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camber area and aspect ratio

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  • B.J.Slotboom
    Hi, I have done some more tests recently on the relations between camber, area and aspect ratio and for me the pieces of the puzzle are falling into place at
    Message 1 of 11 , Jul 1 5:51 PM
      Hi,
       
      I have done some more tests recently on the relations between camber, area and aspect ratio and for me the pieces of the puzzle are falling into place at last. I was a little surprised when I read Victors post saying that you don't need more than 6% camber as most western sails have 10% or more, but then I saw the light. Modern bermudan sails are the product of class rules and rating rules that impose a limit or penalty on sail area. That means the only option you have to increase performance is to go for maximum camber the conditions allow for. In Marchaj's book 'Sail performance' there is a graph that shows the relation between lift coefficient Cl and camber for a rectangular sail of AR=5 . Increasing camber from 5% to 10% gives an increase of Cl from 1.0 to 1.4 at 10 degrees incedence angle for about the same lift/drag ratio. This means that to get the same lift at 5% camber as at 10% camber you need 40% more sail area. So at 6% camber you need some 30% more area. The extra 30% area gives a better downwind performance so it may well be that 6% camber is the best choice for good performance upwind and downwind. Another good reason is that you don't need much camber in drifting conditions and strong winds and camber is not easily reduced once it is in, but sail area is.
      That leaves the question of aspect ratio. I have tested 4 rectangular models with AR=2.8 , 2.0 , 1.2 and 1.0 . When aspect ratio (height) goes down the center of effort goes down so area can go up. I used rectangular models because it is easier to apply uniform camber than in more complex shapes. A rectangle is not a bad shape for high AR ratio models but it is not so good for low AR ones as I will show later but to demonstrate the effects of aspect ratio they do just fine. I have uploaded a sketch of the models and a lift/drag graph to a subfolder 'aspect ratio at 6% camber' in the folder 'windtunnel' in the files section. From the graph it is clear that the high AR models generate more lift at low angles than the low AR models despite the smaller area. That's because air flowing from over the top and around the bottom from the high pressure windward side to the low pressure leeward side reduces the lift in the areas near the upper and lower edges of the model (I test the models in a horizontal position but let us assume they stand vertical like real sails) and it is obvious that a larger part of the total area is affected for low AR models. This overflow creates vortices that are responsible for the induced drag. At low angles and lift the induced drag is low but when the incedence angle and lift go up the induced drag increases rapidly (it is proportional to lift squared and inverse proportional to height squared) and the lift/drag ratio goes down.
      An incedence angle of 10 degrees is supposed to give the best combination of lift and lift/drag ratio for windward performance so I have drawn two lines in the graph through the point of 10 degrees incedence angle of the AR=2.8 model. A straight line for the lift/drag ratio and a curve for the lift-drag force vector. The lift/drag line shows that a low AR model can point as high at reduced angle and power. The force vector curve indicates you can have the same lift-drag vector by pointing some degrees lower. The difference in pointing angle between AR=2.0 and AR=1.2 is 3 degrees for the same forward drive and sideways force but AR=1.2 will have a lower heeling moment due to the lower center of effort.
      For AR=2.8 and 2.0 stall sets in at angles near 20 degrees and the performance drops but AR=1.2 and 1.0 continue to generate more power up to angles of 30 - 35 degrees. This is due to the fact that the vortices at the top and bottom (that are closer together on low AR models) grow so strong that their influence over the middle part of the model delays stall to high angles.
      So a high AR model gives the best windward performance and a low AR model gives the best reaching and running performance. The best overall performance has to be a compromise between the two with an AR between 2.0 and 1.2 .
      This brings me to the junksail model with AR=1.55 I tested earlier. This time I gave it 6% camber and I tested it against a rectangular model of AR=1.5 with the 6% camber and about the same area. I uploaded a sketch an lift/drag graph to the same subfolder I mentioned above. The junksail model is clearly superior at higher incedence angles. Its lift/drag curve is very similar to the one of the rectangle with AR=1.2 so aspect ratio (height/average chord) may not be such a good measure to compare models of different shapes. A better ratio is perhaps height/chord at 50% height. For a rectangle that changes nothing but for the junksail model that ratio is close to 1.2 so that explains the behaviour at high angles better.
      My conclusion is that a sail with about the same proportions as the junksail model will give the best overall performance for cruising (I am more and more convinced the Chinese knew their business). A low sail means a low mast that saves weight and drag. Also mast interference on the sail is reduced as it is proportional to mast diameter/sail chord.
      Last subject is the position of maximum camber. For sails the best position is at 50% chord, however varying the position between 1/3 and 2/3 of chord has little or no effect on lift and lift/drag ratio at incedence angles around 10 degrees used for windward sailing. There are some differences near the point of stall but as low AR sails have a high stall angle anyway these differences are negligible.
      I used to give the models maximum camber at around 40% of chord but this time I put it at 50% for the high AR models and at 60% for the low AR ones. The reason for this aft postion is that you can set the sail at a lower angle to the wind without luffing to get the best lift/drag ratio for a low AR sail if you want to point as high as possible (at reduced power of course).
      There are also more practical reasons. If you use cambered panels then having the camber further aft means the mast does not interfere much with camber when on the lee side of the sail and when your sail has angled battens so that the panels are wider aft than at the luff it is quite 'natural' to have the camber in a more aft position.
       
