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.

"Capt. Geoffrey Thorpe" The Sea Hawk @See My Sig.com wrote:
"John" wrote in message
...
...
It's supposed to be fairly stable because the thrust reaction point
is well above the CG, so there is a strong pendulum effect. They
claim it's better than a helicopter.

An often made, completely wrong assumption - "pendulm effect" - ain't
no such thing for an object in free flight.

Technically speaking, I don't know under what assumptions one could ever
claim an object moving in a fluid is ever moving "freely". More below on
the importance of this point....

Early rocket experimenters often attempted use "tractor" engines
assuming that it would provide stability - Dr. Robert Goddard's first
liquid rocket is an example. It didn't take them long to figure out
that they were wrong.

But a rocket and an rotorcraft aren't equivalent under all cases of
interest. For example, if your rotorcraft's engine fails, then because it
is traveling through a fluid the craft will rotate so the center of
aerodynamic pressure is above the center of gravity.

So if your craft normally flies with the c.g. below the center of pressure
(e.g. rotors above fuselage) then on engine out I would expect you'd
probably count on little change in attitude while autorotation would
ideally slow your descent somewhat.

But if your craft normally flies with the c.g. above the center of pressure
(e.g. rotors below the fuselage) then on engine out I would expect some
nasty rotations that are unlikely to dampen out before you strike the
ground. Even if they did dampen out, you're upside down and the rotors
would actually have to reverse direction to provide autorotation. drag.

"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.

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.)

"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

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 explanation.

John

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..

"Jim Logajan" wrote in message
.. .
"Capt. Geoffrey Thorpe" The Sea Hawk @See My Sig.com wrote:
"John" wrote in message
...
...
It's supposed to be fairly stable because the thrust reaction point
is well above the CG, so there is a strong pendulum effect. They
claim it's better than a helicopter.

An often made, completely wrong assumption - "pendulm effect" - ain't
no such thing for an object in free flight.

Technically speaking, I don't know under what assumptions one could ever
claim an object moving in a fluid is ever moving "freely". More below on
the importance of this point....

Yes, aerodynamics play a big part in real life.


Early rocket experimenters often attempted use "tractor" engines
assuming that it would provide stability - Dr. Robert Goddard's first
liquid rocket is an example. It didn't take them long to figure out
that they were wrong.

But a rocket and an rotorcraft aren't equivalent under all cases of
interest. For example, if your rotorcraft's engine fails, then because it
is traveling through a fluid the craft will rotate so the center of
aerodynamic pressure is above the center of gravity.

The "jet pack" that is the topic of this thread has two ducted fans. When
they quit, it's game over.

You are correct the center of gravity will align with the aerodymanic
center of effort. But, where are the Cp and Cg on the "jet pack" with
ducted fans in the "tractor position" - and how much stability will it add
in a hover?

--
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.

"Jim Logajan" wrote in message
...
...
Now supposing the engine fails - at that point, which in general is
easier
to make safer: the high rotor or the low rotor aircraft?

Ducted fans "jet packs" don't autorotate - they fall like a brick (at least
stuff like the one that was flown at Oshkosh). So it really doesn't matter,
eh?

:-)

(yes, they have / plan to have a balistic 'chute to slow the brick down
from what I've read)

Looking back up this thread a ways to review the original claim:

"It's supposed to be fairly stable because the thrust reaction point is
well above the CG, so there is a strong pendulum effect. They claim
it's better than a helicopter."

Complete and utter bull droppings.

--
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.

cavelamb himself wrote:
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?



http://en.wikipedia.org/wiki/Gyrosco...ced_precession

--

Richard

(remove the X to email)