Jim Logajan wrote:

Rob Bulaga wrote:
I know I'm opening myself up to all sorts of flaming, but I designed,
built and flew Trek's Solotrek and Springtail aircraft. I think I can
shed some light on your discussion about the "jetpack's" stability.

All hovering aircraft are statically unstable.

Technical nit (not a flame I hope): all lighter-than-air aircraft, many of
which are in the subset of hovering aircraft, are statically stable. At
least so far as I understand these things.

The stability of a high-rotor vs. a low-rotor is a dynamic effect,
analogous to dihedral on a high-wing vs. low-wing aircraft. It does
nothing to promote static (hovering) stability. Hovering these
machines is like trying to stand on a large beachball in the middle of
a swimming pool. Essentially, you're balancing on a column of air.
There is no pendulum effect. When the machine tilts, the force
vectors (columns of air) tilt too. Their relative position to the
c.g. is unchanged. There is no "righting" force.

Now supposing the engine fails - at that point, which in general is easier
to make safer: the high rotor or the low rotor aircraft? (See my reply to
Capt. Geoffrey Thorpe's post on the pendulum fallacy for my reasoning, such
as it is, on why I suspect high rotor is probably safer than low rotor.)

Power off is definitely a different story. With power on, the thrust vector is
always aligned with the vehicle and therefore acts through the c.g. regardless
of the aircraft's attitude. With power off, the drag through the rotor acts
parallel to the direction of travel, which is down. So, with an overhead
rotor, when the vehicle tilts right, the drag vector is shifted to the right
also (relative to the c.g.), creating a left rolling moment, making the
aircraft correct itself. With a low rotor, when the vehicle tilts right, the
drag vector is shifted to the left, creating a right rolling moment, making the
aircraft want to flip over. Either way, in a jetpack-like aircraft you've just
become a giant lawn dart.

You're also right, I had neglected to consider lighter-than-air aircraft in my
statements..