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calledfelaxedstaticstabilit! cactsupward.
                              Fa'vor of 0
           ,oclpe~nu~a?o~e~of~hat the tail \ift a{
 madc ,laxedstaticstabilitY,weha'rc
            With rela:                                                                                           (3.85)
                                                              - No
                                               (ddCCA,x -. -cg -
              (3.86)
             >o
Then                                                       . (3.87)
                              Cr = Crao+ GddCc ~,xc~
                                           = C,"o + (Xcg - No)CL                          (3.88)
   Assunung that the tail/ift is posiUve so that C~  =  C~.w + C~.r, we obtain the
206         .  PERFORMANCE, STABIUTY, DYNAMICS, AND CONTROL
tail-lift coefficient for trim as
CL.r = =-Cmo      i
          N - Cr.,v                (3.89)
As said before, to trim a statically unstable airplane, we must have Cmo < 0.
Therefore, for positive tail lift as assumed,
l- -Cmo
} NI>IC.wl
(3.90)
Therefore, by suitably choosing the magnitude of Cmo and the level of relaxed
static stability, the designer can ensure that the tail lift is positive (upward) for the
desired operating conditions.
    With application of the RSS concept, we note that the airplane is in an unsta-
ble trim. In other words, it has no inherent or open-loop stability to counter any
extemal disturbance in angle of attack because of wind ~usts or air turbulence. A
pilot has to continuously monitor the airplane by operating the control surfaces
to maintain a trim condition. To fly such an airplane safely, the airplane must be
made closed-loop stable by a suitable design of the longitudinal control system,
which we will study later.
    For a conventional airplane with wing aerodynamic center ahead of the center
 of gravity, the tail lift is upward for trim. The wing lift acting ahead of the center
 of gravity develops a destabilizing moment. However, for a modern highly swept-
 back-wing airplane, the wing lift acting behind the center of gravity is stabilizing,
 and the tail lift acting downward generates a destabilizing pEtitching moment. To
 understand how both these vehicles are in stable trim, consider what happens when
 the lift coefficient increases by (ACL) because of a disturbance in angle of attack.
For the conventional airplane, the stable pitching moment induced by the hori-
　
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