[Daniel Jones]

Plugging in the numbers and doing the unit conversions, noting that:

1 slug = lbf * ft/sec**2 (needed for doing the conversion factor)

Yields:

Cd = 0.30 for the MkI GT40

That's in a clean configuration with narrow tires. As the original
design evolved a variety of scoops, ducts, spoilers, and vertical
stabilizers were added or enlarged, and tire width increased. It's
quite possible that later GT40's had Cd in th 0.43 range.

> Seems like I read somewhere that the Pantera (in it's original non-flared
> fender/giant wing etc) form has a drag # somewhere around .27

I show a 1972 Pantera Pre-L as having:

Cd = 0.34
A = 18.23 square feet
Cd * A = 6.20

The Pre-L Pamteras had the chrome bumperettes and narrow tires. The later
L models had the rubber safety bumpers and may have been a bit slicker.
GT5-S had the Countach-like wings and flares. Those would have considerably
higher drag as well as larger frontal area.

Understand that testing a model and testing a full size vehicle in a wind
tunnel with a rolling road surface are likely to give to very different
drag numbers.

> Mike, I have a Pantera GT-5 and that wing really does use the wind. My
> hatch opened up at about 50 and the gas shocks shot it stright up. Pulled
> up to a buck twenty on the speedo and it not only pushed the hatch down,
> but at 140 it clicked it back shut.

Most Pantera wings are primarily cosmetic. The way they are attached,
they would bend the decklid if they produced meaningful downforce.
The reason your wing (and the decklid) pushed the hatch down is because
it was at negative angle-of-attack. Even a flat plate will generate lift
(or downforce) if it is inclined relative to the air flow.

> Doc Stenhouse, I would be interested in the aero-comparison of the GT-40
> and the Pantera... while the '40 looks sleeker, the Panty does look less
> "busy" in the wind.

Ford actually had both in the wind tunnel plus a Boss 302 Mustang.
The results were published in an Italian design magazine called
"Style Auto" (Issue #29). The results were in terms of kilograms and
kilometers per hour but I've converted the numbers to pounds and miles
per hour below:

Vehicle Speed Speed Lift Lift Lift Drag HP required
(KPH) (MPH) Front Rear Total (Kg) due to drag
(Kg) (Kg) (Kg)
-----------------------------------------------------------------
260 162 136 51 187 252 238
225 140 104 39 143 193 159
Pantera 190 118 77 28 105 142 100
160 99 52 22 74 99 58
130 81 34 15 49 63 30
-----------------------------------------------------------------
260 162 120 -14 105 231 217
225 140 92 -11 81 177 146
GT40 190 118 68 - 8 60 130 92
160 99 44 - 5 39 91 54
130 81 27 - 3 24 60 28
-----------------------------------------------------------------
260 162 254 -75 179 344 324
Mustang 225 140 194 -57 137 263 217
Boss 302 190 118 143 -42 101 193 137
160 99 99 -29 70 137 81
130 81 60 -19 41 89 42
-----------------------------------------------------------------


Vehicle Speed Speed Lift Lift Lift Drag HP required
(KPH) (MPH) Front Rear Total (lb) due to drag
(lb) (lb) (lb)
-----------------------------------------------------------------
260 162 300 112 412 556 238
225 140 229 86 315 426 159
Pantera 190 118 170 62 232 313 100
160 99 115 49 164 218 58
130 81 75 33 108 139 30
-----------------------------------------------------------------
260 162 265 -31 234 509 217
225 140 203 -24 179 390 146
GT40 190 118 150 -18 132 287 92
160 99 97 -11 86 201 54
130 81 60 - 7 53 132 28
-----------------------------------------------------------------
260 162 560 -165 395 758 324
Mustang 225 140 428 -126 302 580 217
Boss 302 190 118 315 -93 222 426 137
160 99 218 -64 154 302 81
130 81 132 -42 90 196 42
-----------------------------------------------------------------

The article was about the Pantera and the photos show the original
prototype (a so-called pushbutton Pantera) setting visually level
on about 8" of of individual load cells in the wind tunnel. Few
details were given about the GT40 and Boss 302 Mustang. Given the
rear downforce, I would guess it had the pedestal mounted rear wing.

> is there a point where the downforce is "too much" thus reducing top
> speed potential?

Absolutely. Any downforce generates drag, just as any lift does.
In circuit racing, most teams will trade drag for downforce as long
as it results in a quicker lap time.

> Anyway it is just me carrying the Avanti torch since they have always
> injustly been left out of the discussions about american cars and
> speed. Especially in the circles of people my age.

Not to mention the Raymond Loewy Studebaker Starliner. Often seen
at the Bonneville salt flats due to the streamilned shape.

> Based on Ford's claimed top speed of 195 mph for their new Ford GT,
> the drag coefficient works out to be around 0.38.

That assumes ideal gearing and a drag-limited top speed.

> From this data and some rather complex mathematics, I calculate the
> drag coefficient to be 0.29. (referring to the Cobra Daytona coupe)

What assumptions did you make about the rolling resistance and power
to the rear wheels versus flywheel power? I have some of the old German
equations for rolling resistance but those were developed around very
different tires than are used today. I had a more recent NASA paper on
rolling resistance but I can't seem to locate it.

> If that is the real drag coefficient on the original Daytona, it is just
> another testament to the great skill and intuition of Pete Brock. That
> guy deserves a lot more credit than he gets. A true genius.

Brock was (and is) a great designer but he was relying on more than just
intuition. He has publicly stated his design for the Cobra Daytona coupe
was influenced "by some obscure German papers written by Wunnibald Kamm."
Kamm was one of several researchers who in the 1930's were looking for a
way to make a practical low drag shape for an automobile. In the early
1920's, Hungarian engineer Paul Jaray was able to demonstrate (in the
Zeppelin work's wind-tunnel in Friedrichshafen), drag coefficients as low
as 0.2 for a teardrop-shaped automobile. While aerodynamically efficient,
the Jaray teardrops were long and not easily applied to practical shapes.
Based upon experimental research conducted on buses, Baron Reinhard
Koenig-Fachsenfeld applied for a patent on the chopped tail as a practical
alternative. At around the same time Professor Wunnibald Kamm (head of
the Automotive Research Institute at Stuttgart Technical College)
published a textbook that described a similar truncated tail. Fachsenfeld
was persuaded to sell his patent to the state and Kamm was funded to
develop the concept. Another university professor, Everling was onto the
same idea and his design was among those tested by Kamm. Kamm's research
showed that a properly truncated tail had only a little more drag than
a full teardrop tail.

This truncated tail is what Brock applied to the Daytona Cobra. Brock
is reported to have told Shelby that it would take four times the
horsepower to go 200 mph than it would to go 100 mph. The point was it
would be better to reduce drag than increase horsepower. In reality,
it takes 8 times the power to double the speed (drag is proportional to
speed squared but the power required is proportional to velocity cubed).
I don't have the original article by Brock where he mentions this but
I wouldn't be surprised if he got it right and was referring to drag
and not horsepower.

Dan Jones
Boeing Aerodynamics and Flight Controls
1974 DeTomaso Pantera L