A different way to build a wing

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Apr 10, 2019.

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  1. Apr 10, 2019 #1

    rtfm

    rtfm

    rtfm

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    Hi,
    There are many ways to build a wing: here's a somewhat novel way which is both quick, simple and strong.
    I must begin by saying that this is a thought experiment, and that I don't (at the moment) actually plan to build this wing. Perhaps further down the track I might give it a try, but not till I've built a scale version and tested it.

    1. Make your ribs. I would cut them from foam, since (1) it is WAY quicker and (2) significantly stronger than the traditional method of building ribs from spruce sticks. You could cut all your ribs using a simple template and a hot wire. Those who have a CNC router could use that. Either way, they are very simple and quick to cut. Importantly, both the LE and TE of the ribs are truncated (about 100mm (4 inches) from the nose, and at the aileron line.
    2. Mount the ribs on a cradle in order to get them all perfectly aligned. I'd use something like Fritz' "Rib-o-Matic" - which is essentially a rigid box on which are mounted multiple "arms" to which the ribs are affixed. This ensures that every rib is perfectly aligned. I'd build a 2400mm rib-o-matic box, because the wing will be made in three sections, each of which is 2.4m long.
      [​IMG]
    3. However, I'd double up on the "arms" - ie four per rib. The reason is that we are going to cut the ribs vertically at their thickest point, and remove 5mm. Why? Because we are going to bond a 5mm shear web (not a spar) to join the two halves. Each rib half needs to be supported at two places to ensure the shape remains perfectly aligned.
    4. Close off the LE and TE with 5mm AC grade plywood also.
    5. The questions are probably bubbling up thick and fast at the moment. But stick with it. I'll get there...
    6. OK, so we now have our foam ribs held in place on the rib-o-matic "arms", with a 5mm plywood shear web running down the length of the ribs, and 5mm ply both at the D-tube position and at the aileron hinge position.
    7. Importantly, the 5mm thick plywood will be (1) way over-built strength wise and (2) way too heavy. A full sheet of Gaboon ply weighs 7.6kg. A 200mm wide strip weighs 1.26kg. Removing 75% of its volume results in a shear web of 316g (about 11 oz). The D-Tube and Aileron close-offs will be less than 200mm deep, but at the thickest part of the airfoil, will be deeper. As a rough estimate, let's assume 3x 200mm deep close-off pieces. Total weight = just shy of a kilogram. This is why we remove about 75% of its volume by simply drilling multiple holes with a hole saw before bonding it to the ribs. Do not remove any surface area where the shear web bonds to the ribs. We're all familiar with lightning holes in ribs. So this is what we now do to the excessively thick shear web. If you have access to a CNC router, you can cut the plywood into fancy diagonals. But multiple circles should be just fine.
    8. Why use 5mm thick plywood when 1mm ply would have sufficed for a shear web? Simple - we need at least 5mm width in order to bond it to the wing skin.
    9. The skin consists of 1mm AC grade plywood. Birch is best, but Gaboon is cheaper. though a bit heavier. But even Gaboon ply weighs only 2.3kg per 2400 x 1200 sheet (8x4).
    10. As we all know, a wing needs a spar, but all we have is a shear web, with no spar caps. Ah, but we do.
    11. Before bonding the 1mm plywood skin to the ribs, first bond carbon fibre rods down the length of the 2400mm plywood sheet exactly where the shear web is. When flipped over, the plywood skin now has the spar cap bonded directly to it, which aligns with the shear web. The top skin bonds to the D-tube close-out, the shear web and the TE close out. Three strong bonds.
    12. Flip the wing, and repeat.
    13. Material used for each section: 8 foam ribs, 3 plywood shear webs, 1 full sheet of ply for the skin. Estimated weight of each 2.4m section: under 4kg. 3x sections: under 12kg
    And there you have it. One day to cut the ribs. One day to bond the main shear web. One day to bond the D-tube and one for the aileron close-outs. One day to bond the carbon fibre caps to the skins. 2 days to bond the skins to the ribs.

