Figure 5-32.  Fuselage and fin for directional stability.  Figure 5-31.  Keel area for lateral stability.  A minor improvement of directional stability may be obtained  through sweepback. Sweepback is incorporated in the design  of the wing primarily to delay the onset of compressibility  during high-speed flight. In lighter and slower aircraft,  sweepback aids in locating the center of pressure in the  correct relationship with the CG. A longitudinally stable  aircraft is built with the center of pressure aft of the CG.  Because of structural reasons, aircraft designers sometimes  cannot attach the wings to the fuselage at the exact desired  5-19  If an aircraft is flying in a straight line, and a sideward gust  of air gives the aircraft a slight rotation about its vertical  axis (i.e., the right), the motion is retarded and stopped by  the fin because while the aircraft is rotating to the right, the  air is striking the left side of the fin at an angle. This causes  pressure on the left side of the fin, which resists the turning  motion and slows down the aircraft’s yaw. In doing so, it  acts somewhat like the weather vane by turning the aircraft  into the relative wind. The initial change in direction of the  aircraft’s flight path is generally slightly behind its change  of heading. Therefore, after a slight yawing of the aircraft  This restoring tendency is relatively slow in developing and  ceases when the aircraft stops skidding. When it ceases, the  aircraft is flying in a direction slightly different from the  original direction. In other words, it will not return of its own  accord to the original heading; the pilot must reestablish the  initial heading.  Similarly, the aircraft designer must ensure positive  directional stability by making the side surface greater aft  than ahead of the CG.  [Figure 5-32]  To provide additional  positive stability to that provided by the fuselage, a vertical  fin is added. The fin acts similar to the feather on an arrow  in maintaining straight flight. Like the weather vane and the  arrow, the farther aft this fin is placed and the larger its size,  the greater the aircraft’s directional stability.  In examining a weather vane, it can be seen that if exactly the  same amount of surface were exposed to the wind in front  of the pivot point as behind it, the forces fore and aft would  be in balance and little or no directional movement would  result. Consequently, it is necessary to have a greater surface  aft of the pivot point than forward of it.  The aircraft is then momentarily skidding sideways and,  during that moment (since it is assumed that although the  yawing motion has stopped, the excess pressure on the left  side of the fin still persists), there is necessarily a tendency  for the aircraft to be turned partially back to the left. That is,  there is a momentary restoring tendency caused by the fin.  to the right, there is a brief moment when the aircraft is still  moving along its original path, but its longitudinal axis is  pointed slightly to the right.  Stability about the aircraft’s vertical axis (the sideways  moment) is called yawing or directional stability. Yawing  or directional stability is the most easily achieved stability  in aircraft design. The area of the vertical fin and the sides  of the fuselage aft of the CG are the prime contributors that  make the aircraft act like the well known weather vane or  arrow, pointing its nose into the relative wind.  aircraft’s weight and the pressure of the airflow against the  upper portion of the keel area (both acting about the CG)  tends to roll the aircraft back to wings-level flight.  Directional Stability (Yawing)  CG  CG  CG centerline  Area  forward  of CG  Area aft of CG  CG CG  R e l a ti v e  w i n d  Y a w  y $_{a}$w 