jet pack

"Morgans" wrote in message
...

"Capt. Geoffrey Thorpe" The Sea Hawk @See My Sig.com wrote


You betcha. Ain't no difference at all. Well, not exactly, there can be
differences due to the abilitly to align the thrust axis with the CG, or
the location of any control surfaces and their relation to the CG, or
the location of the CG... But pendulum's have nothing to do with it.

I'm not buying it.

Ok, then can you explain, given the fact that:

A: there is no one holding up on end of a rope
B: Gravity will accelerate the entire jet pack / pilot assembly through the
CG - unlike a pendulm where someone is holding one end. And
C: The thrust points along the axis of the vehicle (not "up")

where the force (moment) comes from that would tend to turn the vehicle
upright once it has been tipped to one side.


Yes, but if you tie a rope on it, to keep it from falling, it will hang
down from the rope.

Yea, if you hang it from a rope. But what happens when you let go of the
rope. Ain't no rope here. Please explain wihhout the rope.
(Hint - you can't)


Don't take what I have said as a personal attack, but instead as a
different viewpoint of the characteristics of the aircraft(?) being
discussed.

Same here - nothing personal. However, it's a matter of physics - not a
viewpoint. If you try and draw a free body diagram to illustrate where you
think the turning momemt comes from that would re-upright the jet pack
thingie after it is tipped a bit, you will quickly find that there isn't
any.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

"Capt. Geoffrey Thorpe" The Sea Hawk @See My Sig.com wrote:
[ Explaining the pendulum fallacy: ]
Same here - nothing personal. However, it's a matter of physics - not
a viewpoint. If you try and draw a free body diagram to illustrate
where you think the turning momemt comes from that would re-upright
the jet pack thingie after it is tipped a bit, you will quickly find
that there isn't any.

Just FYI, there is a Wikipedia page dealing with this:

http://en.wikipedia.org/wiki/Pendulum_Rocket_Fallacy

Which includes a link to a page that actually shows vector diagrams and gif
graphics:

http://www.geocities.com/jim_bowery/pendrock.html