Camber and induced drag

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  • 02 Sep 2024 22:58
    Reply # 13401717 on 13397914

    Thanks for the quick responses.

    I will admit that I may have misled you by talking about keels first, and on that subject I reckon most junkies must simply accept the keel on the hull they end up with. My main interest is in the top of the sail, and a set up to have the lowest induced drag/ vortex drag and hence the best L/D ratio.

    Winglets seem to be the biggest improver for transport aircraft, but somehow I can’t see that they can be practically adapted for sailing rigs. David wrote, “So my hunch (no more than that) is that we have to decide which way to jump.” I guess that sums up what I am thinking, and in many ways that ties in with the question of sail balance.

    Look at the photo below of the elect aeroplane which claims to be the fastest of its type in the world. I’m sure much research will have gone into the wing tip design, and I’ve also seen this shape on many slow speed but high performance model glider designs which also require best L/D ratio.

    I’m not surprised that this is considered to be an efficient tip design as back in the 50’s I was building my own design model aeroplanes and found one design with flared wing tips to perform better and faster than expected. Again, about 1970 there was some problem with the wings of the Trident 2 flown by BEA, and some aeroplanes had modified wing tips fitted. The original tips were flared out to the trailing edge, but the modified ones curved in, only a few inches but enough to require an a recalculation of the increased fuel load.

    The one thing about these tip shapes is that they are suitable for sailing rigs as they can work on both port and starboard tacks, and do not necessarily increase weight aloft.

    Are we to assume that this tip shape pushes the vortex out and back and reduces the drag?

    Graeme’s mention of end plates on the yard would increase the weight aloft, and there’s no guarantee that the yard angle might be already concentration the vortex up to the tip. As he says, it would be difficult to appreciate any gain or loss.

    I’ve been looking at the videos of the Americas Cup racing and it is noticeable that they have removed the gap below the sails, even putting small skirts to completely seal the gap, and have squared of the mainsail head to horizontal. (This latter is probably some rule about no sail above the black band (Black band with blach sails and black mast!) Anything to raise the L/D ratio.

    Just more thoughts!

    Cheers, Slieve.


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    Last modified: 04 Sep 2024 11:30 | Anonymous member
  • 02 Sep 2024 22:08
    Reply # 13401703 on 13397914
    Anonymous member (Administrator)

    David's suggestion of a simple end plate was what I had been thinking of, when I asked Slieve about it ages ago. A simple "fence" same as we might put on a rudder.

    The yard angle of the Amiina rig looks perfect to me, and attaching a simple end plate to the yard would have been a doddle on Serendipity, being SJR, just a double end plate on the part of the yard aft of the mast would put a boundary on the mains panels nicely. A simple plywood plate could be attached at the cost of about 1 hour's work on Serendipity, as the yard is wood, both sides of the sail (sail is sandwiched in between). 

    Slieve says "They [end plates] are not practical for junk rigs, but I believe there are ways it can be done" but can't see why not practical, in the above case, anyway. A piece of cake. Just a bit of plywood and a dozen screws. Could be temporary. Wouldn't interfere with anything.


    The problem would be that any gain would likely be very small, so I can't think of any way in which a difference could be measured. 


    For all I know, that yard, which encapsulates the sail on Serendipity, is offering a modest end plate effect as it is.


    As for Bunny Smith and his "keeper" battens - again I have no idea if it makes any difference, but the battens on Serendipity are attached both sides of the sail (same as the yard) so in effect there may be a slight end plate effect on every panel - would that make any sense? Again, it would be a very simple matter to convert these battens into proper little end plates, one each side (shall we call them "platelets"?). A temporary retro-fit with just a few screws would be no problem at all, and wouldn't interfere with anything. (Would stiffen them, actually).



    Might be better (cleaner) to attach an end plate in two parts on the underside of the yard, rather than on top as shown. Ditto batten platelets.

    I think this topic truly belongs in the category "just messing about" with boats for the pure fun of messing about. I only regret that at the present time I am unable to mess about with Serendipity (and not sure when or if I will be able to do so again). I wish I could do it because it would be easy with the wooden battens, and rather fun.

    I am sure one could convince oneself that performance is improved!


