Page 227 - Aircraft Stuctures for Engineering Student

Principles of stressed

skin construction

With the present chapter we begin the purely aeronautical section of the book, where
we consider structures peculiar to the field of aeronautical engineering. These struc-
tures are typified by arrangements of thin, load bearing skins, frames and stiffeners,
fabricated from lightweight, high strength materials of which aluminium alloys are
the most widely used examples.
As a preliminary to the analysis of the basic aircraft structural forms presented in
subsequent chapters we shall discuss general principles of stressed skin construction
from the viewpoint of materials and the loading, function and fabrication of struc-
tural components.

Several factors influence the selection of the structural material for an aircraft, but
amongst these strength allied to lightness is probably the most important. Other
properties having varying, though sometimes critical significance are stiffness, tough-
ness, resistance to corrosion, fatigue and the effects of environmental heating, ease of
fabrication, availability and consistency of supply and, not least important, cost.
The main groups of materials used in aircraft construction have been wood, steel,
aluminium alloys with, more recently, titanium alloys, and fibre-reinforced compo-
sites. In the field of engine design, titanium alloys are used in the early stages of a
compressor while nickel-based alloys or steels are used for the hotter later stages.
As we are concerned primarily with the materials involved in the construction of
the airframe, discussion of materials used in engine manufacture falls outside the
scope of this book. Before we consider the individual groups in detail it is interesting
and instructive to examine briefly the history of aircraft materials from the birth of the
industry, at the beginning of the 20th century, to the present day.
The first generation of conventional powered aircraft were constructed of wood
and canvas. Spruce and birch were the most widely used timbers with tensile strengths
of 70N/mm2 and 100N/mm2 respectively, specific gravities of 0.4 and 0.63 and
Young’s moduli of 9000 N/mm2 and 14 250 N/mm2. Although these strength/
weight ratios compare favourably with modern heat-treated aluminium alloys,
natural wood had disadvantages. Changes in shape and dimensions resulted from