PSYCHOLOGY of the public, as well as engineering structure and aerodynamics, is involved in commercial aviation. The public has confidence in metal.
With quantity production in view, the author and his associates considered costs of production as related to quantity and also costs of maintenance at airports and in the field, and chose metal as the material of construction.
Structural members are fashioned from sheet duralumin rather than from tubes and a type of construction was evolved that can be made with the minimum investment in tools, that is cheap to put together and that can be repaired with the smallest amount of equipment and labor.
For compression loads, duralumin has a great deal more strength for a given weight than has steel. It cannot be used, however, for compression members in combination with steel in tension members because of the difference in coefficient of expansion.
Advantage is taken of the rigidity of corrugated duralumin sheets to use them as wing covering in a way to form part of the structure and prevent torsion.
Strength of metal of a given analysis and heat-treatment can be determined within 5 per cent, and all duplicate members made of the same metal have equal strength. This is not true of wood and of wooden members.
Parts of a damaged metal airplane can be repaired readily with metal of the same thickness, quality and cross-section and the airplane made as good as when new.
Built-up sections of metal riveted together give warning of coming failure, whereas wood and tubular members do not.
The exterior of a duralumin airplane needs no painting or other protection against corrosion. The sheets used in the author's airplanes are polished before fabricating and are given no other finish.