    Last modified: 02 Sep 2024 22:22 | Anonymous member (Administrator)
  • 02 Sep 2024 12:10
    Reply # 13401555 on 13397914

    As I understand it, an aircraft's winglet is toed in a few degrees, and cambered on its inner face (a bit like bilgeboards and leeboards), and interacts with the tip vortex in such a way as to add a small lift vector in the forwards direction (at the expense of more friction drag). Well and good, we might be able to put one on the lee side of a yard, the low pressure side equivalent to the upper side of a wing. But hang on a minute, we need our sails to work on both tacks, and having such a winglet on the weather side would cancel out the positive effect on the lee side (a bit like leaving both bilgeboards or leeboards down). So I don't think the type of winglet as generally used on today's aircraft is a workable notion when applied to sails. We might go for the fence type of winglet though, more like a delta shape attached vertically to an aircraft's wing, or horizontally to the bottom of a keel or rudder. We're familiar with those, on shoal draught boats. It might work in modifying the tip vortex in the way we want. Trouble is, the yard would have to be very close to horizontal. So my hunch (no more than that) is that we have to decide which way to jump. Do we go for a near-vertical yard, and lengthen our leading edge and increase our aspect ratio? Or do we go for a near-horizontal yard, and hope to decrease the negative effect of the tip vortex? Or do we go for a yard at 45˚, and fall between two stools aerodynamically but get ourselves a rig which is easiest to manage and sail with? Decisions, decisions...  

    I remember David Thomas, the designer of Tystie and no slouch when it came to racing, looking at my cambered panel sail approvingly, and saying "the wind likes to be told where to go". Channeling the wind along the panels with protruding battens might just be a Good Thing. 

    Last modified: 02 Sep 2024 12:14 | Anonymous member
  • 02 Sep 2024 10:55
    Reply # 13401551 on 13397914
    Anonymous member (Administrator)

    Interesting, Slieve,

    but performance when racing around the cans may not be the same as performance in everyday use.

    I have owned two boats with fin keels, two with centreboards and three with long keels. The fin-keeled boats may have been the fastest to windward on flat water, but the keels could easily loose grip in an awkward head sea. My present IF, Ingeborg may not have quite the same top performance to windward, but she is still very, very good, in particular since she keeps going against a head sea. Last but not least, she manoeuvres safely at low speed in a windy harbour, without missing her finger berth. I only wish that her rudder was separate from the keel, to increase steering moment with smaller rudder angle. That would give more precision, less drag and lighter helm forces (Remember Johanna, Slieve?).

    All in all I prefer my IF, with her keel profile similar to that of the Dragon.

    The best yard angle to minimise the end airflow leakage at the top end of the sail (..there is a lower end as well...) is still a matter of debate. I don’t know what that angle is. The yard angle of my sails, nowadays land on anywhere between 60 and 70°. The tallish yards are mainly chosen to get the biggest possible sail area for a given mast length. However, this photo of the sail with the extra long telltales, at least hints about a reasonably good airflow up there.

    Conclusion:
    The ideal rigs and keel shapes must often give way to less efficient designs, just to suit the practical needs of the owner. Realities beat ideals...

    Arne



    Last modified: 02 Sep 2024 10:59 | Anonymous member (Administrator)
  • 02 Sep 2024 09:13
    Reply # 13401537 on 13397914

    It has been good to read some of the recent threads on the website, and they have encouraged me to lift down my copy of Tony Marchaj’s book ‘Sailing Theory and Practice’ from the top shelf, and blow the dust off.

    Page 101 has a good drawing of Jester, but little useful discussion, but not finding the diagram I was looking for I latched onto the photo attached to this mail. (Sorry, but I don't know who to embed it with the text.

    In the book it is described as, “the appearance of this additional resistance can only be due to ‘end leakage’ , which is the flow of water under the bottom of the fin keel from the region of pressure to that of suction. This leads to the formation of vortices, such as those clearly visible in the photo, which shows the water flow lines around a model of a Dragon’s hull. As with the sails, this kind of resistance is called induced resistance.”

    Induced resistance, induced drag, vortex drag are all names for this drag which is formed by anything that produces lift. In the photo the vortex is quite large diameter and is about one third of the height of the rudder. The question is, can this vortex be reduced or pushed out of the way to allow the keel to work more efficiently?

    The answer is yes, but it would not be within the class rules of the boat, however we can possibly learn something from this photo.

    My thoughts on the subject, and these are wide open for criticism, are -

    The high pressure on the far side of the keel is escaping under the keel and diluting (weakening) the low pressure on the near (camera) side and therefore the pressure difference (lift force) across the keel is weakened and more leeway will result. If the foot of the keel slope downward more acutely it would be harder for the water to flow under the keel and in practice there must be an angle when the pressure would not be able to escape and would flow along the keel to the trailing edge. With no leakage there would be a higher total force across the keel, and the inevitable vortex would not be able to form until the trailing edge of the keel, and then it would be of smaller diameter and concentrated at the lower tip of the keel. The net result would be a better lift/ drag ratio, which is the key to better performance.

