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frequency tOspo, 20 1o91o IG,,I  =  ~40 db or IGul  = 0.01, i.e., just la/o change in
the forward speed for a unit defiection of the elevator with a frequency of 3.6 rad/s.
Thus, we understand that forward speed is influenced only by slow elevator inputs
and rapid elevator movements have little or no effect on the forward speed.ln other
words, elevator input is not the best way to quickly change the forward speed.
     The phase angle /Gu is approximately zero at lower values and then suddenly
drops off to -180 around co -. 0.21 rad/s, which corresponds to the phugoid mode.
This type of behavior of ZG" is characteristic oflightly damped systems. As said
above, this implies that only a slow movement of the elevator resultys in a change of
forward speed without any phase lag. For co > COspo, the phase Jag increases, which
is characteristic of heavily damped systems.
~'
The transfer functions of the general aviation airplane based on short-period ap-
proximation [substitute in Eqs. (6.131) and (6.123) and use the relation q(s) =
sAO(s)] are given by
      The frequency response of the airplane is ofinterest because the pilot input to a
control surface can be considered as a signal with a frequency content. Generally,
the bandwidth of a human pilot is about 4 rad/s (Ref. 3). Therefore, it can be
564           PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
co
1,
 t:
o
Phugoid Mode     Short-Period Mode
cn
a)
o
 <,
o
~
           Freqwncy, rad/s
Frequency response : Angle of Attack
                                                                                                                                                       
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