Cracked spar

Charlie Longley · Jan 3, 2020

I have a J-3 in my shop with a cracked spar. I am trying to figure out why it cracked. The bolt holes were pretty tight.
I did a set of L-4 wings years ago but memory is foggy. I am going to do a little research tonight but if anyone's seen this before chime in!

mike mcs repair · Jan 3, 2020

Don’t have answer to your question.

Looks to be a downward force did it, has it been upside down? That would have been the last wing to hit upside down. Probably bent front strut also.

But big hose clamps around spar with crack like that will get you home on ferry flight.

Sent from my iPhone using SuperCub.Org

bob turner · Jan 3, 2020

Sometimes they just crack on the ends. That does look like damage, but there ought to be other evidence. I would look it up - some cracks may be acceptable - that one looks like new spar city to me. In the olden days it would get spliced - an ugly affair on a beautiful wing.

Charlie Longley · Jan 3, 2020

bob turner said:
Sometimes they just crack on the ends. That does look like damage, but there ought to be other evidence. I would look it up - some cracks may be acceptable - that one looks like new spar city to me. In the olden days it would get spliced - an ugly affair on a beautiful wing.

Yeah I am putting in a new spar. There's no other evidence of damage other than a few bent nose ribs. I think they got bent when the leading edge was put on though. I am going to finish pulling off the fabric and take a close look at the rest of the wing.

Hardtailjohn · Jan 3, 2020

Looks more like a deep check to me. They'll do that if they don't get to dry slowly enough.
John

Charlie Longley · Jan 3, 2020

Hardtailjohn said:
Looks more like a deep check to me. They'll do that if they don't get to dry slowly enough.
John

It’s definitely not a deep check. Take a closer look at the pictures. It’s cracked through both sides. You can open the crack by hand...

PerryB · Jan 3, 2020

I'd say the wood grain was compromised (for any number of naturally occurring reasons, or while being dried) and over time the compression of the attach fitting caused it to pop. Basically just rotten luck.

skywagon8a · Jan 4, 2020

That is interesting, makes one wonder what and why? What can be done to prevent a repeat? Have others found this sort of damage after certain maneuvers or incidents? The crack happened after the wing was painted since the yellow paint on the front is also cracked indicating motion within the wood grain. Also the crack comes from the butt to the top of the first hole yet departs the bottom of that hole directly to the top bolt hole of the compression tube. What caused that ding on the butt above the crack? Something hit there. Is that a handling ding or was the wing over stressed in a careless acrobatic maneuver causing a severe down load on the fitting? If it was a handling ding, the wing tip could have been lifted too high hitting the butt on something stressing the wood through that first bolt. Do the holes in the fitting and the matching holes in the spar align without having to force the bolts through?

So many questions, will we ever know? We do know it happened after the wing was last recovered.

WhiskeyMike · Jan 4, 2020

"So many questions, will we ever know? We do know it happened after the wing was last recovered.[/QUOTE]"

Is it possible that someone did a whip stall and overstressed the spar?

flybynite · Jan 4, 2020

Something pushed up on the end of the wing. The lift struts were the fulcrum. The top bolt on the root fitting provided resistance to the downward force on the root until the spar cracked. (IMHO)

BC12D-4-85 · Jan 4, 2020

Every wood spar wing I've had could be manually flexed up and down at the tip. It doesn't take much effort. Any bump or unusual flex due to vertical loads could stress the butt end.

Gary

bob turner · Jan 4, 2020

Yeah - I have a buddy that lifts the wing at its tip. Ok to jiggle, but if you are lifting, do it at the strut.

Charlie Longley · Jan 4, 2020

PerryB said:
I'd say the wood grain was compromised (for any number of naturally occurring reasons, or while being dried) and over time the compression of the attach fitting caused it to pop. Basically just rotten luck.

I am thinking this might be the answer. The spar is quarter sawn a bit don’t know if that made a huge difference.

Charlie Longley · Jan 4, 2020

Anyway I am putting a new spar in. I’ll post some pics of the process.

To start you know all those little nails holding the ribs on? I found an awesome system for pulling them out!

bob turner · Jan 4, 2020

Have her slide a piece of .020 stainless under each rib and tap the nail hole flat. Makes disassembly lots easier.

