Aerofoil shapes and entry angles

  • 02 May 2019 22:30
    Reply # 7317309 on 7302245

    Andrew: From the context, I presume "that balance" in your post does not refer to what is normally meant by "balance" in a junk rig, but rather you are referring to a comparison between the camber in the mains and the camber in the jibs.

    The combinations seem to be a little different in each of the cases which have been reported, the only consistency being that the camber in the mains is usually less than the camber in the jibs. Slieve and Edward, who have done the development work, seem to have progressed their ideas since Slieve's notes were first written and I think the latest thinking is 10% in the jibs and 7% in the mains. I have an idea it is not a crucial question, as other numbers also seem to have been used satisfactorily.

    "Balance" as the term is normally used is somewhat more crucial, so if you are in any way unsure about that, you had better start by reading Slieve's notes (which may be found here https://www.junkrigassociation.org/technical_articles and an important clarification by Slieve in the reply #4850273 on 4793670 24th May 2017 to be found at https://www.junkrigassociation.org/technical_forum/4793670?mlpg=3&rid=4850273#4850273

    A couple of points interested me in the link which Andrew provided (http://jeremyulstad.com/2014/01/split-junk-part-3/) - the evolution of a polytarp split junk sail by CAD which was done in 2014. (1) The balance was reported as 35% which must be near the upper limit - and (2) the sheeting angle for the jibs, which was not discussed, looks from the photographs to be miniscule.

    Martin: Your idea for  "inhauls" as a way of playing around with the jibs, will probably have the effect of changing camber, shape and possibly sheeting angle all at the same time. I have been thinking along somewhat different lines, as I just want to tinker with sheeting angle, keeping the other variables constant if possible. It is sheeting angle which seems to be the least understood of the variables, and it may not even make much difference. It is also still unclear if "slot effect" plays much of a part in this sail set-up - quite possibly not. 

    Incidentally, I would be interested to know what sheeting angle is proposed by Martin for his barrel cut jibs, and if the "foots" of the jibs are attached to the battens, what shape will be used to achieve it. I have an idea it might be better to stick to "shelf foot" for the jibs.

    Recent correspondence with Slieve confirms that he and Edward are preferring larger sheeting angles.

    Last modified: 03 May 2019 01:43 | Anonymous member
  • 02 May 2019 15:23
    Reply # 7316522 on 7302245
    Deleted user

    Funny, another programmer & junk rig enthusiast (I am as well). Another programmer wrote up a python/FreeCAD to draw out the split junk rig. Also introduces the "agile" boat-building concept which I like. His uses the angled shelf-foot, whereas yours uses the barrel cut. It would be interesting to compare the 2.


    I am still wrapping my head around all the numbers being used. Jeremy used 10% camber for the jiblets and 4% for the main sail side (main-lets?). I've mostly heard 10% as the goal for a non-split rig, based on bermudan sails camber of 8-15%. Does anyone have thoughts on what that balance should be with the split version?

  • 02 May 2019 10:48
    Reply # 7316116 on 7302245

    I've written a blog post about how I calculate camber for a junk rig sail using barrel cut panels and a NACA profile. I used this method on my previous dinghy but lost the spreadsheet so this is a new version for the new dinghy. Comments are welcome - particularly if you can spot any errors!

    Post is here: http://mwbrown.org/2019/05/02/calculating-junk-rig-sail-camber/

  • 30 Apr 2019 22:14
    Reply # 7312766 on 7302245

    On the subject of adjustable sheeting angle - I have been wondering whether this could be adjusted by using a barrel-cut sail and an 'inhaul' (replacing the outhaul on a bermuda rig). With barrel cut cloth it is necessary to slacken the sail along the battens to allow the camber to form. By pulling the cloth tighter or slacker along the battens it should be possible to adjust camber during sailing. However my brain can't figure out what effect this would have on the jiblet angle - presumably some effect though not large.

    My current plan is to close the outer ends of the batten pockets and use thin line to pull the jiblet part of the sail towards the main part of the sail - an 'inhaul'. If that line is brought down to the mast base somehow then dynamic camber adjustment should be possible. First version will probably be fixed during sailing.

    Experiments are needed! Fortunately the mast is nearing completion so I hope to have a play with the sail soon.

  • 30 Apr 2019 16:01
    Reply # 7312028 on 7308136
    Anonymous wrote:

    I made a dummy for jiblets and one for mainlets and what you are looking at there is: I just threw them on the table one behind the other to see what it looked like. I suppose it makes an interesting adjunct to the 3-d work you have been doing in cyberspace (which is far more powerful, of course.)


