Date:         Mon, 30 Jul 2007 23:44:38 -0700
Reply-To:     Don Hanson <dhanson@GORGE.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Don Hanson <dhanson@GORGE.NET>
Subject:      aerodynamics..NVC
Content-Type: text/plain; charset="Windows-1252"

The Porsches did have poor aerodynamics.  That wing on the back of the later
928s was an attempt to improve it's poor co-efficient of drag, not to help
with downforce, like the modern racecar aero-stuff is mainly aimed at doing.
  As I recall from trying to improve my 928's aeros, once the angle of a
body panel exceeds something like 12 degrees off the direction of the air
flow over that panel,  the air no longer remains attached and becomes
turbulent, causing more drag..by an astounding amount.  So, when they stuck
that rear wing on the later models, way back by the tail, what they did was
to cause the air to stack up on top of that wing, creating a 'false shape'
that caused the air passing over the whole car to think it was not really a
'fast back" but more like a van, with a longer false roof line... It sort of
worked.  But the stylists at Porsche had to have the traditional Porsche
outline, and they wouldn't give up that inset rear hatch glass...That
accounted for almost a complete negation of the wing, because as the air
left the roof, it hit that hatch recess and went turbulent...
  There is a very good printout of the aerodynamics of that car, which
guided me in doing some good things to my racer, such as flush mounting the
windshield (lexan) the rear hatch (lexan again) revising the front end with
flush mounted headlamp covers rather than those 'shark eye' fold back
lights.  A splitter in front added some downforce, and a rather large wing
(a-la 911s of the 90s) set at roof height with angle of attack adjustment,
some 'dive-planes' on the front fenders to give downforce also, since the
12.5" wide tires were in the airflow anyhow..  The engine fresh air inlet
was put at the rear of the front hood, at the base of the windscreen,
because there is a big high pressure area there..
  All of this made a great improvement in downforce, mainly, but the minor
stuff, like the flush mounting, that worked to reduce drag..
  I don't have the figures in hand, but I recall the original "flag"
mirrors, very similar to a vanagon's took some 25hp to bully through the air
at ~125mph..  A NACA duct in my rear quarter window (lexan also) supplied an
amazing amount of fresh air through a hose right into my helmet..for a cool
head..Minimal aerodynamic consequences to the NACA duct, but it does suck in
lots of air..
  Now, supercar makers know more and they use the underside of the car to
create downforce without adding any real dragy stuff like wings and dive
planes..They shape the bottom like a wing, then use the Bernoulli(sp?)
venturi principal to get huge downforce from the air that is forced to
accelerate past a designed in high (low?) spot on the car's belly pan..It is
said that at speed the downforce on an F-50 Ferrari, or a Saleen, or a Lambo
often far exceeds the total weight of the car, so in theory, you could drive
on the roof of a tunnel...If you had a reason to do that...
  Fun stuff and not totally intuitive..Porsches LOOK pretty slick, right?
But with what is known now, that is a huge marketing concession..keeping the
"911 profile" despite the fact that it sucks.
  Don Hanson

"Streamline"  I was told by this old guy I met who was working on a friend's
D-sport racer, testing some ideas he had for aerodynamics, come from a
process he was using that day, testing at Portland International Raceway..
He had this can of hydraulic oil and a rag.  He would go round his new tail
section and put a tiny drop of this oil in places he wanted to see what the
air was doing.  Then he'd send his buddy out for a lap, bring him in, and
see what had happened to the oil on the bodywork...Yellow paintjob, red
hydraulic oil, little red 'streamlines' showing how the air passed over that
body....And you thought racecars were "high tech"...(not at my level.._$)