Page 9 - Combined_137_OCR

SPACE DIVISION CHRYSLER
CORPORATION

where Rn = Reynolds number

Mn = Mach number

N Froude number

fluid density - slug/ft^

fluid velocity - ft/sec

1 characteristic length - ft

coefficient of viscosity - slug/ft-sec

velocity of sound - ft/sec

acceleration due to gravity - ft/sec^

If a model test is conducted such that it has the same Reynolds number and Mach number
as its full scale counterpart, forces and moments recorded during the test can be directly
scaled. Flow patterns of the two bodies will be exactly similar. Froude number must also
be simulated if free flight testing is conducted. For automobile testing which is captive,
Froude number is of no significance. Also, since automobile speeds are small as com­
pared to the speed of sound in air, Mach number has little significance. Therefore, the
only significant parameter for automobile model tests is Reynolds number. Reviewing the
mathematical expression for R^, it will be seen that the model R^ will be reduced by the
model scale value, due to the characteristic length. One of two things can be done to bring
the model Rn back to the full scale Rn value; the fluid density can be manipulated, (i.e.,
test in a different fluid or at a different pressure level) or the velocity can be manipulated.
Of course, a combination of the two could also be employed. In any event the model Rn
should duplicate the full scale Rn.

In the past, full scale automobile wind tunnel testing could not be conducted
because there simply were not any facilities large and economic enough to warrant full scale
tests. However, with the advent of S/TOL and V/TOL aircraft, several large, low speed
facilities (Vmax 250 mph) have been constructed in the past year to allow low speed air­
craft testing. These facilities are readily adaptable to automobile testing. Therefore, one
of the above restrictions no longer exists. Reportably, scale model wind tunnel cars cost
approximately $8000.00 each. At this price, it may be economically feasible to take a
production line full scale car and modify it for wind tunnel tests. The reduced test article
costs could well offset increased tunnel charges. Of course, other parameters such as
test complexity, styling variables, and tunnel availability must enter into the final decision.

The study will weigh all these various parameters and make some positive
conclusions on both the economic and feasibility aspects of scale model vs. full scale testing.

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