jet pack

[[email protected]]

Sounds like it's actually motor driven.
http://www.cnn.com/2008/TECH/ptech/07/30/jetpack.unveiled.ap/index.html
Karl

[Anthony W]

wrote:
> Sounds like it's actually motor driven.
> http://www.cnn.com/2008/TECH/ptech/07/30/jetpack.unveiled.ap/index.html
> Karl

It's nothing new, this has been around for a while but I don't see much
hope for it working as claimed.

Tony

[Ted[_2_]]

I totally agree with Tony...even in the company's demonstration video 2
people are always there "walking" the flyer....sort-of reminds me just abit
of the Mollier Flying Car or whatever it's called. It DOES look like a lot
of FUN, though!!! I also hope the thing succeeds....30 minutes running on
this as they say is so much better than the 43 seconds that the "Go Fast
Sports" JetPack can fly. Ted
"Anthony W" > wrote in message
news:o19kk.556$JH5.435@trnddc06...
> wrote:
>> Sounds like it's actually motor driven.
>> http://www.cnn.com/2008/TECH/ptech/07/30/jetpack.unveiled.ap/index.html
>> Karl
>
> It's nothing new, this has been around for a while but I don't see much
> hope for it working as claimed.
>
> Tony

[Anthony W]

Ted wrote:
> I totally agree with Tony...even in the company's demonstration video 2
> people are always there "walking" the flyer....sort-of reminds me just abit
> of the Mollier Flying Car or whatever it's called. It DOES look like a lot
> of FUN, though!!! I also hope the thing succeeds....30 minutes running on
> this as they say is so much better than the 43 seconds that the "Go Fast
> Sports" JetPack can fly. Ted

I think this think is pretty much a Moller for one. If it ever gets
more than 5' off the ground it will be very unstable.

Tony

[Paul Dow (Remove CAPS in address)]

I was listening to the EAA webcast, and they said that the FAA required
them to hang on the the thing during that demo because of all the people
standing in the area. There's supposed to be a better demo during today
or tomorrow's airshow where it will lift off away from everyone. In any
case, it isn't a "Jet", but a ducted fan. Sounds like a big leaf blower.

Anthony W wrote:
> Ted wrote:
>> I totally agree with Tony...even in the company's demonstration video
>> 2 people are always there "walking" the flyer....sort-of reminds me
>> just abit of the Mollier Flying Car or whatever it's called. It DOES
>> look like a lot of FUN, though!!! I also hope the thing
>> succeeds....30 minutes running on this as they say is so much better
>> than the 43 seconds that the "Go Fast Sports" JetPack can fly. Ted
>
> I think this think is pretty much a Moller for one. If it ever gets
> more than 5' off the ground it will be very unstable.
>
> Tony

[Paul Dow (Remove CAPS in address)]

I just heard from Martin (through EAA Radio) that they won't be flying
again this week. They say they've been overworked and tired, so they
don't want fatigue causing an accident. There was also talk about
insurance issues, but it seems like the pilot capability issue trumped that.

Paul Dow (Remove CAPS in address) wrote:
> I was listening to the EAA webcast, and they said that the FAA required
> them to hang on the the thing during that demo because of all the people
> standing in the area. There's supposed to be a better demo during today
> or tomorrow's airshow where it will lift off away from everyone. In any
> case, it isn't a "Jet", but a ducted fan. Sounds like a big leaf blower.
>
> Anthony W wrote:
>> Ted wrote:
>>> I totally agree with Tony...even in the company's demonstration video
>>> 2 people are always there "walking" the flyer....sort-of reminds me
>>> just abit of the Mollier Flying Car or whatever it's called. It DOES
>>> look like a lot of FUN, though!!! I also hope the thing
>>> succeeds....30 minutes running on this as they say is so much better
>>> than the 43 seconds that the "Go Fast Sports" JetPack can fly. Ted
>>
>> I think this think is pretty much a Moller for one. If it ever gets
>> more than 5' off the ground it will be very unstable.
>>
>> Tony

[John[_17_]]

