Larry, Theo,
The Physics of Racing series posted by physicist Brian Beckman has some
basics, plus examples on speed versus horsepower. The URL is:
http://members.home.net/rck/phor/06-Speed.html
Back up one slash for his home page.
Using the basic equations, with A = 15 sq-ft and Cd = 3.0, it takes 567 hp
to overcome wind resistance (only) at 260 mph and 850 hp with Cd = 4.5.
That's all well and good. However, Brian points out that there is a
significant amount of power needed to overcome other factors like rolling
resistance, etc. For his example of the Corvette, the calculation gives 145
hp @ 150, whereas it really takes all of the car's 240 hp to go 150, or 95
hp for rolling resistance, etc. This is much more than I would have been
lead to believe, but is more in line with my own experience. I am not sure
how this "other" resistance varies with speed; e.g., v or v^2. If it is
pure friction and goes as v, and we assume the same resistance as the
Corvette, then at 260 mph the 95 hp becomes 165 hp. Then, with Cd = 4.5, a
total of 1015 hp is needed. I'm guessing that the "other" part of the
resistance is partly pure friction and partly a higher order term, so this
would make it even worse. I don't think it could possibly be less than
first order.
I know all of these is pretty "back-of-the-envelope", but that's all we
physicists are really good for. I'll leave the rest as an exercise for the
engineers.
Bob
At 10:58 AM 10/10/98 -0400, DrMayf@aol.com wrote:
>Thanks, Theo. I was gonna redo all the numbers, but suffice it to say that
>NASCAR vehicles generate about 750 hp, weigh 3500 lbs , have large frontal
>areas, and still go nearly 200 mph. But, for the record, I will repost all
>those cals I made just in case I screwed up..
>
Robert L. Palmer
Dept. of AMES, Univ. of Calif., San Diego
rpalmer@ames.ucsd.edu
rpalmer@cts.com
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