Fournier res­ur­rec­tion

Part two: learn­ing about wood­work repairs, ap­ply­ing new skills − and mak­ing a shock­ing discovery

Pilot - - TECH LOG: FOURNIER RESTORATIO­N PART TWO - Words & pho­tos: Bob Grim­stead

After con­sid­er­able fi­nan­cial and le­gal dif­fi­cul­ties I – no kind of en­gi­neer but merely a re­tired air­line pi­lot – faced the prospect of re­pair­ing, re­assem­bling and re-cov­er­ing my beloved but bare and com­pletely dis­man­tled Fournier RF4D G-AWGN, Wagon.

Hav­ing re­trieved Wagon from its four­teen-month se­ques­tra­tion, its wing was now safely stored on my newly­ac­quired ‘£10,000 trailer’ in my Poly­tun­nel hangar, while the fuse­lage, en­gine, canopy, tail feath­ers and a heap of other, smaller com­po­nents vir­tu­ally filled my heated dou­ble garage.

First I needed to take stock of what I had. Out came the Main­te­nance Man­ual – a sparse pub­li­ca­tion, lit­tle more than a book­let, re­ally – plus the more com­pre­hen­sive bilin­gual French/ger­man (but un­for­tu­nately not English) Parts List. Who knew that such a lit­tle aero­plane would have so many parts? Well, of course, ac­tu­ally I did, but it was sober­ing to see all those many bits and pieces laid out on a bench top. Re­gret­tably I didn’t think to take a pho­to­graph.

After much cross-ref­er­enc­ing, I was pleased to dis­cover that, al­though my poor lit­tle aero­plane had been stripped down to its last nut and bolt, it seemed that nearly ev­ery­thing was still there. Un­for­tu­nately, all the ex­pen­sive fab­ric and paint I had bought had dis­ap­peared, and so had sev­eral vir­tu­ally ir­re­place­able old-style met­ric bolts. Also noth­ing was tagged or bagged. I ex­pected that other things might have gone astray too, but I ac­tu­ally felt op­ti­mistic that Wagon would fly again.

Learn­ing about wood­work­ing tech­niques

The next thing was clearly to get proper in­struc­tion in air­craft wood­work­ing tech­niques. Se­rial home­builder, for­mer pro­pri­etor of Swin­don Air­craft Tim­ber Com­pany and all-round great guy, Dud­ley Pat­ti­son runs in­ex­pen­sive one-day cour­ses un­der the aus­pices of Bri­tain’s af­ford­able-fly­ing or­gan­i­sa­tion the Light Air­craft As­so­ci­a­tion (LAA), so I en­rolled in one of these. I found it in­valu­able and, with newly-in­spired con­fi­dence, I started plan­ning the nec­es­sary repairs. Then I called my LAA in­spec­tor to come over and have a good look at ev­ery­thing and dis­cuss a way forward. He was very pos­i­tive and said it all should be within my ca­pa­bil­ity and would be ‘just a mat­ter of time’.

Call me con­ceited − ac­tu­ally, I am con­ceited−but I have al­ways

reck­oned that, given enough re­search and a lit­tle prac­tice, I could prob­a­bly un­der­take most man­ual tasks. So, al­though air­craft wood­work­ing was alien to me, I was pretty sure that, given my painstak­ing (some might say ‘OCD’) na­ture, I could make a rea­son­able hash of Wagon’s nec­es­sary air­frame repairs. Turn­ing over the fuse­lage and lift­ing it on to tres­tles en­abled me to view the dam­age and as­sess the chal­lenge.

One pos­i­tive thing was that Pat and Bobby, a cou­ple of trust­wor­thy and com­pe­tent em­ploy­ees, had re­placed the cock­pit floor. And they had done it prop­erly, us­ing ply­wood, Aero­dux ad­he­sive and spruce I had pur­chased my­self from Dud­ley and By­gone Avi­a­tion’s ever-help­ful Matt Petit.

They had also started, but un­for­tu­nately not com­pleted, re­plac­ing the tail­wheel block in the rear­most lower fuse­lage bay. Again, what had been done seemed to have been done well, but it wasn’t the cor­rect ex­ter­nal shape nor had the four holes been drilled to take the tail­wheel pivot bear­ing holder.

At first I was at a loss as to how to drill those four long holes ex­actly par­al­lel, so that their bolts would fit the back­ing plate in­side when the ex­te­rior of that tail­wheel block was now semi­cir­cu­lar. After a while I re­alised the best way to get those bolt holes in the right place was to push the bear­ing holder into its large cir­cu­lar socket and use it as a tem­plate to drill the smaller holes. But how to en­sure these holes were ex­actly par­al­lel all the way through? It would be easy for a drill bit to wan­der off line. And where could I find such long three- and four-mil­lime­tre bits? The lat­ter ques­tion was the eas­ier to solve. As al­ways, Drill Ser­vice Hor­ley came up trumps with three long bits of the re­quired di­am­e­ters, mailed and re­ceived the fol­low­ing morn­ing.