      Bernard
       
    • schoonerpilger
      ... Bernard: I really appreciate the work that you are doing. Thank you. I hope that you can find the time to summarize your findings into one article and
      Message 2 of 11 , Jul 19 6:45 PM
        --- In junkrig@y..., "B.J.Slotboom" <B.J.Slotboom@h...> wrote:

        Bernard:

        I really appreciate the work that you are doing. Thank you.

        I hope that you can find the time to summarize your findings into one
        article and submit it to this group and to the JRA. (The JRA
        sponsored a Ph. D. student to perform the kind of work that you are
        doing, but I find your results to be much more useful).

        Anyway, the following paragraphs from one of your postings caught my
        eye. They hint at something that I think is quite significant: that
        the position of maximum camber can be as far as 60% aft without
        significant loss of performance.

        > Last subject is the position of maximum camber. For sails the best
        position is at 50% chord, however varying the position between 1/3
        and 2/3 of chord has little or no effect on lift and lift/drag ratio
        at incedence angles around 10 degrees used for windward sailing.
        There are some differences near the point of stall but as low AR
        sails have a high stall angle anyway these differences are negligible.
        > I used to give the models maximum camber at around 40% of chord but
        this time I put it at 50% for the high AR models and at 60% for the
        low AR ones. The reason for this aft postion is that you can set the
        sail at a lower angle to the wind without luffing to get the best
        lift/drag ratio for a low AR sail if you want to point as high as
        possible (at reduced power of course).
        > There are also more practical reasons. If you use cambered panels
        then having the camber further aft means the mast does not interfere
        much with camber when on the lee side of the sail and when your sail
        has angled battens so that the panels are wider aft than at the luff
        it is quite 'natural' to have the camber in a more aft position.

        This seems counter-intuitive to me as I have always imagined a
        cambered junk sail as needing to approximate the same cambered shape
        as a Bermuda sail or an aircraft wing. That is, the maximum camber
        would be about 30% aft of the leading edge.

        Anyway, I hope that you are right because I think that the ability to
        place the point of maximum camber 60% aft could be very advantageous
        for sails with bending battens (i.e. knuckles). This makes it
        possible to make bending batten sails with a reasonable amount of
        sail balance in front of the mast without incurring S-bends in the
        sail.

        I have a question about vertical camber.

        Tom Colvin says that when reaching/running in light airs he adjusts
        his euphroes to give the sail 'belly' or vertical camber. He
        specifies stiff battens and does not have any normal camber in his
        sails.

