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Alternatives to spruce in wooden aircraft construction?

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John wadman

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I always thought that the grain in spars/spar caps should run parallel to the ground too. At Sun n Fun I was looking at a kit aircraft and the spars were running with the grain vertical to the ground and I questioned the representative. He told me that was the correct orientation. On the plans to my first aircraft the cross sectional drawing of the spars showed the spar caps (webbed spars) with the grain running horizontal. I asked two other designers/suppliers of wood kit aircraft via email and neither of them answered my question. I got on a forum and asked the same question and what an argument that started! Then I got on the website of a very popular aerobatic biplane manufacturer and looked at photos of spar tests and many of the spars were laminated of 1/4" thick pieces but some of the tests showed spars laminated with layers of 1/4" with the laminations in the horizontal to ground plane BUT the individual pieces in the laminated spar had the grain lines running vertical. When I tested 3/4" x 3/4" pieces to failure, some solid, some scarfed, some laminated, I tested them in every configuration and found that the pieces made of three 1/4" layers glued in the horizontal but with the grain running vertical to the ground in each stick were right up there as far as strength as the ones that had three layers in the horizontal with grain also in the horizontal. What I don't know is bending strength. I may be wrong but in theory it seems to me that the best would be the later, horizontal laminates of horizontal grain as far as the wings being able to flex and bend. I'm not an aeronautical engineer though so i have to rely on what the pros say. I guess the hardest thing is to figure out just who the real pros are!
 

PTAirco

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I know there is an N.A.C.A report on the subject, bu I haven't found it yet. I had some wartime spars of a Fairchild 24 that were made of vertical laminations, but I don't recall the grain orientation on those.

For general advice on wood construction, find a copy of ANC-18.
 

mcrae0104

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ANC-19 has a section on vertical laminations following the snip I posted yesterday (2.401).
 

SpaceRat

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I was gifted enough clear red maple wood (as it turns out, red maple wood is actually white - I guess it's the leaves that are red).
Anyway, I decided to do a bit of research to determine if this wood would be strong enough to built an airplane... and I came across a very interesting pamphlet from 1950 - "Aircraft Woods: Their Properties, Selection, and Characteristics" in PDF format. Wow, what a treasure trove of information!
As it turns out, this type of maple shares a lot of the same characteristics of Douglass Fir as far as weight and strength. I checked with the folks selling the plans, and he agreed, it would make a strong fuselage and wing ribs.
I'm going to build some structures typical in the airplane and stress test to destruction - Ought to be educational!
By the way, I say I was "gifted" this wood - I actually pulled it out of a burn pile moments before they were going to set it on fire !
What is your opinion on using maple instead of fir or spruce?Burn Pile.jpgMaple Porch.jpg
 

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SpaceRat

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By the way, if any of you want a copy of the above mentioned pamphlet, I will be happy to send to your email.
 

Brünner

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By the way, if any of you want a copy of the above mentioned pamphlet, I will be happy to send to your email.
Thank you SR, would it be possible to upload it here for everyone? If not let me know and I'll shoot you a PM.
 

mcrae0104

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Pound-for-pound, it appears that maple has 95% of the strength of spruce. If the member sizes are designed appropriately, the maple airframe would only be marginally heavier to meet the same load requirements. The "gotcha" that may play into this is that many parts of an airplane are already quite slender, and you may find that buckling or practical constructabily will dictate minimum sizes that do not allow you to fully exploit the higher flexural strength of the maple.

If, instead of modifying the design to take advantage of the maple's higher strength, and just build per plans, it will be 68% heavier than spruce (if that is what the designer intended) and and significantly stronger than it needs to be--probably not a great option to go this way.

Probably the ideal thing to do would be to take advantage of the free material anywhere that "minimum gauge" does not carry a penalty--i.e. spar caps and relatively larger member sections--and stick with the designer's intended species for the rest.

https://ntrs.nasa.gov/api/citations/19930091423/downloads/19930091423.pdf
 

103

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I was gifted enough clear red maple wood (as it turns out, red maple wood is actually white - I guess it's the leaves that are red).
Anyway, I decided to do a bit of research to determine if this wood would be strong enough to built an airplane... and I came across a very interesting pamphlet from 1950 - "Aircraft Woods: Their Properties, Selection, and Characteristics" in PDF format. Wow, what a treasure trove of information!
As it turns out, this type of maple shares a lot of the same characteristics of Douglass Fir as far as weight and strength. I checked with the folks selling the plans, and he agreed, it would make a strong fuselage and wing ribs.
I'm going to build some structures typical in the airplane and stress test to destruction - Ought to be educational!
By the way, I say I was "gifted" this wood - I actually pulled it out of a burn pile moments before they were going to set it on fire !
What is your opinion on using maple instead of fir or spruce?View attachment 106654View attachment 106655
Wooden Aviation- Home is promoting ash in structural areas. Weight penalty can be taken or mitigated with traditional engineering and test.
 

ragflyer

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Apr 17, 2007
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On grain direction, having the edge grain be parallel to ground is driven not by strength (directly) but shrinkage/swelling due to change in moisture. Wood will shrink most tangential to growrh rings. This means a spar aligned with the edge grain parallel to the ground will shrink in thickness while the other way around it will shrink in depth of spar. As the bending strength of the spar is very sensitive to depth( varies by cube) you want to make sure the edge grain is parallel to the ground.
 

TFF

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I don’t think maple is good for bending. I would skip making ribs from it. It is good for blocking, usually preferred. Depending on size and hardness, you could laminate it together and make a nice prop out of it.
 
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