TM 1-1500-204-23-2 4-19.  Dampening and Absorbing Units.    Dampening and absorbing   units   retard   the   motion   of   moving   objects   to which they are attached. These    units    perform    this function  by  removing  some  of  the  energy  of  motion  and converting  It  to  heat.    The  heat  is  then  dissipated  into  the atmosphere.  Dampening and absorbing units used in Army aircraft   are   hydraulic   dampers,   shock   struts,   and   liquid springs. a. Hydraulic   Dampers.        A    damper    is    a    device    for controlling  the  speed  of  relative  movement  between  two connected   objects   Usually,   one   end   of   the   damper   is connected to a fixed member; the other end, to a movable part.      The   reacting   parts   of   the   damper   move   against considerable resistance. This resistance slows the speed of relative movement between the objects.  Hydraulic dampers used in Army aircraft operate either by displacing fluid or by shearing fluid. (1) Displacement-principle dampers. Displacement-principle dampers used in Army aircraft are of two types-piston and vane. Though different in construction,    both    types    have    the    same    basic    design characteristics-a   sturdy   metal   container   with   a   sizeable Inner   space   divided   into   two   or   more   chambers.      The chambers  vary  In  size  according  to  the  position  of  parts within the damper.  The chambers must be completely filled with fluid in order to operate properly. (a)   Piston    damper.    A    typical    piston    damper    is shown in figure 4-162.  Note in figure 4-163 that the piston and   rod   assembly   divides   the   space   within   the   damper housing into two chambers Seal rings on the piston prevent fluid  leakage  between  the  chambers.    An  orifice  permits fluid  to  pass  with  restricted  flow  from  one  chamber  to  the other.    A  filler  port  (not  shown)  provides  for  servicing  the damper with fluid. 1. Operation.    As  the  piston  is  forced  to  the right, chamber 2 decreases in proportion to the distance the piston IS moved.  Simultaneously, chamber 1 increases by a   comparable   size.      The   hydraulic   fluid   displaced   from chamber 2 flows through the restricting orifice into chamber 1.      When   the   piston   is   moved   toward   the   left,   reverse changes occur in the chamber sizes and in the direction of fluid  flow.    The  restriction  of  the  fluid  flow  by  the  orifice shows  the  rate  of  speed  at  which  a  given  amount  of  force can move the damper piston.  The rate at which a damper moves  in  response  to  a  force  is  called  damping  rate  or timing rate.  In some dampers, the opening is of fixed size and  timing  rate  is  not  adjustable.    In  other  dampers,  the orifice size is adjustable to allow for timing adjustments. 2. Nose   landing   gear   damper.      The   nose landing gear of an aircraft has a tendency to shimmy when the aircraft is taxiing at any appreciable speed.  A damper is   used   as   shown   in   figure   4-164   to   eliminate   wheel shimmy    without    interfering    with    the    normal    steering movements on the nose wheel. 3. Rotor blade damper.  Piston-type dampers are  used  on  helicopter  rotor  head  assemblies  and  on  tail rotor  hub  assemblies  to  control  lead-lag  movements  of rotor  blades.    Note  in  figure  4-165  how  the  dampers  are connected  Lead-lag  movements  of  rotor  blades  occur  In cases In which there are three or more blades in a set and the blades are hinged to the rotor head. (b)   Vane damper.  A typical vane-type damper is shown   in   figure   4-166.      This   type   of   damper   consists basically  of  a  cylindrical  housing  having  a  polished  bore with two stationary vanes (called abutments) and having a shaft  supporting  two  movable  vanes.    Together  the  four vanes     split     the     cylinder     bore     lengthwise     Into     four chambers  as  shown  in  figure  4-167.    The  two  stationary vanes   are   attached   to   the   damper   housing   The   two movable   vanes,   along   with   the   shaft,   make   up   a   unit called  a  wing  shaft.    The  wing  shaft  rotates  between  the abutments.    One  end  of  the  wing  shaft  is  splined  and protrudes   through   the   damper   housing.      A   lever   arm, attached  to  the  splined  end,  is  the  means  of  rotating  the wing shaft. 1. Operation.      The   damper   chambers   are completely  filled  with  fluid,  and  at  any  Instant  of  damper motion  the  fluid  is  subjected  to  forced  flow.    As  the  wing shaft rotates, fluid flow between the chambers takes place through     an     opening     within     the     wing     shaft,     which interconnects the four chambers.  Thus a restraining force is developed in the damper, dependent on the velocity of fluid  flow  through  the  orifice.    Slow  relative  movement between the wing shaft and the damper housing causes a low velocity flow through the opening and little resistance to  damper  arm  rotation.    More  rapid  motion  of  the  wing shaft Increases the speed of fluid flow and thus increases resistance  to  damper  arm  rotation.    The  timing  rate  of vane-type     dampers     can     be     adjusted     by     a     timing adjustment  centrally  located  in  the  exposed  end  of  the wing shaft.  This adjustment (not shown) sets the effective size   of   the   opening   through   which   fluid   flows   between chambers  and  thus  determines  the  speed  of  movement with which the damper will respond to an applied force. 4-157