Split or unsplit, that is the question

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  • 17 Jan 2020 22:33
    Reply # 8604856 on 8217505

    Then I see this wing (ignore the pint size jib). The big round pole in the middle must mess with the air flow. It seems that middle pole/mast moves from side to side to help change the wing shape for tacking. Now the wing is higher AR than most junk rigs, but that is about it (Video shows a bit more). These things can go somewhere around 4 times the wind speed because the hull to water interface has much less drag (static drag). I would think though that if they could go even faster by getting rid of that lump they would. Compared to these, all conventional boats are barges. But the sail smoothness seems to be one of the lesser concerns.

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  • 16 Jan 2020 10:18
    Reply # 8589498 on 8217505
    Anonymous member (Administrator)

    Len’s and Slieve’s writing below is very interesting.

    At one end of the scale are the glider planes with super-high AR wings, all smooth and polished, and with wing sections optimised to keep the airflow laminar over a big part of the wing’s chord. The price for that is that the wings can stall very suddenly, and this has lead to quite a few fatalities. In general aviation, such handling is not accepted, and since many wings are very smooth nowadays, quite a few of them have being retrofitted with vortex generators at the outer half of the wingspan. In other words, they sacrifice some low friction to higher alpha-tolerance to avoid fatal wingdrops during landing. I don’t know how much the cruising speed drops because of these vortex generators.

    At the very other end of the scale sits the cambered panel JR. From an air molecule’s perspective, passing over a 5m chord of such a sail must be like running a 500cm hurdle race.
    My guess is that this is why my sail, with its ‘hopeless’ leading edge, still is quite tolerant to high alpha. I relax about this. A sail of most makes have a very poor lift-to-drag ratio compared to most aeroplane wings, rarely better than 4:1, while even a tired old Cessna 172  wing has  a L/D of around 8 to 10.

    My armchair hunch is that the dominating cause of the difference between the Cessna wing and the JR is the difference between induced drag, which probably dominates over friction drag. The junkrig’s low AR and high angle of attack is the big L/D-killer, not surface friction and parasitic drag.

    Arne


    Last modified: 16 Jan 2020 12:48 | Anonymous member (Administrator)
  • 15 Jan 2020 11:24
    Reply # 8579560 on 8217505

    Len Ovens wrote - “You have sparked another dumb question in my mind. How "attached" is the air flow anyway? Is that a good thing?”

    No Len, it is not a dumb question. In practice it could be the the answer to a lot of the questions being asked in this thread.

    In conversations with world class glider pilots I have learned that they get upset if you put your hands on the leading edges of their wings as you will leave greasy fingerprints, and that could ruin their performance. Apparently they are first aware of rain when the first drop of water hits the leading edge and a vibration due to upset flow can be felt through the airframe. As for bugs contaminating the wings, wow! This suggests that they are working with boundary layers only millimetres thick. With rough material sailcloth, seams and the such like on a junk rig we can at best be thinking in terms of boundary layers some centimetres thick, or worse. So how does that effect us?

    Arne is right when he says that I concentrate on the camber at the leading edge, and if I can get a good attached flow over the first section of the rig it will help give good performance. When the luff telltales fitted some 20 to 30 cm back stream cleanly then the boundary layer there must be fairly thin, the the leading edge working well, and the performance is good.

    With the lower sail balance on the non-split cambered sail things are somewhat different. In the past there has been talk of the good tack and bad tack issue. With the rig set on the port side of the mast and on starboard tack the camber can set smoothly but the full drag of the mast must be added to give the overall performance. On port tack the cambered surface may be distorted by the mast but the mast drag may be significantly reduced then the overall performance may be just as good. There are reports of the 'bad tack' being better than the 'good tack' so perhaps we should look more closely at this situation.

    Could it be that Len's comment about 'attached' flow has an answer? The boundary layer must thicken very quickly with the rough surface and many irregularities of the junk rig, but that does not necessarily say that the rig is stalled and losing the large part of its lift. When Arne sails with the leech telltales streaming then the rig is clearly not stalled since a cambered foil stalls as the separation bubble comes forward from the trailing edge. Could it be that a thick boundary layer keeps the overall flow attached and lets Arne's rig keep good flow over the full surface of the rig, despite the 'contamination' of the mast near the luff? It's a difficult one to understand, and would require much effort and time to research, but it would appear that we can look differently at the split and non split cambered rigs.

