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Lite-ply shear webs

Old 01-08-2004, 03:06 AM
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Patriot
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Default Lite-ply shear webs

Building an 80" aerobatic bipe. Was originally going to use 1/8" birch ply shear webs in the center section of both wings, but I am wondering if lite ply would be good enough. I am just looking to save a little weight but still be stronger than balsa webs in the center. Outer wings will be balsa webs. Also thinking about the I-beam concept.
Any suggestions?

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Old 01-08-2004, 03:25 AM
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Default RE: Lite-ply shear webs

If you're putting this inbetween the spars like you should to form a pure I beam why not just use thicker balsa for the center webs? This is what is done in sailplanes all the time. Use firm medium wood that is the same thickness as the width of the spars for the first 1/4 panel out each side and then taper down to 1/8 thick for the last quarter panel. The two middle ones being graduated as required to form an even taper. This iwll be more than strong enough for you.

This is actually a better way to do it than to use lite ply. The thicker balsa will completley support the spars and interconnect the upper and lower over the entire width rather than just the center. And supporting the spars to prevent the buckling of the one under compression is what this is all about.
Old 01-08-2004, 03:41 AM
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Default RE: Lite-ply shear webs

Actually, I am planning on gluing to the rear of the spars, along the length of the wing, the design more or less requires this as the wing dowels etc require it to be this way. And I guess what I really want to know is if I can use lite ply instead of the heavier birch type in the center section to help save some weight. I am thinking it should be ok, but just wanted a little experience with this size of a plane to give some input. I thought about balsa in between the spars but I don't think it will be strong enough for the wing dowels in the center of the wing.

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Old 01-08-2004, 04:04 AM
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Default RE: Lite-ply shear webs

OK, now I am thinking about your thick balsa I-beam concept for the center section of the wing, with ply added to the areas only where the wing dowels go through. This sounds better to me minute by minute. hmmmmm..................


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Old 01-08-2004, 08:34 AM
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Kenny R
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Default RE: Lite-ply shear webs

Heres my .02 cents. Ive been trying to build light weight models, and i have learned that 1/16" aircraft ply is stronger and lighter than 1/8" lite-ply.

Just a thought.
Old 01-08-2004, 04:43 PM
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Default RE: Lite-ply shear webs

When the webs are glued to the edges of the spars rather than being betweeen them the strength is limited both by the glue AND the wood as you are relying on the glue holding the full load rather than just holding the wood in place where it can deal with the crushing forces. In this case you would be better to use separate layers of 1/16 birch ply front AND back in the center section as the birch is a stronger base wood. The glue will then be the weakest link in the chain.

Wing dowels? I gather this is a take apart wing like sailplanes then? In that case consider using the same method that many sailplanes use where the tubes that hole the wing rods go. Build the wings with aluminium tubing set into holes in the ribs and located so the tube is between the wing spars. Mix up a putty of epoxy and microballoons to a consistency like thick peanut butter. Spoon this putty around the tubing and packed into the spars. Cap it while still wet with 1/16 balsa front and back using a bit of left over non putty epoxy to join it to the spars. Clamp in place until cured. The balsa is only to prevent the putty flowing out. The putty around the tubing forms the webbing you want. Then for the other rib bays use the balsa between the spars as I outlined before. If you build the whole wing in one go but put 1/16 spacers between the ribs where the breaks will be you can then use a fine tooth hacksaw blade or razor saw to cut through the wood, spars and tubing to free the wing panels up. The beutty of this method is that since you build the joiner rod receiver tubes into the wing in ONE PIECE and cut it later the wing rods all line up perfectly. No guessing involved. And the epoxy putty forms a super strong and decently light webbing as well as fully transfering the bending loads to the spar system.

HOWZZAT ?
Old 01-08-2004, 06:11 PM
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Default RE: Lite-ply shear webs

I'm sorry, the wing is one piece, what I meant by wing dowels is the two dowels that protrude from the leading edge of the wing and plug into the front of the wing saddle on the fuse. The 3/8" dowels (2) will be anchored through the shear webs in the center section and protrude forward out the leading edge.
I have to say for an 80" long by 15" wide semi-symetrical aerobatic design, I think the balsa blocked shear webs would be enough for the center section. I am sure this would be as strong and weigh less than using 1/8" birch ply. I think I want a little more beef than two 1/16" pieces, ,and the I-beam design really seems to be the answer here.
I can glue to the rear of the spar and taper some 1/8, 3/32, and 1/16 balsa webs for the outer sections to the wing tip. Does this seem like a good idea?
I would then only use some ply on the single spot where I need to anchor the dowels in the wing.

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Old 01-08-2004, 06:58 PM
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Default RE: Lite-ply shear webs

Depending on the spars you have used building can be minimal and still remain extremely strong.

I built a .46 sized 55" shoulder wing funfly plane which was capable of doing full throttle inverted loops within its own length yet when people saw the wing being built they said it would never hold.

I simply used spruce 1/4in spruce spars on the top and bottom at the 30% mark and 1/4in strips for the le and te. Between spars I simply used single 1/4in x 1/2in strips running vertical in the center of each pocket with 1/8in light ply braces on either side of the spars at the center section (The spars on the first 2 ribs on each side were also filled with 1/4in sheet).

