Foil construction concept trial balloon

[bistros]

All:

Quality high performance foils are an expensive and skill-intensive component of (home) building a boat. There is not much latitude for error, and hand made efforts just can not compete with CNC milled 3D cores (foam, wood or otherwise) or CNC milled female molds. Perfectly symmetrical hydrodynamically correct foil profile sections are not easily achieved with hand tools and eyeballs.

I was thinking about construction methods where a homebuilder could possibly consistently build hydrodynamically sound lightweight foils.

Here's my idea:

Build foil profile "stations", mounted on a straight "spine", that are subsequently vertically stripped with a skin of wood (or foam), following the curve set by the NACA compliant stations. I imagine the leading edge would probably have to be solid. Foil stations could be gradually reduced in section from top to bottom, allowing for tapering and rounded tips.

This would allow creation of a (mostly) hollow core, NACA compliant foil that could then be laminated with glass/carbon/kevlar or whatever after final surface prep. Vacuum bagging the resulting foil should result in a very lightweight, very strong quality foil without the need for highly precise three dimensional milling activities.

I'm at the "blue-sky" stage with this concept, and will probably take a stab at it to do a proof of concept if I don't get outright derision and (too much) laughter from the feedback I'm seeking here in the forum. What do folks think? Has this been done before?

--
Bill

[Tim B]

The model (and full-size) aircraft world has been using it since.... well, for quite some time. Known to modellers the world over as "Built-up" construction it is very strong, very light and if it's done carefully gives accurate results.

I did propose using the method for home-built yachts (on this forum, somewhere), but the difficulty is in building the foil strong enough. If you're using it for a centreboard or keel, just think about the proposed loading. It's not inconsiderable. Rudders are less of a problem, but you still need to think about it.

Structurally, analysing a standard foam foil is not too hard, and engineer's beam theory will probably be ok. with a built-up structure, the pure bending case is ok, but it is very complex torsionally. Sadly it's torsion that reduces performance and rips things apart.

Anyway, give it a shot, see what you end up with,

Tim B.

[bistros]

I had not given torsional stress on the vertical axis a huge amount of thought, as what I was proposing is a 4-5' dinghy daggerboard + rudder. Assuming a high aspect ratio foil, there would not be significant torsional loads - the lever arm from centre is measured around 8-10 inches at best on the vertical axis.

Actual force applied would be the lift of the foil - which is not averaged far off center. It would be interesting to figure out how to measure the various loads and deflections at various points of the foil. I'm sure one of you naval archtiect folks understands the issues better.

My biggest concern with the construction technique was deflection load while righting the boat from capsize, although I do not have a grasp of whether the operational load at maximum lift going upwind, or righting load from capsize is greater.

The deeper you get into this stuff, the deeper you find the water to be!

I guess if this stuff was easy, none of the senior folks here would be gainfully employed.

[RHough]

Originally Posted by bistros

I had not given torsional stress on the vertical axis a huge amount of thought, as what I was proposing is a 4-5' dinghy daggerboard + rudder. Assuming a high aspect ratio foil, there would not be significant torsional loads - the lever arm from centre is measured around 8-10 inches at best on the vertical axis.

Actual force applied would be the lift of the foil - which is not averaged far off center. It would be interesting to figure out how to measure the various loads and deflections at various points of the foil. I'm sure one of you naval archtiect folks understands the issues better.

My biggest concern with the construction technique was deflection load while righting the boat from capsize, although I do not have a grasp of whether the operational load at maximum lift going upwind, or righting load from capsize is greater.

The deeper you get into this stuff, the deeper you find the water to be!

I guess if this stuff was easy, none of the senior folks here would be gainfully employed.

You might be surprised at the torsional loads.

Building small foils that are not ballasted should be very easy. Built up construction as you describe is difficult to get right. Solid foam, cut with a hot wire gives very accurate foils. The outside of the foam creates a bed to hold the panel for laminating.

Do a search on building foam model airplane wings. The process would be the same. For a daggerboard, you can inlet a spar structure near maximum thickness that gets bonded to the outer skin. I've seen 7% thick foils with 50-60 inch spans and 9 inch chords built this way that have no flutter and can handle 10+G's. A 48-60 inch daggerboard with a 10-12% thick foil and a 12"+ chord would be easy to build.

[Tim B]

You can use "eggbox construction" for greater torsional stiffness. Generally, the righting moment from capsize will be much greater than the bending moment from the lift force. Have a look around at the aeromodelling sites. There are some people who still do it properly :-) far too many people use foam these days.

Tim B.

[Tim B]

An Example for you. This is very standard construction, no D-box, not sheeted and "straight rib" construction.

Just had a look at a few model aircraft sites. Few of them are really hot on the theory, but a few have good construction photos.

Tim B.

[farjoe]

I am currently preparing the materials to build just such a foil for a NACA10,10in chord and 50in span.

