Although vibration can be caused by the propeller, there are numerous other possible sources of vibration that can make troubleshooting difficult. If a propeller vibrates, whether due to balance, angle, or track problems, it typically vibrates throughout the entire rpm range, although the intensity of the vibration may vary with the rpm. If a vibration occurs only at one particular rpm or within a limited rpm range (e.g., 2200– 2350 rpm), the vibration is not normally a propeller problem but a problem of a poor engine-propeller match. If a propeller vibration is suspected but cannot be positively determined, the ideal troubleshooting method is to temporarily replace the propeller with one known to be airworthy and then test fly the aircraft if possible. Blade shake is not the source of vibration problems. Once the engine is running, centrifugal force holds the blades firmly (approximately 30,000–40,000 pounds) against blade bearings. Cabin vibration can sometimes be improved by reindexing the propeller to the crankshaft. The propeller can be removed, rotated 180°, and reinstalled. The propeller spinner can be a contributing factor to an out-ofbalance condition. An indication of this would be a noticeable spinner wobble while the engine is running. This condition is usually caused by inadequate shimming of the spinner front support or a cracked or deformed spinner.
When powerplant vibration is encountered, it is sometimes difficult to determine whether it is the result of engine vibration or propeller vibration. In most cases, the cause of the vibration can be determined by observing the propeller hub, dome, or spinner while the engine is running within a 1,200- to 1,500-rpm range and determining whether or not the propeller hub rotates on an absolutely horizontal plane. If the propeller hub appears to swing in a slight orbit, the vibration is usually caused by the propeller. If the propeller hub does not appear to rotate in an orbit, the difficulty is probably caused by engine vibration.
When propeller vibration is the reason for excessive vibration, the difficulty is usually caused by propeller blade imbalance, propeller blades not tracking, or variation in propeller blade angle settings. Check the propeller blade tracking and then the low-pitch blade angle setting to determine if either is the cause of the vibration. If both propeller tracking and low blade angle setting are correct, the propeller is statically or dynamically unbalanced and should be replaced, or rebalanced if permitted by the manufacturer.
Blade Tracking
Blade tracking is the process of determining the positions of the tips of the propeller blades relative to each other (blades rotating in the same plane of rotation). Tracking shows only the relative position of the blades, not their actual path. The blades should all track one another as closely as possible. The difference in track at like points must not exceed the tolerance specified by the propeller manufacturer. The design and manufacture of propellers is such that the tips of the blades give a good indication of tracking. The following method for checking tracking is normally used:
1. Chock the aircraft so it cannot be moved. 2. Remove one spark plug from each cylinder. This makes the propeller easier and safer to turn. 3. Rotate one of the blades so it is pointing down. 4. Place a solid object (e.g., a heavy wooden block that is at least a couple of inches higher off the ground than the distance between the propeller tip and the ground) next to the propeller tip so that it just touches or attaches a pointer/indicator to the cowling itself. 5. Rotate the propeller slowly to determine if the next blade tracks through the same point (touches the block/ pointer). Each blade track should be within 1/16 inch (plus or minus) from the opposite blade’s track. 6. An out-of-track propeller, may be due to one or more propeller blades being bent, a bent propeller flange, or propeller mounting bolts that are either over- or undertorqued. An out-of-track propeller causes vibration and stress to the airframe and engine and may cause premature propeller failure.
Checking and Adjusting Propeller Blade Angles
When you find an improper blade angle setting during installation or is indicated by engine performance, follow basic maintenance guidelines. From the applicable manufacturer’s instructions, obtain the blade angle setting and the station at which the blade angle is checked. Do not use metal scribes or other sharply pointed instruments to mark the location of blade stations or to make reference lines on propeller blades, since such surface scratches can eventually result in blade failure. Use a bench-top protractor if the propeller is removed from the aircraft. Use a handheld protractor (a digital protractor provides an easy measurement) to check blade angle if the propeller is installed on the aircraft or is placed on the knife-edge balancing stand.