I even­tu­ally made a three-inch square wooden block, traced on it

the tail­wheel fit­ting’s back-plate com­plete with holes, and drilled through this block with my pil­lar drill. I then pre­cisely po­si­tioned this block on the tail­wheel fit­ting and drilled through the whole lot with each of those long bits in suc­ces­sion to make the holes. To my de­light, when I of­fered up the back-plate in­side the fuse­lage it fit­ted per­fectly!

Now I had to shape the out­side of this block and the stern plate to con­form with the fuse­lage’s con­vex belly, but mi­nus the two-mil­lime­tre thick­ness of ply skin that I would next be glu­ing onto it. I started this process by of­fer­ing up the fin and bolt­ing it into place to en­sure that the fi­nal sur­face of the fab­ric-cov­ered fuse­lage would align with it. Then, us­ing a three-foot length of rigid Dex­ion rack­ing as a straight­edge with glass­pa­per at­tached to it, I sanded back and forth over the block, cen­tral spruce fillet and stern plate un­til they nearly con­formed to the re­quired shape. Fi­nally, un­able to find my en­gi­neer­ing blue, I smoth­ered one side with felt-tipped pen ink to re­veal the high spots, gen­tly sand­ing them down un­til I had the shape ab­so­lutely per­fect.

I had to skin this re­shaped area with suit­ably-curved air­craftqual­ity Fin­nish birch ply­wood. I would have used the re­mains of that ply sheet I had bought from Dud­ley, but was told it had been burned, like all the other wood I’d orig­i­nally sup­plied. But then I had a bet­ter idea. Why use new ply­wood of un­known mois­ture con­tent, when in my garage was some good Fin­nish ply of the same age as Wagon, and thus sim­i­larly well sea­soned? Some years pre­vi­ously I had been given the sawn-up re­mains of F-BMKC, the French-built, third pre-pro­duc­tion RF4. Al­though the ac­tual tail skin sec­tion that ’GN needed had al­ready been cut out of this, the wider piece im­me­di­ately ahead of that was still there. Made from the right thick­ness of ply­wood, with its grain cor­rectly aligned, it was even al­most suf­fi­ciently curved.

So I cut it out and care­fully chis­elled away most of the fir in­ter­nal struc­ture. Next, I used sand­ing flap-wheels and a Dremel tool to grind away the re­main­ing frag­ments even

more cau­tiously. Ob­vi­ously this lib­er­ated a lot of dust, in­clud­ing a good pro­por­tion of re­sor­ci­nol glue pow­der, so face-mask and gloves were es­sen­tial.

Hav­ing cleaned out the in­te­rior of this old tail skin, I needed to strip off the fifty-year-old ex­te­rior cot­ton fab­ric and its em­bed­ded ni­trate or bu­tyrate dope. I didn’t like the idea of us­ing paintstrip­per be­cause I couldn’t be cer­tain of wash­ing off all the residue, and the con­sen­sus of opin­ion on the very in­for­ma­tive Amer­i­can Fournier fo­rum was to use a heat gun. Un­for­tu­nately I didn’t have a heat gun so−shhh, don’t tell my wife−i used her hair drier. It doesn’t get as hot as a heat gun, so it took longer than I hoped. Also, after an hour it stopped work­ing (shhh again) so I sub­sti­tuted the spare one from our guest room.

Now I had a clean, curved piece of ply­wood, the right size but only half as tightly bent as I needed.

Paul Hendry-smith of The Light Aero­plane Com­pany told me they made their Sher­wood Rangers’ D-sec­tion lead­ing edges by soak­ing ply sheets in hot wa­ter overnight, and then leav­ing them in an ap­pro­pri­ately curved jig for a day to dry out to the cor­rect shape. So I ran half a bath of hot wa­ter, put a length of three-inch alu­minium tub­ing in­side the skin, wrapped it around with half-adozen strong elas­tic bands, and left this as­sem­blage in the bath overnight.

In the morn­ing it was more curved, but not yet ideal, so I

tight­ened the elas­tic bands and changed the wa­ter, adding ex­tra boil­ing wa­ter from a ket­tle and sev­eral saucepans. Four hours later it was vir­tu­ally per­fect. I put it in place on the up­turned fuse­lage, held in po­si­tion with bungee straps and one G cramp to make the sides par­al­lel, leav­ing it to adopt its fi­nal, cor­rect cur­va­ture while it dried out.

