-----Original Message-----
From: John Beckett
And how does one go about calculating the center of pressure?
.............
Well, the original post (which took such a drubbing I've been afraid
to mention it again) went something like this.
If you take a picture of the side of your car and then cut out the
profile, the point at which the cutout balances is the center of pressure.
It appears that this might work for a simple symmetrical shape like a
rocket but, as was pointed out on the list, race cars are rarely such
simple shapes. Also, rockets fly in clear air, a race car has ground
effect to deal with.
Another model rocket trick. Tie a string around the model and
balance the rocket on the string (string will be at the center
of gravity). Now whirl it around your head (or put it into some
sort of air stream). If the nose points forward, then the center
of pressure is behind the center of gravity.
It was also discussed that if one could build a suitable scale
model of ones car - complete with correct proportional weight
distribution (!), hang it from a string and put it in an
air stream (out the window of a car I think was the example),
one might be able to check center of pressure/center of
gravity/stability/lift issues.
I believe it was Mayf who pointed out that this method was fraught
with inaccuracies, unknowns, and scale effects that rendered any
results (Dr. I don't want to put words in your mouth) as I recall
pretty useless. He is correct that a full mathematical analysis
would be extremely complex and time consuming.
My 2 cents. I think that the more symmetrical your body the more
something like this 'might' tell you (streamliner = better, roadster
= very dim relationship). But it will only be an indicator and
will have to be looked at very carefully. If you have a good model
that flips upside down and goes backwards when you put it out the
car window - this might indicate that you want to look at some
things carefully. I'm going to try the 'cut out the profile' thing
and at least put the results in my 'pieces of string to short to tie'
data bank. I think there are some super-computer tools that could
do the full mathematical analysis. But the cost is out of range
for me. All of this kind of stuff is interesting and provides data
points - the trick will be to figure out which ones mean something,
and which ones don't.
I understand there is a place to test the assumptions in Utah.
Thanks,
Jim W.
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