Advantages/ Disadvantages of Curved Lifting Foils on Monohulls/Multihulls

Discussion in 'Hydrodynamics and Aerodynamics' started by Doug Lord, Apr 6, 2011.

  1. Doug Lord
    Joined: May 2009
    Posts: 16,654
    Likes: 326, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ---
    Very interesting, Brian-thanks!
     
  2. bscally
    Joined: Sep 2011
    Posts: 17
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Seattle

    bscally Junior Member

    Hi All

    I would like to add a use case for people to consider.

    On hydro foiling tri-s it is not uncommon to have the windward foil partially retracted to cant the boat to windward.

    My assumption is that the foils now have an upwind component to their lift as well as the sail being more efficient. All this works to improve the VMG.
    Certainly the VMG in this mode is better.

    Truly foiling cats are a little rarer, but the theory should hold true.


    Brian
     
  3. Doug Lord
    Joined: May 2009
    Posts: 16,654
    Likes: 326, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ===============
    Brian, there has been some discussion in various threads about creating "veal heel", which is different than weather heel in multihull foilers. One of those just occured between me and Magnus Clarke(Canadian C CLass Little Americas Cup champ with Fred Eaton) here: http://www.boatdesign.net/forums/multihulls/mayfly-class-catamaran-fischer-39616-2.html posts 17,18,20 21. This happened as part of another discussion but it seems relevant.
    On multifoilers like the Rave which has two flight altitude settings(as you know) I'm wondering what the effect of such a configuration would actually be since the axis of heel would then be between the rudder foil and lee foil-diagonally. Have you tried this on a Rave in racing? On the Hobie Trifoiler I don't think it could be done. It's an interesting idea if both foils were lifting up , but if one foil was developing downforce if might not be of too much value. What do you think? Bradfield and I have never discussed this. On the Osprey,like the Rave, it has a slight heel to leeward.
    I came up with a multifoiler solution that might work: a trimaran with a bi-foiler arrangement on the main hull(on wand controlled main foil and a rudder foil) coupled with a "power foil" pulling down on the windward ama. Hope to test this in the next year.


    Rough Sketch: veal heel with "power foil" on tri-

    click on image-
     

    Attached Files:

  4. bscally
    Joined: Sep 2011
    Posts: 17
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Seattle

    bscally Junior Member

    ok so taking the Rave as the example....

    the rudder needs to be kept in the water to have lift and steerage.
    if the stern is too low the AA is steeper generating more lift. If the stern is too high the AA is low there is less lift and the stern settles.
    Obviously the AA of the rudder needs to be adjusted for the crew weight and the wind conditions. This is done by shimming the pintals.

    The racing rudder ( hand operated flaps ) makes takeoff easier, and allow trimming for in session variations - at the cost of some drag.

    Running with the windward foil locked at one position "up" rotates the hulls about 8 deg to windward, causing about a 1% drop in vertical force from each of the lift surfaces. However the gains off the rig are substantially greater than that. The issue is will the lift sections of the foils ventilate - and they appear not to.

    --

    We all know that - after takeoff - a hydrofoil has the potential of lower drag.
    The issue is how to get there, if you can't get to take off there underwater profile is just a heavy drag.
    - The weight needs to be minimized. This includes CREW and equipment. It all needs to be paired.
    - Thrust needs to be maximized - wing sail? stayless mast? Kite? low turbulence drive..
    - The drag has to be low enough to get you to take off speed.
    - Once flying the thing needs to be kept in the water.

    --
    This brings me back to the original topic ... Curved Foils

    They have a unique property of only being partially inserted and still offering almost all of their lateral resistance, but without the drag of the lifting section in pre-flight conditions.

    This seems to be a unique set of advantages for one simple foil shape.

    However back to my first point any significant up force forward of the center of mass should be used with caution.

    Mother Nature and Father Sea like to teach us respect. And if we are lucky it only hurts.

    Brian
     
    Last edited: Oct 4, 2011
  5. Doug Lord
    Joined: May 2009
    Posts: 16,654
    Likes: 326, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    MPX-curved foils eliminating t-foils.

