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#1
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compound curves in plywood
I'm considering building a replica of a 30's glider that was originally
built in two halves, like a plastic model, but with pressed plywood halves. Duplicating this has been outside the reach of a homebuilder without some sort of press, and any made since have either been made out of fiberglass or segments of scarfed plywood as were the prototypes. Here's a pic of one here. The fuselage on the production originals was, as I say, made of molded plywood in two halves and assembled over formers and stringers. some of you might be familiar with the Bowlus Baby albatross. http://www.fiddlersgreen.net/AC/airc.../info/info.htm My question is, would it be possible to laminate your own shell using vacuum bagging? Seems to me even the wettest veneers would split if you tried to force them into or around a male or female mold. to date, the only sort of manipulation I've done with ply is to wrap 1/16 leading edge pieces over a hot pipe. I just don't see how Hawley Bowlus got the compound curves.. Any insights? |
#2
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Earlier, BA-100 wrote:
My question is, would it be possible to laminate your own shell using vacuum bagging? Seems to me even the wettest veneers would split if you tried to force them into or around a male or female mold. Well, it's pretty easy to try it and see. Get some strips of thin veneers (I'm thinking 1/64" to 1/32" or so) and try it out. For a test part you can use any compound curved surface of about the curvature of your porposed surface. I'd be inclined to try it out using VW beetle (the old ones) fenders or roof sections from a junkyard. That'll give you both internal and external test curve surfaces. If you can make that work, it should give you enough experience and confidence to proceed. There are still folks around who know exactly how Bowlus made his fuselage pods, it shouldn't be too hard to explore that. I believe that Jeff Byard even has photos of the Bowlus tooling. There are also a multitude of photos of the Lockheed Vega tools that used (I belive) concrete molds and pressurized rubber bladders. Thanks, and best regards to all Bob K. http://www.hpaircraft.com/hp-24 |
#3
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"Bob Kuykendall"
egroups.com: Earlier, BA-100 wrote: My question is, would it be possible to laminate your own shell using vacuum bagging? Seems to me even the wettest veneers would split if you tried to force them into or around a male or female mold. Well, it's pretty easy to try it and see. Get some strips of thin veneers (I'm thinking 1/64" to 1/32" or so) and try it out. For a test part you can use any compound curved surface of about the curvature of your porposed surface. I'd be inclined to try it out using VW beetle (the old ones) fenders or roof sections from a junkyard. That'll give you both internal and external test curve surfaces. If you can make that work, it should give you enough experience and confidence to proceed. Sounds like a plan! I've located a place that sells bags for vacuum forming that would certainly be big enough and I think Icould get an old compressor to do the honors if I mess it around enough. There are still folks around who know exactly how Bowlus made his fuselage pods, it shouldn't be too hard to explore that. I believe that Jeff Byard even has photos of the Bowlus tooling. There are also a multitude of photos of the Lockheed Vega tools that used (I belive) concrete molds and pressurized rubber bladders. Thanks, and best regards to all Yes, that's right, that's exactly how Bowlus did it. I did a lot of research on it years ago when i started the project the first time around, (even talked to the funder of a lot of Hawley Bowlus' creations, Steven DuPont) but making a molded pod was pretty much out of the question at the time. the preferable option was to do like they did with the protottypes and pretty much every other glider of the period and make a series of scarfed rings (why did i think this would be less work?) but when I found out about vacuum bagging, I saw it as a possible way around this. A male mold would be the easiest to build of course. I figure a series of ply formers, filled in with foam and sanded to shape, then maybe a layer of resin and voila, a half a fuselage. Has the aded advantage of being easily replicated left and right. If that didn't work, I suppose I could use that as a plug to make a female mold out of concrete. (yipes, what would I do with it afterewards? ) and use vacuum bagging on that. Might actually give it a try... |
#4
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Luthiers commonly steam wood and form it over jigs, clamps hold it in place
until it drys out. May be the same kind of construction used to build this sailplane. Just a thought and maybe something to check out. Patrick student SPL aircraft structural mech |
