Hydrofoil winglets?

Discussion in 'Multihulls' started by Stefan H, Jan 5, 2010.

  1. Stefan H
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    Stefan H Junior Member

    Hydrofoil wingtips

    Then, why not just change the angle of the whole foil instead of using angled "down" wingtips?

    There is still no answer why the wingtip is angled "down" on almost all racing boats surface piercing foils. And I find it hard to believe it is just because it is fashionable.

    If you don´t know the answer please don´t tell me it´s difficult. I got a basic understanding how hydrofoils work but I´m not an engineer so before start building my own hydrofoils I though I could get some more answers here.

    Just to summarize what I already know about surface piercing foils thanks to Tom Speer and others that share information:

    1.) If possible, increasing the span of the wing reduce the total drag more compared to adding winglets.

    2.) Vorticies developes bihind the entire wing, not just at the tip. (There are others that believes the opposite though...http://airlineworld.wordpress.com/2008/10/01/aircraft-winglets/)

    3.) A curved C foil has less vertical lift vector close to the surface compared to a straight foil which is beneficial, although they more likely ventilate.

    4.) The angle of attack shifts when going upwind which increase drag and lift

    5.) The healing of the entire boat change the angle of the foil creating either more or less vertical/horizontal lift.

    /Stefan
     
  2. daiquiri
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    daiquiri Engineering and Design

    Take it easy, please. Why cannot I say a simple phrase like "Winglet design is a tricky job"? It is.

    Did you notice that I had given you some quantitative info? It's up to you to see whether or not you want to take note and use it.
     
  3. Stefan H
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    Stefan H Junior Member

    Sorry, nothing personal.. I really appreciate your input. Thankyou! Although it does not answer my question...

    ...I still take note.
     
  4. RHough
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    RHough Retro Dude

    Would you believe that they were not part of the original design and were added to fine tune it? Easier than changing the angle or span of the whole foil. :)
     
  5. daiquiri
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    daiquiri Engineering and Design

    Stefan, in the meanwhile I was thinking about your original question...
    Basically, in order to visualize and understand the pressure distribution around winglets, you need to understand what they essentialy do: they enlarge the wortex wake width behind the wing (or foil, in the case). For a wing with a fixed wingspan, the final result is similar to having wingtips shifted further apart, with the consequent reduction of induced drag.
    Knowing that, it means that following cases can be distinguished:

    A) winglet bended towards suction side of the wing (common upward-pointing winglet on airplane wings)
    This configuration will have the following properties:
    - the suction (low-pressure) side of wing and the suction side of winglet coincide, thus the lift loading is increased in proximity of wing-tips
    - the same is valid for pressure (high-pressure) sides of both wing and winglet, so the wortex sheet is effectively spread outwards.
    BUT, being the lift loading increased in proximity of wing tips, it means that the peak low pressure is strenghtened, when compared to the case of wing with no winglet. Take a note of this, because it will lead to another important conclusion later.

    B) winglet bended towards pressure side of the wing (that is the case of Hydroptere's foil)
    This one will have the following properties:
    - the suction (low-pressure) side of wing and the suction side of winglet are facing the opposite sides, thus the lift loading is decreased in proximity of wing-tips
    - the same is valid for pressure (high-pressure) sides of both wing and winglet, so this configuration is not effective in spreading the wortex sheet outwards.
    BUT... being the lift loading in this case decreased near wing tips, it means that the peak low pressure will also be reduced in the zone around the winglet, when compared to the basic-wing case.

    So what happens in case of Hydroptere is that a reduced peak pressure around the wingtip (foil tip) will delay the onset of cavitation in this zone, and I believe this is the most important reason why the Hydroptere's design team have chosen to adopt it. They have somewhat sacrificed the overall wing efficiency in therms of lift distribution, but have probably gained it on the other side - through a reduction of cavitation, particularly in the area around the tip.
     
  6. Stefan H
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    Stefan H Junior Member

  7. jehardiman
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    jehardiman Senior Member

    If it is a fine tuning device, you would still have it on the next design, it would just be smaller.

    Anyway, I believe daiquiri answered your question and I concur that it is down most likely to prevent tip cavitation. Hoerner points out that that it doesn't matter which side (or both) the wignlet is on. Aircraft generally rolled them up for the same reason airliners got retrofitted with them, clearence issues. The 787 doesn't have them because it wouldn't fit any existing jetways anyway so why take the drag and weight hit just for fashion.
     
  8. Stefan H
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    Stefan H Junior Member

    Tip cavitation

    Very good explanation Daiquiri, Thanks!! :)

    So they are simply sacrificing some wing efficiency in lower speeds in order to increase the top speed? Then for low speeds (<15 knots) a straight foil is a better choice?
     
  9. Stefan H
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    Stefan H Junior Member

    Good point
     
  10. daiquiri
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    daiquiri Engineering and Design

    I think that you need to analyze the probability of cavitation onset at a given speed, in order to answer that question. A more comprehensive conclusion could possibly sound like this: for speeds at which the extent of cavitation is within acceptable limits, a straight foil would be more efficient. ;)
     
  11. Doug Lord
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    Doug Lord Flight Ready

    I'm fairly certain that I read someplace that the downward vertical fin used on Hydroptere and some other surface piercers was used to help with lateral resistance* at high speed-does that make sense to you, daiquiri? That would be in addition to the other benefits such a fin would have....

    * perhaps unloading whats left of the main foil area at high speed thereby reducing the chance of ventilation?
     
  12. jehardiman
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    jehardiman Senior Member

    I've thought about that also at first, but their latteral effect would be small compared to the foil itself. It is obvious that the winglet is designed to be nearly vertical and straight ahead while foilborne. It is a possibility that they are to compensate for the increasing (or decreasing...depends on exact foil axis to CL) effect of the windward foil lift as the vessel heels.

    Another explanation may be that winglet gives positive directional stability to the foil and therefor steering stability while foilborne. Normally, there would be a signifcant moment about the foil root in yaw. Additionally as you point out, if the foil had some flex, the winglet could aid in maintaining AoA by twisting the foil across it's span as speed increases. Or it could also act as a vibration damper...or a little of all those things.

    But the big thing I believe is that the winglet binds the tip rollover vortex off the main foil to prevent back cavitation on the main foil which would be devistating at speed.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Doug, Jehardiman, I believe that a truth is the sum of each one of the points expressed till now. Sexy-looks theory inlcuded. :D
     
  14. Doug Lord
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    Doug Lord Flight Ready

    Beauty and the Beast-if it works it must be pretty....

    Guys, I finally remembered where I was told that the tips were used as lateral resistance(as well as directional stability aids as well as so on and so forth). It was from Brett Burvill about five years ago. Brett designed a surface piercing Moth foiler which was the first Moth foiler to win a race. Bretts foils were mounted on the wing racks and were ruled out of the Moth class as a "catamaran" configuration. For surface piercers, the foils were pretty close together-63% of main hull length apart on the Moth where, for reference, Hydroptere's foils are more like 1.33 times hull length apart. This was one of the factors that made the system very difficult to control downwind. The vertical tip fins made it more controllable.
    The picture below tells most of the story:
     

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  15. daiquiri
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    daiquiri Engineering and Design

    In that case they are obviously functioning on a completely different premise respect to winglets intended to reduce induced drag. That is a news to me, thanks for sharing that info.
     
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