Spar question

[waltermrich]

In AC 43.13 text on spars it says to use Spruce plywood for doublers on spars, Spruce plywood is not available from the sources I have looked up,( Aircraft Spruce ,& Wicks) what has been used for the root spar doubler? Is there a source for spruce plywood ?

[otrcman]

Re: Spar question

In AC 43.13 text on spars it says to use Spruce plywood for doublers on spars, Spruce plywood is not available from the sources I have looked up,( Aircraft Spruce ,& Wicks) what has been used for the root spar doubler? Is there a source for spruce plywood ?

Can you give the paragraph number that specifies spruce plywood to be used on spar splices ? I can't find it in my copy.

Dick

[waltermrich]

Re: Spar question

Maybe your not familiar with the section look at figure1-5 and 1-6 of section 3 . "Reinforcement plates may be of plywood, the same material of the spar, or of material of equal or higher quality.

[Edwin Otha]

Re: Spar question

Use Birch it'll work just fine.
EO

[otrcman]

Re: Spar question

Maybe your not familiar with the section look at figure1-5 and 1-6 of section 3 . "Reinforcement plates may be of plywood, the same material of the spar, or of material of equal or higher quality.

In your first post you referred to "text". I read all the text and found nothing specifying reinforcement plate material. In you second post you specified Figures 1-5 and 1-6 of Section 3. OK, I see it now. That's a Note on a Figure, not text.

Specifically, the Note that you quoted is on Figure 1.6. Unfortunately, Figure 1-6 is intended for routed spars, not rectangular spars as used on the Taylorcraft. Figure 1-5 is the correct application for Taylorcraft spars and carries a somewhat different Note. It says, in part, "Reinforcement plates may be spruce or plywood ......."

So you don't need to go find spruce plywood. Either spruce planks of the dimensions shown or aircraft grade plywood of the dimensions shown are acceptable.

You also commented that I may not be ".... familiar with the section look at figure 1-5 and 1-6 of section 3." Actually, I'd have to look in Section 4 to find Figures 1-5 and 1-6. Section 3 covers inspection methods. Section 4 covers repairs.

Am I familiar ? Well, yes. If you check the title page of the Circular, paragraph 4 acknowledges the authors who wrote the document for the FAA. In it you will find my name. That's because I wrote Chapter I, Wood Structure.

Dick Fischer

[Guest]

Re: Spar question

Wow that's neat Dick.

Its very good to have an expert on these discussions.

I hope that you are not offended by any comments or questions and that you were not recognized for your expertise.

The first time that I saw your full name was in the one post above so we would not be able to recognize you.

I have been working on an Aeronca wing and it has factory splices (or perhaps laminations) is the better word not only in the spar height but also in the width. It is amazing to see.

Dave

[Guest]

Re: Spar question

Hi Dick,

After seeing your full name on the t-craft site I think that I recall a few years ago you were looking for a Heath tailwheel hub.

I was not ready to sell then but I am cleaning out my shelves now and have one on ebay.

Do you still need one?

Dave

[waltermrich]

Re: Spar question

Thanks for your response, it is helpfull.

[otrcman]

Re: Spar question

Wow that's neat Dick.

I hope that you are not offended by any comments or questions ...........

Dave

Offended ? No. But often reminded of the frustration in being limited to a page count when we did the major re-write of 43.13. The FAA was adding large areas to the document -- most significantly in composites and metal repairs. But they didn't want to increase the overall page count. So they pressed those of us who were working on the "old world" technologies such as wood and fabric to keep our page counts down. I always felt that I could have made things a lot clearer if I could have had a few more pages. Walter's confusion is a good example of where I could have done a better job.

Expert ? Hardly. I just spent a lot of hours talking to smart people and then translating it all into an understandable format.

The worst frustration was not being able to really address the question of acceptable adhesives. We know that some of the old products are going away, either because they have proven inferior over the years or because they contain toxic chemicals. But you can't just say something like "epoxy is acceptable" because there are so many variations in the quality of stuff marketed under the generic term, "epoxy". So I spent a good deal of time writing a performance based specification. The intent was that a manufacturer could produce a product that met the spec just as an oil manufacturer produced to a Mil Spec or an ASTM. I wrote and delivered the adhesive spec to Oklahoma City in 1998. Unfortunately, all my work is rotting away on some bureaucrat's desk back there and has never been published.

Dick

[waltermrich]

Re: Spar question

Thank you for your response, I did not intenend any unwarrented slight, it is remarkable that we have a resourse of so much experience and knowlege right on this Forum, Perhaps a better question would have been "What did Taylorcraft use for plywood on the spar doublers ?" Really its pretty amazing that you are the author of part of of the text. Thanks for being available.

[Robert Lees]

Re: Spar question

Hello Dick,

Thanks for your input here.

Are you able to offer an opinion on the merits (or demerits) of Aerodux and Aerolite over say the "modern" epoxy adhesives such as West?

It's a loaded question, because I use Aerodux (resourcinol) for everything, but Aerolite (formaldehyde-activated) is also used here in the UK a lot, and I have no experience of it.

(As an aside, Casein (milk)-based glues caused a lot of English aeroplanes such as a lot of the Miles types, to be scrapped in the 50's due to the instability of the glue).

