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   -5
      o
-1.0 -.8 -6 -.4  -.2  O  .2  .4  .6  .8  1.0
        y/s
~g.8.15    Effect ofMach number on vortex flow.14
      M o,
o 0.20
a 0.30
o 0.40
A 0.60
b 0.70
o 0.80
o 0.90
wing vortex as characterized by the suction peak is quite prominent at low Mach
numbers and gradually disappears as the Mach number is increased.
8.4.2  Controlof Vortex Breakdown
    Several attempts bave been made to control the vortex breakdown phenomenon
using various methods, including blowing on the upper surface. Essentially, such
methods aim in postponing the vortex breakdown to higher angles of attack and
soften its impact on the aircraft stability and control. Typical examples of upper
surface blowing are shown in Fig. 8.16.
      The ejection of high-cnergy air in the spanwise direction (Fig. 8.16a) energizes
the vortex core and delays the vortex breakdown to higher angles of attack. Simi-
larly,concentrated blowll;:g from the wing apex (Fig. 8.16b)in the axial direction of
the core also energizes the core and delays the vortex breakdown LO higher angles
of attack. The benefits of upper surface blowing on vortex lift arc schematically
shown in Fig. 8.16c.
8.5 Leading-EdgeExtensions
         A significant enhancement of aerodynamic characteristicsin terms ofan increase
in the maximum lift and reduction in the drag of delta wings at high angles of
attack can be obtained using'Ieading-edge extensions (LEX). Usually, the LEX
has a higher sweep angle to minimize the supersonic wave drag, and the (main)
wing has a lower sweep to improve subsonic lift-to-drag ratio. A forward-placed,
close-coupled canard also functions in a similar way. The schematic arrangement
of these surfaces-is shown in Fig. 8.17.
       To understand how such devices enhance the maximum lift and reduce the drag
 at high angles ofattack,consider the flow field over a delta wing with LEX as shown
 in Fig. 8.18. As expected, the flow separation occurs over the LEX, and separated
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK   687
CL
LEFT L.f:.
WITHOUT  8LOWIN G
R~GHT L.E.
WfTH BLOWING
a) Distributed leading-edge blowing
b) Concentrated leading-edge blowing
With BLowing
\lo Blowing
c) Effect of blowing on ,lift coeffiaent
Fig. 8.16    Pneumatic vortex controlof delta wings.lS
flow rolls up to form a pair of spiral vortices. The core of each of the LEX vortices
stretches downstream. There is "kinkin the wing leading edge at thejunction ofthe
LEX and the wing. At this kink, a second 'vortex, which is the main or wing vortex,
originates and also stretches downstream. With increasing downstream distance,
the wing vortex moves inboard and upward. The LEX vortex moves outboard and
downward towards the wing surface. Downstream of the Ieading-edge kink, the
LEX vortex is no longer fed by the vorticity shed from the leading edge. Therefore,
the strength of the LEX vortex aft of the leading-edge kink can be assumed to
remain more or less constant if viscous dissipation is rgnored. On the other hand,
the wing vortex is continuously fed by the vorticity shed from the leading edge
and, therefore, its strength will increase until the wing trailing edge is reached. The
LEX and wing vortices interact with one another, and the degree of che interaction
688             PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
a)
rd
Fig. 8.17   Schematic sketches ofvarious devices to enhance aerodynanuc character-
istics of delta wings.
l'
a) Plan view
LEX
Vo rt e,t
"
b) Side view
Fig. 8.18    Schematic illustration of vortex interactions.
LEX
 Vo rt e ,r
WinSI
Vort tji
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK   689
depends on the relative strengths of the two vortices and the distance between
them* Eventually, the two vortices merge to form a single vortex stretching in the
downstream direction as shown in Fig. 8.18.
  The LEX 'vortex induces downwash over the inboard wing section and ujy
wash/sidewash over the outboard wing sections. The induced downwash over the
inboard wing sections reduces the local angle of attack, hence the lift on the in-
board wing sections.In other words, the addition of the LEX simulates a positive
　
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