As this climb continues, the airspeed again decreases, causing  the downward force on the tail to decrease until the nose  lowers once more. Because the aircraft is dynamically stable,  the nose does not lower as far this time as it did before. The  aircraft acquires enough speed in this more gradual dive to  start it into another climb, but the climb is not as steep as  the preceding one.  After several of these diminishing oscillations, in which  the nose alternately rises and lowers, the aircraft finally  settles down to a speed at which the downward force on the  tail exactly counteracts the tendency of the aircraft to dive.  When this condition is attained, the aircraft is once again in  balanced flight and continues in stabilized flight as long as  this attitude and airspeed are not changed.  A similar effect is noted upon closing the throttle. The  downwash of the wings is reduced and the force at T in  Figure 5-23  is not enough to hold the horizontal stabilizer  down. It seems as if the force at T on the lever were allowing  the force of gravity to pull the nose down. This is a desirable  characteristic because the aircraft is inherently trying to  regain airspeed and reestablish the proper balance.  Power or thrust can also have a destabilizing effect in that  an increase of power may tend to make the nose rise. The  aircraft designer can offset this by establishing a “high  thrust line” wherein the line of thrust passes above the CG.  [Figures 5-26  and  5-27]  In this case, as power or thrust is  increased a moment is produced to counteract the down load  on the tail. On the other hand, a very “low thrust line” would  tend to add to the nose-up effect of the horizontal tail surface.  Conclusion: with CG forward of the CL and with an  aerodynamic tail-down force, the aircraft usually tries to  return to a safe flying attitude.  The following is a simple demonstration of longitudinal  stability. Trim the aircraft for “hands off” control in level  flight. Then, momentarily give the controls a slight push to  nose the aircraft down. If, within a brief period, the nose rises  towards the original position, the aircraft is statically stable.  Ordinarily, the nose passes the original position (that of level  flight) and a series of slow pitching oscillations follows. If the  oscillations gradually cease, the aircraft has positive stability;  if they continue unevenly, the aircraft has neutral stability;  if they increase, the aircraft is unstable.  Stability about the aircraft’s longitudinal axis, which extends  from the nose of the aircraft to its tail, is called lateral  stability. Positive lateral stability helps to stabilize the lateral  or “rolling effect” when one wing gets lower than the wing  on the opposite side of the aircraft. There are four main  design factors that make an aircraft laterally stable: dihedral,  sweepback, keel effect, and weight distribution.  Figure 5-27.  Power changes affect longitudinal stability.  Figure 5-26.  Thrust line affects longitudinal stability.  Lateral Stability (Rolling)  5-17  Below center of gravity  Through center of gravity  Above center of gravity  Thrust  CG  Thrust  CG  Thrust  CG  Cruise power  Idle power  Full power  Lift  Thrust  CG  Lift  Thrust  CG  Lift  Thrust  CG 