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control authority in the low-speed, low dynamic pressure conditions experienced
at high angles of attack when the traditional aerodynamic control surfaces will
have lost most of their effectiveness because of their immersion in low-energy
stalled flow. The application of multiaxis TVC provides increasing control author-
ity at high angles of attack, leading to improved low-speed agility. The TVC can
eliminate the limitations on the maximum attainable roll rate capability imposed
because of insu:fficient aerodynamic control authority at high angles of attack.
Some typical multiaxis TVC concepts currently being studied are shown in
Fig. 8.79. .
The primary benefit of TVC is the availability of control power at high angles
of attack, low dynamic pressure conditions where the conventional aerodynamic
STABILITY AND CONTROL f'ROBLEMS AT HIGH ANGLES OF ATTACK 747
Ot
a)
b)
Fig. 8.77 Slot blowing for forebody vortex control on the F-16 aircraft.49,50
PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
Cn
Fig. 8.78 Forebody vortex control using suction for RAE FIIRM1 aircrat:t.so,sl
Fig.8.79 Mulhaxis thrust vectoring concepts.4
STABILITY AND CONTROL PROBLEMS AT HIGH ANGLES OF ATTACK . 749
Yawing
moment
coefficient
Angle of attack, deg
Fig. 8.80 Example ofsmooth blending of aerodynamic and thrust vector controlsl
control will havelost most ofits effectiveness. However, another significant benefit
offered by the TVC is that it leads to smaller aerodynamic control surfaces. Tra-
ditionally, the size of an aerodynamic control surface is determined from control
effectiveness at low-speed considerations. The fact that the TVC can provide the
necessary controlauthority atlow-speed orlow dynamic pressure conditions makes
it unnecessar}r to size the aerodynamic surfaces from low-speed considerations.
However, at high-speed or high dynamic pressure conditions, the aerodynamic
control surfaces become effective, and much smaller surfaces can be used com-
pared to those determined from low-speed considerations. The use of small-size
aerodynamic control surfaces leads to lower structural weight, reduced trim drag,
and improved overall performance.
An ideal scenario would be one in which the aerodynamic controls and the TVC
are smoothly blended as shown in Fig. 8.80, the TVC providing most of the contrrol
power at high-angle-of-attack/low dynamic pressure conditions and the aerody-
namic surfaces taking over at low-angle-of-attack/high dynamic pressure con-
ditions. As shown in Fig. 8.80, 10 deg of thrust vectoring on a typical current
generation fighter aircraft matches or exceeds the effectiveness of the ideal control
at low-speed conditions.29 In view of the enormous potential of TVC concept,
extensive research is being conducted by NASA using the F/A-18 aircraft and by
NASA/Boeing using the tailless X-36 research airplane as test beds for evaluating
the benefits of TVC technologies for future combat aircraft designs.
8.14 Summary .
Recent interest in unlimited high-angle-of-attack/poststall maneuver capability
has led to extensive research and technology development in high~angle-of-attack
aerodynamics, stability, and control. Modem combat aircraft feature thin, highly
swept delta wings and long, slender fuselages. The airflow over such configu-
rations at high angles of attack is quite complex and is characterized by exten-
sive separated flow regions dominated by the forebody and LEX/wing vortices
and their mutual interactions. The existence of such complex flow fields leads
to aerodynamic nonlinearities, loss of lateral-directional stability and control,
roll reversal, departure from controlled flight, and unsteady motions like wing
750 PERFORMANCE, STABILITY, DYNAMiCS, AND CONTROL
References
IChambers, J. R., "High-Angle-of-Attack Technology, Progress and Challenges:' NASA
CP-3149, Part l, Vol. 1, 1990, pp. 13-22.
2Nguyen, L. T., Arbuckle, P. D., and Gera, J., "Progress in Controls Technology for
High-Angle-of-Attack Apphcations:' NASA CP-3149, Part I, Vol. I, pp. 117-156.
3 Skow, A. M., and rfiMga, A,, Jr., "A Survey of Analytical and Experimental Techniques
to Predict Aircraft Dynamic Chiuacteristics at High-Angle-of-Attack:' CP-235, AGARD,
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