NAVWEPS OO-ROLRO  STARIUTY AND CONTROL  greater aerodynamic force and, generally, a  continued destabilizing influence.  angle of the surface must be sufficiently great  to prevent stall and subsequent loss of effec-  tiveness at ordinary sideslip angles.  The high  Mach numbers of supersonic flight produces a  decrease  in lift curve slope with the consequent  reduction in tail contribution to stability.  In  order to have sufficient directional stability at  high Mach numbers, the typical supersonic  configuration will  exhibit  relatively  large  vertical tail surfaces.  Figure 4.23 illustrates a typical buildup of  the directional stability of an airplane by  separating the contribution of the fuselage  and tail.  As shown by the graph of C. versus  6, the contribution of the fuselage is de-  stabilizing but the instability  decreases  at  large sideslip angles.  Tbe contribution of the  vertical tail alone is highly stabilizing up to  the point where the surface begins to stall.  The contribution of the vertical tail must be  large enough so that the complete airplane  (wing-fuselage-tail combination) exhibits the  required degree of stability.  The flow field in which the vertical tail  operates is affected by the othei components  of the airplane as well as powe; effects.  The  dynamic pressure at the vertical tail could  depend on the slipstream of a propeller or the  boundary layer of the fuselage. Also, the  local flow direction at the vertical tail is in-  fluenced by the wing wake, fuselage crossflow,  induced flow of the horizontal tail, or the  direction of slipstream from a propeller.  Each  of these factors must be considered as possibly  affecting the contribution of the vertical tail  to directional stability.  The dorsal fin has a powerful effect on pre-  serving the directional stability at large angles  of sideslip wliich would produce stall of the  vertical tail.  The addition of a dorsal fin to  the airplane will allay the decay of directional  stability at high sideslip in two ways. The  least obvious but most important effect is a  large increase  in the fuselage stability at large  sideslip angles.  In addition,  the effective  aspect rario of the vertical tail is reduced  which increases  the stall angle for the surface.  By this twofold effect, the addition of the  dorsal fin is a v  useful’ device.  The contribution of the wing tb %tatic direc-  tional stability is tisually small:  The swept  wing provides a stable contribution ’ depending  on the amount of sweepback  but the contribu-  tion is relatively weak when compared with  other components.  Poluer effects  on static directional stability  are similar to the power effects on static  longitudinal stability.  The direct effects are  confined to the normal force at the propeller  plane or the jet inlet and, of course, are de-  stabilizing when the propeller or inlet is  located ahead of the c.g. The indirect effects  of power induced velocities and flow dirkccion  changes at the vertical tail are quite significant  for the propeller driven airplane and can pro-  duce large directional trim changes. As in  the lontitudinal case, the indirect effects are  negligible for the jet powered airplane.  :.  The contribution of the fuselage  and  nacelles  is of primary importance since these compo-  nents furnish rhe greatest destabilizing in-  fluence. The contribution of the fuselage and  nacelles is similar to the longitudinal  case  with the exception that there is no large in-  fluence of the induced flow field of the wing.  The subsonic center of pressure  of the fuselage  will be located at or forward of the quarter-  length point and, since the airplane c.g. is  usually considerably aft of this point, the  fuselage contribution will  be destabilizing.  However, at large angles of sideslip the large  destabilizing contribution of the fuselage di-  minishes which is some relief to the problem  of maintaining directional stability at large  displacements. The supersonic pressure,.  dis-  tribution on the body provides a relatively  The contribution of the direct and indirect  power effects to static directional stability is  greatest for the propeller powered airplane  and usually slight for the jet powered airplane.  In either case, the general effect of power is  287 