#5
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"W P Dixon"
: Luthiers commonly steam wood and form it over jigs, clamps hold it in place until it drys out. May be the same kind of construction used to build this sailplane. Just a thought and maybe something to check out. Yes, I've done this in fact. Have a guitar and a violin under my belt, but no compound curves. Most flattop guitars have a small radius which is compound, but nothing like what I'm talking about (a complete teardrop) This sort of technology realy died out when fibreglass hit the scene, though i'm sure there are some masters still around.Most likely source of info are boatbuilders, of course, but their curves wouldn't even appraoch the radius I'm talking about..... |
#6
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"BA-100" wrote in message ... "W P Dixon" : Luthiers commonly steam wood and form it over jigs, clamps hold it in place until it drys out. May be the same kind of construction used to build this sailplane. Just a thought and maybe something to check out. Yes, I've done this in fact. Have a guitar and a violin under my belt, but no compound curves. Most flattop guitars have a small radius which is compound, but nothing like what I'm talking about (a complete teardrop) This sort of technology realy died out when fibreglass hit the scene, though i'm sure there are some masters still around.Most likely source of info are boatbuilders, of course, but their curves wouldn't even appraoch the radius I'm talking about..... The key to bending wood with steam is how hot it is. Superheated steam is best, which takes a re-heater after the water is boiled. Make sure the plywood is made with water proof glue, which is anything of A grade or better, or marked withEXT after the grade. It might be B grade, but I don't remember, right off. For testing, that type of plywood is fine, but for building a plane, use at least marine grade, and best is aircraft grade. -- Jim in NC |
#7
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("Morgans" wrote)
For testing, that type of plywood is fine, but for building a plane, use at least marine grade, and best is aircraft grade. "...and best is aircraft grade" I know about marine grade plywood, but aircraft grade? Speaking of marine grade plywood (I paid $77 w/tax for a sheet of 3/4" last year) is there such a thing as marine grade + green treated that they sell? That's what I wanted, couldn't find any. Something about varnish + green preservative doesn't do well together I was told. (???) It was for a carpeted-over floor in a 16 ft boat. We ended up spar varnishing our sheet a number of times. Montblack |
#8
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"Montblack" wrote in message Speaking of marine grade plywood (I paid $77 w/tax for a sheet of 3/4" last year) is there such a thing as marine grade + green treated that they sell? That's what I wanted, couldn't find any. Something about varnish + green preservative doesn't do well together I was told. (???) It was for a carpeted-over floor in a 16 ft boat. We ended up spar varnishing our sheet a number of times. Montblack Yea, I know what you mean. A friend wanted to do the same thing. We found a place that treats wood, and got him to throw in some sheets of marine grade into his treating thingy, the next time he was running a load. We then (after waiting for it to dry out some) put it down and put a layer of fiberglass cloth and epoxy over it all, before laying the carpet. It looks like new, about 5 years later. -- Jim in NC |
#9
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Montblack wrote:
Speaking of marine grade plywood I just finished up a stitch-n-glue (4 mm plywood with single layer fiberglass inside and out) kayak yesterday, and took it out for it's maiden run at the local lake. It's 17' long, single seater sea kayak. Weighs about 35 LBs, but can take an ocean entry driving out through heavy surf, so it's not a frail structure. While paddling around, I got to thinking about this thread, and imagining wings either side of me, tail feathers behind... A bright finished plywood glider would be a true thing of beauty. |
#10
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I would guess that the construction techniques used
were similar to those used in boatbuilding. http://www.albacore.org/USA/members/...g_albacore.asp http://www.boats.com/content/default...contentid=1184 http://www.albacore.org/USA/members/...g_albacore.asp May help. There is a remarkable wooden flying boat hull in the RAF Museum at Hendon that was built in the early 1920s. It has a 'moulded' hull which appears to have been constructed using boatbuilding techniques of the time. http://www.rafmuseum.org.uk/supermarine-southampton.htm With its varnished finish it is simply stunning to look at. |
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