The strip method or other?

Discussion in 'Composites' started by Rom, Jun 18, 2008.

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  1. Jun 18, 2008 #1

    Rom

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    All you composite builders out there have looked into verious methods of building your fuselage. Since mine will have complex curves and be one of a kind I don't plan on building any complex jigs in which to layup the fuselage. The simplest method I can think of and one I am experienced with is the core made up of strips applied the to formers such as wood strips in a canoe.
    The airplane with of course will be PVC foam strips laid up in female plywood formers @12 inches OC.

    Does anyone have any thoughts on building this way.
     

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    Last edited: Oct 5, 2011
  2. Jun 18, 2008 #2

    orion

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    I'd think that it'll be much easier if you do this over a plug rather than trying to work inside a female cavity. Then, when you pull it off the framework you can then do the inside. This is called the One-Off method of construction and it was developed originally for the boating industry. Type in One-Off construction in Google - it should give you a lot of examples.
     
  3. Jun 18, 2008 #3

    Rom

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    I still am considering the pros and cons of female vs male forms. Doing the inside first, there would be a more stiffer and stable outside to complete and put a nice surface on. On the otherhand actualy doing the strip placement on the ouside of the forms would be much easier to install by virtue of being able to see the shape as the strips are being attached. Apples and oranges I guess.
     
  4. Jun 18, 2008 #4

    Rom

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    After looking at the ins and outs (pun intended) of form building, I think the method that you described is probably the way to go Orion. Having built with the male mold before, the results were quite satisfactory.
    Now as far as the process of puting it all together. The strips will be hot glued in place, the whole fuselage will be sanded to create nice smooth curved surface, then the hole thing will be microed to fill all the joints between the strips and create a hard shell.

    Since the joints will be very small, could I use just a micro mix or should flox be used to reinforce the joints?

    I kind of think the two layers of 7725 on each side should be enough reinforcement to take care of the micro filled gaps.
    The spar passthrough and other stressed areas have to be calculated for number of additional layers beyond the 4 layers of 7725 mentioned of course.
     
  5. Jun 18, 2008 #5

    orion

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    I probably wouldn't recommend filling with flox since that will form a ridge that will be difficult to make even with the foam (the foam will sand away faster than the hard ridge). The best filler for the gaps in the foam would be a soft material, preferably close in properties to the foam itself. I think even the micro mix might give you a bit of trouble but that will do a good job of bonding the strips together for a more or less homogeneous shell so it's a good compromise.

    Putting the strips down with hot glue is a good idea but I'd guess you might have to physically pin the ends down to keep the strip from springing back up (hot glue is good for positioning but might not hold the foam if under tension). As you lay the strips down, use the micro between the edges to bond the strips together. Having a stable shell that way will make it easier to sand and shape.
     
  6. Jun 21, 2008 #6

    litespeed

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    From experience in using foam for one off's in small and large scale
    - rc Models

    Hot glue sucks-

    -you will use a suprisingly large large amount of it
    -It will weigh heaps more
    - it leaves a difficult to remove ridge- ever tried sanding hot glue?

    It creates a very dissimilar composite of materials- very unlike the actual foam, and just like flox (like ORION said)It sands away the foam more than glue but the extra bonus is it clogs the paper or forms rolls of glue which then gouges the foam- more work!

    The other pet hate of the stuff is, Hot melt glue means hot melt foam, unless you do it "just" right-depends on the foam. With the potential size of joints you may find the early flow is too hot and by the time you get to the other end the bugger is cooler

    I reckon your best bet is to use micro to join for where there is no gaps.

    Here comes the trick, remember when your sanded the foam down last, that's the trick, collect all that fine foam you sanded off- the pure stuff only, so that means you make a clean area for sanding with plastic below. the sanding.


    So now mix up a batch of epoxy and add a ratio of 1:2 micro mix really well then add the sanded foam into the mix until your happy with a peanut butter- extra thick mixture and apply to the gaps.

    You must use micro or a fairing mix available from Wests and others, this helps to completely cover the foam 'molecules' and does not leave and possible spaces of pure epoxy in the mix.

    For low density foam, micro is best replaced with fairing mix, as it is heaps lighter and sands much faster, experiment and you should get a perfect recipe to suit you foam, change the micro/fairing/epoxy ratio to suit.
    You should end up with a mix that as closely as possible resembles the foam in structure, sands lovely!

    It may take a bit of practice on some test bits until your happy with the consistency- using virgin foam with the coarsest paper you have is probably the best.

