NAVWEPS OO-BOT-80  STABILITY AND CONTROL  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  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  effects of the slipstream and other associated  flow which alter the forces and moments of the  aerodynamic surfaces. The direct effects of  power are illustrated in figure 4.11. The ver-  tical location of the thrust line defines one of  the direct contributions to stability.  If the  259 