Procedure for Spar lamination?

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wmax351

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I'm currently in the planning stages of building a flybaby. I'm considering a laminated spar, since it would allow me to be more certain of the quality of the wood, as I could inspect it every 3/4 inch or so.. I would likely purchase the wood at public lumber in Detroit. Options include "Spar and mast" grade sitka spuce, Doug fir, white ash (and any other suggstions).

The spars are 3/4 x 6nom and 4.5 nom. x 13 feet

My initial plan is below, all dimensions are exact. I'd appreciate any suggestions and tips.

1. Rip 1x3/4 or 1/2 pieces. Inspect, test some 1/2 x 3/4 or 1/2 samples to destruction from each board.
2. scarf together as needed for ~14' or 15' lengths. Scarfs will be offset by >3 scarf lengths, or 2 layers.
3. prepare a straightedge form, using metal stock or very straight wood/mdf covered in duct tape and saran wrap. Spar will be built with the long edge flat on the table. A mold board will be used on top, screwed to the table to provide a flat side. The clamping will be provided by numerous 1x2 wedges, opposed to each other in pairs, pressed against a board running ~3 inches past the dimension of the spar. One will be placed and tacked with a Brad nailer, the other will be tapped into place to clamp, and also tacked in place.
4. glue will be t-88. Allow to cure fully at 70+ degrees.
5. when complete, will be inspected, planed to 3/4 exact with thickness planer (I can use my father's nice one for the cost of greyhound freight to ship it), and then reinspected.
6. ~ 1 foot sections will be removed from one or both ends, for destructive testing. I am planning to find a shop with a hydraulic press with a guage, and it will need to be loaded to about 5-6 tons in the middle of a 1 foot span. After that, I will test for destruction at the glue joints by the hammer and vice method with 1 inch blocks from the end of the sample.

I figure I will get a nice Spar that I can be very confident in this way, for a lot less than A/S (~600 dollars plus shipping).
 

BJC

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Steen Aero Labs laminates spars for Pitts and Skybolt areobatic biplanes. They told me that they typically use 1/4” laminations, and they vacuum bag the spars. They are very open about how they do it. You may benefit from a phone call to them.


BJC
 

TFF

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Spars can be laminated like Steen 1/4" and full height, stacked up the web, or a combo.
 

TFF

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Use lots of extra messy glue. I think the way Steen does it ,works well with epoxy. Lots of area so the spread has a harder time dripping out. 3/4-1" squares can cavitate with gravity if it's not a sweet joint. Just use care. I would build a practice spar from some junk wood and cut it up to look for flaws. I would also keep the scarfing to a minimum. These spars are too short to have to deal with that. It's not a Lockheed Vega. Laminating is enough of a challenge on its own
 

Kyle Boatright

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Use lots of extra messy glue. I think the way Steen does it ,works well with epoxy. Lots of area so the spread has a harder time dripping out. 3/4-1" squares can cavitate with gravity if it's not a sweet joint. Just use care. I would build a practice spar from some junk wood and cut it up to look for flaws. I would also keep the scarfing to a minimum. These spars are too short to have to deal with that. It's not a Lockheed Vega. Laminating is enough of a challenge on its own
Interestingly, my "vintage" Aeronca Champ (Circa 1946) spars have multiple factory scarfs, presumably using casein glue. They flew safely for 50-60 years.
 

dcstrng

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I'm currently in the planning stages of building a flybaby. I'm considering a laminated spar, since it would allow me to be more certain of the quality of the wood, as I could inspect it every 3/4 inch or so.. I would likely purchase the wood at public lumber in Detroit. Options include "Spar and mast" grade sitka spuce, Doug fir, white ash (and any other suggstions)...

I've been reading about everything I can find on this topic in the past couple months and I hope you'll be so kind to post how yours works, out and what process you adopt in the end -- at this point, I think I need to admit I'm a tad confused. Most of the material I've read more or less follows your outline -- interestingly enough, although EAA published several articles on spar laminating techniques back when folks actually used to build wood planes from scratch, the bulk of the written material was focused on the box, cantilever spar -- rather than the Pitts, Cub, Aeronca solid wood spar alternative.

Almost universally T88 was mentioned as the epoxy of choice, haven't seen where WEST was even mentioned although in the marine world WEST has done a gazillion more laminations than T88/System Three -- must be something peculiar to aircraft ??? Other differences I observe was some articles suggested light sanding to ensure laminations mated a perfectly as possible, where other articles (and the recent HomeBuilt Help vid on wood building) say sanding of the gluing surface is a big no-no... The Gougeon brothers book on marine laminations probably has the most extensive description of the scarfing process (although marine seems to use 8:1-12:1 scarfs rather than the 15:1 I observe for aviation's) in the epoxy realm, most of the rest of the material seems to either copy or adopt the techniques used in the 1920s and 1930s when wood building was common -- and then SWAG their way to modern adhesives I'm beginning to suspect.

