Alternatives to spruce in wooden aircraft construction?

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cluttonfred

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Aircraft plywood seems like it will be around for a long time, if nothing else the Finnish birch type from sustainably managed forests, but spar quality spruce seems to be getting harder and harder to find. Yes, there are solutions like laminated spars, but at some point even that will be problematic. What if we want to keep building little airplanes largely of wood/plywood and fabric but without the spruce? I see at least four approaches, there are probably more:

1) Use other woods and take the weight penalty

2) Build up plywood laminations into substitute spars, longerons, etc.

3) Use hybrid plywood/aluminum construction with aluminum spars, longerons, etc. and plywood ribs, bulkheads, etc.

4) Use selective composite reinforcement to give the required strength to an otherwise all-plywood design.​

Yes, I know that there are also the alternatives of going all-aluminum or all-composite, but let's keep this thread on topic for the "Wood Construction" section.

Cheers,

Matthew
 

TFF

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There is no substitution. Aviation wood is aviation wood. Everything changed needs to be a redesign. You cant get away no matter how much you try.
 

cluttonfred

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I guess I wasn't very clear.

It is quite possible that at some point the diminishing supply will drive up the cost of aircraft spruce to prohibitive levels and/or the tiny market will make it no longer economically viable to produce. It that second scenario even good spruce for lamination will be harder to find. I suspect that museum-quality restorations will always be possible, but the cost of aircraft grade spruce may still become prohibitive for the ordinary homebuilder.

I am not talking about replacing the spruce spars in an existing design with some other material. I am talking about new designs of homebuilt aircraft of primarily wooden construction that use something other than spruce for key structural members.

For example, as we've discussed in other threads, the early Avid/Kitfox models used plywood ribs on tubular aluminum spars as did Mike Whittaker's MW series, among others. Would it be possible to take that a step further and eliminate the spruce from a box-type fuselage as well end up with kit aircraft that comes of CNC-cut plywood like a big balsa model but with aluminum tube for spars, struts, longerons, etc.? That's just one idea.
 

Mad MAC

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It may be possible to come up with a direct replacement, although sales volume will always be a problem. The design properties published in ANC18 are based on the range of properties of the timber after inspection i.e. the worst that can pass visual inspection. By supplementing inspection with modern techniques (machine grading, etc) it may be possible to lift the design allowables for a more common timber to that of spruce.
 

TFF

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That material would already be used if it was good. Yes some day it might require to go fell your own tree if you really want wood or China finds a sub for violins and piano boards and it gluts. With all the French wood designs, where do they get wood? No European equivalent? I know hoop pine is in Australia. Saw some at Home Depot, not good enough for real planes In what they had but I bought some for models. Somebody got some good bundles. Find someone you can piggyback some Doug Fir in a container to Europe. On one of my RC sites, a US expatriate has to import and sneak stuff in carry on to get stuff where he lives when he makes trips back to the US. Even the stuff made in Japan is not available direct sometimes. My best friend in high school's dad carried around about a 500 lbs of milled cherry lumber for 40 plus years. I don't know if he ever found a project he thought it worthy of. I know I helped move it twice. You have to become a hoarder and start collecting stuff. It's just part of the furniture until you start building.
 

pictsidhe

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If you redesign, there are other woods that can be used. If I build from wood, I'll use tulip poplar. Properties are similar to spruce, large one tend to be knotless. Most importantly, I have thousands of them... I also have oaks, mostly red, but some white. They may be good for props?
 

Victor Bravo

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This has been addressed many years ago, and it is FAA accepted practice even on certified aircraft. You can substitute Douglas Fir and take a small weight penalty as Matthew mentioned... BUT I believe Douglas Fir is also a little stronger than Spruce.

Soooo this means you can do a little extra machining (mill the Fir into a quasi I-beam)and make a Douglas Fir spar equally strong and equally heavy as a Spruce spar. You mill down the thickness of the plank in the middle, where the shear web would be in a built up I-beam spar, leaving the "cap" areas at full thickness/weight/strength. The wood removed from the "shear web" area will bring the weight of the spar down. The wood be ing stronger will likely provide equal "strength" (stiffness).

So if you are building a traditional Cub/Taylorcraft style wing with solid wood planks for spars, you have a fairly easy task. The previously flat ends of the wooden or metal wib sections will have to be re-shaped so they match the new quasi-I shape of the spar. Or you would cut some semi-structural filler blocks to put at the rib locations and use the stock ribs.
 
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cluttonfred

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For what it's worth, according the LAA's aircraft wood guide (link to .pdf file), the typical aircraft wood density is 34 lb/cubic foot at 15% moisture content for Douglas fir compared 27 lb for spruce so a 26% penalty. For something like a VP-2 with two 12' x 5-1/2" x 3/4" spars and two 12' x 5" x 3/4" spars, the total weight penalty is under 10 lbs and that could be reduced by routing as VB described, though these are just rough estimates, of course.

I wonder, is there any sort of rule of thumb for routing out spars like that? I understand the principle, which is to remove less wood where stresses or higher and more where they are lower, like the tips. I would also leave them full thickness at the root fittings, strut fittings, and perhaps at the compression struts and ribs, but how much is too much? Is there an ideal I-beam proportion (cap thickness, web thickness) for a homogenous material when you have the overall width and height already determined? How would you do it in practice without having to re-engineer the spar?
 

Chris Young

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If you build a wooden spar in Douglas fir the French way (laminated caps with plywood webs), it will be lighter than a solid Spruce beam like on so many American designs (Cassutt, Pitts, ...), for the same strength.

In France we have Epicéa which is inbetween both (closer to Spruce but hard to find in the same lengths without knots, hence the laminated spars being more common).
 

pictsidhe

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Douglas fir may be 26% heavier than spruce, but it's 29% stronger and 31% stiffer. No need for a fir plane to be heavier.
 

Himat

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That material would already be used if it was good. Yes some day it might require to go fell your own tree if you really want wood or China finds a sub for violins and piano boards and it gluts. With all the French wood designs, where do they get wood? No European equivalent?
...
You have to become a hoarder and start collecting stuff. It's just part of the furniture until you start building.

From the woods.;)
I do not know France, but in Norway aircraft quality wood is probably available locally with the right knowledge. That is, to get aircraft grade wood you must know how to grade the wood and the whereabouts of local farm sawmills. Then visit the sawmills to inspect and buy wood. That’s what the people restoring old boats and houses do, buy wood from small sawmills that cut timber that may have grown very slow.
 

mcrae0104

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I went to your website looking for specs on the ash but didn't find it. Do you have the info as mentioned on the site?

I didn't watch the video on the 'testing' page of that site, but you will find what you are looking for in ANC-18 table 2.6. A quick look says ash (black or white, take your pick) is roughly similar in strength and stiffness but considerably heavier.
 

Chris Young

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Ash is good for highly loaded parts but not for long stiffeners. It is especially very good in compression across the grain and in bearing (so ideal for a bolt going through).

It was used on the Stampe for the front of the fuselage longerons, scarfed with spruce at the back. I think this was done to have higher strength with the same dimensions to make assembly simpler.
 
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