Date:         Wed, 01 May 1996 10:42:18 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Ed Mellinger <meed@mbari.org>
Subject:      Re: aerodynamics (Seriously...)

Thomas Johnson wrote:
> 
> >Posit two Beetles. Both are identical mechanically, they have identical
> >bodies and they are driving down identical, perfectly flat roads. One
> >Beetle weighs a hundred pounds and the other weighs a thousand pounds.
> >
> >Disregarding the friction in all internal mechanical
> >components--considering inertia only--once they reach a certain steady
> >speed, will they get identical gas mileage?
> 
> Yes.
> 
> -Tom

Considering inertia only -- identical mileage (Newton's second (?) law). 
But were you thinking of *tires* as "internal mechanical components"?   
Their rolling resistance is proportional to load.  This resistance is 
smaller than aero drag at most speeds, and probably smaller than 
drivetrain losses, but it is still a significant part of the overall 
drag... witness the tires on electric vehicles, which are low-drag, 50 
psi specials.  Or witness your own tires right after an extended 60 mph 
run... noticeably warmer than the ambient air. (And if one of 'ems low 
on air, MUCH warmer!)  The 1000-lb bug would experience much more tire 
drag than the 100-lb bug.

Conerning bus aerodynamics...   While I was at NASA-Ames they were 
testing an 18-wheeler in the big wind tunnel, with simple slab-like 
extensions of the sides and roof sticking about six feet aft of the rear 
doors of the trailer.  Something about cleaning up the flow separation 
at the rear of the truck, anyway they were able to cut drag by 20% or 
so, plus the turbulent vortices behind the truck were less of a hazard 
to following motorcyclists and vanagonauts.  I think there were safety 
problems with their acceptance (not that I understand why anyone was 
worried about the *sides*, given the ridiculous range of truck bumper 
heights allowed in this country).