jet pack

"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.

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?

"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.

"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 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.