Paul Dow (Remove CAPS in address) wrote:
> I just heard from Martin (through EAA Radio) that they won't be flying
> again this week. They say they've been overworked and tired, so they
> don't want fatigue causing an accident. There was also talk about
> insurance issues, but it seems like the pilot capability issue trumped
> that.
>
> Paul Dow (Remove CAPS in address) wrote:
>> I was listening to the EAA webcast, and they said that the FAA
>> required them to hang on the the thing during that demo because of all
>> the people standing in the area. There's supposed to be a better demo
>> during today or tomorrow's airshow where it will lift off away from
>> everyone. In any case, it isn't a "Jet", but a ducted fan. Sounds like
>> a big leaf blower.
>>
>> Anthony W wrote:
>>> Ted wrote:
>>>> I totally agree with Tony...even in the company's demonstration
>>>> video 2 people are always there "walking" the flyer....sort-of
>>>> reminds me just abit of the Mollier Flying Car or whatever it's
>>>> called. It DOES look like a lot of FUN, though!!! I also hope the
>>>> thing succeeds....30 minutes running on this as they say is so much
>>>> better than the 43 seconds that the "Go Fast Sports" JetPack can
>>>> fly. Ted
>>>
>>> I think this think is pretty much a Moller for one. If it ever gets
>>> more than 5' off the ground it will be very unstable.
>>>
>>> Tony

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.

I spoke with one of the promoters at the convention and asked about the
engine failure issue. The engine is a liquid cooled V4 which I assumed
was a marine engine. Like Moller they will be relying on one of those
"zero-zero" (almost) ballistic parachutes that use an explosive device
to spread the canopy when there is line stretch so that in theory it is
already inflated before you've moved down more than 10 or 20 feet or
something like that. Below the parachute safety altitude, they are
relying on a kind of shock strut that sticks down between your legs to
absorb the fall and save your back, which is supposed to be effective to
about 10 feet.

It's the middle zone between the parachute minimum altitude and the
shock strut that they don't account for, and they haven't tested a
parachute yet to see just what the "dead man's" altitude range is. In
any event, anybody flying the thing will be working to spend as little
time as possible between 5-10 feet and the altitude that the parachute
is effective. You sure wouldn't get me in the damn thing.

They sold one of them by Thursday. 100 grand. Crazy.

John

[Capt. Geoffrey Thorpe]

"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.

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.

--
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.

[John[_17_]]

Capt. Geoffrey Thorpe 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.
>
> 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.
>

So much for the roll stability advantage of high wing aircraft...

Perhaps the word should be controllability, not stability. Are you
saying the thing would have the same controllability with the thrust at
the top or bottom?

[Capt. Geoffrey Thorpe]

"John" > wrote in message
...
> Capt. Geoffrey Thorpe 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.
>>
>> 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.
>>
>
> So much for the roll stability advantage of high wing aircraft...
>
> Perhaps the word should be controllability, not stability. Are you
> saying the thing would have the same controllability with the thrust at
> the top or bottom?

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.

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.

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

--
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.

[Morgans[_2_]]

"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

[cavelamb himself[_4_]]

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)

[Morgans[_2_]]

"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

[cavelamb himself[_4_]]

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)

[Geyser]

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>

[Morgans[_2_]]

"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

[Geyser]

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.

[Morgans[_2_]]

"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

[Geyser]

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.

[Rob Bulaga]

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.

[Geyser]

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.

[Jim Logajan]

"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.

[Capt. Geoffrey Thorpe]

"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.

[Jim Logajan]

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.)

[Jim Logajan]

"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

[John[_17_]]

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

[Rob Bulaga]

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.. :)

[Capt. Geoffrey Thorpe]

"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.

[Capt. Geoffrey Thorpe]

"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[_4_]]

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/Gyroscopic_precession#Torque-induced_precession

--

Richard

(remove the X to email)

[Jim Logajan]

"Capt. Geoffrey Thorpe" <The Sea Hawk @See My Sig.com> wrote:
> 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?

Obviously the Cp and Cg are close enough together that it doesn't matter
much where they are - it appears it would be unstable with or without
power.

In fact I should confess that I only posted my comments in a very modest
effort to promote lighter-than-air (LTA) as a more viable option for such
things. Of course the "open" technical challenges and usage tradeoffs of a
personal "portable(!?)" airship are quite different from rotorcraft.

[Capt. Geoffrey Thorpe]

"Jim Logajan" > wrote in message
.. .
<...>
> In fact I should confess that I only posted my comments in a very modest
> effort to promote lighter-than-air (LTA) as a more viable option for such
> things. Of course the "open" technical challenges and usage tradeoffs of
> a
> personal "portable(!?)" airship are quite different from rotorcraft.

Would this sort of concept use He and a compressor to recover it after a
flight, or hot air?

He would, I think, take a lot of compressing to make it portable - Right?
Wrong?

I assume you are talking about something with power - one fuel could
provide power and the heat for hot air. The engine cooling / exhaust would
help maintain the temperature inside the envelope?

--
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.