    Graeme--

    The mesh plot in pic 1 is as close to 3D (split junk jiblet) as I get until I sew up the cloth. Pic 2 is a jpg of the CAD file (3 pieces that will be sewed together along the seam lines) that is emailed to the canvas shop to be cut out. I get the pieces in the mail and sew it together.

    Last modified: 30 Apr 2019 16:11 | Anonymous member
  • 29 Apr 2019 18:53
    Reply # 7309692 on 7308136
    Graeme wrote:

    By the way, in regard to sheeting angle, you wrote: 

    A downside for split rig is that it must be fixed.

    I am not sure if you are right about that. I am thinking of having a little play around with sheeting angle - at first I thought this might be possible only with a horizontal shelf configuration, which I would also like to try - but I have a hunch now there might also be a way of making an angle shelf jiblet with an adjustable sheet angle - practical at least for a little sail with a trial panel. I am half way through making my sail now, and already it is headed for the waste bin - but before it gets there, I might use it to try a couple of things.

    Well, I think there is fixed and there is fixed  ;)  There is fixed at build time and there is fixed while sailing. I would guess it is possible to have mini battens that can change angle in real time but I think it would be too complex and not robust enough for that to be practical. All of the split rigs I have seen pictures of are fixed at build time. I can think of two ways of making the jiblet sheeting angle adjustable though I don't think the adjustment could be changed while the sail is is in use. The first is to use a "hing system" much like Mingming 2 where the hing pieces are adjustable length (line instead of fabric?). The other way is to put a line of gromets on the sail foot in line with the cord and "reef" it varying amounts tighter either with individual ties or with one line zigzagged through. I think the zigzag method would tend to be a two setting approach because I think any other setting other than fully loose or fully tight would tend not to keep the air foil correctly shaped. Of course with individual ties one could experiment with different foil shapes as well.

    The details of changing foil shape or sheeting angle are that for 5 panels there are 10 sets of adjustments to make and they have to be accurate. For each tie the length would be different less at the luff of the sail more at the leech. This would likely mean calculating the lengths for at least one sail foot ahead of time. I suspect making a chart ahead of time to try out different angles would make sense.

    What would the results that would be looked for expected?

    • power. Ya but that may not be the most important...
    • helm balance. This seems to be the most common reason for varying head to main sail sheeting.

    In the end I would guess it would be a mix of the two. Part of the reason for using a split rig is for more power from the same wind. However, by the time there is a sail on the boat, mast placement is probably pretty hard to move and with a split rig, moving the boom fore and aft is less of an option than with normal (whatever that is) junk rig. As such balance would become a greater priority.

    On sheeting angles and the split rig, It seems to me that when I look at boats with two or three head sails (those triangle things) the foresail farthest forward  seems to have the least difference from the mainsail with the chord of the airfoil being sort of in line with the leading edge of the mainsail. The smaller foresail closest to to the mast seems to rather have the trailing edge farther out so that the shape of this foresail if continued back would be parallel to the mainsail. So the closer the foresail is to the mainsail the greater the sheeting angle. Having said that, this aft foresail when used in this fashion does shadow the wind that the forward foresail sees to some extent. All of that to say, I would be interested in the difference jiblet sheeting angle makes.

  • 29 Apr 2019 01:35
    Reply # 7308136 on 7302245

    Yes Robert, that's right. It was at first just to get a feel for the geometry, having never worked with cloth before. The plywood dummies were quick and easy to make, and now serve a useful purpose in helping me (a novice) to baste the curved parts together.

    I made a dummy for jiblets and one for mainlets and what you are looking at there is: I just threw them on the table one behind the other to see what it looked like. I suppose it makes an interesting adjunct to the 3-d work you have been doing in cyberspace (which is far more powerful, of course.)

    The biggest mistake I made was not to bother about the width of the models (eg I did not bother about vertical height of panels, I was only interested in the curve of the seams.) This means having to shift the sail across and lay it all out again to do the other seam. If it is worth making a model (which it probably is not, for most people) then it is worth making it full size in all directions, which I have done on the most recent model. You can then build a sail panel on it all in one go. My little sail panel fits on a 8x4 sheet of plywood so for me it is a no-brainer to do it this way and it helped build up the confidence to start cutting cloth and actually making a sail.

    By the way, in regard to sheeting angle, you wrote: 

    A downside for split rig is that it must be fixed.

    I am not sure if you are right about that. I am thinking of having a little play around with sheeting angle - at first I thought this might be possible only with a horizontal shelf configuration, which I would also like to try - but I have a hunch now there might also be a way of making an angle shelf jiblet with an adjustable sheet angle - practical at least for a little sail with a trial panel. I am half way through making my sail now, and already it is headed for the waste bin - but before it gets there, I might use it to try a couple of things.

