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.3       .4       .5
  (-,,: :.o Oerrad)   .
        M t0
Fig. 4.28    The parameter (C,r/CL)W for subsonic speeds.
The data to evaluate the parameter (Clr/CL)CL = O,M =0  are- presented in Fig.  4.28.
Furthermore,
  A12  ) _ (l/2) (A +A4"oAA.,,)~rad'              (4.617)
The vertical tail contribution is given by7
(Clr)V : b2(lycos a-+zysina)(zycos oc -lysina)Cyp,v       (4.618)
    The value of (CLr)V given by Eq. (4.618) is per radian.
    For sup        iic speeds, no general. method suitable for engineering purposes is
availablupersonic
     Estimation of Cnr.     This derivativeis a measure ofthe yawing momentinduced
due to the yaw rate experienced by the aircraft and is known as the damping-in-yaw
derivative. This is one of the most important lateral-directional dynamic stability
416                PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
derivatives. Generally, the contributions of the fuselage and horizontal tail are
small and can be ignored so that
                                                     Cnr = (Cnr)W + (Cnr)V
The vertical tail contribution is given by
                        2
                                     (C ;r)V  =  b:?,(IU ,oS ct + zu sirity)2Cyp.v
(4.619)
(4.620)
The value of (Cnr)V given by Eq. (4.620) is per radian.
    An approximate estimate of (Cnr)W can be obtained using the strip theory as
follows.
   Consider once again the elemental strip RT on the right wing of an aircraft
undergoing yawing motion with a positive yaw rate r (Fig. 4.27b). As a result of
this yawing motion, the relative velocity experienced by the wing sections varies
along the span, but the angle of attack can be assumedjto remain approximately
constant. The relative air velocity experienced by the left wing sections is higher
compared to that of the right wing sections. Consequently, the drag of the-left
 wing is higher than that of the right wing,leading to a negative or restoring yawing
　
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