Apart from the metallic construction of aeroplanes an enormous
amount of work was done in the testing of different steels and
light alloys for use in engines, and by the end of the War
period a number of aircraft engines were in use of which the
pistons and other parts were of such alloys; the chief
difficulty having been not so much in the design as in the
successful heat-treatment and casting of the metal.

An important development in connection with the inspection and
testing of aircraft parts, particularly in the case of metal,
was the experimental application of X-ray photography, which
showed up latent defects, both in the material and in
manufacture, which would otherwise have passed unnoticed. This
method was also used to test the penetration of glue into the
wood on each side of joints, so giving a measure of the
strength; and for the effect of 'doping' the wings, dope being a
film (of cellulose acetate dissolved in acetone with other
chemicals) applied to the covering of wings and bodies to render
the linen taut and weatherproof, besides giving it a smooth
surface for the lessening of 'skin friction' when passing rapidly
through the air.

An important result of this experimental work was that it in
many cases enabled designers to produce aeroplane parts from
less costly material than had previously been considered
necessary, without impairing the strength. It may be mentioned
that it was found undesirable to use welded joints on aircraft
in any part where the material is subjectto a tensile or bending
load, owing to the danger resulting from bad workmanship causing
the material to become brittle--an effect which cannot be