Aerofoil shapes and entry angles

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  • 26 Apr 2019 15:07
    Reply # 7305189 on 7304294
    Anonymous wrote:

    (Robert, any comments on sheeting angle for the jibs?  Slieve’s early notes suggest 8 degrees, but I understand they have progressed from there and now use 12 degrees, which is what I am going to use – that’s a whopping big change. Never having seen any kind of “balanced rig” in action, its going to be interesting to just try it and see what happens. I have no idea if there is any theoretical basis for choice of sheeting angle.)


    Hi Graeme--I've also drawn my ideas from pics of bermuda jibs (pic 1 below), Slieve's posting and recent pics of Amiina (pic 2). You can see how the entry of a bermuda jib changes vertically. I'm not sure if this is designed in or if there are material/geometric constraints that force this vertical transition of the leading edge. I don't know which one is best. I've stuck with ~ 30 deg. However, given the junk rig is reefed down you could flatten the entry angle vertically. 30+ degree at the bottom going to 7 deg? at the top?

    The thing about the sheeting angle on a bermuda rig is it can be changed in real time to fit the conditions so I can see why there is no optimal. The optimal is whatever makes the boat go the fastest in the current conditions. A downside for split rig is that it must be fixed. Whatever the builder picks there will be a wind/sea condition where the selection won't work very well.

    The pic of Amiina is a screendump from the Tammy, Amiina, Emelene video. If you look closely at the jiblets there are sheetlets supporting the leech and stiffeners along the midline.  Pic 3 & 4 is a CAD excercise varying the sheeting angle but keeping the other dimensions pretty constant. Pic 3 has a 14 deg angle and pic 4 a 11 deg angle. 14 deg might be extreme but it shows why the material along the leech might have to be supported. There is just more material along the curve of the leech; the weight of which needs more wind to inflate.  I've stuck with the pic 4 version. However, note the difference in the gap. This may confer an advantage to a larger sheeting angle format.

    Last modified: 26 Apr 2019 16:03 | Anonymous member
  • 25 Apr 2019 22:33
    Reply # 7304294 on 7302245

    Thanks Robert I can get the gist of what you have done with Excel.

    In my first attempt I went about it in a different way, and I have even tried just sketching free-hand. Comparing all these with the profiles shown in the previous post, the remarkable thing is how close to identical they all are. It seems (and Slieve already hinted at it) if you define an entry and exit angle, strike in a tangent to the point of maximum camber and then try to bend a spline to fit within these constraints, not only is there a practical limit to the entry angle possible – also you always seem to end up with about the same shape.

    Thanks to Arne putting the whole thing into perspective and adding a dose of reality. I think I will stop fretting and just proceed with something that looks about right. By the time it has gone through my sewing machine there will be no hope of perfection anyway. Arne’s reference to the symmetric square sail makes me feel a whole lot more comfortable!

    Having said that, I must say Robert’s sail looks right in the photo, and the sail that Paul made for Pango, which I saw in action the other day, also did resemble pretty accurately a wing foil, so it can be done.

    (Robert, any comments on sheeting angle for the jibs?  Slieve’s early notes suggest 8 degrees, but I understand they have progressed from there and now use 12 degrees, which is what I am going to use – that’s a whopping big change. Never having seen any kind of “balanced rig” in action, its going to be interesting to just try it and see what happens. I have no idea if there is any theoretical basis for choice of sheeting angle.)


  • 25 Apr 2019 18:43
    Reply # 7303909 on 7302245
    Anonymous wrote:

    I have appreciated lots of useful help and advice from Slieve and don’t want to keep bothering him with emails so I hereby invite any opinions on (this probably not very well understood subject of) aerofoil shapes and entry angles.

    FWIW here are a few pics of my current jiblets going from theory to practice. Because I first create my panels in a spreadsheet (see example XLS below) I define my "entrance angle" as the angle between the diagonal (pic 1 & 2) and the 0 deg to 5 deg line. My camber profile in the spreadsheet is a series of straight lines. Pic 2 is the profile drawn in CAD at 1:5 scale and annotations added. Pic 3 shows the jiblet full scale in force 1-2 wind.
    Last modified: 25 Apr 2019 18:57 | Anonymous member
  • 25 Apr 2019 10:03
    Reply # 7303126 on 7302245
    Anonymous member (Administrator)

    Graeme,

    I haven’t made any efforts on maximising the entry angle of my sails. When I made that diagram with four foils, I was more interested in the sum of the entry and exit angles, the ‘deflection angle’. As can be seen, this came out almost the same on the circular foil and the (hand-drawn) wing foil. I wondered how much more driving force I could expect from a sail with 10% camber instead of only 8%.

    There are several reasons for me thinking along these lines:

    1.      After all, we sail with the mast on the leeside half of the time. Goodbye to perfect foilshape. The sails still work well on this tack.

