Fournier RF4D DIY rebuild: Part Three
PUBLISHED: 17:10 25 June 2019 | UPDATED: 17:10 25 June 2019
'Putting a cover on my Wagon' - Fournier Resurrection Part 3: re-covering the fuselage | Words and photos: Bob Grimstead
I had wanted to replace the ageing fabric and shabby paintwork on my wooden Fournier RF4D 'Wagon's fuselage. I specified and collected fabric and paint of the Polyfiber system from Aircraft Coverings at Henstridge because it's flameproof, stays flexible for a very long time, and that's what I have on my Australian Fournier and Maule.
Now the materials had disappeared (see Part 1) so, having repaired and prepared the Fournier's airframe, I repeated the selection process… but this time I would be doing the work myself.
I don't have a spray gun, compressor or booth, nor the appropriate skills for spray painting, and I was concerned about achieving a reasonable finish. I had also been pondering on the necessary elasticity of the paint coating for the flexible fabric it was going onto; many aircraft restorers use two-pack polyurethane paints now and I didn't know if they would be sufficiently pliable.
Importantly, most of today's paint systems are classified as carcinogenic, so I would need protective clothing and breathing apparatus.
Then I learned about Oratex. I had vaguely heard of it before as 'a German aircraft fabric using water-based adhesive' but, being aware of Ceconite's previously unsuccessful flirtation with water-based glues and paints, had dismissed Oratex from my mind. Now I heard something much more attractive about it: it comes ready-painted!
Paul Hendry-Smith of The Light Aeroplane Company (TLAC) was the British importer of this innovative product. I phoned him for more details. He said a dozen or more British aircraft had already been finished with Oratex (this was 2015) including a big Robin and a Pitts Special, which ought to testify to its strength, and insisted this fabric would be within my unskilled ability to apply.
He told me Oratex was a polyester fabric, but its much finer filaments, threads and weave made it significantly lighter than Polyfiber, Ceconite or Diatex. He referred to this as 'nano technology'. He confirmed it was ready-painted with seven layers of coatings: fabric sealant, primer, undercoat, three colour coats, and a urethane clear top coat, each coating having a combination of UV absorbers and blockers.
Oratex also supplies Oracolour flexible paint which contains no plasticisers, so an aircraft can have a multi-colour scheme, and self-adhesive trim tapes of many shades (brand names Oraline for narrow tape and Oratrim for wider tape).
Today there are eleven primary fabric colour options. Paul did admit that it was expensive, but insisted that it was no more than the cost of a similar area of other polyester fabrics plus the required half-dozen or more layers of paint.
He sent me samples of their white and red fabrics: Insignia White is a nice bright, titanium white, and by chance Fokker Red was precisely the same shade as my Fournier's existing paint. Paul had cautioned me that the finish is not high gloss. The paint is actually quite shiny, but the layers are so thin that the very fine weave shows through imparting a satin-like sheen.
There being no substitute for hands-on experience, my friend John Watkins and I drove up to TLAC at Little Snoring for some practical instruction. Paul himself gave us a four-hour tutorial including some hands-on work, explaining the process step-by-step to re-cover a previously covered airframe.
The application of odourless water-based adhesive is pretty simple. Ironing the fabric in place to activate the glue was just as easy. Shrinking it with a heat gun was a quick pleasure, although we learned that Oratex doesn't shrink as much as Ceconite, Polyfiber or Diatex, so compound curves and corners can be a bit more difficult to cover without wrinkles.
Paul showed us how to remove wrinkles and folds, saying, "Never try to shrink a wrinkle, shrink the area around it. Heat the glue to soften it and then shrink the fabric beside the blemish to pull it flat." He showed us how the fabric can be warmed and stretched around the curves of fins and rudders, recounting that Oratex inventor Siegfried can cover an entire electric light bulb in the stuff without it wrinkling!
The good thing was that I've never been interested in having a superb finish; what I wanted was an aeroplane that didn't look as scruffy as when I bought it, and back in the air as soon as possible.
I soon realised the only significant disadvantage of Oratex is the price of everything. To do the job properly Paul insisted I needed a Steinel German digital heat gun and a Japanese Toko digital iron, because of their very accurate temperature control. Even the felt pad he recommended for pressing the fabric down into place after heating the glue was pricey.
The paint only comes in one litre cans and they cost over £100 apiece! Eventually I came to understand that 'high price' doesn't always mean 'poor value'; indeed I saved so much time and labour expense with Oratex that I've become somewhat of an evangelist for it. Paul still thinks I'm a skinflint though.
Wagon operates on an EASA Permit to Fly, administered by the LAA, so I informed their Chief Engineer Francis Donaldson while the Oratex company reviewed my paperwork submission. Their main concern seemed to be the biggest area of unsupported fabric.
There was apparently a maximum, and the trapezoidal-shaped underside of the second bay out from the wing root had the greatest unsupported area.
Not being any kind of mathematician I simply sent them the drawings so they could make their own calculations, but since all their technical info suggested that Oratex is stronger in warp, weft, tear resistance and burst strength, and is held in place by stronger adhesive than Ceconite, Polyfiber or Diatex, and knowing that Wagon was stripped of thirty-odd year-old Ceconite, I couldn't see how there could possibly be a problem.
I already knew Francis was enthusiastic from his comments in the LAA's magazine, and had presumed that, since the comparatively slow and low wing-loading Fourniers meet all of Oratex's specifications, I would merely have to buy the stuff and use it, as I would with other fabrics. However, the wording of Francis's reply suggested that I had needed his specific approval for its use on my aeroplane, although that was quickly forthcoming.