    Oh and did I mention, I hate hot glue. :gig:

    Litespeed

    Wise man says "man who hot glues overhead, have glue eye":gig:
     
  7. Jun 21, 2008 #7

    BBerson

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    For gluing foam, try Foamofill. The foam in a spray can. If you spray some on a board and mix it to release some gases it will thicken and make a foamy glue.
    Should sand well, but I have not tried.
     
  8. Jun 21, 2008 #8

    Rom

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    Thanks for the inpute litespeed.

    The reason I want to use hot glue is for the instant gratifacation.

    The glue will only be applied to fasten the foam to the plywood forms and one or two stitch welds between the forms.

    That I can can definately agree with. Even with my wood strip boat, the glue joints between the wood strips were a pain to sand flush.

    The hot glue has no affect on the Divinycell H80 that I am using.

    As a side note, the low temp hot glue actually adhears better to the H80 than the high temp glue.

    I never considered using the foam sandings as part of the micro mix, but I will check that out.

    A lot of good info litespeed. every bit of info is helpfull
    thanks,
    Mark
     
  9. Jun 23, 2008 #9

    Scott

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    I think you should look at the following web site:

    krsuper2.com

    This is an example of a fuselage with a curved shape that uses a simple form and the fold-a-plane technique.
     
  10. Aug 19, 2008 #10

    Rom

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    Scott,

    I checked out your krsuper2. Nice way to smooth out the kr2. The kr was the first airplane I wanted to build, but being 6'-1" it was eventually determined that the kr would be an uncomfortable ride. I wanted to modify the design but figured if the change needs to be as drastic as my needs require, Why not design from scratch? You are apparently doing the same.

    This is the method used: 2" x 3/8" divinycell h80, hot glued on a female frame 12 OC. This turned out to be incredibly simple and easy to assemble. The whole fuselage is quite rigid as it sits in the photograph.

    Next step: Glass the interior, Take the cutouts, fit and glue them into the interior, flip, sand, shape and do the same to the exterior.

    Mark
     

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  11. Aug 19, 2008 #11

    AVI

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    More photos of your fuselage?
    Keep us posted - I'd like to follow along as you remove the boat from the female mold to glass the outside. I'm particularly interested in finding out the condition of the outside foam surface after removal from the mold.
     
  12. Aug 20, 2008 #12

    wsimpso1

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    The outside foam will most likely require a hard-shell of dry micro to fix all of the little divots from attachment to the mold, which will be OK. I too am looking forward to seeing how it comes out. The figure of merit on this stuff is weight per square foot on the finished part.

    Billski
     
  13. Aug 20, 2008 #13

    Rom

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    I sort of figured that it will need hard shelling, both inside and out. The div'cell does have large bubbles .
    Once the shape is removed most likely there will be joints between the strips needing filling although mostly very narrow. The spots of hotglue fastening the foam to the form will need to be removed and filled also.
     
  14. Aug 24, 2008 #14

    AVI

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  15. Aug 24, 2008 #15

    Rom

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    The diab group document mentioned a weight advantage over grid scored foam in a female mold. That is probably due to the resin taken up by all the score joints. With the strip method all of the joints get filled with lightweight micro during hard shelling.
     
  16. Dec 23, 2008 #16

    Mac790

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    It's a great news, I was worried that something went wrong. We are waiting for pictures.

    Seb
     
  17. Dec 24, 2008 #17

    rtfm

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    Rom, I'd have thought you would have had this figured out by now...:)

    Happy Christmas,
    Duncan
     
  18. Dec 28, 2008 #18

    Rom

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    Here are the photos as promised.

    This is the left side of the fuselage. The 3/8" PVC foam strips were cut on the table saw at 2"~50mm wide. No fancy rip angles, just square cuts. If you'll notice, the joins were minute, so not a lot of micro was used.
    Airplane 001.jpg

    The black lines are to aid in laying out the fiberglass fabric. The joints where microed after sanding to shape. The white spots are where the foam stuck to the forms. Those were micro filled. The top and bottom has a peal-ply strip as visible on the turtle deck.
    Airplane 002.jpg

    You can see the peel-ply where the firewall gets attached and spar passthroughs are layed out.
    Airplane 003.jpg

    I would not hesitate to use the female forms as a method again if I decide to build another project. The work required to get the right shape and the assembly of the foam was very easy to accomplish. Glassing the interior first realy helps in simplifying the construction, since the interior didn't have to be perfect. The exterior was easy to shape with sandpaper since the structure was already rigid.

    Mark
     
    Last edited: Dec 28, 2008
  19. Dec 28, 2008 #19

    rtfm

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    Hey Mark,
    Looks like you built your fuselage on the ceiling! Interesting approach... :)

    Duncan
     
  20. Dec 28, 2008 #20

    AVI

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    Mark was posting the photos for the Downunder guys in Oz !
     

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