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large favorable pressure gradient exists that tends to promote the existence of a
laminar boundary layer. Beyond the minimum pressure point, an adverse pressure
gradient exists towards the trailing edge. Because of the sharp curvature of the
leading edge of a thin airfoil, the adverse pressure gradientis sufficiently strong to
 cause flow separation. K the Reynolds number is low, this type of flow separation
is of a permanent nature (no subsequent reattachment) and, as a result, the airfoil
stalls. Because the flow separation occurs close to the leading edge, this type of
 stallis usually abrupt and is marked by a sudden loss oflift beyond the maximum
 lift as shown by curve b in Fig. 1.23.
REVIEW OF BASIC AERODYNAMIC PRfNCIPLES                 23
Fig. 1.23    Schematic illustration of stall of airfoil sechons.

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24               PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
a) Plain flap
b) Split flap
c) Fowler flap
d) Slotted flap
e) Kruger nap
f) Leading-edge flap
g) Blown flap
h) Jet flap
Fig.1.24    Schematic diagram of typical flap configurations.
    F/aps.   Flaps are mechanical devices deflected either from the trailing edge
(or close to the trailing edge) or the leading edge. Several different designs of the
fiaps have come into existence, and some of the commonly used configurations
are shown in Fig. 1.24. Flap configurations depicted in Figs. 1.24a-1.24g are
mechanical flaps, and that shown in Fig.  1.24h is the so-called jet flap.
      A deflection of a hinged mechanical fiap basically alters the effective camber of
 the airfoil section, hence its lift-curve slope. The defiection of the flap increases the
value of Cimax without essentially altering the stall angle. Therefore, at the stall,
an airfoil with a deflected fiap will have an incrementallift coefficient ACinax as
shown in Fig. 1.25. The plain fiap, split flap, Kruger fiap, and leading-edge flaps
 belong to this category of plain mechanical flaps.
   In addition to changing the camber, the slotted flap and the Fowler-flap also
act as boundary-layer control devices. A schematic diagram of the operation of a
trailing-edge slotted fiap is shown in Fig. 1.26. High-pressure air fiom the lower
surface leaks through the gap and energizes the retarded upper surface boundary
cl
REVIEW OF BASIC AERODYNAMIC PRINCIPLES               25
Cl, max
flg.l 25    Schematic -variation oflift coefficient for airfoils with and without flaps.
High Energy Flow
Fig.1.26   Schematicillustration offlow around a slotted trailing-edge flap.
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26               PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
layer. This process helps to delay the fiow separation on the upper surface and
henceimprove the value of Ci.   x.
       For the blown fiap, high  pressure air is blown over the upper surface of the flap
as shown in Fig.  1.24g. In this way, considerable energy is added to stabilize the
upper surface boundary layer that delays flow separation. The magnitude of Cl,max
depends on the mass of the added air and the blowing velocity (momentum of the
blown air). A jet fiap (Fig. 1.24h) also functions in a similar way.
      The flap effectiveness depends on many factors. The mostimportant ones are the
fiap-chord-to-wing-chord ratio,shape of the fiap leading edge, and width ofthe gap.
For a finite wing, value of the increment in maximum lift coefficient also depends
 on the ratio of flap span to the main wing span, sweep angle ofthe flap hingeline, etc.
    Boundary-/ayer controL    Another method of increasing the maximum lift
coefficient of the wing is the application of boundary-layer control methods. The
slotted fiap and the Fowler-fiap are some forms of boundary-layer control although
this term"7s usually used for"9ctual flow control using boundary-layer suction or
　
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