Design of flaps on a curved foil.

Discussion in 'Hydrodynamics and Aerodynamics' started by bscally, Oct 10, 2011.

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bscallyJunior Member

Starting a new tread for this so we are not mud-ing another one.

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bscallyJunior Member

The challenge is the implementation of flaps on a hydrofoil.

This has been performed on foils of regular section T and V, but too our knowledge never on J or C foils.

J and C foils offer higher efficiency and are attractive for choices where this is the driving design goal.

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Curved foil flap

Thanks, Brian. Here is a rough sketch of what I need to try to do(top of sketch). There are apparently big gains that could be made in drag reduction if dual curved foils could replace a single "T" foil. The extra hassle of retracting a foil with every tack is already dealt with on catamarans. This is a system for a trimaran. The basic system has a main foil(on the main hull) that is wand controlled and a rudder foil. The point is two fold:
1) small tris that are square or oversquare have a great deal of RM that prevents the main hull from flying until there is a substantial breeze. The main hull foils, using a wand altitude control system, are designed to lift the main hull early 5-7 knot wind significantly reducing drag and increasing speed in relatively light air.
2) the tri could use a foil in the ama and the foils in the main hull would control the angle of attack of the ama foil as well as the running angle of a planing ama, if used.
The beauty of the system is that as speed increases the main hull foils unload reducing drag still further.
===========
The idea is to replace a single "T" foil on the main hull with dual retractable curved foils as shown in the top sketch below. The problem is that I'm finding it exceedingly difficult to incorporate a flap in a curved foil that could be tied in to a wand altitude control sensor. It's probably going to require some sort of elastomeric flap combined with a curved hinge-really, really tough.
I found a patent on a slightly curved hinge but it looks pretty tough to implement. For this to work in my application the curved foil must work like any other curved foil and have an angle of incidence of vertical lift and side force adjustment independent of each other and be 100% retractable.

I found this patent on a curved hinge for hydrofoils: http://www.patentgenius.com/patent/4470366.html

click on image:

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markdrelaSenior Member

What brings you to that conclusion?
One advantage of a T foil is that the vertical part doesn't have the built-in bending moment that the one-sided J foil has. So it can be made smaller and thinner, which reduces wavemaking drag at the surface.

The T intersection also doesn't produce extra "interference" drag if done properly. The main trick is to stagger the strut and the wing, so the strut's max thickness point is well back on the wing, ideally close to the wing's TE. A generous Spitfire-type fillet helps further, and also adds volume to allow a better structural joint. All the foil joints on the Decavitator were staggered like this.

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Curved foil flap

Thanks, Mark. This paper by Greg Ketterman suggests that a t-foil making leeway can have high pressure and low pressure on the same side(see his illustrations) which he describes as causing high drag by reducing the effective aspect ratio: http://picasaweb.google.com/1123490...tTriFoiler?gsessionid=mjrhVAVUivoQwZjwdAd9UQ#
Another factor in favor of a curved foil is that it can be 100% retracted. It seems like a single curved foil would also be easier to build than a t-foil(once the tooling was complete).

--below: illustration from the Ketterman paper

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markdrelaSenior Member

That sketch is a very unconvincing argument. His J configuration has 4x greater bending moment at the bend than what's at the T or + configuration joint, so he's shooting down a strawman.

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bscallyJunior Member

markdrela

I somewhat agree, however ( A-Class cats ) only allow the board to be top removable.

there are a couple of other advantages of curved section foils. - Partial raising - to still maintain the anti leeway force.

My question is the bending moment 4X number that you put out there, why 4X?

I can't rationalize that number in my head (may be it is not enough coffee yet ).

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bscallyJunior Member

The first problem is to evaluate the building the mechanical flap structure. Leaving how to control and other solutions for another day.

Adding some parameters to allow some evaluation of the numbers
These are sub-cavitaion foils.
I am assuming a flap deviation of +-20deg and a flap length of 50mm
I am assuming a total foil height of ~2m
If the foil is C the foil would be a 90deg section of the radius.
Foil needs to provide 2kN of lift.

The obvious issue here is that on a curve the diameter is larger as radius increase.
This means that as the flap tip moves it's training edge needs to "shrink" in length.
And as it moves down the training edge needs to "grow".

The conventional boat response to this is to make the lift foil stiff with respect to the load and the tolerances of the pivot.

The solutions I have come up with:-
1) Making the flaps out of linear sections. These would have to be short relative to diameter of the curve. With a single mid pivot and have shorter trailing edges so they don't bind. This would mean there were V notches in the trailing edge if they are adjacent to one another. Also for larger section lengths to diameter ratios they may not match the foil section nicely.
2) Making the curve out of stiff linear sections. This allows conventional flaps on the straight sections. The flaps would not be adjacent to one another. Very applicable to J shaped foils. For C shaped foils the foil box may be "interesting".
3) Make the flaps out of a "bendable" material. Forces applied are probably great. Material will likely kink.
4) telescoping Flap Sections. Similar to one. But each section has a pocket on one side and an shoulder on the other that slide into one another. Nightmare for dirt and jamming.
5) Chained Flap Sections - these would slide along the flap area of the foil with a near point attachment. They would be joined together at close to the trailing edge.
6) Others??

