Pitts S1 C-GIPS
Jan - July 2009 - Re-fabric Wings
|Dragged out the wings recover as long as I could. Finally
peeled the sagging, stretched, billowing fabric off in December 2008.
Nothing broken inside, just a little woodwork and a recover.
Idler needed a couple of new KP4 bearings, and this time I put a tube spacer inside between the two bearings to eliminate the side load with the bolt torqued up.
|Tough Decision: after drooling the 30% hinge SS ailerons for several years, I looked at the new-condition ailerons and wings with the fabric off, thought about how the airplane rolled and felt with the larger spades.......and couldn't justify another set of ailerons and redoing the aileron bays. If I was starting from stock it would have been a no-brainer, but with most of the performance already there I just couldn't justify the extra time from what I've already got. It will never be an MX-2, and the old wings beat a bunch of them at AWAC.|
|Over the last 20 years one by one all the aluminum trailing edges
had gotten beaten out and
were replaced with wood except for the lower left. I changed the last
one out to match.
Finally measured the wingspan. After decades of telling everyone it was about 15', it turns out to be 15' 9". Now we know. Top ailerons are 52.75" at the trailing edge, bottom ailerons are 49.25"
|Say goodbye to oily smelly shirts. I removed the smoke-oil
tank from the upper wing center-section. Maybe get one of Raven's
fiberglass fairings for the wing attach.
I never did use the wing tank for fuel. 130 knots, 2.5 hours to dry tanks always seemed like enough range and about as long as I wanted to sit at one time.
|Top of upper wing center section with the hole I'd cut out to remove
the wing tank now repaired and filled in. That's plain bondo doing
the fill and working good. Resealed everything with S-1 Epoxy
I noticed the fabric had lifted from some of the plywood panels in the last few years. Last summer I had to lay down strips of screws on top of the fabric like rib lacing just to finish off the season. I think the Polybrush needs a little help, so I'm going to follow Craig Dobesh's advice and lay a strip of #6 screws on tinnerman washers through reinforcing tape, spaced 2" along the plywood edge.
|Top wing getting the fabric Poly-Tac'd on. Bottoms already done. Working in the garage, so I've got to time the Poly-brush application for when the house is deserted. I'm going to test out that old adage of "a family that rib stitches together, stays together".|
|Rib stitching - my experience
I followed the Kimball school of thought: Heavy 3.4 oz fabric, round cord, modified seine knot with the runner on the outside. That 1" spacing means a lot of knots. Takes five arms width pulls of cord for the long ribs, 4 for the short ones.
Had some advice from Peter Groves( who put out the "Precision Wings"
kit, similar to mine): Where the wing transitions from taper to straight at the leading edge,
try to keep the fabric as one piece, it requires careful cutting and
fitting at this point, as otherwise this area tends to work, and crack
(as you well know!) if you can carefully shrink a tape in one piece
over this joint it also helps to avoid any future cracking.
Where the wing transitions from taper to straight at the leading edge, try to keep the fabric as one piece, it requires careful cutting and fitting at this point, as otherwise this area tends to work, and crack (as you well know!) if you can carefully shrink a tape in one piece over this joint it also helps to avoid any future cracking.
|1" stitch spacing everywhere except on the plywood covered edges where I went with 2" spacing. Took strip from a plastic calendar and used a hole-punch to make a line of 1" holes along an edge. Lay it down, mark location with a pencil, then stab the mark with needle.|
|Plywood drilled 7/64" to let the needle through. Stitched the walkways and inside of the I-strut bay like this. Used screws on washers on the center section and outside of I-strut bays.|
|Wing standing leading edge down, working from the bottom .Some backlighting made pushing the needle into the right hole easier to see.|
|Stitching starts at the leading edge. Two loops around to start helps keep the first one tight and stops it from getting pulled forward.|
|Reef knot. Tied with wax cord this easily pulls apart, which is why you then do a...|
|.....half hitch at both ends (around both cords) to lock the knot.|
|Now up and across to the next stitch. If you press your eyeball up against one of those tiny needle holes you can just see the needle tip to poke through the other side.|
|Tried the tinnerman washers with #6 flat head screws and it didn't
work. Stands up too high.
I ended up using the thin 1/2" aluminum washers under and a #4 x 3/8 sheetmetal screw. AS #09-19100, and T4x6 truss head, Type A, stainless.
|Needles - used the straight one mostly. The "hidden hitch" that PolyFiber recommends using the curvy tip needle doesn't work well with 1" spacing and heavy fabric. I ended up bending that one into a bow to lace the compression ribs with the extra wooden beam along the sides. Handy for going around other internal obstructions too.|
|One of the frustrations was getting the rib stitch tight. Sometimes the knot would get tight and the stitch would still be a little loose. With enough wax on the string you could give it a little tug and sometimes it would tighten the last bit - and other times it would break. Here's the knot set up and ready to tighten (pulling the loose string hanging down)|
|Put your thumb over the knot shown above, move the loose string to the 30 degree off top and close to the wing as shown, and pull tight. Perfect every time.|
|Alternate stitching, 2" on top, 1" on bottom. More awkward with Sparcraft routed plywood ribs, probably works OK on the built up ones. PIA.|
|Inspection Rings and Tapes|
|Inspection rings on. Aircraft Spruce doilies work great.
Presoaked the stitching and reinforcing tapes per the Polyfiber manual, now it's tape time.
|I had some questions on taping, so I posted them
to the Oracle:
The 4" tape that goes over the leading edge, anybody got any suggestions on how to get it to form around the sweep center section?
Also manual says to do the lengthway (over the ribs) taping first and the crossway (like along the leading and trailing edge) last. With squared off wingtips, to the edgetapes go on last or at the same time as the lengthways? Same question for the ailerons.
|KKimball came back with:
Shrink the tape at the sweep. Put it in
place and shrink the excess that is at the edges where the sweep starts.
Easy to do. You can practice with a short piece of 4" held in place with
masking tape to get it right before you do the long one from tip to tip.
|OK, taping is all done. Have gone over every seam
and tape with the little iron to get it as smooth as possible before
As you can see I am back in the hangar. After a few nights of MEK induced hallucinations, I came home to find my family had thrown both my wings and my suitcases out onto the front lawn. You need a detached garage to get away with fabric work at home.
|Inside ribs got a second 3" tape on top of the 2".|
|Duh! Thought I had everything inside tightened before the fabric and turns out I forgot the aileron attach fittings on the lower wings that were a little loose. No need for another inspection cover, carved a 3/8" x 1/2" hole (just big enough for the end of my smallest 3/8" box end wrench) in the plywood end plate and reaching the nuts that way. Tiny fabric patch over the hole. Whew, that was easy.|
|Not wanting to exhaust the goodwill of friends to help me transport the wings back and forth to the paint shop, I strapped down a truss made up of a pair of 2"x6"x12' onto the shortbox of my Ridgeline. Couple of foamies and straps and I was in business|
|I didn't think the wings could eat up all 3 gallons of Poly-Brush plus 5 Gallons of Poly-Spray, but with all the cross-coats they did. Punched in the drain holes on the bottom trailing edges with a soldering iron. Then 1 coat of Primer Sealer (about 3/4 gal). Spray the white first (1/2 gal), then mask off the pattern, and two coats of topcoat (1-1/2 gal) .|
|Voila! All done, now let's get back to flying.|
Updated July 4, 2009