To demonstrate the strength of the design before flying it I actually was able to stand my full weight (9st and a bit) and gently bounce up and down on top of the wing. I still have the wing somewhere (most likely my dads house) but the fuselage was binned (unfortunately got in a fight with my 2yr old son and he won[&o])
Old 01-08-2004, 07:32 PM
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Default RE: Lite-ply shear webs

Patriot, if you wish to stick with glueing the webs to the edges of the spars rather than putting the wood between them then your scheme sounds fine. I would modify that to put front and back webs on the first 1/3 of the wing out either side. Two 3/32 webs for this would be all you need. Then for the second third drop the web on the front but stick with 3/32 on the rear. Then for the outer 1/3 go for single 1/16 webs. If you use firm to hard balsa it will have all the compresive strength you require and the glue lines will be stronger as well. Where the dowels go through I'd use 1/16 ply to replace one side of the webbing.

For dowels through the leading edge like you're doing the main loads on the dowel to wing interface are in the LEADING EDGE and not the spar at all. So there is no need to put anything heavier at the spars. In fact you could use 1/8 hard balsa and it would have all the strength for that part that you need. The leading edge, on the other hand, will be getting a LOT of load put onto it. A 1/8 plywood doubler for the back of the leading edge that retains a resonable degree of wood where the dowels go through would be a good idea. Or if there's no room to allow for a nice load carrying doubler behind the leading edge then a couple of patches of 4 oz fiberglass around the dowels and extending back to the spars would be a good idea. The center section ribs should probably be harder wood and perhaps a bit thicker than the rest of the ribs to transfer the panel loads onto the dowels without depending TOO much on the center section sheeting. Better yet would be an extra 1/2 rib on either side of the dowels such that they are touching and glued to the dowel so the loads are transfered to the sheeting in a more direct manner. This is a large model. It's good to try for lightness but it's also important to overbuild specific load bearing points such as this. If you do it with an eye to where the loads are and how they are carried through the structure you can do it strongly but still be light. The old story about "It's not about how much you use. It's about where you put it."

Basically your wing spars are carrying the load. But they are going to use the center section ribs, sheeting and leading edge to transfer the flight loads from the spars to the mounting points. The length of the dowels acts like a lever to reduce the forces so that the load at the webs where the dowels go through is not that high. But the loads at the leading edge and the trailing edge where the screws are IS high. If you are using 2 dowels and two screws those 4 points are taking ALL the static and G loading of the model. Well, 1/2 in your case as it's a biplane. This is fine as long as the loads are UP into the wing saddle but when you do inverted stuff it's hanging from those points and all the load carried by the spar is transffering through that center section to those 4 points.

I saw a guy here write off a lovely big Super Sportster because he did not consider these points. The wing was fine but there were only a couple of little ply donuts over some center section sheeting and a thin rib a short ways away. The guy liked doing negative snaps and the structure only put up with about a 1/2 dozen of them before the wing just gave up at the bolts and let them rip through the sheeting. He only got two or three flights out of it, complete write off. And all because of the lack of care about properly setting up two little hard points to deal with the flight loads.
Old 01-09-2004, 06:19 PM
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Default RE: Lite-ply shear webs

BMatthews,

Actually, I am thinking I will use your method for the center section. I will use 1/4" thick balsa blocks (vertical grain) glued between the 1/4" wide spars to fill the entire gap. Then I will use 3/32 tapered to 1/16" on the rear side of the spars for the outer sections. I will use 1/8" birch ply web in addition to the 1/4" balsa, only in the spot where the dowels are anchored. I will also use 1/8" birch ply backplate on leading edge to anchor the front of the dowel as it protrudes out the leading edge. I think this will be light enough, yet pretty much bullet proof.
If you think this minor decision of how to do my shear webs is bad, wait til' I start installing the N-struts and functional flying wires. Hope you are still around.

Patriot
Old 01-09-2004, 06:32 PM
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Default RE: Lite-ply shear webs

ORIGINAL: Team MPP

Depending on the spars you have used building can be minimal and still remain extremely strong.

I built a .46 sized 55" shoulder wing funfly plane which was capable of doing full throttle inverted loops within its own length yet when people saw the wing being built they said it would never hold.

I simply used spruce 1/4in spruce spars on the top and bottom at the 30% mark and 1/4in strips for the le and te. Between spars I simply used single 1/4in x 1/2in strips running vertical in the center of each pocket with 1/8in light ply braces on either side of the spars at the center section (The spars on the first 2 ribs on each side were also filled with 1/4in sheet).

To demonstrate the strength of the design before flying it I actually was able to stand my full weight (9st and a bit) and gently bounce up and down on top of the wing. I still have the wing somewhere (most likely my dads house) but the fuselage was binned (unfortunately got in a fight with my 2yr old son and he won[&o])
I wouldn't have told you that would break. 1/4" square spruce is more than adequate for a .40 size model. I had a model that had a 50" span and a piped Rossi .40 on it. It was simply a "go-fast" plane. Not legal for pylon racing, but plenty fast. It had 1/8" x 1/4" spruce spars with webs and I never gave it a second thought.

In fact, an engineer at my club kept telling me how he knew better than I did what was strong enough and he kept trying to convince me that the wing would never hold. Well, it held up to everything I did to it including some very high-G turns. What did it in was movers. DOA.

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