L- vs inverted T-hydrofoils on sailboats

Discussion in 'Hydrodynamics and Aerodynamics' started by bjn, Mar 19, 2018.

  1. bjn
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    bjn Senior Member

    I have understood that an inverted T-foil has a drag penalty compared to an L-foil. The T is supposed to have higher drag mainly due to the bad pressure distribution and induced drag around the junction.

    But how large is the difference, if we compare two foils with the same horizontal and vertical area in typical sailing conditions?

    The T should have an advantage due to less bending moments in the structure. Could the drag penalty be equalized by taking advantage of the less loaded structure to increase the aspect ratio?

    The T also has the advantage in that it's easy to put a trailing edge flap to control lift. I'm not aware of any L foil with a flap. Is that due to structural or linkage complexity issues? Would a flap be the best from two worlds, or could it be that the flap will be draggy in this configuration?
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Some refer to uptip foils as "L" foils-the auto stable versions of uptip foils are different than "L" foils. The uptip foil can be designed to control altitude with no moving parts.

    Link to Part 1 and Part 2: America's Cup 2007 2010 2013 - Feature Articles Index - From Cupinfo.com http://www.cupinfo.com/en/featuresindex.php

    Quote from the article,Part 1:

    When we were working on the rule, we knew you wanted to get as much lift as possible when you were going fast downwind,” Melvin says. "For instance, in the 2010 America’s Cup, sailed on giant multihulls, the maximum amount of lift we thought we could get was about 50% of the weight of the boat. At that time, we were still relying on the hull to provide pitch control, so what’s come out of this is the boats all now have elevators (the horizontal foils on the rudders).

    At Team New Zealand, we developed a new type of foil that allows you to keep your height above the water more or less steady. No one had been able to do that before, at least not on a course-racing boat that was not going downwind. We developed that mostly on our SL33 test boats -- they came with the stock constant curvature “C” foils and with those kinds of foils, you can generate 50% boat weight lift before they get unstable. But we noticed that when we could get one boat up fully foiling for a few seconds it would really accelerate away from the other boat – and that got the wheels turning. How, with such a huge potential benefit, can we achieve stable flight downwind? So our design team came up with the “up-tip” type of boards. We refined those on the 33s and our 72 is designed to do that and fortunately it worked right of the box.”
     
  3. Doug Lord
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    Doug Lord Flight Ready

    According to Alan Smith and others canting a wand controlled t-foil outboard 15 to 20 degrees can eliminate most negatives associated with them while still retaining the ability to generate RM.
    Another solution with wand controlled t-foils (which eliminates the ability to generate RM with the foils) is the canted, retractable T-foils on the Vampire cat that have proven fast. The boat sails with a main foil consisting of a single, wand controlled, canted t-foil(and two rudder t-foils).

    Vampire-Euro cat 2017-photo yachtclub carnac-catsailing news.jpg

    This picture clearly shows the cant angle of the leeward wand controlled t foil:
    Vampire2015-Tim Bees catsailingnews.jpg
     
    Last edited: Mar 19, 2018
  4. OzFred
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    OzFred Senior Member

    I think you could theorise forever and not answer any of the above in a meaningful or general way. You should start with your required outcome and parameters and go from there.

    J and L foils were developed for AC boats because they fitted particular rule restrictions (no flaps, restricted articulation, beam constraints, etc.). Moths developed T foils for similar reasons—the first foiling Moth had what we'd now call Z foils and, had they been made class legal, might have delayed (or obviated) the development of T foils on Moths. Quant and Dali foils: same again, the designers set out with a particular set of requirements and designed to meet them.

    The question of whether one configuration had more drag than another was either irrelevant in the choice of which to use, or just one parameter in a mix of competing interests.

    I know of at least one set of flapped Z/C foils, that they haven't been adopted more widely means they didn't succeed (i.e. weren't better than alternatives). Class rules prevented the use of a wand, so the flap was manually operated, maybe a wand would have made them "successful". The A Class has developed Z foils to be very fast and nipping at the heals of Moths, but they aren't working so well for the Nacra 17. That might be because the A Class is a development class and Nacra a one–design. Class rules again.

    Vampire style canted, swing–up Ts might solve some hydrodynamic issues, but create a number of practical issues, not the least of which is that when a T foil ventilates (as in the image posted by DL) all lift is lost and a solid crash is inevitable. Other configurations tend to have less violent results (though the SuperFoiler is showing that J foils + short floats do an excellent job of creating spectacular crashes).
    Vampire2015-Tim Bees catsailingnews edit.jpg
    In considering foil configurations, I suggest starting with a desired big picture outcome, a vision. Once you have that, you can evaluate design decisions based on which achieves the vision best (i.e. meaningful criteria vs theoretical analysis). Otherwise you'll disappear down a rabbit hole of endless alternatives that may take the final design to a very different place from where you want to be.

