NAVWEPS 00-EOT-80  STABILITY AND CONTROL  Various control surface tab devices can be  utilized to modify control forces.  Since the de-  flection of a tab is so powerful in creating hinge  moments on a control surface, the possible  application of tab devices is almost without  limit,  The basic trim tab arrangement is  shown in figure 4.18 where a variable linkage  connects the tab and the control surface. Ex-  tension or contraction of this linkage will de-  flect the tab relative to the control surface and  create a certain change in hinge mon~ent coef-  ficient.  The use of the trim tab will allow the  pilot to reduce the hinge moment to zero and  trim the control forces to zero for a given flight  condition.  Of course, the trim tab should have  adequate effectiveness  so that control forces  can be trimmed out throughout the flight speed  range.  stability.  As shown in figure 4.17, a down-  spring consists of a long preloaded spring at-  tached to the control system which tends to  rotate the elevators down.  The effect of the  downspring is to contribute an increment of  pull force independent of control deflection or  airspeed. When rhe downspring is added to  the control system of an airplane and the air-  plane is retrimmed for the original speed, the  airspeed stick force gradient is increased and  there is a stronger feel for airspeed.  The down-  spring would provide an “ersatz”  improve-  ment to an airplane deficient in airspeed stick  force stability,  Since the force increment from  the downspring is unaffected by stick position  or normal acceleration, the maneuvering stick  force stability would be unchanged.  The bobweight is an effective device for im-  proving stick force stability.  As shown in  figure 4.17, the bobweight consists of an eccen-  tric mass attached to the control system  which-in  unaccelerated flight--contributes  an increment of pull force identical to the  downspring.  In fact, a bobweight added to  the control system of an airplane produces an  effect identical to the downspring. The bob-  weight will increase the airspeed stick force  gradient and increase the feel for airspeed.  The lagging tab arrangement shown in figure  4.18 employs a linkage between the fixed sur-  face and the tab surface. The geometry is  such that upward deflection of the control  surface displaces the tab down relative to the  control surface. Such relative displacement  of the tab will aid in deflection of the control  surface  and thus reduce the hinge moments due  to deflection. An obvious advantage of this  device is the reduction of deflection hinge  moments without  a change in aerodynamic  balance.  A bobweighr will  have an effect on the  maneuvering stick force gradient since the bob-  weight mass is subjected to the same accelera-  tion as the airplane.  Thus, the bobweight will  provide an increment of stick force in direct  proportion to the maneuvering acceleration of  the airplane. Because of the linear contribu-  tion of the bobweight, the bobweight can be  applted to Increase  the maneuvering stick force  stability if the basic airplane has too low a  value or develops a decreasing  gradient at high  lift coefficients.  The leading tab arrangement shown in figure  4.18 also employs a linkage between the fixed  surface and the tab surface. However, the  geometry of the linkage is such that upward  deflection of the control surface displaces the  tab up relative to the control surface.  This  relationship serves to increase the control sur-  face hinge moments due to deflection of the  surface.  The servo  tad shown in figure 4.18 utilizes a  horn which has no direct connection to the  control surface and is free to pivot about the  hinge axis.  However, a linkage connects this  free horn to the tab surface.  Thus, the control  system simply deflects  the tab and the resulting  hinge moments deflect the control surface.  The example of the bobweight is useful to  point out the effect of the control system dis-  tributed masses.  All carrier aircraft must have  the control system mass balanced to prevent  undesirable control forces from the longi-  tudinal accelerations  during catapult launching. 