A cou­ple of days later I was able to re­move this now per­fectly-fit­ting curved ply sheet to make the re­quired scarfs (tapered glu­ing sur­faces) around three of its edges, plus the edges of the mat­ing ply­wood skin on the fuse­lage−not for­get­ting to al­low for the thick­ness of the com­pressed bead of ad­he­sive and al­low­ing an over­lap at the very tail. The ply’s scarfed edges were thin­ner than cig­a­rette pa­per and not much stronger, so to sup­port the sheet while sand­ing away at the scarfs I put it into a length of six-inch drain pipe. Once those scarfs were sat­is­fac­tory I var­nished the rest of its in­te­rior.

Fi­nally I mixed up some Aero­dux, painted it on to the top of the block, the spine and the scarfs, and around the ad­join­ing ar­eas of my new skin, put it in place, and sta­pled the whole thing in po­si­tion. I cov­ered it with a sheet of cling film to pre­vent any ex­truded ad­he­sive from con­tam­i­nat­ing any­thing and fin­ished off by re-fit­ting all the bungee straps to hold it all down while the glue set overnight. I sub­se­quently re­moved the clamps and sta­ples, trimmed off the back end and drilled drain holes to let out any mois­ture from this most vul­ner­a­ble part of the air­frame. Job done!

Now for the wing...

A wet Novem­ber mor­phed into a cold De­cem­ber and I got fed up pad­dling about in an inch of wa­ter with in­ter­mit­tent gales whistling through my wind­tun­nel hangar, clothes and body. Thanks to the gen­eros­ity of Air­field Man­ager David Mcal­is­ter I was able to shift my wing on its trailer into the big ex-hawker Hangar 57 on Dunsfold’s north side. It was dry, there were trans­par­ent roof pan­els and high­power lights, elec­tric­ity, toi­lets and, best of all, a café within walk­ing dis­tance.

I spent many days in that echo­ing hangar with my good friend John Watkins, clean­ing the wing of its thick layer of old Su­per Seam Ce­ment with ni­trile gloves, gal­lons of MEK sol­vent, a heap of rags and tons of el­bow-grease. Once fin­ished, I felt I was mak­ing progress and was very glad to get that eye-wa­ter­ing MEK smell out of my nos­trils.

It was in­ter­est­ing to find so much Su­per Seam Ce­ment plastered all over this wing. I guess it would nor­mally come off with the fab­ric, but I was told

“The fab­ric just fell off when cut along the trail­ing edge”. I can only pre­sume that the ce­ment wasn’t prop­erly sucked up through the fab­ric by MEK in the first place. Pro­fes­sional fab­ric work­ers might throw up their hands in hor­ror, and that cer­tainly isn’t the way it should have been done, but it lasted 31 years like that and in my own­er­ship I had flown more than 300 hours of vig­or­ous aer­o­bat­ics with­out any sign of the fab­ric de­tach­ing from the wing−which I guess goes to show how very cau­tious and con­ser­va­tive we all are in these mat­ters.

I made a cou­ple of small repairs where there had been stor­age dam­age, pushed out four ar­eas of con­vex oil-canned ply, and leached out some oil stain­ing on one lead­ing-edge with naph­tha and di­atoma­ceous earth. I also made a rudi­men­tary stand from four-by-two tim­ber on which to turn over the wing to clean off its un­der­side.

Once I was cer­tain all chem­i­cal residues had been cleaned off

and had thor­oughly in­spected its struc­ture inch-by-inch to en­sure it ex­hib­ited nei­ther long-stand­ing nor re­cent dam­age, I pre­pared to fix the ready-made ply­wood patches to cover iphone sized holes cut into the un­der­side of each wing’s struc­tural D-box for the new out­rig­ger leg sock­ets. It was then I came across some­thing shock­ing.

I was about to var­nish the in­sides of these plates prior to fit­ting them my­self, when their sig­nif­i­cantly lighter colour at­tracted my at­ten­tion. Then, as I stud­ied them more closely, a cou­ple more is­sues be­came ap­par­ent. The ply­wood skin here is 2mm thick, with the outer grains run­ning span­wise. The patches were also of 2mm ply with span­wise grains, but there the re­sem­blance ceased. The wing’s ply was clearly made up of five fine lam­i­nates glued to­gether with red­dish-brown re­sor­ci­nol, whereas these patches had only three plies and were not re­sor­ci­nol-glued. Model aero­planes were be­ing built in the work­shop at the time of this work, so I strongly sus­pected this ma­te­rial was not proper air­craft qual­ity ply­wood at all, but model aero­plane ply.

