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The LCDP for this case of ailerons alone is also called aileron alone departure
parameter (AADP).
Aderon p/us rudder for ro/l. On some aircraft, the ailerons and rudder are
interconnected for proper roll-yaw coordination during velocity vector rolls at
high angles of attack. For such cases, the LCDP is given by
k Cn8,) (8.5)
LCDP=C,rp-Ctp((CC,, k )
8.12.2 Yaw Departure
Many attempts have been made to derive simple expressions for predicting the
yaw departure at high angles of attack. One expression that is commonly used in
aircraft dynamics is the so called Cnp DYN criterion. The expression for Cnp DYN iS
derived from the open-loop lateral-directional quartic equation43J5 and is given
by
CnpDYN = CnpCOS - ;CLpsinct (8,6)
For directional stability at high angles of attack, C~ DYN must be positive. This
is the equivalent of the static stability criterion Cn~ > 0 used for low angles
of attack. If CnpDYN is negative, the aircraft may expenence yaw departure, and
the Dutch-roll mode may become unstable:~ Modern combat aircraft usually
have Iz > lx. A loss of static directional stability at high angles of attack makes
such configurations highly susceptible to yaw departure. One way of improving
STABILITY AND CONTROL PROBLEMS AT HtGH ANGLES OF ATTACK 733
resistance to yaw departure is to increase the dihedral effect. However, a high
dihedral effect makes an aircraft very sensitive in roll to gusts and small control
inputs (aileron or rudder).
The yaw departure to aileron and rudder control inputs can be predicted using
the following approach called f/-plus-8 axis stability indicator:43
1) Calculate a'_p and a8 given by the following expressions:
tx_p=cr-tan-l Cn xp ]
Cwlz j
Ct.8= -t -'[C~ ;]
(8.7)
(8.8)
2) Plot the graphs'a_p vs a and a8 vs a.
For stability at high angles of attack, at_p > 0 and a_p > aa. In other words,
the curve a'_p must lie above the ty axis and above the a8 curve.
There is a direct relationship between p-plus-8 axis stability indicator and the
CnpDYN > 0 and LCDP criteria. When a'_p = 0, we find that Cnp DYN - 0. Fur-
thermore, if cy_p = a8, LCDP - O.
8.12.3 Application of Departure Criteria
Let us first consider the application of Cnp DYIV and LCDP criteria for predicting
the aircraft departure/spin susceptibility at high angles of attack. This exercise
consists of plotting the values of CnpDYN and LCDP for selected altitude and
Mach number on a typical chart as shown in Fig. 8.61. This chart has four regions,
A, B, C, and D. In region A, the aircraft has good handling characteristics, and
the departure is not likely to occur. In region B, the aircraft may experience a mild
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