Decklid Aerodynamics - was [DeTomaso] Pantera Kid at Bonneville

[Mad Dog Antenucci]

Dr. Dan,
  As usual you have to mix up our inconclusive road tests with engineering facts...but sure as hell appreciate the support. Hear ya on drag reduction. 
   
  Thanks
  MD

Daniel C Jones <daniel.c.jones2 at gmail.com> wrote:
  > I think Dan Jones and Gray Gregory touched on the problem about a week
> ago as well that supports that the high speed aero problem is UNDER the
> Pantera at the back and there ain't much of a problem on top in
> the back....but I could be wrong...I remember being wrong once ;-]

Mad Dog, fairing in the rear of the Pantera to delay the onset of pressure
separation would be done primarily for drag reduction, not necessarily for
stability. It could have a beneficical side effect of reducing rear lift
but that may depend upon other factors. I do agree that front lift is first
problem that needs to be addressed. If you recall the Style Auto Issue 29
wind tunnel results:

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.

> If you want to experiment just put that very strong clear packing tape
> over the opening on the engine side to block the air rerlease and see
> what happens.

You can fair the rear in and still provide for engine compartment venting.

> You can put some wool tufts on various parts of the lid area about three
> inches long. Run the car and have someone take pictures on both modes for
> analysis.

You can also use a hang glider airspeed indicator to measure the boundary
layer thickness across the roof and at the sides.

Dan Jones
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Mad Dawg Antenucci 
Team Pantera Racing 
  The 1st & still the only vintage race team in open road racing 
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