NAVWEPS oo-s01-80 
STABILITY AND CONTROL 
Mach number. As a corollary of this increase 
in stability is a decrease in controllability and 
an increase in trim drag. 
The static directional stability of an air- 
plane decreases with Mach number in super- 
sonic flight. The influence of the fuselage and 
the decrease in vertical tail lift curve slope 
bring about this condition. 
The dynamic stability of the airplane 
generally deteriorates with Mach number in 
supersonic flight. Since a large part of the 
damping depends on the tail surfaces, the 
decrease in lift curve slope with Mach number 
will account in part for the decrease in damp 
ing. Of course, all principal motions of the 
aircraft must have satisfactory damping and 
if the damping is not available aerodynami- 
cally it must be provided synthetically to 
obtain satisfactory flying qualities. For many 
high speed configurations the pitch and yaw 
dampers, flight stabilization systems, etc., 
are basic necessities rather than luxuries. 
Generally, flight at high Mach number will 
cake place at high altitude hence the effect of 
high altitude must be separated for study. 
All of the basic aerodynamic damping is due 
to moments created by pitching, rolling, or 
yawing motion of the aircraft. These moments 
are derived from the changes in angles of 
attack on the tail surfaces with angular 
rotation (see fig. 4.15). The very high true 
airspeeds common to high altitude flight 
reduce the angle of attack changes and reduce 
the aerodynamic damping. In fact, the aero- 
dynamic damping is proportional to & 
similar to the proportion of true airspeed to 
equivalent airspeed. Thus, at the altitude of 
4O,C00 ft., the aerodynamic damping would 
be reduced to one-half the sea level value and 
at the altitude of 100,000 ft. the aerodynamic 
damping would be reduced to one-tenth the 
sea level value. 
High dynamic pressures (high $I can be 
common to flight at high Mach number and 
adverse aeroelastic effects may be encountered. 
If the aircraft surfaces, encounter significant 
deflection when subject to load, the tendency 
may be to lower the contribution to static 
stability and reduce the damping contribution. 
Thus, the problem of adequate stability of the 
various airplane motions is aggravated. 
PILOT INDUCED OSCILLATIONS 
The pilot may purposely induce various 
motions to the airplane by the action of the 
controls. In additron, certain undesirable 
motions may occur due to inadvertent action 
on the controls. The most important con- 
dition exists with the short period longitu- 
dinal motion of the airplane where pilot- 
control system response lag can produce an 
unstable oscillation. The coupling possible 
in the pilot-control system-airplane combi- 
nation is most certainly capable of producing 
damaging flight loads and loss of control of 
the airplane. 
When the normal human response lag and 
control system lag are coupled with the air- 
plane motion, inadvertent control reactions 
by the pilot may furnish a negative damping 
to the oscillatory motion and dynamic in- 
stability exists. Since the short period motion 
is of relatively high frequency, the amplitude 
of the pitching oscillation can reach dangerous 
proportions in an unbelievably short time. 
When the pilot induced oscillation is en- 
countered, the most effective solution is an 
immediate release of the controls. Any at- 
tempt to forcibly damp the oscillation simply 
continues the excitation and amplifies the 
oscillation. Freeing the controls removes 
the unstable (but inadvertent) excitation and 
allows the airplane to recover by virtue of 
its inherent dynamic stability. 
The pilot induced oscillation is most likely 
under certain conditions, Most obvious is the 
case of the pilot unfamiliar with the “feel” 
of the airplane and likely to overcontrol or 
have excessive response lag. High speed flight 
at low. altitude (high 4) is most likely to 
provide low stick-force gradients and periods 
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