    ==================
    You've got it! Good thinking, Brian.
    I'm working on a solution that would allow curved or "J" foils to replace t-foils in every case. To do that there must be a way for a wand to connect -viably- with a curved flap on a curved foil or "J" foil. I'm going to test this on my model(see the thread).
    1) First my "MPX" system is designed to allow a trimaran main hull to take off very early-in very light air. Most wide (square or over square) tris have so much RM that they won't fly the main hull until its really blowing. The main foil would be wand controlled so that the angle of heel and/or main hull altitude is preset and the main foil and rudder foil control the pitch attitude of the boat which intern sets the angle of attack of the curved ama foil.(no ama altitude control system(so far in my thinking). What happens as the boat starts to move is the ama foil lifts the ama clear of the water with or without crew assistance(movement) and the main foil lifts the main hull before it normally would due just to sail force. Next as speed builds the main and rudder foil unload up to a certain speed and the ama foil loads up. So only one curved foil handles the majority of the weight.
    2) The idea is to come up with a curved foil system to replace the mainhull t-foil and rudder t-foil-see the rough sketches below. For the curved or "j" foil to replace the main "T" foil there would have to be two of them-raised and lowered each tack. There would have to be a system to connect them to a wand. These foils are significantly more efficient than the T foil so may be able to be smaller. See Greg Kettermans paper. Brian, here is a link to a post with a link to Kettermans paper where he discusses the superiority of a "J"(or curved foil) particularly upwind. See his illustrations.
    Post 54 http://www.boatdesign.net/forums/sailboats/dinghy-design-open-60-influence-36401-4.html
    3) The t-foil rudder could be replaced by a rudder stock that supported two pivoting "J" foils-would have to be switched each tack.
    ---Both the new main and rudder foils would require the same work as on a cat-the t-foil eliminates that hassle. But for pure speed on foils this may be the way to go.
    --- To make the curved foil work with a wand and maintain all the other advantages of a curved foil is a very, very, very difficult task-and I don't have a solution yet. If I have to use the t foil on my 18 the good news is that it starts to unload almost immediately. The rudder "j" foils may be worth their extra weight -time will tell.

    Rough Sketch- shows two curved foils inserted in relative center of trimaran main hull and bottom sketch roughed out concept of a single rudder stock supporting two J foils to replace one rudder t foil.
    click on image for more detail:
     

    Attached Files:

  6. bscally
    Joined: Sep 2011
    Posts: 17
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Seattle

    bscally Junior Member

    Doug

    Greg Kettermans' paper is interesting, but there are some assumptions that I have never seen good answers to.

    For example there is an assumption that lift at high speed is 3 times greater than lift at take off speed. Upper lift is bounded by velocity - which is bounded by power - so where does the magic 3 number come from.
    Having a max flying speed of 3 times take off speed is not a bad thing, it is just an odd assumption.

    This is used to justify the foil area and structure - but has no real basis.

    Being the devils advocate - I would content that this is an irrelevance.
    - The foils and system need to be designed for a given takeoff speed.
    - The foil control system should be designed to generate positive and negative lift
    - The foils and system should be designed to minimize drag in all of it's forms since power is limited.

    To the last point the WPD gives the maximum force obtainable from the wind. In the surface boundary zone 200W/ m.m is an often quoted figure.

    That means an a-class has 2.8kW - 3.75hp to get it to speed. USA-17 has the order of 300kW - 400hp. These are theoretical (unobtainable) numbers.
    Since power is SO limited drag is our evil.

    --

    I am interested why you are planning on flaps rather than rotating the lift foil - aka the Trifoiler?
    Also have you given any thought to prefoil fins to stabilize the sub hull boundary layer in a preflight condition?

    Brian
     
  7. Doug Lord
    Joined: May 2009
    Posts: 16,654
    Likes: 326, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    =======
    Brian, Greg is very approachable and might answer your questions. It's hard, however, to argue with his obvious success with the Hobie T and Longshot.
    --
    A flap allows a wand to be used(vs "feelers") and allows the whole assembly to be retracted.
    I would much rather use a system that had flaps on a curved foil(s) rather than a T foil because then the whole assembly could be fully retracted. The T-foil can be pulled up but not 100% retracted like the curved foil. I am, however, finding it extremely difficult to design a system where the curved foil could be used with a wand in place of a t-foil-it is exceedingly difficult. A dual "J"-foil system might work(with the advantages Ketterman speaks of) if the lifting surface could be big enough but it still wouldn't retract; the dual "J" would work ok on the rudder albeit with a weight penalty. And I don't like the "looks" of the "feelers" out in front of the trifoiler.
    The SRT model is first and foremost designed to test the self-righting system and in that case the T-foil is required because ballast can be "hidden" at the juncture of the daggerboard and main foil.
    ---
    What are "prefoil fins"?
     
