If you recall, some propeller blades are fitted with blade cuffs to improve airflow over the blade shank and cooling airflow through the engine. When this is the case, the blade cuffs must also be inspected and checked for proper clearance. Longitudinal clearance of constant-speed pro peller blades or cuffs must be at least 1/2 inch between propeller parts and stationary parts of the aircraft. This clearance must be measured with the propeller blades feathered or in the most critical pitch configuration. -
GOVERNORS
As mentioned earlier, you, as an aviation mainte- nance technician, are somewhat limited as to what you can do with propeller systems. Nowhere is this more evident than with propeller gover- nors. Inspection of governors is limited to check- ing for oil leaks and security of mounting. Maintenance consists of properly rigging the gov- ernor controls and verifying freedom of motion. Although you may remove and install propeller governors on an engine, inspections and repairs which require governor disassembly must be accomplished by a properly equipped and certi- fied repair station.
BALANCING
Exact propeller balance is critical to proper engine and propeller performance. Any time maintenance is conducted or a repair is made that adds or removes weight from a propeller, you must check the propeller's balance. For example, if a wood pro- peller is refinished, the new varnish can create an imbalance if it is unevenly applied. In another
example, a metal blade that is shortened because of tip damage requires shortening of the opposite blade to maintain balance.
Propellers are balanced both statically or dynami- cally. A propeller is statically balanced when the propeller's center of gravity coincides with its axis of rotation. On the other hand, a propeller is dynam- ically balanced when the centers of gravity of the blades rotate in the same plane of rotation.
STATIC BALANCE
Static balancing is accomplished by using either the knife-edge method or the suspension method. Of the two static balancing methods, the knife-edge method is simpler and more accurate.
To balance a propeller using the knife-edge method, a test stand consisting of two hardened steel edges must be used. In addition, the test stand must be located in a room or area that is free from any air motion or heavy vibration.
Before you check a propeller's balance, you should first verify that the blade angles are all the same. If the blade angles are correct you can check a pro peller's balance by following the listed sequence of
operations: : .
1. Insert a bushing in the propeller hub bore hole. 2. Insert a mandrel or arbor through the bushing to
support the propeller on the balance knives. 3. Place the propeller assembly so that the ends of
the arbor are supported on the test stand. The propeller must be free to rotate.
V Once in the test stand, the propeller should be i checked for horizontal and vertical balance. To check a two-bladed propeller assembly for vertical balance, position one blade in the vertical position. Next, repeat the vertical position check with the blade positions reversed from the first vertical check. If the propeller is balanced vertically, it will remain in a vertical position regardless of which blade is pointing up. On the other hand, if a vertical imbalance exists, the propeller will have a tendency to come to rest in a horizontal position. [Figure 12-71]
To check a two-bladed propeller assembly for hori- zontal balance, position the propeller in a horizon- tal position with both blades sticking straight out. If
Figure 12-71. In a vertical balance check, the propeller blades are aligned vertically and an imbalance condition causes them to move to a horizontal position.
the propeller is horizontally balanced, it will remain in a horizontal position. On the other hand, if a horizontal imbalance exists, one blade will tend to move downward causing the propeller to come to rest in a vertical position. [Figure 12-72]
A two-bladed propeller that is properly balanced will have no tendency to rotate in any of the test positions. 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. Static balancing of a three-bladed propeller requires placing the propeller in three basic test positions. First, rotate the propeller until blade number one is pointing downward. Similarly, place blade number
Figure 12-72. To check horizontal balance, the propeller is positioned horizontally. Any rotation from this position indicates a heavy blade.
two in the downward position, then blade number three. A properly balanced three-bladed propeller has no tendency to rotate from any of the three posi- tions. [Figure 12-73]
In the suspension method for checking static bal- ance, the propeller is hung by a cord. A disk is firmly attached to the cord and a cylinder is attached to the propeller. Any imbalance is deter- mined by the eccentricity between the disk and the cylinder.
Out-of-Balance Repairs
When a propeller assembly exhibits a definite ten- dency to rotate, certain corrections to remove the imbalance are allowed. The addition of permanent fixed weights is permitted at acceptable locations when the total weight of the propeller assembly is under allowable limits. Likewise, the removal of weight is permitted from acceptable locations when the total weight of the propeller assembly is equal to the allowable limit.
The location for removal or addition of weight on a propeller is determined by the propeller manufac-
Figure 12-73. A three-bladed propeller is properly balanced when each blade can be placed in the six o'clock position with no tendency to rotate.
12-60
turer. The method and point of application of bal- ance corrections must be in accordance with the manufacturer's instructions. Typically, vertical imbalance is corrected by adding a metal weight on the light side of the hub 90 degrees from the pro- peller's horizontal centerline. On a wooden pro- peller, horizontal imbalance is corrected by adding or removing solder at the propeller blade tips. Horizontal balance correction on an aluminum pro- peller often involves the removal of small amounts of metal by filing.
DYNAMIC BALANCE
A propeller exhibits dynamic balance when the cen- ters of gravity of similar propeller elements, such as the propeller blades, rotate in the same plane of rotation. A dynamic imbalance resulting from improper mass distribution is usually negligible if the blades on a propeller track within limits. One reason for this is that the length of the propeller assembly along the engine crankshaft is very short compared to its diameter. Another reason is the fact that the blades track the same plane perpendicular to the axis of the crankshaft.
Modern methods of checking dynamic balance require the propeller, spinner, and related equip- ment to be installed on the aircraft. With the engine running, electronic equipment senses and pinpoints the location of an imbalance. In addition, the test equipment typically determines the amount of weight required to correct the condition.