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turn rat". can no longer be improved by conventional methods. The longer the turn
lasts, the more time an opponent has to set up an attack. On the other hand, the
capability of controlled fiight at high angles of attack wellbeyond the stall angle of
attack offers distinct and virtually unlimited tactical advantage over the adversary
in WVR encounters. The Herbst maneuvcr (see Chapter 2) is one such example.
Thus, the technical requirements for flight at high angles of attack became directly
opposite of those advocated in the 1960s. Whereas in the 1960s, the high-angle-
of-attack regime was regarded as extremely hazardous and to be avoided at all
cost the current requirements for fighter aircraft demand a high degree of agility
and unlimited capability at high angles of attack. With the advent of highly lethal,
all-aspect air-to-air missiles, the capability to get a first shot has become extremely
importantin air combat.The F-15 STOL and Maneuver Technology Demonstrator
(S/MTD), the F/A-18, and experimental programs like the X-29 and the X-31 are
examples of fighter aircraft designed and developed with this philosophy.
A summary of usage of high-angle-of-attack controls technology in the design
and evolution of combat aircraft in the United States is schematically presented in
Fig. 8.1.2
8.3 Brief Ovenriew of High-Alpha Problems
Typically, a modern fighter aircraft features along, pointed slender fuselage and
thin, sharp leading-edge, highly swept delta wings. Furthermore, they also feature
wing strakes, or leading-edge extensions (LEX), and close-coupled canards. Such
aircraft configuratrons attain their maximum lift coefficients in the angle of attack
range of 25-3s5 deg. The high angle of attack problems encountered by these types
of aircraft are schematically illustrated in Fig. 8.2.3
As the angle of attackis increased, the aircraft experiences the onset of buffeting.
The buffetingis'characterized by heavy fiuctuationsin aerodynamic forces over the
horizontal tail because of the separated, turbulent wing and fuselage wake passing
over it In this angle of attack range, the aircraft may also experience pitch-up.
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK 675
Control shakers
Control pushers
a-limiting
Control
Interconnects
Control
limiting
rstato damper
Spln prevent
--r Wlng rock -w Thrust
i suppression i vectoring
I Rollpertormlnnt.e L ll,ontro's
f enhancement - INS-based
l- Roll/yaw rate sensing
limlters
Inertla coupling
compensation
Fig.8.1 AppLication ofhigh angle-of-attack technology.z
.~
~
~3
(-)
4s
~
Angle of Attack
Fig. 8.2 Various limitations on the use of maximum lift coefficienL3 (Courtesy
AGARD.)
676 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
With further increase in angle of attack, the aircraft is likely to experience the
following problems.
Wing rock is an oscillatory motion predominantly in roll about the body axis.
Wing rock can be highly annoying to the pilot and may pose serious limitation
to the combat effectiveness. In addition, the wing rock can be a safety problem
duringlan ing.
8.3.2 Roll Reversaland Yaw Departure
The roll reversal phenomenon is one in which the aircraft rolls in the opposite
direction to aileron input. The roll due to adverse yaw overpowers the proverse
roll due to ailcrons.
The directional instability/directional departure may cause the aircraft to depart
from controlled flight and enter into a spin. Whethe; the aircraft can develop a
steady spin depends on the balance between the inertial and aerodynanuc moments
as we discussed in Chapter 7.
In tliis chapter, we will study the problems of wing rock, rollreversal, directional
instability/departure, and the control concepts that are currently explored for -flight
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