Bell Spanload Implications for Sailing?

Discussion in 'Hydrodynamics and Aerodynamics' started by lunatic, Nov 19, 2017.

  1. lunatic
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    lunatic Senior Member

  2. Erwan
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    Erwan Senior Member

    Thank you for sharing the workpaper, I will read it with great attention, but I am not the CFD guru who can make relevant observations.
    Cheers
     
  3. Erwan
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    Erwan Senior Member

    Lunatic,

    The optimization presented in this workpaper, is performed around an aeronautical constraint: The Weight
    That is why the benchmark is similar weight wing structure, but it is clearly mentionned that in case of span constraints, the elliptical lift distribution will provide the least induced drag.
    The bell shaped lift distribution remains interesting for the upwash phenomenon.

    In 1985 I bought a German little book :
    "Nurflügel-modelle" by Dipl.-Ing Martin Lichte" ISBN 3-88180-203-7
    Of course the Horten brothers' airplanes and the bell lift distribution were the core of the book

    That is why I feel this NASA workpaper is border-line condescending and infantilizing with regards to L Prandlt and Horten bros achievements 80 years ago.
     
  4. Mikko Brummer
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    Mikko Brummer Senior Member

    On the contrary Erwan, this is totally relevant to sailing. Root moment is comparable to the heeling moment in sailboats, and therefore the triangular sail plan is not at all bad. The current (impractical) trend for fathead sails is due to rating rule limitations, not performance.
     
  5. HJS
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    HJS Member

    [QUOTE="Mikko Brummer, . The current (impractical) trend for fathead sails is due to rating rule limitations, not performance.[/QUOTE]

    Explain this a little more with concrete examples.
    JS
     
  6. Mikko Brummer
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    Mikko Brummer Senior Member

    If the mast height is limited (ex. IMOCA 60 or TP52 or Moth), obviously a fathead is in order to get more sail area into the sail plan. Or as in the ORCi rule, the aero model is such that it may favour lightly the fathead. If there is no limitation on mast height, it pays to make a higher mast and a more triangular sail, with a gain in induced drag as stated in the paper posted by Lunatic. Although it's not quite so straightforward, the longer mast will be heavier, and that will influence performance.
     
  7. HJS
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    HJS Member

    .... moreover, the wind speed varies over the height above the water surface.

    js
     
  8. fastsailing
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    fastsailing Junior Member

    In sailing most masts are stayed. The paper only applies for the case of unstayed mast, with a very different correlation between weight and bending moment.
    In a stayed mast, the weight is mostly influenced by stiffness to avoid in column buckling, not stress as it is in case of unstayed masts or wings of an aircraft.
    Hence the longer mast with bell spanloading will not be lighter as assumed, it will be heavier.
     
  9. lunatic
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    lunatic Senior Member

    Seems past research involves aircraft, where lift and control must be balanced against thrust, if thrust were the main objective, it might lead to some interesting upwind rigs.
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    HJS: The wind gradient helps the case for a triangular sail, as it loads more the head. Fastsailing: Yes, masts are stayed, the longer mast is heavier and that's a disadvantage. You don't associate the airplane wing's bending moment with weight, but with heeling moment, which is an issue for most boats in say over 10-12 knots of wind. Lunatic: In light winds, the criteria for sail efficiency is indeed thrust, with a little penalty on sideforce, as it increases leeway. In light winds, an elliptical lift distribution is desirable. In heavier winds, performance is governed by the ratio thrust/heeling moment, and here bell-type span loading has an advantage. Sailors handle this trade-off with sail trim: In light airs, the sails are fuller in the head and untwisted, for a nearly elliptical loading, and in heavier winds the sails are flattened and twisted off in the head.
     
  11. lunatic
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    lunatic Senior Member

    I assume the trim recommendation to have the top leech telltale not stream aft is an indication of upwash, not stall. Nice to see a vortex, even if weakened, manipulated to advantage, if only the intense tip vortex could be manipulated onto a forward facing surface. I posted a thread "lift without downwash" seems there is lift with upwash, at least outboard in bell span loading.
     
  12. CT249
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    CT249 Senior Member

    For almost all of the time that people have been researching aircraft, they have also been researching sails. Starling Burgess was an aircraft pioneer; Ted Wells, Snipe champion, designed a couple of the world's most iconic private planes; windsurfer inventor Jim Drake was an aeronautical engineer who was involved in the X-15 rocket plane and the Tomahawk cruise missile.

    Guys like that wouldn't have been ignoring major basic differences between aircraft design and sail design, would they?
     
  13. lunatic
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    lunatic Senior Member

    In the NASA paper there are associations with bird flight yet the wing tested and bird wings are quite different. Fig. 2 shows a bulbous LE, perhaps exaggerated, as continuous to the tip, in birds, the handwing has a sharp LE. The herring gull has some impressive gliding abilities especially in turbulent conditions, impossible to judge ambient air flow but long periods of thrust seem to be produced. 2/3 of the gull's outboard wing has a sharp LE and often sweptback, good, depending on AOA, for a high lift and drag LEV. Is another load distribution working here? Another difference: few, if any birds have as high an AR as the model tested.
     
  14. Erwan
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    Erwan Senior Member

    Thank You Mikko and everybody for taking time to post.
    I think the document attached will provide an academic approach of this issue, it was probably downloaded here on BDN.

    The main differencve between boat and airplane is the moment of inertia which is a fonction of : f(0.5*M*R^2) with M in kg and R in meters/R= height of rig's CoG.
    That is why I believed a 22% greater span for the same weight would increase the Moment of Inertia significantly, and is likely to spoil the induced drag theorical advantage of this higher Aspect Ratio sailplan.

    Please find attached the workpaper above mentionned.

    Regards

    Erwan
     

    Attached Files:


  15. fastsailing
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    fastsailing Junior Member

    1) I do associate the airplane wing's bending moment at all sections with weight of the wing, but not the root bending moment alone. But surface area also affects weight of the wing as well as design G-loading, flaps, slats, etc.
    2) I do see heeling moment very relevant for most sailboats when wind is up, which for some boats might be 10-12 knots, while for others it might be 15-20 knots.
    3) I do not see any connection between 1 and 2 , except you talk about both in your post in the very same sentence, which makes no sense to me at this time. Perhaps you could explain what you intended to express?
    4) What advantage specifically are you talking about in the second part of your quoted text in bold, and compared to what? Surely linear span loading has more advantage for thrust/heeling moment criteria with optimal amount of span for the conditions than any bell-type. No non-dimensional span loading has any advantage, if amount of span is totally wrong for conditions with said criteria, which is presumably obvious to you as well.
     
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