NAVWEPS 00-8OT-90  STABHITY AND CONTROL  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.  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.  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 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 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  fixed at a deflection of 10” up, the airplane  static stability is unchanged but the trim lift  coefficient is increased. A change in elevator  or stabilizer position does not alter the tail  contribution to stability but the change in  pitching moment will alter the lift coeflicient  The combined direct and indirect power  effects contribute to a general reduction of  264 