262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
aerodynamic fields a little further in order to show the nature of the 
difficulties that have been met and the methods by which they have been 
overcome. 
STRUCTURAL DEVELOPMENTS 
In 1917 the great majority of aircraft structures were made of wood 
and steel. Light alloys were little used. Wing surfaces were covered 
with fabric, and torsional stiffness requirements were met by the bi- 
plane wing structure. Today, with few exceptions, we use light alloy 
for the primary structure, and torsional stiffness is derived in most 
cases from the light-alloy sheet wing covering. The very con- 
siderable improvement that has been made in aluminum-rich alloys 
contributes chiefly to the wing spars. There is as yet no marked sign of 
a development in their properties or application which will reduce the 
weight involved in meeting torsional stiffness requirements. This is, of 
course, because these involve stability rather than strength character- 
istics. 
I do not suggest that the enormous effort that has been put into im- 
proving aircraft materials has not contributed to the maintenance of 
structure weight at a remarkably low figure in spite of increases of 
speed, strength requirements, and size. But it is significant that the 
Mosquito airplane, which is made almost entirely of wood, has a struc- 
ture weight as low as that of the equivalent metal airplane. 
One feature of the, modern aircraft which has undoubtedly con- 
tributed to a more economical wing structure, in particular, is the great 
increase of wing loading and therefore of wing weight per unit area, 
which has made it possible to employ the material to much greater ad- 
vantage—i. e., to have a smaller percentage of relatively lowly stressed 
material. This brings me to one of the outstanding contributions of 
research to aeronautics—namely, that derived from the investigation 
of the strength of actual structures in close association with theoretic 
analysis. It is by such work that it has been possible to increase greatly 
the useful load of practically all aircraft now in use. The most 
thorough mechanical testing of aircraft structures undoubtedly pays a 
high dividend. These tests have not only shown us that our methods of 
design have led to general forms of structure well adapted to meet the 
demands on them, and fundamentally economical in character, but have 
enabled us to discover where our knowledge of the detailed distribution 
of stresses is inadequate and at the same time to improve that knowl- 
edge and to strengthen the structure against unforeseen local weak- 
nesses. 
The determination of the loads that the structure is called upon to 
bear is fundamentally a more difficult problem. We are greatly in- 
debted to such methods as the V. G. recorder, but these give us only 
over-all figures that, useful as they are, throw little light on the load 
