point. If they had to mount the wings too far forward, and at  right angles to the fuselage, the center of pressure would not  be far enough to the rear to result in the desired amount of  longitudinal stability. By building sweepback into the wings,  however, the designers can move the center of pressure toward  the rear. The amount of sweepback and the position of the  wings then place the center of pressure in the correct location.  When turbulence or rudder application causes the aircraft to  yaw to one side, the opposite wing presents a longer leading  edge perpendicular to the relative airflow. The airspeed of  the forward wing increases and it acquires more drag than  the back wing. The additional drag on the forward wing pulls  the wing back, turning the aircraft back to its original path.  The contribution of the wing to static directional stability is  usually small. The swept wing provides a stable contribution  depending on the amount of sweepback, but the contribution  is relatively small when compared with other components.  Dutch roll is a coupled lateral/directional oscillation that is  usually dynamically stable but is unsafe in an aircraft because  of the oscillatory nature. The damping of the oscillatory mode  may be weak or strong depending on the properties of the  particular aircraft.  If the aircraft has a right wing pushed down, the positive  sideslip angle corrects the wing laterally before the nose is  realigned with the relative wind. As the wing corrects the  position, a lateral directional oscillation can occur resulting in  the nose of the aircraft making a figure eight on the horizon as  a result of two oscillations (roll and yaw), which, although of  about the same magnitude, are out of phase with each other.  In most modern aircraft, except high-speed swept wing  designs, these free directional oscillations usually die out  automatically in very few cycles unless the air continues to  be gusty or turbulent. Those aircraft with continuing Dutch  roll tendencies are usually equipped with gyro-stabilized yaw  dampers. Manufacturers try to reach a midpoint between too  much and too little directional stability. Because it is more  desirable for the aircraft to have “spiral instability” than  Dutch roll tendencies, most aircraft are designed with that  characteristic.  Spiral instability exists when the static directional stability  of the aircraft is very strong as compared to the effect of its  dihedral in maintaining lateral equilibrium. When the lateral  equilibrium of the aircraft is disturbed by a gust of air and a  sideslip is introduced, the strong directional stability tends  to yaw the nose into the resultant relative wind while the  There are two general means by which the designer can  change the planform of a wing and both will affect the  Aspect ratio is the ratio of wing span to wing chord. Taper  ratio can be either in planform or thickness, or both. In its  simplest terms, it is a decrease from wing root to wingtip in  wing chord or wing thickness. Sweepback is the rearward  slant of a wing, horizontal tail, or other airfoil surface.  Understanding the effects of different wing planforms  is important when learning about wing performance and  airplane flight characteristics. A planform is the shape of the  wing as viewed from directly above and deals with airflow  in three dimensions. Aspect ratio, taper ratio, and sweepback  are factors in planform design that are very important to the  overall aerodynamic characteristic of a wing.  [Figure 5-33]  Much research has gone into the development of control  devices (wing leveler) to correct or eliminate this instability.  The pilot must be careful in application of recovery controls  during advanced stages of this spiral condition or excessive  loads may be imposed on the structure. Improper recovery  from spiral instability leading to inflight structural failures  has probably contributed to more fatalities in general aviation  aircraft than any other factor. Since the airspeed in the spiral  condition builds up rapidly, the application of back elevator  force to reduce this speed and to pull the nose up only  “tightens the turn,” increasing the load factor. The results  of the prolonged uncontrolled spiral are inflight structural  failure, crashing into the ground, or both. Common recorded  causes for pilots who get into this situation are loss of horizon  reference, inability to control the aircraft by reference to  instruments, or a combination of both.  Many aircraft are affected to some degree by this characteristic,  although they may be inherently stable in all other normal  parameters. This tendency explains why an aircraft cannot  be flown “hands off” indefinitely.  comparatively weak dihedral lags in restoring the lateral  balance. Due to this yaw, the wing on the outside of the  turning moment travels forward faster than the inside wing  and, as a consequence, its lift becomes greater. This produces  an overbanking tendency which, if not corrected by the pilot,  results in the bank angle becoming steeper and steeper. At  the same time, the strong directional stability that yaws the  aircraft into the relative wind is actually forcing the nose  to a lower pitch attitude. A slow downward spiral begins  which, if not counteracted by the pilot, gradually increases  into a steep spiral dive. Usually the rate of divergence in the  spiral motion is so gradual the pilot can control the tendency  without any difficulty.  Free Directional Oscillations (Dutch Roll)  Effect of Wing Planform  Spiral Instability  5-20 