[DeTomaso] Cobra / Shelby Club Race at Willow Springs / Rear DeckLid Needed

Daniel C Jones daniel.c.jones2 at gmail.com
Thu May 15 11:18:17 EDT 2008


>> Aerodynamic research by Dr. Andrew Wortmen has evidenced poor aerodynamic
>> performance around the rear decklid exacerbated by the sugar scoop design.
>> He champions that a tremendous improvement in Aero could be gained by a
>> cover that encloses it.  He also supports the idea of a belly pan.

> Can you elaborate? Was this for Panteras or just in general?

It is a general principle that applies to the Pantera.  If the pressure
on the aft end of the car were equal to the pressure on the front end of
the car, the vehicle would have no form drag.  Unfortunately, the abrupt
step at the sugar scoop causes pressure separation which causes the
pressure on the aft face of the car to be very low, resulting in increased
drag.  Fairing in the sugar scoop can forestall pressure separation and
reduce drag.

One of nature's best streamlined shapes is the tear drop shape that a
water droplet assumes as it falls under the pull of gravity.  For subsonic
speeds, a tear drop is a very low drag shape and has a blunt, rounded,
leading edge with a long gently tapered, pointy, tail.  Hungarian engineer
Paul Jaray was the first to promote the full-on teardrop shape for an
automobile.  Jaray had designed a new series of Zeppelins that featured
the tear drop 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 royalities 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 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.

When fairing in and truncating the tail, you want to do it in a manner
that raises the base pressure (the pressure acting on the aft end of
the vehicle) while making the base area (where the pressure acts) as
small as possible.  There's a point of diminishing returns where
increasing the tail length has progressively less effect.  Kamm's
research led him to the conclusion that you should find the point where
the tail is half as wide as the maximum width of the vehicle and cut it
off there.  This Kamm truncated tail is what Pete Brock applied to the
Cobra Daytona from above and from the side, you'll see it tapers in both
dimensions.  Fairing in the Pantera sugar scoop will help in one dimension
only so will not be as effective as a true Kamm tail.

Be aware that aero is concerned with much more than just drag.  A low
drag shape is of little use on an automobile if it is unstable or generates
too much lift (or not enough downforce) or doesn't allow for cooling.
The old Style Auto (Issue 29) wind tunnel results had data for both front
and rear lift:

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
-----------------------------------------------------------------

Notice that front lift is nearly 3 times that of rear lift.  Remember
that lift acts in conjuction with the weight of a car.  Using Pantera
specification information from the August 1971 issue of Car and Driver
(curb weight = 3123 lbs, weight distribution = 40.9% front, 59.1% rear)
you'd have 1277.3 lbs of weight on the front and 1845.7 lbs on the rear.
At 162 MPH, subtract the aero lift and you'd have 977.3 lbs total on the
front and 1733.7 lbs total on the rear.  Couple that with the angle of
attack changes that happen at the front when you crest a hill or encounter
a bump and it's not hard to see the front needs to be addressed first.
Several caveats apply here: we're using curb weight of a stock vehicle
without driver, the wind tunnel used a fixed ground plane and not a
rolling mat, and the numbers for 162 MPH were extrapolated from lower
speed data but I think the trend is still obvious.  It should also be
obvious that ballasting the front can be as big a help as reducing lift.

Dan Jones



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