Foiler Design

Discussion in 'Sailboats' started by tspeer, Nov 12, 2003.

  1. Doug Lord

    Doug Lord Guest

  2. tspeer
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    tspeer Senior Member

    Winglets are most useful when there's some artificial constraint on the span, such as class rules. A properly designed winglet can spread the shedding of the tip vortex out and reduce the induced drag - the foil acts as though it were longer than it is. But it also adds parasite drag, so getting a net drag reduction is not trivial. For the same increase in effective span, you can increase the physical span with less added area than the winglet. So you're better off just making the foil longer. It's easier, you get a lower stall speed, and the drag at high speed is less. And this is for the case where the flow is from straight ahead, which is not the case with a sailboat.

    Whether or not an elliptical planform, or even elliptical loading, produces the minimum drag is not a given, either. That's because an elliptical planform produces minimum drag only for the case of an isolated planar foil. But these foilers don't have an isolated foil, they have a big strut in the middle and they're operating with a leeway angle. Instead of two trailing vortices at the foil tips, there will be three trailing vortices - two at the tips and one in the center from the strut. The center vortex will increase the lift on the windward half of the foil and decrease the lift on the leeward half of the foil. So the loading is going to be far from elliptical.

    It's interesting that all the photos of Moth foilers going to windward show the pilot heeling the boat to windward. This tilts the lift vector from the foils, providing a sideways component that reduces the side load on the strut. With the right heel, the strut can be unloaded and the center vortex eliminated. Given the large span of the foil, this is bound to be a more efficient way of producing the necessary side force than depending on the strut. It also means the strut loading isn't changing as the wetted area of the strut varies with the boat's flying height and waves.
     
  3. yipster
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    yipster designer

    stepped foils

    did you foil freaks ever heard of a "stepped foil"?
    Georgia Tech school of aerospace engineering has a paper:
    the only other reference i found was this one.
    i find this interesting but thought all foils step by definition?
     
  4. Doug Lord

    Doug Lord Guest

    stepped foils

    Yipster, thanks-very interesting!
     
  5. tspeer
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    tspeer Senior Member

    A classmate of mine did wind tunnel testing of a Kline-Fogelman airfoil for his senior project. The faculty brought in a panel of people from industry to hear our papers - one of them was Richard Whitcomb.

    After seeing the results (which showed it to be inferior to conventional designs), Whitcomb agreed that, "Yes, the step can be considered a boundary layer control device. But it's a very inefficient one!"
     
  6. jimb
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    jimb Junior Member

    design for controllability

    Some observations from a Moth sailor who has not sailed a foiler. The technical discussions about foil profiles are very interesting but for a Moth, it is all about control. Originally T foil rudders were developed to give a margin of safety for skiffs, to trade off drag to provide attitude control and avoid nose diving. Now Moth foilers are running foils which probably provide way too much lift at maximum speed and which use lift destroying features such as negative incidence flaps or foils running at negative angles of attack to maintain height. Most skippers will operate within their comfort zone with positive or neutral angle of attack and negative flap deflections where large lift and lift destroying forces are counterbalanced so that small perturbations will not trigger a major crash. As with T foil rudders, this trades off drag for stability.

    Of course there are a very few skippers who are comfortable riding the knife edge of catastrophe using very little flap and just enough negative angle of attack to counterbalance the excess lift of the foil. Not surprisingly, they go lots faster and become World champions. Can the foils be developed to be fast for normal people?

    Can a symmetrical foil generate enough lift with angle of attack to lift out at reasonable speeds? This means that lift does not change as much with speed and would imply running with very slightly positive AoA at maximum speed which would accommodate small changes in attitude before generating downforce and triggering a crash.

    While a flap on a fixed foil is lower drag and more efficient on a computer screen, would a pivoting symmetrical foil with sensor control be easier to control and hence faster in real life? This would aim to avoid using negative flap on a high lift profile to provide stability while still providing an adjustable lift force without changing the boat’s attitude.
     
