alignment with a degree scale graduation line. This reading  should always be made on the vernier scale. The vernier  scale increases in the same direction that the protractor scale  increases. This is opposite to the direction of rotation of the  moving element of the protractor. After making any necessary  adjustment of the blade, lock it in position and repeat the same  operations for the remaining blades of the propeller. Propeller unbalance, which is a source of vibration in an  aircraft, may be either static or dynamic. Propeller static  imbalance occurs when the center of gravity (CG) of the  propeller does not coincide with the axis of rotation. Dynamic  unbalance results when the CG of similar propeller elements,  such as blades or counterweights, does not follow in the same  plane of rotation. Since the length of the propeller assembly  along the engine crankshaft is short in comparison to its  diameter, and since the blades are secured to the hub so they  lie in the same plane perpendicular to the running axis, the  dynamic unbalance resulting from improper mass distribution  is negligible, provided the track tolerance requirements are  met. Another type of propeller unbalance, aerodynamic  unbalance, results when the thrust (or pull) of the blades is  unequal. This type of unbalance can be largely eliminated by  checking blade contour and blade angle setting. Check the blade angle by determining how much the flat  side of the block slants from the plane of rotation. First,  locate a point to represent the plane of rotation by placing  the protractor vertically against the end of the hub nut or any  convenient surface known to lie in the plane of propeller  rotation. Keep the protractor vertical by the corner spirit  level and turn the ring adjuster until the center spirit level is  horizontal. This sets the zero of the vernier scale at a point  representing the plane of propeller rotation. Then, lock the  ring to the frame. The knife-edge test stand has two hardened steel edges  mounted to allow the free rotation of an assembled propeller  between them.  [Figure 7-38]  The knife-edge test stand must  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 sequence of operations: While holding the protractor by the handle with the curved  edge up, release the disk-to-ring lock. Place the forward  vertical edge (the edge opposite the one first used) against  the blade at the station specified in the manufacturer’s  instructions. Keep the protractor vertical by the corner spirit  level and turn the disk adjuster until the center spirit level  is horizontal. The number of degrees and tenths of a degree  between the two zeros indicates the blade angle. If the propeller is properly balanced statically, it remains at  any position in which it is placed. Check two-bladed propeller  assemblies for balance: first with the blades in a vertical  position and then with the blades in a horizontal position.  Repeat the vertical position check with the blade positions  reversed; that is, with the blade that was checked in the  downward position placed in the upward position. In determining the blade angle, remember that ten points on the  vernier scale are equal to nine points on the degree scale. The  graduations on the vernier scale represent tenths of a degree,  but those of the degree scale represent whole degrees. The  number of tenths of a degree in the blade angle is given by the  number of vernier scale spaces between the zero of the vernier  scale and the vernier scale graduation line nearest to perfect  Check a three-bladed propeller assembly with each blade  placed in a downward vertical position.  [Figure 7-39] 7-24 Static Balancing Propeller Balancing 3.  Place the propeller assembly so that the ends of the  arbor are supported upon the balance stand knife- edges. The propeller must be free to rotate. 2.  Insert a mandrel or arbor through the bushing. 1.  Insert a bushing in the engine shaft hole of the  propeller. Figure 7-37.  Universal proptractor.  3 0 2 0  $_{1}$$_{0}$  $_{0}$ 1 0 2 0 3 0 4 0 5 0  60  7 0 8 0 9 0 1 0 1 0 1 2 0 1 3 0 1 1 5 1 6 0 1 7 0 1 8 0 1 7 0 1 6 0 1 5 0 1 4 0 1 30 1 2 0  1 0 1 0 9 0 $_{ }$8 0 $_{ }$7 0 6 0 5 0 1 0 0 1 0 Degree scale on disk Vernier scale on ring Disk-to-ring lock Ring adjuster Center spirit level Disk adjuster Ring Ring-to-frame lock Corner spirit level Disk Edge used for checking against the propeller blade Edge used for checking against the propeller blade Edge used for checking against the propeller blade Nut—loosen to rotate face of level