Lowest Drag for hydrofoil designs

Discussion in 'Hydrodynamics and Aerodynamics' started by lohring, Oct 25, 2013.

  1. lohring
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    lohring Junior Member

    Is an L, J, or similar shape foil lower drag than a T foil? A quick look at Hoerner, Fluid-Dynamic Drag seems to indicate that two surfaces joined by a mean radius of double the section thickness might actually have a negative interference drag while even the best T faring still increases interference drag. This would seem to indicate that L foils like those in current cats should have better lift to drag than moth style T foils. Are L foils the future now that we can solve the structural problems?

    Lohring Miller
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Hydrofoils

    Lohring, if you haven't seen this it is worth a look. It's a comparision of a T foil with "V" foil and ladder foil. I'm hoping Tom will update this when he gets the time:

    http://www.tspeer.com/Hydrofoils/generic.pdf
     
  3. tspeer
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    tspeer Senior Member

    There are symmetry and stability considerations that drive the choice of foil type. For example, the Moth has to sail on both tacks and can only have the foil attached to the centerline. So that drives it to the T foil. (The original foiling Moth, Mark Pivac's surface piercing design, used inclined foils mounted on the hiking racks, but these were later deemed illegal, as they made the boat a trimaran.)

    Two factors drive catamarans to L foils. One is a limitation on maximum beam. T foils would either extend past maximum beam or require the hulls to be set more inboard, as with Off Yer Rocker, the unsuccessful C-class cat. The other factor is exploiting the coupling between leeway and vertical lift from the L foil, so as to provide positive heave stability.

    But the dihedral angle required to get substantial positive heave stability also increases the drag, in much the same way lee helm increases the drag. So the design of an L foil becomes a balance between structure (limits foil span), heave stability and drag.
     
  4. bcv99
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    bcv99 Junior Member

    open L-foil with flap

    What about a L-foil with a more open angle than today foils, more like 120-150°, and getting the heave stability by an active controlled flap system a la moth. Idea would be to get all the side force and lift from the angled submerged part, which makes the surface piercing, vertical part of the foil a strut (beside maneuvering). Could that be a way forward?
     
  5. lohring
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    lohring Junior Member

    Some of the earliest foilers with submerged foils used T foils with flaps while the Trifoiler used an L where the whole foil angle changed. If total width wasn't an issue and you were free to use any control system, what would be "best"? If L foils are lower drag, why didn't the AC 72s use L rudder foils?

    Lohring Miller
     
  6. tspeer
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    tspeer Senior Member

    You need to get heave stability from somewhere. For the AC72, the Class Rule did not allow sensors or movable control surfaces, and the windward foil had to be retracted to within 0.5 m of the hull. So there weren't many options to get the stability to where the crew could control the boat.

    There is a range of dihedral angles that provides positive heave stabilty. But there's also a range of dihedral angles that is destabilizing. If you have, say, a 120 deg angle, then as the boat rises, and the leeway angle increases, the vertical lift on the angled board increases, raising the boat further. This is very destabilizing.

    The induced drag is also going up because the span is shrinking. Whether the net drag is increasing or decreasing depends on whether the reduction in wetted area can offset the increase in induced drag. I suspect not. The AC72 experience was the best performance was obtained at the lowest flying height that allowed the hull to clear the water.
     
  7. tspeer
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    tspeer Senior Member

    Loads on the rudders were a concern. The rudders needed to be thin for low drag and long to stay in the water when foiling. Maximum beam limitations meant that the rudder wings were asymmetric, so they were already part way to being L foils. There was a limit to how far off center the lift on the rudder wings could be and not overload the rudders.

    BTW, maximum loading on the rudder wings wasn't when flying. The rudders and wings had to be designed to take cavitation-limited maximum downward lift on the rudder wings during a near-pitchpole event.
     
  8. tspeer
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    tspeer Senior Member

    This is a structural issue. L foils put a large eccentric loading on the daggerboards. The loading from a T foil can be centered on the strut. This allows a larger span for the T foil for the same root bending moment of the strut. The increase in span will reduce the induced drag.

    You can't look at any foil design in isolation, and there is no solution that is the best for all cases. Foils need to be optimized in the context of the whole system and the requirements for performance, stability and control.
     
  9. lohring
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    lohring Junior Member

    Thanks for all your answers. There's nothing like real experience with all the issues. You are very patient with us engineering hobbyists who like to study what's actually happening and design thought boats.

    It still seems to me that L foils will have advantages in small boats because the aspect ratio includes both the foil providing vertical lift and lateral force. Scale effects really help with structural problems as things get smaller.

    Lohring Miller
     
  10. Karl Wittnebel
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    Karl Wittnebel Junior Member

    Not sure I buy the structural argument. Just make the strut thick enough where it needs to be thick. It is out of the water most to the time anyway.
     