PerryB · Jan 4, 2020

flybynite said:
Something pushed up on the end of the wing. The lift struts were the fulcrum. The top bolt on the root fitting provided resistance to the downward force on the root until the spar cracked. (IMHO)

You might be onto something but I think your scenario would cause it to crack at the strut attach point. A cursory "look" at the geometry and load paths in my mind makes me think the strut fitting would be the more vulnerable location for a fracture. In my opinion, over-zealous jacking while replacing a strut seems a more likely candidate.

RVBottomly · Jan 4, 2020

Charlie Longley said:
I am thinking this might be the answer. The spar is quarter sawn a bit don’t know if that made a huge difference.

View attachment 46347

I'm looking closely at all this.

My WagAero wooden wing is designed with plywood faceplates both sides at the wing attach point. I'm wondering why there wouldn't be one on the J3?

Hardtailjohn · Jan 4, 2020

Charlie Longley said:
It’s definitely not a deep check. Take a closer look at the pictures. It’s cracked through both sides. You can open the crack by hand...

A check is a longitudinal crack in the radial direction of the wood and usually on one end of a board or the other. It can go all the way through. It's usually caused by uneven shrinkage when drying. IF there were some evidence of damage or rough handling, I'd say it wasn't a check.
John

Hardtailjohn · Jan 4, 2020

Charlie Longley said:
The spar is quarter sawn a bit don’t know if that made a huge difference.

You must mean Plain sawn...Quarter sawn is what results in "vertical grain", which is what you want. From what I can see in the picture, the grain slope is well within spec, according ANC-19.
John

Charlie Longley · Jan 4, 2020

Hardtailjohn said:
A check is a longitudinal crack in the radial direction of the wood and usually on one end of a board or the other. It can go all the way through. It's usually caused by uneven shrinkage when drying. IF there were some evidence of damage or rough handling, I'd say it wasn't a check.
John

“Checks, shakes, and splits. Checks are longitudinal cracks extending, in general, across the annual rings. Shakes are longitudinal cracks usually between two annual rings. Splits are longitudinal cracks induced by artificially induced stress. Reject wood containing these defects.”

It’s a split not that it really matters. It’s quarter sawn which I realize is better after a little research.

Hardtailjohn · Jan 5, 2020

Charlie Longley said:
“ Checks are longitudinal cracks extending, in general, across the annual rings. .”

That's on a plain sawn board....but it doesn't matter....you're replacing the spar, which is the proper fix. The only reason I specified that, is that it ties in with wanting to know why it did it. I'll be interested to see if you find any evidence of damage when the spar is completely out so you can examine it. I'm not trying to argue.

Charlie Longley · Jan 8, 2020

Nothing but the finest Prime Rib of wood used in my hangar!

N86250 · Jan 8, 2020

I wonder if that split happened during installation. Bolt through the front fitting and then the wing slipped, dropping the trailing edge and twisting the front spar. I would think such a twist would bend the attach fitting. Maybe it did and the fitting replaced but the crack or start of it was overlooked.

bob turner · Jan 8, 2020

How do you slide the new spar in there with the ribs still attached to the rear spar? Gonna add the plywood plates after sliding it in?

Charlie Longley · Jan 8, 2020

bob turner said:
How do you slide the new spar in there with the ribs still attached to the rear spar? Gonna add the plywood plates after sliding it in?

Haven’t done it yet but that’s my plan. I took the spar out in three pieces.

skywagon8a · Jan 9, 2020

bob turner said:
How do you slide the new spar in there with the ribs still attached to the rear spar? Gonna add the plywood plates after sliding it in?

That's what I did on a 7GCB without even uncovering the wing. Just a slice in the fabric over the spar area.

Steve Pierce · Jan 9, 2020

N86250 said:
I wonder if that split happened during installation. Bolt through the front fitting and then the wing slipped, dropping the trailing edge and twisting the front spar. I would think such a twist would bend the attach fitting. Maybe it did and the fitting replaced but the crack or start of it was overlooked.

Steel fittings are bolted over the spar.

Steve Pierce · Jan 9, 2020

bob turner said:
How do you slide the new spar in there with the ribs still attached to the rear spar? Gonna add the plywood plates after sliding it in?

That is how I have done it as well, Champ and J3.

Charlie Longley · Jan 9, 2020

skywagon8a said:
That's what I did on a 7GCB without even uncovering the wing. Just a slice in the fabric over the spar area.

That’s not possible on a J-3. The leading edge has to come off.

bob turner · Jan 9, 2020

I admire you guys. Drilling those holes while the spar is in the wing takes great skill, and probably a really good jig.

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