stances, the 4 at the tail is usually less and this 
reduces the efficiency of the tail. 
When the airplane is given a change in angle 
of attack, the horizontal tail does not expe- 
rience the same change in angle of attack as 
the wing. Because of the increase in down- 
wash behind, the wing, the horizontal tail will 
experience a smaller change in angle of attack, 
e.g., if a 10" change in wing angle of attack 
causes a 4O increase in downwash at the hori- 
zontal tail, the horizontal tail experiences 
only a 6’ change in angle of attack. In this 
manner, the downwash at the horizontal tail 
reduces the contribution to stability. Any 
factor which alters the rate of change of down- 
wash at the horizontal tail will directly affect 
the tail contribution and airplane stability. 
Power effects can alter the downwash at the 
horizontal tail and affect the tail contribution. 
Also, the ~downwash at the tail is affected by 
the lift distribution on the wing and the flow 
condition ,on the fuselage. The low aspect 
ratio airplane requires large angles of attack 
to achieve high ,lift coefficients and this posi- 
tions the fuselage at high angles of attack. 
The change in the wing downwash can be 
accompanied by crossflow separation vortices 
on the fuselage. It is possible that the net 
effect obviates or destabilizes the contribu- 
tion of the horizontal tail and produces air- 
plane instability. 
POWER-OFF STABILITY. When the in- 
trinsic stability of a configuration is of interest, 
power effects are neglected and the stability 
is considered by a buildup of the contributing~ 
components. Figure 4.10 illustrates a typical 
buildup of the components of a conventional 
airplane configuration. If the c.g. is arbi- 
trarily set at 30 percent MAC, the contribu- 
tion of the wing alone is destabilizing as indi- 
cated by the positive slope of CM versus C,. 
The combination of the wing and fuselage 
increases the instability. The contribution 
of the tail alone is highly stabilizing from 
the large negative slope of the curve. The 
contribution of the tail must be sufficiently 
NAVWEPS OO-BOT-80 
STABILITY AND CONTROL 
stabilizing so that the complete configuration 
will exhibit positive static stability at the 
anticipated c.g. locations. In addition, the tail 
and wing incidence must be set to provide a 
trim lift coefficient near the design condition. 
When the configuration of the airplane is 
fixed, a variation of c.g. position can cause 
large changes in the static stability. In the 
conventional airplane configuration, the large 
changes in stability with c.g. variation are 
primarily due to the large changes in the wing 
contribution. If the incidence of all surfaces 
remains fixed, the effect of c.g. position on 
static longitudinal stability is typified by the 
second chart of figure 4.10. As the cg. is 
gradually moved aft, the airplane static sta- 
bility’ decreases, then becomes neutral then 
unstable., The c.g. position which produces 
zero ,slope and neutral static stability is re- 
ferred to asp the ~“neutral point.” The neutral 
point may be imagined as the effective aerody- 
namic center of the entire airplane configura- 
ration, i.e., with the c.g. at this position, all 
changes in net lift effectively occur at this 
point and no change in pitching moment 
results. The neutral point defines the most 
aft c.g. position without static instability. 
POWER EFFECTS. The effects of power may 
cause significant changes in trim lift coefficient 
and static. longitudinal stability. Since the 
contribution to stability is evaluated by the 
change in moment coefficients, power effects 
will be most significant when the airplane 
operates at high power and low airspeeds such 
as the power approach or waveoff condition. 
The effects of power are considered in two 
main categories. First, there are the direct 
effects resulting from the forces created by the 
propulsion unit. Next, there are the indirect