Close-Out Ribs, Fill and Fair or Leave Them?

Discussion in 'Composites' started by wsimpso1, Apr 26, 2018.

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  1. Apr 26, 2018 #1

    wsimpso1

    wsimpso1

    wsimpso1

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    All of my control surfaces are hotwired cores, vacuum bagged skins, and then I rout a little foam from the very ends and apply a 2 BID closeout rib to keep the top and bottom skins from peeling from the foam. Pretty standard stuff. But, which is the right way to finish the job? What is best for overall aerodynamics? I wished that I had paid attention to this at fly-ins...

    Should I leave the 1/2-3/4" indented closeout rib naked? With everything else looking sooooo smoooth on the outside of a composite bird, this seems incongruous.

    Should I insert a chunk of foam, attaching it with dry micro, then fill and sand the ends smooth? This seems more in keeping with the smooth outside of composite birds, but I wonder if it does not increase the air leaks across the ends of the surfaces.

    Then, If I do fill and fair the ends, I suspect that I put the smallest radius on the corner I can - correct?

    Some folks, notably AutoReply, have advocated for end plates on control surfaces. Anybody with knowledge on this?

    Am I wondering over a simply appearance item? If that is all we are talking about, how are the various end treatments viewed by the judges?

    Data, science, and opinions welcome.

    Billski
     
  2. Apr 26, 2018 #2

    gtae07

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    I was fortunate enough to talk to Dave Anders a few weeks ago. As I recall he faired in all the end ribs of his (metal) control surfaces. I'd expect less leakage across the ends. You could also fit a small flexible foam/rubber seal in the gap.

    There was a thread on VAF at one point where someone had endplates on his wing control surfaces. I believe he reported a small improvement in speed and handling. I plan to try it whenever I finally get my project finished...
     
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  3. Apr 26, 2018 #3

    wsimpso1

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    Ooo, good point. Foam seals were already a part of the leading edge of the flaps when stowed, could add seals on the ends too...
     
  4. Apr 26, 2018 #4

    BJC

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    I’ve seen flaps and ailerons done both ways. They look better to me when closed.

    Another closure option is to use 1/4 “ foam with one layer of glass on the outside. Sand the foam to slide into the open end of the surface and bond in place. Sand the one-ply overlap to the profile. Drill a small vent hole.


    BJC
     
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  5. Apr 26, 2018 #5

    Hot Wings

    Hot Wings

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    If I'm visualizing this correctly you are doing something like the left side (Red)?

    I've done it inverted like the right side. Kind of like a flox corner with 2 oz cloth stuffed in around the perimeter then fill to level with flox (Green). Makes a nice strong and sharp edge - if that is what you want. Next time I think I'd just bond on a plate of cured glass for an end plate.

    close out.jpg
     
  6. Apr 27, 2018 #6

    wsimpso1

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    I asked if there is good reason either to leave it as is or to fill the recess flush with the skins, and why. Two answers in favor of filling with reasons to do so from people I have reason to trust. COOL!

    Now onto how to do the filling, given that these parts are already made. First, how the control surfaces are made. Mine are built like Hot Wings' drawing on the left, which is typical in hotwired core construction to tie the skins together strongly and keep the skins from delaminating from the foam. Control surfaces with solid cores and skins bonded on have worked the skins loose, thus the need for closeout ribs. These were built by starting with hotwired cores that extend the full length of the intended control surface, the cores are glassed both sides with overlaps, the recesses cut in the ends by removing a modest depth of foam, sanding the "inside" surfaces of the skins, and laying up a close-out rib using 2 BID. We end up with a strong laminated rib bonded to 1/2 to 3/4" of glass skin all of the way around each end of the control surface.

    The two fill methods I have heard of to fill the resulting recess are rock simple:

    First is to carefully cut 2 pcf foam that can barely be inserted into the recess, bond it in with dry micro, trim the foam off flush and fair the end with dry micro. It weighs a little more than the foam removed earlier, around 1.5-2 ounces per flap or aileron, or 6-8 ounces for the whole wing. Not too bad.

