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cracks in skin around rivets on rudder

blackbeard10

Well Known Member
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During condition inspection I found these two cracks in the rudder skin on the right side. None visible on the left side and the rest of the tail showed no cracks either in the skins or structural members. My plan so far is to use MEK or acetone to remove all primer and paint around these two crack to better visualize the extent of the cracks. If they are limited to around .75 inch or 1 and are only over the ribs, I will drill stop holes and widen the crack lines with dental burr, apply fairing compound and repaint. 400 hours on my rv 4. Comments appreciated.


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Hey Mark! How?s it going (other than the cracks) ? What thickness is the skin? Original was .016, but yours is newer, so I assume .020. There were problems originally with elevator skin cracks at the trailing edge where the stiffener angles were riveted, and a fix was to add a dollop of RTV in that area on each stiffener trailing edge (which you already know about). I haven?t heard of that problem with the rudder, but it may be the same cause. Being that it?s on the right side, I wonder if it is in the same vicinity as a rudder trim wedge maybe on the left side of the rudder? Stop drilling and filling may work if the stress in that area is relieved by that. Cleaning it up with a dental tool first - good idea..... where did you get one of those?
 
Hi Mark,
Cracking of the rudder skin is not uncommon on early RV-6 models that used .016 thickness skins for the rudder which could be similar to yours. However they have also showed up on RV-7s, 9s, .... The cracks often occur on the end rivet of the rudder stiffner. Suspected cause is preload + vibration, ..... Some just drill stops holes while others will take additional actions. You can search the archives for various approaches others have taken.

See this thread
 
Keep in mind that the primary cause of these cracks is insufficient bending of the trailing edge. There are MANY .016" control surfaces that have been flying for over 25-30 years without cracks.
 
My rv-9 developed cracks

After around 300 hours. I rebuilt the entire rudder and used proseal on all the stiffeners. I'm 300 hours since the rebuild and no signs of cracks returning.
 
Stop drill at each end of the crack and keep an eye on it.
This should stop their migration. Once you are sure they are stabilized, you can fabricate a simple .016 patch, the size of a quarter or less, and glue them over the area with thinned proseal and paint. You won?t notice them.
I had one crack in my first 20 hours, then went 800 before my second one showed up, exactly like yours in exactly the same place. I pulled the patch on the one that occurred twelve years ago and it hadn?t changed. I am monitoring the second after stop drilling and may or may not patch.

The issue was well known when I built my rudder. I followed all the protocols in fabrication, trailing edge bend to reduce preload, RTV to tie stiffeners, etc...

If I get anymore, I will have to remain or rebuild. If I do, I will use .020.

However, as Mel says, lots of .016 rudders with no cracks on airplanes with high hours.
 
The rudder on my -4 developed similar cracks after a few hundred hours in service, probably for the reasons previously noted. I stop-drilled the 0.016 skins and flew for a couple more years before a built a new rudder using .020 skin material. I bent more carefully and prosealed the stiffeners to the skin. Twelve years later, no cracks.

Now, I making new elevators using .020 skins. (For the same reason.)

Good luck,

Dean
 
Mark, looks like something else is going on with that rudder.

There are creases and deformities around the lower rod end bearing (on the skin). Did the rudder hit the rudder stop hard due to wind?

I had a similar crack on one of the rivets around the elevator trim hinge. Drilled it and glued a piece of 020 over it. Worked great.
 
I have the same cracks. Two rivets on rt side fwd last one in stiffener . I got a disposition from Van?s. Stoped drilled and have not seen anymore migration! I plan on patching with a 1 1/2round piece of aluminum and pro seal!
 
Diagonal wrinkles at the bottom of the skin looks like it took a big whack hitting the rudder stop. Maybe in the wind when the rudder slammed over to the stop?
 
Me Too

Two cracks on our RV-6 rudder with the oh too thin skins (OK, .016"). Both on the right side of the rudder, but higher up than yours. One is near the upper hinge, the other is some lower, both at the end of the ribs (stiffeners? battens?). I made a couple of patches of .016" 2024T3, picking up a couple of existing rivets thru the stiffeners, dimpling where I could, plus a couple above and below. Removed paint and primer, installed with 3/32" pulled rivets and ProSeal. About 100 hours ago, no more problems... so far. They are a bit unsightly, especially since they're still bare metal. One o' these days I'm gonna touch up all those little bits that need paint matches...
 
Two cracks on our RV-6 rudder with the oh too thin skins (OK, .016"). Both on the right side of the rudder, but higher up than yours. One is near the upper hinge, the other is some lower, both at the end of the ribs (stiffeners? battens?).

Same here (RV-6). The cracks around a rivet at the end of the stiffener appeared about a year after the plane started flying. I decided to just watch the cracks to see if they would propagate.

That was nineteen years ago.....the cracks are still there and haven't spread any further.
 
Rv4

This is very common on 0.016 skins. Had RV4 in my hangar that had cracks since 1992. Stop drilled almost 30 years ago.
 
Keep in mind that the primary cause of these cracks is insufficient bending of the trailing edge. There are MANY .016" control surfaces that have been flying for over 25-30 years without cracks.

I've not heard this before. How are the cracks at the forward end of the stiffeners caused by the trailing edge radius? There is an unsupported (at the front) stiffener floating on a sheet of .016" aluminum, so there is naturally a very large stress concentrator there. If I were to rebuild mine, I'd tie that stiffener into the rudder spar somehow.

