Mast Building Method

[kitexpert]

If manually shaping it will make for an easier transition and wont expose the core when shaping the mast.
Also it will provide impact resistance if the mast is hits something. There would be less likelyhood of core being exposed.

Valid points, but not essential for the main issue which is achieving enough stiffness for the strut. I don't think hitting the strut is a very big thread, front wing/fuselage comes first and takes most of the hits.

Yes, a carbon tube is fine but again, not so for torsion. There is no 45 degrees carbon tube that I can find.

I meant weaved carbon material, a kind of sock of carbon which can adapt to different thicknesses. Then fiber direction will change but it is not so precise, in "neutral" position fibers are at 45 degrees. I think this material makes it possible to have a very nice outer surface (seamless) for tubular carbon parts, and of course it gives strength/stiffness. But I haven't tried it.

We can definitely wrap with more 45 degrees carbon but 1 layer each side is not enough from my tests.

Yes, then two layers, it makes layers crossing on both sides which is good, torsional stresses dividing symmetrically in both sides of the strut. I have done some glass/carbon parts on foam core and noticed 45 degree UD wrapping gives as much torsional rigidity as needed.

a box type tube of biax the fibres of the carbon are better aligned to support shear (sliding of one side up or down compared to other side as strut flexes). Also I think having those fibres wrap onto the skin is helpful.

For the box beam I agree, but at the skin of the strut shear is not a problem. So I wouldn't use more than one layer of biax there and even that mostly for aesthetics. Straight and 45 degree UD should be enough, 45 degree carbon gives chordwise strenght very probably enough. Now I just don't see why straight fibers from LE to TE should be there.

If some internal structure for the strut is needed I would suggest the hollow beam in the center, then adding LE and TE parts (with or without carbon) and finally wrapping UD carbon straight and 45 degrees to finish. I think it is better than surface structure, but also more laborous.

[foam-n-fibre]

The only thing about a hollow tube as a stringer down the middle is that the way I see this, a stringer helps prevent shear (slip between each side) and also prevents collapsing of the cross section, the kind that in a metal pipe would eventually be seen as buckling. In this buckling case the stringer is in compression as the column wants to collapse to allow more bending. Having the stringer as a hollow tube will not have the same compression strength.

Another option, if one favours a beefy hardwood, is to have a strip of good hard wood at the thickest part of the foil, and foam to fill out the front and back shape. That might be easiest - a good old fashioned stringer!

Peter

[kitexpert]

Yes, but hollow beam has quite a lot torsional stiffness and of course it resists shear. I don't know if there is a lot difference between one hollow beam or two stringers (which can be understood as two sides of a beam).

Foam will resist collapsing, I counted a foam core as a hollow one because it is so light weight. I very much think it is better to have something under the carbon skin, also to not let water a possibility to fill.

Your suggestion of wooden strip in the middle is a good one, but then there is not a very long way to full wooden core. If both light weight and stiffness is wanted you can't have very much material in the core.

[plummet]

If manually shaping it will make for an easier transition and wont expose the core when shaping the mast.
Also it will provide impact resistance if the mast is hits something. There would be less likelyhood of core being exposed.

Valid points, but not essential for the main issue which is achieving enough stiffness for the strut. I don't think hitting the strut is a very big thread, front wing/fuselage comes first and takes most of the hits.

Yes, a carbon tube is fine but again, not so for torsion. There is no 45 degrees carbon tube that I can find.

I meant weaved carbon material, a kind of sock of carbon which can adapt to different thicknesses. Then fiber direction will change but it is not so precise, in "neutral" position fibers are at 45 degrees. I think this material makes it possible to have a very nice outer surface (seamless) for tubular carbon parts, and of course it gives strength/stiffness. But I haven't tried it.

We can definitely wrap with more 45 degrees carbon but 1 layer each side is not enough from my tests.