    Of course, there are other things to do. This wing envisages a folding hinge-mechanism, where the outer panels fold upwards (from the Flying Flea designs). A simple hinge, but with inner reinforcements to anchor it. But not a lot else.

    Anyway - that's my idea - so please feel free to comment and offer suggestions.

    Regards,
    Duncan
     
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  2. Apr 10, 2019 #2

    TFF

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    I see it comes down to one thing, the bond footprint of your rod to the web. If that is strong enough for the Gs it pulls, asuming all is sized right, you are in. My guess though is its not going to be wide enough. In general it is how most wings are made, you are jut doing the caps last instead of first. Fancy jig or table top is all pretty much the same. The spar caps web bond is probably the most critical part of building a wing with a built up spar. Doing it last makes quality inspection harder.
     
  3. Apr 10, 2019 #3

    Vigilant1

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    Maybe it can be made simpler yet? Do away with the spar in the middle and use the two end closures as the spars. Have a D-tube at front to perhaps 25% chord and the rear closure goes to perhaps 75% chord (measured from the front). Bond the CF rods to the top and bottom of these two spars, wrap with a layer of CF or FG at 45 degrees on the webs to hold the rods and link them together (in compression and tension) to provide stiffness. The center of the wing is just foam ribs with a sheet of skin and a layer of glass if needed for stiffness,
    Yes, the rather short spars are less structurally efficient than a deeper spar web would be, but that was a much more significant factor before we had amazing graphite rods to use on the caps. Wood or even AL caps are heavy for their strength. With graphite rods, the strength and stiffness is tremendous and they weigh very little, so just use a few more--easy. In the bargain you get a good place to attach flaps and ailerons at the back, a good way to link the wing to the fuselage to take pitching loads (two widely separated spars), and an easy to build wing (D-tube at front, flat spar at back, bond in a bunch of one-piece ribs between them, wrap all with the skin). And I'd bet that the finished wing would be very weight efficient even with the extra rods that would be needed, because we don't have the center spar web and all the bonding that goes with it--one less piece.
    I'm sure this is not a new idea, but I'm just putting it out there.
     
    Last edited: Apr 10, 2019
  4. Apr 10, 2019 #4

    wsimpso1

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    Duncan,

    You know I hate to do this...

    First I have to agree with TFF. In both composite and plywood spars is, you have to get the connection between the shear web and the spar caps strong enough, and I suspect a 5mm wide glue joint will not do it. You will have to do the shear flow calcs at whatever your worst case loading will be, then see how the glue line is doing on stresses - most thickened epoxies are only good to about 7000 psi, so this approach might require more width at the top and bottom of web. In pure composite spars, the shear web material just wraps onto the caps as a continuous structure and carries this load nicely between caps and web. Compounding the analysis is that this is very much a composite spar, with a whole bunch of different stiffnesses in it, yet the beam has to deform as one piece. I can help with this - you have my email.

    A second issue comes up - how would you do your hardpoints in such a scheme? You have to collect the loads toward the connections. Making it a one-piece wing will help bunches, but you will still need hard points at the fuselage and at any strut attachements.

    These two points are, by the way, the reasons that spars are usually made first, then the wing built around it.

    Billski
     
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  5. Apr 10, 2019 #5

    Victor Bravo

    Victor Bravo

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    I am having trouble seeing the usable improvement of this method, compared to the known method of making a foam main spar with CF on top and bottom, all wrapped in 45 degree glass, then foam ribs are bonded to the front and rear of the (completed) spar, then everything is sheeted with plywood.

    It seems to me that the number of operations, jigging/fixturing, and guaranteeing reliable load paths all favor the "spar completed first" method. Billski and a handful of others here can easily debunk, verify, or quantify whether I'm correct...
     
