NAVWEPS 00-BOT-80  STABILITY AND CONTROL  control system is greatly magnified. In addi-  tion, response  lag of the controls may add to  the problem of attempting to forceably damp  the oscillation. In this case, should an oscilla-  tion appear, the best rule is to release  the con-  trols as the airplane stick-free will demonstrate  the necessary  damping,  Even an attempt to  fix the controls when the airplane is oscillating  may result in a small unstable input into the  control system which can reinforce the oscilla-  tion to produce failing flight loads. Because  of the very short period of the oscillation, the  amplitude of an unstable oscillation can reach  dangerous proportions in an extremely short  period of time.  can be assumed to take place with negligible  changes  in velocity.  The second  mode consists  of a pitching oscillation during which the air-  plane is being restored to equilibrium by the  static stability and the amplitude of oscillation  decreased  by pitch damping. The typical mo-  tion is of relatively high frequency with a  period of oscillation on the order of 6.5 to 5  seconds.  For the conventional subsonic airplane, the  second mode stick-fixed is characterized by  heavy damping with a time to damp to half  amplitude of approximately 0.5 seconds. IJsu-  ally, if the airplane has static stability stick-  fixed, the pitch damping contributed by the  horizontal tail will assume sufficient dynamic  stability for the short period oscillation. How-  ever, the second mode stick-free  has the possi-  bility of weak damping or unstable oscilla-  tions. This is the case where static stability  does not automatically imply adequate dy-  namic stability.  The second mode stick-free is  essentially a coupling of motion between the  airplane short period pitching motion and ele-  vator in rotation about the hinge line.  Ex-  treme care must be taken in the design of the  control surfaces  to ensure dynamic stability for  this mode. The elevators must be statically  balanced about the hinge line and aerodynamic  balance must be within certain limits.  Control  system friction must be minimized as it con-  tributes to the oscillatory tendency. If insta-  bility were to exist in the second mode, “por-  poising”  of the airplane would result with  possibility of structural damage. An oscilla-  tion at high dynamic pressures with large  changes  in angle of attack could produce severe  flight loads.  The third mode  occurs in the elevator free case  and is usually a very short period oscillation.  The motion is essentially one of the elevator  flapping about the hinge line and, in most  cases, the oscillation has very heavy damping.  A typical flapping mode may have a period of  0.3 to 1.5 seconds  and a time to damp to half-  amplitude of approximately 0.1 second.  Of all the modes of longitudinal dynamic  stability, the second mode or porpoising oscil-  lation is of greatest importance. The por-  poising oscillation has the possibility  of  damaging flight loads and can be adversely  affected by pilot response lag.  It should be  remembered that when stick-free the airplane  will demonstrate the necessary  damping.  The problems of dynamic stability are acute  under certain conditions of flight.  Low static  stability generally increases the period (de-  creases frequency) of the short period oscil-  lations and increases  the time to damp to half-  amplitude.  High altitude-and  consequently  low density-reduces the aerodynamic damp-  ing.  Also, high Mach numbers of supersonic  flight produce a decay of aerodynamic damping.  The second mode has relatively short periods  that correspond closely with the normal pilot  response  lag time, e.g., 1 or 2 seconds  or less.  There is the possibility that an attempt to  forceably damp an oscillation may actually re-  inforce the oscillation and produce instability.  This is particularly true in the case of powered  controls where a small input energy into the  In order to accomplish the stability and  control objectives, various configurations of  control systems are necessary. Generally, the  MODERN CONTROL SYSTEMS  ?Bl 