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Streffpilot

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Hi, I am working on building a Minimax. Several parts of the aircraft call for Mahogany or Birch plywood (aircraft grade of course). Other locations call for just mahogany. Is there a reason for this, or is birch substitutable in all mahogany places?
 

cluttonfred

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I believe that mahogany is stronger but a little heavier than birch, while birch is a little easier to bend. So it would make sense that where strength is critical, the plans would specify mahogany, but otherwise either is fine.
 

Wayne

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These guys will know - you register through the system and it might take a while but well worth the wait ETLB Squawk Forums - ETLB Squawk Forums.

ETLB is an awesome website for 'Max builders of all shapes and sizes.

You could also call Dave at Minimax, even if you downloaded free plans I'm sure he would be MORE than happy to help. Also - you never know, it might be cost effective and ring piece of mind to order your wood from him. USA Office Phone: 1-269-262-0866 Direct Cell Phone: 1-574-214-3660

Looking forward to seeing progress on your bird!

Wayne
 

Rockiedog2

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Actually, mahogany is lighter and birch is stronger but heavier. It sounds like the plans have taken into account the differences in the types. I would just go with that.
 

Streffpilot

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Actually, mahogany is lighter and birch is stronger but heavier. It sounds like the plans have taken into account the differences in the types. I would just go with that.
My conundrum: I can get Birch locally from WagAero, but not Mahogany.
I am waiting for a reply from Minimax. I was hoping to be able to use Birch in place of the Mahogany to avoid shipping costs/time delays. I HATE HATE HATE paying for shipping for something I can get locally. Especially since I can get it from an actual aircraft parts "store"
 

fly2kads

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Shipping on plywood gets expensive, too! If you're going to order some and pay for shipping, you might as well order a bunch of it.

Plywood species get specified for a lot of different reasons: strength, weight, availability, the designer's personal preference, etc. You'd really have to find out from the designer what they had in mind. Generally speaking, though, you can substitute birch for mahogany pretty safely, recognizing that you're taking a weight penalty for doing so. For small things like gussets, the weight difference will be minimal. For larger areas like fuselage sides, it will really add up.

Doing it the other way around, using mahogany in place of birch, requires more care and judgement. If birch is specifically called out, it is sometimes because the higher density and strength really is required for that part.

Another thing to check on is what core is used in the birch plywood that WagAero is selling: birch or poplar (or something else altogether). I don't see that specified on their site, or in their catalog. The all-birch plywood is a bit heavier than the poplar-core variety, but is only marginally stronger. (It's great for highly loaded areas, though, where crushing or bearing stress is a concern.)

Just to give some idea of the difference in weight, ANC-18 lists the following weights for 1/8 thick plywood (lbs./sq. ft.):
  • birch/birch: .494
  • birch/poplar: .414
  • mahogany/poplar: .352
(I listed these three, because these are what you'll see in Aircraft Spruce's catalog as Finnish birch, domestic birch, and domestic mahogany, respectively.)
 

Rockiedog2

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what fly2kids said.
you may be able to downsize if you go from mahogany to birch but that's not necessarily a casual decision, depending. For instance, rather than using the often standard 1/16 mahogany for rib gussets I used 1MM birch on this last plane. That's .06" for the mahogany vs .o4" for the birch. I didn't engineer that I just knew from experience that it would be fine in the application I was working with. Using fly2kids comparisons above and if I did the math right the birch in that case is about 5% lighter. But in most cases that probably wouldn't be advisable. As in spar reinforcements etc. I'd follow the plans in those cases and probably most others too. Can't go wrong following a proven set of plans. Or do what you did and contact the designer.
 

Streffpilot

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Actually, mahogany is lighter and birch is stronger but heavier. It sounds like the plans have taken into account the differences in the types. I would just go with that.
So, In theory, I can use Birch for everything, but will sacrifice some weight?

Minimax has not yet gotten back to me. :(
 

Jon Ferguson

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So, In theory, I can use Birch for everything, but will sacrifice some weight?

Minimax has not yet gotten back to me. :(
Dave Cooper is best reached via phone or Facebook messenger. His family suffered a loss very recently which could account for the holdup. But I assure you he is still very much engaged with TEAM and getting those orders out. He took minute to chat with me today and he was attending a funeral later.
 

Rockiedog2

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So, In theory, I can use Birch for everything, but will sacrifice some weight?

Minimax has not yet gotten back to me. :(
I would think that generally that is correct but there may be cases where not so...I dunno. Your call.
I would hate to add ANY weight that could be avoided somehow. I guess if the $ are very close(I been there) well OK to keep things moving along. But I would look for some other place to make it up if possible before adding the cursed weight. Last resort around here...
 

Little Scrapper

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You live within 20 miles from me. McCormick lumber in Madison has aircraft plywood as well as Spruce and Fir.

The Pietenpol guys do a lot of shopping here.
 
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