[VACList] Re: VACList Digest V3 #247 - Aerodynamic drag

[waymark1@xxxxxxxx]

Aerodynamic drag is generated mostly by the partial vacuum drawn behind 
a road vehicle. The shape of the front end influences turbulence. Big 
ole buses that have nearly flat front ends with smoothly rounded corners 
and edges, along with smoothly rounded edges at the rear, make for 
relatively low drag. Sharp pointed front ends don't make any significant 
difference - look at the nose of an airplane as compared to its tail. It's 
not practical to have long tapering rear ends, like airplanes have, on 
road vehicles. A German by the name of Kamm discovered that the long 
tapered rear could be truncated pretty short and add only a small amount 
of drag while making for a much more practical road vehicle.
The Airstream and similar trailers (and motorhomes as well) that have 
generously rounded edges have low drag as compared to the sharp-edged 
crop of travel trailers.
Another serious generator of drag is the gap between the tow vehicle and 
the trailer. The gap makes a lot of turbulence, which interferes with the 
air closing in behind the trailer. 5th wheelers are much worse due to the 
open pickup bed and the gap between the trailer front extension and the 
PU box.
A smooth belly pan helps a lot. Air dams don't work well on trailers due 
to the great variance in road clearance at the front of a trailer. An 
effective one would scrape on the road, and one that was high enough to 
clear the road when going through whoop-de-dos at fuel station driveways 
would have minimal effect.
Most of our trailers have a lot of external protuberances that don't help: 
roof air conditioners, awnings, TV antennas, steps, etc., though the middle 
of the roof is somewhat of a dead spot.
Keep your speed down and aerodynamics won't be as influential on total 
drag, plus we are safer in these big rigs. You won't see me going 75 mph.
Reference work: Automobile Aerodynamics. Large format paperback available 
from some auto parts and accessories stores.
Al Grayson