Materials and Methods of Construction in Light Structures Page: 29 of 38
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N.A.C.A. Technical Memorandum No. 515 27
all due to considerations regarding the suitability of the
material, but that it is simply a question of the diffi-
culty of obtaining wood in war time, In the event of war,
however, the quicker and cheaper supply of wooden airplanes,
in comparison with the use of metal, would play a decisive
role, since the possible longer life of metal airplanes
would be more than offset by their rapid destruction and
by the types becoming obsolete.
2. The greater cost of a metal airplane cannot be
due alone to the extensive preliminary work of a construc-
tive nature. This is demonstrated by the fact that, in
metal construction, the necessity of further division of
the structure into many small parts directly affects the
working times even in quantity production. For obvious
reasn-o. Dr. Rohrbach does not give the absolute working
times in metal construction, so that no direct comparison
is possible in this respect. An approximate idea can be
obtained, however, from the prices of similar airplanes
built on about the same scalo, This applies, for example,
to the Junkers K 16 and the Focke-Wulf A 16, the s az1
prices of which bear approximately thne ratio of 2 : 1.
.3, As regards weight, wood construction still has the
advantage, naturally with, the fulfillment of the same
strength requirements. Here also we can compare Junkers
K 13 and Focke-Wulf A 16. With the same engine (75 hp
Siemans) the K 16 carries only one pilot and two passen-
gers and has a correspondingly smaller wing area than the
A 16, which carries one pilot and three passengers at a
somewhat higher speed, Nevertheless, the ratio of the
dead load to the pay load is about the same for both air-
4. One of the fundamental faults which can be imput-
ed to the light metals of to-day, is their fatigability
under varying stresses, though the danger from this phe-
nomenon has always been contested, especially in Germany,
but without any counter-evidence. American and Dutch ex-
periments show that, under some circumstances, the figures
.obtained for varying stresses (vibration strength) are on-
ly 40f of those for static loading. Though Dr. Rohrbach
says that fatigue phenomena vere observed only above the
proportionality limit, there always remains the considera-
tion that no absolute definition pf the proportionality
limit can be nade for any material. It has been found
that accurate measurements enable the recognition of very
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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/m1/29/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.