    As Steve Clark has said, take the "catamaran" out of C Class and you'd likely end up with a kite foiler.
     
  5. Doug Lord
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    Doug Lord Flight Ready

  6. Doug Lord
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    Doug Lord Flight Ready

  7. Erwan
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    Erwan Senior Member

    Intuitivly, I'd say the Moth foilers are a perfect exemple of the best use of T foils
    As the boat is heeled windward, so both horizontal and vertical forces to balance the boat
    are generated by the "horizontal foil", as a result: the vertical foil (daggerboard) is just a streamlined strut, its main role is to link the boat's hull with the lifting foil.

    For a Catamaran sailing flat, T-foils are at best sub-optimal if not irrelevant, depend on the AR of the foil.

    Cheers
     
  8. bjn
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    bjn Senior Member

    I think that is true. The ideal would be to have the strut angled in line with the force vector. In that case the only "unnecessary" drag would be from an unloaded strut.

    I agree, but I enjoy theorizing. I have tried to simulate and compare L- and T-foils in XFLR5, but I think the SW is not capable enough with both alpha angle (lifting) and beta angles (leeway) with two foils in the simulation (which is the case with the T-foil).

    Thanks, looks like an interesting read. I'm surprised of the high induced drag of the V-foil with constant cord.
     
  9. Erwan
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    Erwan Senior Member

    In addition to L and T foils, IMHO Straight canted foils should be considered too.
     
  10. OzFred
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    OzFred Senior Member

    On a Moth, the horizontal foil lift is normal to the span of the foil. The only "balancing" related to the foil is through the wand automatically adjusting the flap based on height above the water, which balances the apparent weight of the boat with the vertical component of the foil's lift (more or less).

    The force generated to windward (the lateral vector of the foil's lift) is a side effect of having to sail the boat with windward heel (for the same reason as a sailboard) when going to windward. The resulting component of lift that counters leeway is serendipitous. When going downwind, the boat is usually sailed flat an can be sailed healed to leeward.

    This ignores the effects of the rig and sail trim, which is significant. Putting it all into one equation would result in an extremely complex equation. :)
     
  11. Doug Lord
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    Doug Lord Flight Ready

    Veal Heel increases righting moment substantially as well as unloading the daggerboard and more. A difference with a windsurfer is that with windward heel the board CG is not moved to windward like it is on a Moth hull with Veal Heel:
    from Beavers paper--
    Windward heel, shown in Figure 25 has multiple benefits:
    • it increases the righting arm between the foil lifting force and the hull and helm weight
    • it produces a side force component from the lifting foils that augments the side force generated by the struts to reduce leeway. This is critical if strut area (and drag as we have seen) is minimized by flying high
    • it produces a lift component from the sail force that reduces the lifting foil loading
    • it shifts the helmsman closer to the water decreasing the impact of his wind shadow on the sail and putting him in a slower moving portion of the wind boundary layer


    Fig 25-
    Veal Heel.jpg

    Veal Heel dl 2012 N.jpg
     

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    Last edited: Mar 22, 2018
  12. OzFred
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    OzFred Senior Member

    As indicated in your your diagram, the horizontal foil provides no righting moment (RM). It comes from the displacement of the centre of gravity from the centre of lift, which you've labelled "righting arm".

    Moths have been sailed healed to windward since they were narrow skiffs, long before they started foiling and probably before Rohan Veal started sailing them. It's been a feature of many narrow skiff classes since they were narrow skiffs. You're the only person I know of who refers to it as "Veal Heel".
     
  13. Doug Lord
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    Doug Lord Flight Ready

    Windward heel on a boat or board that is not on foils is entirely different than Veal Heel*-that's why there is a different name since about 2002 or so. Like uptip foils are not "J" or "L" .
    * which substantially increases righting moment.

    Frank Bethwaite's book:
    Higher Performance Sailing https://books.google.com/books?id=iQQoAAAAQBAJ&pg=PA400&lpg=PA400&dq=Veal+Heel+for+foilers&source=bl&ots=5TPOZtu3iA&sig=bOzG7UqJs3wZJ9KpgINx2UYXc5Y&hl=en&sa=X&ved=0ahUKEwin9rze-4DaAhXJzFMKHWXlDXoQ6AEIXjAK#v=onepage&q=Veal%20Heel%20for%20foilers&f=false

    Doug Culnane's Blog: Veal Heel on a C Class Cat. http://dougculnane.blogspot.com/2007/08/veal-heel-on-c-class-cat.html

    Rohan Veal – former Moth world champion from Victoria returns to the fold http://www.foilingweek.com/blog/2015/01/rohan-veal-former-moth-world-champion-from-victoria-returns-to-the-fold/
    "He first took the foiler to the Worlds in 2003, in France. “I went on the water and did what the English referred to as the ‘Veal Heel’, an aggressive windward heel and bow up technique,” Veal explained in 2005."
     
