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

Morgans wrote:

"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.



I'm skeptical too.

Unlike the huge gyroscopic forces on a helicopter rotor this thing has
two puny ducted fans. Good thrust efficiency, but not much stabilizing
force.

In addition, there are two fans - side by side.
I believe the torque reactions would be in fore/aft pitch.

Hmm?


--

Richard

(remove the X to email)

"cavelamb himself" wrote

In addition, there are two fans - side by side.
I believe the torque reactions would be in fore/aft pitch.

Hmm?

I don't think torque is going to be a show-stopper. I believe these are two
counter-rotation, fixed pitch propellers, and the only collective (so to
speak) is the RPM of the engine. The RPM stays mostly constant, and changes
slowly, so the fore and aft reaction should be pretty slight.

I don't see that this thing will work without some kind of fly by wire, or
more precisely, some type of electronic stability system. It is common
practice for remote control helicopters; a couple rate gyros, and a
connection to a couple servos to keep things from wobbling out of control so
much.

Another problem could be the pilots position on the machine. With the real
jet pack, the pilot's legs and free to move around to allow the pilot to
give some "body english" small corrections to the flight path. That does
not look to be possible, for this particular (S)mall (M)otor (U)pwards
(R)otor (F)lyer, or SMURF, for short. ggg
--
Jim in NC

Morgans wrote:
"cavelamb himself" wrote


In addition, there are two fans - side by side.
I believe the torque reactions would be in fore/aft pitch.

Hmm?


I don't think torque is going to be a show-stopper. I believe these are two
counter-rotation, fixed pitch propellers, and the only collective (so to
speak) is the RPM of the engine. The RPM stays mostly constant, and changes
slowly, so the fore and aft reaction should be pretty slight.

I don't see that this thing will work without some kind of fly by wire, or
more precisely, some type of electronic stability system. It is common
practice for remote control helicopters; a couple rate gyros, and a
connection to a couple servos to keep things from wobbling out of control so
much.

Another problem could be the pilots position on the machine. With the real
jet pack, the pilot's legs and free to move around to allow the pilot to
give some "body english" small corrections to the flight path. That does
not look to be possible, for this particular (S)mall (M)otor (U)pwards
(R)otor (F)lyer, or SMURF, for short. ggg

Sorry Jim,
My bad.

What I meant was the torque reaction bewteen the two gyroscopic preseccions.

You are right, obviously not torque like from a prop or rotor.


--

Richard

(remove the X to email)

cavelamb himself wrote:
Morgans wrote:

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


... But pendulum's have nothing to do with it.


I'm not buying it.


I'm skeptical too.

"Charles Zimmerman, to the amusement of his engineering peers, proved
the theory that rotors on the top (i.e. helicopters) are inherently
unstable."
http://www.hiller.org/flying-platform.shtml

"Geyser" wrote

"Charles Zimmerman, to the amusement of his engineering peers, proved the
theory that rotors on the top (i.e. helicopters) are inherently unstable."
http://www.hiller.org/flying-platform.shtml

So, what would you expect an article to say, that is trying to build support
for a rotor on the bottom craft? Of course that is what they would say.

Also, it is taken out of context, since the next paragraph talks about the
fact that they believe a person over the rotor will be able to use shifting
body weight to make the rotor under craft stable.
--
Jim in NC

Morgans wrote:
"Geyser" wrote

"Charles Zimmerman, to the amusement of his engineering peers, proved the
theory that rotors on the top (i.e. helicopters) are inherently unstable."
http://www.hiller.org/flying-platform.shtml

So, what would you expect an article to say, that is trying to build support
for a rotor on the bottom craft? Of course that is what they would say.

Nobody is trying to build support for it. The Hiller Flying Platform is
a relic, 50 years old. Hiller built many helicopters since that time,
with the rotor on the top.

Also, it is taken out of context, since the next paragraph talks about the
fact that they believe a person over the rotor will be able to use shifting
body weight to make the rotor under craft stable.

But stability and controllability are different things. Weight shift
acts against the stability.
The relative wind hitting the draggy form *on top* keeps the platform
from tilting further-n-further and running away. It "wants" to
straighten up and return to a low speed.
If the drag were underneath, it would weathervane toward horizontal and
might be unrecoverable.

Anyway, the article also says that the duct's bellmouth leading edge
generates 40% of the lift. Wow! I wonder why the Martin jet pack missed
that.

"Geyser" wrote

Nobody is trying to build support for it. The Hiller Flying Platform is a
relic, 50 years old. Hiller built many helicopters since that time, with
the rotor on the top.

This was an quote from an article written 50 (your number) years ago.

Don't take my lack of further comment as agreeing with you.
--
Jim in NC

Morgans wrote:
"Geyser" wrote

Nobody is trying to build support for it. The Hiller Flying Platform is a
relic, 50 years old. Hiller built many helicopters since that time, with
the rotor on the top.

This was an quote from an article written 50 (your number) years ago.

The Hiller Flying Platform is 50 years old.

The Hiller Aviation Museum wasn't around 50 years ago to write about it.

The article appears to have been written November 26, 1999.

Don't take my lack of further comment as agreeing with you.

Don't worry.

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.