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How much power do you need to hover? (human powered helicopter)

  1. Jul 12, 2013 #1
    This question, or some form thereof, has come up a lot in this forum.

    The answer is that if you can have a propeller big enough, you can hover with almost zero power. But you can only build so big before physics and material limitations get in the way....

    These guys didn't seem to give a hoot :thumbup:

    I thought this was pretty cool:

    http://www.bbc.co.uk/news/world-us-canada-23293676
     
    Last edited by a moderator: Jul 12, 2013
  2. jcsd
  3. Jul 12, 2013 #2
  4. Jul 12, 2013 #3

    berkeman

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    This thread from the PF may be of help: https://www.physicsforums.com/showthread.php?t=338502

    It's quite long, so start reading at the last page.
     
  5. Jul 12, 2013 #4

    berkeman

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  6. Jul 13, 2013 #5

    sophiecentaur

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    You need to take drag into account here. Lift / Drag ratio will never be infinite and a long rotor blade will introduce high drag - imposing a fundamental power requirement. It's no surprise that the only animals that can truly hover are very small. At that scale, the electric forces start to be more relevant than g forces.
     
  7. Jul 13, 2013 #6

    rcgldr

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    Kites are about the same size as a pigeon or dove, and they can hover. I see them quite often a local site for radio control gliders. Link to a video of a kite (not mine):

     
    Last edited by a moderator: Sep 25, 2014
  8. Jul 13, 2013 #7
    It seems as if it's trying to go against the wind and it's not trully hovering. I've seen the same with many different birds.
    I think the only bird that can really hover is the hummingbird.
     
    Last edited by a moderator: Sep 25, 2014
  9. Jul 13, 2013 #8

    sophiecentaur

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    Yes, they use the wind and up-draughts to keep up. Kestrels are masters of this and can stay stationary for minutes on end but that's not a true hover.
    As you say, a hummingbird is the only real 'hoverrer'. But birds have a totally different respiration system because of the incredible cost in Energy of the immense demanded for flight with large animals. I looked up the respiratory system for birds, once, and found that they work in a very different way - just to shift the necessary volume of gases through. Hummingbirds, of course, use pretty high grade fuel - nectar (four star unleaded quality!!)

    I heard David Attenborough making the point that, as soon as they find themselves isolated from big predators (on an evolutionary timescale, that is) large birds soon develop a flightless lifestyle because the energy demand drops significantly if you just walk around on a safe island!
     
  10. Jul 13, 2013 #9

    rcgldr

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    There's not much wind in that particular video, and no real source for an updraft. I've seen them hover on windless days. Wiki article for species group:

    http://en.wikipedia.org/wiki/Elanid_kite

    more articles:

    http://www.birdinginformation.com/birds/falcons-and-kites/white-tailed-kite

    http://www.bird-friends.com/BirdPage.php?name=White-Tailed Kite

    Another video:

    http://ibc.lynxeds.com/video/white-tailed-kite-elanus-leucurus/adult-hovering

    Ospreys are bigger still and can hover (over water since they hunt fish), but I don't know if they can hover on windless days.
     
  11. Jul 13, 2013 #10
    I'm not particularly interested in ornithology but thanks anyway ;)
     
  12. Jul 13, 2013 #11

    rcgldr

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    Getting back to Berkeman's post, in the other thread, there's a video of a 4 rotor human powered helicopter that does quite well, but the rotors are still operating in ground effect. I'm not aware of any human powered helicopter that doesn't involve ground effect, although with the model shown in the other thread, it might be possible to get that model out of ground effect for a brief moment if that model could be tested in a much larger building.
     
  13. Jul 13, 2013 #12

    A.T.

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    From their website: http://www.aerovelo.com/projects/helicopter/tech-info/ [Broken]

     
    Last edited by a moderator: May 6, 2017
  14. Jul 13, 2013 #13

    sophiecentaur

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    Any site that is suitable for r/c gliders is going to have a useful amount of lift to keep totally un-powered models in the air so that evidence is not very conclusive, surely.
    And the video has loads of wind noise on the microphone, too! Plus the tree is shaking a fair bit.
     
    Last edited by a moderator: Sep 25, 2014
  15. Jul 13, 2013 #14

    rcgldr

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    At the site I was referring to, when there is no wind, the people there fly electric powered models. When there is wind, it's about 60% powered / 40% gliders, depending on the group.

    As far as the kites behave versus the wind, if there's a headwind and updraft, they mostly soar with little flapping. If there's a headwind and no updraft they flap a bit more. If there's no wind, then they are flapping quite rapidly, with full swing flaps. Regardless of the wind, when "hovering", they're virtually motionless with respect to the ground (relying on vision to track a spot on the ground), which is quite impressive.

    The other video I posted showed a kite hovering close to the ground (2nd part of video), where there can't be much wind, due to the small shrubs, and what wind there is appears to be blowing from behind the kite. Link to that video:

    http://ibc.lynxeds.com/video/white-tailed-kite-elanus-leucurus/adult-hovering
     
    Last edited: Jul 13, 2013
  16. Jul 14, 2013 #15

    sophiecentaur

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    There's little point in this conversation because I think we are talking at cross purposes and we are only differing in 'degree' and not in principle. Any video you post can't be real evidence because there are no measurements of actual wind speed (particularly not where the bird is hovering). Kites, Eagles Buzzards etc. etc, will soar on invisible thermals on a totally 'calm' day, getting their energy from the Sun. Of course, most birds can hover for a short while, particularly when they can use ground effect. It often goes with the process of landing and taking off. The point is that it is very seldom 'sustained' hovering, because it is so costly. If it were, in fact, no trouble for birds to hover then they would be doing it much more often and they would not bother to face into the wind, run, leap off cliffs etc.. They would just VTOL.

    The only convincing evidence that kites actually hover in static air would be a video of two kites facing each other and hovering for a significant length of time - showing that their air speed is zero. Hummingbirds can often be seen, clustered around flowers, facing in all directions - supping their high energy drink. I would say that was good evidence about hummingbirds.
     
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