    Last modified: 29 Apr 2019 02:21 | Anonymous member
  • 29 Apr 2019 01:12
    Reply # 7308126 on 7307123
    Anonymous wrote:

    Split junk dummy in plywood.

    45 degree shelf.  10% camber, 12 degrees sheeting angle followed by 8 degrees camber, zero sheeting angle. I have no idea if this is an optimal combination.




    Hi Graeme--I must admit I'm not entirely sure I know what I'm looking at in these pics. But I'll take a guess that you have built a 3D mold of jiblets and main panel over which you stretch material; add 25mm seam width to the edges; cut the material out; sew the panels togather to make the final sail. If I'm correct, another unique approach to the problem.

    robert self

    Last modified: 29 Apr 2019 01:14 | Anonymous member
  • 28 Apr 2019 06:24
    Reply # 7307123 on 7302245

    Split junk dummy in plywood.

    45 degree shelf.  10% camber, 12 degrees sheeting angle followed by 8 degrees camber, zero sheeting angle. I have no idea if this is an optimal combination.




  • 28 Apr 2019 02:36
    Reply # 7306947 on 7302245

    Robert I envy the software skills you have at your fingertips and there is much we can learn from computer modelling. Its quicker for me to work in wood than find and get familiar with CAD software, so I have gone down a different road and mucked around a bit with thin plywood which I happen to have a stack of.

    I made a model of a jiblet with 8 percent of camber and 8 degrees of “sheeting angle”. Later I found out from Slieve that he and Edward had moved on since those figures were used, and the latest sail is 10 percent camber and 12 degrees of sheeting angle. I wasn’t so concerned about camber but did at the time wonder if sheeting angle might have been another factor contributing to the lee helm problem Bert had with his Farthing a couple of years ago (“Sail Balance – Position Relative to Mast” thread) so I decided to make another one, this time with the new figures.

    I was amazed at how different they looked and thought at first I must have made a mistake somewhere.


    Regarding exit and entry angles – by the time I made the models (pretty rough and ready) and one of them I had to tear apart and re-do – and due to the bending characteristics of the plywood, I doubt if they ended up how they were designed to be, so I tried to measure the result, approximately. The entry angles are about 30 degrees and the exit angles about 10 – 12 degrees – these are the angles between the tangents to the curve, and the plane of the sheeting angle (not as shown on the sloping side panels.)

    I am past fussing about trying to be exact, having been (correctly) diagnosed by James G (River Rat) as suffering from “analysis paralysis” – and I know that the hoisted sail won’t look like the plywood model even if my sewing is accurate (which it isn’t) – but I do think this theoretical messing around has value – we need a target to aim at, and we need numbers for comparison, or how can we make developments?

    I also had in the back of my mind, the daunting prospect of using basting tape (for the first time) and sticking cloth together around those curves, and although I was told it was not necessary, I thought a plywood model might be useful as a “taylor’s dummy” or three dimensional basting table. And indeed it was. Here is an experimental Tyvek mainlet panel, and another being made from a chopped up spinnaker.



    I think both cloths are too light and will probably make replacements later – if the shape turns out OK it will be easy, using the dummy, half the work is already done. (You can drape the cloth and cut it directly on the table by slicing around the "chine" with a hot knife - and then by some happy ordination of Providence, the cloth sticks there. It can be basted, (darts cut if necessary) and the other part of the seam then just has to be folded back and patted down - you don't need two pairs of hands. By the way I stapled the cloth down initially, and that was OK, but cheap, poor quality double-sided tape for easy removal seems a better idea. Don't use basting tape for that!

    By the way, on the subject of taylor's dummies - it didn't take long to knock this one up, spent more time lofting the curve, and fooling around with 45 degree angles. It occurs to me now that horizontal shelf is 100 times easier to visualise, and would be to make. Having admired the (horizontal shelf) sail that Paul made for Pango I think next time I would be inclined to try  horizontal shelves, at least for the mainlets. The dummy would be a cinch to make. The sail might not inflate quite as quickly in very light wind, but Pango's sail seemed pretty good to me. What about the jiblets - would they stand, without being supported at the leech? I think it needs to be tried.



    ****************

    One can only marvel at the creativity, faith and courage of people like Slieve, Arne and David who have been able to start these new ideas with absolutely no guidance and nothing provided as a starting point.


    Last modified: 28 Apr 2019 04:30 | Anonymous member
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