BMW Oracle Wing

Discussion in 'Multihulls' started by Inquisitor, Jul 2, 2010.

  1. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Has anyone seen anything more about the aerodynamics of the wing? I just saw a picture that shows the flap hanging off the back of the wing. The flap's trailing edge not only is off the centerline, but the whole flap is DEFINITELY off the centerline. So it either articulates around the wing or the wing is non symmetric. Any thoughts?
     
  2. Alex.A
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    Alex.A Senior Member

    Why the interest? pure racing rig - can't furl/reef or pack away!!! Bummer parked if windy...:p
     
  3. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Well Alex... its kind of like this... I took this magazine to my Library (the Admiral calls it that) for the morning constitution... and I had an Epiphany. :D

    Actually... I was starting to look at the rigging for the Z40½. (of course... the broken record again)... Sloop, Schooner, Easy Rig, Leeward, Windward... just mentally checking things out.

    I used some data from Shuttleworth's site to estimate sail lift to drag ratios. The data appeared to be for a racing Cat, so I'm assuming they're pretty high-end sails... definitely laminates. So the data is probably at or near state of the art.

    I used that in an analysis with COE and weights of my boat to determine what kind of thrust I could expect at the point where the windward hull started flying. Using this with Michlets numbers I came up with the prediction of 27 knots.

    Just for shits and grins I pulled out an old Aerospace book and used some data from Dr. Lazauskas' site and created a spread sheet calculating thrust to drag ratios. It is so much better than the sail, I can see why BMW/Oracle was doing it. I'm getting over twice the thrust at the same heeling loads...

    Unfortunately, I only ran the Michlet analysis up to 30 knots. So I don't really know how fast it can go. I will by the end of the weekend though! :D

    Besides, freely whether vaning wings have far less drag than the bare free standing mast. So even with the wings up all the time, they have less windage!

    So... for right now... its just a mental thing... and an Epiphany.
     
  4. Alex.A
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    Alex.A Senior Member

    As soon as race was over they pulled the whole thing down - scary while parked!!!!!!! And apart from the odd fun in really decent conditions you are not going to cruise at thos speeds - are you? Seriously......
    FUN but not practical?
     
  5. Doug Lord
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    Doug Lord Flight Ready

    Wing

    This is a bit more from another thread:

     
  6. Alex.A
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    Alex.A Senior Member

    Did you look up what this technology is going to cost?
    Read more of dougs threads before getting too excited.....
     
  7. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I believe that a soft wingsail is more suitable for cruising yachts, like the one made by Omer Wingsails:
    http://www.omerwingsail.com/
    It is a variable-geometry wingsail which can be reefed, twisted and cambered in function of wind force and direction.

    It would be interesting to see if anyone on the forum has used it on a real boat, and how much more it costs when compared to traditional rigs.

    By the way, Ilan Gonen, the owner of Omer WS and the inventor of that rig, is a member of this forum:
    http://www.boatdesign.net/forums/profile/ilan-gonen.html
    but it's been long ago since his last post...
     
  8. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Thank you Doug. I did do the obligatory search... but with "BMW/Oracle Wing" I got tons of hits with no substance that I was looking for. I saw that you were likely to be knowledable and hoped you'd help focus me!
     
  9. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Sail cost...

    I don't really have a clue... how much does a set of high performance, laminated, kevlar this, carbon that... sails cost for something in the 40' to 50' class?

    I heard someone who recently bought a main sail for a 28 footer and paid upward of $5000 and it wasn't even a laminate.
     
  10. Alex.A
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    Alex.A Senior Member

    More than my boat in total - but thats my problem - not yours....
     
  11. Doug Lord
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    Doug Lord Flight Ready

    "It may look different but it really is a sloop rig."

    More from Tom Speer:

    Quote:
    Originally Posted by Chris Ostlind
    Perhaps it is important to note that the entire wing aft section is actually made up of many unique and independently controlled sections that do not connect to one another at all. ...

    Reply:
    "That's not quite right. Each flap is pinned at the trailing edge to its neighbors. The control arm at the base of each flap thus controls that flap and the one below it. The gaps between the flaps were covered with a stretchy material to seal the gaps, forming one continuous surface from an aerodynamic perspective.

    In order to reduce weight and maintain stiffness, the spar was straight and located at the maximum thickness of the wing. That meant the hinge line had to be curved. Which meant that there had to be some gap between the flaps because they would come together when deflected.

