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Aviation History
1941
1941 - 0913.PDF
APRIL 17TH, 1941. 289 DOPE" (Continued) taken to run through at a controlledtemperature gives a measure of the viscosity of the liquid. Reference has been made previouslyto the finger-tapping test of doped panels in the early days of flying. Inmodern times a more scientific method is used. The doped fabric on a stan- li'tlard frame of about ift. side is testedfor tautness by measuring the depres- sion (usually in thousandths of an ..inch) given to the centre of the panel• "by the application of a known weight. Flexibility is an important qualityof a dope. Examination can be made of the dope film itself, prepared by evaporation on a bath of mercury or•* by spinning a flat surface coated with the dope. Usually tests can be applied to the doped fabric. For instance, thestandard doped fabric frame can be subjected to a series of heavy blows,of the order of 30 lb. /sq. in., followed by examination for cracking. An-other method is to bend the fabric backwards and forwards at a high rateuntil signs of failure appear. Such tests are made in rooms in which thetemperature and humidity are con- trolled. It is also necessary to knowhow dope behaves under load in the; low temperatures encountered by ser-vice aircraft flying at great heights or in wintry weather. Such atmospheresare obtained by the use of solid carbon dioxide, temperatures as low as —50deg. to —60 deg. being reached. In the early flying days inflamma-bility was something of a bugbear, and it was customary to demonstratefire-resisting qualities by placing a lighted match on the fabric frame.That was not a fair test, because the smooth surface helped greatly. Now-adays a more severe test is applied. A small fire is started at one end of adoped fabric frame, and the time The despatch bay. Lorries being loadedwith Cellon dope and other products. taken for the flame to travel across,or to extinguish itself, is noted. The durability which can be ex-pected from a given dope is ascer- tained by accelerated weatheringtests. In these the doped fabric is r A triple-roll paint mill in whichadjacent rollers run at different speeds. (Leftj running off dope through a glass-fronted metering device. subjected to extremes of heat andcold, to ultra-violet rays, and to mois- ture. In a few hours the results areknown, but actual tests on an aircraft are a safer guide. Satisfactory dopes must be suitablefor use in atmospheres containing as much as 90 per cent, relative humid-ity, and at temperatures as low as freezing point, without blushing orchilling. So much for the dopes themselves.And now a few words about the matt colours applied over them as finishingcoats. The protection given is de- pendent on a very thin coating of suchmaterials. Among the qualities of im- portance is lack of gloss, as obviouslya surface which reflects too much light may reveal the whereabouts of an iiircraft. A so-called gloss meter is used for testing this. The paint is appliedto a glass plate, and the reflected light is obtained from a standard source oflight. This is transformed, via a
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