Topic of the Day: Controllability Factors
(1) Static stability is the inherent flight characteristic of an aircraft to return to
equilibrium after being disturbed by an unbalanced force or movement.
(2) Controllability is the ability of an aircraft to respond positively to control surface
displacement, and to achieve the desired condition of flight.
(3) At high-flight altitudes, aircraft stability and control may be greatly reduced.
Thus, while high-altitude flight may result in high TAS and high MACH numbers, calibrated airspeed is much slower because of reduced air density. This reduction in density means that the
angle of attack must be increased to maintain the same coefficient of lift with increased altitude. Consequently, jet aircraft operating at high altitudes and high MACH numbers may
simultaneously experience problems associated with slow-speed flight such as Dutch roll, adverse
yaw, and stall. In addition, the reduced air density reduces aerodynamic damping, overall
stability, and control of the aircraft in flight.
(I) Dutch roll is a coupled oscillation in roll and yaw that becomes objectionable when roll, or lateral stability is reduced in comparison with yaw or directional stability. A stability augmentation system is required to be installed on the aircraft to dampen the Dutch roll tendency when it is determined to be objectionable, or when it adversely affects the control stability
requirements for certification. The yaw damper is a gyro-operated autocontrol system installed to
provide rudder input and aid in canceling out yaw tendencies such as those in Dutch roll.
(ii) Adverse yaw is a phenomenon in which the airplane heading changes in a direction opposite
to that commanded by a roll control input. It is the result of unequal lift and drag characteristics of
the down-going and up-going wings. The phenomena are alleviated by tailoring the control design
by use of spoilers, yaw dampers, and interconnected rudder and aileron systems.
(4) Supersonic flow over the wing is responsible for:
(i) The formation of shock waves on the wing which result in drag rise.
(ii) An aft shift in the center of lift resulting in a nosedown pitching moment called MACH tuck.
(iii) Airflow separation behind the shock waves resulting in MACH buffet.
(5) Swept wing and airfoil design alone, with boundary layer energizers such as the vortex
generators described earlier, has reduced the hazardous effect of the problems described above.
However, these problems are still encountered to some extent by the modem turbojet airplane in
high-altitude flight.
(6) In general, this discussion has been confined to normal level, unaccelerated 1.0 G-flight.When turning or maneuvering about the pitch axis, however, acceleration of G-forcescan occur while maintaining a constant airspeed. As G-forces increase, both the aircraft’s aerodynamic weight and angle of attack increase. The margin over low-speed stall buffet decreases, as well as the margin below MACH buffet, because of the increased velocity of the air
over the wing resulting from the higher angle of attack. This, in effect, could lower the aerodynamic ceiling for a given gross-weight. Increased G-loading can also occur in non maneuvering flight because of atmospheric turbulence or the lack of fine-touch skill by the pilot.Pilots flying at high altitudes in areas where turbulence may be expected must carefully consider acceptable safety margins necessary to accommodate the sudden and unexpected vertical accelerations which may be encountered with little or no warning.
