jet pack

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. When a fixed wing aircraft is
perturbed from level flight, a measure of its stability is how quickly the
perturbation damps out; its "time-to-half". For a hovering aircraft, a measure
of its instability is its "time-to-double"; how long it take that pertubation to
get twice as bad. For a Huey helicopter, time-to-double is over 4 seconds, well
within a pilot's ability to react. For the Harrier, time-to double is just over
2 seconds; without the onboard stabilization system the Harrier was a handful.
The Hiller Flying Platform had a time-to-double of 1.2 seconds; it had a
mechanical gyro-stabilization system to make it flyable. The
Solotrek/Springtail aircraft have a time-to-double of 0.8 seconds; it has an
onboard computer-driven stabilization system. What you'll note is, as moment of
inertia (mass) goes down, time-to-double also goes down. The Martin JetPack is
even lighter and smaller than Trek's machines, its time-to-double must be very
quick. I'm sure they have some sort of stabilization system on their machine.

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.

On Trek's machines, close to 50% of the static lift is produced by the airflow
over the ducts. Martin's design is somewhat less efficient, so he's probably
seeing a 20-30% benefit. This helps get the machine up, but causes lots of
headaches when you transition to forward flight. In forward flight, the airflow
over the leading edge of the duct produces even more lift. That lift, however,
is forward of the c.g and causes a pitch-up effect. This was very apparent on
the Hiller Flying Platform. Until you can effectively counter the pitch-up
problem, you'll be limited to forward flight speeds of 6-8 mph.

Mr. Martin appears to be where Trek was 6-7 years ago. He has achieved a lot in
his garage, but he still has a long way to go before his machine is ready for
anything but test flights.

Good post! You will be flamed soon as someone can find an angle.

The pendulum idea probably could have been tossed out early by noting
that a pendulum inside the cockpit won't work as an attitude indicator.