You were supposed to do it with LESS than 72 hp :)
In short, at any speed you are traveling over a distance in a time.
While you travel that distance, you are applying a force to overcome
drag. Force applied over a distance is work, and work per time is
power. Power determines speed, maximum power determines top speed.
You have the drag equation right, but have a few errors from there.
1/2*CD*A*rho*V^2 = 0.5*0.6*0.5*1.2*67.056^2 = 809 N of drag force.
Newtons don't convert to foot-pounds, but would convert to pounds force
(675N is about 151 lb).
809 Newtons at 67 m/s = 54273 Watts, or 72.78 hp
Can't go that fast with that car with less than 72 hp.
Your example (72 hp @ 2500 RPM, 1.0 gearing and 10.1" tire radius) gets
you there within rounding errors, but that's because you started with
the 72 hp limit I gave you.
If we take your 151 ft*lbf assumption, and apply it to 50 hp instead of
72 you can see the problem. 151 ft*lb at 1739 RPM is 50 hp. 151 ft*lb
times the 1.0 gear ratio divided by the 10.1" radius gives us 809 N of
force, so we have enough force. But 1739 RPM times 1.0 gear ratio times
10.1" times 2pi only give us 104 MPH. If we change the gearing to give
us 150 MPH, we only have 556 N - not enough grunt. Can't get there with
only 50 HP.
Like I said, power is what's important, torque was invented to confuse
and misdirect :)
Chuck Voboril wrote:
Red Doggie,
Don't really need a steam engine afterall.
Assuming zero acceleration at steady state speed (and no mistakes in the
SI vs English conversions)
Using classic equation for drag force in Newtons using your Cd, frontal
area, density of the medium, and speed in m/s.
N=1/2*Cd*A*rho*v^2
I came up with 675.4 Newtons SI which converts to 151 ft-lbs. English
Then, a motor which produces 72 HP at 2500 RPM will have the required
151 ft-lbs.
10.1" radius tires, 1:1 gearing at 2500 rpm
speed(MPH)=RPM*tire radius/(168*gearing)
You do have a very valid point that HP cannot be ignored.
It is totally tied to HP and RPM.
However, since RPM was free, I used it to my advantage.
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