NAVWEPS 00-8OT-90 
STABHITY AND CONTROL 
at the inlet of a jet engine contributes a similar 
destabilizing effect when the inlet is ahead 
of the c,g. As with the propeller, the magni- 
tude of the stability contribution is largest at 
high thrust and low flight speed. 
The indirect effects of power are of greatest 
concern in the propeller powered airplane 
rather than the jet powered airplane. As 
shown in figure 4.12, the propeller powered 
airplane creates slipstream velocities on the 
various surfaces which are different from the 
flow field typical of power-off flight. Since 
the various wing, nacelle, and fuselage surfaces 
are partly or wholly immersed in this slip- 
stream, the contribution of these components 
to stability can be quite different from the 
power-off flight condition. Ordinarily, the 
change of fuselage and nacelle contribution 
with power is relatively small. The added 
lift on the portion of the wing immersed in 
the slipstream requires that the airplane oper- 
ate at a lower angle of attack to produce the 
same effective lift coefficienr. Generally, this 
reduction in angle of attack to effect the same 
CL reduces the tail contribution to stability. 
However, the increase in dynamic pressure at 
the tail tends to increase the effectiveness of 
the tail and may be a stabilizing effect. The 
magnitude of this contribution due to the 
slipstream velocity on the tail will depend on 
the c.g. position and trim lift coefficient. 
The deflection of the slipstream by the nor- 
mal force at the propeller tends to increase the 
downwash at the horizontal tail and reduce 
the contribution to stability. Essentially the 
same destabilizing effect is produced by the 
flow induced at the exhaust of the jet power- 
plant. Ordinarily, the induced flow at the 
horizontal tail of a jet airplane is slight and is 
destabilizing when the jet passes underneath 
the horizontal tail. The magnitude of the 
indirect power effects on stability tends to be 
greatest at high Cr, high power, and low flight 
speeds. 
The combined direct and indirect power 
effects contribute to a general reduction of 
static stability at high power, high CL, and 
low 4. It is generally true that any airplane 
will experience the lowest level of static longi- 
tudinal stability under these conditions. Be- 
cause of the greater magnitude of both direct 
and indirect power effects, the propeller pow- 
ered airplane usually experiences a greater 
effect than the jet powered airplane. 
An additional effect on stability can be from 
the extension of high lift devices. The high 
lift devices tend to increase downwash at the 
tail and reduce the dynamic pressure at the tail, 
both of which are destabilizing. However, 
the high lift devices may prevent an unstable 
contribution of the wing at high CL. While 
the effect of high lift devices depends on the 
airplane configuration, the usual effect is de- 
stabilizing. Hence, the airplane may experi- 
ence the most critical forward neutral point 
during the power approach or waveoff. Dur- 
ing these conditions of flight the static stability 
is usually the weakest and particular attention 
must be given to precise control of the air- 
plane. The power-on neutral point may set 
the most aft limit of c.g. position. 
CONTROL FORCE STABILITY. The static 
longitudinal stability of an airplane is defined 
by the tendency to return to equilibrium upon 
displacement. In otherwords, the stable air- 
plane will resist displacement from the trim or 
equilibrium. The control forces of the air- 
plane should reflect the stability of the air- 
plane and provide suitable reference for precise 
control of the airplane. 
The effect of elevator deflection on pitching 
moments is illustrated by the first graph of 
figure 4.13. If the elevators of the airplane are 
fixed at zero deflection, the resulting line of 
CM versus C’s for 0’ depicts the static stability 
and trim lift coefficient. If the elevators are