    Having been fortunate to have sailed on Johanna a couple of times it is not possible to say which gives the best performance. Only by building two rigs and fitting to identical hulls and sailing them together would we find the answer. Even then we would still have to evaluate the different physical characteristics.

    Until we do that then we will always be asking the question, to split or not to split. Perhaps we should concentrate more on mast position or ease of handling when selecting a rig.

    Cheers, Slieve.

  • 13 Jan 2020 17:45
    Reply # 8561789 on 8558637
    Arne wrote:PS: I have no idea of how the airstream can re-attach on the leeside of my cambered panel junksails, on port tack, but the telltales at the leech anyway say that it does.

    You have sparked another dumb question in my mind. How "attached" is the air flow anyway? Is that a good thing?

    I don't see that a junk sail with not only seams but lazyjack/sail catcher lines on the surface as being a laminar flow wing. Perhaps the air flow stays quite close to the sail but the attachment is quite loose such that so long as the airfoil is "about" the right shape, things work. Perhaps being only loosely attached is a good thing creating less drag? I know an air layer between the hull and the water in a planing boat does this but am not sure how well this translates to a sail... maybe not at all. In fact this may explain why a flat sail still goes to windward at all.

    I am remembering that a sail is a low speed airfoil. Most planes that use a laminar wing need to go faster than any wind I would care to be on the water in just to get off the ground.

  • 13 Jan 2020 09:08
    Reply # 8558637 on 8217505
    Anonymous member (Administrator)

    David,
    You surely are right there. I don’t expect to perform that well to windward, offshore. That is badly uncomfortable, anyway, so better avoid it if one can ( yes, even I have tried it...). Besides, the shape (and size) of the vessel plays a bigger role for going to windward, offshore.

    Still, on our sheltered fjords, the fetch may be up to 10-15 miles on places, and a very steep, short chop may build up rapidly. This chop can stop a small boat dead. Again, the hull shape is as critical as the sail.

    Ingeborg, with her slim lines and large, heavy keel, is well suited for these conditions. In addition, the ‘groove’ of my sail seems to be just as wide as on a Bermudian sloop, so it doesn’t  add to the problems.

    Arne

    PS: I have no idea of how the airstream can re-attach on the leeside of my cambered panel junksails, on port tack, but the telltales at the leech anyway say that it does.


    Last modified: 13 Jan 2020 15:22 | Anonymous member (Administrator)
  • 13 Jan 2020 08:35
    Reply # 8558456 on 8554934
    Rudolph wrote:
    Arne wrote:

    I focus on the trailing edge...

    You are right Arne, keeping the leech telltales flowing means your angle of attack is ok. So that's the most important part of getting good performance. But by making the luff of the more tolerant for small windshifts (or negligence at the sheet) you would have even better performance as the sail would be more forgiving.

    I do agree that the result of all this may be small, but in pointy sails all these small improvements together make the difference when racing. Cruising a very different game.

    Rudolf


    ... and the other factor to be taken into account is pitching in a seaway, something that doesn't affect Arne too much, sailing as he does within Stavanger Fjord. Pitching causes large variations in alpha, particularly near the top of the sail, and good alpha tolerance is to be desired. A sail that is more forgiving , less demanding, in a seaway is my aim, just as much as that last few % of windward speed. As a cruising sailor, I want to know that I can engage the vane gear and leave the boat to sail herself, confident that she's going well.

  • 12 Jan 2020 22:18
    Reply # 8554934 on 8554338
    Anonymous wrote:

    I focus on the trailing edge...

    Here we go again! David and Slieve agree on the importance of having a well rounded leading edge on the sail, this or the other way.

    I am not so sure about that. The leading edge of my sails should then be aerodynamic nightmares:

    ·         On the port tack, the mast turns my sail into a flat junksail, made worse by the Hong Kong parrels.

    ·         On the SB tack, the sail is undistorted, but the big mast is standing proud in the wind.

    Still, my boats with cambered panels points well, and the VMG to windward is less than 5% worse than in a well-sailed Bermudian sister boat.

