Aerodynamics of Flight

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