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counters a crosswind, the resultant 'velocit)t vector falls out of the airplane's plane
of symmetry, producing sideslip. An aircraft with positive directional stability
(Cnp  >  O) will tend to realign itself with the direction of the resultant wind so that
the sideslip is eliminated. Takeoff with this kind of aircra:ft orientation (Fig. 3.85a)
during the ground run can pose safety problems. To prevent this, the rudder should
be capable of generating a yawing moment to counter that due to directional stabi-
lity so that the aircraft sideslips but is properly oriented with respect to the runway
(Fig. 3.85b). However, the takeoff or landing performance will be below normal
because the sideslipping aircraft experiences a higher drag.
     Adverse yaw.     In alevelcoordinatedmrn,the aircraftis moving approximately
in a circular path in a horizontal plane as shown in Fig. 3.86. The angular velocity
Q about the vertical axis passing through the center of the turn is equal to V/R.
During the turn, the outer wing is moving with a higher velocity compared to
the inner wing. As a result, the outer wuy; experiences relatively higher drag,
and this imbalance in drag (AD) induces an yawing moment that tends to turn
the nose of the aircraft away from the center of the turn. This phenomenon is
known as adverse yaw. Because of this adverse yaw, a rolling motion may also
be induced because of the dynamic derivative CLr (which is usually positive) as
we will study in Chapters 4 and 6. This rolling motion caused by Clr iS called
adverse roll because it tends to bank the aircraft away from the direction in which
the aircraft is turning. Generally, the magnitude of the adverse yawing moment
is small. However, for rapid turns at lugh angles of attack, it can pose problems,
particularly if the adverse roll overpowers the proverse roll because of ailerons
causing a roll reversal. Therefore, the rudder should have sufficient control power
to prevent the development of adverse yaw during a tum.
      Asymmetric power.      On multiengined furcraft, either partial or total failure of
one or more engines grves rise to an asymmetric power situation that can generate
a significant yawing moment as schematically shown in Fig. 3.87. If this yawing
moment is not countered by the rudder, the aircraft will develop a sideslip and, in
 some cases, may go out ofcontrol because of aerodynamic roll-yaw coupling. The
rudder must be designed to have sufficient control"authority to maintain controlled
flight under such conditions.
    Spin recove,)r.   Generally, in spin, the airplane is operating at high angles
of attack with wings and horizontal tail surfaces more or less completely stalled.
Quite often, the rudder may be the only control that has some effectiveness under

580           PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
1-5
0.5
            Dutch Roll Approximation
--  Complete Fitth-Order System
                                                                                                                                                                                                           
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