A few of my recent planes ( last 10 years) have cracks in the fillets & around the stab, not all but some, I don't see any difference in how I built the ones the crack & don't crack, My newest plane the Olympus has the stab cracking away from the fuse in a big way. I cant fly it until I remedy the cracking.
  Any help in construction of the fuse & such is welcome. I've built many planes& it isn't in all but seems more frequent than ever & no I'm not building lighter either.
  John

I know it would look ugly, but 1/4" triangle balsa on the outside, under the stab, provides plenty of strength.
That has been my "fix" whenever the pushrod and clevis doesn't leave enough room for interior doublers.
F.C.

  Tim, you 've seen most of my planes  Big boy planes 700 squares . I think you guys might have something in the fillet construction.. I've been making them lighter every year ..I also believe I might want to build up in the area.  The Olympus is pretty minimal over the top of the stab.. maybe I should of built it up.. now I have to figure how to save the plane,

    This is a pretty common problem the "my airplane is 1 ounce lighter so it's 1 ounce better" crew. You just can't take a design approach that was developed for 35-sized planes and 7 lb balsa, put a 60 or more in it, use 4 lb balsa, and expect it to last.

     The problem is flex along the axis of the grain right below the mount. I use a medium 1/32 vertical-grain balsa doublers underneath the stab, and generous fillets to spread the load.

     Brett

  I have an idea , since I started reducing fillet size in the rear of the plane & that is when the cracking became more common that might be the cause or part of it.
  Thanks everyone, John

Oddly enough, widening the fuse a little doesn't hurt either. Since I went to Ball-Links, I've got into the habit of sticking a tail post in the back when I pinch the fuse sides together to give me more room for the ball link and the adjustment hatch.

Larger fillets with a good material like SuperFil helps too... and it doesn't matter if it's foam or built up, I always use an inner trailing edge doubler. With modern horns you might not have to notch the stab trailing edge spar, but even so, there is a stress riser there and you probably use the brass horn clips that cut into it as well... a hard balsa doubler thats around 6" X 3/8" X the stab thickness, prolly wont hurt. You also get the added benefit of it not cupping (hopefully) when you apply silkspan...

EricV