NAVWEPS OO-BOT-80 
STABILITY AND CONTROL 
Since wing dihedral is so powerful in pro- 
ducing lateral stability it is taken as a common 
denominator of the lateral stability contribu- 
tion of all other components. Generally, the 
contribution of wing position, flaps, power, 
etc., is expressed as an equivalent amount of 
“effective dihedral” or “dihedral effect.” 
The contribution of the fadage alone is 
usually quite small depending on the location 
of the resultant aerodynamic side force on the 
fuselage. However, the effect of the wing- 
fuselage-tail combination is significant since 
the vertical placement of the wing on the fuse- 
lage can greatly affect the stability of the com- 
bination. A wing located at the mid wing 
position will generally exhibit a dihedral effect 
no different from that of the wing alone. A 
low wing location on the fuselage may con- 
tribute an effect equivalent to 3’ or 4’ of nega- 
tive dihedral while a high wing location may 
contribute a positive dihedral of 2’ or 3’. The 
magnitude of dihedral effect contributed by 
vertical position of the wing is large and may 
necessitate a noticeable dihedral angle for the 
low wing configuration. 
The contribution of wccpback to dihedral ef- 
fect is important because of the nature of the 
contribution. As shown in figure 4.28, the 
swept wing in a sideslip has the wing into 
wind operating with an effective decrease in 
sweepback while the wing out of the wind 
is operating with an effective increase in 
sweepback. If the wing is at a positive lift 
coefficient, the wing into the wind has less 
sweep and an increase in lift and the wing out 
of the wind has more sweep and a decrease in 
lift. In this manner the swept back wing 
would contribute a positive dihedral effect and 
the swept forward wing would contribute a 
negative dihedral effect. 
The unusual nature of the contribution of 
sweepback to dihedral effect is that the con- 
tribution is proportional to the wing lift 
coefficient as well as the angle of sweepback. 
It should be clear that the swept wing at zero 
lift will provide no roll due to sideslip since 
there is no wing lift to change. Thus, the 
dihedral effect due to sweepback is zero at zero 
lift and increases directly with wing lift 
coefficient. When the demands of high speed 
flight require a large amount of sweepback, the 
resulting configuration may have an excessive- 
ly high dihedral effect at low speeds (high CL) 
while the dihedral effect may be satisfactory 
in normal flight (low or medium C,). 
The vertical tail of modern configurations 
can provide a sign&ant-and, at times, un- 
desirable-contribution to the effective dihe- 
dral. If the vertical tail is large, the side force 
produced by sideslip may produce a noticeable 
rolling moment as well as the important yaw- 
ing moment contribution. Such an effect is 
usually small for the conventional airplane 
configuration but the modern high speed 
airplane configuration induces this effect to a 
great magnitude. It is difficult then to obtain 
a large vertical tail contribution to directional 
stability without incurring an additional con- 
tribution to dihedral effect. 
The amount of effective dihedral necessary 
to produce satisfactory flying qualities varies 
greatly with the type and purpose of the air- 
plane. Generally, the effective dihedral should 
not be too great since high roll due to side- 
slip can create certain problems. Excessive 
dihedral effect can lead to “Dutch roll,” 
difficult rudder coordination in rolling maneu- 
vers, or place extreme demands for lateral 
control power during crosswind takeoff and 
landing. Of course, the effective dihedral 
should not be negative during the predominat- 
ing conditions of flight, e.g., cruise, high 
speed, etc. If the airplane demonstrates satis- 
factory dihedral effect for these conditions of 
flight, certain exceptions can be considered 
when the airplane is in the takeoff and landing