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 zontal tail exceeds the destabilizing pitching moment caused by the wing and the
fuselage and, as a result, the net induced pitching moment is stabilizing. For a
 modern highly swept-back-wing airplane, the stabilizing moment induced due to
 wing exceeds the destabilizing pitching moments caused by the horizontal tail and
 the fuselage. These concepts are schematically illustrate/in Fig. 3.33. Thus, for
 a conventional airjlane, the tail plays the dualvrole of providing both stability and
 trim, whereas, for a modern highly swept-back-wing airplane with aerodynamic
 center aft of the center of gravity, the wing prorrides stability, and the horizontal
 tail functions as a trimming device.
    By longitudinal control, we mean the capabOity to change the equilibrium or
trim-lifi coefficient within the permissible range (O < CL < CL.max). In other
words, the airplane shoulcf be 7apable of flying at any desired angle of attack
within its aerodynamic limits. To explore the methods of longitudinal control,
let us take a look at the expression for the equilibrium lift coefficient, which is
obtained from Eq. (3.65) and the condition Cm.cg = 0.
C~  = -Cmac.vu - Cm f  +ac(ay, - /\U +/r - e) Virlr                 (3.91)
                                   xcg - xac
+
STATIC STABILITY AND CONTROL
 Cmo
Crn  O
+
┏━━━┳━━━━━━━━━━━━━━━━━━━━━━┓
┃      ┃                                   ACmw* ,  ┃
┃4     ┃---                                         ┃
┃      ┃        Tco                                 ┃
┣━━━╋━━━┳━━━━━━━━━━━━━━━━━━┫
┃      ┃      ┃                                    ┃
┣━━━╋━━━╋━━━━━━━━━━━━━━━━━━┫
┃\ cLo ┃ACLi  ┃      CL                            ┃
┃      ┃      ┃ .                                  ┃
┃      ┃      ┃I       jACmtl > } ACmt N           ┃
┣━━━┻━━━┫                                    ┃
┃ '~           ┃ACmt                                ┃
┃              ┃~                                   ┃
┗━━━━━━━┻━━━━━━━━━━━━━━━━━━┛
 Cmo
Cm  O
a)  Conventional air )lane
b) Modern swept-back-wing airplane
Cmt
AC,r,w I
AC mt
ACmtl
Fig. 3.33    Schematic illustration ofstability and trim concepts.
207
Here, we have assumed CL ~ CL.w- In other words, the tail lift, even though con-
tributing to trim drag penalty and structural weight considerations, is assumed
　
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