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  6. Apr 10, 2019 #6

    rtfm

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    Thanks for the comments guys. Early days in the concept stage, of course, and obviously a lot more thinking needs to be done. And no actual analysis of bond strength has been attempted at all. If I read the comments correctly, the points raised (so far) are:
    1. Bond footprint between rods and web? Including calcs of bond strength.
    2. One web or two?
    3. What value in the spar first vs spar last approach?
    Point 1:
    Easy enough to bond a 20mm x 20mm capstrip to the upper and lower edges of the web to provide much larger footprint. While we're at it, since 5mm isn't sufficient width, replace the 5mm ply with 1mm ply, and don't bother with the removal of material. This added real estate should take care of the bond strength.

    Point 2:
    One web good, two webs better (in this case). If the LE web were to remain (say) 100mm (4 inches) from the nose, then it would provide an unbroken skin surface over most of the chord, which appeals to me. The SD-1 guys simply then bond a 100mm piece of foam along the LE, and sand it to shape with an accurate template, and glass over it, faring that into the plywood. A very elegant way to get the LE done.

    Also, having no central web cuts out the need for (1) four-arms on the jig, and (2) the need to cut the rib. Instead, a single rib can be cut, and then mounted. Much simpler.

    Point 3:
    Fritz' rib-o-matic jig allows all ribs to be set up, and his CF (or aluminium) tubular spar to be threaded through the ribs efficiently. I don't like tubular spars (they are kinda unobtanium here in Oz, anyway), but the jig does provide a very convenient way to bond the LE and TE webs. The question remains, however - what is to be gained by bonding the spar cap directly to the skin before bonding the skin to the ribs/webs? Mmmm Not sure.

    Seeing that It would be necessary to recess the spruce "web caps" by the thickness of the CF rods anyway to ensure continuity of contact along the entire surface bond with the rib, I'm not sure it matters much either way. Bonding the CF rods to the plywood skin first, however, does allow one to wrap the rods in glass, which the alternate method doesn't allow. So I think, on balance, the spar-last approach might be preferable.

    So the method would be:
    1. Cut the ribs (include recesses for the web caps)
    2. Mount ribs on the jig
    3. Bond the LE and TE webs (with web-caps pre-bonded)
    4. Bond the spar-cap rods to the skin
    5. Bond the skin.
    6. Flip and repeat step 4
    This is shaping up quite nicely. Certainly far simpler.

    The two major advantages would be: (1) time savings and (2) weight savings.

    One final thought re: hard points.
    One would need to bond sufficient reinforcement for the strut/hinge attachments. I haven't thought about what material to use, and haven't considered sizing either. But it would need to be completed before the skins are bonded, and adequate cut-aways in the outer ribs would need to be made.

    Regards,
    Duncan
     
  7. Apr 10, 2019 #7

    Vigilant1

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    The spar webs need to have the caps directly attatched to them. A spar works as a truss, with a top "plate" and a bottom "plate". It is stiff in bending because the top cap and bottom cap are directly connected by a spar web that puts the top cap in compression and the bottom cap in tension.
    Move your carbon rods to the top of the front and rear spars and everything else you have envisioned should work okay. It will not work to attach them to very thin skin and have that skin transfer these very substantial loads to the spar webs and then to another cap located across another expanse of thin skin.
     
    Last edited: Apr 10, 2019
  8. Apr 10, 2019 #8

    rtfm

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    Hi. Not sure I follow. The web caps, CF rods and wing skin are all bonded together into a single unit. The only significant difference with this method is that the final bonding happens at the end. The benefit is that the CF rods can be securely mated to the skin not only by direct bonding, but also by the addition of (say) 100m of glass tape.
    Oh, and let's not forget the other benefits, which include:
    • Much quicker build time
    • Lighter
    • Stronger
     
