Nails backing out of spar? (merged with "spar varnish" thread)

[TerryM76]

I would certainly recommend drilling, unless there is some compelling reason not to drill.

 

[Hambone]

Here's a photo showing the raised area above the spar, with a nail inside the red circle. You can see the gap between what I think is the spar cap and the spar. Looking closer, I'm hoping that the black area underneath the nail isn't damp or rot!

 

[Hambone]

I would certainly recommend drilling, unless there is some compelling reason not to drill.

The plans specification for the nail seems useless. They're only 1/2" long, and are specified to be every 2" along the spar, and every 1" along the top of the ribs. I bought a bag of the nails, and they're above as strong as a staple. I can't see them going through the aluminum and all the way into the spar. They're too short.

 

[Victor Bravo]

The photo appears to show that the nails are only there to hold the aluminum onto the wood strip behind the spar.

1) The presence of a non-structural wood strip and very short nails seems to indicate it's not structural.
2) The fact that here are two wing struts on each side of the Baby Ace tells me that there is not a structural D-tube being used. A structural D-tube would allow a single strut on each side. If the D-tube was structural, it would have been made out of plywood and glued on to the rest of the wood.

However, these are my assumptions as an amateur... and there is already at least one trained engineer participating on this thread who knwos a lot more than me about structures.

If, and I say if, the aluminum sheet and these nails are not structural components, then you can likely put the aluminum back down onto the top of the wood strip using new nails more closely spaced, or very very small short wood screws. and the wing sould be fine.

 

[lr27]

Where is it on the wing? If it's out near the tip you don't have to worry as much as if it's near a strut attachment or something. Crooked grain isn't as strong as straight grain. If there are spar caps, with straighter grain, it may not matter quite as much. I'm no expert, though.

 

[TFF]

Aluminum leading edges are not structural on wood wings. Most people today make the leading edges structural with the ply leading edges. Done for looks and done for a little more stiffness. It’s really what you call overbuilding but ply takes hangar rash better.
Normal leading edge aluminum is just roof flashing. A nail should go right now through. Airplane ones are copper plated for corrosion. You will look like a shoe cobbler knocking them in top and bottom. Forceps or needle nose pliers holding the nail and a tack hammer. Whack to get it started and whack to sink it. Maybe a set to get it right height. AIRCRAFT NAILS | Aircraft Spruce

 

[Dan Thomas]

If those nails won't stay put, the wood might be decaying around them.

 

[TFF]

More likely it has split the ribs. Years ago a Skybolt lost the whole upper leading edge. The pilot got lucky and survived. A whole can of worms needs to be considered. How stressed was the fabric along that line? Eddie Rickenbacker survived survived the fabric ripping right there in N28. That is just about where the lift is concentrated; it was coming off. It’s not that it can’t be ok. It just needs to be looked at hard. Might sink a couple of extra rib stitches behind that area for luck.

 

[wktaylor]

A couple of tricks for bonding or coating composites and woods...

Ensure Your wood is kept dry. Moisture interferes with all bonds/coatings. and wood is subject to expansion and contraction with moisture content. the intent of finishes is to stabilize moisture content and prevent damage due to tramp moisture/fungus/mold intrusion over the expected life time. One handbook stated that wood should have no more than 20% moisture content, ever... about 10-to-15% preferred... depending on species.

MIL-STD-171 indicates that bare wood should be sealed with epoxy primer [after assembly/bonding], non-chromated preferred; and then over-coated with a layer of polyurethane [over epoxy primer ] as a very useful finish stack for enhanced environmental sealing/exclusion. Recommended primers [non-chromated] for are per: MIL-DTL-53022 or MIL-DTL-53030 and polyurethane per MIL-PRF-85285 [specs readily translate to commercial numbers].

FYI NOTE.
Primer/paint that is 1-part [base and thinner as required] has a relatively simple deposition... apply 'as-is' or 'thinned' and allow to dry [evaporate-off thinning solvents]... forming a coating that is resistant to most environmental factors for a short term... but is not resistant to long-term environmental and chemical breakdown in service. Aggressive solvents can usually degrade or remove these finishes... and are used as 'paint strippers'.

On the other-hand, primer/paint that has 2-or-more chemical parts [base + catalyst and thinner as needed], chemically cures over several hours/days to have high physical resistance to moisture absorption/penetration, the environment over-all... and most chemicals/solvents/detergents/abrasion encountered during cleaning. Specially formulated chemicals are used to breakdown coating chemical bonds structure... to degrade/remove these finishes. Generally speaking the harder the chemistry of these type finishes to degrade... the tougher/longer-lasting the paint system.

 

[harrisonaero]

I did an STC for Rainbow Aviation that replaced nails with screws for a series of airplanes. You might contact them for details and a fix.

 

[TerryM76]

For whatever reason I have a copy of the Ace Aircraft drawings wing construction.
Item 23 is a 1/4 x 1/4 inch filler strip on top of the spar and between each rib.
Item 38 is .020" aluminum for the leading edge
Item 57 is 1/2" x 20GA nails.

