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Would this two-rotor hovercraft design work? The bold black line is weighted metal on the lower to stable it. If it helps, this is a human sized hovercraft.

Would it work if:

  1. prop1 lift > prop2 lift
  2. prop1 lift = prop2 lift
  3. prop1 lift < prop2 lift

The difference between prop1 and prop2 lift should be in a range of 0-30%

diagram

NOT TO SCALE, assume that lower weighted bars are equidistant from middle of hovercraft.

Pondlife
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Gabriel Kusiak
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    A hovercraft rides on an inflated air cushion and is inherently stable. Do you mean a twin-rotor helicopter instead? – Camille Goudeseune May 13 '20 at 23:00
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    If all you want to do is fly it around a few inches off the ground, then putting a skirt around it reduces the power requirement dramatically. And voila! There's your hovercraft. – John K May 14 '20 at 00:59
  • In other words, use the ground effect, too much weight, it won't fly. Good one, Camille. I would also explore the stabilizing effects of the rotor "disks". They seem to help on bicycles. – Robert DiGiovanni May 14 '20 at 11:27
  • As a matter of interest, are hovercraft on topic on Aviation? They are normally considered ships. – DJClayworth May 14 '20 at 16:20

2 Answers2

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Yes - it can work. Colin Furze (UK inventor & TV personality) built an essentially similar device. Skip to about 1:40 to see some of the problems he had.

CatchAsCatchCan
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    What a death trap! – John K May 14 '20 at 00:58
  • What kind of motor do you think he uses? – Gabriel Kusiak May 14 '20 at 01:15
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    You can see that there are two rather cantankerous and unreliable two strokes. Double the risk of failure! And when one goes, even if you stay close to the ground, it flips you over and if you're lucky, you won't have a limb removed when it goes through the running engine's flimsy netting that is all that is between you and your future nickname, "stumpy", during the crash. This Fruze chap seems to have about the same risk tolerance profile as BASE jumpers, who are addicted to near death experiences. – John K May 14 '20 at 01:25
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    However the answer to “would this be stable” is a NO – the thing is not stable, and it cannot be made stable. It is controllable (barely), but not stable. No helicopter is stable in hover. It could be made stable by converting it into a hovercraft, that is adding the skirt all around it, which would also reduce the power requirement. – Jan Hudec May 14 '20 at 08:01
  • @Jan Hudec. Would not the 2 rotating "disks" add stability. They seem to on a bicycle. Yes, this is a hovercraft. A little high drag would give a "righting" force while in motion. The main issue seems to be engine reliabilty. Great start for this inventor. – Robert DiGiovanni May 14 '20 at 11:24
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    @RobertDiGiovanni, bicycle is not stable. When moved out of the centred position, the deviation grows unless corrected. The gyroscopic effect slows down the deviation increase a bit, which makes it more controllable, but cannot change the way the feedback works. The same applies to rotorcraft. – Jan Hudec May 14 '20 at 21:31
  • @Jan Hudec was hoping for a full answer from you, sorry they closed this one. The gyroscopic effect does indeed make it more controllable, and that is the key. – Robert DiGiovanni May 15 '20 at 01:39
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If the weight is located low enough, it should have "pendulum stability", just like this toy --

enter image description here

If the props are running at different speeds and generating different amounts of lift, then (assuming that is a top view not a side a view), the pilot's weight will need to be off-center to compensate.

quiet flyer
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    "Pendulum stability" doesn't work. The toy you mention is stable because the center of drag is behind the center of mass. – Mark May 14 '20 at 02:47
  • We're going to get to the bottom of this pendulum thing. Too much "fallacy" and Pavlovian dv's. Needs work. – Robert DiGiovanni May 14 '20 at 05:28
  • In other words, an object in an unaccelerated state will have a "pendulum" effect, but Mark is correct, drag, and thrust line also play a role. – Robert DiGiovanni May 14 '20 at 05:31
  • Put a little drag above the prop and watch what happens to hover stability. May be another reason the Army liked to put a radar bulb there. – Robert DiGiovanni May 14 '20 at 11:33
  • @Mark -- sure "Pendulum stability" works. This related answer goes into some detail about how the "pendulum effect" of a low CG contributes to roll stability-- https://aviation.stackexchange.com/questions/77971/when-a-paraglider-pilot-applies-brake-on-one-side-and-maintains-the-brake-pressu/77996#77. See main body, 3rd footnote, and addendum. It's also not hard to see how putting the center of drag way above the CG would contribute to pitch stability or speed stability -- in fact arguably this is essentially the only source of pitch stability or paragliders, yet they can be flown hands off. – quiet flyer May 14 '20 at 12:48
  • @Mark -- I don't know the exact mechanism at play in the case of the helicopter toy, but obviously there is one. The rocket case is totally different, nothing to do with what I am talking about, because the aerodynamics of the rocket nozzle don't change significantly when the rocket is tilted relative to the flight path, but that's not true of the rotor disk. Amazing that Goddard would suffer from such a misconception. Anyway this link is more relevant-- see especially the 1st paragraph -- https://en.wikipedia.org/wiki/Keel_effect – quiet flyer May 14 '20 at 12:48
  • @RobertDiGiovanni -- re your last -- more likely because that allows the radar to see over the ridgeline that the helicopter is hiding behind. – quiet flyer May 14 '20 at 13:43
  • That's the primary reason, it seems to work. – Robert DiGiovanni May 14 '20 at 15:37
  • @Mark -- consider also the question -- why do pilots find a model airplane to be more stable in a nose-up "hover" maneuver (see https://www.instructables.com/id/Want-to-fly-3D/ , https://www.youtube.com/watch?v=aqOEwCzR3YQ ) if the CG is well AFT? Do the dynamics have anything to do with the dynamics of the rotor-on-the-stick toy? Could be grounds for another ASE question, to explore one of the many facets of "pendulum" stability. The amazing thing about the rotor on stick toy is that it is stable both climbing and descending-- fascinating-- – quiet flyer May 14 '20 at 16:08
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    @quietflyer, the keel effect, airplane pendulum stability, and the like are all about the relative positions of the center of mass and various aerodynamic surfaces. But your answer is suggesting that the relative positions of the center of mass and the center of thrust are important, which is the classic rocket pendulum fallacy. – Mark May 14 '20 at 20:31
  • @Mark -- "all about the relative positions of the center of mass and various aerodynamic surfaces"-- is not the same true of the prop-on-a-stick toy?. Consider-- would these other toy helicopters fly just as well if the rotor blades were below rather than above the center of mass? https://www.youtube.com/watch?v=sV5EFlAmfIA , https://books.google.com/books?id=IyEDAAAAMBAJ&pg=PA148&lpg=PA148#v=onepage&q&f=false (pages 148-150) – quiet flyer May 15 '20 at 14:52
  • @Mark -- I don't fully understand the mechanism at play here (could be the basis of another ASE question?) but I think it is fundamentally different than a jet or rocket engine. – quiet flyer May 15 '20 at 15:05
  • @Mark -- I my try to formulate some of the above comments into a question for SE, maybe I'll post in Engineering rather than Aviation to try to get a different community's perspective, haven't decided yet. Or Physics. Not sure which would be better. – quiet flyer May 15 '20 at 16:37