Mark(Red Doggie?),
I sent another post after that that corrected the ft-lbs to pounds on the
Newtons,
but you are right, I dropped rho somehow and didn't multiply it on my
spreadsheet.
So air density was 1 and not 1.2 kg/m^3 and Newtons were 20% low.
One (me) can be fooled into believing that in steady state one only needs
energy to keep the car there at a steady speed.
When it finally dawned upon me that it would only be true for an instant, I
realized that power is involved since energy is used over time.
You put it very well "Force applied over a distance is work, and work per
time is
power. Power determines speed, maximum power determines top speed."
'course now that we have had this conversation, probably no one will take
the bait in my last post to reveal what happened when they tried clutching
with a falling RPM curve (to top end on the torque peak).
I think that it was Q that was supposed to have tried that.
Chuck
From: Mark Osterbrink <[EMAIL PROTECTED]>
Reply-To: [email protected]
To: [email protected]
Subject: Re: [F500] Torque at 150 MPH
Date: Wed, 02 Aug 2006 20:06:24 -0400
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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