to sideslip. If an airplane has favorable rolling 
moment due to sideslip, a lateral displacement 
from wing level flight produces sideslip and 
the sideslip creates rolling moments tending 
to return the airplane to wing level flight. 
By this action, static lateral stability will be 
evident. Of course, a sideslip will produce 
yawing moments depending on the nature of 
the static directional stability but the consid- 
rations of static lateral stability will involve 
only the ‘relationship of rolling moments and 
sideslip. 
DEFINITIONS. The axis system of an 
airplane defines a positive rolling, L, as a 
moment about the longitudinal axis which 
tends to rotate the right wing down. As in 
other aerodynamic considerations, it is con- 
venient to consider rolling moments in the 
coefficient form so that lateral stability can 
be evaluated independent of weight, altitude, 
speeds, etc. The rolling moment, L, is defined 
in the coeflicient form by the following equa- 
tion : 
or 
L=C,qSb 
* 
+I 
0 
where 
L=rolling moment, ft.-lbs., positive to 
the right 
4 = dynamic pressure, psf. 
S=wing area, sq. ft. 
b = wingspan, ft. 
C,=rolling moment coeflicient, positive 
to the right 
The angle of sideslip, 8, has been defined 
previously as the angle between the airplane 
centerline and the relative wind and is positive 
when the relative wind is to the right of the 
centerline. 
The static lateral stability of an airplane can 
be illustrated by a graph of rolling moment 
coefficient, Cl, versus sideslip angle, 8, such 
as shown in figure 4.27. When the airplane 
is subject to a positive sideslip angle, lateral 
stability will be evident if a negative rolling 
NAVWEPS 00-8OT-80 
STABILITY AND COI’ITROL 
moment coefficient results. Thus, when the 
relative wind comes from the right (+-a>, 
a rolling moment to the left (-Cl> should be 
created which tends to roll the airplane to 
the left. Lateral stability will exist when 
the curve of C1 versus p has a negative slope 
and the degree of stability will be a function 
of the slope of this curve. If the slope of the 
curve is zero, neutral lateral stability exists; 
if the slope is positive lateral instability is 
present. 
It is desirable to have lateral stability or 
favorable roll due to sideslip. However, the 
required magnitude of lateral stability is deter- 
mined by many factors. Excessive roll due to 
sideslip complicates crosswind takeoff and 
landing and may lead to undesirable oscil- 
latory coupling with the directional motion of 
the airplane. In addition, a high lateral sta- 
bility may combine with adverse yaw to hinder 
rolling performance. Generally, favorable han- 
dling qualities are obtained with a relatively 
light-or weak positive-lateral stability. 
CONTRIBUTION OF THE AIRPLANE 
COMPONENTS. In order to appreciate the 
development of lateral stability in an airplane, 
each of the contribution components must be 
inspected. Of course, there will be interference 
between the components which will alter the 
contribution to stability of each component on 
the airplane. 
The principal surface contributing to the 
lateral stability of an airplane is the wing. The 
effect of the geometric dihedral of a wing is a 
powerful contribution to lateral stability. As 
shown in figure 4.28, a wing with dihedral will 
develop stable rolling moments with sideslip. 
If the relative wind comes from the side, the 
wing into the wind is subject to an increase in 
angle of attack and develops an increase in lift. 
The wing away from the wind. is subject to a 
decrease in angle of attack and develops a de- 
crease in lift. The changes in lift effect a rolling 
moment tending to raise the windward wing 
hence dihedral contributes a stable roll due to 
sideslip. 
295