NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/electron/21729

Internet Message Format | 1992-12-30 | 2.9 KB

Xref: sparky sci.electronics:21729 sci.energy:6500 rec.autos.tech:17248
Path: sparky!uunet!zaphod.mps.ohio-state.edu!darwin.sura.net!seismo!skadi!stead
From: [e-mail] (Richard Stead)
Newsgroups: sci.electronics,sci.energy,rec.autos.tech
Subject: Re: Flywheel batteries as EV power source
Message-ID: <[e-mail]>
Date: 30 Dec 92 13:08:34 GMT
References: <[e-mail]>
Sender: [e-mail]
Followup-To: sci.energy
Lines: 40
Nntp-Posting-Host: skadi.css.gov

In article <[e-mail]>, [e-mail] writes:
> >Now, let's say you take 5 seconds to make a 90 degree turn.  Torque is
> >the change in angular momentum over time.  The vector direction of the
> >change in torque is tangential to the curve, and the torque is 1.414*L/5
> >or about 40,000 N-m.  How about the bopper?  Let's say he's 200 kg (big),
> >and is able to swing his body back and forth (180 degree) once a second
> >and is 2 meters tall (we'll assume he's standing, and all his weight is in his
> >head).  That's a heck of a bopper.  Ok, L=200*2*2*pi = 2500 J-s, and torque
> >would be 5000 N-m, 1/8th of the slow-rotating big flywheel in a turn
> for the FB.  I have seen 400 pound men oscillating on dance floors at more
> than 1 Hz, and if the precessional torgue was reduced by a factor of eight
> by a combination of flexible mounting and reduced rotor diameter, your treat-
> ment of this issue appears to support my original position.  Thanks for your

Not really.  I deliberately made the bopper unreasonable.  The 400 pound guy
you saw was not a massless stick 2 m long hinged at one end and with 200 kg
at the other, and oscillating a full 180 degrees.  So, let's make him more
reasonable.  he bends at the waist, so that's only 1 m; and he's more of
a cylinder in terms of mass distribution.  He also flops at most 90 degrees in
1 s, not 180.  I'll still grant that all 200 kg are in his upper body.  So this
gives L=200*1*1*pi/3 = 208 J-s, and torque is 416 N-m.  So it isn't a factor
of 8, it's a factor of 100.  Not only that, but a normal car turn takes about
1 second or less, not 5 seconds.  So it's more like a factor of 500.  The factor
of 8 was to show that even with unreasonable comparisons, the amount of
torque is about an order of magnitude different between a human swaying as
hard as they can, and the vehicle making a slow 90 degree turn.  As far as
flexible mounting goes - that only spreads the force over time - it is unlikely
to be able to spread the action over 5 seconds for a turn that took 1 s.
Thus, even with an incredible mount, you'd have to reduce the flywheel size
enough to get a factor of 100 reduction in precessional force.  The problem
is that the flywheel used for this comparison was the smallest design
at the limit of tensile strength of carbon fiber that could store the
energy equivalent of 2 gallons of gas.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
[e-mail]