    2.      My first cambered sail was a flat sail, modified with hinged battens (see NL 24). This was very powerful and close-winded, even though the shape of the resulting foil was more like a circle arc than a proper wing-shape. I dropped this concept, partly because I didn’t like the idea of the mechanic hinges, partly because the luff and leech (still) didn’t get very taut, so sometimes fluttered, and partly because this symmetric camber moved the CP aft in the sail, so increased weather helm.

    3.      There has been lots of workboats with square-sails in use in my country. These sails need to have a symmetric camber, since the airflow is reversed from tack to tack. They still perform well to windward, if the vessel and crew allow it to do so.

    I can see the point in finding the perfect airfoil shape for a split JR, but for my ‘ordinary’ sails, which have the camber shape badly distorted half of the time, I don’t bother.

    Cheers,
    Arne


     


    Last modified: 25 Apr 2019 10:49 | Anonymous member (Administrator)
  • 25 Apr 2019 01:38
    Reply # 7302619 on 7302245

    In Slieve's McGalliard's public writings, he gives a good explanation of entry and exit angles in the section where he describes camber profiles. P56_66 will get you there.

    Thanks Dave D.

    Ah, here it is. Ueli was correct.

    I had read it, sort of understood it, lost it among the other notes, and forgotten.

    Here it is, entry angle is clearly defined, and its significance explained.


    In the diagram above the entry angle EN and exit angle EX are marked and if these are drawn as well as the maximum camber and its position then it is easier to bend a spline to be a tangent to the angle lines and complete a fair curve. ..... Reading through the various text books it becomes clear that these angles have a significant effect on the performance. C.A. Marchaj, is his book ‘Sail Performance, Theory and Practice’ writes that “for sailing up wind in light conditions a well rounded entry will give improved performance.” Upwind in light conditions is probably the weakest point of the earlier junk rigs so this simple statement makes a very important point….”

    Followed by a page or two of further useful information, including: “All indications are that the entry angle should be as large as practical in an effort to combat the weaknesses in the junk rig’s performance, and equally the exit angle should be as low as possible for the same reasons…..”

    These draft notes of Slieve's are so well-written, referenced and informative that the above quote is worth repeating. 

    I remember now, the above is the reason why I made the entry angle as big as possible.  I may have over-done it. (Slieve has not gone as far as stating the actual entry angle he used. As far as I have been able to discover, so far only Arne has made a suggestion as to what that angle might ideally be.)

    I hope one day Slieve will update, complete and assemble his treatise. 


    Last modified: 25 Apr 2019 01:44 | Anonymous member
  • 24 Apr 2019 21:22
    Message # 7302245

    Here is an aerofoil shape taken from Slieve’s published notes:


    Accompanied by the words:

    The diagram shows a curve with about 10% camber placed about 37% chord and with a large entry angle to help produce high lift and a low exit angle to produce low drag. The shape is substantially flat over the last 40% though has a very slight reflex in the after section in an effort to flatten the camber just in front of the leech. This curve may not be everyone's ideal, but it is offered as a well considered starting point.

    From this I would infer that when Slieve refers to “entry angle” he is referring to the angle between the tangent of the curve at the luff, and the centre line. In Slieve’s drawing it looks as if it could be as high as, perhaps, 40 degrees or more – just eye-balling it.

    It is the equivalent of the angle in Arne’s drawing which is shown to be 30.85 degrees.


    I still have no idea what Paul meant about Pango’s sail having an entry angle of 8 degrees, as the sail has a section which is in the same class as the above two drawings. 

    (Paul kindly sent me his drawings for Pango's sail, I hope he does not mind me taking the liberty of adding to this post his CAD drawing of Pango's 8% cambered section, very similar to Slieve's 10% above. Looks like at least 40 degrees.)


    My guess now is that the "8 degrees" was a typing error – (I say "was" because I notice that bit about entry angle has since been edited out....!)

    My concern (not concern really, just interest) was that I have recently made a set of mainlets for a split junk sail, using Slieve's angle shelf construction and a made-up foil shape with 8% camber which has a somewhat blunter entry than the three drawings above (that is, a somewhat greater entry angle). I completely forgot “less is more” and thought instead that “more would be better”. Well, I guess I will find out the hard way. It was only an exercise in learning how to sew, made from scraps and for a little trailer boat, so no big deal. Maybe we will learn something.

    I am about to cut the cloth for the jiblets (10% camber and 12 degree sheeting angle) and hope to improve, so, before picking up the hot knife, would appreciate any last minute advice on aerofoil shapes. I have appreciated lots of useful help and advice from Slieve and don’t want to keep bothering him with emails so I hereby invite any opinions on (this probably not very well understood subject of) aerofoil shapes and entry angles.

    Last modified: 24 Apr 2019 23:51 | Anonymous member
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