Procedure for Spar lamination?

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wmax351

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Oct 10, 2018
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Ann Arbor, MI
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
Thanks, I'll shoot you a pm.

One of the folks on the flybaby group was suggesting Johnsons workbench in Charlotte, mi where they've gotten spars for several planes. If they've got quality spruce, Doug fir, or ash, that removed the price difference of truck shipping and the aircraft spruce premium, probably makes more sense to go with solid.
 

John wadman

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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...
On my first homebuilt i used douglas fir and West system as i am a boatbuilder/woodworker by trade. i tried resorcinal and west and the west won in destruction tests. However, west is not an approved FAA glue as far as i know (but who cares, its an experimental category). besides, if youre trusting in the federal government to tell you whats good and bad, i'd rethink that. the FDA approves EVERYTHING that'll kill you by heart disease or cancer. also, epoxy strengths are prone to suffering from heat. my planes are all light colored, both are off white. it always amazes me how many "experimental aircraft" builders have the mentality that there is only one wood that can be used and only one glue. Technology has come so far since the Wrights or even Mr Pietenpol. from all of the kevlar high performance engine parts ive seen on the racing circuit i would imagine that the heat issue with epoxies has been solved now too. i have an epoxy paste that i like to use on my boat stuff and i've done destructo tests on it and i'm convinced its a lot better than Resorcinal and maybe even T88. lets put it this way, with both T88 and this stuff i use, the wood fails before the glue joint everytime! so the wood itself becomes the weak link. if its an experimental aircraft, you're allowed to experiment. glue some wood together and then load test the samples. beat on it! hang buckets of kitty litter or sand on it. put it in a hydraulic bearing press and pump it up until it crushes. study the strength increases of laminated spars over solid spars. all of my planes have been lightly loaded wings so my spars were solid caps with ply webs. But if i built a wing with solid spars i'd laminate them and try not to have scarfs. if i did have scarfs, no two would overlap and i'd try to keep them way outboard OR inboard under ply doublers at the root where the tangs mount. best of luck.
 
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Speedboat100

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Nov 8, 2018
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Europe
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.
Compressed laminated spar is 3 x stronger than solid wood one.
 

TFF

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It’s only stronger in averaging to all directions. A spar is one direction at max strength one way. Mixing metaphors.
 

John wadman

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It’s only stronger in averaging to all directions. A spar is one direction at max strength one way. Mixing metaphors.
I'm curious just how much stronger it would be laminated. I'm going to do a simple destructo test with a bucket, sand and a glued up 3/4 x 3/4 of 1/4" layers and a plain 3/4 x 3/4 " piece. I might do one with a scarfed piece in the middle too just to see if the scarfed laminate is as strong as the solid piece. Probably won't happen this weekend but when get some results I'll post them. I have a three foot 1x4 of beautifull vertical grain in the shop. I'll try to do the laminates and the scarf this weekend and let the epoxy cure until next week.
 

TFF

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Pretty plane but CGI pics usually means it is not a finished airplane.
Building a spar like that would bolster the web strength by a good bit, but in that would be a huge weight gain. Top and bottom cap and web is the lightest. Standard I beam look. With the compressed wood you add 4000 more bits of wood and glue that does not need to be there. Solid wood is for ease of construction; most people can build a plank of wood; it’s a plank of wood. In general it’s just a fancy plywood spar. And in that only the span wise wood is contributing strength in that direction, so how thick does it need to be to be equal strength span wise to a 1” board? What would that weight be compared to the 1” board? The numbers have to be at least close to the standard in use part.
 

Lendo

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Feb 6, 2013
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Brisbane
Vertical Marine Ply would be a good substitute for wooden beam, it may well need caps as well. I know they used this approach for house under floor Joists and ceiling beams. They would be heavier and probably have less flex. There could be a balance point, in terms of weight and strength (and flex) where it would be beneficial.
George
 
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