Carbon Cub Preemptive Modification

I keep finding cracks in the boot cowl where the pulley cover blister is attached to the cooling tunnel. They seem to emanate from the sharp corners of the the Tinnerman clip nuts. Today, instead of waiting for my buddy's new FX3 to crack I went ahead and drilled for Tinnerman nuts plates, dimpled and rivited them in. Should have used my blind dimple dies that work with a pop rivet gun and some counter sunk 3/32 pop rivets since it is hard to get the river squeezer in there but it worked and he shouldn't have any cracks. Just an FYI for those Carbon Cub owners out there.
Cracked SS I patched.

The culprit.

The fix.

Thanks for the tip. I'll take a close look at next condition inspection which is due soon.

Where can I get a "river squeezer"

Antennas. Sigh....

Web

No doubler? Seems like the age old battle between weight and strength.

Do pulley cover blisters w/o an antenna also crack? That round stinger exposed to the prop blast can wobble

Gary

WhiskeyMike said:

No doubler? Seems like the age old battle between weight and strength.

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And built in stresses in the sheet metal base material due to the method of forming the shape under the blister before and/or after the hole was cut. If the assembly has built in stresses when the screws are tightened, the vibration over time can still create cracks. Might take more time. I'm waiting for Steve to report back in a couple of years that his well intentioned fix is still working.

skywagon8a said:

And built in stresses in the sheet metal base material due to the method of forming the shape under the blister before and/or after the hole was cut.

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All current Carbon Cub panels are cut out, and have all holes made, in sheet stock on a NC router. Any shaping is done later. On my FX-3 Carbon Cub the panel that has the pulley cutout is flat except for the 90 deg flanges at the edges. This panel is formed separately from the "wrap-around" boot cowl and is riveted to it.

Do earlier Carbon Cubs have curvature in this panel?

frequent_flyer said:

All current Carbon Cub panels are cut out, and have all holes made, in sheet stock on a NC router. Any shaping is done later. On my FX-3 Carbon Cub the panel that has the pulley cutout is flat except for the 90 deg flanges at the edges. This panel is formed separately from the "wrap-around" boot cowl and is riveted to it.

Do earlier Carbon Cubs have curvature in this panel?

Click to expand...

When all the fasteners which hold this panel are removed, does it maintain it's fastened shape without any distortion whatsoever? If it does not, then when the fasteners are installed and the panel is subject to in use vibrations, it will have built in stresses. When a built in stress has an edge, a crack can start. Not always, but enough times for a person to pay attention how the parts are put together.

In this particular case, there is a small hole in the skin next to another larger hole. The skin is curved. Curved skins create stresses when forced into that curve. The amount of material between the screw hole and the edge of the skin is small. With the curved stresses on the skin that small amount of material will crack first. After that piece cracks, it's support for the material on the other side of the hole will be gone, so the crack will continue as it did in Steve's picture. Also, does the curvature of the blister cover exactly match the curvature of the panel? If it does not .. even just a little ... the tightened screws will impart a stress into the panel. Actually those three screws, when tightened may introduce further stresses.

This design may look absolutely wonderful on an engineer's drawing board, in actual use the result shown in Steve's picture takes place. This particular example is just one of many in many airplanes ... and not just airplanes. Stresses create cracks .... sometimes even with glass smooth edges.

Perhaps if that sheet had been roll formed into shape rather than just being held in place with screws, the crack may not have formed. Also the base of the blister cover, then formed to match the curvature of the rolled sheet.

It is a flat sheet. I have seen the cracks with and without the transponder antenna. All the cracks I have found travel through the corner of the clip nut as seen from witness marks in the panel.

When I built my EX in 2010 I put in nut plates from the beginning. 1100 hours later no cracks. Also no antenna in the middle of the tear drop.