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Pitchup
Fig. 1.56 Schematic iUustration of pitch-up of swept-back wings.
its first fiight on Dec. 14, 1984. The X- 29 aircraft was designated as the Forward
Swept Wing (FSW) Technology Demonstrator aircraft. A schematic three-view
drawing of the X-29 aircraft is presented in Fig. 1.58.18
Forward-swept wings have favorable stall characteristics. Unlike swept-back
wings, the root regions stall first, and the stall progresses from root to tips:PThere-
fore, forward-swept wings do not experience the loss ofrollcontrol at stall and thus
are inherently spin resistant (we will study spinning motion later in the text). Then
why did the forward-swept wing vanish afier the German Hansa 320? The main
reason for this is the fact that the forward-swept wing experiences what is known
as aeroclastic divergence, and the swept-back wing does not face this problem.
Because wing tips of a forward-swept wing are effective in producing lift, the
swept-forward wing experiences larger twisting moments, which tend to twist
the wing sections in a direction that increases their angle of attack further, hence
their lift. As a result, the twisting moment increases further. The lift and twisting
moments are proportional to the square of the flight speed. Therefore, the problem
becomes more severe as flight speed increases. This phenomenon is known as
aeroelastic divergence. When the twisting moments exceed the structural Ioad
limit, the wing fails. The traditional aluminum structure could not resist such
aeroelastic deformations of the forward-swept wing. What made a difference in the
X-29 aircraft was the use of an aeroelastically tailored composite wing. Composite
materials forming the wings ofX-29 are so designed that the wing actuallyTwists
in the opposite direction, i.e., leading edge down under the action of twisting
loads, thereby reducing the angle of attack and effectively preventing aeroelastic
REVIEW OF BASIC AERODYNAMIC PRINCIPLES 57
a) Delta wing at angle of attack
AA
b) Variation oflift coefficient with angle of attack
Fig.1.57 Delta winginlow-speed flow at angle of attack
Lift
divergence. This advance in aircraft materials paved the way for the resurgence of
interest in forward-swept wing technology.
Forward-swept wings also experience pitch- up like swept-back wings, and the
pitch-up tendency is more severe. Because of this problem, the X-29 is statically
unstable and haslabout 35c7o negative static margin. (We will study the concept
of static stability and stability margins later in the text.) Therefore, this inherent
instability requires the use of an automatic fiight controlsystem that has to make the
aircraft safely fiyable. The X-29 aircraftis stabili:z:ed by alughly augmented triplex
digital-analog, fiy-by-wire flight control system. The flight control system has
three modes: normal (primary) mode, digitalreversion mode, and analog reversion
mode.
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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
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