Form all repairs in such a manner to fit the original contour  perfectly. A smooth contour is especially desirable when  making patches on the smooth external skin of high-  speed aircraft. Keep the weight of all repairs to a minimum. Make the size  of the patches as small as practicable and use no more rivets  than are necessary. In many cases, repairs disturb the original  balance of the structure. The addition of excessive weight in  each repair may unbalance the aircraft, requiring adjustment  of the trim-and-balance tabs. In areas such as the spinner  on the propeller, a repair requires application of balancing  patches in order to maintain a perfect balance of the propeller.  When flight controls are repaired and weight is added, it is  very important to perform a balancing check to determine if  the flight control is still within its balance limitations. Failure  to do so could result in flight control flutter. To prevent severe vibration or flutter of flight control surfaces  during flight, precautions must be taken to stay within the  design balance limitations when performing maintenance or  repair. The importance of retaining the proper balance and  rigidity of aircraft control surfaces cannot be overemphasized.  The effect of repair or weight change on the balance and CG  is proportionately greater on lighter surfaces than on the older  heavier designs. As a general rule, repair the control surface  in such a manner that the weight distribution is not affected  in any way, in order to preclude the occurrence of flutter  of the control surface in flight. Under certain conditions,  counterbalance weight is added forward of the hinge line to  maintain balance. Add or remove balance weights only when  necessary in accordance with the manufacturer’s instructions.  Flight testing must be accomplished to ensure flutter is not a  problem. Failure to check and retain control surface balance  within the original or maximum allowable value could result  in a serious flight hazard. Flutter and Vibration Precautions Maintaining Original Contour Keeping Weight to a Minimum 4-89 Notes Engineering Notes a. For stringer in the upper surface of a wing, or in a fuselage, 80 percent of the number of rivets shown in the table  may be used. b. For intermediate frames, 60 percent of the number shown may be used. c. For single lap sheet joints, 75 percent of the number shown may be used. a. The load per inch of width of material was calculated by assuming a strip 1 inch wide in tension. b. Number of rivets required was calculated for 2117–T4 (AD) rivets, based on a rivet allowable shear stress equal to  percent of the sheet allowable tensile stress, and a sheet allowable bearing stress equal to 160 percent of the sheet  allowable tensile stress, using nominal hole diameters for rivets. c. Combinations of shoot thickness and rivet size above the underlined numbers are critical in (i.e., will fail by) bearing  on the sheet; those below are critical in shearing of the rivets. d. The number of AN–3 bolts required below the underlined number was calculated based on a sheet allowable tensile  stress of 55.000 psi and a bolt allowable single shear load of 2.126 pounds. Figure 4-169.  Rivet calculation table.  6.5  6.5  6.9  8.9 10.0 11.1 - -  - -  - -  - -  - -  - -  4.9 4.9 4.9 4.9 5.6 6.2 7.9 9.9 12.5 - -  - -  - -   - -  - -  - -  - -  - -  - - 3.3 3.3 3.3 3.3 3.3 3.3 - -  3.9 3.9 3.9 3.9 4.0 5.1 6.5 8.1 9.1 10.3 12.9  - -  - -  - - 3.3 3.3 3.3 3.6 4.5 5.7 6.3 7.1 8.9 - -  - -  - -  - -  2.4 2.4 2.4 2.5 3.1 3.5 3.9 4.9 .016 .020 .025 .032 .036 .040 .051 .064 .081 .091 .102 .128 Thickness “t” in inches No. of Bolts No. of 2117–T4 (AD) protruding head rivets required  per inch of width “W” Rivet size 1/4 3/16 5/32 3/32 AN–3 1/8