Compression dents on wood spars

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Jerry Jackson
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Compression dents on wood spars

Post by Jerry Jackson » Mon Dec 01, 2014 10:44

My IA doesn't have any guidelines for judging what compression dents are acceptable in a set of fir spars for a 65TC so I want to ask for recommendations and opinions here.

The dents are located where parts are bolted to the spar face--compression ribs, root fittings, ect. They are unremarkable in appearance, free of cracks, discoloration, or broken fibers. The dent depth ranges from about 0.015 inches to about 0.045 inches. They are shallow around holes fitted with bushings.

I need to replace the doublers at the root. Should the doubler material be installed flat, with the underlying dent filled with glue? Or should the doubler material be deformed to match the shape of the underlying dent?

What is the recommended tightening torque for bolts in wood spars?
Jerry Jackson
San Antonio
7ECA 65TC
8T8

Paul Agaliotis
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Re: Compression dents on wood spars

Post by Paul Agaliotis » Mon Dec 01, 2014 17:13

Jerry,
Most of the time these defects are caused by overtightening of the attach hardware. I'm not sure if there is any specific inspection criteria for this condition, I've never looked. If you plan to replace the butt plates, I would try some distilled water on the area to try to restore the dishing. Add a little ammonia to the water if needed. This allows the grain to relax a little more.
Paul
Mailing Adress : Paul Agaliotis 2060 E. San Martin, San Martin,Calif. 95046

Jerry Jackson
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Re: Compression dents on wood spars

Post by Jerry Jackson » Wed Dec 03, 2014 05:51

Thanks for the response, Paul.

I will try using water and ammonia to reduce the dents. I plan to install the doublers flat, allowing glue to fill the dents. The only quantitative information I have found for bolt torque is from propeller manufacturers. They recommend 10-12 ft-lbs for 1/4" bolts and 11-13 ft-lbs for 5/16".
Jerry Jackson
San Antonio
7ECA 65TC
8T8

brink
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Re: Compression dents on wood spars

Post by brink » Sun Nov 18, 2018 15:10

Water and ammonia how does that work on wood that’s been varnished usually with two coats of spar varnish, ya you lost me, I’ve used that on oak rifle stocks and a wet towel and a hot clothes Iron,after the entire stock has been boiled in water that works, but the stock usually has been rubbed with linseed oil, and no varnished. Hum confusing. If you can locate a Northrop manual their is a really good section on wood composite and spar repairs, using T-88 is also an excellent glue for all types of wood repairs. I am framilure with pinch pockets. The Sensenich torque values apply to hard wood,our spars have closer grain but are softer than a wood prop, use caution with those values. Not to offend just my thoughts.

Paul Agaliotis
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Re: Compression dents on wood spars

Post by Paul Agaliotis » Sun Nov 18, 2018 22:46

In most repair conditions the surface finish will be removed prior to additional work. A glue joint doesn't have sufficient strength if installed on a varnished/glued surface. The question was how to relax the grain deformation, forming complex wood structures will require steam, water soaking. The addition of Ammonia will assist in the grain to relax.
Paul
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brink
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Re: Compression dents on wood spars

Post by brink » Mon Nov 19, 2018 17:23

I’ve used ammonia to bend balsa on my RC model aircraft, you use to much and it destroys the hard wood and balsa fibers, ya I would think a guy would know to sand an remove what ever finish is on wood spars before he would make a repair of any kind. I know it’s very hard to remove a finish especially 2 part Epoxy varnish. On any wood wing surface spar, tell me Paul I may learn something here, how do,you determine a scratch in the finish from a crack in the Spar? What do,you use to,developer a scratch or is it,a crack,?

Paul Agaliotis
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Re: Compression dents on wood spars

Post by Paul Agaliotis » Tue Nov 20, 2018 10:15

Cracks typically will follow the grain and cause it to separate. It is impossible to get a compression crack in kiln dried wood. Thee compression cracks happen in green wood that has been mis-handled, like dropping the tree on another tree. What the AD is referring to is a cracked spar by compression, not compression wood.

Any cross grain cracking should be looked at real close. I've been asked to evaluate a spar crack and determined it was not a crack by this criteria. Looking through small lightning holes can be challenging and open to interpretation. The operator decided to open the wing, and it was a pencil line.

Cracking occurs at only a few highly loaded areas of the spar. The greatest load will be at the doubler plate where the cantilever section starts. Right where the AD tells you to look. The last statement in the AD is where many repairable spars were scrapped, due to other defects noted.

I 've only found 1 spar cracked in the manner described in the AD. This was found during rebuild and the wing had been repaired after a ground strike. I would suggest if the wing gets into the dirt, strip the fabric and do the inspection properly.

Paul
Mailing Adress : Paul Agaliotis 2060 E. San Martin, San Martin,Calif. 95046

brink
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Re: Compression dents on wood spars

Post by brink » Tue Nov 20, 2018 20:44

Okay I read your post but you didn’t answer my question, I’ve done the same on spar issues but never found a viable crack on a 7AC spar. Again what do you use when you see a pencil, scratch, or possible crack, how do you determine what your looking at? Paul I have a way from Old school,But I want to learn something here, what do you do,to Determine what your seeing or looking at? A fellow flyer!

Paul Agaliotis
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Re: Compression dents on wood spars

Post by Paul Agaliotis » Wed Nov 21, 2018 09:46

A seperation in the grain will indicate the spar is cracked.
Paul
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brink
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Re: Compression dents on wood spars

Post by brink » Fri Nov 23, 2018 10:17

Okay Paul I get it!

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