jet pack

"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.

Take the wife's heirloom grandfather clock and throw it off the roof - you
will observe that the "heavy end" of the pendulm doesn't "hang down" or
fall any faster than the rest of the clock once you have let go of it.

Yes, but if you tie a rope on it, to keep it from falling, it will hang
down from the rope. Same would go with a rotor disk suporting the weight,
like a helicopter, or two small rotors, like the so called jet pack.

A helicoper is basicly stable, once you get constant torque, and cancel out
the torque. There is turbulent air flow though the rotor that needs minor
corrections. Would you presume to say that a helicopter would fly as good
with the rotor underneath the cockpit and engine? I would hope not.

A rocket is a different beast, because it is in ballistic flight. Its
aerodynamic characteristics as the most dominant forces. You need to get
more side surface area behind (below) the center of gravity or else be
prepared to change the direction of thrust very rapidly, and precisely.

Any difference between tractor and pusher aircraft controllability that
can't be explained by the change in airflow over the control surfaces?

Same thing as the rocket example. You have to have more area behind the
center of gravity, then it will fly straight. The prop is not supporting
the weight, the wings are. That is why a high wing plane's wing is many
times straight, because the weight below the wing makes it naturally stable.
Low wing planes tips are higher to promote natural stability. High wing
planes many times have the tips lower than the middle to promote more
instability, thus maneuverability.

The jet pack has to have better stability while hovering with the rotor
above the CG. Even then, the small volume of air being moved so rapidly
creates more turbulence and instability.

Once it starts to try and transition to forward flight, all bets are off,
with stability. It will still be hanging from the rotors, but at a certain
point in gaining speed, the airflow past the machine and pilot will start to
change the stability, and then some control surfaces better be thinking
fast, as in gyro stabilized moving surfaces. It is this problem that may
ultimately make this machine unsuccessful, as have many others of similar
design.

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.
--
Jim in NC