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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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