Fournier RF4D DIY rebuild: Part Four
PUBLISHED: 17:09 16 August 2019 | UPDATED: 17:18 16 August 2019
Time to wing it… Part 4: re-covering the wing | Words and photos: Bob Grimstead
After re-covering Wagon's fuselage, I set about the wing. My good friend John and I had dissolved off the excess old Super Seam Cement with MEK, so it only remained to sand it all over, paint on two coats of varnish, let it dry fully, and then scuff it with fine Scotchbrite pads to provide a good key. I decided to start with what seemed the most difficult and complicated areas−the concave aileron cut-outs.
The work was fiddly and time consuming but I felt the Oratex went on pretty well, adhering nicely with my small modeller's iron, despite being on a multi-faceted and uneven concave surface. I added a second patch of fabric around each of the aileron hinge arms, for extra protection.
Anticipating the second most difficult bit would be fitting fabric around the compound-curved wingtips, I decided to tackle them next, before starting on the larger, more straightforward areas; I could always remove that wingtip fabric and start again if it didn't work out well. I wasn't too concerned about getting a superb finish because they would eventually be covered by my smoke pods.
I understood that the Oratex adhesive becomes tacky when heated with the hot air gun to about 80°C, and the fabric can be pulled around on it. Once pressure is applied (three kilos or seven pounds of pressure) together with more heat above 90°C by an iron, I presume the little microballoons filled with hardener in suspension in the glue get popped, hardening the glue and making it impossible to shift thereafter.
For my first attempt, I painted glue on two large squares of fabric (with adequate excess) and the varnished-balsa-and-ply left wingtip as far as the inboard edge of the first rib. Then I tacked down the central upper three square inches around the outer edge of the wingtip to give me some purchase to pull against.
After allowing that area of glue to cool and set, I used the heat gun to stretch it out from there. I wish I had a picture, but with the heat gun in one hand, the fabric in the other and the felt smoothing pad in my teeth, I had no way of taking a photo!
I heated and pulled, heated and pulled until the fabric was nice and smooth on the wing's upper surface (which was actually underneath my inverted wing on its stand). It was a bit of a struggle but I did get the material fairly flat there.
But I could not stretch the fabric enough to remove lots of wrinkles on the very visible compound-curved lower surface. So I increased the heat gun's temperature in an attempt to shrink the residual wrinkles.
I texted a couple of photos to Paul Hendry-Smith at TLAC and asked his advice. He said, "Start on the top, heat it and stretch it and pull it down over the tip, and then shrink the wrinkles out of that. Use the silicone-coated paper to prevent it sticking to the glue until you want it to."
That was easier said than done! Holding the heat gun in one hand, the fabric in another, the felt smoothing pad in my 'third' hand and the silicone paper in my teeth, I knelt on one knee while bracing the wing from sliding across the floor with the other.
Luckily there was nobody within earshot, and swearing through silicone paper does rather muffle the sound! Eventually I got it all nicely smoothed out on the top surface and curved over three-quarters of one tip. Then, pulling from the mid-chord forwards and rearwards, I managed to get rid of most of the wrinkles, but was still stuck with one fairly big crease in mid-chord, a thin, wide one around the trailing-edge, and a right bunch of glued-together rubbish under the leading-edge. More photos to Paul and another phone call.
"Ah, no. If you're going to try to cover the whole tip with a single piece, you need to shape it first over the dry tip before applying glue. You have to heat it and stretch it into a sort of bucket shape around the wing-tip before painting on the glue and ironing," says Paul.
"I know the manual says it shrinks by up to eighteen per cent, but how much does it stretch?" I ask.
"Oh heaps. Maybe 25 per cent."
"What temperature do I use?"
"As much as you need."
My buddy John and I heaved and sweated, with me pulling the fabric using the best grip I could muster, and John running the heat gun back and forth around the tip, being careful because the manual cautions not to exceed 180°C over a wooden surface.
All we achieved was pushing the wing along the floor on its integral stand again, plus a slightly wrinkled sheet of Oratex! I gave up and took home the heat gun, a couple of fabric panels and some off-cut strips for experimentation.