  7. boogie
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    boogie Member

    hi guys,

    wasn't it earlier in this thread that alan's kooee design concept used symmetrical foils with only flap deflection?
    kinda makes sense to me with those very small AoA's at very high speeds.
    plus the windward foil on his concept had to pull down at speed to create the righting moment.


    but jimb brings up a good point by asking for increasing the control of the moth configuration using different design features. has anyone in the moth foiler world built any configurations with dihedral [up to the angle of windward heel] and maybe even sweep [foils and strut]?

    i guess quite a bit of control could be gained by sweeping the strut back a bit.
    with a vertical strut every little deflection on the strut creates an even larger force that creates an even larger deflection.
    with sweep it should return to a neutral position.

    cheers
    boogie
     
  8. Wardi
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    Wardi Senior Member

    Fully pivoting foils

    I agree with Jim B completely!

    There are several people in Australia and Germany currently developing solutions with the aim of improving longitudinal stability, height control, efficiency and simpler construction etc., but as far as I know, none of these attempts to date have come anywhere near the success of John Ilets design, which works extremely well over a wide range of conditions. John's design does however seem to have some of the inherent inefficiencies that you mentioned and it would be great to come up with a more forgiving solution, making it easy for average sailors, and I think you are on the right track with your proposal.

    I have been using a fully pivoting symmetrical foil on my canard since 1998 and find that works extremely well in all conditions, so I can confirm that the arrangement you propose does work. The most difficult part is to get the main lifting foil to work in the same way. This week I have built a fully pivoting foil with integral sensor in the board. The current arrangement is a rudder foil and bow foil, no lift on the centreboard. First trials are encouraging, but I have had some structural problems...broke some internal supports in the boat and snapped the the bow foil strut off today..... so if you are going to build a fully pivoting lifting foil, make sure the boat and it are really strong!!

    In my previous post in this forum I asked Tom Speer whether a foil with a flap, or a symmetrical foil with angle of incidence only was more efficient. His answer was that the flap was more efficient. He did not consider the inefficiency of the condition of high incidence on a fixed foil with negative flap. Perhaps a compromise of high lift foil shape with full incidence control ie; optimum angle of attack at all times, would be a better overall solution. I would be interested in any comments.

    Dihedral, has been trialled by both myself and Phil Stevenson. I tried several dihedral and anhedral angles. The conclusion from both trials was that a straight Tee is the best compromise.
     
  9. kenwstr
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    kenwstr Junior Member


    Hi

    Very simple:

    L = 1/2 * p * V^2 * S * Cl

    S = L / ( 1/2 * V^2 * Cl)

    L = Lift
    p = Density of fluid = 1000 Kg/m^2 (fresh water)
    S = area m^2
    Cl = coefficient of lift

    You need some lift drag polars to find the best Cl for a given foil.

    I think the T foils have a basic design flaw in that your stuck with the take off area and therefore have to wear more area and therefore more drag than necessary at higher speeds.

    To support a 250 Kg boat at Cl = 1, you need:

    Speed Area
    Kts m/s
    5 0.8
    10 0.2
    20 0.05
    40 0.0125

    The drag force is going to be roughly proportional to area so the extra area is really going to kill top speed. You can reduce drag with apropriate Cl and flap but nowhere near tha same as reducing area. A V foil with the tip above water automatically reduces area as it rises and therefore gives almost constant form drag at any speed. However I don't know how interference drag is affected at the interface.


    Regards, Ken
     
  10. GDelerm
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    GDelerm Junior Member


    I think there is a mistake :

    You have to read : S = L / (1/2 * p * V^2 * Cl)

    L = Lift in Newtons (equal the weight of the boat)
    V = Speed in m/s

    __
    Gérard
     
  11. boogie
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    boogie Member

    wardi,
    what were the characteristics or behaviour that made the straight Tee the best or rather what made the others worse?

    cheers
    boogie
     
  12. Wardi
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    Wardi Senior Member

    As reported in other sections of this forum:

    Dihedral provides very good initial stability and tends to rolls the boat in to windward. Initially this is great, but it does not stabilize and you simply keep rolling in to windward.

    Anhedral is initially very stable and provides no windward heeling moment. The problem is that when you heel more than about 10 degrees, it takes over entirely and tries to flip the boat over in the direction of heel. It is completely unstable and a disaster to foil with.