- From these 2 appears to be the one that is build-able

Brian

Last edited: Oct 12, 2011
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=============
Brian, I'll be doing the tooling shortly for the (curved) ama foil for the SRT model. I'll make a foil out of that mold and use it to experiment with a flap(s).
The flap will be connected in some fashion to a wand for altitude control. On my boat there will be dual wands used one at a time and like the Bradfield Osprey the wand will remain connected as the foil is retracted. The "t-foil+ballast" version will be made first and after testing the curved foil will be experimented with. It is remotely possible, down the line, that two models will be made and tested simultaneously with the dual curved main foils and "J" rudder foils and the "T" foil version(minus ballast).
Thanks for giving this some thought.

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bscallyJunior Member

So I think I have a foil design for a retractable C curved flapped foil.

There are some interesting features in the design.

1) For the retractable length the foil is constant span. The cord is not constant.
2) The foil is 100deg arc, with flaps on about 35deg of the arc.
3) The foil is primarily supported on 2 planes (top and bottom of the foil box) with 2 sets of 3 or 4 runners ( or rollers ). These do not form a foil shaped grip - thus a gasket will be needed.
4) The training and leading edge of the flap section of the foil are linear approximation to the curve.
5) The flap sections are separated by solid sections to allow trailing edge transitions.
6) The foil section in the flapped region is not constant.

--

The big issues are ...

How much additional drag will the crinkle foil section have. Simulations running now YAWN.
Does the crinkly foil have any additional "interesting" properties?

Brian

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-----------------
Sounds interesting Brian. Sketches would be nice-rough ones ok. HOWEVER, if you think the system may be patentable AND you're interested in that sort of thing don't post the sketches.
--A constant chord is pretty much necessary to be able to reliably adjust the angle of incidence of the lifting portion of the foil independent of the lateral resistance portion-at least I think it is.
-- A foil section that is not constant will have some effect on spanwise flow but I'm but sure what-this is probably a big deal to research. I'm a lot more comfortable trying to do this with an already proven foil section like the 63412 or H105(? Tom Speer)-rather than having to invent a new type of foil. But maybe you're on to something.....
-- Fairly soon I will have a foil I can experiment with hands-on -might make a bit more progress then.
--Thanks for your effort-please share all the details you feel comfortable in publishing.

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WavewackerSenior Member

This is all Greek to me, but when I was a kid we built a small hydroplane, a neighborhood project. I thought that putting a J sideways on each side of a hull so that in a tight turn the outside edge would lift and the inside would cup the water holding it in the turn. I also thought if the J deepened aft that it could also form a planing surface. Turning kinda like driving a tractor.

Another idea was to lower these "brakes" on the inside corner as needed in turns. Is that something like you guys are talking about? Sorry to interupt, but this has been up awhile and just had to ask.

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J Foils

=========
This thread is about attempting to work out a system to add a flap that can be deflected up or down on the back end of a curved foil.
Using a "T"-foil, curved foil or "J" foil on a sailboat has to take leeway into consideration. Greg Ketterman, designer of the Hobie Trifoiler hydrofoil determined that a "J"- foil was less drag than a "T" foil, as far as he was concerned, in a paper mentioned earlier. He also discovered, thru practical experiments based on the theory he developed in that same paper, that the tip of the "J" foil had to point inboard on a sailing hydrofoil. He explained that if it didn't, it would develop more drag since the effective aspect ratio was significantly reduced. He tried foils on a prototype trifoiler pointing in both directions and the boat was measurable faster with the foils pointing inboard.
The same theory that applies to a "J" foil applies to a curved foil but I'm not sure about your application......
Hope that is a little help....

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bscallyJunior Member

Doug

Profiles I used were E836 got the non lift section.

In the flapped lift section I have an E836 blended to a E817 alternating ripple.

I hadn't though to use the NACA 63-412. But I thin the general principal will work for most profiles. So I am not getting hung on it.
As for the H105 it is a nice profile. The E817 has slightly lower top end CL, but comes on a little sharper.

http://www.tspeer.com/Hydrofoils/h105/h1053.gif

As for PA I believe that a similar technique was claimed in prior to 1940 by Supermarine Aviation Works and has thus expired. But I have put an email into my IP lawyer ( my brother ) to confirm the non claim ability. There may be some IP in the actuation method I plan.

If it proves logically effective I will go through the establishment process and throw it to a "GPL" model.

I am not publishing till ....
a) the sims come back at least OK ( I couldn't log into my cuda cluster from the day job )
b) Untill I have confirmed the IP situation as I won't open myself / us to having lawyers breathing down our necks because I thought up an idea later than somebody else.

<soap box on>
This is an area where we need brains on the problem, not on patent dancing.
<soap box off>

--
Brian

Last edited: Oct 18, 2011

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