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740 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
Actuated strake
Jet blowing
sT;oatngl~n;i;:~
Jet suction
Fig. 8.69 Forebody rrortex control concepts.Z
when the conventional rudder loses its effectiveness. The forebody vortex control
also offers the potential to improve the directional-stability at high angles of attack,
augment roll-yaw damping, and prevent departure/spin entry.
The important forebody vortex control concepts that are currently studied are
actuated forebody strakes and forebody blowing and suction. These concepts are
schematically illustrated in Fig. 8.69.
Actuated forebody strakes. The use of a fixed symmetric pair of forebody
strakes produces a pair of symmetric vortices suppressing the naturally occurring
vortex asymmetry at high angles of attack and eliminates the associated side force
as shown in Fig. 8.70a. On the other hand, the deployment of a single strake can
systematically manipulate the forebody vortex system to produce a side force with
large moment arm in a controlled manner for generating the much needed yaw
control at high angles of attack as shown in Fig. 8.70b.
The magnitude ofthe side force depends on the axial and circumferentiallocation
of the strakes and height of the strakes. For example, a strake located at a given
axial location is found to produce the highest levels of yaw control if located
circumferentially at 4 = 120 deg as shown in Fig. 8.71. It may be noted that
the value of 4 for maximum strake effectiveness also depends on the fOfebody
cross-sectional shape, fineness ratio, angle of attack, andivs"ideslip. As shown in
Fig. 8.71, the conventional rudder begins to lose its effectiveness (as measured by
ACn) around a = 20 deg and, atjust about this angle ofattack, the forebody strake
starts to develop side force/yawing moment. The peak yawing moment coeyfficient
of the forebody strake occurs around cy - 50 deg and is more than twice the
maximum yawing moment produced by the rudder at its peak efifectiveness. The
strake effe9tlveneFss is higher if located (axially)'closer to the apex because it is
here that the forebody vortex system originates.
The strake height has a prominent effect on the direction rather than the magni-
tude of the side fosrce. If the strake height is below a certain critical value, the strake
acts like a boundary-layer tripping device and causes a transition from laminar to
turbulent fiow in the boundary layer. The turbulent boundary layer adheres to the
body surface to a much greater extent and delays the flow separation. As a result,
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK 741
Section A A
SF=O
a) Symmetric pair of strakes
.14
.12
.10
.08
.06
Cn .04
.02
o
-.02
-.04
-.06
-.08
n
Section A A
b) Single strake
Fig.8.70 Conceptsofforebodystrakes.
o 10 20
30 40 50 60 70
a,deg
Fig. 8.71 Effects of radiallocation offorebody strakes.CZ
r0]
742 PERFORMANCE. STABIUTY, DYNAMtCS, AND CONTROL
a) h < ha
ation
nt
b) h < ha
Fig. 8.72 Effect of strake height on the forebody flow pattern.49
kt
the vortex on the side of the strake lies closer to the body. On the other side where
there is no strake, the flow pattern essentially remains the same as the basic case
with the vortex positioned further away from the body surface. As a result, the
suction is relatively higher on the side of the strake, and a side force is produced in
the direction of the deployed strake (Fig. 8.72a). It is to be noted that this type of
flow pattern is likely to be sensitive to flow Reynolds numbers, On the other hand,
if the strake height is above the critical value, it promotes flow separation on its
side and produces a forebody side force in the direction opposite to the deployed
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