Date:         Fri, 4 Nov 1994 07:01:29 -0800 (PST)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         ray landers <rayl@netcom.com>
Subject:      Re: mpg enhancements


On Thu, 3 Nov 1994, Thom Fitzpatrick wrote:

> >
> >the shapes all have a blunt front and a long tapered tail.
> >
> >this prevents the low pressure area in the rear from forming.
> 
> Then why are the tails the big thing on 911s?  I had a turbo tail on mine
> for a while, and I noticed the biggest difference about 90MPH.  I now
> have a ducktail, and while it is an improvement over stock, its not quite
> the same.  I put the ducktail on for aesthetic reasons - the turbo taile
> looked funky since I only have 7" flares...
> 

i'll try to keep this short again.

the angle of the stock slope on the back of the 911 looks like it is
optimized for a speed around 100kph (i would guess that this is somewhere
in the 55 to 60 mph range).

i think there's some spiritual atraction to the number 100, specialy on 
speedometers. and since a 100 in the U.S. is bigger that a 100 anywhere 
else, it means that you will be moveing faster.

at higher speeds the length of taper on the trailing cone shaped rear of
the body being pushed through the fluid (air in this case) needs to be
extended out to a greater distance to allow the air (fluid) time to 
return to the space it has been forced out of for the body to pass 
through the fluid.

if you increase the speed beyond the point where it can rush back into 
the vacency left buy the moveing body, a low pressure area will develop 
and that low pressure area will tend to retard the passing of the body 
through it.

sort of "sucking" on the back of the vehicle and slowing it down.

the tail.

the theory of what the tail helps was developed by somebody named kham, 
and that's why for a while there were vehicles with what were known as a
"khamback".

the idea being that it would form a cone shaped turbulant area directly 
behind the vehicle that would act like the body itself extended on back 
to allow the air to smoothly return to the hole punched in it by the 
passing vehicle. this would reduce, not eliminate but reduce, the "suck" 
tending to hold back the car at high speeds.

i'm not sure, but i think that kham was/is from pakistan or india. i'm 
not sure maybe somebody else remembers beter than i do.

ray

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