    The other method is to simply to fill the recess with dry micro. I get that at about 8 ounces per flap or aileron or 2 pounds for the wing. You have to be concerned over exotherm too with a thick casting of dry micro, so you will probably have to do it in a couple steps, so this approach's simplicity does not save steps and is heavier.

    Let's also remember that net weight gain behind the hinge line on ailerons hits you again, because the control surfaces on most fast airplanes are mass balanced, so you add at least that much mass in again. I am going with the foam and then fill. While weight is the enemy, the added effectiveness that this allows (through sealed control surfaces) and reduced drag seems worth 10 ounces. To go 2-1/2 times as heavy for the same function seems silly to me.

    So Hot Wings, did you start with a hotwired core to the tip of the surfaces? Then removing a little foam around the periphery, laminating cloth over the end foam and down into the space, followed by flox to attach the rib to the skins? Hmmm, could work nicely, and might be lighter than what I have... Too late for me, but not for others.

    Now a question of the method Hot Wings proposes - are you not worried that simply laminating over the end will not adequately tie the skins together and could result in delamination of both the end plate and the skins? I can see a simple solution though. Flox corner all of the way around, then laminate with glass. That seems like it would be adequate, and may be the lightest way yet to close out the ends. Again, too late for my parts, but for other folks...

    Billski
     
  7. Apr 27, 2018 #7

    Hot Wings

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    Thought about that as I wrote it. A flox corner was part of what I was thinking but it still might be too little surface for the secondary bond?

    Edit:

    "So Hot Wings, did you start with a hotwired core to the tip of the surfaces?"

    Yes. Stock Quickie. The conventional wisdom at the time was to just paint the ends with micro slurry to seal. Didn't like that idea even though it is the lightest - other than raw foam. Dug out about a 30 deg triangle of foam about 3/8" deep and abraded the inner surface of the skin with a Dremel. Stuffed the edges of wet BID down in the crevice and then filled it in with flox. Does add weight but probably not as much as the dirt on my shoes and the lint in my pockets. :gig:
     
    Last edited: Apr 27, 2018
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  8. May 2, 2018 #8

    wsimpso1

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    Ok, I had one close-out rib yet to do with foam out to the end of the part. I inlet the foam at about 30 degrees and abraded the glass, then micro-slurried the foam and applied two BID to the foam, down into the inlet, and then up along the glass of the structure. It worked really neat, with rough trim letting me get it all in place. I finished up by adding more microballoons to my slurry, and filling the 30 degree space level with the end of the aileron. When I sand the end flush, I think it will require another pass of dry micro, but oh well. Neat trick, it works, and it is less steps and most likely less weight than what I did elsewhere.

    Billski
     
  9. May 3, 2018 #9

    Hot Wings

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    Glad it worked out for you!

    Brought back some memories. I only used 1 BID and now remember sanding through to the foam in a couple of high spots. Pasted some 2oz model cloth on top to finish. Overall more work than trying to do it with 2 BID.
     
  10. May 3, 2018 #10

    wsimpso1

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    I am in the camp where 2 BID is the minimum thickness of anything on the bird. Too easy to peel it apart or poke a hole in it otherwise.

    Billski
     
  11. May 3, 2018 #11

    Hot Wings

    Hot Wings

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    Probably a good R.O.T.. The aft fuselage of the Quickie is just one BID on the outside, one on the inside - thus my minimalist procedure. A second ply would be better than the 10# of tail lead used for the Rotax conversions? :gig:
     
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  12. May 5, 2018 #12

    proppastie

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    off topic, but workable, 1 layer glued and heat shrink fabric (bleed cloth Dacron) (hardly any weight)....probably tape(extra layer) the edges , still hardly any weight.
     

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