Drill stopping a crack in my aluminum Turtle Deck?

[Narwhal]

2022 Carbon Cub Experimental with 220 hours. Unfortunately the warranty just expired. A crack developed in the turtle deck which I believe is constructed of painted aluminum. The crack is on the left side of the turtle deck, near the machined gap that I presume was put there to try and preven this? It's presently about 1/2" long but it seems to get about 1/4" longer every 10 hours. Tac-Aero reports this is caused by a vibration due to the antenna mounting location in the center of the turtle deck. It has happened to all of their airplanes with this design and will happen again after repair or replacement about every 200 hours according to them. Factory rep said they'd never heard of this happening.

Anyhow, I was advised to do a "stop drill" repair by the factory and others. The turtle deck is non-structural. I am not a mechanic but since this an experimental airplane.....can anyone offer any kind of advice or resource on how someone might go about making this sort of repair. CubCrafters quoted about $500 for a new painted turtle deck but I'm guessing it wouldn't be a lot fun installing a new one, sealing it it with adhesives, reinstalling the antenna, etc etc etc. I am not even sure what is directly under this spot below the turtle deck if I were to drill, although I realize the idea is to go no farther than the metal's thickness with a stop. I'd love to pay a shop to do this but I have a lot of difficulty finding people with any desire to work on my airplane up here. I had a good independent mechanic but he took a shift job for the insurance/retirement benefits so back to square 1. The response I got from CC was just to reference AC43.13-1b but the only drill-stop/stop drill I can find referenced in there was for plexiglass repair.

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[FatCub300]

Once a crack like that propagates, it's going to be hard to isolate. stop drilling will buy you some time, a more proper fix would be a patch on the inside. Start by drilling a hole at the visible end of the crack. Generally 3 x material thickness for the hole (drill) diameter is adequate. you will center the new hole you are going to drill right at the point of the current end of the crack. Of perhaps more importance is very carefully de-burring the new hole, once you've drilled it.

If it were me...I'd place a round silver dollar sized patch inside surface of the skin with 6 holes around the periphery and I'd use PRC Pro-Seal on the patch. I think if you are correct that the antenna vibration is causing the panel to vibrate enough to work harden the existing relief slot and hole, then...it's gonna keep doing it and move to an adjacent location. Seldom to things like this just go away.

My thought would be to add a doubler on the undersize of the antenna mount out of .063 2024 T3 maybe 2 inches a side larger than the antenna base with a double row of rivets and I'd PRC that as well...

a lot of work that the factory should have dealt with...hope that helps.

Steve

[Narwhal]

Once a crack like that propagates, it's going to be hard to isolate. stop drilling will buy you some time, a more proper fix would be a patch on the inside. Start by drilling a hole at the visible end of the crack. Generally 3 x material thickness for the hole (drill) diameter is adequate. you will center the new hole you are going to drill right at the point of the current end of the crack. Of perhaps more importance is very carefully de-burring the new hole, once you've drilled it.

If it were me...I'd place a round silver dollar sized patch inside surface of the skin with 6 holes around the periphery and I'd use PRC Pro-Seal on the patch. I think if you are correct that the antenna vibration is causing the panel to vibrate enough to work harden the existing relief slot and hole, then...it's gonna keep doing it and move to an adjacent location. Seldom to things like this just go away.

My thought would be to add a doubler on the undersize of the antenna mount out of .063 2024 T3 maybe 2 inches a side larger than the antenna base with a double row of rivets and I'd PRC that as well...

a lot of work that the factory should have dealt with...hope that helps.

Steve

Thanks Steve! I should've opted for the 2nd comm radio because that option results in no antenna in the turtle deck, instead placing them on the wing root, but i was trying to avoid a 1200 lb cub. Thanks again, I will try this and see what works.....maybe by the time the crack reproprogtes after stop drilling they'll redesign this antenna mount or I will try to fabricate something like you describe for a new turtle deck.

[skywagon8a]

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Notice the crack originates from a factory installed "stop" drilled hole at the end of a manufactured straight "crack". (Sloppy manufacturing) That "straight" crack ought to have something bridging the space to stabilize the material on either side. By not stabilizing the material, the vibration bending loads concentrated at the location of the original stop drilled hole. Thus when the material at the edge of the hole became work hardened, the new crack began. Stop drilling the end of a crack is the recommended fix. However unless the original cause of the crack is eliminated, a new crack can form somewhere on the perimeter of the stop hole. Thus the reason for the patch which Steve recommends. In addition to Steve's patch, the original manufactured slot ought to be stabilized as well. If not, the new patch itself could crack. If a crack has not yet started on the right side, there is a good possibility one will start. A pre-emptive patch there while you are fixing this one would be useful. The use of the PRC is an excellent idea in that it helps by locking all the parts together removing a stress concentrated location for further cracks while including a vibration absorbing material.

