Page 228 - Aircraft Stuctures for Engineering Student

21 2 Principles of stressed skin construction

moisture absorption and loss caused by changes in atmospheric humidity, while its
structural properties exhibited the inconsistency common to natural products.
Further, pronounced anisotropy caused by its grain structure gave a variation in
the value of Young’s modulus in the ratio of 150: 1 depending on the direction of
loading in relation to the grain. Associated effects were ratios of shear modulus
and Poisson’s ratio of the order of 20: 1 and 40: 1 respectively.
The introduction of plywood and the development of synthetic resin adhesives
brought improvements in the strength of spars and skins and enabled anisotropy to
be eliminated or at worst controlled. However, the large amount of wood required
for military aircraft construction, during the 1914-18 war, revealed one of its most
serious limitations. The most suitable forms were imported from overseas, requiring
a large volume of shipping which was otherwise needed for the transport of food and
troops. To avoid a similar critical situation arising at a future date the Air Ministry, in
1924, prohibited the use of wood for the main load carrying parts of the structure.
This decision obviously hastened the introduction of alternative metallic materials
in airframe construction, although wood continued to make a significant contribution
for many years. In fact, during the 1939-45 war a particularly successful high perfor-
mance aircraft, the de Havilland Mosquito, was built entirely of wood. It must be
admitted, however, that the special circumstances of the time were the cause of
this. There was a shortage of factories and skilled workers for metal fabrication,
whereas the furniture industry was able to supply manpower and equipment. More-
over, wood could be adapted to rapid methods of construction and designers had
acquired a substantial amount of experience in dealing with the problem of
anisotropy in bulk timber. Furthermore, improvements in adhesives, for example
the introduction of the Redux adhesives based on phenolformaldehyde thermosetting
resin and the polyvinyl formal thermoplastic resin as a composite adhesive system, led
to improved wood-wood, acceptable wood-metal and even metal-metal bonds.
Despite this relatively modern successful use of wood it became inevitable that its
role as an important structural material should come to an end. The increased wing
loadings and complex structural forms of present day turbojet aircraft cause high
stress concentrations for which wood is not well adapted. Its anisotropy presents
difficult problems for the designer while wooden aircraft require more maintenance
than those constructed of metal. It is particularly unsuitable for use in tropical
conditions where, as we have noted, large changes in humidity have serious effects
on shape and dimensions. Attacks on the wood by termites is an additional problem.
The fist practical all-metal aircraft was constructed in 1915 by Junkers in Germany,
of materials said to be ‘iron and steel’. Steel presented the advantages of a high
modulus of elasticity, high proof stress and high tensile strength. Unfortunately
these were accompanied by a high specific gravity, almost three times that of the
aluminium alloys and about ten times that of plywood. Designers during the 1930s
were therefore forced to use steel in its thinnest forms, the usual preference being
for a steel having a 0.1 per cent proof stress of 1000 N/mm2. To ensure stability against
buckling of the thin plate, intricate shapes for spar sections were devised; typical exam-
ples of these are shown in Fig. 7.1. Common gauges of the material were 33 to 16 SWG
(i.e. approximately 0.25 mm to 1.63 mm), with a composition of 0.5 per cent carbon,
1.5 per cent manganese steel to Specification DTD 137, a nickel chrome steel to
Specification DTD 54A or a 12 per cent chromium steel to DTD 46A.