    I have developed a complete respect for Rich Miller in being able to foil on his inverted Y foils on a sailboard. There is no doubt that the ability to instantly sense and balance in all directions while standing up as well as moving the rig is the only way this could be achieved.

    It is concluded that a straight Tee or slight dihedral, like an aircraft wing is the best compromise.

    All of these results are quite fortunate, as it means the best arrangement seems to be a simple Tee foil in a standard centrecase.
     
  13. jimb
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    jimb Junior Member

    So, we use big foils with high lift to get lift out at low speeds then have the problem of controlling these foils and the added drag at high speeds. The aircraft industry uses slots and flaps to enlarge the wing and change its lift characteristics at low speed or uses delta wings at huge angles of attack and lots of power to lift off. At the risk of making the whole thing too complex to be drivable, can a Moth foil be made to act as two different foils with manual intervention to select the required format?

    I am thinking of a pivoting symmetrical foil to have low lift and drag from its profile at speed and low variation of lift with speed. This would be controlled by a sensor. It would have a manually controlled flap just to form a high lift profile at lift out. Normally the flap would trail freely in the wake of the foil, producing minimal form drag. This would have benefits in light air regattas.

    At low speed, you would run the foils in minimum drag mode until lift out speed was reached then deflect the flap to pop the boat up out of the water. Quickly set the flap back to neutral before the foils get too close to the surface and let the sensors control the boat’s height.

    If Wardi’s bow and stern foils with sensor control on both foils works nicely, the Ilett tiller control could manage the flap control on the stern foil. How the linkage is going to cope with flap control on a pivoting foil, I have not thought about yet, but it is important that the flap can just trail freely when not in high lift mode. The flap is going to generate large rotation forces in the foil about its pivot which will want to reduce the angle of attack so some sort of reaction stop will be required. Perhaps careful placement of the pivot could let the sensor provide the reaction or the manual control could temporarily lock the pivot. Of course this gives the skipper something else to do during a tack so maybe foiler scows are the answer.

    It is all so much easier from the armchair.
     
  14. Wardi
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    Wardi Senior Member

    JimB
    I am sure it will be possible to build variable area foils either by extending the section width, swing wings etc. You are certainly on the right track. The mechanics and construction will of course be a big issue but not insurmountable. At least one part of the arrangement you are considering can work, see this site.... http://www.portchestersc.co.uk/sailm4/index.htm

    An alternative is a ladder foil arrangement where you simply fly higher to get some of the foil area out of the water. Extremely simple, but I am unsure if it is really workable and effective.

    One concern I have is that if we start making extremely complex variable area foils, they can become very difficult to build and expensive, which would certainly not be good for the class, especially at this early stage of venturing into foiling.

    I think it is much more important to get a relatively simple, easily handled foil arrangement first, before going into complex refinement.
     

  15. kenwstr
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    kenwstr Junior Member


    Hi

    Problem with the lader foil idea is induced drag form the combined effects of all those tips. A single T foil with tips near the surface would experience very little induced drag as the vortices have not space to develop.

    This is Also one advantage of V or dihedral foils. Interesting above post re heel to windward. That effect is created by side slip increasing the angle of attack on the lee foil and decreasing on the winward side. The strength of this aleron effect is determined by dihedral angle and side load.

    Dihedral also wastes some energy due the opposed inboard reaction vectors but not serious at small angles. Another problem with many early dihedral designes is that as the area reduces with speed much of the upper foil surface comes out of the water so your back to planing on the lower surface so loose L/D efficiency. Tapering to a small chord at the dihedral centre helps keep the foil covered.

    I know of 2 vairable area solutions in the aero industry. Extending/retracting flaps and teloscopic span flying surfaces. Both offer variable aspect ratio. The later being a solution for reducing induced drag at low speed, low loading.
    Better for min trim but very difficult ot make strong and operate smoothly.

    I agree the 1st might be the better option for foils operating near the surface as it can offer reduced camber and incidence as well as area at high speed.
    Minimising both Cdf and the area its acting on. Still not reducing area enough though.


    Ken
     
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