NAVWEPS 00-BOT-80  STABILITY AND CONTROL  tail is subject to an increase in angle of attack  and the elevators tend to float up, the change  in lift on the tail is less than if the elevators  remain fixed and the tail contribution to  stability is reduced. Thus, the “stick-free”  stability of an airplane is usually less than the  stick-fixed stability.  A typical reduction of  stability by free elevators is shown in figure  4.14(A) where the airplane. stick-free demon-  strates  a reduction of the slope of CM versus Cs.  While aerodynamic balance may be provided  tu reduce control forces, proper balance of the  surfaces  will reduce floating and prevent great  differences  between stick-fixed and stick-free  stability.  The greatest floating tendency oc-  curs when the surface is at a high angle of  attack hence the greatest difference between  stick-fixed and stick-free stability occurs when  the airplane is at high angle of attack.  at which equilibrium will occur. As the ele-  vator is fixed in various positions, equilibrium  (or trim) will occur at various lift coefficients  and the trim CL can be correlated with elevator  deflection as in the second graph of figure 4.13.  When the c,g. position of the airplane is  fixed, each elevator position corresponds to a  particular trim lift coefficient. AS the c.g. is  moved aft the slope of this line decreases  and  the decrease  in stability is evident by a given  control displacement causing a greater change  in trim lift coefficient. This is evidence that  decreasing  stability causes  increased  controlla-  bility and, of course, increasing stability de-  creases  controllability.  If the c.g. is moved  aft until the line of trim CL versus elevator de-  flection has zero slope, neutral static stability  is obtained and the “stick-fixed”  neutral point  is determined.  If the controls are fully powered and actu-  ated by an irreversible mechanism, the sur-  faces  are not free to float and there is no differ-  ace between the stick-fixed and stick-free  static stability.  Since each value of lift coefhcient  corresponds  to a particular value of dynamic pressure re-  quired to support an airplane in level flight,  uim airspeed can be correlated with elevator  deflection as in the third graph of figure 4.13.  If the c.g. location is ahead of the stick-fixed  neutral point and control position is directly  related to surface deflection, the airplane will  give evidence of stick podion mbility.  In  other words, the airplane will  require the  stick to be moved aft to increase the angle  of attack and trim at a lower airspeed and to  be moved forward to decrease the angle of  attack and trim at a higher airspeed. To be  sure, it is desirable to have an airplane demon-  strate this feature. If the airplane were to  have stick position instability, the airplane  would require the stick to be moved aft to trim  at a higher airspeed or to be moved forward to  trim at a lower airspeed.  The control forces in a conventional air-  plane are made up of two components.  First,  the basic stick-free stability of the airplane  contributes an incremem of force which is  independent of airspeed.. Next, there. is an  increment of force dependent on the trim tab  setting which varies with-the dynamic pres-  sure or the square of  ‘ equivalent airspeed.  Figure 4.14(B) indicates the variation of  stick force with airspeed and illustrates the  effect of tab setting on stick force. In order  te trim the airplane at point (1) a certain  amount of up elevator is required and zero  stick force is obtained~  with ’ the  nse of the tab.  To trim the airplane for higher speeds  corre-  sponding to points (2) and (3) less and less  nose-up tab is required. Note that when the  airplane is properly trimmed, a push force is  required to increase airspeed and a pull force  is required to decrease  airspeed. In this man-  ner, the airplane would indicate positive stick  force stability with a stable “feel”  for air-  There may be slight differences  in the static  longitudinal  stability  if the elevators are  allowed to float free. If the elevators are  allowed to float free as in “hands-off”  flight,  the elevators may have a tendency to “float”  or streamline when the horizontal tail is given  a change in angle of attack.  If the hot&ma1  246 