    To get the best out of my sails, I keep an eye on the leech telltales. With attached airflow on both sides, the telltales are standing fine, despite that distorted luff. Best of all; it is not difficult at all to keep the boat somewhere in the groove. The sail’s alpha-tolerance must therefore be OK.

    It could be that the lack of the last 2-5% speed to windward is caused by my not-advanced (or primitive) leading edge. Still, I stress that the big gain in performance to windward (10 – 50%) is achieved when replacing a flat sail with the cambered one. Anyone who have sailed their boat with a flat sail and then with a ‘plain’ cambered one, will verify this.

    Improving the airflow over the leading edge, and hiding the mast, may possibly provide the final 2-5% improvement. I wouldn’t expect more than that.

    Arne


    You are right Arne, keeping the leech telltales flowing means you angle of attack is ok. So that's the most important part of getting good performance. But by making the luff of the more tolerant for small windshifts (or negligence at the sheet) you would have even better performance as the sail would be more forgiving.

    I do agree that the result of all this may be small, but in pointy sails all these small improvements together make the difference when racing. Cruising a very different game.

    Rudolf

  • 12 Jan 2020 21:52
    Reply # 8554780 on 8553051
    Anonymous wrote:

    Oops Len! Could it be that you are sitting too deeply in your armchair and thinking too deeply?

    Well I figured that was probably the case which is why I warned people to take it that way :)

    Saving space by not repeating stuff...

    important. A lifetime in aviation tells me that leading edge devices produce high lift at high angle of attack and that where trailing edge flaps may produce a small increase in lift, they always produce a greater increase in drag and a decrease in the L/D ratio (area for area, as some flaps increase the area before angling downwards).

    Overall it would appear that it is the leading edge shape that effects the upwash and allows the higher α-tolerance, so that seems to be the best area for experimentation.

    Cheers, Slieve.

    That is the best part at the bottom. The SJR is closer to a leading edge device than a jib might be. With the wind on the beam, the jib is far away enough from the main that they start to act more like separate airfoils (a biplane even) but the SJR always presents the two parts of the sail as part of the same wing with the jiblets  acting as a part of the main rather than on their own at all times. It is a different animal.

    Thank you for clearing that up. It has made it much easier to let go of the whole line of thought... here I was thinking I might be able to get away with one pattern instead of two  :)

  • 12 Jan 2020 20:34
    Reply # 8554338 on 8217505
    Anonymous member (Administrator)

    I focus on the trailing edge...

    Here we go again! David and Slieve agree on the importance of having a well rounded leading edge on the sail, this or the other way.

    I am not so sure about that. The leading edge of my sails should then be aerodynamic nightmares:

    ·         On the port tack, the mast turns my sail into a flat junksail, made worse by the Hong Kong parrels.

    ·         On the SB tack, the sail is undistorted, but the big mast is standing proud in the wind.

    Still, my boats with cambered panels points well, and the VMG to windward is less than 5% worse than in a well-sailed Bermudian sister boat.

    To get the best out of my sails, I keep an eye on the leech telltales. With attached airflow on both sides, the telltales are standing fine, despite that distorted luff. Best of all; it is not difficult at all to keep the boat somewhere in the groove. The sail’s alpha-tolerance must therefore be OK.

    It could be that the lack of the last 2-5% speed to windward is caused by my not-advanced (or primitive) leading edge. Still, I stress that the big gain in performance to windward (10 – 50%) is achieved when replacing a flat sail with the cambered one. Anyone who have sailed their boat with a flat sail and then with a ‘plain’ cambered one, will verify this.

    Improving the airflow over the leading edge, and hiding the mast, may possibly provide the final 2-5% improvement. I wouldn’t expect more than that.

    Arne


  • 12 Jan 2020 19:46
    Reply # 8554053 on 8217505

    Rudolph,

    In between each batten of my wingsail, I put three of these luff formers, which only weigh ~22 grams:

    This may be a way into an experiment. I can sort of visualise how it might be done, but it would be a complex bit of sailmaking and would need to be done accurately. As ever, the Pareto principle comes into play: that, for many events, roughly 80% of the effects come from 20% of the causes. That is, you have to put in a lot of extra work/time/expense/complexity/weight/other negatives, to get that final 20% of available effect - in this case, performance to windward. 

    Last modified: 12 Jan 2020 19:48 | Anonymous member
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