Anyway, I wish I had something insightful to contribute, but from your description it appears we're reading more or less the same material -- do wish Steen would publish something on modern laminations since they appear to have the most active experience, that would be much appreciated even if for a small audience. I look forward to reading your progress -- especially since you intend destructive testing... Ironically, after reading and reading, I came to the conclusion that ACS wasn't too bad on the cost of their spar blanks -- since at a common hourly rate, I'd already consumed more time/money trying to learn how to compete with the high-cost of spruce blanks...
 

TFF

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You answered the question yourself, factory. Good glue joints are hard enough without having one angled in the mix. Talking amateur homebuilding. Now if you pick building something like a Spacewalker or a Falco, you are stuck with very advanced woodworking problems.
 

TFF

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On a side note. If I was building laminated spars, I would step up to recorcinol. Epoxy can be squeezed out too much. You can clamp the dog poo out of recorcinol. And it's temp and water resistance is superior. Ribs, even ply reinforcements I'm ok in epoxy.
 

pictsidhe

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I'd be inclined to take a different tack. A solid spruce spar is likely there to make construction simpler. That falls out of a very tall window when you need to build it from dozens of thin laminations.
A box section with spruce/fir/whatever caps and ply webs would be much easier to make. It'd be lighter, too. You'll have to redesign the spar, though. Off the top of my head, you may need caps around 1x1. That's a size you should be able to find clear wood in.
 

Kyle Boatright

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Steen Aero Labs laminates spars for Pitts and Skybolt areobatic biplanes. They told me that they typically use 1/4” laminations, and they vacuum bag the spars. They are very open about how they do it. You may benefit from a phone call to them.


BJC
I have the impression that their 1/4" laminations are generally "stacked" to build the thickness of the spar - in other words, four 1/4" laminations to build a 1" thick spar.
 

BJC

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I have the impression that their 1/4" laminations are generally "stacked" to build the thickness of the spar - in other words, four 1/4" laminations to build a 1" thick spar.
Their process, as they explained it to me, is to build up, for example, a 3/4” X 5” spar by stacking 20 each 3/4” X 1/4” strips of wood.


BJC
 

TFF

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The pictures of Steen spars I have seen were front to back laminations. Top to bottom is supposed to be slightly stronger before breaking. Front to back may be stronger if you knew you broke it in the air and were stuck still flying. That's a lot of glue making up the spar thickness if going top to bottom at a 1/4".
 

BJC

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The pictures of Steen spars I have seen were front to back laminations. Top to bottom is supposed to be slightly stronger before breaking. Front to back may be stronger if you knew you broke it in the air and were stuck still flying. That's a lot of glue making up the spar thickness if going top to bottom at a 1/4".
The early ones that I saw were the way you described, but they explained their current method makes it easier to utilize the wood available today. See examples of both stacking schedules here: http://www.steenaero.com/Skybolt/construction_gallery_12.cfm#Pic0003


BJC
 

TFF

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So it is. The top to bottom one is a chunk of a spar though. Not tall and thin. I wonder how they choose. I'm sure getting the grain direction they want is hard to find.
 

wsimpso1

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The hard parts of making a structure this way are: Straight stock; straight caul plate; and uniform clamping.

Straight stock is usually handled with a planer after the splices are done.

A straight caul plate may be best done by making a torsion box for assembly of your wings then glue on a plastic or aluminum skin (in one piece) that is longer and wider than the spar.

Uniform clamping is tough to assure unless you use a vacuum press.

Yeah, the whole thing looks like a vacuum press is in your future.

I have a suggestion... I live 8 minutes from Ann Arbor Muni Airport, send me a private message with your contact info. With that, we can meet up, and if I like you, I can bring you to the house and show you how to do the laminating with epoxy and vacuum bagging.

Billski
 

Aerowerx

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I'd be inclined to take a different tack. A solid spruce spar is likely there to make construction simpler. That falls out of a very tall window when you need to build it from dozens of thin laminations.
A box section with spruce/fir/whatever caps and ply webs would be much easier to make. It'd be lighter, too. You'll have to redesign the spar, though. Off the top of my head, you may need caps around 1x1. That's a size you should be able to find clear wood in.
Yes, but....

By cutting his stock into thin strips and gluing them back together, particularly if each layer is rotated 180 degrees from the next, any imperfections and/or variations (such as grain direction) in the wood property is "hidden". That is, they become less significant. And the resulting structure will be pretty darn close to the theoretical values for the type of wood used. This is how you can get a good piece of wood out of a mediocre.
 
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