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#21
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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. |
#22
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jet pack
"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. |
#23
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jet pack
"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. |
#24
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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. 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.) |
#25
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jet pack
"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 |
#26
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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 explanation. John |
#27
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jet pack
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.. |
#28
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jet pack
"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. |
#29
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jet pack
"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. |
#30
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jet pack
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) |
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