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to experience moderate initial yaw divergence, followed by a roll reversal, and
this tendency is likely to become worse in region D, where a strong directional
divergence coupled with roll reversal may occur, leading to a spin entry.
The above definitions of various regions A-D is derived by the application of
Cnp DYN and LCDP criteria to various aircraft.43 Examples ofthis application for the
F-8, the F-102, the F-106, and the SAAB 37 are shown in Fig. 8.62. The flight tests
on the F-8 aircraft modelindicate that beyond 26-deg angle of attack, the departure
is likely to occur. Full-scale flight tests confirm this and further indicate that the
aircraft experience a mild to moderate rolling departure. The motion immediate,ly
following departure (from controlled fiight) is primarily a rolling motion. From
Fig. 8.62, we note that the points for the F-8 aircraft at ct = 25 deg and a - 30 deg
fall in region B. Therefore, the observed departure of the F-8 aircraft is correctly
predicted by the Cnp DYN and LCDP criteria.
Model tests on the F-102 indicate that the aircraft experiences a directional
divergence above 28-deg angle of attack, and the severity of this divergence in-
creases with further increase in the angle of attack. From Fig. 8.62, we observe
that this behavior is correctly predicted because the point for ct = 30 deg falls in
the overlapping area of regions C and D.
The full-scale F-106 aircraft indicates a departure followed by poststall gyrations
for angles of attack between 34 and 38 deg. The correlation shown in Fig. 8.62
appears to be quite satisfactory as the points between 33- and 40-deg angle of
attack fall in region B.
The SAAB 37 aircraft has good flying qualities up to a = 35 deg. For higher
angles of attack, the aircraft experiences a departure to lateral control inputs and
exhibits spin susceptibility. Application of p:57us-8 axis stability indicator to the
SAAB 37 is shown in Fig. 8.63. Up to a = 35 deg, a'_p > 0 and ry_p > 'a8,
indicating good fiying qualiries. At a - 35 deg, a'_p = ry8, which implies that
STABILITY AND CONTROL PROBLEMS AT HtGH ANGLES OF A71-ACK 735
Q- p.Q 6
(Deg)
Fig. 8.63 P-plus-8 stability indicator, SAAB 37 airplane.43
L.
LCDP = 0. Beyond a = 35 deg, a'_p < cta and LCDP becomes negative, indicat-
ing departure to lateral control inputs as experienced by the aircraft.
In summary, both.Cntr DYN-LCDP and f3-plus-8 axes stability indicators corre-
late well with experimental data on model and full-scale fiight tests. Therefore,
these criteria can be used with some degree of confidence r.o predict the depar-
ture characteristics during preliminary design stages. Although both criteria are
equivalent, the Cnp DYN-LCDP criterion is better suited for practical application.
8.13 Control Concepts at High Angles of Attack
8.73.rf Pitch-Up
The first phase of a typical high angle of attack, poststall maneuver is a pitch-up
to a high angle of attack orientation. To generate the required pitch- up as rapidly as
possible, sufficient pitching moment capability must be available throughout the
poststall maneuver envelope. The effectiveness of the conventional aerodynamic
pitch control surfaces, as measured by the pitch acceleration, decreases with angle
of attack and is far below the required level as schematically shown in Fig. 8.64
for the F-15 aircraft. This decrease in longitudinal control effectiveness occurs
because of the immersion ofcontrol surfaces in the low-energy stalled flow at high
angles of attack. ,
736 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
Fig. 8.64 Schematic illustration of pitch control requirements at lugh angles of
attack/6
8.I3.2 Ve/ocity Vector Rol/
Suppose an aircraft executes a pure rolling motion about the x-body axis as
shown in Fig. 8.65a. From Eq. (8.1), we have
sin p = sin a sin 4
For small values of p and ~, we have
so that
p a p sinor
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