        I often adjust the sails on our Gazelle in this fashion. They take
        up a very nice looking shape, but I don't know if it makes any
        significant performance difference.

        My question is: Is there any techical value to being able to adjust
        vertical camber on a junk-rigged sail?

        Regards,

        Don Taylor
      • De Clarke
        dumb question from the newbie: if you cut camber into your panels, for better windward work in lighter air, then how do you flatten again if you have to sail
        Message 3 of 11 , Jul 19 6:52 PM
          dumb question from the newbie:

          if you cut camber into your panels, for better windward
          work in lighter air, then how do you flatten again if
          you have to sail in heavier air?

          I'm working my way through a sampling of JRA newsletter
          back issues (am in the mid-80s right now). there's much
          discussion about bendy battens at this period in JRA's
          history. I'm trying to follow the debate (and innovations)
          chronologically to understand it all better.

          am interested to note more than one person recommends
          decreasing batten diam and stiffness from top to bottom,
          i.e. topmost panels are stiff and flat, lower panels
          have bendier battens. the theory is that you drop the
          bendy panels first when reefing, leaving only the flat
          panels. one author even recommends a bendy boom!

          do those who have cut camber into their sails also put
          more camber in lower panels and none in topmost panels?

          what about those who use hingeing battens for more camber --
          how do you flatten out when the wind kics up? do you use
          hingeing battens on lower panels and stiff battens higher
          up?

          camber's nice until the wind pipes up and you want less
          of it :-) how do you get rid of it again? on a marconi
          rig one often has the flattener cringle which takes quite
          a bit of fullness out of the main. no equivalent of this
          on a junk, is there?

          de

          .............................................................................
          :De Clarke, Software Engineer UCO/Lick Observatory, UCSC:
          :Mail: de@... | :
          :Web: www.ucolick.org | Don't Fear the Penguins :
          :1024D/B9C9E76E F892 5F17 8E0A F095 05CD EE8B D169 EDAA B9C9 E76E:
        • Tim Dunn
          The authority Marchaj says that the ideal spot for maximum camber to windward is 50%, (in bermuda/marconi rigs) and for reaching, is aft of 50%. He says that
          Message 4 of 11 , Jul 19 9:17 PM
            The authority Marchaj says that the ideal spot for maximum camber to windward is 50%, (in bermuda/marconi rigs) and for reaching, is aft of 50%.  He says that the idea of the optimum camber being foreward of 50% is probably based on assumptions that sails are exactly like airplane wings.   He says that testing has proven this idea incorrect.
             
            Tim Dunn
            www.steelsilhouettes.com
            "Cut metal art, crafts, & gifts"
            ----- Original Message -----
            Sent: Friday, July 19, 2002 6:45 PM
            Subject: [junkrig] Re: camber area and aspect ratio

            --- In junkrig@y..., "B.J.Slotboom" <B.J.Slotboom@h...> wrote:

            Bernard:

            I really appreciate the work that you are doing.  Thank you. 

            I hope that you can find the time to summarize your findings into one
            article and submit it to this group and to the JRA.  (The JRA
            sponsored a Ph. D. student to perform the kind of work that you are
            doing, but I find your results to be much more useful).

            Anyway, the following paragraphs from one of your postings caught my
            eye.  They hint at something that I think is quite significant:  that
            the position of maximum camber can be as far as 60% aft without
            significant loss of performance.

            > Last subject is the position of maximum camber. For sails the best
            position is at 50% chord, however varying the position between 1/3
            and 2/3 of chord has little or no effect on lift and lift/drag ratio
            at incedence angles around 10 degrees used for windward sailing.
            There are some differences near the point of stall but as low AR
            sails have a high stall angle anyway these differences are negligible.
            > I used to give the models maximum camber at around 40% of chord but
            this time I put it at 50% for the high AR models and at 60% for the
            low AR ones. The reason for this aft postion is that you can set the
            sail at a lower angle to the wind without luffing to get the best
            lift/drag ratio for a low AR sail if you want to point as high as
            possible (at reduced power of course).
            > There are also more practical reasons. If you use cambered panels
            then having the camber further aft means the mast does not interfere
            much with camber when on the lee side of the sail and when your sail
            has angled battens so that the panels are wider aft than at the luff
            it is quite 'natural' to have the camber in a more aft position. 

