The properties of aluminum alloys make aluminum propellers durable and relatively inexpensive to maintain. However, some types of damage can be severe enough to cause blade failure. Therefore, alu- minum propellers must be carefully inspected at regular intervals. In addition, if any damage is dis- covered that jeopardizes the integrity of a propeller, it must be repaired before further flight.
BLADE INSPECTION
A requirement for both annual and 100-hour inspec- tions includes checking for cracks, nicks, and prop- erly torqued or safetied bolts. As with a wood pro- peller, most inspections of a fixed-pitch aluminum propeller are conducted without removal. However, if operational problems such as vibrations occur, it may be necessary to remove the propeller for a detailed inspection of the hub area.
Removal procedures for a fixed-pitch aluminum propeller are the same as for a fixed-pitch wood pro- peller. However, the aluminum propeller may be heavier than a comparable sized wood propeller. If so, obtain help when necessary to support the pro- peller during removal and prevent damage to the propeller or personal injury.
Prior to an inspection, an aluminum propeller should be cleaned with a solution of mild soap and water using a soft brush or cloth to remove all dirt and grease. Acid or caustic cleaning materials should never be used on aluminum propellers because their use could lead to corrosion. Furthermore, avoid the use of power buffers, steel wool, steel brushes, or any other abrasive that may scratch or mar the blades. If a propeller has been subjected to salt water, it should be flushed with fresh water until all traces of salt have been removed. This should be accomplished as soon as possible after exposure to salt spray.
Once clean, aluminum blades are inspected for pit- ting, nicks, dents, cracks, and corrosion. Areas that are especially susceptible to damage include the leading edges and the blade face. To aid in the inspection process, you should inspect the entire propeller with a four-power magnifying glass. In addition, if a crack is suspected, a dye penetrant inspection should be performed. In many cases, a dye penetrant inspection will show whether visible lines and other marks are actually cracks or only scratches, saving the time and expense of unneces- sary repairs.
Inspect the hub boss for damage and corrosion inside the center bore and on the surfaces which mount on the crankshaft. Also, inspect the bolt holes for cracks, excessive wear, and proper dimen- sions. Light corrosion can be cleaned from the hub boss with sandpaper. The affected area may then be painted or treated to help prevent further corrosion. Propellers with damage, dimensional wear, or heavy corrosion in the boss area should be referred to a repair station for appropriate repairs.
REPAIRS
When an inspection reveals surface damage such as nicks, scratches, or gouges on an aluminum alloy propeller blade, repairs should be made as soon as possible. By making prompt repairs, you help elim- inate stress concentration points which, in turn, helps to prevent cracks and fatigue failure. Defects on a bladeis leading and trailing edges may be dressed out by using a combination of round and half round files. When a repair is complete, it
should blend in smoothly with the edge and should not leave any sharp edges or angles. In all cases, the repair of surface defects on aluminum propeller blades must be made parallel to the length of the blade. In addition, the approximate maximum allowable size of a typical repair on a propeller edge is 1/8 inch deep by no more than 1 1/2 inches in length. [Figure 12-66]
Repairs to the face and back of a blade are per- formed with a spoon-like riffle file which is used to dish out the damaged area. The maximum allowable repair size of a typical surface defect on a blade face or back is 1/16 inch deep by 3/8 inch wide by 1 inch long. In addition, all repairs must be finished by polishing with very fine sandpaper, moving the paper in a direction parallel to the length of the blade. Once sanded, the surface should be treated with Alodine, paint, or other approved protective coating. [Figure 12-67]
Damage in the shank area of a propeller blade can- not be repaired in the field and should be referred to an overhaul facility for corrective action. Since all forces acting on the propeller are concentrated at the shank, any damage in this area is critical. Furthermore, transverse cracks of any size render an aluminum alloy blade unrepairable.
Contrary to popular belief, bent blades can often be repaired. To determine if a blade is repairable, begin by measuring the thickness of the blade where the
Figure 12-66. When making a repair to either the leading or trailing edge of a propeller, ensure that the repair will not exceed the propeller blade's minimum dimensions or change the profile of its leading edge.
Figure 12-67. Repairs on the blade face and leading edge should blend into the blade profile to maintain smooth air- flow over the propeller.
bend is located. Once this is done, determine the blade station of the bend by measuring from the cen- ter of the hub to the center of the bend. With the center of the bend located, mark the blade one inch on each side of the bend and place a protractor tan- gent to the one inch marks to determine the bend angle. [Figure 12-68]
Figure 12-68. A protractor is used to determine the amount of bend in a propeller blade. Place the hinge over the center of the bend, set the protractor legs tangent to the blade one inch on each side of the bend centerline, and read the amount of bend in degrees on the protractor.
Figure 12-70. Cracks in the blade retention area of the hub of a ground-adjustable propeller are critical defects. Dye-penetrant inspection methods are used to detect such cracks.
Figure 12-69. This chart shows the amount of bend damage at a given blade station that can be repaired by cold bend- ing. Values that fall beneath the curve are repairable. For example, a 10 degree bend at blade station 22 can be repaired by cold bending. However, a 15 degree bend at sta- tion 22 cannot be repaired.
Many propeller manufacturers furnish charts that help a technician determine if a bend is repairable. In most cases, the chart consists of a graph with the blade station on one axis and the degree of bend on the opposite axis. When reading this type of chart, any bend below the graph line is repairable while any bend above the line is unre- pairable. [Figure 12-69]
If the proper chart is not available, take the mea- surements and contact a propeller repair station for a decision before sending the propeller to them for straightening. Cold straightening repairs to a bent aluminum propeller must be accomplished by an appropriately rated repair station or the manufacturer.
Once a repair has been made to an aluminum pro- peller, the propeller should be cleaned with an approved solvent. This helps remove all traces of dye penetrant materials used during an inspection and subsequent repair. If the propeller was painted, repaint the face of each blade with one coat of zinc chromate primer and two coats of flat black lacquer from the six inch station to the tip. The back of each blade should have the last four inches of the tip painted with one coat of zinc chromate primer and two coats of a high visibility color.
If a high polish is desired, a number of good grades of commercial metal polish are available. However, after completing the polishing operation, all traces of polish should be removed.