Alternatives to spruce in wooden aircraft construction?

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John wadman

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That is how the Sopwith planes did it to.
My first airplane was a light minature aircraft 87% 2-place Taylorcraft replica. I built it of Douglas fir and left the dimensions as per plans. It’s supposed to be 25% heavier than Sitka Spruce but about 25% stronger too so in theory you can reduce dimensions. Douglas Fir is a perfectly acceptable alternative to Sitka Spruce. I didn’t worry about the extra few pounds in the entire airframe as I’m 10 pounds lighter than the “average pilot” (170 pounds) and judging by most of the other builders I’ve talked to the “average pilot” is in reality 20 pounds plus lighter than the actual pilots flying these homebuilts. Being a woodworker by profession I’ve worked with many types of wood. For my current build I researched every type of tree that was similar to the Sitka Spruce. I looked up the various strengths and loads of Sitka spruce and then did a comparison. I also researched builders sites such as the pietenpol builders site to see what has been used. One type of wood that comes extremely close to Sitka Spruce in every category is Bald Cypress. It grows in the southern states, it is relatively rot resistant, grows in tall straight trees, nice straight tight grain, is light, and withstands season after season of tropical storms and hurricanes. It works very well and at least here in the southern states is easy to find and reasonably priced. I would highly recommend that anyone building in the southern states get on line and look up the specs on Sitka Spruce, then do the comparison. You’ll be surprised. Then get yourself a nice board of it and cut into sample sticks and do some tests on it. You’ll be even more surprised!
 

Heliano

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Allow me to poke my nose into this interesting discussion since I am building a wood aircraft: Other factors which must be considered: grain direction, glue joint quality, and - do not laugh at me - termites.
Ash is in fact a viable option. I came to know it when I spent a couple of years in the States. However for those who look for more exotic options, here in my country we use a wood called freijó. I am not exagerating: thousands of aircraft and gliders have been built with this wood. I've flown aircraft more than 50 year old which have freijó spars. As a matter of fact the certification authority here decided to verify the structural integrity of a 40-year old spar made of freijó, so they disassembled a wing and did a destructive bending test. Surprise: the 40-year old spar was found to be stronger that a newly built one!
 

Jerry Fischer

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All good information. As a cabinetmaker and familiar with various woods & their properties I'll share what I do know:
1. a substitute we used on the restoration of an Aeronca 7AC was straight grained mahogany. weighed about same as spruce & very strong. Sealed with 2 hand rubbed coats of tung oil.
2. Bald cypress is a substitute as long as it is NOT SEALED. Why? because it needs to breathe
(absorb some moisture) or it will split. This wood would be suitable in a less humid Environment as long as it is inspected periodically for condition (no splitting).
I hope this is useful information.
 

John wadman

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All good information. As a cabinetmaker and familiar with various woods & their properties I'll share what I do know:
1. a substitute we used on the restoration of an Aeronca 7AC was straight grained mahogany. weighed about same as spruce & very strong. Sealed with 2 hand rubbed coats of tung oil.
2. Bald cypress is a substitute as long as it is NOT SEALED. Why? because it needs to breathe
(absorb some moisture) or it will split. This wood would be suitable in a less humid Environment as long as it is inspected periodically for condition (no splitting).
I hope this is useful information.
Curious what part of the aeronca you restored using the mahogany?
 

John wadman

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Did you do 337s on the substitutions?
I didn’t do 337s. I was scratch building an experimental so I just did various glue and destruction tests. Or were you asking if Jerry did 337s on the use of mahogany on the Aeronca? I kind of have a different opinion about the sealing of wood in an aircraft! I’d seal ALL wood in the aircraft. I actually DON’T want the wood to breathe or it can grow mold, rot, expand and contract more and get heavier by absorbing moisture. I’ve never heard of NOT sealing wood in an aircraft! What’s your take on this TFF?
 

TFF

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My question was for the Aeronca. The sealing and wood choice in my opinion would require a 337 for alternative materials. For Homebuilt im just an interested bystander; use what you want would be my answer. I know someone who built a Home Depot airplane. Fisher like. Made Sport Aviation. Very interesting. I would have wanted to see the wood selection when he made it to see what is inside it. He is gone, very sad. I hope they cut it up. Not because it’s bad, but it is really an unknown for anyone else.
 

Charles_says

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Strength. That's the one good reason I am using it in my laminated spars. Shipping Sitka is cost prohibitive. I got my White Ash locally.
wood strengths.png
 

mcrae0104

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That table isn't saying anything about the relative strength of spruce and ash. It lists strength changes relative to changes in moisture.

The relevant figures are in a different table (the tables are numbered differently depending on the edition). Ash is 44% stronger in bending (Fbp) and is 12% stiffer but about 55% heavier than spruce. This is not to say it doesn't work or that it's a bad choice--other considerations may inform your decision. Would love to see some pictures of your project sometime.
 

Charles_says

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That table isn't saying anything about the relative strength of spruce and ash. It lists strength changes relative to changes in moisture.