    Really, the wing is not that much different from the soft sail rig. There's a rotating mast and a mainsail attached to that. For the wing, the mast is larger in chord and the mainsail is thicker. But the cross section topology is the same for both rigs - a teardrop shaped mast, a small gap, and a much thinner mainsail that is articulated to be cambered relative to the mast. The hinge points where the flaps attach to the main element/mast are just like batten cars that are fixed in position instead of being on a track. The flap ribs are just thicker battens. The covering is also a woven material, so USA 17 could be said to have a more traditional sail material than A5's molded 3DLs!

    Despite its size, the main (forward) element of the wing can be considered to be a leading edge device for the mainsail, just as the mast is for the soft sail rig. Deflecting the flap one degree while holding the main element fixed relative to the apparent wind will produce more than 80% of the lift one would get by rotating the whole wing one degree while holding the flap fixed relative to the main element. Which also means that rotating the main element while holding the same orientation of the flap relative to the apparent wind will only change the lift by less than 20% compared to rotating the whole wing. So qualitatively, it's where the flap points that matters.

    That's one reason why twist control on USA 17 is all done with the flap instead of by twisting the main element as was done with Cogito. It was a simpler and lighter way to go for a wing that size. This is another way that the wing is similar to the soft sail, where twist is all in the mainsail, too.

    It may look different, but it really is a sloop.
    "
    __________________
    Tom Speer
     
  12. Doug Lord
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    Doug Lord Flight Ready

    Tom, once again

    More:
    Rigid wing rigs have these advantages over soft sails:
    - The section shape and twist match the design shape
    - Slotted flaps can develop a higher maximum lift coefficient
    - Larger cross sectional moment of inertia for greater stiffness and strength
    - Less drag at low angles of attack due to avoiding windward side separation

    Rigid wing rigs have these disadvantages over soft sails:
    - Inability to reduce area as the wind increases
    - Heavier (generally)
    - Difficult to transport and handle
    - Must be "flown" 100% of the time, making mooring problematic
    - More expensive (many more parts, labor to construct)

    The higher maximum lift coefficient of slotted flaps has been a key factor in their success in C class catamarans, with their sail area limit that precludes the use of spinnakers off the wind.

    AFAIK, the most widespread use of rigid wing rigs has been in landsailing. They have dominated the C class cats, but there are comparatively few of those compared to the larger number and greater variety of landyacht wings. The fastest of the rigid wing landyachts run neck-and-neck with the fastest of the wingmast/soft-sail landyachts, so one can't generalize the superiority of one over the other.

    In bad weather, rigid wings are taken down and stored indoors. Landyachts at a regatta are typically "moored" by using dollies under the rear wheels that have their axles aimed at the front wheel. This allows the yacht to pivot freely about the front wheel, like a boat on a mooring, but the toed-out dolly wheels resist backward motion. It's not uncommon to see tracks in the morning that indicate the yacht has swung through 360 degrees during the night.

    In principle, a rigid wing yacht could live on a conventional mooring. However, when a gust or wind shift hits the wing, it will develop lift and drive, causing it to sail forward and possibly capsize. Aerodynamic control of the wing using a tail to make it aerodynamically balanced allows the wing to react much faster than the hull can move, which will alleviate gust loads. However, the wing must be able to swing through 360 deg (and for an unlimited number of rotations) if the hull is to remained fixed. Aerodynamic control has been successfully used offshore in the form of the Walker Wingsail, although I don't know how extensively they have sailed offshore.

    The experience of sailing a rigid wing has some big differences from sailing soft sails. A wing doesn't "talk" to you the way a soft sail does because it doesn't luff. Instead, it backwinds, which can slow you down quickly or even pitchpole backwards bow over stern. When tacking, if the wing is not rotated quickly enough to follow the hull, the angle of attack will become negative and the wing will backwind as the hull turns. If the wing is rotated too quickly during the tack, the wing will backwind. A wing that has positive control (manually rotated instead of being free to turn) takes some practice to tack properly. An aerodynamically stable wing that is free to turn is easier to tack because it can be allowed to feather itself, like a luffing sail. It can also be sheeted conventionally, pulling in on the wing and allowing aerodynamic moments to rotate it outboard.

    Especially for high-speed craft like landyachts, flutter is a definite possibility. This is a self-sustained oscillation due to coupling between rotation of the wing about its pivot axis and the side-to-side translation as the wing rotates laterally about its mast step due to flexibility or slack in the rigging. Positive control is sometimes used to minimize flutter, but moving the center of gravity forward, preferably on or ahead of the pivot axis, is a more effective remedy.

