Jetpacks currently suck. What are the flaws, and how can we overcome them?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Doggzilla, Nov 25, 2017.

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  1. Nov 29, 2017 #41

    pictsidhe

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    Air filters are available for just about every turbo. The monster turbos need a pretty bulky filer, though, A model jet could need one 12" diameter, 12' long. Not very compact. A jet pack woud need several larger ones. Drag engines aren't built for longevity, so they don't care. There are paperless filter systems, Husqvarna uses a centrifugal pre-filter. I can confirm that it works surprisingly well for what it is, the main filter doesn't get half the crud on it that a regular saw does. This idea could be combined with pre-whirl on a jet for a small pressure ratio boost.
     
  2. Nov 29, 2017 #42

    Topaz

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    The method used by the Ryan X-13 was a simple transition right through the stall. A VTOL aircraft has to have sufficient thrust and control power so that even through transition, there's strong and positive control in all three axes, despite any aerodynamic asymmetry. This is where most VTOL projects (and especially amateur ones) fail, because the necessary control authority is usually underestimated by a great deal. A simpler option, requiring a little less control power, planned or used by the Convair XFY-1, Lockheed XFV-1, and Northrop N-63 , is to climb vertically from takeoff until airspeed was above stall and then slowly pitch over into level flight. For landing, pitch up into a vertical climb and then slow until the vehicle is totally thrust-borne, then back down to a landing. You simply take the altitude gain (especially for landing) as part of the process.

    The X-13 project video has a great sequence of transition from vertical flight starting at 3:52, and the landing procedure in the minute or so before that. The operational procedure, taking off and landing from the actual trailer, is shown at starting at 4:57. Pilots of the Convair XFY-1 reported vertical landings to be extremely challenging. The difference is probably that the X-13 "landed" on a vertical trailer bed using a hook on its nose, while the XFY-1 landed on the ground and the pilot had to look over his shoulder all the way down.

    A "winged jet-pack" is essentially a "tail-sitter", in terms of overall geometry. The fact that the pilot would be in a standing position at takeoff and landing ought to make it easier than the situation with the XFY-1. Making it into a tilt-wing is possible, but complicated.
     
    Last edited: Nov 29, 2017
  3. Nov 29, 2017 #43

    Giggi

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    If the mission is to fly long distances fast in a VTOL light enough to carry, I'd just abandon the jetpack concept and put those engines into a hoverbike or maybe tiny fabric-covered airplane, with a seat and a windshield, that folds up when you need to carry it. Likely no heavier than a VTOL jetwing, but with better aerodynamics and engine placement, not to mention comfort. And at that point the concept would go into the "motorcycle of the air" thread instead of this one.

    Jetpacks are good for one thing only, and that's... I don't actually know, cause nobody's ever built one that's useful in any practical way. But I'm pretty sure that one thing isn't cruising.
     
  4. Nov 29, 2017 #44

    bmcj

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    According to a reliable source (a movie I once saw... ok, more than once), they are useful for foiling Nazi plans of world conquest.

    Rocketeer.jpg
     
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  5. Nov 29, 2017 #45

    12notes

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    For a far less comfortable, but much more adrenaline inducing ride, you could mount the jets on 2 position, 180 degree rapid swivels and mount the fixed wings so the leading edge points straight down on takeoff. The jets lift you to a sufficient altitude, you throttle down and swivel them while gravity accelerates you to flying speed. Don't forget to pull up! To land, you just swivel the jets back, the reverse thrust direction slows you down to a stall, as the nose of the plane drops, the jets will end up in position for the vertical landing.

    Practical, safe and efficient are just some of the words that don't apply here. But, man, would it be cool to watch someone else attempt.
     
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  6. Nov 29, 2017 #46

    Aesquire

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    Jetpacks are made so a soldier can jump up a cliff, trailing a rope for others to follow. Jump across a stream with rope, ditto. And for James Bond to escape the Villain's Death Trap.

    The idea of scouting with them got dropped when it was pointed out that they were so noisy you couldn't sneak up on anyone.

    Other than that last one, they haven't been used in service yet. ;)
     
  7. Nov 30, 2017 #47

    Mad MAC

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    To quote a web comic (Schlock mercenary)

    What do you call flying infantry?

    Skeet!
     
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  8. Nov 30, 2017 #48

    markaeric

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    @Topaz

    Watching that X-13 slowly hover away from its launch platform then roll around prior to transition looks hair raising and downright alien to me. The transition modes you mentioned seem exactly like the kinds of things you'd want to avoid, as it seems it would make performing the necessary maneuvers difficult and dangerous, or complex to implement in an automated system.

    When I hear of tiltwing, I think of planes like the X-18, which isn't what I meant, but rather a freely pivoting wing (similar to http://www.freewing.com/) so that the wing angle doesn't necessarily transition with the rest of the pack. That makes the pitch of the pilot separate from the AoA of the wing (which would be controlled by aero surfaces).


