Module 17.6_b1_rev 00.pdf

Module 17 – PROPELLER Sub Module 17.6 – PROPELLER MAINTENANCE

CATEGORY B1– MECHANICAL

MODULE 17 SUB MODULE 17.6 PROPELLER MAINTENANCE

Rev. 00 Oct 2006

17.6 For Training Purposes Only

Module 17 – PROPELLER Sub Module 17.6 – PROPELLER MAINTENANCE

CATEGORY B1– MECHANICAL

Contents

Page

Static Balancing ............................................................................2 Dynamic Balance ..........................................................................6 Aerodynamic Balance ...................................................................8 Propeller Track............................................................................10 Blade Indexing ............................................................................12 Assessment Of Metal Propeller Blade Damage ..........................14 Removing Damage......................................................................16 Assessment Of Composite Propeller Blade Damage ..................18 Composite Blade Repairs............................................................20 Overspeeding...............................................................................22 Post Installation Propeller Testing ..............................................24

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Module 17 – PROPELLER Sub Module 17.6 – PROPELLER MAINTENANCE

CATEGORY B1– MECHANICAL

“The training notes and diagrams are compiled by SriLankan Technical Training and although comprehensive in detail, they are intended for use only with a Course of instruction. When compiled, they are as up to date as possible, and amendments to the training notes and diagrams will NOT be issued”.

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Module 17 – PROPELLER Sub Module 17.6 – PROPELLER MAINTENANCE

CATEGORY B1– MECHANICAL STATIC BALANCING When the weight distribution about the propeller axis is equal, with the propeller in any position, it is said to have static balance. On fixed pitch propellers an unbalanced condition (Figure 17.95) can be rectified by the removal of material from heavy blades or by the addition of extra coats of paints on the lighter blades. Static balance is checked and corrected at a propeller repair shop. The propeller is mounted on a mandrel and placed across perfectly level knife-edges. The balance is check in two planes, one with the blades horizontal (Figure 17.96) and one with them vertical (Figure 17.97). Fixed-pitch metal propellers are balanced in a propeller repair station by removing some of the metal from the heavy side and then refinishing the propeller. On variable pitch propellers, balance is corrected by the addition of weights at the hub, or by the installation of lead wool in the hollow blade roots or nuts, bolts and washers on the spinner backplate.

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Fig 17.96 Fig 17.97 Fig 17.95

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CATEGORY B1– MECHANICAL Another method of achieving static balance is described in the following procedure. Balancing Procedure -

Place alignment markings between balance arbour (2) and balance weight (5), and also between flanged adapter (7) and arbour (2) to provide proper orientation during 180° balance check as illustrated in Figure 17.98.

-

Attach a hoist to the cable loop on the balance indicator and raise the propeller. Ensure blades are correctly set to position recommended in the AMM.

-

Balance the propeller by adding washers (item 170), screws (item 180) and nuts (item 190) illustrated in Figure 17.99 until the balance indicating bushing and disc are centred as illustrated in Figure 17.100.

-

Repeat procedure with alignment marking rotated 180°.

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Balance indicator circles concentric. (assembly in balance).

Fig 17.99 Balance indicator circles slightly eccentric (assembly slightly out-of-balance).

Fig 17.98

Fig 17.100

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CATEGORY B1– MECHANICAL DYNAMIC BALANCE

Testing for Dynamic Balance

A propeller possessing static balance may cause vibration due to the non symmetrical disposition of the mass within the propeller (Figure 17.101). Unequal weight distribution about the propeller axis can only be corrected by repeated ground runs following the addition of weights to the propeller.

The propeller can be tested for dynamic balance either on ground or in flight using a dynamic balancing test set (Figure 17.102). The test set is plugged in to appropriate connections on the flight deck, and appropriate cables are attached to the reduction gearbox (Figure 17.103). Readings obtained will determine the adjustments (if any) required to balance weights in order to dynamically balance the propeller.

Dynamic: Balanced when the blades’ centres of gravity are in the Plane of Rotation. On some aircraft dynamic balance is tested and adjusted off the aircraft on a specialist balancing machine. On these aircraft the only method used to check dynamic balance is to ensure the tracking of the blades is within limits.

