
Ch07-H6927.tex 20/6/2007 16: 34 Page 193
Lateral–Directional Dynamics 193
and by assuming, quite correctly, that both I
x
and I
z
are usually much greater than
I
xz
. It then follows that
2ζ
d
ω
d
∼
=
−
⎛
⎝
◦
N
r
I
z
+
◦
Y
v
m
⎞
⎠
ω
2
d
∼
=
⎛
⎝
◦
N
r
I
z
◦
Y
v
m
+V
0
◦
N
v
I
z
⎞
⎠
∼
=
V
0
◦
N
v
I
z
(7.40)
Comparing the damping and frequency terms in the expressions in equations (7.40)
with those of the mass–spring–damper in Appendix 6 it is easy to identify the roles
of those aerodynamic stability derivatives which are dominant in determining the
characteristics of the dutch roll mode. For example,
◦
N
r
is referred to as the yaw
damping derivative and
◦
N
v
is referred to as the yaw stiffness derivative, and both are ...