    The actual angle required for this situation unfortunately would depend on a number of variable so would inevitably end up being a compromise. As a keel for the Dragon it would probably be too steep for the boat to dry out on, and the draft would probably be out of all proportion.

    We are told that action and reaction are equal and opposite, so we can turn the diagram upside down, change its aspect ratio as air is less dense than water, and think of it as a sailing rig. Imagine a rectangular (junk) rig made of a number of parallel panels with a horizontal top (and no batten rise just for convenience). As in the above photo, the air at the top would curl over from high pressure on the windward side to the low pressure on the leeward side, and form a large diameter vortex. But if the top of the rig, the yard, was angled upwards, peaked up, the vortex could be decreased and even get pushed right to the peak of the yard, reducing the pressure leakage and the diameter of the vortex. To me this suggests that there should be a compromise yard angle that could reduce induced drag and give a good L/D ratio.

    Conversely, if the head of the rig sloped down, as the leech of a Bermudian mainsail does, then the vortex will spread down the sail and be of quite large diameter. Some of the diagrams on Tony Marchaj’s book do indicate rotating airflow over the upper area of a Bermudian mainsail, and certainly not flying of the top.

    Does anyone have any thought on this idea?

    Modern airliner wing tips are fitted with winglets to achieve this tight vortex right at the tip to achieve much higher L/D and better fuel efficiency. They are not practical for junk rigs, but I believe there are ways it can be done.

    What say you?

    Cheers, Slieve.


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    Last modified: 02 Sep 2024 09:20 | Anonymous member
  • 27 Aug 2024 04:32
    Reply # 13398141 on 13397914
    Anonymous member (Administrator)

    Some homework for Paul

    Regarding luff webbing, winglets and battens, and the "bad tack"...

    The above does not apply to Paul, it applies to me. Paul, your exploration of theory (and your beautiful diagrams) IN ADDITION TO your out-on-the-water gaining experience, is an inspiration.

    Paul wrote: "But wait, what I saw right in front of me, were already winglets. Winglets not located at the wingtip, but spread all over the sail. It was the battens in their batten pockets, which force the air horizontally along the panel, limiting any vertical airflow (which creates the tip vortex)."

    This perked my interest. At the present moment I am doing some “homework” : critically studying all of the early newsletters and magazines. I am overwhelmed by the amount of thinking and debate which was taking place, and debated so hotly, despite the snail-like pace of the pre-internet and pre-forum era.

    These were early days in the development of ideas we now take for granted, people were only starting to come to grips with the various ways in which camber can be given to a junk sail and the advantages which might ensue from it – keep this in mind as background to what follows.

     In 1989 (long standing members will know all this) a talk was given by Group Captain “Bunny” Smith (DFC and Bar, OBE, CBE) and I mention his “gongs” so as to clarify that he was not some sort of over-imaginative “flake” – he had a seriously respectable war-time record as an aviator and leader – perhaps in the same league as Blondie Hasler. He must have been a real character – at the age of 70 he took up junk rig sailing and declared “I am jolly well going to find out how this rig works and I haven’t got long” !

    The Newsletter published an article - an introduction to Bunny Smith’s “I.F.T.” (Insect Flight Theory) as applied to the junk rig. He was interested in low speed aerodynamics and had done considerable testing on his 26’ Saddler Fenix – investigating such possibilities as battens attached to both sides of the sail to create vortices, (“turbulators”) and also something along the lines you seem to be suggesting, he refers to these double battens as “keep battens”.

    In addition, he utilised quilting of the sail which we see on some of the photographs of traditional Chinese junks (always a mystery to me). These are all details visible in insect wings, an inspiration to him due to the fact that a bee can carry 50% of its weight aloft, and in a number of ways defy conventional aerodynamic theory. As it happens, he was also experimenting with bendy battens, carefully designed and built to provide camber in the way we want it to be – but his main focus was on his theory, that turbulent airflow is able to re-attach itself more quickly than laminar flow, and the benefit thereof.

      

    (He also had his answer to your question as to why the mast, on one tack, does not seem to degrade the lift generated by the sail – the mast is buried in that bubble of turbulent air, which re-attaches itself further down the foil before exiting.)


    Needless to say, much discussion was generated. He also had something to say on planform, and the induction of camber from the controlled twist of fanned panels. Unfortunately, so many things were going on at the same time, that it would be difficult today to analyse which particular innovation led to the improved windward performance he was able to demonstrate. (The camber from his bendy battens, as much as anything else, it seems most likely to me).