  9. Apr 11, 2019 #9

    Vigilant1

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    I don't know why gluing the rods to the inside of this thin ply skin (top and bottom) is easier than gluing them to the tops of the spar webs.
    But it won't be as strong.
    I recommend that you spend some time with some web trusses and consider the load paths.
    Below is an example of a typical Warren bridge truss, it works just like a wing spar (though in a wing the load distribution is different, and it is held at just one end). The bottom is in tension, the top is in compression ("negative tension" in the diagram below). Notice the diagonal pieces that directly connect the top to the bottom: some are in tension (like a rope), some are in compression (like a column). This works well, as long as the diagonals are strong and straight. Now, what if the diagonals didn't run directly between the top cap and the bottom cap, but instead (if viewed from the approach end of the bridge) they were giant "C" shapes? Will a rope shaped like a "C" keep its shape if pulled at both ends? Will a "C" shape stand up to compression across the opening as well as a straight piece would? No, the entire shape will be unstable and will rack. If we make it a solid "box" (out of very thin plywood) and support it with occasional foam formers, we've still got mainly a monocoque shell that might work fine under perfect conditions, but may not hold up well to even a little dent or damage (like standing on an empty aluminum can--it will support more than the weight of a person, until it is even slightly dented).
    I think you'll be much better off with conventional spars (one or two) with the caps connected >directly< by suitable webs, but certainly you should do it the way you want.
    [​IMG]
     
  10. Apr 11, 2019 #10

    rtfm

    rtfm

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    Sorry Vigilant. I still can't see your objection. I know how spars work. You seem to be assuming that the CF spar caps aren't bonded to the web, which of course, they are. So what's your point?
    For me one of the benefits is that the ribs are held securely in place throughout the build. And it's an easy build.
     
  11. Apr 11, 2019 #11

    Vigilant1

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    Duncan,
    If this means that carbon rod spar caps will be located directly over and under the shear webs that are located near the front and back of the wing, then that addresses my primary concern. I was reading things such that you still intended to affix the bundle of carbon rods to the highest (top surface) and lowest (bottom surface) of the wing skin, and transfer that load to the shear webs through this 1mm plywood skin.

    On your build sequence: The bonding of the spar caps to the webs, and thus to each other, is the most critical bond in the airplane. My personal preferences: I would make it by wrapping the reinforcement cloth up the web, over the rod bundle, down the other side of the web, then around the other cap. I would not be satisfied with a simple adhesive bond (spar caps to the top of the web with no reinforcement fabric across them) and I would not do it as a blind bond (i.e. press the top skin with the cap down onto the web top and hope/trust that there are no gaps).
    The importance of the bond between the CF rods and the wing skin is virtually zero. The importance of the bond between the CF rods and the web (and each other) is paramount. I'd give myself every chance to maximize the strength of the cap-to-web bond--including use of reinforcement fabric and direct visual observation of the process.

    Certainly others may differ on these points.

    Mark
     
    Last edited: Apr 11, 2019
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  12. Apr 11, 2019 #12

    Victor Bravo

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    If you are using a rib alignment jig as shown in the graphic, and assuming that a correctly designed rib jig will hold the ribs in whatever alignment is necessary... what is the advantage of rtfm's proposed method, versus the other method of dropping a completed and glassed and bagged foam/carbon/glass spar plank into the gap between the (supported and aligned) ribs?

    Honestly and sincerely I am not trying to trash anyone's dreams or shoot down new ideas for the sake of shooting them down. But consider these advantages:

    1. The spar is built out of a sawn or hotwired rectangular plank, and is built on the bench.
    2. The carbon rods, core material, fiberglass shear web, and fiberglass "capture-wrap" are all done in one layup on the bench.
    3. The monolithic spar construction guarantees the load path, no voids or fillers between the structural components of the spar.
    4. The monolithic spar layup can be vacuum bagged o n the bench, guaranteeing a full-strength bond between the caps and the web.
    5. Bagging the spar will always result in weight savings compared to an old-school wet layup.
     
    Last edited: Apr 11, 2019
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  13. Apr 11, 2019 #13

    rtfm

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    Hi Vigilant1,
    I hear what you're saying, but I'm not sure I can agree. The purpose of the web is to keep the caps apart. They need have no intrinsic strength themselves, which is why Marske uses a single layer of cloth to do the job. All the strength is in the spar caps. I dare say that even if one bonded the CF rods to the outside, and didn't even bother bonding the inside of the skin to the web, the wing would lose no rigidity. As long as the CF caps are kept apart, the wing would be rigid.