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Fig 17.102 Fig 17.101

Fig 17.103

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CATEGORY B1– MECHANICAL AERODYNAMIC BALANCE When all the blades of a propeller are producing equal thrust, it is said to posses aerodynamic balance (Figure 17.104). To achieve this it is necessary to adjust the blade angles relative to one another, by a few minutes of a degree when setting the initial blade angles on assembly. Note: Balancing can only be carried out by approved propeller repair organisations using approved balancing test apparatus. Aerodynamic: Balanced when the aerodynamic forces on all the blades are equal.

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Fig 17.104

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CATEGORY B1– MECHANICAL PROPELLER TRACK An out of track propeller will suffer an imbalance caused by the propeller being out of Dynamic and Aerodynamic balance. Propeller track is the path followed by a blade segment on one rotation. If one blade does not follow in the same track as the others, its angle of attack and thus the thrust it produces, is different to the remaining blades, and vibration will result. It centre of gravity will also be out of alignment, which will also cause vibration. A simple blade tracking check would entail, chocking the wheels to prevent the aircraft from moving. Place a board under the propeller (Figure 16.105) so the blade tip ‘nearly’ touches it. Mark the board at the tip of the propeller, and then rotate the propeller until the next blade approaches the board; mark the second blade position. Repeat for all blades. It can be observed from the marks generated (Figure 16.106) the extent of tracking deviation between blades. The amount that blades can be out of track is specified in the relevant Aircraft Maintenance Manual (AMM). For information only, an example of an average ‘maximum’ permitted deviation in track would be 0.25 inches.

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Fig 17.106

Fig 17.105

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CATEGORY B1– MECHANICAL BLADE INDEXING Slight differences in blade shapes produce unequal aerodynamic forces on the propeller. These inequalities can be corrected for by slight adjustments to the individual blade angles to produce a specific thrust. See Figure 17.107. Aerodynamic balancing can be achieved in two ways, thrust balancing or torque balancing. The adjustment or index is termed the Aerodynamic Corrected Factor (A.C.F). This can be measured in two ways. -

The thrust produced by the individual blade.

-

The torque produced by the individual blade.

The blade’s ACF is usually painted on the blade close to the root. Torque balanced blades and thrust balanced blades cannot be fitted to the same hub. Thrust balanced blades will be marked with ‘T’ and then an angle, Torque balanced blades are marked ‘Q’ with an angle. The ACF is the amount to be added or subtracted from the basic setting when assembling the propeller. The process is often referred to as ‘Indexing’ as shown in the table 17.108 below.

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Fig 17.108

Fig 17.107

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CATEGORY B1– MECHANICAL ASSESSMENT OF METAL PROPELLER BLADE DAMAGE

LIGHTNING DAMAGE

The most frequent major damage to a propeller is bent blades. No straightening is allowed by anyone other than the propeller manufacturer or an approved repair station that must be approved for the particular operation. It is, however, the responsibility of the licensed technician to know the repairable limits of a propeller, so that a decision can be made to either remove/replace the propeller or to send it to a repair station.

If a metal propeller is struck by lightning, burn damage to the blades is likely to occur. In removing this damage the normal repair limits apply, but after cleaning out all physical damage, a further specified thickness of metal must be removed, and the depression blended to a smooth contour. The damage area should then be chemically etched, and inspected with a magnifying glass to ensure that there are no signs of material abnormalities. Any electrical circuits in the propeller should be checked for continuity and insulation resistance.

Blades which are bent, twisted or cracked, or have severe surface damage, are to be considered unserviceable, and the propeller must be removed and returned to the manufacturer or an approved overhaul organisation. Minor surface damage may be blended out within the limitations laid out in the relevant AMM (Figure 17.109). As a general rule: a. The rework depth of the face or camber sides must not exceed 0.060”. b. The reduction of section thickness must not exceed 25% of blade thickness in the area of rework. c. The final blend area must not extend over more than 25% of chord, or 4” whichever is less. d. After removing visible damage, remove further 0.002” for gouge rework, or 0.020” for burn rework with polished finish. e. The length of any one (combined) blending shall not exceed 7”.