I intend to attach foam to the fore and aft side of a 2 inch wide piece of wood planed to the correct width and then use a hot wire across 2 NACA10 profiles set at either end of the wood(hopefully perfectly aligned) to remove excess foam and then cover with glass (or perhaps carbon).

There will be 2 boards for a 16ft cat.

Any comments will be appreciated.

Regarding torsional loads can anyone explain further to us poor amateurs how these come about and how to approximate their values.

I know that the proper theoretical analysis is probably complex but I find that many people ( excluding ACuppers of course) use shortcuts anyway to obtain a value for most problems and then add in a factor of safety.

regards

[Tim B]

Torsional loads are incurred because of the pressure distribution on the foil. With a foam cored foil, there's not too much cause for concern. Just put at least some plies per side on at +-45 degrees. Foam foils are generally quite good in torsion anyway. The bending load is much easier to predict, being that of the expected crew weight on the end of the foil. Normal engineers beam theory will be fine here, though I would advise you to use a CAD package to work out the second moment of area.

The full analysis for a foam cored-foil is relatively easy if you assume the foam is non-structural, and the front and rear sections act like torsion boxes. The fully-built-up foil needs FEA to get the full answer. Engineers beam theory is ok for a bending analysis.

Tim B.

[Tim B]

Just for interest, here are two FEA examples. The standard straight-rib construction is obviously worse in torsion.

Nb. The load cases are only there to prove a point, they are not actually correct!! it would take a few days to do the whole problem.

Don't assume them to be right,. I'm just trying to show a difference.

Tim B.

[messabout]

You can build a spectacularly strong and torsion resistant foil using the egg crate system. Do a layout of the Nasa section of your choice, projected to 45 degrees. These will be ribs. The ribs lie at 45 degrees to the leading and trailing edges. One set of ribs lean to the right and the other set leans to the left. There will be two intersections where the ribs cross at right angles. The ribs are notched to accept one another in this configuration. The structure is to be covered with plywood or GRP sheet. Do a doodle on a scrap of paper and you'll get the picture.

I have made several of these, probably because it was fun, but also because it makes a tremendously stiff structure with very light weight. The skin can be fairly thin because you have a lot of cells to support the skin over small spans which diminishes skin deflection. If you use an uncolored GRP skin, the rib work will be semi visible and you can have superior bragging rights for having created a seemingly complex structure.

[farjoe]

Interesting concept.

If I understand correctly you will need 18 ribs for a 10in by 60in foil.

How do you manage to keep these aligned accurately until the resin sets?

How do you ensure that the skin glues completely to all points on the ribs?

regards

[RHough]

Originally Posted by farjoe

Interesting concept.

If I understand correctly you will need 18 ribs for a 10in by 60in foil.

How do you manage to keep these aligned accurately until the resin sets?

How do you ensure that the skin glues completely to all points on the ribs?

regards

You have identified some of the pitfalls of the construction method. Alignment jigs and possibly a foam bed allow them to be overcome. Since most dagger boards have near neutral buoyancy there is no overriding need to make them light or hollow. A simple high density foam core with laminated skins is simpler and more than strong enough.

[messabout]

Farjo; If you make the rib slots, for the intersections, a snug fit, the structure almost aligns itself. Of course you are going to use a leading and trailing edge upon which the ribs will land. The process is easier than it sounds.
RHough: The demonic scheme that I have desribed is not really difficult and it can have a high degree of accuracy. I admit that this method may constitute a form of self flaggelation because there are a lot of parts to be carefully made. I agree that the foam core idea is more practical and probably faster as well.
Bistros: I contend that CNC machining is NOT more accurate than a part done without benefit of exotic machinery. Peoplle were building swiss watches of impeccable quality and accuracy long before CNC was dreamed of. To be sure, the automated machine is faster and easier.

[RHough]

Originally Posted by messabout

RHough: The demonic scheme that I have desribed is not really difficult and it can have a high degree of accuracy. I admit that this method may constitute a form of self flaggelation because there are a lot of parts to be carefully made. I agree that the foam core idea is more practical and probably faster as well.

I agree, there is great satifaction id building light, strong stuctures. I've enoyed every built up wing I ever built.

I also think that the translucent skin over a honey comb structure would be a great "look". It would be just radical enough to make other sailors question their equipment and would be an advantage on course even if the actual performance was exactly the same.

[Steve Clark]

I'm afraid I kind of disagree with your initial premise. Hand shaping can be a very effective way to build foils. Particularly if they are one offs. Good templates, sharp planes, and a good sense of what you are about all the way from shaping the blank through final finish makes stuff that can be just plain perfect.
I have built the way you suggest, and it wasn't any more accurate than hand shaping a blank and it didn't save any time. I have also built quick and dirty molds by flipping your idea around, Setting up templates in a tray like rack and shaping the mold out of plaster. As the shapes get more complex, this gets harder and harder to achieve.
SHC