Re: Aero rule of thumb (was: RE: More three wheeler stuff)

[Bryan Andersen <bryan@visi.com>]

Chris Tromley wrote:
> 
> Peter Vanderwal wrote:
> 
> > There was a fellow (can't remember his name) who discovered
> > that if your
> > back end is too close to taper all the way to a point (at no
> > more than 17
> > degrees), you are better off tapering at 17 degrees as far as
> > possible and
> > then sharply cutting it off (flat back end).  This actually
> > works well for
> > vehicles because it gives you a place to put tail lights and
> > license plates,
> > etc.
> 
> The fellow's name was Kamm, I believe.
> 
> I've seen this 17 degree rule of thumb before, and I'm curious where it
> comes from.  I'm not challenging it, I'd just be more comfortable with a
> source before using it to design a body.  To what range of road speeds does
> the rule apply?  And just to clarify, do you mean 17 degrees included
> (between upper and lower, or right and left sides of the body) or 17 degrees
> to the centerline (i.e. between the roof and the ground)?

>From what I know of aircraft design it would only be for general 
automobile speeds.  17 degrees would be a general rule of thumb.

The 17 degree taper would be from any point on the rear taper to 
the point of convergence.

The reason one does not wish to go to more than an X degree taper 
to the rear point is boundary layer separation.  As one goes 
faster, the angle you can taper to a point decreases.  There are 
things you can do to increase the angle.  They can be either 
passive or active.  Most methods either employ adding directional 
energy to the air flow or siphoning some of it off.  Either works, 
and can work very well if implemented properly.  For a car I'd go 
for siphoning off some of the air flow and using it for cooling, 
then exhaust it out the back in a reduced size point.  If you wish 
to find out more about this, look up boundary layer control on 
aircraft wings.  You'll need a good aeronautical engineering 
library to find much.  I learned most of what I know back in 
84-85 time frame from articles I had to read off of microfiche.

The basic principal is that every body moving through a fluid 
ends up with a thin layer of the fluid next to it that is either 
smoothly flowing or turbulent.  Generally if the effective size 
of the object is increasing as it goes by a point the the fluid 
the boundary layer will be flowing smoothly.  As one transitions 
to efectively decreasing in size the pressure exerted by the 
neighboring fluid decreases and may not be able to push the 
boundry layer fluid in fast enough to keep up with the efective 
decrease in size.  When this happens, the boundry layer flow of 
fluid becomes turbulent unless you do something to either 
increase the neighboring fluid's pressure on the boundary layer 
fluid, or remove the turbulent boundary layer fluid.  Where the 
boundary layer is turbulent, there is increased drag.  This is 
why there has been so much boundary layer control research for 
aircraft.  With boundary layer control they have been able to 
get a sail plane that had a 55 to 1 glide slope ratio up to 117 
to 1, or in other words it would glide over twice as far for a 
given drop in altitude.

PS: Any thing that disturbes the smooth flow of air can cause a 
boundary layer separation.  Think edges of windows, trunks, etc.  
It dosen't take much.

-- 
|  Bryan Andersen   |   bryan@visi.com   |   http://www.nerdvest.com   |
| Buzzwords are like annoying little flies that deserve to be swatted. |
|   -Bryan Andersen                                                    |