    So we need to move past the traditional paradigm of thinking that the CF rods must be bonded with great gusto to the web.p under stress, the CF rods will be under compression/tension with no thought or attention paid to the web, other than the fact that it is keeping the two caps apart.

    In fact, Jim Marske only has CF spar caps with a layer of cloth keeping them apart. I'm providing the spruce doublers not for strength, but merely to provide surface bonding area. I could just as easily have used 25mm thick ply, cut away sufficient surface to reduce weight and have done with it.

    Again - (and going to an unnecessary extreme) the wing would lose no rigidity even if there were no bond between the skin and the web at all. A bond would add nothing to the CF rods ability to withstand either compression or tension.

    I dunno - maybe I'm just trying to be different for difference sake. Possibly. As I said, this was something I thought of in the bus, and considered it worth exploring.

    VB, you might be right.
     
    Last edited: Apr 11, 2019
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  14. Apr 11, 2019 #14

    Vigilant1

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    Hi Duncan,
    The web does much more than just keep the caps apart (i.e. maintains a fixed displacement between them). I know this terminology is common in literature describing composite panels. But the web also prevents "sliding" of the top cap relative to the bottom cap. Look again at the Warren truss--those diagonals are under tension and compression because of these forces. Or, visualize a stack of flat sticks in bending: If the sticks aren't fastened together at all, they are still being "kept apart," but they can still be bent easily. If we bind them together (so they can't slide relative to each other) then the middle sticks can sustain tension or compression loads from the top and bottom sticks and the bundle is much stiffer. That's how you want your spar(s) to be.
    Marske, Rutan and others who successfully use a "single layer of cloth" as their spar webs are using a very strong (in tension and compression) high-tech carbon (or fiberglass) epoxy matrix that is kept >very< straight between the caps so they can successfully carry these significant loads. Marske, Rutan, et al also wrap this reinforcing matrix from the web around the spar caps for (much) greater strength than an adhesive bond alone can provide. If it were just a matter of "keeping the caps apart," I suppose they would just use foam. That would be a much easier way to make a VariEze wing, but it is not what they do.

    Mark
     
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  15. Apr 11, 2019 #15

    wsimpso1

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    We do not know that, but I suspect it to be true. Analysis of the beam and determination of the internal shear between web and caps is needed, then the adequacy of the bond joint and how much (if any) improvements are needed can be determined.

    Usually in really light airplanes, the web, like many other parts is subject to min gage issues. We can only get our materials in certain thicknesses. We can calculate the minimum required thickness, but in reality, we usually end up with the next increment possible. And usually in light airplanes, we do not need a lot of torsional stiffness in the spar because the rest of the wing provides plenty of torsional stiffness, so a single web is fine. Yeah, the Long Ez and all of its derivatives used a broad box spar, but that was because it did need torsional stiffness and torsional strength in the connection from wing to center section. A different problem...

    Anyway, if the wing is strut or wire braced, you can produce big compression in the main and auxiliary spars. While the spars will have to resist buckling, the webs may be elastically unstable and thus leave you with only the caps and skins resisting compression and buckling. If you need two plies to resist other loads and two plies together to prevent elastic instability, but two plies applied separately won't prevent elastic instability, two separated webs are worse not better...

    It seemed to me that the big gain was speedy construction. If it can be shown to be adequate in terms of strength and stiffness, and it is much faster to build, and it only weighs a little more than conventional construction, I can see the advantage.

    My concern was and still is that it may not be adequately strong and stiff as presented, and by the time it is adequate, it will be heavy. WEIGHT IS THE ENEMY. IMHO, weight minimization is worth extra build fuss...

    Duncan, I love the out of the box thinking. Let's make sure that your spar caps and shear web will stay connected at max g flight loads.