            This seems counter-intuitive to me as I have always imagined a
            cambered junk sail as needing to approximate the same cambered shape
            as a Bermuda sail or an aircraft wing.  That is, the maximum camber
            would be about 30% aft of the leading edge.

            Anyway, I hope that you are right because I think that the ability to
            place the point of maximum camber 60% aft could be very advantageous
            for sails with bending battens (i.e. knuckles).  This makes it
            possible to make bending batten sails with a reasonable amount of
            sail balance in front of the mast without incurring S-bends in the
            sail.

            I have a question about vertical camber. 

            Tom Colvin says that when reaching/running in light airs he adjusts
            his euphroes to give the sail 'belly' or vertical camber.  He
            specifies stiff battens and does not have any normal camber in his
            sails.

            I often adjust the sails on our Gazelle in this fashion.  They take
            up a very nice looking shape, but I don't know if it makes any
            significant performance difference.

            My question is:  Is there any techical value to being able to adjust
            vertical camber on a junk-rigged sail?

            Regards,

            Don Taylor





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          • Tim Dunn
            It would be easy to have less and less camber farther up the sail with a hinge arrangement. Each hinge could have a different angle built into it. Still,
            Message 5 of 11 , Jul 19 9:27 PM
              It would be easy to have less and less camber farther up the sail with a hinge arrangement.  Each hinge could have a different angle built into it.  Still, with the effect of sail twist on the angle of attack, it may not be such a big deal as all that.  Typically, wind speeds are higher as you go farther aloft, so the natural tendency to twist may not be such a bad thing.  (As higher wind speeds call for sails to be closer to parallel to the apparent wind.)   Also, with designs published to date, there is no hinge in the yard, so the top panel would be flatter than the others, even if all battens had the same hinge angle.
               
              I wouldn't make battens too bendy in a junk rig, as the effect of the sheet is to "stay" the sail instead of the rig.  It battens are too bendy, you might loose the staying effect of the sheets and thus throw more strain on the mast.  (Of course, I have experience only with the Hasler type rig, which has no camber and fairly rigid battens.)
               
              Tim Dunn
              www.steelsilhouettes.com
              "Cut metal art, crafts, & gifts"
              ----- Original Message -----
              From: De Clarke
              Sent: Friday, July 19, 2002 6:52 PM
              Subject: Re: [junkrig] Re: camber area and aspect ratio


              dumb question from the newbie:

              if you cut camber into your panels, for better windward
              work in lighter air, then how do you flatten again if
              you have to sail in heavier air?

              I'm working my way through a sampling of JRA newsletter
              back issues (am in the mid-80s right now).  there's much
              discussion about bendy battens at this period in JRA's
              history.  I'm trying to follow the debate (and innovations)
              chronologically to understand it all better.

              am interested to note more than one person recommends
              decreasing batten diam and stiffness from top to bottom,
              i.e. topmost panels are stiff and flat, lower panels
              have bendier battens.  the theory is that you drop the
              bendy panels first when reefing, leaving only the flat
              panels.  one author even recommends a bendy boom!

              do those who have cut camber into their sails also put
              more camber in lower panels and none in topmost panels?

              what about those who use hingeing battens for more camber --
              how do you flatten out when the wind kics up?  do you use
              hingeing battens on lower panels and stiff battens higher
              up?

              camber's nice until the wind pipes up and you want less
              of it :-)  how do you get rid of it again?  on a marconi
              rig one often has the flattener cringle which takes quite
              a bit of fullness out of the main.  no equivalent of this
              on a junk, is there?

