Bryan Cox (builder and sailor of a Duo 480) and I have been discussing these misfortunes (see comments on prior posts). Most of our discussion has been about how to compress the rudder while the various epoxy steps are completed. While this is important I suspect we are missing something. I don't think you get plywood delaminating itself as above because you chose an awkward way to compress the rudder faces. This delamination is a function of the geometry at work.
I hold Bernd's design as above reproach. So without changing the geometry, and putting aside for the moment the question of how to compress the rudder. What could cause the plywood to come apart like this? My theory of the case is that the design is fine, the jig is fine, and even the wedges are fine. I think the issue is that my rudders must not reflect the design in some meaningful way. Let me interject an observation that the cured epoxy goop is very rigid (feels like a hard thermosetting plastic). It may be hard to tell from pictures, but I know I'm a bit of a mess when putting the epoxy goop in the groove. I'm thinking I'm leaving a small radius of goop at the edges, and given the remarkable rigidity of the cured goop, I effectively still have a goop bead that is too wide.
Another observation, is that the rudder faces only delaminated where the goop did not leak through and cover the leading edge. So next time after glueing in the jig, I will fill the outside leading edge joint. Similar to what I'm doing with R1.
Lastly Bryan is right that pre-gluing the spar into the rudder seriously increases the stiffness of that face (how could it not). Having one side stiffer than the other is likely creating some stress concentrations that should not be there. Locating the spar in a compressed rudder presents some challenges, but right now I'm just producing broken parts.
BTW: The link to Bryan's Blog is in the links section.
You could always make the spar wider by the width of a saw blade and cut it down the center. Then you could glue half spars to both sides or the rudder and glue the parts together when you compress it. If you align the grains correctly you can end up with an stronger spar (you would have to make two spars if you want to play with grain).
ReplyDeleteInteresting thought. I think the problem would be tat the rudder faces are not exactly aligned when they come together. This is due to a rather inexact method of tying them together at the leading edge (duct tape) and somewhat due to each siding taking a slightly different amount of bend. This shows in slight misalignment when closed at the trailing edge. I don't think this is a big deal, but these misalignments could be a bigger deal if you have to marry up a split spar as you suggest. I am intrigued to learn how you could arrange the grain to get more strength. It's already clear vertical grain material, I understand that making a laminate can improve properties, but why should the grain (given that it is vertical in any case) matter? I don't think this application needs the marginal strength improvement, (three layers of carbon fiber are on the way) but I am curious to hear more about the grain alignment idea?
ReplyDeleteThanks,
-Esteban