C foils

Discussion in 'Multihulls' started by GrahamR, Jan 14, 2018.

  1. GrahamR
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    GrahamR Junior Member

    Hi,
    I would like advice from anyone who has sailed beach cats with c foils, so A class, Nacra 17 etc.
    I'm in the process of completing a 15' single handed cat which I designed a couple of years ago. Amongst the gear I have picked up are a pair of c foils from an A class, these may possibly be regarded as a bit old hat amongst the sailing elite nowadays but I'm thinking they might prove to be a useful halfway stage before graduating to a fully foiling set up. I would add that the boat has removable daggerboard cases so that trying different foils will not involve major surgery.
    I've been told that c foils are best used in conjunction with rudders that have tiplets, and watching footage of the first iteration Nacra 17, which as I understand didn't, would seem to support this view. They seem to spend a lot of time with bows pointed skyward, so my concern with a shorter, lighter cat is that this tendency could cause problems.
    Any advice gratefully received.
     
  2. bjn
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    bjn Senior Member

    Hey!

    Sounds like a cool project!

    I will start by saying that I have zero experience to share, can only share thoughts from looking at retrofitting foils on my small catamaran... =)

    I think c-boards seems like a great start. And if board rake and cant is adjustable, you will probably be able to find a setting for full foiling with the c-boards. I think rudders needs elevators, and with adjustable rake, to find the right angle on the elevators.

    I think rudder elevators will also help preventing pitch poles (not only preventing flying up out of the water). So for only that reason I will fit them on my cat. And then maybe lifting daggerboards later.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    The only thing about "C" foils is that they don't have the intrinsic, automatic altitude control of uptip foils-so even with rudder t-foils they are not likely to fly well.
    One problem is that as the foil goes faster and lifts more it develops more lift- not just due to speed but also due to the angle* of the foil. Also, taper of the foil can reduce this effect. The "C" foil works for foil assist if the lift is limited so the hull the foil is attached to doesn't lift clear of the water.
    *Note the angle of the foil at the three different flight waterlines in the sketch below:
    C Foil.png
    Farrier foil assist ama "C" foils:
    Curved foils-Farrier- tight curve 3.jpg
     
    Last edited: Feb 13, 2018 at 5:07 PM
  4. UpOnStands
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    UpOnStands Senior Member

    not seeing this.
    The angle of the near horizontal component of the foil remains constant as the foil lifts (assuming vertical lift) and so generates exactly same lift force per unit area at the same speed until
    the lifting surface approaches the water's surface and loses effectiveness. Maybe you mean the near horizontal component becomes a larger percentage of the submerged foil as the foil lifts.
    If both foils have the same profile along their entire length, we can expect the near vertical sections of port/starboard foils to develop opposing inward forces which would be very draggy and raise the speed at which the hulls fly.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    The point of the sketch was to show that as the "C" foil lifts the angle of the foil at each waterline becomes closer to horizontal with a greater component of vertical lift. When you combine that with the natural increase in lift with speed , you have a potential crash in the making. If the foil is not designed carefully for its application the thing could have a tendency to breach the surface. It has no altitude control, so it has to be designed with that in mind. A surface piercing foil like Hydroptere's foils would maintain the same lift at different speeds(with less foil in the water as it speeds up)-the "C" foil with a rectangular planform can show an increase in lift as it goes faster along with less foil in the water!
    It just points out what a revolutionary development uptip foils are for many applications.

    PS- before uptip foils many racing tri's were equipped with "C" ama foils that had to be designed for a certain top speed and maximum lift. When speed exceeded what was designed for, the foil could breach and cause a crash.
     
  6. UpOnStands
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    UpOnStands Senior Member

    ??
     
  7. Doug Lord
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    Doug Lord Flight Ready

    The thin red lines represent an approximation of the increased proportions of vertical lift as the "C" foil rises. The "C" foil has no intrinsic altitude control as does a surface piercing foil or an uptip foil.

    C Foil.png
     
  8. UpOnStands
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    UpOnStands Senior Member

    Yes, that is what I thought you were saying. But is your idea of "approximating the foil" correct?
    Consider wl#1. Most of the submerged foil is generating strong vertical lift as it is nearly horizontal.
    That lift is not opposed or offset by negative lift from the foil nearer the waterline.
    As your red arrow indicates, the section of near vertical foil close to the waterline generates more lateral force than lift; but the majority of the foil is still generating lift.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    Thats why the foil has to be carefully designed so that it doesn't lift the hull of the boat its attached to. Once it lifts the boat clear it is close to being out of control. Even rudder t-foils wouldn't help when it loses control with runaway lift. So, again, carefully designed "C" foils can make good foil assist foils but they are not good full flying foils.......

    PS- I had no idea of "approximating the foil"? No idea what you meant.....I approximated the thin red lines and their length* and direction....
    *as an indicator of the greater proportion of vertical lift at different flight waterlines.

    ====================
    UPDATE-2/16/18
    -----
    ProBoat article on Daggerboards (note this article was written in 2010-before uptip foils. Some people today refer to uptip foils as "J" foils. In this article Pete Melvin talks about real "J" foils which most definitely are not uptip foils!)
    Daggerboard Debate - Professional BoatBuilder Magazine https://www.proboat.com/2010/04/daggerboard-debate/
    ------
    Flexible "C" foils on A Cats: Brett Burvill on Windrush's alternative C foils http://www.a-cat.org/?q=node/326
    ------
    NACRA 17--see page 26
    Dropbox - Meeting Presentation.pdf https://www.dropbox.com/s/x44cb22wlw5zfvb/Meeting%20Presentation.pdf?dl=0
    ------
    Dario Valenza-- Imagineering Part 1 - Carbonix http://carbonix.com.au/imagineering-part-1/
     
    Last edited: Feb 16, 2018 at 9:29 PM
  10. UpOnStands
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    UpOnStands Senior Member

    this is the statement that is causing confusion.

