Date:         Tue, 30 Apr 1996 12:39:00 -0700 (PDT)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         "Maher, Steve (SD-MS)" <SMAHER@gi.com>
Subject:      RE: drag coefficient


>        Drag coefficient is the value you multiply the ACTUAL frontal
>area of the bus by to get the actual drag of the bus.

Uh, not quite. You multiply the drag coefficient by the frontal area
of the bus... and then by the speed, and then by the speed again, and
then by the density of the air (0.002378 pounds/cubic foot standard).
Then divide by 2, and THAT's the drag of the bus at whatever speed it's
going.

Use consistent units, of course: area in square ft, speed in ft/sec,
air density in pounds/sq ft, drag in pounds.

For you formula-oriented types:


     Drag = 0.5 * Density * Speed^2 * Area * DragCoefficient



>Aha you say-
>so a bus might have a lower coefficient of drag than a beetle, but
>still have MORE drag, because it's got a larger frontal area? Exactly.

Right on!


>        A drag coefficient of 1 means the object has the same drag,
>*per square whatever*, as the proverbial brick wall.

Well... sorta. If it's the world's biggest brick wall, extending left,
right, and up forever without any end in sight.

Real brick walls have edges, of course. Air can flow around the edges,
which causes less drag; and it can also burble and eddy behind the wall,
which causes more drag. And no, the two usually don't cancel each other
out, so most brick walls *don't* have a drag coefficient of 1.

If a brick wall (or anything else) could derive all of its drag from
bringing the air to a complete stop in front of it, with no edge effects,
turbulence, etc., then it would have a drag coefficient of exactly 1.


Dontcha hate these anal-retentive college geeks who have to nail down
every last little thing?

                         /\
                        /  \
                        \   \__
            _            \   \o\
           |_0            \   \o\=
~~~~~~~~~~~||~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
______________________________________________________________________

 Steve Maher         smaher@gi.com           '80 V6anagon w/Chevy 2800
                                             '66 Mustang Coupevertible

          Check out the cars at http://www.wp.com/IrishMafia

              *** It takes all kinds to make this world.
                  That's why we aren't all Irish. ***
______________________________________________________________________

P.S. Hey, at least I didn't mention that density units are more properly
called slugs/ft^3.   :^)