Birch and Poplar Plywood Aircraft Grade MIL-P-6070

[Tre Landrum]

OK, I am ready to buy my wood (plans built Hatz Classic). No problem, just order some 1/4" birch plywood right? Well, not so much. Look at the webpage and what they have is Birch and Poplar Plywood. It is aircraft grade MIL-P-6070 (I which I have found it really isn't so much about the wood as it is about the glue).

There is a huge difference in birch and poplar, but some things I have found make it sound like all birch ply made in the USA are combined with either poplar (or Basswood, which is even worse... eg lighter, but not as strong). Can someone tell me any info on this? The guy at the wood supplier (supposedly the best guys in the business) say the birch poplar is the way to go, but I would just like some more info... like why the poplar, is that the specifications when it says birch plywood? Should I go with imported ply? Is the imported stuff really all birch? Is it better?

I have spent many hours combing through wood density tables, mil spec docs, grading specs, but still can't come up with a benefit to mixing poplar with birch, other than maybe cost and weight, both of which are fairly negligible. So please tell me there is a wood guru on here who can help me out!

Tre'

[WLIU]

As a woodworker, I can comment on a couple of aspects of your question.

First, Poplar is less expensive than Birch. When you are mass producing, pennies count.

As boards, Poplar is less dimensionally stable than Birch, but when rotary cut into thin plies, this is not an issue.

If you are looking at plywood that is thin enough so that you only have 3 plies, the face plies carry the load and the middle ply is under much less stress. This allows the use of a wood like Poplar that has approximately half the crushing strength and modulus of Birch.

When you start to look at thicker plywood, it gets more interesting. For 1/2" and up, the highedst quality birch plywood is generically calld "Baltic Birch" because it is manufactured in Scandinavia and western Russia and the small countries around the Baltic Sea. Those sheets are 5+ plies of nothing but Birch. We use that product to make drawers and cabinets.

If you are looking for thinner plywood that has high quality inner plies, then you have likely found that you are looking at marine grades of Okume. Great stuff. Boulter Plywood in Mass will happily ship you 1/4" Okume in 4x4 sheets. http://www.boulterplywood.com/

But the question today is what you are using plywood for? You can replace it with S-glass and carbon fiber in epoxy to make a stronger structure that tolerates weather and fatigue better.

Best of luck,

Wes

[Tre Landrum]

I am looking at 5 ply 1/4" for making ribs noseblocks, false ribs, and the one full plywood rib in the plans.

So I am somewhere between. I am really not even sure how many plys are birch (assumed 2) and how many are poplar (assumed 3).

I knew I would have trouble with how to build some of this... but I sure never thought I would have this much trouble with the wood... just call up "hey, I need some 1/4" aircraft grade plywood.... nope... guess nothing in airplane building is that simple!

[WLIU]

So looking at the Aircraft Spruce web site, you want their Finnish Birch.

Best of luck,

Wes
N78PS

[Tre Landrum]

I have mostly been looking at Wicks... but I am thinking about ordering the ply from AS, and the cap from wicks... may cost me some more for shipping... but I have a lot more questions than there are answers on the wicks page.

Thanks

[Tre Landrum]

So now I am leaning toward going ahead and getting the Finnish Birch... 12ply over 5 is clearly better (in my mind). But now I notice there is no MIL-P-6070 compliance... Which I have found is more to do with the glue than it is the wood... but at least it is a aircraft plywood specification. And some say metric with 1/4 cap is a big deal... others don't seem to think it matters!??!

While I am pretty sure none of this matters at all... all of these woods will work and work great... wanting to know which one is right is driving me nuts!!!!!

LOL.... Thanks all.

[Clarke Tate]

Tre,
What is the specification for the 1/4"
ply on the plans or Hatz materials list. I assume there is a Hatz builders group online. What is their input regarding ply? Mil Spec and Germanischer Lloyd may be a different weight.

Having looked into the French Jodel plans, while not a Hatz, utilizing Baltic Birch ply would lead to an unnecessarily overweight aircraft if substituted for the Okume ply specified.