Can superstall apply to sailing

Discussion in 'Hydrodynamics and Aerodynamics' started by lunatic, Mar 6, 2011.

  1. lunatic
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    lunatic Senior Member

  2. Tim B
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    Tim B Senior Member

    Does nobody bother thinking about the questions they post? Unless I'm much mistaken, we usually try to reduce separated flow. Deep stall condition produces huge separation, and consequently immense drag. Ideally, we'd like attached flow on everything and minimum drag all the time. Failing that, we'll live with drag as long as it's contributing positively to driving force (eg. sails when sailing dead downwind).

    So no, I wouldn't recommend it. With aircraft, if you can maintain the deep stall stably (I have done it on a tailless RC glider) then what you get is a very steep descent, which could be very useful for STOL operations, but is not appropriate for us.

    Tim B.
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    In short, no, probably not. :)

    There might be some advantages if the flow separating from the leading-edge and
    side edges can re-attach on the sail. This so-called "vortex lift" is important
    on fixed low aspect ratio delta wings, however, it is not all that apparent
    on high aspect ratio cambered "wings" like yacht sails.
    (I'm not sure if this is exactly the same as your term "super stall".)

    There are some hints in the short compendium for flat wings I attached to
    a post in another thread:
    http://www.boatdesign.net/forums/sailboats/myth-aspect-ratio-36836-7.html#post446638

    If you look at Figure 16, you will see that the lift coefficient for flat
    delta wings is much greater than the "potential" lift for low aspect ratios,
    but the advantage decreases as the aspect ratio increases.
    (Yacht sails have an even larger aspect ratio than the wings in those plots.)

    There are many complicated reasons for this behaviour. If you are a really
    keen nerd, search for papers by W. H. Wentz Jr. on the NASA Technical
    Reports Server, e.g.
    http://naca.larc.nasa.gov/search.jsp?No=0&N=4294950890&Ns=ArchiveName|1&as=false
    There are also some recent papers put out by organisations like AGARD
    and NATO.

    Of course, the situation is far more complicated on real, cambered,
    yacht sails (which are similar to half delta wings), but I think some of the same
    sort of arguments hold.

    Good luck!
    Leo.
     
  4. tom28571
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    tom28571 Senior Member

    In some cases you want high drag along with lift (landing) and is some cases you tolerate high drag to get high lift (takeoff) but neither is good for efficient lift in a wing or a sail.
     
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  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks for the clarification, Tom.
     
  6. DCockey
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    DCockey Senior Member

    I suspect sailors have been trimming sails in something like a "super stall" condition for a very long time on some points of sail. Based on the description in lunatic's attachment "superlift" is not an unusual phenomena which requires a particularly special lifting surface, but rather what happens with a high angle of attack, particularly a lifting surface with considerable camber such as a sail.

    A note on the definitions of aerodynamic lift and drag as I'm going to use here. Drag is the component of force in the same direction as the apparent wind. Lift is the aerodynamic force perependicular to the apparent wind. Lift from sails is generally much closer to horizontal, parallel to the water, than vertical as long as heel angle is relatively small. Any vertical component is neglected in the following analysis.

    When sailing directly downwind optimum sail trim will maximize aerodynamic drag of the sail. Any lift force will tend to heel the boat and is not beneficial. Generally optimum sail trim directly downwind will have separated flow.

    When sailing close-hauled aerodynamic lift provides the driving force. Drag both opposes the forward motion of the boat, causes leeway and a heeling moment. Optimum sail trim is close to that which maximizes lift and minimizes drag. At apparent wind angles closer than 45 degrees optimum trim will generally be at a lift less than that of the maximum lift to drag ratio because an increment of drag takes away from the driving force more than the same increment of lift contributes to it.

    When sailing on a reach (apparent wind abeam) aerodynamic lift provides the driving force. The inevitable aerodynamic drag pushes the boat sideways and tries to heel it but it does not directly oppose the forward motion of the boat. Optimum sail trim will be that which results in the most lift possible without too much drag producing excessive heeling from aerodynamic drag. (I'm assuming the boat has an efficient keel/centerboard.) This is likely to be with separated flow from the leading edge which reattaches along the sail.

    Now, let's consider sailing on a broad reach, with the apparent wind between abeam and aft. Here the both aerodynamic lift and drag provide both the driving force while pushing the boat sideways and trying to heel it. Optimum sail trim depends on the angle of the apparent wind. As the apparent wind moves aft lift becomes less "good" and drag less "bad". With the apparent wind at 45 degrees aft of abeam (135 degrees from the bow) equal magnitude increments of lift and drag contribute equally to both the driving force, and leeway and heeling. So optimum sail trim will generally have separated flow while still producing lift.

    Conclusion - a condition similar to that described as "superlift" is probably the optimum on a reach to a broad reach.
     
  7. lunatic
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    lunatic Senior Member

    Some clever telltales might reveal if "superlift" actually exists in sailing and its effect on performance. My question arises from a slow realization that vortices are ubiquitous and spontaneous but hard to exploit in sailing. An interesting non sailing example at ht
    tp://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19860017728_1986017728.pdf I have sailed heavily telltaled, LEV driven, low aspect, sweptback rigs. Some were a delight to sail despite their flow patterns, which I hope to post soon.
     
  8. DCockey
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    DCockey Senior Member

    There may be two different phenomena being discussed in this thread. The references in the original post are for wings which are close to unswept and at very high angle of attack, which have massive separation from the leading edge which reattachs to the upper surface. The separation results in a large separation "bubble" on the upper surface. The flow in the bubble has large recirculation and is highly turbulent. The flow just above the wing surface is upstream over a significant portion of the bubble.

    The second phenomena occurs at high angle of attack due to separation from highly swept leading edges or wing tip. A shear layer from the leading edge or tip rolls up into a longitudinal votex above the surface and inboard of the leading edge or tip. The flow under the vortex is generally attached with no reversal of direction. This phenomena is frequently assumed to be essentially inviscid for purposes of analysis and modeling. I believe this is the phenomena which Leo discusses in Post #3.
     
  9. lunatic
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    lunatic Senior Member

    Yes, there are 2 different vortex systems. The first is on wings of moderate AR and sweepback, some is needed for vortex drainage? This is a planform close to some sailboard rigs. Where is the downwash in this condition? In best sailing of second (Leo) phenomena a half delta will have a spanwise flow approx 90* to freestream, even pulling TE streamers up into LEV. Confusing but fun to sail? What is going on here?
     

  10. DCockey
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    DCockey Senior Member

    The "vortex" system in the first can be essentially a ring within the separation bubble. The may not be any trailing vortex "drainage", at least in a reality. Even if there is a trailing vortex from the sides no leading edge sweepback is required, at least in reality. Some approximate models may need it to avoid degeneracy. The flow in the separation bubble is very far invisicid, even though it would be much simplier to model and predict if it was.

    The concept of downwash is difficult to extend to such flows.
     
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