NAVWEPS 00-8 ’ X-60  STABILITY AND CONTROL  leading edge (unshielded) or partway to the  leading edge (shielded). Aerodynamic balance  can be achieved by the provision of- a hinge  line aft of the control surface leading edge.  The resulting overhang of surface area ahead  of the hinge line will  provide a degree of  balance depending on the amount of overhang.  Another variation of aerodynamic balance is  an internal balance surface ahead of the hinge  line which is contained within ,the surface.  A flexible seal is usually incorporated to in-  crease the effectiveness  of the balance area.  Even the bevelling of the trailing edge..of the  control surface is effective also as a balancing  technique. The choice of the type of aerody-  namic balance will  depend on many factors  such as required degree of balance, simplicity,  drag, etc.  attack airplanes. A large patrol or transport  type airplane would ordinarily show a much  higher maneuvering stick force gradient be-  cause of the lower limit load factor.  When the airplane has high static stability,  the maneuvering stability will  be high and  a high stick force gradient will  result. A  possibility exists that the forward c.g. limit  could be set to prevent an excessively high  maneuvering stick force gradient.  As the  c.g. is moved aft, the stick force gradient de-  creases  with decreasing maneuvering stability  and the lower limit  of stick force gradient  may be reached.  The pitch damping of the airplane is obvi-  ously related to air density.  At high altitudes,  the high true airspeed reduces the change in  tail angle of attack for a given pitching velocity  and reduces the pitch damping.  Thus, a de-  crease  in maneuvering stick force stability can  be expected  with increased  altitude.  TAILORING  CONTROL FORCES.  The  control forces should reflect the stability of  the airplane but, at the same time, should be  of a tolerable magnitude. The design of the  surfaces and control system may employ an  infinite variety of techniques to provide satis-  factory control forces.  Many devices can be added to a control  system to modify or tailor the stick force  stability to desired levels.  If a spring is added  to the control system as shown in figure 4.16,  it will tend to center the stick and provide a  force increment depending on stick displace-  ment. When the control system has a fixed  gearing between stick position and surface  deflection, the centering spring will provide a  contribution to stick force stability according  to stick position.  The contribution to stick  force stability will  be largest at low flight  speeds where relatively large control deflec-  tions are required. The contribution will be  smallest at high airspeed because  of the smaller  control deflections required. Thus, .the stick  centering bungee will  increase the airspeed  and maneuvering stick force stability but the  contribution decreases  at high airspeeds. A  variation of this device would be a spring  stiffness which would be controlled to vary  with dynamic pressure, 4.  In this case, the  contribution of the spring to stick force  stability would, not diminish with. speed.  Aerodynamic balance must be thought of in  two different senses.  First, the control surface  must be balanced to reduce hinge moments due  to changes in angle of attack.  This is necessary  to reduce the floating tendency of the surface  which reduces the stick-free stability.  Next,  aerodynamic balance can reduce the hinge  moments due to deflection of the control sur-  face. Generally, it is difficult to obtain a high  degree of deflection balance without incurring  a large overbalance of the surface for changes  in angle of attack.  Some of the types of aerodynamic balance  are illustrated in figure 4.16. Thesimple horn  type balance employs a concentrated balance  area located ahead of the hinge line.  The  balance area may extend completely to the  A “downspring”  added to a control system  is~  a means ~of increasing airspeed stick force  stability without a change in airplane static  2,70 