[Steve Pierce]

I would push the warranty issue. That is a known issue that Cub Crafters is well aware of. I have not seen them crack in quite some time so I though what the factory was doing must be working. I have replaced several and been contacted on several others. Cub Crafters adds more 3M tape under that area. I made a bracket to attach the antenna to the V structure under the turtle deck. Watch the turtle deck in flight, it drums and those cut outs are the flexing point.
Bracket I made to reinforce the antenna mount to keep from flexing the turtle deck.

[skywagon8a]

Proof that a stop drill hole is not the only answer, since the stop hole also cracked. The flexing must also be stabilized.

Nice solution for the antenna Steve.

[RaisedByWolves]

They are super fun to take off with all that double sided sticky tape. Fun fact on the SS with light weight fabric, the tape is stronger and can tear the fabric when taking it off. This has been an issue for a long time, but no surprise they used there old line of never seen that before.


Sent from my iPhone using Tapatalk

[stewartb]

Warranty on an E-AB airplane? Who built that plane?

[Steve Pierce]

Cub Crafters built it. Owner spent two weeks there but it is a builder assist of Cub Crafters airplane.

[tzfred]

So mostly curious here but when did they started making the turtle decks out of aluminum? Mine is a carbon layup. I have an antenna on it but with aluminum tape star under it. Works well and the composite layup is likely more up to the task of issues in that area. The drumming did cause a couple of the screws in that area to start backing out, a little blue loktite did the job. I assume they did it to allow antennas without a taped ground plane. Again just curious.

[stewartb]

Narwal, take it to Carbon Concepts and have an improved part made with reinforcements added where necessary. Enjoy the experimental part of ownership!

[wireweinie]

65804[/ATTACH]

Did they actually mount the flat base of an antenna directly on a fold line? I'm obviously no structural expert, but how was that supposed to work? Was there a wedge spacer under each side? Anything to fill the gap on each side of the fold?

Web

[FatCub300]

My only two cents worth on Steve Pierce's fix is this: In general, transferring a load from one entity to another, is not always sound. In this case, the loads are small, but over time...vibrational loads are probably some of the worse loads to transfer. It would be better to re-inforce and create a pad for the antenna to sit on and allow the loads to absorb/ dissipate into the sheet, than to dump them into one single cluster. Just being picky here, not arguing...but my background in structural aircraft engineering and manufacturing reminds that this can often have unintended consequences...the way Steve Pierce did it is pretty damn good. The use of the Adel clamps with rubber padding, does provide a bit of absorption and allow for a little movement. The loads don't all descend into one entity...

As for the "go see Randy and Carbon Concepts" comment....don't kid yourself that carbon is always the right answer either...Tension certainly...but torsion, compression and vibration...?? It does seem that a minute ago Stewart you were espousing the virtues of AD's affecting your Experimental Aircraft, (which they don't)...and the next...you're recommending an completely un-certified, un-engineered, un-tested "fix".

I personally would stick with Aluminum, as it more closely approximates ac43.13 replete with all its ambiguity...


Respectfully,


Steve

[Steve Pierce]

So mostly curious here but when did they started making the turtle decks out of aluminum? Mine is a carbon layup. I have an antenna on it but with aluminum tape star under it. Works well and the composite layup is likely more up to the task of issues in that area. The drumming did cause a couple of the screws in that area to start backing out, a little blue loktite did the job. I assume they did it to allow antennas without a taped ground plane. Again just curious.

I started seeing them with the G3 touch panel in the FX3. I believe it was a ground plane thing for the antenna for that system.

[Steve Pierce]

Did they actually mount the flat base of an antenna directly on a fold line? I'm obviously no structural expert, but how was that supposed to work? Was there a wedge spacer under each side? Anything to fill the gap on each side of the fold?

Web

The antenna mount is flat but the antenna flexes the turtle deck in flight and the cut outs at the flap intersection transitions to an upturn in the trailing edge of the turtle deck. You can see the turtle deck flexing on flight if you look back there under the headliner.

This is screwed onto my wall of shame.

