Working out a decent laminate for a rudder

More worried about the foil section, getting it right down the ellipse. Is there a better way than by eye?

 

There may well be,this is almost identical to the technique I use https://charlesriverrc.org/articles...ithout-templates/markdrela_airfoilshaping.pdf .You will see why I strongly recommend marking a good centreline and clearly the thickness of the blade needs to be tapered in proportion to the planform.Once that is accomplished you will need to calculate the proportions of the chord at which your facets will intersect the surface and having marked those points at suitable locations,you use a spline to join the dots.Then just remove the waste material and go on to the next facet.Eventually you will have to fair the surface to prepare it for sheathing.

 

That is the 1000 AU$ question for this project.

Small stuff <60cm /24" I would run off on the CNC, large stuff I try to make it look/feel good with a electric planer and sanders (8"/200mm disk and 5" /125mm random orbital).

The article makes a good case for solid wood foils and our smaller boats all have those. The one I had with plywood foils did not fail dramatically but the glassing cracked along the edges without a bias treatment. What is nice about plywood is the layers produce a contour map as they are shaped that helps one see where the foil is going.

If I were doing this one with the current material set, I would have a plan as to wrapping the wood or letting the glass form the trailing edge and develop a paper or Mylar drawing as a template for the profile/side view and thin rigid templates to capture the progression of the foil from root to tip. You may have a section that you could treat as a straight foil for the sake of roughing.

 

Yes, I think I will 3d print some templates. Getting the ex laminate shape is a little thought experiment. Reducing the cord until the thickness is correct shortens the blade too much, shortening the cord by the depth of the laminate then using the percentage reduction in the foil plotting tool seems to be a reasonable answer.

<edit> Strike that thought, that shrinks everything, no worky!

I'm playing with this tool Airfoil plotter (n63412-il) http://airfoiltools.com/plotter/index

I assume it's one used by many. Seems handy!

 

Does your CAD give you a volume estimate on that drawing? I'm considering some other materials and I want to get an idea of weight.

Cheers
Z

 

The Freecad example I posted quite a bit earlier in the thread has a volume of 0.00346 cu.m. If you get the FCInfo add-on it will determine these things quite easily but be aware that the numbers will be in the chosen system units,which in my case are millimetres.In an ideal world you would have a laminate thickness to refer to and could offset the surfaces by that amount to arrive at the core shape.

 

Volume does not pop directly out of the print.

**redacted for errors, see post below**

 

If I bring in Wet Feet's data, convert units, merge into a solid, etc...
Volume = 1448.59407 (+/- 2.2e-06) cubic inches

And then go back to mine in Fusion, find mistakes, etc:

GRP=volume 319.9 in^3, density 1.012 oz/in^3, mass 323.6 oz
Paulownia=volume 1356.7 in^3, density 0.165 oz/in^3, mass 223.9oz
Total= volume 1676.586 in^3, density 0.327 oz /in^3, mass 547.5 oz

This leads me to believe that there is something amiss in my previous numbers in posts 13-15 and that my CAD is carrying a bit more volume than Wet Feet's. Looking at the data side by side it all seems plausible. Wet Feet data is in purple on the left, Tops is in gray on the right.

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You're getting what you pay for, at least from me

 

My excuse is that I swept the NACA 0012 along the two rails formed by the leading and trailing edges,which leads to a proportionate reduction in thickness as the planform tapers and I think it shows quite clearly in the edge on views.I'm pretty sure that if Tops split the outline at the lowest point and swept the foil section along those edges,the results would be similar-but in different units.

 

Aghhhh... sick as dog today so I will keep it brief.

I'm trying to work out what an Oregon core (I think what we get as "Oregon" is in fact Douglas Fir) would weight. I get up near 15kg or down near 2kg both of which seem off. I will come back to it when I can concentrate again!

As always, thank you all for the input.

 

Seems a little low when I work out the weight. I will get the add-on as suggested. Many thanks.

 

@Zoro hope you feel better soon!
@wet feet thanks for the vote of confidence. I ran a new NACA0012 out and did a Sweep 2 edges and attached my previous cheek with a new loft and got this:
Volume = 1419.38363 (+/- 1e-06) cubic inches

 

Thanks, @Tops, just got out of hospital. As soon as I can think straight, I will be back on the job.

 

That works out to ~11.5 kg of timber core. Now that seems very realistic to me! ~ 15kg is about the target weight in an ideal world.

Cheers
Z

 

I had wondered why we hadn't seen progress reports, I hope you soon return to being in shape for sailing or boatbuiding.