> >>Was it 50's cars that turned out to be MORE aerodynamic when going in
> >>reverse rather then forward? :)
> > It's not urban legend, it's just difficult to measure more of it. There
> > have been a lot of testing in exactly those types of things.
> For a excellent
> > reference Look at Hoerner's Fluid Dynamic Drag and chapter 7
> pages 12-5 and
> > 12-6 shows measurements for exactly what has been discussed.
>
> Dave, since most (all?) of us don't have that reference handy,
> would you care to summarize and address the postulates? A/C on or off
> vs windows open drag.
>
> R

Meant to post this last night but we got range clearance today on my project
and got to shoot all day. Beats freezing ur butt off in winter to get
trajectory tables.

Oook now for something completely different...

Basically the results showed that on a typical average car body opening
the windows would increase drag by 4%. To compare the side view mirror
if its one of the round types will increase the overall drag by about
3%.

Typical car has a CD of about 0.4, something slick will be down about 0.2
You can figure out how much hp it takes to go down the road at a certain
speed
with the formula HP=(CD*Q*A*(V^3)*6.8)/10^6

CD = drag coe say 0.4 to 0.2 range
Q = density of air in slugs or 0.0024
A = frontal area of the car basically the cross section area. For example a
small SUV might be 4.5 feet wide and 6 high with 1 foot of ground clearance
so the area would be 22.5 The VW Vortex was aprox 13.45.
V = speed to test at in mph. Ie if you want to see what hp it requires at
highway speeds then use 70 or whatever. The amount of power climbs quickly
from 45 on up as the air pressure builds as the cube of the speed.

The result gives a fairly accurate number to within a few percent. For
example
that VW Vortex that got over 200mpg had a 8.5hp diesel engine. And the
highest
speed they could achieve was about 75mph. CD was 0.19. If you run the
numbers
you get the hp of about 7.331. The reason for the difference is the friction
loss from gears in the tranny etc. So if they lost 5-7% the hp number is
quite close to the rated hp.

Now if you know the cd of a specific car you add or subtract the mirror or
mirrors cd number. On the VW they didn't have mirrors at all they used rear
looking cameras. So if you had a mirror on each side you would add about
0.0075
to the CD for a total of 0.1975 (about the same for opening the window).
That
requires 1/3 of a hp more (7.61 hp) or 3.8% just to go the same speed. Drag
reduction is always passive so once you have it, it is there for good ie
free hp. Using more hp to go faster instead of lowering the drag costs money
in fuel
etc.

Dave

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