Dynamic balance is that condition in a rotating body wherein  all rotating forces are balanced within themselves so that no  vibration is produced while the body is in motion. Dynamic  balance as related to control surfaces is an effort to maintain  balance when the control surface is submitted to movement  on the aircraft in flight. It involves the placing of weights  in the correct location along the span of the surfaces. The  location of the weights are, in most cases, forward of the  hinge center line.  Repairs to a control surface or its tabs generally increase the  weight aft of the hinge center line, requiring static rebalancing  of the control surface system, as well as the tabs. Control  surfaces to be rebalanced should be removed from the aircraft  and supported, from their own points, on a suitable stand,  jig, or fixture.  [Figure 2-61] Sometimes a visual check is all that is needed to determine  whether the surface is balanced or unbalanced. Any trim tabs  or other assemblies that are to remain on the surface during  balancing procedures should be in place. If any assemblies  or parts must be removed before balancing, they should be  removed.  Trim tabs on the surface should be secured in the neutral  position when the control surface is mounted on the stand.  The stand must be level and be located in an area free of air  currents. The control surface must be permitted to rotate  freely about the hinge points without binding. Balance  condition is determined by the behavior of the trailing edge  when the surface is suspended from its hinge points. Any  excessive friction would result in a false reaction as to the  overbalance or underbalance of the surface.  Several methods of balancing (rebalancing) control surfaces  are in use by the various manufacturers of aircraft. The most  common are the calculation method, scale method, and the  balance beam method.  When installing the control surface in the stand or jig, a  neutral position should be established with the chord line of  the surface in a horizontal position. Use a bubble protractor  to determine the neutral position before continuing balancing  procedures.  [Figure 2-62]  The calculation method of balancing a control surface has one  advantage over the other methods in that it can be performed  without removing the surface from the aircraft. In using the  calculation method, the weight of the material from the repair  area and the weight of the materials used to accomplish the  repair must be known. Subtract the weight removed from  the weight added to get the resulting net gain in the amount  added to the surface. The distance from the hinge center line  to the center of the repair area is then measured in inches. This  Rebalancing Methods Rebalancing Procedures  Dynamic Balance Figure 2-62.  Establishing a neutral position of the control surface. Support stand Hinge center line Bubble protractor Chord line Figure 2-61.  Locally fabricated balancing fixture. 2-38 Outboard hinge fitting Inboard hinge fitting