600 MPH, we had supersonic airflow in some areas and
sub sonic airflow in other areas. The car was actually breaking the sound
barrier in several places and not in others, causing a myriad of problems.
Obviously, aerodynamics is a major part of going for a Land Speed Record.
We knew a lot and yet we were just scratching the surface. We had a 15,000
HP weenie roaster to play with and a car that wanted to take off
(aerodynamic lift). Not good.
One funny thing that happened during our LSR attempts in 1965 (the year we
set a new world record with a 2-way average of 600.601 MPH) involved NASA
engineers that were sent from Washington D.C. to see what we were up too.
President Johnson had the idea of creating a network of extreme high speed
trains around the country (remember the Great Society?). The engineers
just hung around, sitting in their rent-a-cars and watched. Eventually we
talked to these guys about some of the issues regarding “ground effect”
and asked their advice. Their response, “We don’t know enough about it
either and that’s why we are here, to see what you guys find out.” Great.
When the Government is trying to learn something from a bunch of beer
drinking hot rodders, you know we’ve got problems.
OK, enough about 600 MPH. Assuming that you will be running in the 150 MPH
to 200 MPH range, the wind tunnel results for a race car are somewhat more
accurate and realistic, so how can you at least take advantage of some
possible gains or benefits regarding the aerodynamics of your race car?
There may still be something important to learn about your specific car,
even without the “aid” of a wind tunnel.
The proper use of aerodynamic principles will usually give you increased
speed without the expense of additional horsepower. Air is free. The trick
is to make it work for you. From an aerodynamic perspective, you would
like the air to get out of your way while at the same time, having the
airflow assist in keeping the car stable.
First order of business is to take a fresh look at your car, from an
aerodynamic point of view. If it’s a true “door slammer” race car, do you
still have your side mirrors in place? Are there a lot of surfaces with
things sticking out into the wind? Things like rear tires protruding from
the fenders, ugly fender flairs, fuel pressure gauges on the hood, even
hood pins instead of flush fasteners can effect airflow adversely. How
about your body panels? Are the panels and windows as flush and as tight
with each other as possible? Are there little “air traps” that don’t need
to be there?
And what about engine air inlets? This is an area that many racers seem to
disregard, or worse, play “follow the leader”. Big air scoops on your hood
may look racy but may be costing you horsepower. Other than a very few Pro
Stock type hood scoops, many of the scoops I see are mounted in the wrong
area to maximize positive airflow. Even Pro Stock scoops costs horsepower.
The best estimate is that it takes 80 to 100 horsepower to offset the
aerodynamic drag of the Pro Stock scoop at 200 MPH. If it wasn’t necessary
to clear the carburetors and tunnel manifolds on a Pro Stock car, I can
guarantee you that they would not be using those big scoops.
Do you know what areas of your car have positive airflow? Do you think
that cutting a hole in your front bumper for your air inlet will bring in
positive airflow? You would be mistaken. Cooler air, yes, but
high-pressure air, negative on that. Surprised?
The actual high pressure areas on the front of your car, the places where
you can get increased or high speed airflow, are typically at the front
part of the hood (not the middle of the hood like the old Pontiac Firebird
or Mach I shaker scoop) and the area where the rear of the hood meets the
windshield (cowl area). The cowl is the area where your fresh air or vent
inlets are located from the factory, and it’s not an accident. The center
of the hood is actually a low-pressure area, unless the scoop opening is
mounted extremely high. The air comes over the leading edge of the hood
and arches up (typically over the top of most low scoops) and then turns
down toward the cowl area.
If you have an air opening or low scoop in or on your hood and you are not
sure if it’s in the best area, give it the “free wind tunnel” test. Just
before you make a pass down the track, put some dabs of a viscous liquid
in several locations in front of the scoop or opening. I’m cheap and I’ve
always used Heinz Ketchup. Works great as long as you don’t mind cleaning
up the mess after the run. It’s thick enough to stand there as a “dollop”
and yet it will flow with the wind as it goes over the hood, and leaves a
nice trail (not recommended if your car is painted red). I’m sure there is
other stuff that would work equally well. And if the flow lines don’t lead
directly into your inlet opening, guess what? The opening is in the wrong
place and you are not getting the benefit of pressurized airflow.
There is nothing that you can do about all the air surrounding your car.
It’s there and it’s going to stay there. The trick is to push your blob
(race car) through it with as little effort as possible and at the same
time, use the air to help you go fast.
Spoilers, particularly rear spoilers, are a totally different subject and
we will leave that discussion for another time.
