Page 9 - Combined_22_OCR

TEST CONDITIONS

The model was mounted to an external six-compartment pyramid type balance
by attachment bolts through each wheel. An air gap of . 120-inch front and .200-inch
rear existed between the wheels and the ground plane. This clearance is the minimum
allowable to avoid fouling between the car and ground plane.

A ground plane was utilized to lessen the boundary layer effects on the car.
Previous tests (Reference 2) have shown the boundary layer thickness on the ground
plane to be approximately . 5 inch in the vicinity of the car.

Body rake angles were obtained by pivoting the car about the front axle. In
all circumstances the car was positioned at NASCAR, minimum clearance specifications
which are: 6-1/2 inches for any part of the body, 5 inches for the oil pan and 4 inches
for the exhaust pipes. An exception to this was body rake angle of zero degrees where
the exhaust pipes were allowed to be below minimum clearance. Model abnormalities
were felt to cause the exhaust pipes to be an unrealistic control factor in this case so
that the rocker panel was used as the critical dimension. As the car was raked to
-1.5° the oil pan became the controlling factor. At -2.5° the "K" member was the
determining parameter. The minimum clearances model scale included the added
dimensions for wheel gap.

The balance system measures forces and moments in the wind axis system
about a balance moment center (BMC) which is .63 inch above the tunnel centerline.
A computer program has been written which transfers these forces and moments to
the car c.g. a.nd body axis system. A complete description of data reduction pro­
cedures is given in Reference 2. The same reference gives complete details about
corrections applied to the data for tunnel effects.

To compensate for dynamic effects of the data system, five readings were
taken at each data point and an average value used for data reduction purposes. With
this consideration it is felt that the balance sensitivity on the forces as measured are
accurate with the following tolerances:

Lift + . 6 lb

Drag + .375 lb

Sideforce + .8 lb

Moments + 10 in lbs

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