I also know that West is banned in Australia, because of their high temperatures, and that in the UK one is not allowed to use it with a dark-coloured paint scheme, for the same reason.

Best wishes,
Rob

The worst frustration was not being able to really address the question of acceptable adhesives. We know that some of the old products are going away, either because they have proven inferior over the years or because they contain toxic chemicals. But you can't just say something like "epoxy is acceptable" because there are so many variations in the quality of stuff marketed under the generic term, "epoxy". So I spent a good deal of time writing a performance based specification. The intent was that a manufacturer could produce a product that met the spec just as an oil manufacturer produced to a Mil Spec or an ASTM. I wrote and delivered the adhesive spec to Oklahoma City in 1998. Unfortunately, all my work is rotting away on some bureaucrat's desk back there and has never been published.

Dick

[otrcman]

Re: Spar question

Hello Dick,

Thanks for your input here.

Are you able to offer an opinion on the merits (or demerits) of Aerodux and Aerolite over say the "modern" epoxy adhesives such as West?

It's a loaded question, because I use Aerodux (resourcinol) for everything, but Aerolite (formaldehyde-activated) is also used here in the UK a lot, and I have no experience of it.

(As an aside, Casein (milk)-based glues caused a lot of English aeroplanes such as a lot of the Miles types, to be scrapped in the 50's due to the instability of the glue).

I also know that West is banned in Australia, because of their high temperatures, and that in the UK one is not allowed to use it with a dark-coloured paint scheme, for the same reason.

Best wishes,
Rob

Hi Rob,

'Fraid I can't be of much help on your specific questions. The resourcinol family of adhesives is a long standing product which has a good history of not deteriorating over time. If you have used it, you know that is incredibly messy to work with and requires good clamping and climate control. The other problem with resourcinol type adhesives is that the bond line has to be extremely thin, requiring excellent machining methods on the mating parts. The reason for the thin bond line is that the adhesive itself shrinks while curing. This shrinkage causes tension stresses in the adhesive and at the interface between wood and adhesive. If the cured joint line is too thick, the amount of shrinkage will pull the adhesive away from the wood and you will have no strength at all.

A major concern with all bonded joints is moisture intrusion while in service. If the joint is not impervious to moisture entry, swelling and shrinking of the wood fibers puts undue stresses on the bond. Bond line separation eventually begins to take place. Painting varnish over resourcinol joints can be problematic if the squeezed out (glue fillet) adhesive chips or cracks. When the adhesive chips, you have a natural moisture entry point.

I don't know much at all about Aerolite. I always thought it was part of the urea-formaldehyde family, similar to the "plastic resin glues" that have been so common in the U.S. The U-F's have been shown to deteriorate over time, particularly under high humidity / high temperature conditions.

My personal preference is for a high quality epoxy. I'd like to see the temperature capability of epoxies get a little better, but the good stuff seems to work very well. Of course the elevated temperature curing epoxies have an advantage in not softening until they get quite hot. But we're pretty much stuck with room temperature curing products when we work with wood. The really premium grade products, such as the West Pro-Set line, have quite good elevated temperature performance.

One thing to consider when talking about elevated temperature softening of epoxies is that the wood itself softens as the temperature increases. Nobody seems to be worried about elevated temperature performance of the wood itself, even though its strength may well be inferior to the adhesive.

Dick

[otrcman]

Re: Spar question

I should have added one more comment on epoxy adhesives. Many epoxy products are simply awful stuff, having poor elevated temperature resistance and low initial strength. I would group all the the "five minute epoxy" products in the awful category. If you are going to use epoxy, do your homework. Find a product with a long standing record of performance and a company to back it up. I mentioned West in my prior post. There may be many other brands out there that are just as good or better, but West has a well equipped laboratory and a professional staff of test engineers who can answer my questions and point me in the right direction. They also have a lot of product in service out there with a good history of success.

Dick

[N96337]

Re: Spar question

The resourcinol family of adhesives is a long standing product which has a good history of not deteriorating over time. If you have used it, you know that is incredibly messy to work with and requires good clamping and climate control. The other problem with resourcinol type adhesives is that the bond line has to be extremely thin, requiring excellent machining methods on the mating parts. The reason for the thin bond line is that the adhesive itself shrinks while curing. This shrinkage causes tension stresses in the adhesive and at the interface between wood and adhesive. If the cured joint line is too thick, the amount of shrinkage will pull the adhesive away from the wood and you will have no strength at all.

A major concern with all bonded joints is moisture intrusion while in service. If the joint is not impervious to moisture entry, swelling and shrinking of the wood fibers puts undue stresses on the bond. Bond line separation eventually begins to take place. Painting varnish over resourcinol joints can be problematic if the squeezed out (glue fillet) adhesive chips or cracks. When the adhesive chips, you have a natural moisture entry point.

This is exactly my point when speaking to the original poster about the time it takes to get a "CORRECT" splice. It's alot more involved than just getting it "about right" and getting a saw and doing a couple angle cuts and throwing a few "C" clamps on it.... In my past experiences, it's been less time consuming to just replace... not to mention less frustrating.
Dick, I wish the FAA had given you more space to comment about adhesives and wood too... stuff like you've just posted NEEDS to be included in the AC, so people might actually understand the importance of what's being said. Thank you!
John