Well, after 65 hours TT both the rudder and elevator's started developing cracks around the stiffener rivets. This seems to be a well known problem with the old style bent edge control surfaces on the other RV models, but there is not much information out yet about this happening with the new style double flush surfaces.
In my case I suspect that the rudder cracking was caused by a couple of factors. First I was just learning how to rivet and more specifically, how to back rivet. I over drove the stiffener rivets until the actually are a little bit proud of the skin surface on the outside. Thus I suspect that too much metal was mashed and started the stress cracks from the rivet lines.
|It is hard to see without looking in person, but the lines of rivets are just a bit proud of the surface|
However the issue is not just rivet technique since the only rivets that are failing are the first and last holes at the ends of the stiffeners. There is still the same dymanics working of the skin and stiffener moving and flexing at those locations. The second spot I found was an area of oil canning on one side of the rudder. It would lightly pop in and out. Well over the 65 hours it is starting to form a 1 inch long stress area that will soon crack out also. This is just off of the rudder spar.
Here are some pictures of the rudder problem areas.
Next I also noticed similar problems with the elevators. On these I didn't over drive them as on the rudder, and the skins are a bit thicker... So I'm not yet sure why they are cracking out. The cracks again are starting at the ends (both front and rear) of the stiffeners. Here's a picture of one of the cracks to give and idea of what is going on...
At this point I have stop drilled all of the cracks as I find them. The rudder is worse than the elevator so I have ordered the replacement parts to build another rudder. Hopefully with my new found skill from finishing the plane I can do a better job next time and have better luck. I'm still considering ways to tie the stiffeners better in place to eliminate this problem in the future.
Update: I took the plane down to Vans to have them take a quick look at it. Seems that on the elevators I may have cut the stiffeners too short. If the ends of the stiffeners are trimed too close to the last rivet hole, you don't get proper support for that rivet and they crack due to stress on the skins. Looks like I have a few elevators in my near future also!
I have started building a new rudder and this time decided to proseal the stiffeners onto the skins along with back riveting them. This should help keep the stiffeners attached to the skins and not moving. It was easy to do and I think will help solve this problem. It also came out looking oh so much better than the first one! I prosealed the entire length of the stiffeners onto the new rudder skins. I also turned down the air pressure, properly set the rivets, preped the holes, and added globs of proseal between the 2 sides at the rear stiffener locations while closing up the TE. With all of this there is no oil canning at all, and the 2 skins are very stiff when squeezing on the sides near the TE. I think this one will go the distance.
Second Update: I have now built new Elevators and also prosealed the entire stiffener's in place before back riveting. The rudder has held up great for 350 hours now with no further problems so I expect that the problem has been solved.