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                                                         Min <W,,,a    Min Wn,
Level Category . Class  Min<a   rad/s    rad/s
-----
I      A     I,IV    0.19    0.35     1.0
I      A     n,m    0.19    0.35     0.4
I       B      All    0.08    0.15     1.0
I      C   I,II_C,bIV  0.08    0.15     1.0
I      C    II_L,bIIJ[   0.08    0.15    0.4
II      All     All    0.02    0.05     0.4
III     All     All    0.02    ~     0.4
           == ==:==:: ====:: ::=  __  -    -, .r--~77-:-
:;rhhe ~etteerns Cg dda:ping requirement is that yielding the larger value of <.
                              d L denote the carrier-based and land-based aircraft.
   The natural frequency and damping ratio should be generally better or exceed
the minimum values given in Table 6.5 in terms of the minimum time to double
the amplitude.
     hSpiralmode.     The spiral mode flying qualities4 are given in Table 6.6 in terms
of minimum time to double the amplitude.
6.5  Closed-Loop Flight Control
    In the previous sections, we have discussed longitudinal and Iateral-directional
free and forced responses of the aircraft. These responses can be classified into two
broad categories: 1) t,hose involving mainly rotational degrees of freedom such
as short-period mode, the roll-subsidence mode, and the Dutch-roll oscillation
and 2) those involving flight path changes such as phugoid and spiral modes.
The responses belonging to first category are of high frequency. Therefore, it iS
essential that they be adequately damped. If not, it will be difficult for the human
pilot to take corrective action especially if the frequencies of the unstable modes
are higher than the bandwidth of the human pilot, which is about 4 rad/s. For such
cases, it is necessary to provide an automatic control system that ensures that these
modes are adequately damped in the closed loop sense. Such a controJ system is
called a stability augmentation system.
   There is another class of fiight control systems called autopilots. The autopi-
lots'are used for establishing and maintaining desired flight conditions. Once the
autopilot is engaged, it continues to hold that flight condition without fufiher in-
tervention from the pilot.
Table 6.6   Time to double amplitude for spiralmode
                          =-_. 7
Class    Category  Levell  Levelll  Levellll
I,IV       A     12s    12s     4s
I,IV     BandC   20s    12s     4s
IIandlll    All     20s    12s     4s
--
596            PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
   In the following, we will discuss some of the typical stability augmentation
systems and autopilots.
6.5.1  Pitch Stab,7ization System
      The pitch stabilization system is used when the short-period mode is unstable.
Typically, modern high-performance aircraft experience these problems because
such aircraft are intentionally made statically unstable for bettert'performance. As
we have seen earlier for the relaxed static stability rrersion of the general aviation
airplane, such aircraft experience exponential instability in pitch. Here, we will
　
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