Spar cracks

[Dalton]

So I'm rebuilding a set of wings and found cracks at the butt end of the old rear spars starting at the end of the end and going thru the bolt holes in a couple different spots. I was replacing the spars anyway, so I peeled off the plywood doubler there just to see how far they were cracked. I've probably taken a dozen or so wood spar wings apart ,and most all of them have these cracks in one or both spars. Just curious, how many of you all have found this same condition. Some of the ones I've taken apart, the plywood doublers almost fall off as the glue has obviously failed. What say Y'all?

[taylorukuk]

We have just rebuilt out BC12 wings (we had 4 wings) - All had spars multiple cracks and were unusable.

Ours were cracked just after the doublers.

[MBednarek]

No ply doubler failures, but I had similar spar butt cracks in my left main and righ rear (1946 factory originals). Wings are now in pieces in my garage.

[Hank Jarrett]

Always wondered about the logic on split wood. It is legal to laminate wood and the laminated spars are actually tested as STRONGER than the single piece of wood ones. Why can't a split spar be glued as a repair? Yea, I know it makes all our hair stand up on the back of our neck, but why is it OK to laminate two different pieces of wood but not glue back together a single piece that has become two? Before anyone jumps in with the expected ""ARE YOU NUTS!!" comments, I am looking for a logical explanation, not an attack on the sanity of even asking. How did the FAA decide a spar split couldn't be repaired?

Hank

[Dalton]

Hank, I am in the same camp as you. I seem to remember someone, possibly on the Aeronca site, that had actually had an approved repair for the very thing we are discussing. It just seems like there should be some way to make a repair, other the than a scarf with a new length of spar. Makes one wonder how many of these aircraft with original spars are actually in the same shape, and nobody knows it, cuz there isn't any way to really get a good look at them, other than possibly a mirror and light to view the end grain .

[Hank Jarrett]

And if the crack is really tight at the end you still might not see it! I understand the feelings of those who say scrap it if there are root cracks but I also think it would be valuable to take some cracked spars, repair them and then test them to destruction to see if they lost any strength. With the cost of new Stitca Spruce we are talking about substantial savings and waste elimination. On the other hand I would NOT use a repaired spar unless I had done a statistically significant number of spar tests. I think the problem would be convincing FAA managers with no technical knowledge that the spars were safe. Some of the WORST decisions at the FAA are made by business majors instead of engineers. Or worse, politicians!

[N96337]

Take a saw and rip it at the crack, and laminate. AC43 doesn't want you splicing under an attachment point, but other than that, it's pretty open. My spars had over a dozen different pieces in each from the factory. Times were tough. I have new spruce up above the paint booth now, waiting to be worked and installed. All the old hyde glue was failing in mine. New ones will be one piece.
John

[Scott]

As dope shrinks over the life of the cover, the fabric pulls the root rib outboard. The two tiny nails holding the support block top and bottom rust away or are just sheared out.

The spar butt ends then poke through the fabic, exposing them to the elements under the wing root fairings. So even if the spars are one piece, the exposed butt ends wick moisture. Wood and moisture eventually always end in trouble.

I believe a worthwhile improvement is to bond a plywood cover onto the butt end of the spars before urethane varnish to offer the grain more protection in this vulnerable area.
S

[Hank Jarrett]

Take a saw and rip it at the crack, and laminate. AC43 doesn't want you splicing under an attachment point, but other than that, it's pretty open. My spars had over a dozen different pieces in each from the factory. Times were tough. I have new spruce up above the paint booth now, waiting to be worked and installed. All the old hyde glue was failing in mine. New ones will be one piece.
John

When I first read this post I thought you were saying to just open up the crack and glue in a piece of filler. Kind of made my hair stand up again. Then I realized you were saying rip the full length of the spar, which is a great way to fix it! Most of the cracks I have seen were from the butt end through a fitting hole. Alignment of the holes with the spliced piece could be a problem, but putting in the higher GW bolt hole bushings would take care of that.

[Dalton]

Scott I see your point there on the plywood cover being bonded to the end grain, as this would be a better way than the plywood strips that are nailed to the end grain. Hard to tell if they were ever glued too. Most of the ones I've seen are either partially rotted away or just hanging on by a few rusted nails. I have to question the wisdom of nailing anything into that end grain area ( plywood cover or butt rib) as the nails would seem to act like a splitting wedge there. Seems like there should be a better way to attach that butt rib, possibly a slotted flange to wing attach fitting bolts? I believe that is how the Aeronca ones are done.

[Edwin Otha]

There is some information in AC 43.13 about end grain splitting, which this defect is.

EO

[Scott]

During the little woodwork I was taught,(in England) I learned that nails are to hold parts in place while adhesive dries. Ie nails were not considered fasteners.

Taylorcraft and many other aircraft manufacturers apparently learned woodwork from the construction industry, as nails ended up as the only attachment in several cases, in shear and tension, and for the most part the joints, thankfully mostly non-critical, fail.

But these aircraft were meant to last maybe 10 years!

Even where adhesive was used, the joints have either failed or are about to fail as we approach double the life expectancy of the glues available to the factories in 1946.

[Robert Lees]

It might be argued that the nails that attach our ribs to our spars are only there to aid assembly of the basic structure. Once the drag/antidrag wires and compression struts are fitted, the sole purpose of the ribs is to form the aerofoil section.

There is no need for them to be nailed at all (apart from keeping them in place vertically & in torsion, and the spars should do that; but for the sloppy spar tolerances prevalent in the day).

If I had a spare Taylorcraft/Cub/Aeronca wing (whichever) and the time, I'd love to try to assemble one without nailing/riveting/screwing the ribs to the spars.

Thoughts?

[taylorukuk]

On our wing rebuild we disassembled the wings to get the old spars out and interestingly there were no defects / splits / cracks etc near any of the nails. Lots elsewhere though and there were a lot of nails!

New bigger bushes glued in this morning to the new spar ...

[Dalton]

In response Robert, my first thought is the nails would transfer the lift/load to the spar, maybe I'm not thinking about that correctly tho. On the subject of nails, I have some that I bought from ACS, but they are not the same shank size as what I've pulled out and the heads are much smaller too. What's the consensus on replacement nail size? The originals measure around .040 and the head around .100. Spruce nails are around .035 shank and .070 on the heads.