I was warned by an ex­pe­ri­enced aero model builder that they looked like obechi ply, which has very lit­tle strength, so I took them to the lo­cal model shop for con­fir­ma­tion. “Oh no, they’re not as good as that,” said the guy. “They’re what we call ‘light ply’. See that cen­tral layer? That’s balsa wood. It’s about as strong as wet card­board.” I’m not of­ten ren­dered speech­less, but that day I was!

A wooden wing’s lead­ing-edge D-box is of course vi­tal tor­sion-re­sist­ing struc­ture, and these holes had been cut im­me­di­ately out­board of rib No 9−co­in­cid­ing with the outer end of the spoil­ers’ torque tube, the aileron cut-out’s in­board end and the out­rig­ger leg at­tach­ments. This was where the pro­to­type wing broke dur­ing struc­tural test­ing (al­beit un­der a mas­sive 13.8g) so it was im­por­tant these plates and their ad­he­sion were at least as strong as the orig­i­nal struc­ture. This is not the thin alu­minium lead­ing-edge skin of a Cub or Champ’s strut­ted wing, which is merely a fair­ing, but the se­ri­ously im­por­tant tor­sion box of a can­tilever wing. I had no idea how much weaker those patches would have made my wings, but was re­ally glad that they had not al­ready been glued into place. Other­wise I would have had no clue that such a sub-stan­dard re­pair had oc­curred un­til I found my­self tum­bling earth­wards.

Fur­ther­more, the air­craft main­te­nance ‘bi­ble’ AC 43.13-1B stip­u­lates scarfs of at least 1:12 for struc­tural com­po­nents, so with 2mm thick ply these scarfs should be 24mm or more (one inch) wide. Mea­sure­ment with a vernier cal­liper soon re­vealed that they were much nar­rower than this, their gra­di­ents rang­ing be­tween 1:6.5 and 1:3.5, mean­ing that the finest scarf was only half the ta­per it should have been, and the worst was only a quar­ter. This meant there would be noth­ing like enough glu­ing area for safe ad­he­sion.

So I spent the evening con­vert­ing the edges of a cou­ple of larger pieces of proper air­craftqual­ity, five-lam­i­nate ply­wood from F-BMKC into fine dust with a block of wood and some 240 grade glass­pa­per. It was time-con­sum­ing but strangely sat­is­fy­ing, and very re­as­sur­ing to know things had now been done prop­erly.

I couldn’t just glue those patches onto the wing’s skin be­cause the clamp­ing weights might dis­tort the sur­round­ing ply­wood, re­duc­ing the glu­ing area, so I next had to make two over­lap­ping re­in­forc­ing squares of ply to be glued un­der the edges of those holes. Then I could fi­nally glue the patches in place, be­ing sure not to use ei­ther too lit­tle or too much glue so that the patches would set level with the ex­ist­ing skin. Hold­ing them down with a stack of old gel-cell bat­ter­ies, I left it all for 48 hours and was de­lighted to see that the end re­sult was ab­so­lutely flush with the wing’s orig­i­nal skin.

So now the air­frame was ready for its fab­ric cov­er­ing....

"That's balsa wood. It's about as strong as wet card­board"

FROM LEFT: skin donor sec­tion of F-BMKC along­side; drill guide block, spare bits act­ing as lo­ca­tion dow­els; and trial fit­ting of the tail­wheel pivot bear­ing

‘Wagon’s fuse­lage and other com­po­nents await at­ten­tion in Bob’s heated garage/work­shop

Us­ing a flap-wheel to grind away the re­main­ing frag­ments of a lower bulk­head from the ply ‘skin graft’

Shap­ing the tail­wheel mount­ing block, us­ing sand­pa­per at­tached to a length of Dex­ion

The base of the fin (white sec­tion) acts as a guide, al­low­ing for thick­ness of ply skin, fab­ric and paint

first overnight bend­ing in a bath of hot wa­ter around a three-inch alu­minium tube for­mer

Skin clamped in place with bungees to make it con­form pre­cisely to the fuse­lage as it dries out

Sup­port­ing the re­place­ment skin in a four-inch drain pipe to pre­vent dam­age to the pa­per-thick­ness scarf

ABOVE: the new skin in place, with the sta­ples yet to be re­moved

BE­LOW: Job done! The new rear un­der-fuse­lage skin in place

Inad­e­quately scarfed holes in the un­der­side of Wagon’s lead­ing-edge tor­sion box and the un­der­sized balsa ply­wood plate the ‘pro­fes­sional air­craft en­gi­neer’ had made to cover it up

BE­LOW: Bob us­ing a sand­ing block to make re­place­ment wing tor­sion-box plates of the cor­rect ma­te­rial and size, and with the cor­rect ta­per of scarfs

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