  8. bscally
    Joined: Sep 2011
    Posts: 17
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Seattle

    bscally Junior Member

    Going to pop a new thread. - Design of flaps on a curved foil.
    So it is obvious what we are up to.


    Though I firmly believe that lift foil rotation is a better way to go - due to drag.
     
  9. Doug Lord
    Joined: May 2009
    Posts: 16,654
    Likes: 326, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ----------------------
    The concept of an all moving foil has been tried without success -so far- in the Moth class which is a great inqubator of foil tech. One of the reasons Kettermans foils work is the short height relative to the "feeler" length. Also, the Osprey and Moth fly much, much higher than does a Trifoiler. But the impracticality of a foil that can't be retracted or flown in more than one position kills it for me.
    Both the Rave and Osprey foils are designed to be retracted and to be able to be flown in two positions( two different altitudes)-which is a good thing on a boat that would be a production sailing hydrofoil. "Low Drag" as a driver can only go so far when things like convenience are considered. However, I could be "driven" to try a dual curved foil(+flap) setup even though one would have to be retracted with each tack for a lower drag system on my boat(after testing the SRT). See rough sketch in post 35.

    Pictures: the value of retracting foils when beach launching, note the "notch" in the back edge of the foil about half way down-that is the way the foil is retained in its 50% draft(altitude) position.

    click on image-
     

    Attached Files:

  10. P Flados
    Joined: Oct 2010
    Posts: 603
    Likes: 33, Points: 28, Legacy Rep: 390
    Location: N Carolina

    P Flados Senior Member

    Doug, have you really considered all the options?

    A cassette arrangement could be engineered to allow both rotation and retraction. The parts and pieces would need to be robust, but it could work. A mostly circular curve would even allow wetted surface area adjustments as needed.

    The bigger limitation for full foil rotation is the sheer energy associated with the dynamic forces and torque required for control. This takes either the TriFoiler style sensors way out front, or some form of power assisted control .
     
  11. Doug Lord
    Joined: May 2009
    Posts: 16,654
    Likes: 326, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ================
    Thanks, PF ! I think that one of the reasons that the Trifoiler flys so low compared to the Osprey is that if you went for the higher altitude the forces on the foil might be hard to control with feelers.
    Power is out and "feelers" are out ,for now anyway.
    Do you have any ideas on how to make a flap work on a curved foil?

    Pictures: note difference in flying altitude between the Osprey and the Trifoiler:

    click on image-
     

    Attached Files:

    Last edited: Oct 11, 2011
  12. P Flados
    Joined: Oct 2010
    Posts: 603
    Likes: 33, Points: 28, Legacy Rep: 390
    Location: N Carolina

    P Flados Senior Member

    With a true curve and traditional methods/materials, I come up blank.

    If you were to let the main portion (constant cord and constant curve section) terminate some distance from the end, there would be a couple of choices. A straight section with a larger than normal flap would be one choice. An entire tip that rotates would be another choice. With the entire tip rotating, you would actually gain an advantage with planform freedom and the ability to select an airfoil optimized for control requirements as compared to high lift to drag performance. This also frees you up to select foil secitons for the main portion that do not need the control aspects.

    If you really wanted a chalenge, a foil could be made with the ability to twist on demand. The outer shell would need to hold the foil section shape and it would need to provide bend stiffness but at the sme time have a low torsional stiffness. An inner element (speedometer cable on steroids) would connect to the tip and would be free to rotate inside the shell until it exits at the opposite end.

    A smaller than normal "active" (continuously modulating) control surface could be allowed if you went with a cassette or equivalent approch that allow a small range of on-the-fly AOA adjustments for the entire foil. This adjustment would only be used infrequently to deal with things like take off in marginal conditions, overall setting for heavy vs. light wind, boat loading, etc.
     
  13. Doug Lord
    Joined: May 2009
    Posts: 16,654
    Likes: 326, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ==================
    PF, thanks for taking a look at it.
     

  14. Doug Lord
    Joined: May 2009
    Posts: 16,654
    Likes: 326, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.