Materials and Methods of Construction in Light Structures Page: 19 of 38
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.A.C.A. Technical Miemorandum NTo 515 17
These difficulties do not exist for large airplanes,
where there is generally an abundance of time at one's
disposal and where the cost of the first airplane is not
generally so important, so that wood is eliminated.
As regards the relative merits of steel and light
metal, I wish to call attention to the fact that, in vari-
ous cases for the dimensioning of certain parts, not the
strength but the stiffness is the determining factor. For
example, in heavily loaded cantilever monoplane wings,
torsion corrugations for ailerons, etc., the ratio
modulus Iofelasticity or sear is determinative for the
behavior of the various metals with respect to stiffness.
This ratio is approximately the same for steel and
light metal, whereby it must, however, be remembered that
the modulus of elasticity always has approximately the
sxame value of 2,150,000 for nearly all kinds of steel,
even those of less strength. Hence if one is compelled,
for the sake of rigidity, to make a part larger than would
be necessary for strength alone, a poorer quality of steel
of 50-60 kg/mm2 (71,118 to 85,340 lb./sq.in.) can safely
be soed, without making the part heavier than light metal,
with the advantage of being considerably cheaper.
As regards the advantages and disadvantages of opon
and closed =profiles, I agree e entirely with the speaker.
It cannot be denied, however, that for very large airplanes,
due to the given structural possibilities, the closed. pro-
file, cs-ocially in the f orm of tubes, may have decided
advzntr, es, when it is possible to avoid the disadvantages
otherwise inherent in the closed profile. This can be
easily accomplished by special constructional devices.
The chief advantages of a tube over a combination of two
open profiles are:
i. Greatest utilization of the cross section, espe-
cially for heavily stressed compression struts;
2. Elimination of the lon-i;tudinal seam;
3, Convenient worability and hence gradtual adapta-
bility of the cross section to the generated forces
by simply screwing one section over another;
4. The butt joints can be made with simple screw
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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/m1/19/?rotate=270: accessed June 27, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.