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58 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
NOSE STRAKE
Fig.1.58 X-29 forward-swept wing airplane.18
Ob/ique wings. As said earlier, if we need good low-speed characteristics
combined with good high-speed characteristics such as low-wave drag, then we
have to use a variable-sweep wing. However, one of the main disadvantages of
the variable-sweep wing is the large weight penalty associated with the additional
structure required to handle structuralloads. With oblique wings,19 a straight wing
is rotated in fiight so that, at low speeds, it is essentially at zero sweep and, at
high speeds, one wing is swept forward and the other is swept back as shown in
Fig. 1,59. An oblique wing is continuous from tip to tip and is attached to the
fuselage at one point only-4he pivoL The bending moment on one half of the
wing is reacted by the other half. As a result, the pivot carries only the lift load;
hence the wing structure is much lighter.
Wing
F/g.1.59 Obliquewmg.
REVIEW OF BASIC AERODYNAMIC PRINCIPLES 59
Another significant advantage ofthe oblique wing is the absence of aerodynamic
center shift at transonic speeds. On a conventional variable geometry wing at
transonic speeds, the aerodynanuc center moves aft as the wing sweep increases
to reduce the wave drag. This creates more nosedown moment and requires a
large horizontal tail surface to trim the aircraft. This increases the trim drag and
hence affects the performance. With the oblique wing, this problem is considerably
alleviated as one half of the wing sweeps aft and the other half sweeps forward and
the overall aerodynamic center hardly moves. Therefore, a large horizontal tail is
not needed to trim the aircraft in transonic speeds.
The benefits of the oblique wing wereodveumonstrated in flight using the F-8
Crusader aircraft during the mid-1980s.
1.12 Area Rule
Area ruling18,20 is a systematic method of minimizing the transonic/supersonic
wave drag of airplane configurations. Fundamental to this method is the assumption
that, at Mach numbers close to unity and at large distances from the body, distur-
bances and shock waves are indepen~ent of the arrangement of the components and
are only functions of the longitudinal variation of the cross-sectional area. In other
words, the wave drags ofa given wing-body and an equivalent body having an iden-
tical longitudinal cross-sectional area variation (Fig, 1.60) are essentially the same.
For most airplane configurations, adding cross~sectional areas of wing to that of
the fuselage results in a bump in the overall area distribution as shown in Fig. 1.61a.
To obtain the minimum wave drag, the overall distribution should be that for a
smooth body with minimum wave drag. The most obvious way to achieve this is
to remove the cross-sectional area of the wing from the fuselage in that region
where the fuselagejoins the wing. This results in a modifled shape that looks like a
coke bottle. The cross-sectional areas of other components like"vengines, nacelles,
and tail surfaces can be treated in a similar manner:y:Che wave drag of the modified
body is lower than that of the basic configuration as shown in Fig. 1.61b.
For supersonic Mach numbers, cross-sectional areas are to be obtained by taking
planes inc.lined at an angle /r, = sin-l(l/Moo) to the axis of the given body.
The application of area rule resulted in significant transonic/supersonic drag
reductions of several aircraft, One particular example was the Convair F-102
a) Basic wing-body
b) Eqruvalent body
Fig. 1.60 Concept of equivalent body for area rule.
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