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Aviation History
1926
1926 - 0059.PDF
JANUARY 28, 1926 11 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT form, and if it can be developed to an entirely successful conclusion, the advantage of rigid and non-water-absorbing plane surfaces should lead to improved performance and the longer life of the planes. Light alloys open up, therefore, an interesting field of development, which is barred to metals of much greater specific gravity. Corrosion. Nothing has done more to retard the development of light-alloy aircraft construction in this country than the one word " corrosion." We have become familiar with the word rust from childhood, and equally we have come to take it for granted that its effects are not so serious as to deny to us the use of the metals subjected to it. The word " corro- sion " has a more fearsome sound, yet the two words have the same meaning and may be applied alternatively. Some metals corrode more rapidly than others, but in any case, certain -definite conditions are required to bring about this manufacturers who use metal for construction purposes. The writer's experience of Duralumin in connection with^the machines built by Short Brothers, and the numerous experi- ments carried out at our Rochester works, extending over nine years, on samples of steel and Duralumin, is as follows :— Duralumin, without protective covering of any kind, exposed for long periods to ordinary atmospheric conditions, compares very favourably, so far as corrosive effects are concerned, with the ordinary high-tensile steels used in aircraft manufacture ; the latter, indeed, appear to suffer far more severely than the former under such conditions. A long series of tests also seem to indicate that with similar unprotected plates immersed in sea water the light alloy is less affected than is the steel of the non-rustless variety. When Duralumin is protected with a suitable paint and exposed to atmospheric conditions, it appears to suffer no deterioration whatever even after several years' exposure, and one is led to the conclusion that A Short Duralumin spar on test. Built up from corrugated strips, this wing spar showed excellent strength qualities. Under a load 4-8 times normal, maximum stresses of about 40,000 lbs. per square inch were realised. The weight of the spar was 4-25 lbs. per foot run. phenomenon. The prevention of corrosion consists in avoiding these conditions as far as is possible. The composition of the metal itself is important. The composition and treat- ment of Duralumin has made it less prone to corrosion than the metal aluminium which forms, perhaps, 93 per cent, of its composition. Rustless steel is a further example of this fact, and indicates one line of research to which metallurgists are, of course, quite alive, and in which we may expect progress to be made. Corrosion may be said to be due to electrolytic action, and the presence of an electrolyte in contact with the metal is a condition of the process of corro- sion, where the cause is external. If the electrolyte can be insulated from the metal, corrosion is prevented. The writer does not profess to have a knowledge of the intricate processes connected with the problem of electrolyte action, a pheno- menon which is indeed a long way from being completely understood even by those whose work involves a special study of the subject. Nevertheless, the above remarks show the nature of the problem as it affects the aircraft under these conditions Duralumin may be expected to outlast wood and ordinary high-tensile steels, as used in aircraft manufacture. When a Duralumin hull or floats are protected in a similar manner, the coating appears to be effective in preventing serious corrosion, so far as tests of 12 and 15 months go to prove. In parts immersed in sea water, care must be taken to keep the protective covering intact by repainting any abrasions, the part having first been carefully washed with fresh water, to remove any traces of salt on the metal. Care must be taken also in the treatment of the metal in the process of manufacture ; where a salt bath is used for anneal- ing and normalising purposes the metal must be thoroughly cleansed, and running warm water has been found to be most effective for this purpose. The use of a suitable electric furnace for heat treatment will, of course, dispense with any necessity for this washing process. Such furnaces are being used in France, where the manufacture of Duralumin spars and fuselages is being carried out on a production basis.
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