              de

              .............................................................................
              :De Clarke, Software Engineer                     UCO/Lick Observatory, UCSC:
              :Mail: de@... |                                                     :
              :Web: www.ucolick.org |             Don't Fear the Penguins                 :
              :1024D/B9C9E76E           F892 5F17 8E0A F095 05CD  EE8B D169 EDAA B9C9 E76E:






              To unsubscribe from this group, send an email to:
              junkrig-unsubscribe@egroups.com



              Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
            • Victor Winterthun
              If you use parallel battens in the Hasler fasion, it is usual to use the same camber in all the panels of the same shape, for simplisity. Less in the
              Message 6 of 11 , Jul 20 2:33 AM
                If you use parallel battens in the Hasler fasion, it is usual to use the same camber in all the panels of the same shape, for simplisity. Less in the triangular top panels.
                So far, I have not seen a practical and cheap way to reduse camber when sailing, so we have to compromise.
                On big junks, the crew probably added bamboo when the wind were strong, and took it away in light wind.
                Tecnically it is not difficoult to make hinges that will change their angle in stronger wind, but that might turn the junk rig into a high tec. expensive rig.

                Victor.
              • Stephen Wandling
                This is a very interesting idea! It is consistent with our knowledge that wind speed increases the further you are above the water. It s also an idea that
                Message 7 of 11 , Jul 20 11:19 AM
                  This is a very interesting idea! It is consistent with our knowledge
                  that wind speed increases the further you are above the water.
                  It's also an idea that might well be derived from traditional junk rig
                  methodology.

                  Given the sketchy information that's out there, I had anticipated
                  that I would have to experiment with battens of various cross sections
                  in order to obtain an optimum sail shape in a range of winds. I now
                  plan
                  to incorporate this idea as a starting point.

                  Stephen

                  De Clarke wrote:

                  > Am interested to note more than one person recommends
                  > decreasing batten diam and stiffness from top to bottom,
                  > i.e. topmost panels are stiff and flat, lower panels
                  > have bendier battens. the theory is that you drop the
                  > bendy panels first when reefing, leaving only the flat
                  > panels. one author even recommends a bendy boom!--
                • schoonerpilger
                  ... I have thought about this and it does bother me. One answer is that since you can reef so easily then you just take out an extra reef to reduce power.
                  Message 8 of 11 , Jul 20 7:14 PM
                    --- In junkrig@y..., De Clarke <de@u...> wrote:
                    >
                    > what about those who use hingeing battens for more camber --
                    > how do you flatten out when the wind kics up? do you use
                    > hingeing battens on lower panels and stiff battens higher
                    > up?
                    >
                    > camber's nice until the wind pipes up and you want less
                    > of it :-) how do you get rid of it again? on a marconi
                    > rig one often has the flattener cringle which takes quite
                    > a bit of fullness out of the main. no equivalent of this
                    > on a junk, is there?

                    I have thought about this and it does bother me. One answer is that
                    since you can reef so easily then you just take out an extra reef to
                    reduce power.

                    However, I can think of another (untried) solution for hinged battens
                    made of aluminium pipe. The hinge is made by cutting the batten pipe
                    and inserting some sort of rod or cone-pair that bridges the two
                    halves of the batten. (Victor published a hinge design to the Files
                    section of this group some time ago). The batten bends at the hinge
                    because the inner device has some space between it and the battens
                    that let the battens take up the desired angle. It seems to me that
                    you could also have a pipe sleeve that could be slid over the hinged
                    area that would prevent this bending when desired. The down-side of
                    this is that you would have to reef the sail down sufficiently to be
                    able to reach and adjust each batten. Not a big deal if done early
                    enough, but it I don't know if I want to complicate the sail power
                    reduction mechanism - after all this is one of the delights of having
                    junk-rigged sails.

