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From: rob@ireta.cynic.wimsey.bc.ca (Rob Prior)
Newsgroups: rec.skydiving
Subject: Re: bungee jump from plane
Message-ID: <40BXw1w163w@ireta.cynic.wimsey.bc.ca>
Date: 6 Feb 91 01:50:02 GMT
References: <1991Feb12.024253.21786@murdoch.acc.Virginia.EDU>
Organization: Federation of Sentient Planets, Burnaby BC
Lines: 146

ds4a@dalton.acc.Virginia.EDU (Dale Southard) writes:

> First off -- I am NOT saying that bungee jumping from aircraft is a possible
> or good idea, but...

Agreed.
       
> >First off, connecting the bungee cord to any point other than
> >the center of gravity is going to cause the plane to be unstable
> >at best, or tumble out of the sky at worst.
>
> Why Dave?  I seem to remember hanging off the far tail of a skyvan with
> seveal others -- [text deleted]

Not the same thing.  A bungee jump imparts a sudden torque on the CG, while
hanging off the tail of the aircraft requires edging out to it.  The pilot
compensates as the CG shifts when you guys get out there.

> >SECOND, as each person leaves the plane, the center of gravity
> >changes.  Thus the connecting point is going to move.
> 
> Yea so?  I don't move the connecting point for the static line in-between
> students, and I haven't lost any yet.

The static line has a very low force applied to it (unless the pin jams :)
and hence has a very low effect on the flight characteristics of the
jump plane.

> >THIRD, it would be impossible to connect the bungee to the
> >"true" CG, but it would have to be connected to the frame
> >or fuselage at the nearest point. Thus, when a force is
> >applied on the connecting point, it will cause the plane
> >to rotate. Initially in a direction perpendicular to the
> >line segment between the connecting point and the true CG.
> 
> Again, SO WHAT???  Bungee jumpers regularly connect the bungee to their 
> ankles.  The rate of deceleration that a bungee causes is obviously within
> that which the human knee can handle (or there are a lot of spare lower leg
> parts hanging around connected to a lot of used bungees :-)  ).

Actually, Bungee Jumping started as Bridge Jumping, at the end of a _rope_,
not a rubber band... The human knee can take quite a jolt and still hold up.

> So, you
> are saying that the torque caused by this much force on a lever arm 1 meter
> long (we will assume that a sutable hardpoint can be found that close to
> the CG) is going to be more than the pilot can counter.

No, but the sudden effect of a torque applied unexpectedly to the CG could
cause the pilot to lose control.

> Before you respond,
> remember that the force is not really perpindicular to the lever arm--if
> it does displace the plane via rotation, the lever arm is shortened and the
> torque reduced -- assuming that the attachment is located behind and below
> the actual CG, I would probably term the torque negligible.

Only negligible if the aircraft is HUGE and the jumper SMALL.  Also, what
if the jumper catches his pilot chute on the way out?  Suddenly jumper
is supporting the weight of an aircraft over the length of his body.
(providing nothing gives... :(

> >FOURTH, the pendulum effect of a mass hanging at the end
> >of a long elastic cord is going to cause it to bounce around.
> >What effect do you think this will have on the pilot's efforts
> >to keep the plane stable?
> 
> 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.

> Since the
> effective force constant of elastic is increased with its length, a body at t
> end of a long piece of elastic will bounce up and down more slowly than one
> on a short piece.  For the proposed 100 meter elastics, the bounces would be
> very slow, probably on the order of one bounce every ten sec (but I am really
> guessing here)

And how close will the jumper bounce to the plane?

> >How big a plane did you plan to do this from? I've noticed that
> >even when a skydiver shifts his position from sitting on his
> >(or her, excuse me) butt to a kneeling position in the back of
> >a Twin Beech, that the pilot immediately feels it and has
> >to make small adjustments.  Now imagine the torque applied
> >by a 200 pound object at the end of a 300 foot elastic cord.
> 
> 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.

> >Maybe, just MAYBE, such a stunt would be possible from a C-130
> >hercules, where the weight of one jumper would make only
> >a very small difference in the CG.   But anything the size
> >of a DC-3 or smaller would be courting disaster.
> 
> Well, I will be sure to keep that in mind the next time someone suggests
> square-dancing in the Mr. Douglas.

I have to agree with you on this... :)

> Seriously, I don't mean to flame Mr. Apple...

Then why did you?

> ...for anything other that a failure
> to understand physics.

Seems as though he's not too far off... maybe _you_ should give _us_ some
facts/figures?

> There are lots of reasons not to bungee from a plane,
> amoung them is the severe spacial disorientation casued by spinning at the
> end of a bungee (you can't fly your body as well with your feet tied together
> and they can't cut you away from the plane without risking a bungee/chute
> entanglement).  Or how about getting tangled/hung in the bungee when you
> bounce back up into its slack?  Or how about how stupid/trivial it makes
> skydiving look?  But I would be of the opinion that aircraft stability/CG
> concerns would not preclude bungee jumping from something as small as even
> a 182.

The potential for accident is too great to bother with this kind of silliness.
But, how about doing this from a _baloon_ at about 10,000 AGL?  (can baloons
go this high?  I know nothing about them).  This I could see as being possible.
After all, the baloon would only bounce up and down... possibly creating
seasick occupants... barfing overboard right down on top of... 'oh! I thought
you had cut away already!'... :)

> Flame away -- but don't bother unless you have HARD FIGURES, or at least a
> couple of equations to back you up.

I don't think equations are really necessary here... Most people, I think,
have a gut feeling about this form of physics (if they've studied it at the
post-secondary level).  Perhaps you would like to supply your own equations?

+------------
| rob@ireta.cynic.wimsey.bc.ca
| Rob Prior, President, Still Animation Logo Design
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