Aerodynamics of 'overlapped' staysails

Discussion in 'Hydrodynamics and Aerodynamics' started by Revboat, Jun 23, 2021.

  1. Revboat
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    Revboat Junior Member

    I'm working out the sail suite for a Class 40 under construction, and am wrestling with a problem that seems to bother no one but me.

    The problem is that it's common for racers to use multiple headsails in such a way that the leech of the A-sail extends further aft than the leech of the aft-most headsail -- thereby ruining the various 'slot effect' benefits of the inner sails. It doesn't seem sensible aerodynamically.

    Since reading Gentry, my (amateur) understanding is that the leech of the trailing sail must extend further aft than the leech of the leading sail, or else the singular foil is compromised. A staysail that is 'buried' by the leech a big A-sail might help a bit with mainsail flow attachment... or might reduce the drag of the mast... but is more likely to simply add drag itself or to generally interfere (particularly if its twist cannot be controlled to match that of the A-sail and main, which is usually the case in the low clew jibs and genoas used on most racers).

    Staysails that are 'overlapped' by A-sails don't add upwash, lift, etc.

    But no one seems to pay attention to this. Very smart people are violating it everywhere -- on Maxis, VORs, Class 40s...

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    With a Class 40 on a TWA broad reach, it's typical to furl the Solent so as to not blanket the A-sail, and to sail with the overlapping A-sail and 'buried' staysail as in the above photo. But this can't be right.

    What we should see instead is a large staysail with perhaps a fairly high clew so the foot and LP can be extra long, extending beyond the leech of the A-sail and overlapping the main quite a bit. Or so I think.

    But so far, I'm only getting blank stares and shrugs from designers. Conventional practice ignores the issue. I just can't figure out why I'm wrong.

    Thoughts and/or recommended resources very much appreciated.
     

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  2. Remmlinger
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    Remmlinger engineer

    I think you described the problem well. Search the forum for comments by Mark Drela. He explained the theory and benefits of slots:
    sail aerodynamics https://www.boatdesign.net/threads/sail-aerodynamics.457/page-36#post-744698
    sail aerodynamics https://www.boatdesign.net/threads/sail-aerodynamics.457/page-18#post192017
    sail aerodynamics https://www.boatdesign.net/threads/sail-aerodynamics.457/page-19#post192023

    The grand-prix-racing teams use sophisticated CFD and many computer hours to investigate the interaction of sails. Essential is the correct modelling of the boundary layers.
    The only member in this forum I know of, who can handle this kind of CFD, is Mikko Brummer.
     
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  3. Doug Halsey
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    Doug Halsey Senior Member

    Also essential is the precise modelling of the shape, which changes continually with every variation in wind conditions & sail trim and is not generally known very accurately.

    Even the most talented CFD practitioners would be hard-pressed to improve on the results obtainable from more empirical approaches.
     
  4. Revboat
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    Revboat Junior Member


    Thanks, helpful stuff. Somewhat clarifying, although I find myself thinking in circles a bit.

    Slots and shape of slots/overlaps don't determine lift but rather the aft-most section of the foil determines lift -- or, perhaps better said, realizes the lift of the overall foil. OK.

    But also... in that "(t)he primary function of slots is to better manage boundary layers, and thus allow larger rear-sheet incidence angles and hence larger lift without separation," the slots do help determine what happens with that aft-most foil section... which determines lift.

    We know from experience that slots change effective angles of attack (and thus boost lift) -- but I guess the proper way to think about why is to focus on boundary layers. OK.

    Still sounds a little like upwash and downwash to my primitive mind, but OK.

    That still leaves questions of whether staysails, overlapping foresails, or parallel leech twists, etc. do a better job of managing boundary layers than alternatives, or perhaps when they might do a better job.

    Dr. Drela indicates that overlap of foils in the sense I'm talking about makes no appreciable difference -- e.g., aircraft foils barely overlap. He further indicates that staysails have a very small percentage lift effect per sail area increase. OK. (Although I notice the model analysis he offers in the discussion doesn't involve much overlap.)

    But even buying that, I can't figure out why so many boats are using these entirely overlapped staysails -- often multiple entirely overlapped staysails -- except maybe to reduce profile drag from the mast, which seems like not much benefit given all that sail area and complication. These Grand Prix yachts must be seeing some payoff.
     
  5. Revboat
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    Revboat Junior Member

    Thank you, Doug. Somehow it comforts me to think that empirical approaches have their place! (I also take to heart your comment about modeling of shape. When I stumble through 2D foil analyses, I often think about twist and how hard we work to manage it and parallel it from one sail to the next, and how we cut sails to make it just so, and what profoundly complicated shapes it creates in our sails -- and how birds' wings are full of it and aircraft wings have none of it.)
     
  6. Remmlinger
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    Remmlinger engineer

    I would not additionally complicate the problem. I would at first limit the analysis to the flying shape. How this shape is achieved by the sailmaker and trimmer is another question and a problem in itself. It helps in the analysis, if these two problems are kept separate.
     
  7. Remmlinger
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    Remmlinger engineer

    This is the essential effect. Multi-element airfoils allow larger angles of attack without separation and therefore can create larger lift.
    There is the famous 8-element airfoil by Handley Page, depicted in fig.4 in the attachement. It achieved a lift coefficient of almost 4.
    In this paper: https://openaccess.city.ac.uk/id/ep...igation of a Triple Slotted Aerofoil_2016.pdf
    the measurements show how the boundary layer is energised and the importance of the position of the slot.
     

