Aerodynamics of Flight

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