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9A elevator skin crack

N

NM Doug

Well Known Member
Our 9A has about 400 hours on the airframe. It's usually hangared, but it had been outside with some snow while waiting for a 500 hour mag inspection/overhaul. I don't know if I just had missed this before under a smudge of dirt, or if the temperature changes caused it to advance.

This is near the outboard edge of the trim tab, on the elevator skin. A quick search on the forum turned up a couple of relevant posts, here and here, though I haven't yet spent much time looking.

I'll probably start another thread on repair strategies; this is just a heads up and further encouragement to look at this area on preflights!

fqtzq.jpg
 
Suggestion

Hi Doug,
I thought someone smarter than me would have answered by now.

I'd suggest you "stop drill" the crack to hopefully prevent it from spreading. Then just keep an eye on it during your pre-flights.

It looks like you may not be a builder so I'll give some details.

Drill a hole in the skin at the end of the crack. The radius of the hole will spread the stress over a larger area and should prevent the crack from going further.

Drill at least a #40 hole. That's about 3/32". Much larger and it's more of an eyesore.

The crack as it is now is not a safety of flight issue.

Have Fun!
Mark
 
Mark is spot on and don't let his "smarter than me" stuff fool you. The only thing I would add is I like to drill just beyond the crack, a tiny fraction. I do this for two reasons, the crack usually projects slightly beyond what is visible. Also, the crack can make the bit wander a tad. If you drill just beyond, it will "grow" into the hole, and stop. That's why they call it "stop drilling".
I do not consider this kind of repair a detractor. It is a badge of honor worn to show your diligence in inspection.
 
If this were me I would drill out the rivet, push some pro seal between the skin and the spar and reinstall the rivet along with anything else you do.
Larry
 
BillL said:
Does that hinge look a little sloppy?

Edit: I checked your post references. You did a good job in your search. This one by Vern has a good reinforcement fix. http://www.vansairforce.com/community/showpost.php?p=827520&postcount=3

It seems the 9 has this issue in the 400-600 hr range, at least that is where it begins. Personally, I would stop drill AND add a stiffener like Vern.
Click to expand...

I found my problem before a crack appeared by investigating the root cause of a smoking rivet. Now that there is more than one person reporting this problem (I had about 450 hrs), I hope Van's develops an SB to prevent any problems and one for repair.

Alas, I have sold my 9A, but it's still local so I can check the integrity of my 'fix'.

I will urge any one interested in my fix to copy the images off this site as soon as you can... I have been contemplating taking my Rv-9a site down permanently, so many of my posts that have images will be compromised.

The internet giveth and the internet taketh away...
 
9A trim tab issue

I too had the same problem with my elevator skin near the trim tab at about 450hrs on my 9A and I would like to thank Vern for bringing it to our attention. My fix was too drill out the same damaged rivet to an AN4, then drilled a small hole in the skin to stop a tiny crack. Lastly I bent a piece of .040 into an L shape. This was then riveted to the trim tab spar and elevator rib area. Our 9's do not have the trim tab spar extending into the elevator area like the 7's do. If you grab the trim tab and the elevator and twist them you will see some flexing in that area that is only supported by the elevator skin. I agree that van should address this and incorporate a mod into the plans to prevent this flexing and cracking to take place. Hope this helps this is just another way of handling the fix. So far no additional problems and it's been about 70 hours of flight time since the correction last summer. it's definitely worth looking into. And yes this area is always part of my free flight. Steve
 
inspection ability

...and how would this appear if the rivet line was filled with bondo, or covered with vinyl wrap material????

I second that motion to have Van's issue a notice on this, and it would be great if they provided the reinforcing tab, as many of us don't have the wherewithall to accurately bend up the material.

just my $.02
 
Well if it was bondo, there's a good chance it would crack. If it was vinyl I think you'd still see it. Fibers thinner than a hair show up under vinyl and most cracks will too. I think.
 
Hinge?

