17.6 Propeller Maintenance Flashcards
All propellers vibrate to some extent during operation. Assuming that the engine itself is not at fault, propeller roughness can be caused by:
Bent blades
Blades out of track due to an improper mounting of the propeller on the engine shaft
Imbalance
Propeller loosely mounted on the engine shaft
Blade angles between blades out of tolerance with respect to each other
Spinner imbalance due to improper mounting or to dirt, snow or ice inside the shell
When working with propellers three different types of balancing are of importance. These are:
Static Balancing
Dynamic Balancing
Aerodynamic Balancing
Static and dynamic imbalance are caused by unequal mass distribution while aerodynamic balancing is to ensure each blade delivers an equal amount of thrust.
In most cases, static balancing is sufficient. Only with larger propellers or with fast running propellers can dynamic balancing be necessary.
Static Balance
A body capable of rotating about a fixed point is said to be in static balance when its centre of gravity lies on the axis of rotation. If a body is in static balance, each time it is rotated it comes to a stop in a random position.
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Static balancing is carried out ‘off the aircraft’, either at the manufacturer, over hauler or in an approved propeller repair facility. The propeller is mounted on a mandrel which is placed across stable and perfectly levelled knife edges or rollers. The balance is checked in two planes, one horizontal and the other vertical
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Fixed Pitch Wood Propellers
(Balancing)
Horizontal imbalance can be corrected by
applying small amounts of varnish or paint to the light blade or if larger amounts are required then solder can be applied to the metal tipping of the lighter blade.
Fixed Pitch Wood Propellers
(Balancing)
Vertical imbalance can be corrected by
adding weights to the hub.
Fixed Pitch Metal Propellers
(Balancing)
Fixed pitch metal propellers are statically balanced by removing some metal from the heavy side and then refinishing the propeller using a non-anodic protective coating such as Alodine ®.
For propellers of aircraft up to 5700 kg (12 566 lbs), the general permissible tolerance is 2 gm (0.070 oz). However, the specifications of the manufacturer are binding.
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Metal and Composite Variable Pitch Propellers (Balancing)
These types of propellers are balanced by placing lead washers onto a balancing stud within the hollow blade root/shank for horizontal balance while lead washers are placed in recesses of the hub for vertical balancing.
Metal and Composite Variable Pitch Propellers
Minor adjustments to the propeller balance are corrected by placing lead wool into the balancing tube located within the hollow root of the blade.
.On smaller propellers the lead wool can be placed in the counterbored area of the hub bolt heads. These bolts are known as Welch bolts because of the Welch plug used to cap the bolt when lead wool is installed. Welch plugs must be in the bolt head regardless of whether or not lead wool is inserted in the bolt.
If a Welch plug is missing then the propeller is considered out of balance.
Dynamic Balancing
Balancing the propeller to reduce the moment of imbalance is known as dynamic balancing. This must be carried out on a running engine.
The term ‘balancing’ is not completely correct, because it cannot be done on all inertial axes as a change in mass is only possible in specific areas (blades, hub, spinner).
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One method is a vibration pick-up on the engine and a defined weight which is attached alternately to each blade. In accordance with the result, a polar diagram is drawn.
The balancing weight is then attached at the position of greatest imbalance and a further ground run is carried out to ascertain the lowest level of vibration. For this balancing cruise RPM and cruise power are chosen.
Small propellers are not normally dynamically balanced as any shift of the centres of gravity is only small.
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There are four steps used to dynamically balance a propeller:
- Obtain in-flight vibration information.
- Ensure vibration is greater than permitted limits.
- Calculate the mass and location to reduce vibration to an acceptable level.
- Install balance weight(s) and confirm vibration levels are within limits.
Propeller Balancing Systems
Modern electronically controlled Turbo prop systems use some form of Propeller Balance Monitoring System (PBMS).
The information required to provide a balance solution is derived from data provided by a Magnetic Pickup Unit (MPU) and the Active Noise Control Unit (ANCU) controller. This information together with engine RPM and aircraft altitude is processed to determine the level of propeller imbalance.
Aerodynamic Balancing
Propellers can be affected by vibrations because of the differing aerodynamic loads of the blades resulting from the different blade angles. No two blades are manufactured to 100% accuracy. This means that each blade can produce slight variations from the standard theoretical values for torque and thrust
A propeller is described as being in aerodynamic balance when the aerodynamic forces acting on the blades result in no periodic vibrations in the mountings. This means that each blade will be producing an equal value of thrust.
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one blade of a four bladed propeller is producing more thrust than each of the other blades. As this propeller rotates, it will experience an alternating force. When the subject blade is at the top, it will try to push the propeller shaft down. When the blade rotates through the bottom, it will try to force the shaft up
this vibration does not increase with increasing engine speed. It does, however, become worse with increasing pitch and thrust.
The forces of thrust and torque are perpendicular to each other. This makes it difficult to correct for both forces simultaneously
The blades are therefore balanced for either thrust or torque. The choice will be marked on the blade as an Aerodynamic Correction Factor (ACF), preceded by the letter ‘T’ if the correction is for thrust, or ‘Q’ if it is for torque.
All the blades on a single propeller are balanced for the same force and must not be mixed with blades that have been corrected for the other force.
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Aerodynamic balancing is only necessary for propellers with high performance. Manufactured blades are compared individually with a master blade and receive, according to deviation from the zero lift angle, an aerodynamic correction factor in the form of a reference to the blade angle difference necessary to the basic setting.