                    Don.
                  • Bernard Slotboom
                    Hi Don, Having the position of maximum camber at 30% makes the sail slightly less sensitive to oversheeting but form drag is a bit higher, a 60% position has a
                    Message 9 of 11 , Jul 23 6:09 AM
                      Hi Don,

                      Having the position of maximum camber at 30% makes the sail slightly less
                      sensitive to oversheeting but form drag is a bit higher, a 60% position has a
                      somewhat lower form drag but the sail is also a bit more sensitive to
                      oversheeting so the best compromise is at 50%. However the differences are
                      quite small.
                      As to vertical camber: a hollow shape (like a spinaker) has a higher drag
                      than a flat plate so a little vertical camber will help as long as you don't
                      sacrifice to much frontal area but I don't know if it makes a significant
                      difference in performance

                      Bernard


                      schoonerpilger wrote:

                      > This seems counter-intuitive to me as I have always imagined a
                      > cambered junk sail as needing to approximate the same cambered shape
                      > as a Bermuda sail or an aircraft wing. That is, the maximum camber
                      > would be about 30% aft of the leading edge.
                      >
                      > Anyway, I hope that you are right because I think that the ability to
                      > place the point of maximum camber 60% aft could be very advantageous
                      > for sails with bending battens (i.e. knuckles). This makes it
                      > possible to make bending batten sails with a reasonable amount of
                      > sail balance in front of the mast without incurring S-bends in the
                      > sail.
                      >
                      > I have a question about vertical camber.
                      >
                      > Tom Colvin says that when reaching/running in light airs he adjusts
                      > his euphroes to give the sail 'belly' or vertical camber. He
                      > specifies stiff battens and does not have any normal camber in his
                      > sails.
                      >
                      > I often adjust the sails on our Gazelle in this fashion. They take
                      > up a very nice looking shape, but I don't know if it makes any
                      > significant performance difference.
                      >
                      > My question is: Is there any techical value to being able to adjust
                      > vertical camber on a junk-rigged sail?
                      >
                      > Regards,
                      >
                      > Don Taylor
                    • Victor Winterthun
                      Hi Bernard. Long time ago, I read that a cat-rig (only one sail), shound have max. camber at 50%, but when there were a foresail, the max. camber in the
                      Message 10 of 11 , Jul 23 12:18 PM
                        Hi Bernard.

                        Long time ago, I read that a "cat-rig"(only one sail), shound have max. camber at 50%, but when there were a foresail, the max. camber in the mainsail should be ca. 30% from the luff. This refer to gaff. rig. May be the same is the case for the junk rig?.

                        Victor.
                      • B.J.Slotboom
                        Hi Victor, Marchaj in his book Sail performance says that the performance differences when you vary the position of max. camber from 1/3 to 2/3 of chord do
                        Message 11 of 11 , Jul 24 12:43 AM
                          Hi Victor,

                          Marchaj in his book 'Sail performance' says that the performance differences
                          when you vary the position of max. camber from 1/3 to 2/3 of chord do not
                          exceed 4%. This applies to a single sail. For a mainsail with a headsail in
                          front of it he says the positon of max. camber depends on many factors such
                          as overlap, camber distribution of the headsail etc. so he gives no specific
                          value. I do not think the (lack of) accuracy of my simple test equipment
                          allows me to detect such small differences with any level of significance.
                          Anyway the effect on performance of the amount of camber itself is much
                          greater.

                          Bernard

                          ----- Original Message -----
                          From: Victor Winterthun <victor.winterthun@...>
                          To: <junkrig@yahoogroups.com>
                          Sent: Tuesday, July 23, 2002 9:18 PM
                          Subject: Re: [junkrig] Re: camber area and aspect ratio


                          > Hi Bernard.
                          >
                          > Long time ago, I read that a "cat-rig"(only one sail), shound have max.
                          camber at 50%, but when there were a foresail, the max. camber in the
                          mainsail should be ca. 30% from the luff. This refer to gaff. rig. May be
                          the same is the case for the junk rig?.
                          >
                          > Victor.
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