This article by Dudley Pattison is from Light Aviation, the magazine of Light Aircraft Association, the British equivalent to EAA, RAA [Canada] RSA [France] and so on. The Flitzer is a biplane of all wood construction including the cowl over the cockpit where plywood is curved over laminated bows. The first part is wasted on KR builders but the latter section about applying aluminium to the plywood may be of help to KR builders suffering from Mustang envy.
?I described my method of fixing heavier duty aluminium panels to a wooden structure with the use of 'nutserts'. Here I want to describe the preparatory work I do in readiness for affixing a very lightweight aluminium skin to a structure. The non-modellers among you may not know the glass cloth and epoxy form of skinning. With a little care and a fair bit of elbow grease, a very good, totally weatherproof, light and strong surface finish can be achieved. I covered the forward upper decking area of the Flitzer ZIR using this method as, later, aluminium litho plate was to be stuck to this area for aesthetic reasons. To get a really good finish you could see three coats being required with a wet and dry rub between. For my purposes. only the fact that the skinning will tougher and seal the plywood surface is important, so two coats would suffice. lt is also an ideal preparation for a high quality paint finish on ply or wooden surfaces. *THE METHOD* 1] Lay the glass cloth over the area to be covered. used 0.6 ounce per square yard. Don't worry. if all you can get is 0.9 or 1 oz as it will end up at about the same weight. The lighter cloths are more open weaves so a little more epoxy is required to fill it brings the finished job to about the same weight. 2] Mix lay-up epoxy resin [not the five minute variety] at the correct mix ratio and pour a little in the centre of the cloth. Using an old credit-card as a squeegee, spread the epoxy in all directions out from the centre pool. All whiteness will disappear from the cloth as it turns transparent, indicating that it has been wetted right through. If you have any ?nutserts? take care not to fill them with epoxy. Do not try to mix enough epoxy to do the job in one go; mixed epoxy left in quantity gets fed up with waiting and exotherms meaning it get super-hot and quickly sets as it boils! 3] Pay particular attention to the edges of the job, ensuring that the cloth is wetted all the way to the edge - a little beyond doesn't hurt. 4] Check the area all over for dry spots and, more importantly, 'over wet' areas. The wet areas show up as being very glossy and are an unnecessary weight addition. Keep squeegeeing them, scraping the excess epoxy off the edge of the squeegee back into the mixing pot. 5] After allowing it to cure, trim the edges of the cloth with a sharp craft knife. A sanding block will remove excess epoxy at the edges. Sand the area all over, removing the gloss finish. Any runs can be scraped down with a sharp wide chisel held at 90. to the surface, or use a proper scraper if you have one. 6] Wipe down with a tack cloth to remove dust. Make another mix of epoxy and apply another thin coat all over. Much less will be required for this coat as the weave of the cloth is already nearly full and none will be soaking into the plywood surface. A very thin coat should give a gloss finish this time. When cured, flat down and remove dust. Voila! You have a sealed surface that will accept a contact adhesive to fix the litho plate aluminium or a two-pack hi-build primer surface for your paint job. A smallish amount at a time, which is poured out on the job quickly so as not to allow the possibility of excess heat build-up, is the way to go. To do my front decking I made four mixes. This could have been cut to three if my quantity estimation had been better. I used this method of applying aluminum litho plate for my Isaac's Fury. Thinking back, however, I realise that it wasn't Iitho plate on the Fury, but thin commercial grade aluminium. This proved to be the wrong decision as the aluminium had a surface 'grain' that took many hours of sanding and polishing to remove, whereas with litho plate a polished finish is readily achieved with a small polish and far less effort. *What is litho plate* Litho plate is the thin [0.3mm] aluminium sheet that comes in drawing paper size ? A3, A2. A1, A0 and so on. Offset litho print shops use it as part of the print process. Normally, when the print run is finished the litho plate is considered scrap and can often be procured at no charge - one more benefit So, the second-hand plate comes to you etched on one side with whatever the printer was producing and with a clean aluminium surface on the other. The plate is very light and of a grade that is maleable which is why aeromodlers started using it years ago. I use 3M Scotch-Weld 10 after reading that it was the choice of car homebuilders for sticking aluminium to plywood to make their vintage-style car bodies look more authentic. *APPLYING LITHO PLATE* Some thought needs to be given as to how you are going to accurately reposition the plate after it has been cut (which is easily done with scissors) and the adhesive applied As most readers will know, contact adhesive means just that: when it touches it sticks. There is no second chance, especially with something as fragile as 0.3mm aluminium. The system I developed involves drilling a couple of 1.5 mm diameter holes in strategic positions through the plate and into the plywood and structure when the plate is where you want and nicely taped down. Then draw your perimeter cut line, after which you can remove he plate. Open out the holes in the plate to say 2mm diameter and drive 1.5mm pins into the Two holes in the structure, leaving them standing up by say 25mm. Pop rivet stems are perfect for this. After nailing together a Groppo Trail recently I have a few thousand of those! So now you have a register that will allow you to put the plate back into position, theoretically to within a quater of a millimetre - good enough for most folk methinks. The plate may now be cut but. again, do stop and think about it before diving in with the scissors. The chances are that you will want to 'burnish' the edge of the plate over the edge of the structure which you will have put a small radius on (won't you). lt wouldn't be good to leave a flat edge that would be prone to lifting every time you caught it. I turn an edge about 3mnm wide using a hardwood dowel, the round plastic handle of a screwdriver or maybe even a steel dowel (which could be a screwdriver blade), Now, with the register dowels (rivet stems) in place, the plate cut to finished size ? plus edges to be 'burnished' over - and with any hatch areas also cut out of it to the finished size, the plate and the epoxied area it is to be affixed to may be cleaned with cellulose thinners or a proprietary spray cleaner, and then the adhesive applied. Do your best to avoid lumps in the adhesive film, which is applied to both surfaces, as anything under the aluminium will telegraph its presence through to the surface. *CAREFUL POSITIONING* Now you need to take your courage in both hands and carefully locate the aluminium component over the register dowels and drop into position, A Haynes manual at this stage would probably say'simply drop into position'. ln fact, you need extreme care here and to drop the forward upper decking section on to the Flitzer fuselage I enlisted the aid of Jane, my long-suffering wife, and Martin Sims, who now helps me organise the homebuilder tent at the Rally (serves him right for volunteering, I say). So, with Jane on one side and Martin on the other, the piece was carefully located on to the two pins I had positioned on the centreline, while ensuring that the panel didn't sag in the middle and prematurely make contact. Then it is a question of lowering the central point to make contact and gently running a hand along the 'ridge' line of the decking before wiping down each side. At this stage a wallpaper roller was used to firmly fix the alumrnium. Finally, burnish any edges over. lt is a good idea to practice this first on a piece of scrap litho. lt doesn't need gluing. For a very small weight penalty the above system can give an aircraft a really authentic look, so it is well worth the extra effort.? Bill Weir