    He was convinced that the interrupted and re-attached flow of air over the sail (as he saw on the wings of insects) was of great benefit. I wonder what he would have made of your offset luff webbings, given that your sail seems to perform so well. (Rather than try to criticise your sail-making, (the luff issue which seems incompatible with aerodynamic efficiency) I suspect he would have looked instead for a reason why this quirk seems to work so well! Something to ponder. [The Schnabel Luff Turbulator. Don't fix it if it works ! ])

    Smith was happy to disregard aerodynamic theory if it did not seem to fit the results he was getting (nothing wrong with that). It seems he was in good company.


    Smith was greatly offended by a fluid dynamics PhD student who was researching at the time and reporting back to the JRA (Joddy Chapman). Chapman's reports did not support Smith’s findings. Smith wanted him off the payroll (JRA funds). It seemed to more or less come to a head in Newsletter number 34 in which Chapman published a detailed academic paper which, among other things, was a serious reply to Smith – though it did not “take the wind out of his sails”. If you have not already read that report of Chapman's, I believe you would be interested to do so.

    Bunny Smith died less than a year later. His Insect Flight Theory still appears from time to time but he is probably now remembered more for the “Fenix” planform, an ancestor along with the planform of Reddish, of some of the fanned sails we still see today.

    Bunny Smith's 26' Saddler, Fenix, with his trademark "wu liu" (56) on the sail.

    If you have the time to ponder, may I suggest you download JRA Newsletter 24 which outlines his theory, and also 25 and 26. Number 34 which is Chapman’s paper – and you might like also to read Robin Bains’ obituary to Smith in issue 37.

    Finally, you may also find interesting a short Forum discussion on Smith which took place in 2011 – just 10 posts, each one excellent and worth reading – here https://junkrigassociation.org/technical_forum/484001

    Apologies Paul, if you have read all this, and to longstanding members who were there at the time. Those of us who were not, ought to know a little of the history.

    Last modified: 27 Aug 2024 12:23 | Anonymous member (Administrator)
  • 27 Aug 2024 04:06
    Reply # 13398136 on 13397914
    Anonymous member (Administrator)

    Paul.

    I don’t know how relevant this is to the tip of a junk sail.

    Tip vortices are hard for me to imagine, but I saw a video clip some months ago which set me wondering if a square topped sail might benefit from an “end plate”.

    I saved the link to this rather beautiful little video clip. It is here.

    It shows a piper cub turning in close to the ground, for a landing, through a layer of fog just above the runway. As the aircraft cuts through the fog, the wingtip vortices show up very clearly. I guess you have seen plenty of this sort of thing.

    In case the link doesn’t work I captured a few freeze frames.


    It seemed to me that it would not be too difficult to add a thin plywood “end plate” to the yard – at least to that part of the yard which is aft of the mast. It would be easy enough to do. I asked Slieve and he did not see much value in the idea and, too many projects anyway, so I left it at that.

    Last modified: 27 Aug 2024 08:50 | Anonymous member (Administrator)
  • 26 Aug 2024 22:15
    Reply # 13398030 on 13397914
    Anonymous member (Administrator)

    Fences?

    I remember when I showed my cambered panel sail to a local sail maker, in the nineties. He at once pointed at the horizontal panels, and his view was that the battens would act as aerodynamic fences, preventing the air from moving spanwise.

    Many years later, Slieve McGalliard asked if I could fit a number of long telltales to the upper two panels in Ingeborg’s sail. I did so, but any anticipated vortexes failed to materialise. The telltales stood fairly straight aft. My guess is that smoke generators would be needed to get a clearer view.


    My conclusion is that I have not concluded on this one  -  in particular since the sail with hinged battens (1991-92) also worked well (see JRA NL 24)...

    Arne


    (Full size photo in my Photo Section 8-22)

    Last modified: 26 Aug 2024 22:19 | Anonymous member (Administrator)
  • 26 Aug 2024 18:34
    Message # 13397914

    One of the major unsolved questions that remain with the cambered junk rig is, why the rig draws so well even if on the “bad tack” with the mast distorting the camber profile totally.

    (At least I read this between the lines while reading through JRA forum threads. Please be so kind and direct me to the answer to that question, if it was already found and I am writing here about something already solved. Much appreciated!)

