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A Dozen Rules of Thumb for Composite Repair
Posted by J.S. Decker on Dec 7, 2007, 18:19

A little mid-air mishap.

Having enough experience to have rules of thumb for repairing composite model airplanes is not a positive statement about my piloting ability.  Be that as it may, these rules of thumb are based on my experience and what make sense to me.  You will probably never have need to consider them, but just incase you do, they might be worth a gander.

  1. The damage is usually not as bad as it first appears.  Take a good look at it and then put it somewhere out of sight for awhile.  Come back to it after you are done being pissed off about crashing it, sometimes this takes me a few years, and after your subconscious mind has thought about how to repair it for awhile.  It will not look as bad to you and how to repair it will come easier.

  2. Figure out what requires a structural repair and make the structural repairs first.  If the spar is broken, that requires a structural repair.  If the tail cone of the fuse is broken, that is a structural repair.  If the top of the cross tail fin is broken off, that is not a structural repair.  Here’s the test that I use to determine if a break is structural:  imagine that the repair is made with just tape or with foam carved to shape that is attached with tape.  If you could fly the plane safely with this repair, it is not a structural repair.

  3. For structural repairs, repair with the original material.  While you might have a real carbon fiber fetish, you should repair that broken fiberglass fuselage with fiberglass and not carbon fiber.  The goal is make repairs with materials with same stiffness as the material that is broken.

    Fiberglass stab repair made with fiberglass.

  4. Scarf and overlap for development length.  Laminate materials are much stronger than the resins used to bind them and it takes length/area to transfer loads from the original material to the new reinforcing material.  In this context “development length” is the minimum length required to anchor the new reinforcing material on each side of a break on the original part.  There are a lot of situations where there might not be enough development length available to make a repair, e.g., a spar break near the root.

  5. Avoid voids in sandwich construction.  Laminates that are applied to substrates, e.g., carbon fiber vacuum bagged onto foam, depend on the substrate to hold the laminate in place under load.  With substrate to hold the laminate in place, the laminate can resist higher loads.  So, fill the voids in the foam and sand it nicely to shape before applying the reinforcement for repair.

    Filling voids in the foam.

  6. Consolidate the reinforcing laminate.  Even fiberglass is much stiffer and stronger than resin, so 1) do not apply so much excess resin that your laminate floats in the resin, and 2) if there is any way to apply pressure, even a little bit, to consolidate the laminate, do it.  The best repair would have the reinforcing material pushed into contact with the original part and just enough resin to bind (matrix) the laminate and attach it to the original part.  The weave of the reinforcing laminate can always be filled later.  Learning how to apply a vacuum bag to a localized area is a skill worth learning.

    Local vacuum bag repair.

  7. For structural repairs, apply repair reinforcement to both sides of a break.  Even after you apply a repair to one side of a break, the break is a discontinuity on the other side of the repaired surface that weakens the structure significantly under specific load orientations.

    Consolidating the laminate, tape around the nose and clamps on the sides.

  8. Use precured laminate to reinforce where necessary.  Use this technique with hollow fuselages and wings when you can not apply laminate inside a part and then wet it out effectively, e.g., repairing a hollow molded stab.

  9. Apply material 0/90 and on bias.  Alternating the laminate weave direction helps distribute the stress in with the laminate structure and resist multiples type of loading, e.g., bending and torsion.

  10. Use multiple layers of light material instead of a single layer of heavy material.  Instead of grabbing the 9 oz fiberglass to repair that broken fuselage boom, use six layers of 1.5 oz fiberglass – three layers on one side and three on the other side.  You can stagger the application of the material so that the application of fiberglass is thickness tapered and lighter weight, you can change the weave orientation between layers, and the repair will be stronger.

  11. Fill before touching a repair with sandpaper.  Do not sand the laminate.  Sandpaper will cut the fiber that you need to reinforce the repair.  Apply a filler or primer over the repair area before picking up the sandpaper.  There are a couple exceptions:  1) if a sacrificial layer(s) of laminate is applied, and 2) circumstantially where scarf joints are used.

  12. Fill then feather the fill to make it pretty.  For repair to be effective, you have to leave the new reinforcing material in place, i.e., you can not sand it off in order to make the repair smooth and pretty again.  To make it smooth and pretty again, you need to fill around the repair then sand the filler to feather that filler into the surface of the original part.  Yes, even if your repaint the whole part, you will probably be able to see evidence of the repair – get over it.  It looks great a few feet away.

    Wing leading edge repair.

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