 

[fly2kads]

Whether you want to call it structural or not, the aluminum leading edge is seeing pretty high air loads trying to peel it off the wing. Those nails are what's keeping it on. It sounds like since the current number of nails is inadequate to keep the aluminum fixed in place, it is flexing up and down and "working" the nails in the ribs, enlarging the holes. Sounds like TFF's advice to secure the existing nails with a drop of glue, in addition to adding the missing nails, should work.

 

[BBerson]

There don't seem to be many nails along the spar

I think those missing nails every 2" is a big defect.
The few remaining rib nails carry the load and have failed. The entire leading edge could separate in flight.
First thing is to install a standard lower wing inspection hole if needed for inspection.

 

[Hambone]

A couple of tricks for bonding or coating composites and woods...

Ensure Your wood is kept dry. Moisture interferes with all bonds/coatings. and wood is subject to expansion and contraction with moisture content. the intent of finishes is to stabilize moisture content and prevent damage due to tramp moisture/fungus/mold intrusion over the expected life time. One handbook stated that wood should have no more than 20% moisture content, ever... about 10-to-15% preferred... depending on species.

MIL-STD-171 indicates that bare wood should be sealed with epoxy primer [after assembly/bonding], non-chromated preferred; and then over-coated with a layer of polyurethane [over epoxy primer ] as a very useful finish stack for enhanced environmental sealing/exclusion. Recommended primers [non-chromated] for are per: MIL-DTL-53022 or MIL-DTL-53030 and polyurethane per MIL-PRF-85285 [specs readily translate to commercial numbers].

FYI NOTE.
Primer/paint that is 1-part [base and thinner as required] has a relatively simple deposition... apply 'as-is' or 'thinned' and allow to dry [evaporate-off thinning solvents]... forming a coating that is resistant to most environmental factors for a short term... but is not resistant to long-term environmental and chemical breakdown in service. Aggressive solvents can usually degrade or remove these finishes... and are used as 'paint strippers'.

On the other-hand, primer/paint that has 2-or-more chemical parts [base + catalyst and thinner as needed], chemically cures over several hours/days to have high physical resistance to moisture absorption/penetration, the environment over-all... and most chemicals/solvents/detergents/abrasion encountered during cleaning. Specially formulated chemicals are used to breakdown coating chemical bonds structure... to degrade/remove these finishes. Generally speaking the harder the chemistry of these type finishes to degrade... the tougher/longer-lasting the paint system.

Very informative! Thank you.

Any recommendation on a particular product? There is only about 1 sq ft to treat.

 

[lr27]

When applying epoxy on wood, one trick is to make sure that the wood is cooling off, rather than warming up, so you don't get little bubbles. You can apply the epoxy and then judiciously warm the epoxy and the wood with a heat gun. This reduces the viscosity of the epoxy and makes some of the air bubble out through it. As it cools off, epoxy is drawn back in where some of the air was. Or, at least, this works nicely on boats.

If I was applying epoxy for something like this, I'd consider a sample kit from Raka, System Three, or West. Have used Raka quite a bit on boats, with good results. Garden variety Raka, with light colored latex paint over it, has held up for years exposed outside, in New England, on a couple of my boats. If I anticipated having to glue something structural onto it, on an aircraft, I might consider something a little more temperature resistant. Or else I would test Raka after some extra curing. Especially for a plane that wasn't white. Hobby shops carry small amounts of epoxy "finishing resin". My guess is that they'd hold up about as well, but it's only a guess. OTOH, this stuff is inside the airplane.

 

[bmcj]

Where in the span is that deviation? If it is near the top, it may not matter as much since the loads will be low out there.

 

[Hambone]

Where in the span is that deviation? If it is near the top, it may not matter as much since the loads will be low out there.

It is roughly mid-span on the wing, not far from the strut attach point.

 

[Hambone]

For whatever reason I have a copy of the Ace Aircraft drawings wing construction.
Item 23 is a 1/4 x 1/4 inch filler strip on top of the spar and between each rib.
Item 38 is .020" aluminum for the leading edge
Item 57 is 1/2" x 20GA nails.

Thanks! I managed to source the same plans. I then ordered a bag of 1/2" 20 GA nails, which are tiny!
Can I nail them through the fabric, aluminum, and into the spar cap/spar without uncovering the wing, then patch over the nails? Even this will be challenging my limited skills!

 

[Hambone]

I think those missing nails every 2" is a big defect.
The few remaining rib nails carry the load and have failed. The entire leading edge could separate in flight.
First thing is to install a standard lower wing inspection hole if needed for inspection.

My thoughts exactly! There only seems to be one nail along the spar, at each rib, unless there are smaller nails under the fabric which I can't see.
There are a number of lower wing inspection holes already in place.
The plans specify a nail every 2" along the spar, and every 1" along the nose ribs.

 

[Hambone]

I did an STC for Rainbow Aviation that replaced nails with screws for a series of airplanes. You might contact them for details and a fix.

Screws do seem a stronger solution.