put through the same
evolutions as a small machine, and therefore could safely be built with
a lower factor of safety. Owing to the fact that a wing section which is
adopted for carrying heavy loads usually has also a somewhat low lift
to drag ratio, and is not therefore productive of high speed, these
machines are not as fast as light scouts; but, nevertheless, they proved
themselves capable of achieving speeds of 100 miles an hour or more in
some cases; which was faster than the average small machine of 1914.

In one respect the development during the War may perhaps have proved
to be somewhat disappointing, as it might have been expected that great
improvements would be effected in metal construction, leading almost to
the abolition of wooden structures. Although, however, a good deal of
experimental work was done which resulted in overcoming at any rate the
worst of the difficulties, metal-built machines were little used (except
to a certain extent in Germany) chiefly on account of the need for rapid
production and the danger of delay resulting from switching over from
known and tried methods to experimental types of construction.
The Germans constructed some large machines, such as the giant
Siemens-Schukhert machine, entirely of metal except for the wing
covering, while the Fokker and Junker firms about the time of the
Armistice in 1918 both produced monoplanes with very deep all-metal
wings (including the covering) which were entirely unstayed externally,
depending for their strength on internal bracing. In Great Britain cable
bracing gave place to a great extent to 'stream-line wires,' which are
steel rods rolled to a more or less oval section, while tie-rods were
also extensively used for the internal bracing of the wings. Great
developments in the economical use of material were also made in the
direction of using built-up main spars for the wings and interplane
struts; spars composed of a series of layers (or 'laminations') of
different pieces of wood also being used.

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 conne