  11. lohring
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    lohring Junior Member

    However, in an L foil rising out of the water reduces the total aspect ratio increasing induced drag if the coefficient of lift stays the same. The advantage of an L foil would be that the aspect ratio of both the lifting and the lateral force producing foils add together to give the total aspect ratio. That is the sum of the areas divided by the sum of the spans squared gives the aspect ratio.

    With a "strut" acting to provide lateral force and the foil producing lifting force, the aspect ratios are separated. Thus only the lateral force producing foil has its aspect ratio reduced as the boat rises. If the boat rises as speed increases off the wind, less lateral force would be needed so the net drag might be lower. Since this also happens with L foils, it isn't clear to me that either type of foil provides an obvious advantage. It probably needs analysis for each individual situation.

    Lohring Miller
     
  12. tspeer
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    tspeer Senior Member

    Aspect ratio is misleading. The lift coefficient is not held constant as the flying height is varied. Instead, what is held constant is the side force and vertical force. Because the vertical area is changing the leeway angle changes to maintain the side force required to oppose the loading from the sail rig.

    The induced drag is inversely proportional to the square of the effective span. So the induced drag rises as the boat rises. However, the wetted area is also decreasing, which reduces the parasite drag. Initially, there may be a reduction in drag, but there will come a point where the increasing flying height results in enough induced drag that there is no benefit in flying higher.

    The change in leeway angle has a different effect on an L foil than for a T foil. The change in lift on T foil is more balanced than for an L foil. The end of the horizontal lift distribution on the vertical strut induces a downward change in angle of attack on the leeward panel of the T foil and an upward change in angle of attack on the windward panel. If you add dihedral to the T foil, the two panels are again affected in opposite directions by leeway. This results in a rolling moment, but not a big net difference in lift.

    With an L foil, the load induced by the strut on the horizontal panel is in the upward direction. This can make an L foil unstable in heave, as the vorticity shed at the end of the strut can result in more lift on the horizontal L foil as the boat flies higher and the remaining strut area becomes more highly loaded. However, with dihedral, the horizontal panel of the L foil can experience a reduction in angle of attack with leeway. This is stabilizing in heave. It is also detrimental to performance because the lift on the horizontal panel has a leeward tilt to it, which requires more horizontal load on the strut.

    So there's a big difference in the stability and performance between L and T foils.

    Making a foil thicker leads to more drag and a lower incipient cavitation speed. If you assume the boundary layer is fully turbulent, the shape of the foil is almost irrelevant to the drag, and the drag varies in the same way with thickness for nearly all section shapes. There's a big incentive to make the foil as thin as possible consistent with the structural demands. L foils have large bending moments that make the structure particularly difficult compared to the more balanced T foil.
     
  13. Karl Wittnebel
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    Karl Wittnebel Junior Member

    I think you misunderstood my comment.

    The "structural argument" refers to Dr. Speer's comment on L vs T. Obviously the L puts more bending moment into the vertical part of the foil (aka strut). So the strut needs to be thicker to have the same deflection while sailing as a T foil, if they are equal length. Apparently Oracle thought thickening the strut would create more drag than using a T foil. I am challenging that assumption.
     
  14. Karl Wittnebel
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    Karl Wittnebel Junior Member

    Sorry I did not see tspeer's reply before my last post.

    I suspect it is a lot easier to keep a radiused bend on an L foil from cavitating than to keep a T-foil intersection from cavitating. The heave stability thing is interesting and another example of accepting increased drag to obtain heave stability. I would think any decent heave control system on the main foil would be able to keep an L-foil sufficiently immersed to prevent ventilation, but obviously these boats didn't have very good mainfoil heave control, so perhaps the T helped the rudders stay in the water.

    When I mention thickness I am not referring to % section thickness, but to absolute thickness, which is the relevant parameter for stiffness. So for an L foil I would simply scale the strut dimension up until it was thick/stiff enough in the right places, which would be the root (again not immersed at speed) and perhaps the bend, which could be faired if required.

    I was not aware that using a larger foil of the same section would lead to lower cavitation speed; if true that seems counterintuitive!

    Interesting problem to think about.
     

  15. tspeer
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    tspeer Senior Member

    I was referring to physical thickness, too, but keeping the chord the same and thus changing the thickness ratio. You're right that a larger section of the same thickness ratio would have the same cavitation speed or higher (because it would be more lightly loaded).

    There were effective limits to how large the chord and thickness could be on the AC72 foils, too. The limitation had to do with the definition of a hull. The hull had to displace at least 45% of the weight of the boat when in measurement condition. Public Interpretation 22 clarified that this calculation had to be done with the daggerboards in the full down position. If the daggerboards had too much volume, they would reduce the displacement of the hull below 45%, and the big long things on each side would no longer be hulls.

    In addition, the upper portions of the daggerboards were in the water most of the time on the upwind legs, and upwind speed was still relevant despite the time spent foiling. So it wasn't possible to simply increase the size of the daggerboards willy-nilly to meet the structural demands.
     
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