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Transition
I
B
Section BB
Right Vortei
Sucked Down
due to Suction
on Right Wing
Lt~t Vortex
Sucktd Down
due to Suction
on Left
t = t3 tAt
Fig. 8.53 Ericson's conceptual explanation offorebody-induced wing rock.3r
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK . 725
wing rock on aircraft configurations is different from that assumed in Ericson's
hypothesis. In spite of this, Ericson's hypothesis provides a physical insight into
the possible flow mechanism causing the forebody-induced wing rock.
8.11 Suppression of Wing Rock
Several attempts have been made in recent years to find solutions to the wing
rock problem. These approaches can be broadly classified into two groups: passive
aerodynamic methods and active control.
8.71.1 Passive Aerodynamic Methods
As we have seen above, the wing rock is caused by the wing vortices or the
forebody vortices or by an interaction between these two vortices, The loss of
damping in roll at high angles of attack initiates the wing rock, and some form
of vortex-asymmetry-switching mechanism sustains it as a limit cycle oscillation.
Therefore, the key to suppressing the wing rock by passive aerodynamic methods
is to manipulate the vortices so that the vortex-asymmetr3r-switching mechanism
is suppressed and the vortices are forced to assume a symmetric disposition.
The basic process thatleads to the formation ofleading-edge vortices on the lee
side of a delta wing is the flow separation at the sharp leading edges. Therefore,
a direct manipulation of this process holds the key to suppressing the wing rock.
One such example is the concept of spanwise tangential jet blowing along the
leading edges as shown schematically in Fig. 8.54. An application of this concept
for suppression of wing rock is shown in Fig. 8.55.38 The test model used38 is
a 60-deg delta wing with a LEX of 78-deg sweep, The wing rock of this model
is primarily induced by the LEX vortices. The Coanda effect induced by the jet
energizes the boundary layer in the vicinity of the leading edge and, thereby, delays
the flow separation. This process leads to smaller and better organized vortices.
The symmetrical blowing on both wings leads to symmetrically disposed vortices.
As a result, the wing regains roll damping, and wing rock is suppressed as shown
in Fig. 8.55b.
Fig.8.54 Tangenhalleading-edge spanwise blowing concept.38
726 PERFORMANCE, STABIL17Y, DYNAMICS, AND CONTROL
41
--
{
Q
=
&
s-
}
a) No blowing
b) With blowing
Fig. 8.55 Suppression of wing rock with tangentialleading-edge blowing.38
An example of suppression of forebody-induced wing rock using the blowing
concept37 isrpresented schematically in Fig. 8.56. The model tested was a 60-deg
delta wing with a conical nose, and the angle of attaclc was 45 deg. The model
exhibited wing rock with an amplitude of about 22 deg as shown in Fig. 8.57a.
Note that at a - 45 deg and p - 0, the wing rock of this model is induced by
the forebody because an isolated 60-deg delta wing is not likely to exhibit wing
rock. At high angles of attack, such a wing exhibits the roll attractor phenomenon
as described earlier.
The blowing slots were located on either side of the conical nose as shown in
Fig:. 8.56. rfhe wing rock was suppressed by asymmetrical (one-sided, right or left)
tangential aft blowing as shown in Fig. 8.57b. However, the disadvantage of one-
sided blowing is that it is accompanied by significant amounts of undesirable side
force and yawing moments. To avoid this,it is preferable to employ simultaneous
symmetric, tangential aft blowing from both sides. However, this approach of
symmetrical blowing was not much effective in suppressing the wing rock.39
Another interesting example of the application of the blowing concept to the
suppression of forebody-induced wing rock is discussed in Ref. 40. The model
tested was a generic aircraft configuration with a circular-cross-section forebody
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK 727
/
/
Fig.8.56 Schematic arrangement offorebody blowing.39
/
lio r
of fineness ratio 6 and a delta wing of 78-deg sweep. This model exhibited wing
rock above a 22-deg angle of attack. The wing rock amplitude reached 40 deg
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