each airplane is outlined in the pilot’s hand- 
book and it is impcrativc that the specific tech- 
nique be followed for successful recovery. 
PITCH-UP 
The term of “pitch-up” generally applies to 
the static longitudinal instability encountered 
by certain configurations at high angle of 
attack. The condition of pitch-up is illustrated 
by the graph of CM versus C, in figure 4.33. 
Positive static longitudinal stability is evident 
at low values of Cs by the negative slope of the 
curve. At higher values of Cs the curve changes 
to a positive slope and large positive pitching 
moments are developed. This sort of in- 
stability implies that an increase in angle of 
attack produces nose up moments which tend 
to bring about further increases in angle of 
attack hence the term “pitch-up” is applied. 
There are several items which may con- 
tribute to a pitch-up tendency. Sweepback 
of the wing planform can contribute unstable 
moments when separation or stall occurs at 
the tips first. The combination of sweepback 
and taper alters the lift distribution to produce 
high local lift coefficients and low energy 
boundary layer near the tip. Thus, the tip 
stall is an inherent tendency of such a plan- 
form. In addition, if high local lift coefficients 
exist near the tip, the tendency will be to incur 
the shock induced separation first in these 
areas. Generally, the wing will contribute 
to pitch-up only when there is large sweepback. 
Of course, the wing is not the only item con- 
tributing to the longitudinal stability of the 
airplane. Another item important as a source 
of pitch-up is the downwash at the horizontal 
tail. The contribution of the tail to stability 
depends on the change in tail lift when the air- 
plane is given a change in angle of attack. 
Since the downwash at the tail reduces the 
change in angle of attack at the tail, any in- 
crease in downwash at the tail is destabilizing. 
For certain low aspect ratio airplane configura- 
tions, an increase in airplane angle of attack 
may physically locate the horizontal tail in 
NAWEPS DD-EDT-89 
STABILITY AND CQNROL 
the wing flow field where higher relative 
downwash exists. Thus, a decrease in stability 
would take place. 
Certain changes in the flow field behind the 
wing at high angles of attack can produce large 
changes in the tail contribution to stability. 
If the wing tips stall first, the vortices shift in- 
board and increase the local downwash at the 
tail for a given airplane C,. Also, the fusel~age 
at high angle of attack can produce strong 
cross flow separation vortices which increase 
the local downwash for a horizontal tail placed 
above the fuselage. Either one or a combiua- 
tion of these downwash influences may provide 
a large unstable contribution of the horizontal 
tail. 
The pitch-up instability is usually conlined 
to the high angle of attack range and may be 
a consequence of a configuration that otherwise 
has very desirable flying qualities. In such a 
case it would be necessary to provide some 
automatic control function to prevent entry 
into the pitch-up range or to provide synthetic 
stability for the condition. Since the pitch-up 
is usually a strong instability with a high1 
rate of divergence, most pilots would not be 
capable of contending with the condition. At 
high 4, pitch-up would be of great danger in 
that structural failure could easily result. At 
low q, failing flight loads may not result but 
the strong instability may preclude a successful 
recovery from the ensuing motion of the, air- 
plane. 
EFFFCTS OF HIGH MACH NUMBFB 
Certain stability problems are particular to 
supersonic flight. While most of the problem 
areas have been treated in particular in previous 
discussion, it is worthwhile to review the 
effects of supersonic flight on the various items 
of stability. 
The static longitudinal stability of an air- 
plane increases during the transition from sub- 
sonic to supersonic flight. Usually the prin-