I clamped a two-foot, one-inch wide strip in the vice, heated it and pulled. I held the heat gun against the fabric and ran it back and forth, starting at 100°C, increasing in 20°C increments, pulling hard. It remained static. I pulled harder and increased the heat.
The manual says Oratex's strength is degraded above 200°C, but it still wasn't stretching. At 350°C I finally smelled burning and it snapped in two. So now I know that, like most other fabrics, Oratex only stretches on the bias.
I realised I was being too ambitious for a novice. Next day I neatened up the right tip, trimmed the excess fabric around its edges, and covered the left tip using two pieces of fabric. This worked better and, after gluing down the folds and cleaning up the area, it looked more or less acceptable after final trimming.
I am neither artist nor artisan, simply a pilot who wanted to fly my Fourniers, something I had been unable to do for more than two years. My priority was to get Wagon flying again as soon as possible. Making it lighter was of secondary importance, and 'prettiness' was way down the list.
However, what I learned was: you can have the same difficulties with any make of fabric; proper tuition and more experience would have enabled me to do a much better job (I've seen examples of wingtips that have been perfectly covered with a single Oratex panel by more expert fabric workers); and I should have left the most challenging areas to the end.
Also, the silicone paper was only used to protect the fabric's painted surface from discolouration, so I knocked up covers for both my irons from offcuts of an old flannelette cotton bed sheet.
With both wingtips now adequately, if not beautifully, completed, I moved on to the much easier stage: covering the large, flat areas of wing between tip and root. I started with the underside again, as its appearance wasn't as important as the upper surface, which needed cut-outs for the spoilers and might be tricky.
The Oratex application manual says to drape a sheet of fabric over the area it is going onto, peg or clip it into place, and then go around the other side to mark with a soft pencil where the solid parts of the airframe will attach to the fabric, so you can apply the adhesive there and only there, to avoid both waste and unsightly areas of excess adhesive through the thin, semi-translucent material.
Working alone, I found this too easier said than done in Dunsfold's big, airy and well-lit Hangar 57, which is also pretty breezy when someone opens one of the doors.
I had intended using one large, un-seamed sheet per wing panel with the weave parallel to the chord line for neatness−making four big sheets of the sheer, glossy, slippery fine-weave Oratex. Unfortunately, I found that however carefully I secured the sail-like eighteen-feet by five-feet sheet to the wing's undersurface it would try to slip off all the time I was securing it, and then flap in the breeze even once pegged into place.
Fitting the sheet around the necessary cut-outs for the aileron pushrods, pitot tube and outrigger legs, and then shrinking it without these holes shifting relative to one another, as well as accurately joining it with the existing wingtips, would be beyond my ability.
Taking a break, I realised that, with my Fournier's efficient tapered wing, it was going to be very difficult to ensure the weave was always aligned spanwise. But, except up close, it wouldn't be possible to tell in which direction that weave actually ran, so I reassessed my use of this big roll of fabric.
Rather than try to cover a whole upper or lower wing panel at once with one eighteen-foot sheet, I decided to use several smaller pieces that would be easier to handle solo, joining them chord-wise with the recommended minimum two-inch overlap.
There would now be four pairs of symmetrical components: the tips, the aileron area between ribs one and nine, the central spoiler and outrigger area, and the inner wing panels, plus a single underside centre-section piece, making nine much smaller sheets in all.
This used the expensive fabric more efficiently. The total length for both the wing and the fuselage (excluding all tail units and ailerons which had recently been re-covered) was 52 feet of the six-foot wide fabric.
Per Oratex's instructions, I wrapped these panels from the trailing edge all the way under and over the wing via the leading edge and back to the trailing edge again, with at least a two inch overlap underneath, where it shouldn't be too obvious. This left me with a twenty-foot roll of spare fabric. Was I dismayed at this waste of money? Well, being me, yes at first I was. But then I had a brainwave: it was more than enough to re-cover the old, cracked and unsightly paint on the fuselage of my Australian RF4D!
The routine for applying these sheets was much more straightforward than the difficult areas I had tackled first. Mark on the underside of the fabric the areas on which the adhesive will be painted; paint a couple of coats of adhesive on to both the marked areas and the corresponding wing framework; allow them to dry. In very low ambient temperatures this drying can be slow.