    T-foils provide no incremental feedback to control height. If boat speed is high enough to carry full displacement then T-foils will rise until they broach. [minimal drag and excellent hull clearance: not removable from the top]
    Slant flat foils, say 45% angle, provide good incremental feedback to control height. As they rise, the surface area generating lift falls in direct proportion to hull height above the water. They greatly reduce broaching. [worst drag and moderate hull clearance: can be withdrawn]
    C-foils provide only weak incremental feedback. Their total effective lifting area starts to fall only when the main lift area is close to the surface. Broaching is likely. [intermediate drag and poor hull clearance: can be removed from the top: partial lift alters lift characteristics greatly.]
     
    Last edited: Feb 13, 2018 at 9:05 PM
  11. tspeer
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    tspeer Senior Member

    What you're seeing in the boats' behavior is unstable pitch-heave coupling. Basically, the boat is rotating about the stern. This leads to an unstable situation, where lift from the C foil raises the boat, but the stern stays at the water surface. This means the pitch attitude increases, which increases the angle of attack on the foil, and increases the lift on the foil. Which makes the boat rise up more, pitch up more, and get still more lift.

    With a horizontal foil on the stern, when the pitch attitude increases, the increase in lift on the stern is enough to raise the stern. This reduces the unstable coupling between the lifting of the boat by the C foil (heave) and the pitch attitude. What you'd really like to see as the boat rises up is a more bow-down pitch attitude. This is a stabilizing influence, as it reduces the lift on the daggerboard as the boat flies higher.

    If you have a shorter, lighter cat, you might be able to tame it with larger foils on the rudders.

    As for the nonsense that's been posted about C foils not being able to self-regulate in heave, I suggest you take a look at this presentation. You need to include both the leeward and the windward foils together when you consider how they behave.
     
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  12. UpOnStands
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    UpOnStands Senior Member

    Very complicated as heave can be triggered by momentary or persistent factors? Boat has fwd speed and so its momentum is significant.
    Momentary: If an external force, say wind gust on sails, pitches the boat bow up, the momentum will heave the boat momentarily but with a commensurate loss of speed, lift is reduced, heave is reduced.
    Persistent: Increase in sail loads due to wind strengthening drives boat faster and thus foils provide greater lift. Boat heaves for far long duration.
    Is it possible to design main foil and rudder foil so that as boat speed increases the rudder foil generates proportionally more lift and thus tends to pitch the hull bow down?
    That is at 15 k main foil generates 130 kg/sqm, rudder foil 80 kg/sqm. At 20 k main foil generates 150 kg/sqm, rudder foil 120 kg/sqm.

    As regards height stability, I made the statement that C-foils provide only weak incremental feedback. Is this right? My height stability is your self-regulation in heave?
     
    Last edited: Feb 16, 2018 at 7:48 PM
  13. tspeer
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    tspeer Senior Member

    That happens somewhat automatically, by virtue of the boat's pitch stability. The daggerboard rake is controlled by the crew to keep the vertical lift in equilibrium. The rudder foil needs less angle of attack to maintain the pitching moment equilibrium at higher speeds, and if the pitch attitude and rudder rake were kept constant the rudder foil would generate excess bow-down pitching moment. Instead, the boat pitches down until the rudder foil reaches its equilibrium angle of attack.
     
  14. UpOnStands
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    UpOnStands Senior Member

    I was thinking more in general terms, not for this particular boat. My interest is not active controls but more of a built-in passive control system. Something that does not need large budget, lots of maintence or lots of crew.
     

  15. tspeer
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    tspeer Senior Member

    Three conditions are needed for static stability in the vertical plane. Starting from equilibrium flight:
    - An increase in height at constant pitch attitude should result in a decrease in lift,
    - A bow-up change in pitch attitude at constant height should result in a bow-down change in the pitching moment,
    - An increase in height at constant pitch attitude should result in a bow-down change in pitching moment.
    These conditions are satisfied by:
    - heave stiffness of the foils (possibly moderated by leeway coupling),
    - the forward foil having a greater loading than the aft foil (by virtue of the center of gravity being forward of the neutral point),
    - the forward foil having greater heave stiffness than the aft foil.

    In aircraft, for stable speed stability an increase in speed should result in a nose-up pitching moment, which also comes from placing the center of gravity ahead of the neutral point. This makes the aircraft climb and slow down when it speeds up from equilibrium. I'm not sure the same condition is needed for sailing hydrofoils, because of the strong influence of heave on the dynamics. An increase in speed at constant height and pitch attitude will result in an increase in lift. When the three conditions above are satisfied, the increase in lift will be offset by an increase in the flying height, and the pitching moment equilibrium will be satisfied by a bow-down change in the attitude. There is also an external bow-down moment that increases with speed due to the height of the center of effort of the rig.

    So it is possible to get the behavior you want without active control. Even for the AC catamarans the medium speed and fast dynamic modes were naturally stable. The instabilities showed up in the long-period modes, which were well within the bandwidth that could be controlled manually.
     
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