[DeTomaso] Pantera Kid at Bonneville

[Mad Dog Antenucci]

Dan,
   
  A few years back Scott Black made an intro for me to Pete Brock. I talked to Brock for about an hour about the Daytona Coupe and La man and the Pantera drag and some of the stability issues Junior Wilson had had at Silver State above 190 MPH ...
   
  Three areas he spoke about that needed work were the under hood pressure under the front end which Junior solved with the Gurney flap on the hood and the sugar scoop on the back and the need for ;a longer tail preferably one that went straight out from the end of the decklid another 18 inches off the back deck. Brock said he wasn't convinced rear wings did anything but add addtional drag.
  fwiw
   
  MD
   
   
   
  
Daniel C Jones <daniel.c.jones2 at gmail.com> wrote:
  > He said he had been at Bonneville the prior week and seen the Pantera run
> there. He had an interesting observation, that there was major dust trailing
> the car. He said that the dust trail tells you how efficiently the rear of
> the body slips through the air, and that a lot of cars that look slippery,
> don't do all that well. He said the really good body designs (and I assume
> this is a function of speed) don't leave a lot of dust in their wake.
>
> Can anyone comment on this?

The wake is likely due to separation that occurs at the sugar scoop step.
Pressure separation (not to be confused with a turbulent boundary layer)
is bad from a drag perspective. Pressure acts perpendicular to a body.
When the air flow detaches from a body, the pressure on that part of the
body reverts to near atmospheric. Higher pressure on the front than the
back increases drag. If flow did not detach over a body that is symmetric
front-to-back (and top-to-bottom) like a cylinder, there would be no pressure
(or profile) drag component. If you plot the (theoretical) pressure
distribution along the surface of the cylinder (remembering that pressure
acts perpendicular to a surface) and decompose it into horizontal (drag)
and vertical (lift) components, you'll find that the pressure on the front
face of the cylinder (from -90 to +90 degrees) and the pressure on the rear
face ( from +90 to +270 degrees) are equal in magnitude but opposite in
direction, exactly cancelling each other out. Therefore, there should be
no drag (or lift). However, if you actually measure the pressure
distribution, you'll find there are considerably lower pressures on the
rear face, resulting in considerable drag.

The same thing happens on a Pantera when the flow detaches over the sugar
scoop. You get a large drag increase. There was a lot of work early on
to design automobile shapes with little or no pressure separation at the
aft end. Hungarian engineer Paul Jaray was the first to promote the full
teardrop shape for an automobile. Jaray had designed a new series of
Zeppelins that featured the teardrop shape and applied his ideas to
automobiles, applying for a patent in 1922. Jaray tested a series of
streamlined automobiles in the Zeppelin work's wind-tunnel in
Friedrichshafen, achieving drag coefficients as low as 0.2. He went
on to design a variety of aerodynamic bodies for Tatra, BMW, Benz,
Adler, Mayback, Audi, and Hanomag and influenced a number of others.
Chrysler was forced to pay royalties for the Airflow to Jaray, as was
Peugeot (for the 402).

Jaray's patent was contested by another aeronautical engineer, Edmund
Rumpler but was ultimately upheld. Rumpler had debuted a mid-engined,
aerodynamic automobile (the Tropfen) at a 1921 show in Berlin. Benz
used Rumpler's ideas in a 1923 race car but Rumpler returned to aviation.
Rumpler was later arrested by the Nazis because he was Jewish but was
protected by Goering who knew of his aircraft designs. Rumpler's
design was wind tunnel tested in the late 1970's at VW and recorded a
Cd of 0.28.

While aerodynamically efficient, the Jaray teardrops were long and not
always easily applied to practical shapes. Based upon experimental
research conducted on buses, 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 Jaray tail had less drag than a shortened tapered
tail. The full length tear drop is still a lower drag shape, of course,
but the Kamm-back is more practical from a packaging standpoint.

Pete Brock would later stumble upon Wunnibald Kamm's research prior
to designing the Cobra Daytona coupe...

Dan Jones
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