    I’ll try to keep the theoretical introduction to this issue short: When you look at a sail, you look at an airfoil. If you slice our beautiful junk rig in the direction of airflow, so more or less horizontally, you get a 2D profile (please, only slice virtually in your head…). There you go, with different profiles for different junk concepts:

    Now with these 2D-profiles, you can draw lift and drag vectors, discuss upwash, alpha-tolerance, laminar/turbulent flow, boundary layer characteristics, attached and detached flow and all the like. Though these are interesting issues, one is lacking one important factor: induced drag.

    Induced drag is not explicable on a 2D profile, as it is a 3D effect: The pressure on the windward side of the sail and the suction on the leeward side of the sail create a huge vortex at the highest tip of the sail. This vortex creates a lot of issues with airfoils, for example the induced drag. I do not want to go into detail here, but with airplanes this induced drag can be contributing up to about 40% (or higher, depending on the kind and speed of the airplane). I do not know the exact numbers for sails on a boat, but the induced drag is potentially the second highest contributor to overall sail drag – after friction. A short (really short) google research offered me the following web page from NorthSails:

    The science behind your sail trim

    Quite an interesting read, concerning sailing aerodynamics! Though it deals with Bermudan rigs, the physics are the same for the junk rig. The article also shows some very understandable views taken from CFD simulations, which are able to explain that tip vortex at one sight.

    Okay, so much for the background of induced drag. It results from air flowing from the windward side to the leeward side – in vertical direction, so to say. I tried to sketch it up (sorry for the Bermudan rig, it is just simpler to draw) in the following. In that sketch we are looking from infront of the boat towards its bow, so the boat is sailing straight into the viewer.

    Again, the tip vortex induces a significant amount of drag onto the sail: the induced drag. Induced drag has a very, very high influence on overall sail drag, close after friction drag.

    Now, windward performance is strongly dependant on drag. Slieve perfectly explained why, in his “some thoughts” document. So, as the induced drag from the tip vortex has a significant effect on the overall drag, this induced drag also has a significant effect on windward performance. Right?

    Now, in aviation the induced drag can be significantly reduced by wingtips. Everyone who ever sat in a airplane (fast passenger jet, not slow propeller machine) will remember the little vertical wings at the end of the main wing. They are winglets (or wingtip fences, or endplates, …), and they are there to limit the unwanted airflow from the pressure side (lower side) to the suction side (upper side). It works so good, that it is commonly seen on most airplanes (sorry to any aviator for such a generalisation!).

     

    We don’t have that on sails of a sailboat, do we? I’ve seen endplates on ruder tips, I think on fotos from Arne. But never on sails… or do we?

    It was when we were tacking up the Kalmarsund against F5-6 yesterday, when it struck me. We were sailing heavily reefed, upwind, making good progress in one of the most ugliest, choppy seastates the already very choppy Baltic sea has to offer. By the way, we reefed and unreefed 16 times that day, with me singlehandedly sailing Ilvy while Toni was under deck due to her hurt ankle. It was the most easiest exercise, and really showed off the junk rig capabilities. A lot of huge, white, shiny sailing yachts with carbon/kevlar sails motored past us while we steadily kept sailing, and after we tacked even into the harbour, we were the talk of the evening in the Kalmar harbour. I think it we produced some good junk rig advertisement that day.

    Enough of that side story, back to the topic: Being at the tiller, heavily reefed, looking at Ilvy's sail drawing us beautifully forward, my brain kept rolling ideas in my head, visualizing the airflow right in front of me and imaging winglets on a sail. But wait, what I saw right in front of me, were already winglets. Winglets not located at the wingtip, but spread all over the sail. It was the battens in their batten pockets, which force the air horizontally along the panel, limiting any vertical airflow (which creates the tip vortex). Though distortet in their horizontal profile, the sewn-in camber still creates huge pockets which limit any vertical air flow:

    The battens might act as winglets, only not located at the wingtip but distributed vertically along the sail. Thus limiting vertical airflow, thus decreasing the tip vortex, thus lowering the induced drag, thus lowering overall drag significantly, thus increasing windward performance significantly. And as those camber pockets form on port as well as on starboard tack, this tip vortex lowering effect makes for nice upwind performance on both tacks.

     

    The theory I describe here explains the good upwind performance of sewn-in cambered sails. It is a theory, yet, not verified by experiments/simulations. I do not know the actual numbers/percentages involved, and I do not intend to risk an educated guess. It might be that the tip vortex lowering effect of the cambered junk rig batten pockets is negligible, it might be that it is significant if not dominant. I try to stay on the non-speculative side. My explanation here is rather a hypothesis, as long as not backed by experiment or simulation. It would, however, be an explanation for the unexpectedly good performance of the “bad tack”.


    What do you think? Please do not hesitate with critics!

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