TM 1-1500-204-23-5 b.  Tip Damage.  When the removal or treatment of defects on the tip necessitates shortening a blade, shorten each  blade  used  with  it  and  keep  such  sets  of  blades together (see figure 2-4 and 2-5.).  In making the repair, it is not permissible to reduce the propeller diameter below the    minimum    diameter    limit    shown    on    the    airplane specification. c.    Bends  and  Twists.    Repair  methods  for  bends and twists are explained in the following paragraphs. (1) Cold straightening.  Repair bent blades in accordance   with   the   instructions   of   the   manufacturer. Carefully check the extent of a bend in face alignment by means of a protractor similar to the one shown in figure 2- 6.    Only  bends  not  exceeding  20  degrees  at  0.15-inch blade thickness to O degrees at 1.1-inch blade thickness may  be  cold  straightened.    Straighten  blades  with  bends exceeding these values only based upon recommendation of   the   manufacturer,   and   only   at   the   facilities   having proper  heat-treatment  equipment.    In  all  cases,  inspect the  blades  for  cracks  and  other  injuries  both  before  and after straightening. (2)  Annealing  method.    Repair  by  annealing consists of heating the alloys to an elevated temperature, holding or soaking them at this temperature for a length of time   depending   upon   the   mass   of   the   metal,   and   the cooling  in  still  air.    This  method  leaves  the  metal  in  the best condition for cold-working. (3)  Hub  damage.    Repairs  to  steel  hubs  and parts  shall  be  accomplished  only  in  accordance  with  the recommendations   of   the   manufacturer.      Welding   and remachining     is     permissible     only     when     covered     by manufacturers service bulletins. (4)  Spinner  damage.    Check  for  evidence  of dents,    cracks    or    other    visible    damage.        Replace    if damaged.    Dents  are  permissible,  provided  they  do  not affect propeller appearance while it is turning. (5) Marking repaired blades.  Mark the blades to  correspond  with  the  manufacturer's  system  of  model designation to indicate propeller diameter. (6)    Painting.        Paint    in    accordance    with instructions contained in TM 55-1500-345-23. 2-11.  Propeller Balancing.  Propellers must be balanced statically, dynamically, and aerodynamically. a.     Static   Balancing.      Propeller   static   unbalance occurs when the center of gravity of the propeller does not coincide with the axis of rotation.  Two methods of static balancing are described in the following paragraphs. NOTE During  a  propeller  static  balance  check, all   blades   must   be   at   the   same   blade angle.      Before   conducting   the   balance check, inspect to see that each blade has been set at the same blade angle. · Unless otherwise specified by the manufacturer, an acceptable balance check requires that the propeller assembly  have  no  tendency  to  rotate  in any of the positions previously described. If  the  propeller  balances  perfectly  in  all positions, it should also balance perfectly in    all    intermediate    positions.        When necessary, check for balance in intermediate positions to verify the check in the originally describe positions. (1)  Knife-edge method.  The knife-edge test stand, as  shown  in  figure  2-7,  has  two  hardened  steel  edges mounted   to   allow   the   free   rotation   of   an   assembled propeller  between  them.    The  knife-edge  test  stand  shall be  located  in  a  room  or  area  that  is  free  from  any  air motion,   and   preferably   removed   from   any   source   of heavy    vibration.        The    standard    method    of    checking propeller assembly balance involves the following procedures: (a) Insert a bushing in the engine shaft hole of the propeller. (b)   Insert   a   mandrel   or   arbor   through   the bushing. (c)   Place   a   propeller   assembly   so   that   the ends  of  the  arbor  are  supported  upon  the  balance  stand knife-edges.  The propeller must be free to rotate. (d) Check two-bladed propeller assemblies for balance,  first  with  the  blades  in  a  vertical  position  and then with the blades in a horizontal position, as shown in figure  2-7.    Repeat  the  vertical  position  check  with  the blade positions reversed; that is, with the blade which was checked  in  the  downward  position  placed  in  the  upward position. 2-10