> The GT-40 was not designed in a wind tunnel.
Actually it was but it was a learning experience. 76 individual wind
tunnel tests were carried out in the Maryland University wind tunnel on
a 3/8 scale model to arrive at the original GT40 MKI body shape. Based
upon those tests, a full-size model was built and tested at Ford's own
wind tunnel in Dearborn. Ford used the Lola GT as a baseline, comparing
it to various revisions of the baseline GT40 shape. The forum software
may screw up the formatting but some results are presented below:
Vehicle Yaw Speed Lift Lift Lift Drag
(MPH) Front Rear Total (lb)
(lb) (lb) (lb)
-----------------------------------------------------------------
Lola GT 0 200 528 168 696 503
15 200 768 384 1152
-----------------------------------------------------------------
GT40 with 0 200 540 108 648 519
High Nose 15 200 844 362 1206 614
-----------------------------------------------------------------
GT40 with 0 200 445 199 644 507
Low Nose 15 200 704 422 1126 596
-----------------------------------------------------------------
Front Spoiler 0 200 326 266 592 513
#1
-----------------------------------------------------------------
Front Spoiler - 200 --- --- --- 531
#2
-----------------------------------------------------------------
Front Spoiler 0 200 236 272 508 488
#3 15 200 309 343 652 591
-----------------------------------------------------------------
Front spoiler #1 was 2.67 tall and was added below nose, behind the
air intake. Front spoiler #2 was in the same location but twice as
tall. It reduced lift but was deemed to not have enough ground
clearance. Front spoiler #3 was 3 1/2" tall and faired in. Recessed
headlights were chosen as raised headlights resulted in "marked increases
in lift and drag".
Many of the tests were directed towards trying to find the lowest drag
way to provide air flow for the cooling system, the induction, engine
compartment ventilation, interior ventilation, and brake and shock
absorber cooling air. The designers originally wanted to use twin side
radiators mounted in the engine compartment but tests on the full scale
wind tunnel model indicated 8000 CFM would be required which was deemed
not possible with the side intake duct layout. A conventional front
mounted radiator with intake and outlet beneath the nose was a little
better. The final solution was to take air in at the high pressure
region below the nose, let it flow past an angled radiator and exhaust
out the low pressure region at the top of the nose bodywork. Anti-dive
and squat were designed into the suspension to keep the cars more level
so as to not upset the aerodynamics.
Even with all the wind tunnel work, Ford was learning as they went and
the GT40's airflow management proved insufficient once the cars got out
in the field. The wind tunnel models were not fitted with the internal
ducting so it was only later discovered that 76 horsepower were being
consumed up just trying to ram air through the car at high speed. The
cars were modified in the field to fix one problem like cooling only
to change another, like the aerodynamic balance resulting in yet another
problem. Ken Miles spoke of the problem:
"The aerodynamic problems we've had with the car were essentially ones of
air flow within the car being affected by external details. For example
we were getting very little air flow to the brakes, although they had huge
ducts ostensibly directing vast quantities of air at them. In fact, the
brakes were overheating badly. The engine was getting too hot. The
engine compartment itself was getting too hot. The cooling water was
getting too hot. The engine and gearbox
oil was gettting too hot. All
this in spite of a large number of aperatures which should have supplied
them with more than enough air. We discovered that what was happening was
that due to design changes that had been made over a period of time,
probably without reference to the original specifications practically all
of the ductwork was at a "stall " condition" i.e. no air was moving in the
ducts".
Ford's aeronspace division Aeronutronics was brought in to instrument a
GT40 with pressure and temerature sensors on various parts of the body
(externally and inside the ducts). From this data, the Shelby team was
able to modify the cars properly. Even when the MKII's appeared, Ford
still had some aerodynamic lessons to learn. Phil Hill wrote "The second
year at LeMans we were in deep trouble when we first arrived, thanks
to a diabolical instability that had been supposedly eliminated in
stateside testing. The MKII's were simply terrifying down the Mulsanne
Straight. We ended up tacking on little eyebrow spoilers as well as an
additional little spoiler across the back to solve the problem. I also
remember early Ferraris with so much front end lift that the steering
became progressively lighter as speed climbed until finally the rebound
stops were a factor... we could have called it up-force."
> Don't know what the drag is for a Coupe, but according to the Nov
> 2003 Car and Driver article about the "new" GT-40, the "old" GT-40
> had a c/d of "about .43". Not as aerodynamic as it appears.
A few things to remember. A low Cd number is only part of the battle.
It gets multipled by the frontal area, so its the product of the two
that determines the overall drag. In addition to minimizing drag, a
primary goal is to reduce lift (or better yet generate downforce) and
have a balance of lift front and rear for stability. Plus you need to
make provisions for cooling flow for brakes, engine, driver, etc. Also
"Aerodynamic" does not necessarily mean low drag. Formula 1 race cars
are very aerodynamic but have horrible drag. That's because they gladly
trade downforce and balance for drag. Note that wings generally do not
decrease drag. Quite the contrary, they typically increase drag.
The earliest of the original GT40's had a Cd of around 0.3, very good
for the time. However, once in the field a number of problems were
encountered that required changes to the original shape and hurt the
drag.
An old GT40 sales brochure mentioned that in 1964, Phil Hill drove a GT40
at LeMans that hit 207 MPH on the Mulsanne straight. That speed was
supposedly achieved with the 4.2 liter Indy engine which made 375 horsepower
at 7300 RPM. A first order approximation estimate of a car's top speed can
be computed using the following formula:
/------------
15 / 1100 P
Vmax = ---- \ 3 / -------------
22 \ / Cd S rho
\/
where
P..........Power in rear wheel horsepower
Cd.........Drag coefficient
S..........Frontal Area in square feet
rho........Density of air in slug/cu. ft.
Vmax.......Speed in miles/hour.
Assume the 375 HP figure is at the crank and maybe 325 of that is
available at the rear wheels:
P = 325 hp
Cd = 0.3
S = 15.8 sq. ft.
rho = 0.002378 slug/cu ft. (standard sea level density)
Assuming ideal gearing, the above formula yields an approximate maximum
of 216 MPH which is in the ballpark. That's quite a low HP number to
run that speed and is a product of a relatively low drag coefficent and
a small frontal area. Perhaps the original GT40's weren't so bad
aerodynamically after all. Even as the cars got more draggy, top speeds
went up and lap times went down. The Ferrari and Chaparral competition
had better power-to-weight ratios but it appears Ford had a better
handle on the aerodynamics and handling.
Note that the above formula makes the implicit assumption that all
drag is aerodynamic and therefore varies with the square of velocity.
In reality, things like rolling resistance vary to a power less than 2.
> When it finally made it to a wind tunnel, they found that the complex
> "airflow managment systems" i.e. scoops and ductwork, consumed more
> than 100 horsepower. The drag coefficient of 0.43 would not suprize
> me.
Aerodynamic drag is proportional to the square of velocity and is
defined as:
D = Cd * A * (rho * V**2)/2
where:
D = drag
V = velocity
rho = air density (a function of temperature and altitude)
Cd = drag coefficient
A = area
From the above, for the early GT40 MKI at 200 MPH, we have a total drag
force of 488 lbs. The references from the day quote a frontal area of
15.8 square feet for bothe the MKI and MKII GT40's. A rationality check
on this number can be obtained by multiplying the height (40") by the
track (57"):
A = 40 inches * 57 inches
= 15.833 square feet
so we're in the ballpark.
I vote for "faster than the GT-40"!
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