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  8. Doug Halsey
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    Doug Halsey Senior Member

    I'm not sure the triple-slotted airfoil is a good one to reference, since the slots are just cuts through an existing single-element airfoil. A rig with overlapping staysails would have a new boundary layer starting at the front of each individual sail, rather than a single boundary layer from front to back of the entire rig. Likewise for most high-lift systems on commercial aircraft, where care is taken to position the elements to prevent the merger of the boundary layers of the separate elements, which would be highly undesirable.

    In any case, you wouldn't necessarily think of the boundary-layer being energized. That comes dangerously close to the mistaken idea that the slots cause some sort of jet effect. The first paper you linked does actually mention jets coming through the slots, which is a very misleading characterization. AMO (the author of the attached PDF) would not approve!
     
    Last edited: Jun 24, 2021
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  9. Remmlinger
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    Remmlinger engineer

    The word "energized" in the linked paper is may be not an appropriate description, but let's not get into semantics. I was interested in the measurements. The crucial point is the fact (as you say) that at each element, whether sail or airfoil, there is a stagnation point from which a new boundary layer develops. This new b.l. is thin and in addition, the staggerd foils or sails can create a (potential) flow-field with mild pressure gradients on each element. This combination of optimized pressure gradients and thin b.l. enables large angles of attack without separation and therfore large lift coefficients. I hope this is the explanation Revboat was looking for.
     
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  10. Revboat
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    Revboat Junior Member

    Thank you, both. Super helpful.

    That Handley Page diagram has always seemed like the Rosetta Stone to me. To the sailor's eye, it's easy to see the eight elements as overlapping foresails working as a unitary foil, and to understand the relation of progressively greater AoA's to lift.

    The Triple Slotted Airfoil article had great visualizations of boundary layers effects. In so far as it seems focused on 'energizing' velocities, it does tend to sound like a discussion of slot jets. But... I recall that even the old Smith lecture had a graph of how edge velocities get 'energized' (not his word) via the foil slots. I'm not smart enough to know the nuances in conceptualizations -- one man's velocity is another man's pressure. But the fundamental point lies not in the velocity boosts themselves, but in the flow attachment. Got it.

    I find it a very effective summary explanation!

    I still have my problem about why so many yachts use 'buried' staysails -- which should not work! The boundary layer approach seems to confirm this even more.

    'Staggered' leeches, in which the leech of the trailing sail always extends beyond the leech of the leading sail, will work. Multiple element sail foils in which the leech of the leading sail doesn't quite reach the luff of the trailing sail might also work (nod to Dr. Drela). But arrangements in which the middle sail is entirely buried by the lead sail, should not add to lift. And yet, they're enormously popular. This bugs me.

    The point about hours of Grand Prix CFD analyses stands: there must be something good happening wth those buried staysails! But surely it has nothing to do with overall CL.
     
  11. Rumars
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    Rumars Senior Member

    I think it's easier to understand if you find pictures showing the same configuration from the lee side. There is a huge gap between the A-sail on the bowsprit and the mainsail, not only fore-n-aft, but also sideways, and down low (from the high clew of the A-sails). There is no chance to have the right slot effect if you only flew the A-sail and the mainsail, you need something in between. The staysail provides exactly that, it cleans the airflow over the mainsail. Staysail drive is secondary here, the main does the work and is to be kept happy at all times, hence the blade, non overlapping staysail you see on all this boats.
    On a class40 there is probably no benefit to also fly a second staysail/jib at the same time, Open 60's and longer can do it in specific conditions.
     
  12. TeddyDiver
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    TeddyDiver Gollywobbler

    First, IMHO, there's no such thing as "slot effect" with soft sails. Second: More sails have higher lift coeffience with the cost of added drag. Overlapping or not has nothing to do with it.
     
  13. Mikko Brummer
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    Mikko Brummer Senior Member

    I concur with @Rumars. The buried staysail helps the mainsail to maintain attached flow, without disturbing the too much the large A-sail. The inhanced flow on the main may help the large A-sail in return. I've had my doubts about these staysail, too, I think a complete 3D simulation would be needed to know better.
     
  14. Remmlinger
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    Remmlinger engineer

    I have tried to make a sketch for the case of two and three sails (attached). The additional staysail allows a closer sheeting angle of the main. On the rear suction side of the main the pressure is negative and has to increase to zero at the trailing edge. This steep pressure increase can cause separation. The additional staysail has its pressure side towards the suction side of the main. The positive pressure induced by the staysail will reduce the negative pressure peak at the main. Therefore, the pressure increase towards the trailing edge at the suction side of the main will be less steep and separation will not occur. The result is, that the sheeting angle and hence the lift coefficient of the main can be increased.

    I cannot prove this with data. XFOIL cannot calculate multi-element airfoils and the other available codes (AVL, XFLR5) assume inviscid potential flow, which is not applicable to sails. Only a 2D-CFD-code (RANS) could prove my theory.
     

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  15. Revboat
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    Revboat Junior Member

    Yes, I can imagine how the small staysail improves attached flow on the main... but the key, as you say, is that it must not disturb the A-sail too much. Conceptually, one could think of the A-sail as one element, and the Staysail/mainsail as one element, and the overall rig as a two element foil. (I believe I recall a wind tunnel experiment in which a small staysail was tacked off-center to windward and evaluated as an 'extension' of the main to good effect.)

    If this is the case, it seems to me that the design of that staysail should be quite particular. A high aspect blade, tacked fairly far aft, etc. Maybe, as Rumars indicates, it is designed as a 'gap-filler.' Makes sense.

    However, it seems to me that the triple foresail rigs on these Grand Prix boats are something different. It really seems as if they're 'slotting' these staysails in the classic sense.

    Rumars, could you say more about the "specific conditions" you mention? E.g., certainly point of sail generates differences in terms of risk of interference among sails. Is that what you mean?

    I'm glad that even experts are bothered! Makes me feel better.
     
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