High Density Foam for Hardpoints

Discussion in 'Composites' started by wsimpso1, Apr 22, 2018.

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  1. Apr 22, 2018 #1

    wsimpso1

    wsimpso1

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    I looked and looked, no joy. For several scratchbuilt composite airplanes, the method of attaching control surfaces is including a high density foam block (20 pcf foam, Wicks stocked it) in the massive cores, glass the surface, then imbed hinges in the clock of high density foam.

    Trouble is I can not find 20 pcf foam anywhere and mine is used up. I did find companies that list that density and others near it, but none of their distributors have any of it, and the maker is only interested if you want a semi-trailer full of it. Anybody know what the currently used options are for this sort of construction?

    I can make syntactic foam using simple molds and dry micro at about 32-35 pcf (about 0.51-0.55 g/cm^3) but using a previously engineered material is always a better option in my mind...

    Billski
     
  2. Apr 22, 2018 #2

    Marc Zeitlin

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    I assume at least one example of what you're talking about how the Rutan derivative canard install their hinges in the canard. Here's an 18 lb/ft^3 foam:

    http://www.aircraftspruce.com/catalog/cmpages/lastafoam.php

    although the LE/COZY etc. use the 6 lb. version for the hinge inserts, and the 0.2" 18 lb/ft^3 for the instrument panel only.

    One could laminate the 0.25" 18 lb. stuff to build up to whatever thickness you want, if the 6 lb stuff isn't dense enough for you.
     
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  3. Apr 22, 2018 #3

    wsimpso1

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    Ah, Burt's "red" foam. Thanks. I had not spotted that.

    Bill
     
  4. Apr 22, 2018 #4

    BJC

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  5. Apr 22, 2018 #5

    pictsidhe

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    Is PVC foamboard, aka 'Sintra' any good? There's also PVC trim 'wood' at your local bigbox aircraft material supplier.
     
  6. Apr 23, 2018 #6

    DeepStall

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    Spruce? Douglas Fir? (possibly as marine plywood rather than lumber)

    Alternatively, tooling board foam comes in that density range, although I don't know how its structural properties compare to the foam you're used to seeing. Sounds like you don't need much, see if a local shop will let you raid their scrap pile?
     
  7. Apr 23, 2018 #7

    wsimpso1

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    Byron,

    Good idea if it would work... I spent quite a bit of time with one of the suppliers, might have been these guys. Their samples are small, I would need about six sample kits to get enough for my ailerons and vertical tail hinge mounts. They were willing to send me one sample kit, but I was going to have to wait as they had none with the right density foam made up and none scheduled for production either. And then none of their distributors had any in stock.

    Tempted to do the design calcs for use of 6 pcf foam, and I can laminate up using 18 pcf or cast some dry micro.

    Bill
     
  8. Apr 23, 2018 #8

    Aerowerx

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    Why not just use a piece of aircraft plywood for the hard point, as DeepStall suggested?

    Or are you concerned about the "purity" of the design?
     
  9. Apr 23, 2018 #9

    wsimpso1

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    Purity? Naw, just doing what works recognizing that WEIGHT IS THE ENEMY.

    The stuff I used to be able to get was 1" polyurethane at 20 pcf. Wish I had bought more. We use it by removing a designed shape of blue foam from massive cores and replace the 20 pcf foam. Easy to machine, shape, fit, install in the massive foam cores, and then hog out the space for the hinge pieces. It serves to collect loads from the control surface skins down onto the hinge pieces that are embedded in the foam. It has to cover some square inches of skin on each side of the control surface and deliver load to the hinges.

    For the elevators, the four blocks in the h stab are about 1"x1-1/2"x3", one on centerline is bigger, and the four in the elevator are about the same, but tapered. Then there are three pairs of similar blocks in the vertical tail, three in each flap, three in each aileron. The rest of the system is a 1/4" thick aluminum alloy hinge pieces, much of it is heavily drilled and embedded in the the foam with flox/epoxy. If you have your preference, you would pick the system with the least weight that does the job. The designs out there either use blocks on the order of the sizes I cited with 18-20 pcf foam, or somewhat larger foam blocks and larger hinge pieces with 6 pcf foam. In 20 pcf foam, I have a couple pounds in the airplane and several more pounds by the time I have the aluminum hinge pieces and flox/epoxy to hold it all together.

