Date: Sat, 17 May 2008 23:58:47 -0500
Reply-To: John Rodgers <inua@CHARTER.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: John Rodgers <inua@CHARTER.NET>
Subject: Re: Belly Pans for speed or MPG
In-Reply-To: <5ebe10a0805160602m7f02e8bcx997077d9f8ad3fe0@mail.gmail.com>
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Hmmmmm. This is a toughy. Everyone wants more speed, better fuel mileage
for their vans. It ain't going to happen - at least to any degree making
the cost and effort worth while.
As every pilot is trained to know - slow it down, extend the range -
increase the speed and get there faster, burn more fuel. Simple fact of
life in a pilots world. It's the physics, and you cannot get around it.
The faster you go, the more aerodynamic resistance encountered (drag) so
what is the only thing that can be done? Increase the horsepower to
overcome the drag. Heeey! - Look at me - I'm going Faaaster! Heeeey!
Look at my gas tank needle - it's going Faaaaster! - towards the empty
mark!!!! So if you increase the horsepower, you are increasing the
amount of energy required and that fuel consumption goes way up. But
now the barn door flys, but the fuel bill is horrendous.
You see - you can make a barn door fly - If you have enough horsepower
to overcome the drag. But increased horsepower requires increased fuel
consumption to generate that extra horsepower. No matter what we do,
we cannot get around that stoichiometric relationship of fuel to air of
12/1 - 12 pounds of air to one pound of fuel to release the energy of
the fuel. (or is is 15/1 - can't remember - brain cells getting old)
Enter the Vanagon Barn Door. It flys along nicely at 55 mph on a little
horsepower - designed to deal with the aerodynamics of a box shape. To
increase the speed of the box it is simple - increase the
horsepower........ OOPs! more horses means more oats, so open the wallet
a little wider please - buy more of the $4/ gallon oats to .feed all
those extra horses who are required drag that un-aerodynamic barn door
faster.
There is no way to win this battle. In Airplanes you deal with
lift-to-drag ratios. Lift is generated by speed which increases the drag
which requires more horsepower which requires more oats. The only place
yo can win is in the realm of aerodynamic efficiency. It is with that
issue that aircraft engineers spend thousands of man hours and millions
of dollars on, to get small improvements. Every little bit counts,
especially when an airplane is going to be around for a lot of years.
Small gains mean a lot of money when you are talking millions. But for
an out of date, no longer manufactured barn door like the Vanagon, the
time, effort and expense aren't going to be worth it IMHO. .
Dr. Porsche had it right. The Van - given it's tasks, it's shape and
aerodynamic inefficiencies, is perfectly mated to an engine that will
push it along at 55 mph all day long nonstop, mile after mile after
mile, relatively maintenance free, at optimum fuel efficiency. Get above
that, and fuel efficiency drops dramatically.
So what to do?
Drive for the way the machine was designed. Slow it down to 55 mph and
get that maximum fuel efficiency. It was not designed to drive on super
highways and keep up with the high performance automobiles
When I had my engine in the old '88 Pomgranite rebuilt, it was done
locally by a guy who I think must have worked on the very first VW
aircooled engine in the US, and every model since - you know, one of
those grand old guru's you love to find still working. in an old shop
somewhere. Not much they don't know about VW engines. - particularly the
flat 4's. He cautioned me - don't try to keep up with the traffic on the
interstates and the freeways and expressway's. It's not built for it.
But drive it right - it will last 200,000 miles plus with no trouble. I
have tried to follow his advice. I've 100,000 on that engine now, and it
is going strong with virtually NO direct engine related problems.
Accessories, yes - alternators, hydraulic pump, water pump - some
problems, but not with the engine itself.
My $0.02,
John Rodgers
88 GL Driver
Chris S wrote:
> Has anyone ever fitted full belly pans under their Vanagon for increased
> aerodynamic efficiency? There's a lot under there that snags airflow and
> causes unnecessary drag. I'm contemplating installing galvanized sheets
> under the van and some in front. With 82hp every bit helps while going 75
> mph.
>
> Here are pics of the underside to help visualize:
>
> http://i16.photobucket.com/albums/b7/misterpolak/Hershey/25.jpg
> http://i16.photobucket.com/albums/b7/misterpolak/Hershey/26.jpg
>
>
> Any engineering thoughts? How would you do it?
>
> Chris S.
>
>
>
