Page 174 - Aeronautical Engineer Data Book

Section 10

Aircraft design and construction

10.1 Basic design configuration

Basic variants for civil and military aircraft are
shown in Figure 10.1 Large civil airliners have
a low wing design in which the wing structure
passes through the freight area beneath the
passenger cabin. Small airliners may use the
high wing design, with a bulge over the top line
of the fuselage so as not to restrict passenger
headroom. Having a continuous upper surface
to the wing (as in the high-wing layout) can
improve the L/D ratio and keeps the engines at
a higher distance from the ground, so avoiding
debris from poor or unpaved runways.
Tailplane configuration is matched to the wing
type and includes high tail, low tail, flat, vee and
dihedral types. Low tails increase stability at high
angles of attack but can also result in buffeting
(as the tail operates in the wing wake) and non­
linear control response during normal flight.
High tails are generally necessary with rear-
fuselage mounted engines and are restricted to
high speed military aircraft use. Figure 10.2
shows variants in tail and engine position. The
rear-engine configuration has generally been
superseded by under-wing mounted engines
which optimizes bending moments and enables
the engine thrust loads to be fed directly into the
wing spars. In contrast, rear-fuselage mounted
engines decrease cabin noise.

10.1.1 Aspect ratio (AR)
The aspect ratio (AR) is a measure of wingspan
in relation to mean wing chord. Values for
subsonic aircraft vary between about 8 and 10
(see Tables 10.1 and 10.2). Figure 10.1 shows
some typical configurations.