    The foam blocks are strong, sturdy, and trouble free. Balsa varies all over the place on density and strength running from a low of 4 pcf to 22 pcf, and at its densest, gives up something like 40% on strength to 20 pcf PU foam, plus is fussier to machine and fit. SPF plywoods run 30-50 pcf and are about as strong as the foam. So, if I use balsa at the high end, I need about 40% more of it and larger hinge pieces and more flox/epoxy, roughly doubling my hinge weights from around six pounds to around 12 pounds. If instead, I use plywood, my weights are similarly increased by the plywood, but not the hinge pieces, while machining and fitting gets another level higher work than the balsa.

    The choice is clear. Using foam makes the lighter airplane, is easier to build with, and is the known quantity in this type of structure.

    Billski
     
  10. Apr 23, 2018 #10

    pictsidhe

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    Last edited: Apr 23, 2018
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  11. Apr 23, 2018 #11

    Rocket29

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  12. Apr 24, 2018 #12

    Slars

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    I bought some PVC Divynicell boat foam that appears to be metric. It is approximately 1/2" thick and is scored on both sides with scores approximately on 1" centers. As far as I can ascertain it is approximately 9 lb foam.

    My plans say to use 1" thick 8 lb urethane to replace the styrene. I used 3 pieces of this on each side of the hinge and filled the scores with micro. I built a test piece like this and could stand on the hinge post without breaking it.

    Billski: If you are interested, I would be happy to send you some. I have way more than I will ever need. Just PM me with how much you need and where to send it.

    Steve
    IMG_5968.jpg IMG_5969.jpg
     
  13. Apr 24, 2018 #13

    wsimpso1

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    Steve - Thanks for the kind offer, but I shall pass.

    Staying high density and have a couple options now, thanks to the folks on here!

    Billski
     
  14. Apr 24, 2018 #14

    BoKu

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    Garolite and carbon fiber.

    The dark side has cookies.
     
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  15. Apr 24, 2018 #15

    wsimpso1

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    That amazon page picks to General Plastics. They had none in stock, no schedule to make more, and IIRC they were one of the outfits that tried to find me a distributor with it in stock and failed.

    Nice try, anyway...
     
  16. Apr 24, 2018 #16

    wsimpso1

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    Bob,

    I know how to make Garolite bushings and bond them into beefy structures for bolting or pinning things together. Maybe my imagination is a bit soft today, but I am having a hard time figuring out how Garolite is applied here.

    What we have historically done in massive foam core structures is apply a somewhat higher density foam bonded to both top skin to bottom skin and then embed a plate using flox/epoxy. The plate has a concentrated load at a spherical bearing or control pushrod, and the load is spread out through plate to the flox/epoxy to the high density foam block, which then spreads the load to the top and bottom skins through both direct bonding and through bonding to the surrounding 2 pcf foam to the skins. The scheme works when we make the bonded area of the aluminum plate large enough and then when we make the high density block large enough that stresses in each element are OK.

    So, how do we apply Garolite to this sort of foam and skin only structure? I am suspecting a paradigm shift, but not seeing it. I can imagine using Garolite for the plate (replacing aluminum) or even using a Garolite plate at full size of the section as a rib for each hardpoint, Then bond Garolite hinge plates to Garolite embedded rib after skinning the cores with Garolite ribs... Close?

    I am certain there is wisdom here someplace, I just do not see it yet...

    Billski
     
  17. Apr 24, 2018 #17

    BoKu

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    For the most part, I was joking. But I think that there is likely the germ of an idea in the humor.

    As I understand what you're trying to do, the high-density foam serves as an interface of intermediate strength and stiffness between an embedded blade of aluminum and the surrounding foam core. You cut a rectangle of foam out of the planform and replace it with two blocks of high-density foam, with room in between the two for your blade. You glue the blade into the slot, and rely on the bond between the blade and the high-density foam and on the bond between the high-density foam and the exterior plies to react the forces applied to the blade.

