Static Balancing

The knife-edge test stand has two hardened steel edges mounted to allow the free rotation of an assembled propeller between them. [Figure 1] 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.

aircraft propeller
Figure 1. Positions of two-bladed propeller during a balance check
The standard method of checking propeller assembly balance involves the following sequence of operations:
  1. Insert a bushing in the engine shaft hole of the propeller.
  2. Insert a mandrel or arbor through the bushing.
  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.
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. Check a three-bladed propeller assembly with each blade placed in a downward vertical position. [Figure 2]
aircraft propeller
Figure 2. Positions of three-bladed propeller during balance check

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 described 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 described positions. [Figure 3]

aircraft propeller
Figure 3. Static propeller balancing
When a propeller assembly is checked for static balance and there is a definite tendency of the assembly to rotate, certain corrections to remove the unbalance are allowed.
  1. The addition of permanent fixed weights at acceptable locations when the total weight of the propeller assembly or parts is under the allowable limit.
  2. The removal of weight at acceptable locations when the total weight of the propeller assembly or parts is equal to the allowable limit.
The location for removal or addition of weight for propeller unbalance correction has been determined by the propeller manufacturer. The method and point of application of unbalance corrections must be checked to see that they are according to applicable drawings.