My recollection is that most of these are on O-360 equipped C/S prop models of 6's and 4's.

I stop drilled 3 of these at about 200 hours. That was 18 years/1500 hours ago, no further progress.
 
I also noticed the other damage on thus rudder. Good point about on ground damage due to no gust locks.

Epoxy will work well for a patch provided the surfaces are well scuffed and cleaned. Not everyone has proseal on hand as it has a shelf life. I learned that the hard way.

I don?t know if u r a builder or not but making a new rudder is not that big of a deal or that expensive.
 
I've not heard this before. How are the cracks at the forward end of the stiffeners caused by the trailing edge radius? There is an unsupported (at the front) stiffener floating on a sheet of .016" aluminum, so there is naturally a very large stress concentrator there. If I were to rebuild mine, I'd tie that stiffener into the rudder spar somehow.

My recollection is that most of these are on O-360 equipped C/S prop models of 6's and 4's.

I stop drilled 3 of these at about 200 hours. That was 18 years/1500 hours ago, no further progress.

The theory is if the trailing edge bend isn?t done aggressively enough the skin will be preloaded when riveted to the spar. I used to think this was the on,y factor with cracking but I have changed my tune.
 
I also noticed the other damage on thus rudder. Good point about on ground damage due to no gust locks.

Epoxy will work well for a patch provided the surfaces are well scuffed and cleaned. Not everyone has proseal on hand as it has a shelf life. I learned that the hard way.

I don?t know if u r a builder or not but making a new rudder is not that big of a deal or that expensive.

I used proseal so I could easily remove the patch and inspect the crack. Having done this after 800 hours in service, I don?t think inspection is necessary. Stop drilling works.
 
Stress

Something that I have not seen mentioned in this thread is the misalignment of the rudder hinge points. If the rod end bearings are adjusted to correct an out of alignment issue then the rudder horns will actually twist the rudder frame work and cause the cracks. Once the cracks are created they relieve the stress, so stop drilling them seems to "fix" the issue.
 
As others have said:
  • stop drill skin and watch (avoid drilling sub structure)
  • stop drill skin and metal bond an external patch, *
  • Re-skin Rudder with 0.020t skin (or best make a new rudder all new parts)
  • Give that flight control a very good inspection it looks rough , may have other damage.

*Cold Bond Patch - BMS 5-95 or "pro seal" will be fine but metal bond epoxy like Permabond ET515 or 3M Ec2216i s better. After stop drilling (1/8" min Dia.) trying to no drill sub structure, remove paint, scuff bare aluminum (both patch and rudder skin) with scotch-brite pad. Clean with solvent (MEK or lacquer thinner) and then Etch and alodine both surfaces (rudder skin and patch mating surfaces). Fab patch from 2024-T3 one gauge thicker than rudder (0.020 on 0.016 rudder skin or 0.032 on 0.020 rudder skin) so patch overlaps past crack about 1" to 1.5 in all directions. If cracks are close make one combined patch. Patch shape as required: round, oval, rectangular with radius corners. Mix epoxy as directed and install patch and provide some pressure. Best if rudder is off plane. Once cured up to 72 hours RT, tap test it and repeat tap testing every other oil change or once a year whatever occurs first. Stay away from hardware store stuff like JB weld and similar. Down side of external patch, if crack is propagating you can't see it. Once you prime and paint it should not be too visible.
 
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rudder cracks fix??

Ok I'll jump in here cause I like my (fix) or maybe temporary. Repairs only work on painted rudder so you can paint over.
My 6a 2001 variety 500 hours cracked and drilled rudder. 6 on the right and 2 on the left. oil canning for sure. Went to VAN's To get new rudder parts and skins. Forget it they don't have. So brain in gear.
Drilled about a 3/16 hole between spar and stiffener. Sanded outside, scraped inside with sharp crooked hooky tool sorta like bent finger.
Next I used (J B weld) My other epoxy was at home. Took the bendy hooky thing and got epoxy inside against spar and over stiffener and thin cap over outside.
Got hard, next day sanded off smooth, When I paint it you won't see the repairs.
Now will it hold???? time will tell. only have 8 hours on repairs. But, it is way faster than rebuilding complete and in my opinion should work real good.
Do I have an out of balance problem?? probably not cause the little weight is right next to the spar and very little at that.
Oh by the way it feels very stiff now. I pushed on one quite hard and no movement.
My repairs are my own and I don't warrant your success. Have fun.
I love this experimental world and this blog.
Art
 
I've not heard this before. How are the cracks at the forward end of the stiffeners caused by the trailing edge radius? There is an unsupported (at the front) stiffener floating on a sheet of .016" aluminum, so there is naturally a very large stress concentrator there. .

I think what was meant to say was the cracks on the forward end of stiffeners is from not pre-bending enough the LE radius. I also have 2 such cracks that are each 1/2" long that I stopped drilled 10 years ago and they have not changed. It is pretty obvious from the shape of my LE that it introduced some stresses at the front end of the stiffener.

Cracks on the last rivet are from not pre-bending the TE enough.
 
update on rudder cracks and apparent wrinkles

Checked the rudder for what looked like wrinkles in the skin. Relieved to find that the two lines are shadows generated by several different led lights and the skin is smooth and with some dirt and grease. Based on comments I will stick to my plan and monitor.

Thanks for all the comments.
 
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