Materials and Methods of Construction in Light Structures Page: 25 of 38
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. A.C.A. Technical Iremorandum 1o. 515 3
and hence with the same tensile strength as the riveted
material, art 1'sed, while it has long been customary in
steel construction to use iron.rivets considerably softer
than the riveted steel. Consequently, the rivet-holO re-
lations can in no case to less favorable for steel with
iron rivets than for duralumin with duralumin rivets. The
relation is unfavorable, however, for duralumin with iron
rivets. As stated by the speaker, the Rohrbach Company
uses steel for spar fittings. It is obvious that those
steel fittings for duralumin can never be lighter than
steel fittings for steel.
I would summarize my conclusions regarding the choice
of materials as follows, The choice of building materials
can be made only from consideration of the given static
relations, economy and practical experience for each indi-
vi dual case. I assume it to be obvious that the whole ma-
terial problem can relate only to the construction of
highly stressed structural parts. As a iattor of oxperi-
ence it is known that the weight of such parts constitutes
bout 8% of the dead load of an airplane and the time re-
aquired to construct them, about 7% of the ti-me required to
build the whole airplane.
Former naval architect Baatz.- Dr. Rohrbach spoke on
the question of airplane materials. On the one hand he
co;w.Dared wood. and metal and, on the other hand, duralumin
an steele. His contention is probably correct that the
dovclo'p.maent of the airplane will follow the course of do-
volopment of all other vehicles, cars, ships, etc., from
wood to metal construction. This is due to the difficulty
of obtaining sufficient wood of uniform structure for the
production of any article on a large scale. As to what
the metal of the future is to be, there is still a great
divergence of o-oinion. Dr. Rohrbach compares steel having
a tensile breaking strength of over 100 kg/mm2 (142,235
,. so.i-.) with .lurminum alloys having a breaking strength
of 40 kg/.mm2 (56,890 lb./sq.in.). The general fact that
metal of specifically great strength unfortunately has a
vory small elongation, has compelled mechanical engineers
i'n general to refrain from approaching the upper limit of
strength and to prefer materials with a relatively groat
olongt3 on. I recall, for oxamrlo, tnSat steol, with a
strength of' 50-50 kg/mm (71,118 to 85,340 lb./sq.in.) a~d
an elongation of 10-I2, can be producod cheaply in any
desired au.ntity. ievertheless, there is used in great
qu entities , in the construction of vehicles steel of low
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Rohrbach, Adolf. Materials and Methods of Construction in Light Structures, report, May 1929; (https://digital.library.unt.edu/ark:/67531/metadc65364/m1/25/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.