Original spar - lifespan?

john-ranger · Post by **john-ranger** » Fri Feb 06, 2009 23:31

Is there a useful life to my 1946 Aeronca 7AC spar? I know about the AD on the wing spar. How frequent has there been a failure? I was flying yesterday back into Las Vegas the wind was blowing hard and gusting to about 32mph at HND (not flying in my Champ but riding left seat in a friends Cessna 182). The turbulence was like I've never seen before. My Champ had one pair of spar replaced in the mid 60's due to ground loop. I cannot remember if it was both wings or just one.

Post by **joea** » Sat Feb 07, 2009 00:00

As long as they are in good shape and inspected from time to time, would have no problems flying them at any age.

The only failures that I know of are those that have been used in an aerobatic airplane or one who has had a ground loop incident, and those failed near the outside of the spar and did not result in an accident or incident.

Nathan K. Hammond · Post by **Nathan K. Hammond** » Sat Feb 07, 2009 01:02

One of the great things about wood, is it gets STRONGER with age; unlike metal that deteriorates from day one. As long as there is no damage, the older spars will be better than new ones.

The catch 22 comes from the glue; they usually fail after _X_ years. That's what condemned mine; 2 glue joints let go. The good news, adhesive technology has made leaps and bounds in the last 20 years. With Resorcinol, T-88 and other 2 part epoxy's; we shouldn't have this problem anymore... hopefully.

But to answer your question... No, there is no service life. Fly them until you see signs of Rot, Cracks, or Glue failure.

nkh

flyingfool · Post by **flyingfool** » Sat Feb 07, 2009 09:07

From what I remember from engineering school in timber design class. Wood also is BETTER than metal for fatigue life.

Mother nature is no fool. Trees are designed to be bent over and over through its life from wind. And hold up well for the most part short of Hurricans and tornados etc..

I once new a guy who did pipeline and electrical transmission patrol. He used Citabria's and would ONLY use Citabria's with wood spars. This guy literally had over 30,000 hours in Citabria's. And said with the low level they have to fly and the severe turbulence that occurs he would only trust wood spars. He thought that metal spars would fail without warning put in this environment. Wood was much more resistant and forgiving.

He showed me pictures of when he was flying near. And I mean NEAR like less than 1 mile away from a Tornado. Amazing guy with a ton of experience. I suspect given the passage of time and his age when I last saw him that he has a set of eternal and permanate wings now and is looking down upon us and being a co-pilot to many!

MikeB · Post by **MikeB** » Sat Feb 07, 2009 17:00

Most of the cracks I've seen were either in the butt end under the attach plate or run from a nail hole on the horizontal plane. I'm quite sure they had probably been flown that way for some time but, of course once it's found a 'crack is a crack

as far as inspection is concerned. I'm also quite sure some of the supposed 'cracks' were misdiagnosed and probably found to be scratches from rib installation or where the original spars were laminated together. One other place they seem to occur is at the wing tip where the bow is attached. Probably from being bumped when moved in or out of the hangar. That being said, a lot of Champs and Chiefs are still flying with original 60 year old spars that are still serviceable. Loose nails and rib rubs seem to be a problem after years of service.

If the plane hasn't been abused and kept under cover it's hard to tell how long these original spars will last. According to the logs, my Champ still has the original laminated spars and other than some loose nails I can't see any problems at this point but I plan to pull the cover once I get my L16 finished.

Mike Berg

gcgilpin · Post by **gcgilpin** » Sat Feb 07, 2009 17:38

How serious an issue is delamination? I've seen a wing 'condemned' because of this. I've always felt that this wasn't necessarily a severe issue (about 4 inches long). Can delaminations be repaired?

Also, my thinking is that laminated spars are stronger than the solid spars. Does anyone have any information on this?

MikeB · Post by **MikeB** » Sat Feb 07, 2009 21:53

My understanding is that laminated spars are stronger than solid stock spars. Think plywood, I guess. Although I'm sure it depends upon how long the laminated sections are. If the laminated sections started to 'spread' it would certainly take some serious consideration. I believe Nate has some experience in this area.

Mike Berg

john-ranger · Post by **john-ranger** » Sat Feb 07, 2009 22:23

Solid spars were standard on the 1946 models? Spruce sells both the laminated & the solid?

Nathan K. Hammond · Post by **Nathan K. Hammond** » Sun Feb 08, 2009 00:49

john-ranger wrote:Solid spars were standard on the 1946 models? Spruce sells both the laminated & the solid?

Generally speaking; No. They used every possible foot of wood they could; if it meant cutting out unwanted sections and gluing up spars, that's what they would do. It was actually a fairly common occurrence after WWII, lots of airplanes rolling off the line, and surplus 'full boards' were long ago used up. My dad's Cub had 15+/- different pieces of wood between the 4 spars.
Spruce only has solid boards that are unfinished. It's up to the builder to do any cutting or laminating needed.

gc; delamination can go either way. If it's near the tip, and isn't growing I would make a note of it, watch it very closely and fly accordingly. But if it's at the strut fittings or inboard, I get real suspicious and start making plans to repair it soon. It's a judgment call, like when do you replace a low-compression cylinder. It's still acceptable, but the seed of doubt is planted and will continue to grow until you finally open the wallet. Plus, a glue joint failure will take a while to show it's self. It takes several years of the wood flexing and allowing dust and dirt into the crack for it to become readily visible. That's why it's always good form to have someone flex the wingtip gently while you inspect your spars.

Can delamination be repaired? I'm of the old school crowd and would say NO; but with the advances in epoxy's these days it's arguable to inject the delam and clamp it back together. Only problem is you're never able to completely fill the void at the ends of the crack, so the problem could continue.

If I had a single glue joint failing I would replace the entire spar. Not because I fear the rest of the glue joint, but because it will be easier AND cheaper to build new than trying to cut out the bad section and scarf in a repair. Plus you'll have issues with a scarf inboard the struts. Been there, tried both ways and new is by far the better option.

You're correct that laminated spars are stronger... to a point. If you had two spar's identical except 1 laminated and the other solid; for the first 20+/- years the laminated would be stronger. But after that 20+/- years, the glue will deteriorate and will be the weak link while the solid board will become stronger with age.

Here's an example of a failed glue joint causing delamination on my Champ.

nkh