# Lowest Drag for hydrofoil designs

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

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### philSweetSenior Member

lohring,

For the large bend angles you are looking at here, it doesn't work that way. You have to calculate an effective aspect ratio for cases where the span geometry is not a flat plane and the ideal lift distribution can't be assumed to be elliptical either. The interest in aspect ratio centers on reducing Induced Drag. Induced Drag can be simply regarded as lift in the wrong direction. In a flat wing, the only wrong direction is tilted back from the perpendicular to to the free stream because there are no lift components in the span-wise direction. But if you have a wing with a span than curves in the y-z plane, you do have lift in the span-wise direction, and that adds to the induced drag. You can't just roll an elliptical distribution along an arbitrary curved foil or map an elliptical distribution to a projection of the foil on the xy plane. You have to set up the integrals that led to flat wing results and redo them in 3-D.

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Phil, Greg Ketterman says that it works exactly the way Lohring described it.
He said(in his paper, I think) that the radius on his Trifoiler/Longshot foils was there specifically to allow the increase in effective aspect ratio.
His exact words are quoted in this* thread ,post 54. And there is a link there to his paper as well:
Greg said: (in reference to whether it was better that the "L" foil point inboard or outboard)
"A simpler way to look at it is the aspect ratio of the horizontal foil adds to the aspect ratio of the vertical foil when the foil points in. Aspect ratio is everything with hydrofoil sailboats."

* http://www.boatdesign.net/forums/sailboats/dinghy-design-open-60-influence-36401-4.html

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### lohringJunior Member

Tom, again thanks. Those are the effects I'm trying to understand. Doug has pointed out my basic thinking. I've been getting some input about stability with L foils from discussions on various forums. At this point I felt the main heave stability from L foils came from surface piercing of the upward angled tip section. See below.

Lohring Miller

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### tspeerSenior Member

Flying so high the tip comes out of the water is inefficient. Look at the hole in the water at the foil. It isn't necessary for the tip of the L foil to be near or above the surface for it to provide heave stability. V foils work by reducing the vertical lifting area as the boat rises, but that's not the way L foils work.

L foils work through a much more indirect mechanism. It's the horizontal lift-producing vertical area that is changing. This results in the leeway angle changing, and it's the leeway that affects the vertical lift from the horizontal surface with appropriate dihedral.

Instead, best performance is obtained when the hull is just above the water and the induced drag is at a minimum. Here's the way to do it:

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### Karl WittnebelJunior Member

Then I suppose one could increase the % thickness at the root to keep the wetted surface area down for the root, which is rarely immersed at cavitation speeds, and proceed from there.

I'm just challenging the notion that having the lateral part of the T tilted/lifting to leeward is less draggy than designing an L foil that is a bit thicker. Perhaps the loads are high enough that this would result in a VERY thick foil?

Of course if an L foil was unstable in heave then you are probably stuck with a T, unless the mainfoil control could be improved.

I'm sure it all ended up in a VPP somewhere.

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### philSweetSenior Member

<edit> Doug, I'm not saying there isn't some good to come of it. I'm only saying that it is going to be less that what lohring was suggesting, which I took to mean he was treating it mathematically as a flat wing, when it isn't. Lohring said "add together to give the total aspect ratio". Kettering just said it was helpful.

Last edited: Nov 8, 2013
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### Mikko BrummerSenior Member

Tom, is the rudder T-foil producing negative or positive lift at this point? Lifting the stern up or pulling it down? Is it most always lifting, except when close to pitch poling?

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### lohringJunior Member

Exactly. However, with video of the C cats as well I see the surface piercing effect as the last resort limiting height, especially as the boat heels and the foil becomes a V surface piercing foil. The AC 72 crews obviously got their manual (really) height control down better than the C cats.

Given all this, it still isn't clear to me whether the L foil could have advantages over T foils, especially if you have enough control power to adjust the angle of the whole assembly.

The L foil needs to run at a constant, low height, especially when carrying a high lateral and lifting loads. As the lateral load changes is the heave stability reduced? Running shallower off the wind reduces the lateral area, the aspect ratio, and the coefficient of lift of both the bottom and the side foils. How does this complex interaction effect drag over the speed range?

With a T foil the aspect ratio of the lifting foil is constant as is its area, creating unneeded profile drag above the design speed. Flap or total angle controls match lift to weight over the speed range. Lateral force is controlled independently by "strut" depth and angle.

Sailboats need to foil over a wide speed range. Both types can be optimized for a small range of conditions, but which has more potential in off design conditions? The Hydros C cat had a lot of possible adjustments on their foils including reversing them in drifting conditions. T foils don't have these options but seem to be easier to analyze.

Lohring Miller

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### Doug HalseySenior Member

When the boats heel far enough, the L foils sure start looking a lot more like V foils, though.

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### Doug HalseySenior Member

It's actually not that simple. In many cases, lift in the "wrong" direction can produce enough favorable interference to make the induced drag lower.

A good example is a conventional wing with dihedral. Increasing the dihedral angle causes a significant reduction in induced drag, even though the left & right halves of the wing are producing opposing side-force components. This is true as long as the total span is held constant. (Reference : Hoerner)

Another example with actual curvature, not just angled panels, is the ring-wing. This shape produces force components in all different directions, yet has only a fraction of the induced drag of a planar wing of the same span.

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### philSweetSenior Member

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Thanks for that paper Phil!

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### tspeerSenior Member

There were times when the boats flew so high that the tips of the L foils broke the water. But that was not their typical flying condition. The drag is extremely high when doing that and the boat is not at all competitive. But it makes for a great picture!

When the boats flew high, they would reach a point where the leeway angle increased dramatically and the boat seemed to skid sideways. That often preceded heeling to windward and the windward hull crashing into the water.

Take a look at these photos made while racing, and see how much more the foils are immersed. They have just enough flying height to allow for variations in height control without touching down.

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