NAVWEPS OtWOT-80  STABILITY AND CONTROL  stability.  Ati airplane with high static longi-  tudinal stability will exhibit great resistance  to displacement from equilibrium.  Hence,  the most critical conditions of controllability  will  occur when the airplane has high sta-  bility, i.e., the lower limits of controllability  will set the upper limits of stability.  Since the only control forces are those of the  tab, this device makes possible the deflection  of large surfaces  with relatively small control  forces.  A variation of the basic servo tab layout is  the sprirzg tab arrangement of figure 4.18.  When the control horn is connected to the  control surface by springs, the function of the  tab is to provide a given portion of the required  control forces. The spring tab arrangement  can then function as a boost to reduce control  forces. The servo tab and spring tab are  usually applied to large or high speed subsonic  airplanes to provide tolerable stick forces.  There are three principal conditions of  fli~ght which provide the critical requirements  of longitudinal  control power.  Any one  or combination of these conditions can de-  termine the longitudinal  control power and  set a limit to forward c.g. position.  MANEUVERING  CONTROL REQUIRE-  MENT.  The airplane should have sufficient  longitudinal control power to attain the maxi-  mum usable lift coefficient or limit load factor  during maneuvers. As shown in figure 4.19,  forward movement of the c.g. increases the  longiturjinal  stability  of an airplane and  requires larger control deflections to produce  changes in trim  lift  coefficient.  For the  example shown, the maximum effective de-  flection of the elevator is not capable of trim-  ing the airplane  ‘ at C,,,,  for c.g. positions  ahead of 18 percent MAC.  The spring Zoadcd  tab of figure 4.18 cotisists  of a free tab preloaded with a spring which  furnishes a constant moment about the tab  hinge line.  When the airplane is at zero air-  speed, the tab is rotated up to the limit of  deflection. As airspeed is increased, the aero-  dynamic hinge moment on the tab will finally  equal the spring torque and the tab will begin  to streamline. The effect of this arrangement  is to provide a constant hinge moment to the  control system and contribute a constant push  force requirement at speeds above the preload  speed. Thus, the spring loaded tab can im-  prove the stick force gradient in a manner  similar to the downspring.  Generally, the  spring loaded tab may be more desirable  because  of greater effectiveness  and the lack of  undesirable control  forces during  ground  operation.  This particular control requirement can be  most critical for an airplane in supersonic  flight.  Supersonic flight is usually accom-  .  panied by large increases in static longltu-  dinal stability and a reduction in the effective-  ness of control surfaces. In order to cope with  these trends, powerful all-movable surfaces  must be used to attain limit load factor or  maximum usable C, in supersonic flight.  This  requirement is so important that once satis-  fied, the supersonic configuration usually has  sufficient longitudinal  control power for all  other conditions of flight.  The various tab devices have almost un-  limited possibilities for tailoring control forces.  However, these devices must receive proper  care and maintenance in order to function  properly. In addition, much care must be  taken to ensure that no slop or play exists in  the joints and fittings, otherwise destructive  flutter may occur.  At takeoff, the airplane must have sufficient  control power to assume the takeoff attitude  prior to reaching takeoff speed. Generally,  for airplanes with tricycle landing gears, it  is desirable to have at least sufficient control  power to attain the takeoff attitude at 80  To be satisfactory, an airplane must have  adequate controllability  as well as adequate  TAKEOFF CONTROL  REQUIREMENT.  LONGITUDINAL  CONTROL  275 