effective and these sections have a small 
spanwise moment arm. Therefore, the changes 
in wing lift due to sideslip occur closer in- 
board and the dihedral effect is reduced. The 
effect of power on dihedral effect is negligible 
for the jet airplane but considerable for the 
propeller driven airplane. The propeller slip- 
stream at high power and low airspeed makes 
the inboard wing sections much more effective 
and reduces the dihedral effect. The reduction 
in dihedral effect is most critical when the 
flap and power effects are combined, e.g., the 
propeller driven airplane in the power approach 
or waveoff. 
With certain exceptions during the condi- 
tions of landing and takeoff, the dihedral 
effect or lateral stability should be positive 
but light. The problems created by excessive 
dihedral effect are considerable and difficult 
to contend with. Lateral stability will be 
evident to a pilot by stick forces and displace- 
ments required to maintain sideslip. Positive 
stick force stability will be evident by stick 
forces required in the direction of the controlled 
sideslip. 
LATERAL DYNAMIC EFFECTS 
Previous discussion has separated the lateral 
and directional response of the airplane to 
sideslip. This separation is convenient for 
detailed study of each the airplane static 
lateral stability and the airplane static direc- 
tional stability. However, when the airplane 
in free flight is placed in a sideslip, the lateral 
and directional response will be coupled, i.e., 
simultaneously the airplane produces rolling 
moment due to sideslip and yawing moment 
due to sideslip. Thus, the lateral dynamic 
motion of the airplane in free flight must 
consider the coupling or interaction of the 
lateral and directional effects. 
The principal effects which determine the 
lateral dynamic characteristics of an airplane 
are : 
(1) Rolling moment due to sideslip or 
dihedral effect (lateral stability). 
NAVWEPS, OO-ROT-80 
STABILITY AND CONTROL 
(2) Yawing moment due to sideslip or 
static directional stability. 
(3) Yawing moment due to rolling veloc- 
ity or the adverse (or proverse) yaw. 
(4) Rolling moment due to yawing ve- 
locity-a cross effect similar to (3). If the 
aircraft has a yawing motion to the right, 
the left wing will move forward faster and 
momentarily develop more lift than the 
right and cause a rolling moment to the 
right. 
(3) Aerodynamic side force due to side- 
slip. 
(6) Rolling moment due to rolling ve- 
locity or damping in roll. 
(7) Yawing moment due yawing velocity 
or damping in yaw. 
(8) The moments of inertia of the air- 
plane about the roll and yaw axes. 
The complex interaction of these effects pro- 
duces three possible types of motion of the 
airplane: (a) a directional divergence, (b) 
a spiral divergence, and (c) an oscillatory 
mode termed Dutch roll. 
Directional divergence is a condition which 
cannot be tolerated. If the reaction to a small 
initial sideslip is such as to create moments 
which tend to increase the sideslip, directional 
divergence will exist. The sideslip would in- 
crease until the airplane is broadside to the 
wind or structural failure occurs. Of course, 
increasing the static directional stability re- 
duces the tendency for directional divergence. 
Spiral divergence will exist when the static 
directional stability is very large when com- 
pared with the dihedral effect. The character 
of spiral divergence is by no means violent, 
The airplane, when disturbed from the equilib- 
rium of level flight, begins a slow spiral which 
gradually increases to a spiral dive. When a 
small sideslip is introduced, the strong direc- 
tional stability tends to restore the nose into 
the wind while the relatively weak dihedral 
effect lags in restoring the airplane laterally, 
In the usual case, the rate of divergence in the 
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