    @Giggi

    Just dropping the jetpack idea and making it more airplane-like did cross my mind but the thrust vectoring does become more complex unless you make it a tail sitter, and then all you gained is a little comfort and weight. But perhaps it is worth further consideration.

    I don't really see any practical application for such a thing either, other than fun... but I think that might be reason enough. A 20-30 minute horizontal flight endurance should be enough for someone to get their rocks off. I wouldn't consider it to be a transportation device.
     
  9. Nov 30, 2017 #49

    jedi

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    A bumble bee (or humming bird) type of flapping wing is what you are looking for. 8 or 12 foot wing span ups the efficiency and drops the downwash velocity to reasonable values. It trades the inflexable physics limitation for solvable strength of materials and engineering problems.
    The hover to forward flight transition is simplified and controls are intigrated with propulsion while power failure issues are reduced with partial power or gliding desent a possibility.
     
  10. Nov 30, 2017 #50

    BBerson

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    Apparently some hummingbirds can glide briefly http://forums.gardenweb.com/discussions/1978536/do-hummingbirds-glide but I don't think they glide to a safe landing. It generally takes a rotor disc about the same diameter as a parachute to equal the parachute sink rate.
    The real problem is the 300 feet needed to transition to autorotation.
     
  11. Nov 30, 2017 #51

    bmcj

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    For the transition through stall as you go from VTOL to level flight might be made easier with a low aspect ratio wing because the stall angle of attack is much greater, leaving you with a smaller risk range.
     
  12. Nov 30, 2017 #52

    Topaz

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    Which, to me, is perfect proof of how we can't rely upon our "feelings" with unusual aircraft like this and instead focus on the math and engineering. The X-13 was one of the most-successful, yet simple, VTOL airplanes ever created, and the transition modes chosen proved easy and reliable for a manually-flown airplane. The "landing" maneuver was arguably difficult, as with all tail-sitters that sat the pilot in a conventional cockpit, but that's another matter from the transition to and from horizontal flight.

    When it has been demonstrated to work consistently and we'll, I think that's a data point to use, not to throw out because it "looks weird" to our eye.

    YMMV, of course.
     
  13. Nov 30, 2017 #53

    Topaz

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    With an airplane that has enough thrust-to-weight to be VTOL, there's no need for high aspect ratio unless you also want great range. Go with stubby wings and all the benefits they provide during transition.
     
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  14. Nov 30, 2017 #54

    Giggi

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    Especially if they're delta wings that give you vortex lift. Though level flight would need to be faster.
     
  15. Dec 1, 2017 #55

    Bill-Higdon

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    "Targets of Opportunity"
     
  16. Dec 1, 2017 #56

    markaeric

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    Ok.. So maybe jetpacks don't totally suck. from: https://zapata.com/air-products/flyboardair/

    [video=youtube;KEDrMriKsFM]https://www.youtube.com/watch?v=KEDrMriKsFM[/video]


    Technical details are pretty much non-existent, other than an interview (https://www.theverge.com/2016/4/15/...acing-jet-powered-flying-hoverboard-interview) claiming four 250HP :)rolleyes:) propulsion units. Not saying it's exactly inaccurate, just that it's a crappy way of quantifying the useful power produced by the system while concealing how it operates.



    Just to be clear, my comments weren't about the engineering and plausibility of the X-13 being unsound or impractical, but rather about the psychological effect (on me) of seeing the footage of something so uncommon as to be radical. Just because I think the take-off and transition appeared strange, doesn't mean I didn't think it was totally awesome, because it was!

    FWIW (not much) I do maintain, however, that I have my doubts that it's the ideal method for a winged jetpack - admittedly without any substantive evidence to back that up.
     
    Last edited by a moderator: Mar 20, 2019
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  17. Dec 1, 2017 #57

    markaeric

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  18. Dec 1, 2017 #58

    BBerson

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    It needs the louvered grill for takeoff (seen in the video). So I don't see how it will be useful for getting the morning bread.:ermm:
    Obviously, the next version will not have jets 6" above the ground.
    Still impressive to watch, however impracticle it is.
     
  19. Dec 1, 2017 #59

    markaeric

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    Pffft. There's grated storm drains all over the place!
     
  20. Dec 2, 2017 #60

    cheesefactory

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    These concepts have all been solved in the RC world:

    https://www.youtube.com/watch?v=L2fgL97rgK0

    It's just a matter of someone putting together a bunch of money and manpower to scale it up.

    I was working on a VTOL myself, but halfway along I woke up and realized I didn't have enough money to see it through to the end. So I put that project on hold so I can go hustle. I'll return to it in the future.
     

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