Presumably, once mine was successfully flying, Oratex would be deemed OK for any Permit Fournier, but I can't say for sure. I believe that everybody will be using it in ten years because the only special equipment required are that iron and heat gun, which most aircraft restorers should already have. Time will tell.
The fabric comes in rolls I measured at 1.854 metres (just over six feet) wide, then costing £63.42 per linear metre. I thought I would need ten metres of white fabric to cover the top and bottom surfaces of my five metre long fuselage, but when I measured around it from top to bottom it seemed the fabric only had to be a maximum of 1.3m wide and a minimum of around sixty centimetres.
By sketching it out, I decided that by 'topping and tailing' tapered fabric sheets I should actually be able to get both fuselage sides out of just the one five metre piece. I did in fact achieve this although the flat, square area under the rear of the cockpit (which fits immediately over the wing's centre-section) needed an additional bit a little less than one metre square which came from a wing offcut.
With a wing of nearly twelve metres span, both surfaces of which needed covering, I guessed I would need 24 metres more for that, plus a few extra metres of red so I could have a double layer to protect the wing's leading edges. I also planned to use red fabric for the wing's upper sunburst stripes.
Paul suggested that I would need two litres of adhesive, although in the end I actually used three. Then there were varnish, Scotchbrite scuffing pads, masking tape, sandpaper and a few other sundries, like a modeller's small digital iron for getting into the awkward tight corners that seemed to abound under the fuselage.
I transferred a significant amount of money to TLAC's account and tried to be patient, filling in the time by labelling all of Wagon's myriad metal fittings and hardware and sorting them out into individual boxes for the various sub-assemblies.
A few days later a sturdy, seven-foot-long cardboard tube and a surprisingly big but light box arrived. The tube of course contained my fabric, while the box turned out to hold mostly expanded polystyrene insulation and gel-filled 'freezer packs' protecting the plastic bottle of adhesive in the middle - not to keep the contents cold but rather to prevent the glue from freezing in the December weather.
In those winter conditions it seemed prudent to start with my fuselage in our heated garage, so with the help of another mate and neighbour, Alan Washington, I hoisted it on to trestles. First, dissolve off the old adhesive. That took a couple of eye-watering days with lashings of MEK and a heap of old cotton rags.
Then roughen the whole surface with 120 grit glasspaper. On my slim fuselage this took just ten minutes. Next varnish with one coat of exterior varnish (I used Ronseal Yacht Varnish) very slightly thinned (ten per cent) with white spirit. That took me half an hour.
The fuselage was tack-dry in a couple of hours in my heated garage, but to be on the safe side I waited two days before the next step, which was to sand it all over again lightly with 240 grit paper−another ten minutes' work.
Now apply a second, full-bodied coat of varnish being very careful not to get any runs, brush strokes or bristles on the surface, because the very slightest blemish shows through that twelve-micron thin fabric. Again I did other stuff (reassembling the main undercarriage) for two days while that hardened.
Finally, I scuffed the varnished surface with a maroon (fine) Scotchbrite pad to key it and provide good adhesion, followed by a thorough wipe over with a rag dampened with white spirit to get rid of any dust particles. Then Alan and I inverted the fuselage.
I used small pieces of fabric to cover the smaller, more complicated underside areas first, and there were lots of them. For the more complex curving shapes I made paper templates and copied them on to the fabric. I took time and trouble to fit them perfectly because I wanted the belly to be completely waterproof.
After stripping it, I had noticed that areas around the firewall, cockpit floor and undercarriage had only been painted, and some had shown signs of dampness, so I wanted to be sure that everything on Wagon's underside was properly covered with fabric and completely sealed.
Initially I resolved that not only would the seam along the upper spine have the required five-centimetre overlap, but the opposite edges of those triangular sheets would overlap under the belly as much as possible to give a double layer of protection.
Then I thought 'What the heck, forget the weight' and, having finished all the fiddly little under-belly bits (several days' work), I decided to cover the whole rear fuselage underside in one continuous sheet of fabric before gluing on the two side pieces, thus giving triple protection.
The upper tail area ahead of the fin and under the tailplane required complex shapes to be covered with multiple small pieces of fabric. The stern plate was easier though, merely requiring a sort of pinking around its edges to conform to the fuselage's curves. I did the same with the front and rear faces of the cockpit.
In view of the original paintwork damage, I also put double layers of fabric around the cockpit sides and edges, and an extra triangular piece just behind the cockpit on the left side, where abrasion occurs from pilots leaning into the cockpit.
Notwithstanding Oratex's claim to be fuel-proof, I also fitted two layers of fabric around the fuel tank ahead of the cockpit, with an additional ring of protecting fabric just around the filler. That whole area had previously been stained and chipped from over forty years of fuel nozzles and I could learn from that.
Once all these small areas were done, the final, overall covering was much less fiddly. It took me twenty minutes to paint the milky-looking water-based adhesive all over the fuselage, with no mess and absolutely no smell.
The main challenge was to brush out any tiny bubbles that might have been left before the adhesive dried out because any blemish is obvious under the thin fabric. I also had to paint a thin, even layer of the adhesive onto the fabric's back, doing this on a wallpapering table my wife Karen made in her DIY night-school course.
The last, big part was easiest of all. Turning the fuselage on its side, I laid the fabric across it, held down with a couple of old gel-cell batteries. Starting from the middle and working carefully and systematically both outwards and lengthwise I ironed on the fabric with one hand while pulling it taught with the other.
"Hey Al, can you come down and give me a hand again turning over the fuselage?"
"We just did that. I've only just got home."
"Yes, but now I'm doing the other side."
"Blimey, that was quick!"
"It was, wasn't it?"
So I did it all again on that side and, hey presto, I had what I'd wanted from the start: a fabric-covered fuselage!