Carbon D-box repairs

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  • #41299
    Anonymous
    Inactive

    There are two repairs to taco shell damage. I attached pictures of the CF cloth repair I did on my F1H and Miles Johnson’s F1A short ship. This works well for smaller cracks in the d-box or were you can re-build the d-box pieces together. The light weight CF cloth I get from Matt Gewain– these are reduced cost scraps from the RC guys cut at 45 degrees.

    – Prep the d-box- 220 sand paper– remove paint, down to the CF– wipe with dry paper towel– use only lightly damp with Lacquer thinner or Acetone– do not make surface wet.
    – Spray a very light coat of 3M adhesive on wax paper and place the CF cloth on it.
    – Cut with scissors the CF/wax paper. Then using a Tire CA or High impact (Gorilla super glue, Impact tough I found in Home Depot) apply and then Roll with a small dowel (don’t spread- this tends to make the CA kick). [Tire CA has fine rubber particles to make it impact resistance makes it black and is in some hobby shops. the Gorilla CA is clear– not sure if it has any particles– both makes the CA better for CF to CF– far less brittle and provides longer working time.]
    – Place CF- wax paper patch on wing and rub to shape, have another piece of wax paper available to keep from sticking yourself to the wing.
    – Here’s the trick– just as the CA is starting to stick / kick, peel the wax paper backing off the CF– use a new piece of wax paper to continue to rube the CF edges down until the CA sets– sometimes takes 2-3 minutes.
    – After full set, sand, use a small amount of thin CA to secure any loos fibers- edges.
    – Over coat with epoxy– good west tech resin or DP– just a bit rubbed into the fibers

    The picture of F1A wing shows a patch ~8 ‘ x .4″ wrapped around the LE. M&K painted these D-boxes with white epoxy paint. For this repair, I used 21st Century paint– can only with stand a single brush stroke of the Stick-It. On my F1H wings, I’ll spray a very light coat of white epoxy primer– just enough to whiten the CF.

    The second repair is using the pre-cured taco shell. It is the shell material M&K and others use on F1A tips, it is .003” thick. I over lap ~25 inch but at a rib location—some times I’ll insert a new rib out of soft 3/16 balsa. The hard part is getting the panel straight. If the spar and TE are in one piece, this is not too difficult. At Sierra Cup I broke a short model 2″ outb’d the dihedral break. I could not get the parts to fit properly so I strip the whole panel, build up a jig to hold the d-box, spliced in new spar material and LE, and TE pieces. Then I replaced the missing taco shell. So far so good with this model– taken full launch loads.

    Thermals, JIM



    #48437
    Anonymous
    Inactive

    Sorry– having trouble getting my photos under 100kb in size– will try later– have to do work now. JIM

    #48438
    JLorbiecki
    Participant

    Hi Jim…

    That is exactly how I repair our stuff. I have used standard thick CA as well as epoxy. The thickest CA is slow cure and work provides enough working time to do a decent job. I then feather the edges with 220 paper and away we go. It is pretty simple when one does it and works out very well…

    #48439
    Bill Shailor
    Participant

    You guys have to learn how to stop breaking stuff…. 😀

    #48440
    Anonymous
    Inactive

    I’d like to see your pics, Jim, but can add my own thoughts having, in a previous life, defined repairs to composite military aircraft.

    For all except the root area, if you can glue the damage back together with CA without any gaps then you get ~80% of the stiffness of the d-box back. D-boxes aren’t strength designed so as long as you get all the stiffness back then you should be OK, so typically after the CA work you only need a thin cover sheet which need not be carbon. I typically use 1oz glass cloth or better still two layers of 1/2oz glass cloth fixed with CA. Very quick and more than sufficient. If there are any dents then use body filler with the glass cloth over the whole lot.

    Repairs at the root I think are more critical and what Jim suggests is sensible.

    Once saw Mike Fantham repair a Kevlar wing that got very damaged at Lost Hills. He cut out a section then inserted thick (6mm ?) ribs to provide a bridging piece between the good material and the damage. He then cut a new piece of d-box, fitted it on the ribs and exposed spars, then put strips of materials (can’t remember what he used) over the joints. The model needless to say flew perfectly afterwards.

    Any impact that damages the spar will probably do lots of nasty internal stuff as well so best to strip back and just build, er, buy another panel.

    CHE

    #48441
    Rene Limberger
    Participant

    jim,

    you can upload your photos to a free photo hosting service like photobucket or flikr and then post the link here.

    -r

    #48442
    Anonymous
    Inactive
    #48443
    Anonymous
    Inactive

    CHE– agree on the stiffness vs strength. I have use fiberglass on tip repairs and likely fiberglass would work on the F1H.
    There are significant loads in the d-box skin and TE. M&K had to beef-up the dihedral joints on thier flapper due to the TE no longer carrying load across the joint. The most important part of the d-box repair is torsional stiffness. I have found that even the smallest crack in the d-box skin will result in inconsistant launches– more critical for those that seek the max launch height. Along that line– need tight fits at the wing fuselage pin fit and wing wiggler– often surprized to see how sloppy these fits can become. JIM

    #48444
    Bill Shailor
    Participant

    Nice repair work! How much of a load are you guys putting on the wings during launch?

    #48445
    Anonymous
    Inactive

    “How much of a load are you guys putting on the wings during launch?”

    Enough to make a witch wince !

    CHE



    #48446
    Lee Hines
    Participant

    BTW, that is one of my fav photos ever.

    Now re Bill’s query of how much load: A test I made gave about 30lbs
    at what I reckoned would be max pull.
    I wager many fast, strong, young types may pull double that. 🙄 ❓

    Any votes for other poundage from the group, or the lurkers? 🙂

    Leeper

    #48447
    DAN BERRY
    Participant

    Please, I read these forums. No comments about ‘poundage’ please!

    #48448
    JLorbiecki
    Participant

    I thought Lee was talking about British money….If pounds count, I should be able to hit the moon….

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