Foil Ideas

On the next boat I'm designing, I want a new radical foil design. I was thinking about making a sort of circle for the daggerboard because I remember making paper "airplanes" that were a circle and I could get them to fly very well. So I was thinking that it would have the same effect as hydrofoils, just a little differently. Any ideas on this?

Tim

 

I like to see new ideas come to fruition.It would be helpful to know what advantage you perceive as attributable to a circular foil.My thoughts are that the aircraft you described were operating in a uniform fluid rather than trying to escape from a dense medium into a less dense medium.In addition,the load paths would be less direct that those found in a more conventional(?) foil configuration.The construction of a strong enough foil would then become challenging if the weight were to be kept reasonable.Please keep us updated on your progress.

 

Foils

Tim, a guy in the northeast built a boat with what were eliptical foils-I can't think of the name-but I'll look for it.
What size boat? Power or sail?

 

Defenitely sail, it could be either though if a person wanted to customize it. If you could find that for me that would be great. Thanks.

Tim

 

Foils

Tim, I've looked in most places and no luck-I'll try some more later.
Why not go with two t-foils and a wand? Just saw one of the most inspirational Moth stories from an Aussie Mothie-Phil Stevensen(?) who is also class measurer: he talked about matching speed with a Tornado cat off the wind just recently. The T was flying it's asy spin.

 

I was just thinking of a new innovative idea...Really if I was to ever build a boat with foils like t-foils I would go with the t-foils for simplicity but I just wanted to see if these would work.

Tim

 

SA Comments by Phil S

Here are Phil S's comments-see post #38:
http://www.sailinganarchy.com/forums/index.php?showtopic=27655&pid=574254&st=0&
Have you considered building an rc model to test it? You can get good results that way a lot cheaper than building the actual thing.. I built a 68" model Dr. Sam Bradfield used to test foils/wand concepts for the Skat 40 footer. And my own F3 is 56"LOA X 72" and works well.
Is the foil system you envision more like a surface piercing , fully submerged or like a ladder foil? In other words, I guess: does it require an altitude control system?

 

I might build an RC model...I would like to. What do you mean by what type of foil system it would be? Sorry for having to bear with me but I am not used to hydrofoils so I'm still learning. And for the altitude part, do you mean that it would control the height that the boat is above the water? If so I could figured a way for the "circle" to get smaller to lower the hull down to the water.

Tim

 

Foils

Tim, Fully submerged foil systems are designed so that foils stay at or below 2.5 times their chord below the surface. Ladder foils are, essentially, fully submerged foils arranged in a ladder-like formation and as the boat goes faster some of the "steps"(foils) of the ladder come out of the water until lift is matched exactly to speed. Surface piercing foils are designed so that they rise out of the water the faster the boat goes. Their advantage is that they sail with nearly the minimum area thats necessary at any particular speed. Both ladder foils and fully submerged foils are self regulating with speed determining the altitude the boat flies at(based on the designers detemination of likely speed). Fully submerged foils require a device like a wand(surface sensor) that can usually be adjusted and that controls the boats' altitude.
Surface piercing foils, while conceivably able to operate at a lower wetted surface than fully submerged foils are more prone to foil ventilation(especially in waves) which is when the foil sucks down air from the surface causing the foil to lose lift.
Go to http://www.foils.org and read! Also go to Tom Speer's site(google it) for very interesting and detailed information.
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edit: Tim, also check out the "Foiler" thread under"Sailboats" on this forum to help build your understanding of present technology. Use the "search" feature....

 

I know about ventilation...I'm going to read the chapter in my design book on foils. Thanks for the help.

Tim

 

Frank Bethwaite on foils

On the Bethwaite site there use to be an article about foils which is very informing. Also on Frank Bethwaite's book "High Performance Sailing" there is a good chapter on foils and why or why not NACA foils are not all you need to go fast.
If you do a google image search on rudders you'll find some old dinghies with round type of foils. They are very old designs and for their speeds may be adequate.

 

Thinking out of the box is definitely good, but I don't think this radical idea would necessarily serve the function properly.

Before, I go to my criticisms, here is a positive note. The research that I did in grad school was rooted inspired by some work that the Germans did on designing jet planes with a circular disk as a wing (research around World War 2). The original mathematical derivations for the aerodynamics were inspirational for some other people that developed the dynamic wake model for helicopter aerodynamics (another circular lifting disk). I continued the development of the dynamic wake model, but never took the time to check out the original German references. My immediate research topic was more than enough to keep me busy. I'll see if I can find these references, though my German is not up to a technical scientific level. Considering all of this, there may be some aero or hydrodynamic advantages to a circular lifting surface.

You mentioned remembering that circular paper airplanes fly well. They fly well due to their stability. When you throw a circular paper airplane, you spin it like a frisbee. It is "difficult" to change the direction of the rotational momentum vector (gyroscope effect). The result of this is that aerodynamic forces that perturb the object have minimal effect. A circular paper airplane would probably not fly as well if you didn't spin it. A boat with a circular foil will not benefit from this type of stability characteristic: no spinning and the boat's weight is not centered in the circular disk like it is in the paper airplane.

The classic high aspect ratio of span to chord length makes for a more efficient lifting surface because the lift to drag ratio is higher. You don't get this benefit with a circular disk foil. The longer chord length compared to planform area will also allow for more turbulent flow to develop and drag increases on wings with turbulent flow.

I believe that swept back wings typically increase the drag. A circular foil will consist mostly of "swept back" regions, so I would guess that the drag be increased. Jet fighters have swept back wings because they fly at supersonic speeds, which is completely different aerodynamics. The advantage of swept back wings at supersonic speeds is that the Mach number perpendicular to the leading edge is reduced, and drag significantly increases in the hypersonic region.

All that being said, I'm sure those Germans had a good reason to consider a circular lifting surface. I'll see if I can find the references when we get together this weekend.