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#11
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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 |
#12
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jet pack
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) |
#13
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jet pack
"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 |
#14
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jet pack
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) |
#15
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jet pack
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 |
#16
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jet pack
"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 |
#17
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jet pack
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. |
#18
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jet pack
"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 |
#19
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jet pack
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. |
#20
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jet pack
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. |
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