Aircraft technical Basics: TM 1-412, Aircraft Propellers, 1941: II. Ground adjustable Propeller

SECTION II. GROUND ADJUSTABLE PROPELLER

6. General.-The ground adjustable propeller is noncontrollable and, as the name implies, can be adjusted only while on the ground. The propeller consists of two or more blades and a hub. Inasmuch as the blade angle is not controllable, it is set at the time of assembly for an angle that is fairly efficient for all service conditions. This angle is determined by flight tests and is specified by Air Corps Technical Orders. Since the advent of the controllable propeller, the use of this type of propeller is confined almost entirely to primary training type of airplanes.

7. Description. of ground adjustable propeller having aluminum alloy blades.-a. The blades are of aluminum alloy and are of conventional design. Shoulders are machined on the blade shank as means of securely attaching them to the hub to take up the centrifugal force on the blades. A concentrically located hole is drilled in the root end of the blade to aid in balancing the propeller.

b. The hub is of chrome vanadium steel, cadmium plated. It is machined in halves with recesses to accommodate the blade shoulders. The blades are retained in the hub by the blade shoulders seating in the hub recesses aided by the friction of the two halves being drawn together by a ring clamp located at each end of the hub.

c. This propeller uses a spacer behind the rear cone to properly locate the propeller on the shaft and to prevent interference with the engine cowling. The retaining nut, front cone, snap ring, and rear cone are standard parts. Instruction pertaining to the inspection and inspection maintenance of these parts is explained in section IX. A typical ground adjustable propeller is shown in figure 5.

8. Description of ground adjustable propeller having solid steel blades.-a. Some types of ground adjustable propellers have solid steel blades and a different means of retaining the blades in the hub. But like the ground adjustable propeller having aluminum alloy blades, their use is confined to the primary training type of airplane.

b. The sections are thin and the shank and root end of the blade small in comparison to the aluminum alloy blade. A change in the airfoil section from the conventional Clark Y or R. A. F. 6 to a radically different section has been made in this type of propeller. A shoulder has been machined on the butt end of the blade which, when assembled in the recess of the hub, resists centrifugal force on the blade during operation. The weight of the blade is approximately that of the aluminum alloy blade.

c. The hub is of the split type and is manufactured from high-strength steel. Replacing the ring clamps as used on the conventional split type hub are four bolts which hold the halves together. A recess is machined in each blade socket to accommodate the shoulder of the blade. Internal threads are cut in the end of each hub blade bore for a split type nut which, when screwed into the hub, helps prevent any movement of the blade. The blade is also held from turning by two cup point set screws which are safetied together.

9. Installation and removal. a. When installing the ground adjustable propeller, the applicable provisions of section IX are complied with in addition to the following :

(1) In the order given, assemble rear cone spacer and rear cone on crankshaft.

(2) Install propeller on crankshaft. Care should be exercised to prevent damage to the threads of the crankshaft. If difficulty is encountered when installing the propeller, check the splines of the hub for alinement. At no time should force be used to install the propeller on the crankshaft.

(3) Assemble the two halves of front cone on flange of retaining nut and screw nut on shaft. The tightening is accomplished by using a bar approximately 4 feet in length. One man of average weight (175 pounds) can tighten the retaining nut sufficiently without additional leverage or the use of a hammer.

(4) Insert snap ring.

(5) Secure retaining nut by installing a clevis pin of correct size in alined holes of retaining nut and crankshaft. The pin is inserted from the center of the crankshaft.

(6) Safety with cotter pin.

b. The procedure of removing a ground adjustable propeller from the crankshaft is as follows :

(1) Remove cotter and clevis pin.

(2) Unscrew retaining nut. This will pull the propeller off the shaft. If difficulty is encountered in removing the propeller, remove snap ring, retaining nut, and front cone and thoroughly clean threaded portion of nut and shaft. Lubricate retaining nut and shaft with clean engine oil and thread lubricant, reassemble, and apply sufficient force to unscrew the nut. Do not damage threads of shaft when sliding propeller from shaft.

10. Inspection and inspection maintenance.-a. A visual inspection is made of the retaining nut lock pin, ring clamp nuts, and hinge pins as to condition and position of the cotter pins. All worn or damaged pins are replaced with serviceable parts. Any interference between the ring clamp bolts and nuts and engine cowling is corrected before flight, but under no circumstances will the position of the ring clamps be changed after the propeller has been balanced.

b. A physical check for looseness of the retaining nut is made as follows :

(1) Remove retaining nut lock pin.

(2) Insert 4-foot bar into retaining nut. One man weighing approximately 175 pounds pulling on the end of the bar is sufficient to check retaining nut for looseness. The force is applied steadily, avoiding jerking on the bar. Do not use additional leverage or force to make the check.

(3) Replace retaining nut lock pin and safety with cotter pin.

c. At the specified periodic inspection, the ring clamps are checked for looseness as follows :

(1) Remove cotter pin from ring clamp nut.

(2) Insert wrench with a leverage not to exceed 12 inches on the nut, and with a normal pull, check nut for looseness.

(3) Safety nut with cotter pin.

(4) Repeat the operations on the other ring clamps.

d. Maintenance problems.-Listed below are a number of maintenance problems with which the airplane mechanic may be confronted. They are of such a nature that the airplane mechanic can make the necessary checks and corrections.

(1) Retaining nut has been tightened but propeller remains loose on shaft. The probable cause for this condition is either the front or rear cones "bottoming." In case the front cone is bottoming, it is corrected by the addition of a locally manufactured spacer which is placed behind the rear cone. In event the rear cone is bottoming, 1/16 inch is cut off the apex of the cone or it is replaced with a new rear cone.

(2) Trouble is encountered when installing propeller on crankshaft; that is, the rear half of the hub will go on the shaft while the front half will not. Splines in the halves of the hub being out of alinement will cause this condition. Loosen ring clamps slightly and tap hub with a rawhide mallet. This allows the front half of the hub to properly aline itself in relation to the shaft and will correct the condition. Care must be taken not to disturb the blade angle during this operation.

(3) Excessive vibration in the propeller may be one or more of several probable causes. The airplane mechanic should check propeller blade for looseness in the hub, the hub for looseness on the shaft, blade angle settings and propeller for track. If the above checks are found correct or within the specified tolerance, the propeller is removed from the shaft and checked for balance. If this fails to correct the condition, there is little or nothing remaining for the airplane mechanic to do except make a report to the proper authorities.

(4) Additional information on inspection, maintenance, and repair is found in Section IX.

FIGURE 5. - Ground adjustable propeller