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Hmm, yes, I did overlook that. You could preserve the Reynolds number of a half-scale model by doubling the airspeed, but it would become problematic for a 1:18 model of a car. It would take air at 1080mph, or Mach 1.4, to mimic the full-sized car at 60mph, and I don't know whether approaching or exceeding the speed of sound invalidates the results. However, if you use water, which is 50 times as viscous, you can run the 1:18 model at 22mph. A 6"x6" water channel flowing at 22mph (or maybe 11mph is good enough) is relatively easy to build.
However, for an Honors project, I think ignoring but acknowledging the lack of dynamic similitude is sufficient. Unless you want to play with your ME department's water trough.
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