[Steve Pierce]

My only two cents worth on Steve Pierce's fix is this: In general, transferring a load from one entity to another, is not always sound. In this case, the loads are small, but over time...vibrational loads are probably some of the worse loads to transfer. It would be better to re-inforce and create a pad for the antenna to sit on and allow the loads to absorb/ dissipate into the sheet, than to dump them into one single cluster. Just being picky here, not arguing...but my background in structural aircraft engineering and manufacturing reminds that this can often have unintended consequences...the way Steve Pierce did it is pretty damn good. The use of the Adel clamps with rubber padding, does provide a bit of absorption and allow for a little movement. The loads don't all descend into one entity...

As for the "go see Randy and Carbon Concepts" comment....don't kid yourself that carbon is always the right answer either...Tension certainly...but torsion, compression and vibration...?? It does seem that a minute ago Stewart you were espousing the virtues of AD's affecting your Experimental Aircraft, (which they don't)...and the next...you're recommending an completely un-certified, un-engineered, un-tested "fix".

I personally would stick with Aluminum, as it more closely approximates ac43.13 replete with all its ambiguity...


Respectfully,


Steve

I think the whole problem has been when they went from carbon to aluminum for ground plane reasons the aluminum flexed more. They added a layer of 3M tape and an Adel clamp, screw and washers to the turtle deck at the intersection, upturn of the flap and until now that seems to have cured the issue. My fix was done in 2019 at just over 400 hours total time and now we are at well over 1000 hours without issue so so far so good.

[Steve Pierce]

[skywagon8a]

I just noticed something else in Steve's pictures which is unrelated to the crack topic. That shoulder harness reel is attached with two screws in a bending load direction. If that reel is needed, there is a possibility that it will peel off those two screws only slowing down your impact. In fact, you could be hit in the back of the head with the reel. Loaded screws should be used in shear for highest strength. That plate to which the reel is attached ought to be welded to those two tubes to which Steve has attached the antenna in a horizontal direction. Then the screws would be in shear.

[spinner2]

I have about 1200 hours on an original Carbon fiber turtle deck with the com antenna in the same location. No cracking or other issues. I put an aluminum plate on the underside to act as a ground plane. I wonder though if that’s needed? Carbon fiber products are a conductor.

[wireweinie]

I have about 1200 hours on an original Carbon fiber turtle deck with the com antenna in the same location. No cracking or other issues. I put an aluminum plate on the underside to act as a ground plane. I wonder though if that’s needed? Carbon fiber products are a conductor.

More like 'somewhat conductive'. The easiest ground plane for a composite panel is either a piece of stainless screen or a star made of aluminum tape.

Web

[Steve Pierce]

Here is the structure behind the reel. They did install a bolt along with the 4 screws on this one.

[FatCub300]

It's hard to argue which is stronger...a 1/4" AN bolt in shear is somewhere around 95,000psi, depending on specifics and the screw is probably 1500 lbs to pop the head off in a heel/toe
load scenario.

Is it strong enough? would need a whole bunch of more data to answer that...but most likely...just barely. They have a few smart engineers and I'd guess they evaluated this.

Do I think think it's the strongest? No, absolutely not. A pair of bent 4130 mounting brackets, heat treated after forming, which placed the bolts into a double shear scenario, would be an infinitely better option...but...this is probably why my planes seem to end up heavier than others. Most likely...if you actually broke that bracket out of the plane in a crash...it absorbed enough energy that it performed its job on the way to failure, so as to aid survivability. Which is the point...time to absorb energy as gently as possible...giving our bodies enough time to decelerate and transfer only survivable loads ie..(bumps and bruises).


Steve

[Steve Pierce]

The second picture is an NX Cub which is part 23 certified so I am sure it is pretty darn strong. The seats are required to withstand 10 Gs.

[skywagon8a]

I guess that I have seen too much over the past 60+ years. I'm recalling a particular certified airplane which in a crash sheared all of the rivets holding the seat belt bracket in the plane. In this case the G loading of the seats has nothing to do with the belt attachments.

[Steve Pierce]

I guess that I have seen too much over the past 60+ years. I'm recalling a particular certified airplane which in a crash sheared all of the rivets holding the seat belt bracket in the plane. In this case the G loading of the seats has nothing to do with the belt attachments.

I am just using the G loading of the seats as an example of what standards a part 23 aircraft is held to. Far cry from the 4 3/16" bolts holding the seat (5/8" thin wall tubing) to the fuselage and the seat belt was tied to the seat.