    Last edited: Mar 22, 2018
  14. philSweet
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    philSweet Senior Member

    Okay, if that's how you want to describe it, but the resulting reduction in drag when set up this way with the strut operating at quite small lift is anything but.
     

  15. tspeer
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    tspeer Senior Member

    The extra junction drag of a T foil is not really the source of the extra drag compared to an L foil, at least not for a sailing hydrofoil. If you're considering L foils, then you most likely have a configuration like a catamaran that is sailed flat, as opposed to heeled to windward like a Moth. This means the shaft of the L foil will be loaded to create the necessary side force, and that changes the loading on the entire foil. A T foil has a discontinuity in the spanwise lift distribution at the junction when you have a side force, with the lift on the leeward panel being substantially less than the lift on the windward panel.

    I think it's useful to look at the foil from an entirely different perspective. Tilt your head so that "up" is in the direction of the total lift vector. The L foil now becomes an asymmetric "V" foil and the water surface is tilted. The effective span of the foil is from the water surface to the projection of the tip on the plane that is perpendicular to the total lift vector. The effective dihedral angle should also be measured from this plane.

    The T foil becomes a "y" foil. It's as though you took the "V" foil and added an extra leg in the middle. This extra panel does not contribute very well to the object of deflecting the flow along the span to produce lift. The span in this tilted coordinate system is determined by the shaft and windward panel, so from an induced drag standpoint it is better to have them carry the lift instead of the leeward panel. Since the leeward panel is not loaded as much as the windward panel, it has excessive area and contributes more to the parasite drag than is warranted by the amount of lift it produces.

    If you want to find out how different a T foil is from an L foil, I suggest you use a tool like AVL or a lifting line analysis. You don't need to model any airfoil section, just use the default flat plate section. Create a T foil and load it symmetrically, then apply a sideslip angle in addition to an angle of attack. You'll see the discontinuity in the lift distribution when you look at the Trefftz plane plot. Compare a T foil and an L foil for the same vertical and side forces, and you'll see why an L foil is preferred for a sailing hydrofoil.

    When it comes to bending moments, the minimum induced drag for a given bending moment is obtained when the spanwise wake wash varies linearly along the span and is proportional to the cosine of the dihedral angle. This holds true for both L foils and T foils, although the details of the loadings that meet this criterion are different for the two foil configurations. In this context, the dihedral angle should be measured from the plane perpendicular to the total lift vector. What you will find when you apply this criterion is the planform becomes more tapered and the leeward panel of the T foil will shrink. You will probably have to constrain the shaft chord to being constant because otherwise the chord will go to zero at the water surface.

    The reason flaps haven't been used in the AC catamarans is because the Design Rule forbade them. Had flaps been legal, we definitely would have used them. There's no reason an L foil couldn't have a flap like a T foil. If you have a sharp junction to the L foil, you could run a flap right up to the junction on both the wing and the shaft. If you want to have a curved elbow, the mechanism is more complicated and you'll probably have to restrict the flap to just the wing outboard of the elbow, but it's definitely feasible. However, a control system that fed back flap angle through an integrator to either the daggerboard rake control or a stern foil to alter the pitch could be used to ensure the steady state flap angle was zero. That would preserve the flap authority for rapid control and minimize the induced drag due to a discontinuity in the camber at the flap/elbow junction.

    It's also possible to have a flap that is spring loaded and entirely passive, so no control system is required, if your goal is to use the flaps to optimize the camber across the speed range. It would be down at low speed for takeoff and tacking, and deflect upward to reduce the camber at high speed. This works because the hinge moment for a flap is only weakly influenced by angle of attack, but is strongly influenced by speed and flap deflection. The net result is a flap deflection that varies almost linearly with speed. Proper selection of the spring constant and the jig shape (zero speed deflection) or preload (if there is a stop) allow you to match the desired flap deflection at takeoff and at maximum speed. Short passive flap segments can also solve the problem of going around a curved elbow. Of course, passive flaps won't help with heave control - that will have to be done with rake or stern control. Although actively controlled flaps and spring loaded flaps are not mutually exclusive. The active control would change the zero deflection angle of the spring, and the active control would take place about the static flap deflection determined by the spring. At high speed, the control effectiveness would drop off compared to a rigid flap, but that might not be such a bad thing because you might want to reduce the gains at high speed anyway.
     
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