Reskinning RV-6 Rudder to Eliminate Cracks

[plehrke]

I built my rudder 23 years ago and has the original 0.016 skins. I had a few cracks develop years ago at the first stiffener rivet just aft of the front spar. During this Covid isolation I have decided to re-skin my rudder with the 0.020 skins.

Lots of advise I have seen over the years on stiffener attachment to keep from getting the cracks, everything from pro sealing the entire length of stiffener to just a dollop of RTV at the trailing edge.

Obviously my rudder plans/drawings are 23 years old. Question I have is what do the current plans say? Do they still just say to just rivet on the stiffeners or are they calling out some type of goo?

[Ed_Wischmeyer]

Pretty sure that the fix is 0.020 skins... Somebody else can confirm.

Ed

[Desert Rat]

It's not specific to the rudder, but on my 7 it's called out in section 5.7 of the build manual. With respect to folded trailing edges, you should stick the aft end of the skin stiffeners together with a wad of pro seal or RTV.

[Sam Buchanan]

But that doesn't address the cracks that appear at the leading edge of the rudder.

Cracks appeared in my 1999 0.016" RV-6's rudder at the spar very early in its life, propagated a short distance and haven't grown in the years since. I've consider a reskin of the rudder and if I do will incorporate some means of stabilizing the stiffeners near the leading edge.

This is challenging due to the way the rudder get closed up. I've considered actual ribs for the rudder that will be closed up with pulled rivets on one side of the rudder skin. Another thought is ribs that only extend about a 1/3 of the way back from the spar, enough to stabilize the problem area. This would also require some pulled rivets on one side.

And yet another thought is to extend the forward end of the stiffeners enough for them to be attached to the spar. This would require shims on the spar so there would be a uniform surface where the skins are riveted to the spar.

What complicates this whole thing is the possibility of moving the CG aft on an RV-6 with a fixed-pitch prop. My plane can't have the CG any further aft if baggage capacity with low fuel is to be retained. But with careful design of the mod this might not present a problem if 0.016" skin is used.

If someone comes up with a clever way to anchor the stiffeners near the spar I'm all ears.

[azrv6]

I developed a small crack in the aft rivet of one of the stiffeners on the 0.016 thick left elevator after only 4 hours of flight time following the first flight in May 1996. Stopped drilled it and hasn't changed in 24.5 years.

My current plan is to let it go another 24.5 years and see how it is then.

[BillL]

Sorry, Phil, my 7 plans are just like your 6. Maybe Vans guys can send you a current PDF of the drawing or give the latest revision.

I remember Fast Bob Mills posted some rudder stiffening mods for his super 6, if you are interested in mods.

[Martyrv6a]

I built my -6A rudder some 20 years ago using the .020” skins. I recall gluing the stiffeners their total length using 3M Scotch-Weld 2216 B/A gray epoxy adhesive. Learned this from the Matronics RV List! I also used some RTV on the trailer edges to dampen vibration. I used a greased 1/8” inch dia rod in the trailing edge to make a hole for moisture to run out when closing up. The RTV didn’t stick to the rod so it was easily removed, once the RTV was cured. Same process on the elevators.

[Ralph Inkster]

If you are replacing the rudder skin, you will also probably be replacing the stiffeners too. I suggest you trim the forward end of the stiffeners different from the plans, extend them so you can apply a bit of proseal between the stiffener and the rudder spar. This will eliminate any chance of the forward cracking you see on some rudders. This will be additional to the proseal glob you apply to the rear end to connect the two stiffeners. Results is a solid rudder that will never crack.

[Dean Pichon]

I built a new rudder for my -4 with 0.020 skins some years ago after I developed a number of cracks. I used Pro-Seal along the full length of each stiffener. My original rudder had globs of RTV at the trailing edge in accordance with the plans at the time. Since the 0.016 skin cracked with the RTV, I did not bother to use it the second time around. This second rudder now been in service for about 800 hours with no cracks. (The first one developed cracks before 200 hours.)

I just finished building a new set of elevators, also with 0.020 skins. Again, I used Pro-Seal along the full length of each stiffener (again, without RTV).

I received new/current plans and instructions with the new tail kit (June 2019). These instructions also direct the builder to put RTV or Pro-Seal at the trailing edge.

Someone on Van's Builder Support line recommended using Pro-Seal to bond the stiffeners to the skin. I had called with an unrelated question regarding this second build and this advice was unsolicited, and had already been incorporated, but was much appreciated, nonetheless.

Good Luck,

Dean

[Ralph Inkster]

I can't see how prosealing the length of the stiffener helps. I do see how skin can flex in the short unsupported area between the front spar & stiffener, and behind the stiffener to trailing edge areas, and how this will cause a crack at the last rivet on the stiffener. So I believe in supporting those unsupported areas as much as I can without adding too much weight.