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strake as shown in Fig. 8.72b. In this case, the direction of the side force is always
fixed and not affected by the flow Reynolds number. Therefore, to ensure a fix"9d
direction of the side force, it is desirable to use a sufficiently large strake height
so that the strake always acts as a flow scparator on its side and produces a side
force in the (fixed) opposite direction. For the example considered in Fig. 8.72, the
critical strake height is around 0.05 diam, where diam is the forebody diameter.49
The forebody strakes can either be fixed or be of actuated'type as shown in
Fig. 8.73. The actuated forebody strakes have the advantage that they can be
retracted and remain conformal with the forcbody shape at low angles of attack
when not required. With this concept, strakcs on either side of the forebody would
deflect individually from their conformal positions depending on the des/ired di-
rection of yaw control. An important advantage of the forebody strakes is that
they produce very little rolling or pitching moment when deployed as yaw control
devices. These favorable characteristics make the forebody strakes a promising
yaw control concept for poststaitl angles of attack. The conformal forebody strakes
are currently fiight tested ori F-18 tMcr2tt.49
The basic fluid flow mechanism responsible for generating suchlarge side forces
and yawing moments is schematically shown in Fig. 8.74. The basic forebody
operating at high angles of attack has a pair of small counter-rotating vortices.
However, the forebody with a single strake of sufficiently large height (exceeding
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK 743
Section A-A
~~
Undeflected
strakes
Confor
~-
Deflected strakes
Fig.8.73 Actuated forebody strakes.4
the critical height) has a much larger and stronger strake vortex, which is displaced
far above the strake. This type of fluid flowspattern gives rise to the pressure
distribution shown in Fig. 8.74v On the left half: even though the vortex is stronger,
the pressure is higher compared to the basic case owing to earlier flow separation
induced by the strake. On7he right half, flow pattern and the pressure distribution
are essentially similar to the basic case. Therefore, the net effect of the strake is
that the suction on the side of the strake is reduced, and the forebody develops
a side force to the right as shown. Here, the forebody strake acts like a spoiler.
The magnitude of this side force and the associated yawing moment depends on
the pressure differentials, which in turn depend on t~e axial and crrcumferential
location of the strake.
One possible limitation to the deployment of the forebody strakes on aircraft is
that they may cause an adverse interference on the operation of the radar, which
is usually housed inside the forebody. In this regard, the pneumatic methods of
forebody blowing and suction offer better alternatives as discussed below.
LEFT HALF
RIGHT HALF
Cp
Cp
Fig. 8.74 Fluid flow mechanism associated with forebody strakes.4~
744 PERFORMANCE, STABILI-fY, DYNAMICS, AND CONTROL
b) Aft jet blowing
c) Slot blowing
Fig. 8.75 Schematic illustration of forebody blowing concepts.
Forebody blowing. Thejetblowingis perhaps one ofthe earliest concepts that
was explored for forebody vortex control.4~The blowing is usually accomplished
using single or multiple nozzles or through slots located at various places on the
forebody. The direction ofjet blowing is either forward (against the airflow) or aft
(along the flow) or slot (tangential to the body surface) as illustrated in Fig. 8.75.
The optimal blowing configuration for a given aircraft usually depends on the
forebody geometry, angle of attack, and sideslip.
An example of the effect of forebody jet blowing for the F-16 aircraft is shown
in Fig. 8.76a. The nozzles on either side are fixed at fuselage station (FS) 5 and
blow the jets aft along the forebody surface. The direction of the side force is
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