Flight International, JANUARY 17, 1914 hat geschrieben:INVISIBLE AEROPLANES.Two views of the Etrich monoplane covered with Emailllte treating and flown by Lieut. Nittner at Wiener Neustadt. It will be noticed especially from the photograph on the right that the inner construction of the planes can be seen through the top surface.
ONE important point which the designer of a military aeroplane has to keep in mind is that his machine, when in the air, must be as hard to discern as possible. There have been several attempts to build an aeroplane with the wings of transparent material, and in May and June of 1912, Lieut. Nittner was flying at Wiener Neustadt, near Vienna, an Etrich monoplane, specially built on such lines for Capt. Petroczy, formerly commandant of the flying corps in the Austrian Army. This machine had the planes covered with a special variety of Emaillite cellulose sheets, and the system has since been developed and patented in all countries by MM. Leduc Heitz, of the Paris House of Emaillite. A photograph is reproduced of the Etrich machine, to which reference has been made, and which those present on the ground were unable to locate in the air when flying at an altitude of between 900 and 1,200 ft. It is stated that at a height of 700 ft. only the framework is dimly visible, and this and the outline of the motor and pilot and passengers present so small an area to rifle or gun fire, that at the rate of speed at which aeroplanes are flown to-day, accurate aiming at such surfaces becomes nearly impossible.
There are also secondary advantages in the use of such transparent sheeting in the construction of aeroplanes. For one thing, it enables the pilot to keep an eye upon the interior framework of the planes, and to detect at once any straining or fracture of the ribs, &c. Another advantage is that the highly polished smooth surface reduces the friction, as was proved in the case of Capt. Petroczy's machine, although, as that was the first machine to be so treated, the material used was not so suitable as the latest product. Trie surface could not be properly tightened, and owing to the sheeting being more or less plastic it presented a wavy surface, while some difficulty was experienced in securely fastening it to the ribs.
As long ago as 1904. Prof. Reisner, of Aachen, suggested that polished celluloid should be utilised for aeroplane sheeting in order to diminish air friction.
Last year, M. W. A. Lebedeff, working in conjunction with the Russian Government, tried to cover a Henry Farman biplane with transparent cellulose sheeting of a somewhat modified composition. This material was not so heavy as that used in Austria, and it was also somewhat stronger (its tensile strength being about 7 kilogs. per square millimetre of section), but the wavy surface of the wings, due to the flexibility of the material, could not be overcome.
After working at the problem for some time the Emaillite firm have developed a better material which was seen at the Paris Show on the planes of the Moreau monoplane. Instead of using ordinary cellulose sheeting, this machine has what might be termed a reinforced sheeting consisting of two layers of Emaillite with a sheet of silk tulle between them, the tulle being specially treated to render it transparent. That the material is to all intents and purposes transparent is illustrated by the photographs of one of the wings of the Moreau monoplane behind which a man can be clearly seen The use of the tulle liner not only strengthens the material but it also prevents it sagging or warping between the ribs so that by its use it is quite possible to obtain a smooth and regular surface on the planes. The tensile strength of the material is about nine to ten kilogs. per square millimetre section and a -35 mm. sheeting is sufficient to ensure a tensile strength of about 2,800 to 3,000 kilogs. of the wing covering, a stress which is never attained with the best fabrics in use. The weight of this new Emaillite material does not exceed 375 grammes per square metre, which is but 40 per cent, more than the weight of good doped linen fabric as generally used, so that the increase of weight m the case of ordinary machines would be between 12 and 15 kilogs. It is claimed for this new Emaillite transparent reinforced sheeting that it has all the advantages of that which is not reinforced without its faults. It can be fastened either by nailing, sewing, or by using an adhesive solution. It will not tear or break when anything such as a tool falls upon it, while should it be pierced by a bullet the fabric liner would prevent the damage extending. The British patents for this invention are held by the Britibh Emaillite Co., Ltd., of 30, Regent Street, W. Extensive tests are shortly to be carried out with machines covered in this way, in order to ascertain the height at which they become virtually invisible.