    Bill
     
    Last edited: Apr 11, 2019
  16. Apr 11, 2019 #16

    wsimpso1

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    Duncan, you appear to have some conceptual errors on beam theory...

    Please get your hands on a textbook on mechanics of materials with chapters on beam theory, and go through those chapters carefully. I am sure that there is either a public or university library someplace in Brisbane that has one. I like Timoshenko, but there are others. C

    Caps held apart by a web do carry the vast majority of bending IFF the beam components all bend together. The web carries the shear load developed from lift and must resist buckling under compression in wings with external bracing wires or struts. There is also internal shear flow between the web and the caps, and your plywood and adhesives (in this scheme) must carry that load between your carbon rods and the web. This is the area of concern. If the load is larger than the adhesive can carry, the adhesive will fracture and the wing will fold...

    There really is no substitute for getting an estimate of the max g shear and moment curves, then analyzing the spar at the worst spot to see if it will hold together, and what it will take to make it adequate...

    My suspicion is that you will be lighter (at strength) if you simply build a foam cored rectangular section spar with carbon rod caps and fiberglass wrapping (as both web material and as connection of caps to web) and insert that in your jig instead of the wooden web, then bond ribs and light forward and aft spars, then bond on light skins. Only analysis will tell - maybe you are in a corner where simple epoxy adhesives can do the job of connecting the caps to the web.

    Billski
     
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  17. Apr 11, 2019 #17

    Sockmonkey

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    I think some of the confusion has to do with the fact that different spar configurations are going to put different stresses on the web.
    Warren truss ribs are critical structural components that get stressed in several different directions so you need a good solid web, as opposed to those "kite" wire braced ultralight wings that just use battens.


    Sort of related, but when making a solid foam wing core, several different types of foam are layered together yes?
     
  18. Apr 11, 2019 #18

    Victor Bravo

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    No, traditional old Rutan style wings use a core made from one soid billet of 2 pound blue foam with glass C-channel spars. (the core is cut into 2 parts for the C channel then glued back together).

    rtfm, our friend Billski is politely trying to say rtfb :) (sorry I could not resist...)

    If I may step in and simplify it for one brief moment - there is a very large amount of spanwise shear in the joints between the spar caps and the web. In the thousands and tens-of-thousands of pounds. This is because the loads on wing spars create a very very BAD case of leverage (mechanical advantage) that is multiplying the forces AGAINST the shape. All the leverage is being multiplied directly through the joint between the web and the caps.
     
  19. Apr 11, 2019 #19

    rtfm

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    Hey Billski. You are not regarded by your peers as a man of wisdom and sanity for naught. Not to mention your good looks!

    Excellent idea, and it neatly sidesteps the major issues.

    Thank you.
     
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  20. Apr 11, 2019 #20

    BoKu

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    Western US
    Not to dogpile this or anything, but in a typical Marske type spar the shear web consists of three plies of cloth, typically 9oz 7725 "Rutan BID," over a stiffener of 1/4" Divinycell H60 PVC foam. We use 6oz carbon biax +/-45 for our spars, but it's the same sort of thing.

    The way it works is that you cut a piece of cloth a little over three times the height of the spar, saturate the middle third of it into the mold, and then arrange the foam stiffener and carbon rods in the mold on the saturated cloth. When the rods and foam are all properly arranged, you fold over one of the two flaps of cloth and saturate it down to the foam and rods. Then you fold over the other flap and saturate it down to the first. Then you bag it and tag it. When you're done, the shear web consists of three plies of cloth; two on one side of the foam and one on the other. And if your spars, like mine, consist of two separately molded sections bonded back to back, you end up with a total of six plies of cloth to the shear web.

    BTW I just talked a team through this at the Spring Akaflieg build session we had up at the shop last week. A team from an aerospace startup who had never worked with pultrusions ever before easily laid down a wing spar section in about an hour, and an hour and a half after starting to mix the first pot of epoxy we had the spar section under vacuum and curing.

    akaflieg_2019_april_copec-124.jpg

    --Bob K.
     

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