Heat doesn't help, but airflow does, so a fan or fan-heater set to 'cold' or 'cool' can greatly assist. Mine took half-an-hour to dry with a cool (50°C) heat-gun in an ambient temp of about 8°C. Then, holding the fabric carefully in place and using a small modeller's iron set to 50°C, press down briefly in a few spots along one edge to tack it in place while pulling it as taut as possible.
Oratex doesn't shrink very much, so the tighter you get it initially the better the eventual finish. Once everything is square and true, and wrinkle-free, you can run the bigger Toko iron set to 100°C slowly all around the remaining edges, again pulling the fabric lengthwise and working along carefully to get out any bubbles, while ensuring the iron is over every bit of structure for the required ten seconds, and pressing down firmly to pop all the little hardener microballoons in the adhesive. It took me about forty minutes for the first piece, but less than thirty for the second and subsequent panels.
Once the glue has gone off trim away the surplus. That's it−no painting of any sort is required and the website suggests waiting a day before going flying. Go home and drink copious volumes of good Australian Shiraz to celebrate a job well done!
Where panels required holes for the pitot probe, outrigger legs, stall warner, air vents, aileron hinges and operating arms, I found it best to melt the hole first with a soldering iron and then position the panel correctly over the projection before tacking down that area and ironing outwards from it.
Melting the hole rather than cutting it seals the edges so the fabric doesn't subsequently rip along the warp or weft. Trailing edge drain holes, plus those under the spoiler boxes were easily burned this way. For the larger holes, like those for the inspection panels, I made a series of radial cuts with my tiny soldering iron.
For the larger apertures for the spoiler boxes, I tucked tongues of the fabric down into the cut-outs and glued them down with my small, modeller's iron. Where there wasn't even enough space for that, I used my Dad's big, old wartime soldering iron.
On some fabric-covered types, chord-wise reinforcing tapes are often installed over the ribs to cover and smooth the stitching or lacing, but because Fourniers have no lacing, they don't need these tapes.
In the end it took me a couple of weeks of part-time working to cover my Fournier's wing, generally completing one bay per day. Some days it was simply too cold to work; either below Oratex's minimum OAT of five degrees Celsius or their recommended fifteen degree minimum because 'of the difficulty of working with cold hands and fingers'. Amen to that brother! If the weather hadn't been so cold, damp and depressing I could definitely have covered the whole wing in a week.
With the wing completely covered from tip to tip with the white Oratex, now came the quick, fun and rewarding bit: shrinking it. I insulated the glued-down areas with strips of corrugated cardboard, cut from old boxes and held in place with masking tape, so that this new heat didn't adversely affect the adhesive.
Then, heat gun set between 120°C and 150°C, I made a succession of slow passes left and right and then up and down across each unsupported panel to ensure as much as possible that all threads were equally shrunk. Finally I ran the iron over everything at a slightly higher temperature, but not applying any pressure.
To ensure the fabric was at the correct tension, I bounced a ten pence coin over every piece of tautened fabric. Allowing for the diminution of bay size towards the wingtips and consequent increase in pitch of the drum-like sound thus generated, I tried to ensure that all panels were similarly tensioned by this simple audio test, slightly tightening by further heating any area that sounded 'flat'.
Evelyn Glennie might not have approved of the timpani-like scales I eventually achieved, but they were music to my ears.
Finally, following Oratex's instructions, I pushed a carefully calibrated finger into the centre of each panel with three kilogrammes of force (tested on my kitchen scales). If the consequent dent recovers to a smooth, drum-taut surface within four seconds the fabric tension is correct. This confirmed my earlier 'musical' assessment, and so suddenly the job was done.
Re-fitting the ailerons is much easier with the wing inverted, so we turned it over again and I fiddled around all day in the very tight gaps for the pivot bolts and their nuts.
Eventually they were mounted and I could connect their operating pushrods and ensure they could move fully and freely. One final inversion of the wing to upright on my trailer and we were able to tow it back to my hangar.
Now all I had to do was finish off a few minor ten-minute jobs and reassemble my aeroplane. That shouldn't take long...