Can anybody comment on the hinge and or hinge pin? It appears to have a lot of wear at the first two eyes? ie. Misalignment?
 
unsupported sub spar

I've ran into a couple RV's with similar elevator skin cracks. My solution (ugly as it may be) was to install triangilar shaped doublers to tie the trim hinge sub spar to the nearest elevator stiffener. My thinking is that the same strain on the top skin is happening to the bottom skin as well, so top & bottom skins got doublers.
Would be nice to to have a way to lock the outboard end of that sub spar so it doesn't flex independent to the rest of the elevator structure.
 
Here's a repost of the image from my original thread.

20131108_0505.JPG


Interesting observation was that no one responded to the original thread, which was a report on a smoking rivet. Once a crack was reported, it got serious!

I guess our tolerance to smoking is higher than our tolerance to crack! :roll eyes:
 
Data Point

I just turned 1350 hours on the meter this week and took a long hard look at this area last night on my -9A. No smokin' or crack.

No idea why mine might be different, but will watch it carefully.
 
elevator trim rod fairing

It would be interesting to understand if elevator cracks are from aircraft that have a trim rod fairing similar to the rudder cable fairing. Would it be beneficial? This is an add-on piece not a plans part. Picture of part from Avery Tool. Pete, do you have this?

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Our fix

With the comments here in mind, I got back to the airplane, and a RV-7 builder friend took a look with me.

There was no slop in the hinge itself. The edge was a little "hard," which is probably why the crack started where it did. On the top, it turned out the crack actually went around the rivet hole. On the bottom skin, there was also a crack, in a similar location, this one going from the edge to the rivet hole.

The real issue/source of the problem did seem to be the relative motion of the left end of the trim tab and the elevator just outboard from that end - holding the elevator and exerting a little force up and down on the left end of the trip tab near the hinge resulted in quite a bit of relative motion, and we could see the skin wiggling.

Here is our fix. This is before we trimmed the side tab, drilled the side holes, dimpled and countersunk on the side, and riveted the brace, but you can see the general idea. We also stop drilled the crack on the top and will keep an eye on the bottom skin in case that needs to be stop drilled later. It's not obvious in the photo, because we hadn't trimmed the side tab yet, but once complete there is still plenty of clearance between the elevator and trim tab; the brace doesn't take up much space at all. There is much less relative motion in the area now.

dnksaq.jpg
 
Currently working on building my elevators. Looks like a great fix to tie in the spar to the stiffeners.

May take the opportunity to do something similar in my build. Thanks!
 
I just bought my -9A with about 300 hours. I like to think I'm careful on preflight but this will make me even more so! thank you for posting and thanks all for the replies. This will give me some comfort IF this happens. What a great support community.:)
 
vlittle said:
Nicely done. I bet that's what the Vans SB will look like.
Click to expand...

Just be a little careful that not all cracks require the same fix. The crack show by the OP is a fore-aft crack on the end rivet that could indicate a fatigue issue between the aft sub-spar and the rib that closes out the trim tab cut out. Fix shown in this thread could be a way to reduce the fatigue.

The cracks that I had (http://www.vansairforce.com/community/showthread.php?t=116167) are across the rivets and indicate either a high cycle fatigue due to oil canning of the skin panel or a high tear out load of the trim tab hinge. The fix shown in this thread would do little to nothing for my skin cracks. They require a skin doubler to effectively transfer load between the skin, spar, and hinge.
 
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Elevator skin carck near trim tab rivet

Hi first time posting, not sure if I've done it in the correct place. Just started rv9 in NH, had this question I sent to Van's support but haven't gotten a reply. Can anyone on the forum help, thanks,

Note I sent to Van's
Hi, I'm at the build stage of the left elevator and trim-tab. In reviewing the van's air force forum, I see a number of references to crack developing at the outboard hinge rivets on aircraft with 400 or more hours flight time. I couldn't find any reference to a reinforcing fix in the plans/drawings.

Is this not a problem? Is there a SB I should be aware of? Should I fabricate a doubler as described in the articles on the forum?

Thanks for your time and help, Steve Briggs in NH
 
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