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From: ds4a@dalton.acc.Virginia.EDU (Dale Southard)
Newsgroups: rec.skydiving
Subject: Re: bungee jump from plane
Message-ID: <1991Feb13.172511.6896@murdoch.acc.Virginia.EDU>
Date: 13 Feb 91 17:25:11 GMT
References: <1991Feb12.024253.21786@murdoch.acc.Virginia.EDU> <40BXw1w163w@ireta.cynic.wimsey.bc.ca>
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Ireallyam: ds4a

Rob -- good post (I like to see lively discussions without the use of
 the word Idiot, etc)

Now, to really beat this thing to death...

OK hanging off the tail of the skyvan is NOT the same thing, but Mr. Appel's
original post maintained the the CG would be moved too far foreard/aft.  My
point is that this is not going to happen.  In days of old (when men were
bold??) the "student in tow" malfunction was not unknown.  People really
did hang below aricraft on the end of a static line.  My point (though I
didn't word it very well) was that the force being generated is not all
that great.  We are not talking about a rope, but an elastic cord.  The longer
the cord, the slower the mass at the end will be decelerated.  The pilot would
not recieve a sharp jerk, but a gradual tug.  I would maintain that this 
should be controlable -- The lever arm is/would be far shorter than the lever
arm on which the control surfaces of the plane are located.

Rob:
>> Well Dave, last I checked, the period of a harmonic oscilator becomes greater
>> when the force constant is decreased (aka the less "stiff" the elastic
>> band is, the slower the mass at the end will bounce up and down).

>It doesn't matter.  The force applied to the lever arm is the same no matter
>_how_ 'stiff' the rubber band is.  When it is stretched fully, you have the
>same force.

That isn't what I said -- I was talking about the PERIOD of the oscilations.
The jumper would bounce up and down slowly, not rapidly.  Not to mention that
the force constant of the rubber band would be of great importance -- If 
the rubber band is too stiff, the force will be transmitted to the plane much
more quickly, and the pilot would have to correct for it in a much shorter
(possibly too quick) time frame.  The important criteria in this example is
the rate at which the jumper's momentum is "absorbed" by the bungee/aircraft
that rate is determined by the force constant of the elastic.

---------

>> I can imagine ... not much.  (note that the length of cord has no effect
>> on the torque as it is not rigid)

>Where did you learn physics?  Pilot loses one jumper as he leaves, conpensates
>accordingly, and suddenly has to compensate the other way as the cord tightens
>when the lost jumper reaches the end of his line and the torque increases.

Well, sorry to point it out, but the length of the cord has no effect.
Velocity  does.
Effective force constant does.
Distance from CG does.
Length doesn't.


To sum up:

We are all making educated guesses here, let's not call each other idiots.


Some questions to ponder:

1) Can the jumper really bounce back up and hit the plane? (I think this
violates some enery conversion law somewhere)

2) Are static lines safe? (It may indeed be that all pilots are just one
student in tow away from tumbling out of the sky)

3) Why the hell am I defending an idea that I really don't care all that much
to try? (Couldn't we discuss the best way to do a triple dragplane diamond
instead?)

But at any rate, thanks for the deversions -- I really hate to do productive
work  :-)


-->  -->  Dale  UVa  (ds4a@virginia.edu)