I use sheeted foam for the stab/elevator, it weighs less than yours even though it is about twice as big. 1 lb foam, 1/32" 6-7 lb, epoxy with carbon veil at the bond line. One of the issues most people have with sheeted foam is that it takes *very very little* epoxy to hold it together. Seal it with nitrate dope at the bond line, and when applying it, put on a lot and then squeege it off with a hard plastic spreader (I use old hotel key cards) until it looks like it was barely dampened. Then use LOTS of weight or press it heavily, maybe 100 ish lbs for a typical stab.
A lot of structural design for model airplanes is pretty questionable, and enabled by either using very strong wood, or very light wood, and not "efficient" in the use of the materials. A common mistake it taking a design that used 7-8 lb wood and just substituting 4.5 lb wood. Sometimes there is enough margin for that to work, and sometimes there isn't. The opposite case (taking an inefficient structural design that was acceptably light only because you had 4 lb wood and using 7 lb instead) ends up like yours.
The "world's lightest Ringmaster" efforts usually wind up with structural failures when someone get the great idea to replace the 15-lb LE, TE, and spar with 5-lb wood. The Ringmaster is the absolute dead worst structural design of any commonly-built airplane, you can save weight if you are *very careful* but you had better know what you are doing - all the wood it right down the center of the wing, the worst place for it, so it had darn well be strong.
Stabs are particularly difficult because it is very clear that they want to be *thin* for performance reasons, but they also need to be stiff for consistency. If I wasn't going to use foam, I would build it up and sheet it, but you have to also take a bunch of wood away around the edges. Maybe built-up LE, and TE just a thin cap (maybe 1/8") cap over the tip 1/32 ribs, a few 1/16 stringers spanwise. This is a lot more complicated than the original but much more efficient since it moves wood away from the center. Look how full-scale wood airplanes are built for some tips. Still more complex, make a mold for carbon shells and a few ribs inside to keep the shape.
All that being said, one of the most amazing performance improvements I ever make was to replace a flat-wedge with an airfoiled stab and the lightest wood I could find (maybe 4.5 lbs which was pretty good at the time) with one made from a single piece of about 7-lb C-grain, 1/2 thick, 4" chord, and maybe 30" span, just airfoiled with a plane and sandpaper. It probably weighed twice what the original did, and took correspondingly more nose weight. The airplane was very unpredictable before, the next week with a new stab it was instantly far more predictable, never did anything even a little bit funny, and flew the same in the calm and the wind. I attribute this to the stiffness increase, it was *remarkably better* even though it ended up maybe 3.5-4 ounces more all-up. 630 square inches, ST46, 45-46 ounces before and right at 50 ounces after the fix.
Point being, do the best you can and there are a lot more things to consider in addition to the weight.
Brett
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