    That's great if you can walk over to a tooling shop and take a couple blocks of high-density tooling foam. I'm not, so here's what I'd do:

    I'd cut away the rectangle of foam just as described above. But then I'd take that rectangle, split it in two, and sand each one so that it is about 1/8" smaller in every dimension. I'd apply five (or however many) plies of 7725 ("Jiran BID") to five of the six faces of each of the two rectangles. Then I'd glue the two rectangles to the blade and glue the assembly into the rectangular cutout. The exterior plies help distribute the load into the surrounding foam core and into the exterior plies that get applied to it.

    20180424_140009.jpg

    That obviously takes longer than cutting the blocks of tooling foam. But it uses materials and processes you already have at hand, and I think it can result in a stronger and more robust structure.

    Oh, and the blade? I'd definitely use Garolite for that. You can use aluminum for embedments like this; Rutan did that a lot. But service history strongly suggests that by the time it's a problem, it might still be your problem. Study the Vari-Eze outboard wing attach plates for a good example of how corrosion-prone, life-critical, yet uninspectable, structural embedments into even mildly hydrophilic resins might be a mediocre to poor idea.

    I was kidding about the carbon fiber. But we'll leave the light on for you. :)

    --Bob K.
     
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  18. Apr 24, 2018 #18

    Marc Zeitlin

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    An interesting idea you posit for load spreading.

    With respect to the Garolite/G10, I'm sure it can be used (with appropriate design to accomodate the lower stiffness and strength from AL) to replace the embedded AL plates for hinges and such. However, I didn't see any reference to NOT using it as the actual bearing material for rotating hinges/components. Even in the AL hinges in Rutan derivative aircraft, a bronze bushing is used to take the rotating loads from the bolts (in the GU canard) or SS hinge pin (in the Roncz canard).

    So I just wanted to caution that.

    Having been castigated roundly on certain canard oriented mailing lists for being Chicken Little and declaring that the sky is falling for VE's due to the wing fitting corrosion issue (a topic for a separate thread), I do not disagree with your statement that using embedded AL in resins has been and can be a terrible idea.

    HOWEVER, the issue with the VE wing attach fittings is that the VE (and LE and COZY) plans never indicated to builders that they should alodine and protect ALL AL parts, including embedded ones (that's what happens when the designers of a product live in the desert, and not in a rain forest). While I have personally seen four sets of very corroded VE wing attach fittings and had a metallurgical analysis done on one to ensure that it was, in fact, made out of the specified type of AL, none of those fittings had any corrosion protection applied.

    With the correct protection on the AL, and (if installed with hardware) a wet install with primer/pro-seal/protectant, there are no instances of wing attach fittings (on VE's) or other embedded AL corrosion of which I'm aware. Now, I HAVE seen pictures of corroded canard lift tabs due to having unprotected AL tabs installed in expanding urethane foam (NOT PER PLANS - the builder was a con man "pro for hire") that apparently was outgassing some sort of acid, but I've never seen or heard of corroded canard hinges.

    In any case, if one currently purchases hinges and/or canard lift tabs and/or other AL parts from the main vendor of such parts for canard aircraft, they come alodined.

    Not saying that AL is BETTER than G-10/Garolite - just that it can be made to work just fine if treated correctly.

    Back to your regularly scheduled programming...
     
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  19. Apr 24, 2018 #19

    BoKu

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    Excellent points, Marc, thanks for posting up!

    Absolutely, I'd recommend against using Garolite G10 and similar green materials in a moving assembly without some sort of bearing or bushing. The glass fibers in G10 are very abrasive, and can cut through an aluminum torque tube pretty quickly. However, I've seen lots of bearing blocks and fairleads made out of brown phenolic. But that stuff is generally cotton fibers in a phenolic resin, and it appears to wear a lot softer than G10.

    --Bob K.
     
  20. Apr 25, 2018 #20

    pictsidhe

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    How is carbon as a bearing material?
     

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