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camber effect for the inboard wing sections. The upwash over the outboard wing
sections increases the local angle of attack and leads to increased lift in that region,
simulating a local negative camber effect. The net effect of this interaction is to
delay the occurrence of the vortex breakdown to higher angles of attack in com-
parison to the wing alone case. As said before, a forward-placed, close-coupled
canard also functions in a similar fashion.
  The F-16 and the F/A 18 aircraft make use of the LEX for enhancement of
their aerodynamic characteristics at high angles of attack. The Swedish (SAAB)
Mggen is perhaps the first successfully operating combat aircraft to feature a
forward-placed, close-coupled canard. For this aircraft, it is reported that, with
the deployment of close-coupled canards,16 the lift on the final approach during
landi:gXcreased by as much as 659to over a simple delta wing. Several other
aircraft such as the Israel Lavi, the Swedish GrippcCn, the French Rafale and the
Mirage III, the Russian Su -35, and'the European Fighter Aircraft (EFA) employ
a variety of close-coupled canard corrfigurations for enhancement of aerodynamic
lift and drag characteristics. An exception is the X-31 airaraft, which uses a long-
coupled canard for the purpose ofpitch recovery at high angles of attack rather than
for enhanced lrft. An example oflift enhancement using optimal canard deflection
at various angles of attack is presented in Fig. 8.19.
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o
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Fig.8.19 Exampleofoptimalcanarddeflectiionsforliftenhancementofdeltaw:ings.rr
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690           PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
  The sideslip has an adverse effect on lateral-directional stability characteris-
tics of delta wings equipped with LEX or leading-edge strakes or close-coupled
canards.3.12 The adverse effect could be more pronounced compared to plain delta
wings. In other words, the very same fluid dynanuc phenomenon that gives rise to
lift augmentation and drag reduction at high angles of attack, zero sideslip condi-
tions, leads to a deterioration in lateral directional stability and control character-
istics if sideslip is experienced at high angles of attack. Therefore, a compromise
may have to be made in the design of slender delta wings with LEX/strakes or
forward-located, close-coupled canards between the benefits in lift and drag and
the degradation in lateral-directional stability at high angles of attack.
8.6   Forebodies at High Angles of Attack
     One of the most puzzling aerodynamic problems encountered in recent years is
the side force developed by an axially symmetr:ic slender body, typical of the fore-
body of a modern fighter aircraft in symmetrical flow at high angles of attack. The
magnitude of the associated yawing moment can far exceed the rudder capability
as schematically shown in Fig. 8.20. If this forebody-induced yawing moment
is of unstable nature, it can have a strong influence on the aircraft stability and
control at high angles of attack. The vort,ices shed from the forebody at high inci-
dence are primarily responsible for this phenomenon through a direct effect on the
 forebody pressure distribution and through secondary effects involving interaction
with other vortices shed from the wings and the LEX.
     At low and moderate angles of attack, the forebody vortex system consists of a
 pair of a steady symmetric vortices as shown in Fig. 8.21. As the angle of attack is
 increased, the vortex pattem becomes asymmetric. The asymmetric vortex pattern
 is of "bistable" nature, i.e., it has two stable patterns that are mirror images of each
 other. For any reason, if one asymmetric vortex pattern is disturbed, it switches to
the other asymmetric pattern. The vortices willvessentially stay stable in the new
  asymmetric pattern until disturbed again.ln its natural form, the microasymmetries
 in the forebody geometry or slight flow asymmetries prompt the vortex system to
 assume any one of the two bistable patterns.
ICn}
Fig. 8.20    Arrailable and required yawing moment at high angles of attack.
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK   691
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