Universal Propeller Protractor
The universal propeller protractor can be used to check propeller blade angles when the propeller is on a balancing stand or installed on the aircraft engine. Figure shows the parts and adjustments of a universal propeller protractor. The following instructions for using the protractor apply to a propeller installed on the engine:
Turn the propeller until the first blade to be checked is horizontal with the leading edge up. Place the corner spirit level at right angles to the face of the protractor. Align degree and vernier scales by turning the disk adjuster before the disk is locked to the ring. The locking device is a pin that is held in the engaged position by a spring. The pin can be released by pulling it outward and turning it 90°.
Release the ring-to-frame lock (a right-hand screw with thumb nut) and turn the ring until both ring and disk zeros are at the top of the protractor.
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.
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.
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 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 Balancing
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.
Static Balancing
The knife-edge test stand has two hardened steel edges mounted to allow the free rotation of an assembled propeller between them. 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: 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 knifeedges. 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. 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.
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.
Dynamic Balancing
Propellers can also be dynamically balanced (spin balanced) with an analyzer kit to reduce the vibration levels of the propeller and spinner assembly. Some aircraft have the system hardwired in the aircraft and on other aircraft the sensors and cables need to be installed before the balancing run. Balancing the propulsion assembly can provide substantial reductions in transmitted vibration and noise to the cabin and also reduces excessive damage to other aircraft and engine components. The dynamic imbalance could be caused by mass imbalance or any aerodynamic imbalance. Dynamic balancing only improves the vibration caused by mass unbalance of the externally rotating components of the propulsion system. Balancing does not reduce the vibration level if the engine or aircraft is in poor mechanical condition. Defective, worn, or loose parts will make balancing impossible. Several manufacturers make dynamic propeller balancing equipment, and their equipment operation could differ. The typical dynamic balancing system consists of a vibration sensor that is attached to the engine close to the propeller, and an analyzer unit that calculates the weight and location of balancing weights.
Balancing Procedure
Face the aircraft directly into the wind (maximum 20 knots) and place chocks at the wheels. When you have installed the analyzing equipment, run the engine up at low cruise rpm; the dynamic analyzer calculates the balancing weight required at each blade position. After installing the balancing weights, run the engine up again to check if the vibration levels have diminished. This process may have to be repeated several times before satisfactory results are achieved.
A dynamic balancing example procedure is listed here, but always refer to the aircraft and propeller manuals when performing any balancing procedure. Dynamic balance is accomplished by using an accurate means of measuring the amount and location of the dynamic imbalance. The number of balance weights installed must not exceed the limits specified by the propeller manufacturer. Follow the dynamic balance equipment manufacturer’s instructions for dynamic balance in addition to the specifications of the propeller.
Most equipment use an optical pickup that senses reflective tape for rpm reading. Also, there is an accelerometer mounted to the engine that senses vibration in inches per second (ips).
Visually inspect the propeller assembly before dynamic balancing. The first runup of a new or overhauled propeller assembly may leave a small amount of grease on the blades and inner surface of the spinner dome. Use Stoddard solvent (or equivalent) to completely remove any grease on the blades or inner surface of the spinner dome. Visually examine each propeller blade assembly for evidence of grease leakage. Visually examine the inner surface of the spinner dome for evidence of grease leakage. If there is no evidence of grease leakage, lubricate the propeller in accordance with the maintenance manual. If grease leakage is evident, determine the location of the leak and correct before relubricating the propeller and dynamic balancing. Before dynamic balance, record the number and location of all balance weights. Static balance is accomplished at a propeller overhaul facility when an overhaul or major repair is performed. Twelve equally spaced locations are recommended for weight attachment. Install the balancing weights using aircraft quality 10-32 or AN-3 type screws or bolts. Balance weight screws attached to the spinner bulkhead must protrude through the self-locking nuts or nut plates a minimum of one thread and a maximum of four threads. Unless otherwise specified by the engine or airframe manufacturer, Hartzell recommends that the propeller be dynamically balanced to a reading of 0.2 ips, or less. If reflective tape is used for dynamic balancing, remove the tape immediately after balancing is completed. Make a record in the propeller logbook of the number and location of dynamic balance weights, and static balance weights if they have been reconfigured.