The relevant figures are in a different table (the tables are numbered differently depending on the edition). Ash is 44% stronger in bending (Fbp) and is 12% stiffer but about 55% heavier than spruce. This is not to say it doesn't work or that it's a bad choice--other considerations may inform your decision. Would love to see some pictures of your project sometime.

Yes, Other considerations made me choose Ash. the first was an article on the "SteenAero" website ( they build Aerobatic aircraft). It was mostly about laminating's strengths, but they included the phrases:
"Laminated spars exhibit no less than 25% greater strength than solid spars" and
"Ash has a favorable strength to weight ratio"
I also knew it was used for Baseball bats, and handles for shovels, having owned both. :)

aaa_Laminated Spar (Steen Aero).jpeg


Spar material.jpg
I used hand picked (by me) 1" x 6" x 12 ft planks, (composite photo) and resawed them into 3/16" x 3/4 inch strips

acclimating after milling.JPG
Cut pieces aclimating to the ambient humidity, for 2 -3 weeks prior to laminating


final spar glue-up (first wing)
Epoxying rear Spar.jpg More than half of the 1x6's went to become mulch in my garden, as I said I used the choicest cuts.... no knots, no defects.



The Left wing finished (structurally), and awaiting rigging.
 

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John wadman

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52 linear feet of 1"x1" sitka longeron material weights about 9.75 lbs. The same in Ash at about 42 pounds per cubic foot weights about 19 lbs. Thats just the 4 longerons. Every wooden component will be almost twice as heavy. You can make a table of weights for every thickness of wooden pieces in the airframe and then add the lengths up and get the weight. Your airframe in ash can easily be 30 pounds heavier than Sitka. Engine choice and power is going to be more critical for your airframe. Will may need to reduce the amount of fuel you carry to stay below gross weight. 30 pounds is about 4 gallons of fuel. Weight affects everything! Climb, speed, etc.
 

John wadman

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I made a comparison chart for various dimensional lumber used in my airframe using Sitka as the baseline. I calculated weights for Douglas Fir and Bald Cypress as they are very close to Sitka in every aspect important to aircraft construction. I didn’t do the calculations for ash because I felt it was too heavy and the grain rings are not as defined as the others. But, it can easily be done. You can measure every piece on the plan sheet and figure out from the chart how much each will weight almost to the ounce. You can get a pretty accurate “guesstimate” of what your airframe will weight in each lumber. I also did dimensional reductions based on strengths. In theory, (but not practicality) Doug fir for instance can be milled to 3/4” x 3/4” in place of 1 x 1 Sitka. In theory, ash longerons could be 1/2” x 1/2” in place of 1 x 1 Sitka. I wouldn’t build a plane with 1/2” longerons! Realistically, even reducing from 1” to 3/4” is a stretch. It would be interesting to do the calculations for ash and see what total weight gain you can expect. My airframe (w/o engine) came in at about 26 lbs heavier than it would have in Sitka. With full tanks (I carry an extra 3 gallons for longer flights) at takeoff I’m only 10 pounds below gross. With normal fuel (5 gallons) I’m about 31 lbs below gross. I’d prefer more! In my case I couldn’t have used ash!
 
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fly2kads

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Those numbers for the reduced sizes are way too low, but your point is still valid. It takes some work to determine the size required to carry a given load when you switch species. From there, you can determine your new weight of the timber components. It's all knowable, it just takes effort.
 

poormansairforce

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Something to keep in mind is that the designer was planning on a certain amount of glue area of gussets, etc for strength so that needs to be checked as well if reducing size. IMHO, its better to use a wood that has properties closer to sitka.
 

John wadman

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Those numbers for the reduced sizes are way too low, but your point is still valid. It takes some work to determine the size required to carry a given load when you switch species. From there, you can determine your new weight of the timber components. It's all knowable, it just takes effort.
Oops! The numbers for Doug fir are correct but not for ash. I was reducing the ash for equivalent weight as Sitka, not strength! My mistake. The thing about the fir is that the percentages for both weight and strength are about the same so by reducing dimensions the weight will come out in the ballpark for Sitka also. I don’t think it works that way for ash. The solid wood portion of the airframe is going to be substantially heavier. All other components, engine, gear, rigging, plywood, covering, etc obviously stay the same. I wonder if there are other Aircraft of his type that are flying that used ash and if the weights can actually be compared. Ash is nice to work as a wood! It’s also a pretty wood!
 

Charles_says

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Perhaps I misled, or confused some. Let me clear that up.
It is the long lengths of Sitka Spruce, which are costly to ship.
Hell even the $38.00 Bargain boxes of Sitka from A/S, cost more to ship,
than the material costs Cost me $84.00 to ship from Atlanta.

I used the Ash only to laminate the Spars. Everything else, was fabricated from the Bargain box.
I cut my own 1/4" rib sticks, and cap strips from the Bargain Box material. And I have pieces left over for repair work, if it becomes necessary.spruce bargain box2.jpeg
I found this to be the most cost effective method., and I get to say:
" Yeah, I built it all, myself!" which for me is the fun part. I truly enjoy building things.

(edit) Oh.... my 12' wing weighs 28 pounds less fabric. ( end edit)
 
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