Yes, then two layers, it makes layers crossing on both sides which is good, torsional stresses dividing symmetrically in both sides of the strut. I have done some glass/carbon parts on foam core and noticed 45 degree UD wrapping gives as much torsional rigidity as needed.

a box type tube of biax the fibres of the carbon are better aligned to support shear (sliding of one side up or down compared to other side as strut flexes). Also I think having those fibres wrap onto the skin is helpful.

For the box beam I agree, but at the skin of the strut shear is not a problem. So I wouldn't use more than one layer of biax there and even that mostly for aesthetics. Straight and 45 degree UD should be enough, 45 degree carbon gives chordwise strenght very probably enough. Now I just don't see why straight fibers from LE to TE should be there.

If some internal structure for the strut is needed I would suggest the hollow beam in the center, then adding LE and TE parts (with or without carbon) and finally wrapping UD carbon straight and 45 degrees to finish. I think it is better than surface structure, but also more laborous.

Some good idea's year. I like the carbon sock idea. However I think fibres on the 45's are not entirely what you want. They are too short and not in line with the torsional twisting, The torsional twist takes place on at angle from top of mast to bottom. So maybe 15 degree? I layed up some ud carbon layers on 15 deg angle with my foil build and it made an exception difference to the torsional twisting. for the record I also have fibers on the 45 too. So fibres at 0, 15, and 45 are ideal for stiffness,

[foam-n-fibre]

When you refer to a tube having torsional stiffness, are you referring to the whole skin of the strut being like a tube? Or do you mean a small tube down the middle like a stringer? The small one down the middle won't do much for torsion, if that's what you mean.

Also, as for the foam resisting compression, yes, its better than it being hollow, but I think small amounts of compression, small amount of flex in shear all add up. Sure none of those on their own might be a large amount, but they all contribute to the overall flex.

Plummet - interestin idea about the 15 degree angle. I'll have to think about that. Did you angle unidirectional, or plain weave? Unidirectional would be best

Peter

[plummet]

Uni on the angle

[kitexpert]

To get torsional stiffness some angle of the wrap is best. If angle is 90 degrees or 0 degrees nothing is gained, of course. Difficult to guess what is the most efficient angle, it may depend also the dimensions, lenght/chord. I would use 45 degree wrapping, but of course some lower angle is good too. 45 degree fibers limit through the whole lenght and they are sufficient. I wouldn't try fully diagonal direction, for sure it is difficult to do and in practice it means the waste of carbon.

Wrapping biax (tape) at an angle has an advantage over uni having both directions stiffened at the same time. If angle is not 45 degrees it covers two different angles simultaneously.

I did mean having a hollow beam in the center for added torsional stiffness. If it is made torsional stiff it makes the strut stiffer, at least more than not having it. And side walls act like two stringers resisting shear and bending.

Again adding thickness gives very much more torsional rigidity, in relation to power of four of it. So if you increase thickness 10% you gain almost 50% more stiffness if everything is scaled up respectively. This is true for round shafts, I don't know if it is true for hydrofoil cross section strut with different inner structures. Difficult questions

[tahoedirk]

hmmm... 15 degrees huh? Sounds good but kind or difficult to accomplish. I have always alternated 20oz biased cloth( biaxial/45/45) and unidirectional and that's hard enough , I am leaning toward more UNI these days for what it's worth.

[Bille]

...
I am leaning toward more UNI these days for what it's worth.

You loose about 20 to 30% in compression and tensile strength , whenever you Weave carbon or glass ...
Fabric is weaved, mostly to make it easier to work with.

Have you tried Stitched carbon ? It's easier to work with than multiple layers of uni-carbon ; and
can be had in a variety of different angles :

https://www.google.com/?gws_rd=ssl#q=st ... rbon+fiber

Bille

[plummet]

hmmm... 15 degrees huh? Sounds good but kind or difficult to accomplish. I have always alternated 20oz biased cloth( biaxial/45/45) and unidirectional and that's hard enough , I am leaning toward more UNI these days for what it's worth.

Uni cut on the angle. Simply lay your mast over the uni until you get fibres running from Le to te the length of the mast. Then cut. Easy peasy