The whole process is also known as
blade indexing
Aerodynamic Correction Factor
Aerodynamic balancing is achieved by adjusting the basic setting angle of each blade relative to the others during the assembly of the propeller. The action is known as ‘Blade Indexing’ and there are two methods used to achieve it, each dependent on the type of propeller.
The amount to be added or subtracted from the basic setting is expressed in degrees and minutes fine or coarse and is known as the Aerodynamic Correction Factor (ACF). This figure is marked on the blade.
One method of making adjustments, for example is by means of a vernier adiustment on each blade root
Universal Blade Protractor
The protractor is used to measure the propeller blade angle at a specific blade station to determine if the propeller is properly adjusted. The blade angle is referenced from the propeller plane of rotation, which is ninety degrees to the crankshaft centreline.
hen using this device and before measuring the angle of a propeller blade, the reference blade station must be determined from the propeller or aircraft manufacturer’s maintenance manual. This reference station is normally set at either the 30-inch, 36-inch, or 42-inch measurement on the propeller blade. The reference station must be marked with
a chalk or grease pencil on the face of each blade.
The reference plane is not based on the airframe attitude because of the canted Installation of some engines.
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Blade tracking is the ability of one blade to follow the other in the same plane of rotation. Tracking is held to reasonable limits to prevent roughness and vibration.
A tracking check may be called up after a propeller has been installed or, if there is any reason to suspect the dynamic balance. A propeller that has suffered an impact from an object would be a classic case, particularly if vibration occurs.
(Blade Tracking)
Measurement is usually conducted at the master station. The fixed reference point can be mounted on the engine or placed on the ground
The distance between a blade and the pointer is measured. The propeller is then turned by hand to enable measurements to be taken of the distances between each blade and the reference pointer
propeller chapter (Chap 61)
It is important that the propeller blade angles are all set at the same value before doing a tracking check. The maintenance manual will specify the actual setting. Usually, the check is done with the blades on the fine pitch stop.
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There are two main methods for check a blades tracking:
Flat Bench Method
Block or Pointer Method
To check tracking, place a smooth board just under the tip of the lower blade
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Flat Bench Tracking Method
On controllable props, move the tip fore and aft carefully through its small range of movement, making small pencil marks at each position.
Centre the blade between these marks and draw a line the full width of the blade.
Rotate the propeller so that the next blade is in the lower position and repeat this procedure at the blade tip.
The lines should be separated by not more than 3 mm (1/8 inch) or by the measurements designated in the maintenance manual.
Block or pointer Tracking Method
rotate one of the blades so it is pointing down.
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 attach a pointer/indicator to the cowling itself
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 the tolerances defined in the maintenance manual (i.e., within 1/16 inch (plus or minus) from the opposite blade’s track.
Blade Damage
Damage to a propeller can be divided into airworthy and unairworthy damage. Airworthy damage can be repaired in the field. Unairworthy damage is any damage that exceeds the limits of the airworthy damage.
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Damage to blades can take many forms. Some of the terms used to describe blade damage are as follows:
Fatigue: Fatigue failures normally occur within a few inches of the blade tip: however, failures also occur in other portions of the blade when dents, cuts, scratches or nicks are ignored
Split: is a delamination of a composite blade extending to the blade surface, normally found near the trailing edge or tip.
Distortion: is an alteration of the original shape or size of a component
Debond: is a separation of the metal erosion shield from the composite material in the blade.
Normal airworthy damage does not affect the flight safety characteristics of the blades, although airworthy damage should be repaired to maintain aerodynamic efficiency.
To determine if the damage is airworthy or unairworthy, the aircraft maintainer should refer to the information in the blade repair manual.
Airworthy damage limits are usually described within AMM ATA chapter 61 of the related aircraft.
Always refer to the manufacturer manuals to determine serviceability and methods of repair.
Some of the inspection techniques that are or may be required are
Visual Inspection
Tap Test
Penetrant Inspection
Ultrasonic Inspection
Eddy Current Inspection
Magnetic Particle Inspection
Magnetic Particle Inspection
Magnetic particle inspection is conducted at an appropriately approved maintenance facility. It is useful for finding cracks, inclusions, and imperfections at or near the surface of ferrous parts
Eddy Current Inspection
Eddy current inspection uses specialised equipment to generate and measure an electric field that detects flaws at or slightly below the surface of the component being inspected. Eddy current inspection is conducted by appropriately certificated engineer.
used on ferrous and non ferrous metals
Ultrasonic Inspection
Ultrasonic inspection uses specialised equipment to send, receive, and process sound waves to detect flaws on or below the surface of the component being tested. Appropriately certified engineers conduct ultrasonic inspections. Ultrasonic inspections are very specific and require specially designed probes and calibration standards to obtain reliable results. Ultrasonic inspections can be conducted on composites, wood, ferrous, and non-ferrous metals.
Penetrant Inspection
Fluorescent penetrant is far superior to non-fluorescing penetrant (visible dye penetrant), particularly for detecting small surface cracks on propeller blades.
The use of visible dye penetrant is not recommended.
Penetrant inspection on propellers is usually conducted in an approved repair station.
Tap Test
Composite blades are inspected for delaminations and debonds by tapping the blade or cuff (if applicable) with a metal coin. If an audible change is apparent, sounding hollow or dead, a debond or delamination is likely
Blades that incorporate a ‘cuff’ have a different tone when coin tapped in the cuff area. To avoid confusing the sounds, coin tap the cuff area and the transition area between the cuff and the blade separately from the blade area.
