I broke my air inlet boxes while replacing the ducts. Can they be had? If not, how can I repair them?
Search strings: duct replacement; air box repair; fix the air boxes; air plenums; inlet air box; screened air inlet; screened air box.
Technical Editor, Mike Rellihan:
No new plenums are available. I have repaired them that were in as many as 50 pieces. You can send me yours for repair, or tackle it yourself. If you do it, you’ll need the following:
– Reinforcement material for the ID of the 5/8″ radio duct hole and the 2″ exit hole.
– Paper-thin fiberglass cloth from an aircraft hobby shop (0.6 ounce to 1.2 ounce range; you can read through it).
– Thick (gel-type/gap filling) cyanoacrylate glue, from the same hobby shop (not the cheap Wal-Mart super glue).
– Thin (watery) cyanoacrylate glue, from the same hobby shop (not the cheap Wal-Mart super glue).
– Cyanoacrylate accelerator (preferably the same brand as the glue) from the same hobby shop.
– Tape sealing putty to re-bed the screen for reinstallation.
DO NOT USE AUTO SHOP FIBERGLASS; NOR CONVENTIONAL RESIN OR EPOXIES!
All six clip-nuts must be gently removed by lifting the edge with a knife blade, to allow the dimpled center-hole edge to slide out of the hole in the plastic. Then run an 8-32 tap through each of the clip-nuts, and set them aside.
You first tack all of the plastic fragments together to regain the overall shape, using the thick glue; leave as few protrusions on the outside as possible. Then cut pieces of fiberglass on the bias (45 degrees across the weave), so that it will conform to the contours when you dab it with the thin glue. As each piece is applied and wetted, you then spritz it with the accelerator to set it instantly.
The 2″ exit hole needs to be reinforced on the inside, either with more fiberglass or another material. So does the exit hole for the 5/8″ radio cooling duct. Any missing pieces must be fabricated from layers of fiberglass or ABS plastic. Once reinforced, it will be far stronger than the original. Open up any screw holes blocked by the repairs (I prefer a stepped Unibit), and snap the clip-nuts back into place. You apply a layer of putty tape around the lip, and embed the screen halfway into the tape.
When you reinstall the cheek box on the airframe, use new AN525-832R10 washer-head screws. If you prefer to use AN526C832R10 stainless-steel truss-head screws, make certain that you use a #8 Nylon washer under the heads of the screws. Snug the screws down to seal the putty to the airframe, but don’t crush the box lip from over-tightening. Take care to get the boxes back on the proper sides; they are mirror images, and it can be surprisingly easy to try to install them backwards.
When you connect the new 2″ duct to the air distribution plenum, make sure you first file or grind off any protruding seam line on the plenum inlet. If the distribution plenums need repairs, use the same method as used on the inlet boxes. Thoroughly tape on the new duct to the plenum, to make certain that the joint cannot leak water (also make sure that the plenum drain opening is clear, and install a new drain line). Do not use any kind of clamp other than a tie-wrap; the tape should be adequate by itself. When you connect the other end of the 2″ duct to the repaired cheek air inlet box, use nothing but a tie-wrap for a clamp. Ditto for all the other duct connections.