Tekfire (oz9064)

About this Plan

Tekfire. Rubber sport model. Free flight rubber power model featuring litho plate fuselage construction.

Quote: "Litho plate and Litespan - fresh materials for Barry Sim's sleek semi-scale rubber sportster.

Last winter, while building a WWII biplane for rubber power, I had trouble with Jap-tissue covering the fuselage, as the amount of dope needed to tighten it tended to bow the thin stringers. So off went an order for two sheets of Litespan and a jar of Balsaloc. In the meantime, I set about covering the nose section with litho plate. Scale panels were cut and carefully polished, but the double curvature of the nose prevented fitting without creases. The only answer was to cut each panel into two or three sections and attach them with Evo-stik, retaining the original correct panel lines and filling the other gaps with model filler. A rub-down with fine wet-or-dry paper, the use of cut-off pins for rivets, a coat of primer and two of aluminium from spray cans resulted in a better-looking cowl than I could have achieved by any other means, but I was sorry to lose the polished metal finish.

I took as a personal challenge the note in the building instructions for this aeroplane that the top surface of the annular wing could only successfully be covered by using several pieces of tissue. I cut single pieces of tissue and applied small dabs of balsa cement to the wing outlines to tack them roughly into position. Jap tissue allows for such tacks to be pulled away again without damage, so by progressive slight repositioning it was possible to obtain a smooth and evenly-tensioned covering. Cement spots disappeared when the two finishing coats of thinned clear dope were applied.

Clearly, this was the method to use for Litespan covering. Although the plastic sheet could not be fully tightened by the 'moving tack' technique, the final heat-shrinking pulled the covering tight and wrinkle-free.

High-tech topics. I now had two new, fairly high-tech materials waiting to be used for the right project - litho plate (that could be polished to a mirror finish) and Litespan, which would provide a light, strong and self-coloured finish for the rest of the model. How about a rubber powered, semi-scale Spitfire? I knew that Litespan would cover the slight dual-curvature of the elliptical flying surfaces, but how to shape the fuselage sections from litho without those curves? A bit of lateral (or tubular) thinking gave the answer - straight lines!

A thin metal tube, straight or tapered, can be shaped at either or both ends by squashing or bending, without creasing the surface area, as the sides of the tube remain straight between the front and rear shapes. This rather crude idea can be improved by making front and rear formers, shaped as required, then rolling a sheet of litho around the formers. So a fuselage could be made in, say, three sections, front, centre and rear, each to match in section where joined, and profiled in straight lines. A session with pad, pencil and ruler indicated that a pleasant looking fuselage shape could be drawn, accommodating the necessary design features for a stable, rubber powered sports model.

The outline drawings for Tekfire were prepared and construction began - a model built rather out of normal sequence - materials dictating design and plans finally drawn from the finished aircraft.

The first, all-litho fuselage looked really impressive, but was too heavy to be practical for a small rubber powered model. This led to another useful design feature. Obviously, substituting 1/32 in balsa sheet for the litho covering on centre and tail sections would solve the weight problem and still be quite strong enough, but it would have to be covered for the sake of appearance and weatherproofing. No trouble with Litespan, as it's all straight lines, but sticking the material to the whole balsa surface resulted, as it would have much more so with tissue and dope, in slight distortion of the balsa and a less than smooth finish. The answer was to stick the plastic covering at the seams and edges only, then heat-shrink for a perfect finish..."

Scan from DBHL, cleanup by theshadow.

ref DBHL-6222.

Direct submission to Outerzone.

Update 14/08/2017: added article, thanks to Pit & Gerrit.

Supplementary file notes

Article.

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