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Attaching the Mylar to the Airframe ----
This is straight forward and will seem pretty familiar to anyone who has used modern covering materials fron the R/C side of the local hobby shop. First though, a little prep work is in order. Since the Mylar has no adhesive on it, some sort of adhesive needs to be applied to the airframe. Before painting on the adhesive, the airframe should be sealed with dope or sanding sealer. One thinned coat, sanded smooth, should suffice.
We have found that various adhesives will secure the Mylar. All that is required is an adhesive that will dry then soften again when heat is applied. The adhesives used to attached standard iron film i.e. Balsaloc or Balsarite (fabric formulae) or SIG Stix-It are all suitable, as are many handyman contact cements, such as Evostick or Weldwood Contact Cement. Some glue sticks are also suitable so try your favourite adhesive. It may be OK.
Many of these products are far too thick and need to be thinned down before application. The adhesive should be thinned down to the consistency of dope. This will allow the adhesive to be brushed onto the structure without stringing. If the adhesive strings, it is still too thick. Just dilute with more solvent. You may find it easier and better if two thin coats of adhesive are applied rather one thicker coat. After coating the airframe check that there are no lumps in the adhesive before covering. you might want to lightly sand before attaching the Mylar. Apply the adhesive to all the areas that the Mylar will touch. This means leading edge, trailing edge, ribs, spars, gussets, everything. Once the adhesive is dry, covering can commence. Because the adhesive has been thinned, it will dry within minutes so there is no long wait before covering can commence. If one is using Mylar in a lightly loaded environment i.e. indoor, the aerosol spray-on adhesives can be used. The airframe can be coated with a sprayed on adhesive, the majority of the adhesive will be wasted on open structures but this is a way to apply a thin and light layer of adhesive. Once sprayed on the covering process is as above. If you go down this route cover up well to avoid the messy over spray! Covering the model Cut a piece of film slightly oversized. With the aluminised finish, in order to protect the finish, the aluminium should be on the inside rather than the outside. It is easy to recognise the aluminised surface as it is slightly shinier, if in doubt, rub gently with thinners to test. Lay the structure onto the film, rather than the film onto the structure. Although the heat shrinking will remove the wrinkles, try to get the film on as neatly as possible. Gentle finger pressure will insure that the film attaches to the structure. You may find it helpful to use a spot or two of fresh adhesive to tack the film in place before activating the dried adhesive with the iron. Re-heat the glue to make the Mylar attach to the structure. A small trim type covering iron is best, as it is very light and damaging the framework is less likely. Gently work around the edges of the structures heating the Mylar and softening the adhesive so that the film attaches. Once you are happy that the edges are firmly in place, trim back the surplus and seal the edges. |
Covering cont'd
With some more of the thinned down Evostick, paint around the edges where there will be any overlap and apply the other surface. Cut another piece of Mylar to size for the top surface. Lay this in place and attach as for the lower surface. Trim off the edges, wrap around and seal. Trimming is best achieved using a sharp new blade, older dull blades may well tear the Mylar. The thinner the Mylar the worse the issue so be careful, but the key is a sharp and clean blade. If care is taken the surplus can be removed with sandpaper, this method needs practice to get a good result.
Now, slowly and carefully, working both top and bottom surfaces, use the iron to melt the adhesive on the rest of the structure and attach the film. Once all the structure has been attached to the film, shrinking can take place. Slowly and carefully shrink the film into position, ironing the wrinkles out to the edge. The adhesive will melt and the film will move to get rid of any wrinkles around the edges.
One of the problems you will observe is the air expanding inside the structure and causing the film to "balloon". You can either ventilate by making pin holes in the covering or put up with the problem because as the film slowly shrinks into place, the "ballooning" will reduce. Beware though that these pin holes give an opportunity at a later date for tears to take place.
Perhaps the best solution is to "vent" each rib bay with small holes in the ribs. A last small hole in the center sheeting will allow the wing to breathe.
A few points:- 1. Any joints you are covering, overlap a reasonable amount, at least 1/8". 2. Undercamber is not a problem. Just make sure all the structure has adhesive on it, and carefully attach the film to the ribs. 3. The heavier grades of film shrink less than the lighter grades. 4. Make sure that all edges and joints are fully sealed. If opened to the air flow, the air can get in and cause the film to lift away from the structure. 5. Clean the finished covering and remove any adhesive that is on the surface with a small amount of solvent. Not only will the covering look better it will also prevent dirt attaching itself to the model when it is out in the fields. Before heat shrinking the film, a quick test will find if your iron turned up to maximum heat will melt the Mylar. Howver, be aware that if attaching aluminised film to carbon, heat can build up in the structure and spoil the aluminised finish. Also keep the shoe of the iron clean free of adhesive and dust, a quick wipe with solvent will do the job. Adhesive on the shoe will attract dirt which may scratch or even tear the film. |