    The relevant telltales are on the lee side, where they are not visible through an opaque rigid wing. Phil Rothrock solves this problem by having a wind vane forward on the body of his landyacht, with the arms that form the references slaved mechanically to the wing so they rotate with the wing. This allows him to gauge the wing angle of attack as he's looking forward to see where he's going. Another form of telltale that would work with a rigid wing is the close-mounted wind vane at the leading edge, like is used by many wingmast sailors to locate the stagnation point.

    Going back to your original question about efficiency, If a softsail rig is given the same height as a comparable rigid wing rig and designed to avoid separated zones, then there's no fundamental reason why it would be less efficient. Those are two big "ifs" though. The flip side is also true. If you design a rigid wing to have a low aspect ratio, it may not give you any performance edge over a soft sail.

    The ability to adapt the soft rig to the sailing conditions can give it the edge over the rigid wing rig - adaptability through wing twist was the key to Cogito's success. The ability to add sail area or to reduce it through reefing makes the soft rig the better choice for most applications.


    Tom Speer
     
  13. Alex.A
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    Alex.A Senior Member

    Yeah do it - oh wha t has doug done.... tspeer is respected - is doug? spend a fortune on something impractical... go for it. We all dream but who has BWO's budget and experience...?
     
  14. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Book worm view...

    I've never been around a wing mast... much less a full wing sail. Just what is the issue of taking a wing down?

    To clarify my question...

    I do a simple, closed form calculation...

    (1) A 40' bare cylindrical mast with an average diameter of 6". It has 6 stays, each a 1/4" diameter.

    (2) A free standing 40' wing foil using a NACA 0012 foil. With a whopping 600 square feet of area and a thickness of 21" versus 6". Let it weather vane all it wants.

    Now say we have a 100 knot breeze... version (1) has a 1000 lb force 20' feet off the deck. Version (2) only has 132 lbs.

    ... so why am I taking down this near perfect fairing? ... just because 600 square feet up in the air is scaring someone else?
     

  15. tspeer
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    tspeer Senior Member

    The sections of the individual elements are symmetrical, but when the flap is deflected it forms a cambered section that looks something like this:
    [​IMG]

    The hinge line is somewhat forward of the forward element trailing edge. This opens up a gap between the flap and forward element, forming a slotted flap.

    This is what the pressure distributions look like as a function of angle of attack for the configuration above:
    [​IMG]

    The flap has the effect of making the forward element act as though it were cambered, adding a lot of aft loading to it. The boundary layer on the forward element doesn't have to slow all the way down to freestream, but comes off the trailing edge at a higher velocity that matches that of the flap leading edge. This means the forward element boundary layer does not have to have as strong an adverse pressure gradient, and is less susceptible to separation.

    It also lets the forward element carry lift without developing a leading edge suction peak. If you look at the pressure distribution for -6 deg angle of attack, the lee side pressures on the forward element form a smooth curve that is almost flat, with a peak velocity ratio that is only around 1.6. Since skin friction is proportional to velocity squared, there is a big friction drag penalty associated with pressure peaks, which can account for the majority of the profile drag. This is why its better to camber up and sheet out than to operate the wing fairly flat.

    The flap's job is to complete the slowing of the flow to near the freestream velocity at the flap trailing edge. This is where the slot helps, because the flap gets to start with a fresh boundary layer that hasn't been tired out by passing over the forward element. So the flap can tolerate a steeper adverse pressure gradient without separating. It's like a relay race, with the forward element handing off to the flap to bring the flow home.

    15 - 25 degrees of flap deflection is typical. When you get to 30 degrees or more, the drag goes up and there's not that much lift benefit. Zero or small flap deflection is used when you're basically feathering the section. In general, it's better to operate with a fair amount of flap and rotate the whole wing to a lower angle of attack if you need less lift.

    The C-class typically have an additional element that is a plain flap on the trailing edge of the first element. Once the desired width of the slot is obtained by deflecting the #3 element (slotted flap), a bracket deflects the #2 element to follow the flap and maintain the same slot. Slot geometry is important to the C-class cats because they have a fixed wing area, and have to go to high lift to get the most out of that area downwind.

    High lift was less important for USA 17 because it could have any amount of area in the wing, and could add area in the form of headsails for downwind. Lift/drag ratio was more the driving factor. But light weight and reliability were very important, so they went with the simpler configuration of just two elements.

    The section design contributed to the performance of USA 17, but profile drag is a small component of the overall drag. People tend to look at the boat and then overlook the most obvious and most important feature of the wing. It was tall. Lift-induced drag is the biggest component of wing drag, and induced drag goes down with the square of the span.
     
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