Episode 80: Big Rig Myths

I have a degree in mechanical engineering. Although I have done no calculations to proove it, I would venture that the Big Rig's fuel mileage was improved, but not to the same extent that the car's mileage improved. The reason for this is that vortex shedding was reduced somewhat. A portion of the low pressure region behind the truck was replaced by car volume, effectively making the truck more aerodynamic. This effect is probably small, but measurable.
On another matter, I wanted to take issue with the conclusion reached by Mythbuster's staff. They measured marked improvements in car efficiency up to 10 feet of drafting, but registered an efficiency drop at closer distances. This is probably not due to accelerator pedal foot flutter, as they concluded. Instead, it is due to air flowing under the Big Rig that now piles up on the front of the car, increasing frontal drag. At 10 feet, the streamlines were more optimal and the Rig-car combination was more aerodynamic. Ten feet is a "characteristic length", that would change with the speed, air density, viscosity, etc.
Mythbusters team: If you ever need some engineering calculations done, just write to me. I specialize in stress analysis, heat transfer, and fluid mechanics. Hobbies: light gas guns and rockets.

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