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Fig 17.109

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CATEGORY B1– MECHANICAL REMOVING DAMAGE

COLD STRAIGHTENING

-

Blending out damage and correction using:

-

Riffler files

-

Scraper

-

Small power grinder (with suitable buffs and grinding discs)

-

Fine abrasive or powder

Cold straightening of the blade is allowed within the limits prescribed in the relevant AMM, provided the blade has not been subjected to impact damage. Impact damage is defined as damage, visible or not, from a blade striking, or being struck while rotating or when stationary. If a blade has suffered impact damage (although it may be within the cold straightening limits of the AMM) the damage details must be recorded and communicated to the manufacturer before any cold straightening procedure is undertaken.

The rework must be carried out in the direction of the major axis of the blade, forming a smooth rounded depression in the blade surface. The junction between edges of the depression and surrounding blade surface must be faired out with a smooth blend. All traces of file or grinding marks must be removed using abrasive cloth and then the worked area finally polished.

The term ‘cold straightening’ has become accepted, by common usage, to mean blades that can be straightened or twisted without prior annealing. Blades damaged beyond the limits of cold straightening will require heat treatment prior to bending or twisting operations and must therefore be returned to the manufacturer for repair.

The rework area should now be inspected for cracks, indentations and tools marks using a magnifying glass. A crack will cause rejection of the blade. Any further marks should be polished out and the inspection repeated. Check that the rework length/depth proportions are within limits. For gouge and dent damage a further -0.002” of material should be removed, beyond the required damaged. Electrical damage or damage with burrs a further 0.02” if material should be removed. It is essential that as soon as a repair has been carried out, the blade is reprotected.

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A blade may be subjected to cold bending or twisting within the prescribed limits on two successive occasions only. Where correction is required for a third time the blade must be returned to the manufacturer for heat treatment.

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CATEGORY B1– MECHANICAL TIP CROPPING The tip of the blade can be cropped within the limits specified in the AMM. A template should be made to the new tip dimensions and the template placed against the face side of the blade. Using a sharp pencil, mark the new tip arc. The portion of the blade outboard of the marking is removed by hacksaw or coarse grinding disc depending on the amount of material to be removed. All file and grinding marks must be removed and the work area polished using fine emery cloth. The blade should then be inspected to determine that the blade length is within permitted limits. The amount of tip cropping must be recorded on the blade butt face in code form (e.g. TC 0.25”).

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CATEGORY B1– MECHANICAL

CAUTION: TAPPING ACROSS BOUNDARIES OF ABRUPT CHANGES IN SHELL THICKNESS OR MATERIAL CHANGE WILL PRODUCE TONAL CHANGES. THIS IS NORMAL AND IS NOT A VOID OR DEFECT.

ASSESSMENT OF COMPOSITE PROPELLER BLADE DAMAGE Damage to the blades of a composite bladed propeller may not be visual using normal inspection methods. Delamination between fibre-glass layers, or between fibre-glass and foam filler (Figure 17.110), can however be deducted using a simple ‘tap test’ procedure.

When tapping, the strike of the hammer should be approximately 0.25” apart. The direction of the tapping should be with the longitudinal axis of the blade because the construction of the blade varies slightly in this direction.

CAUTION - TAP TESTING MUST ONLY BE PERFORMED BY INDIVIDUALS WHO HAVE SUFFICIENT EXPERIENCE AND TRAINING.

When inspecting the blade on the wing, the tap test area should be free of loud noises since the effectiveness of the tap test is related to the sound levels and variations in the vicinity of the tap test.

CAUTION - DO NOT TAP TEST OVER THE INTEGRAL BLADE HEATING ELEMENT.

Any area with a suspected deformity as determined by a tonal change or visual inspection will marked on the blade so as to identify the outline of the damage. These markings will be used to determine limits of repairability (Figure 17.111).

TAP TESTING Tap testing is an auditory test performed by striking the outside surface of a blade with a hammer specifically designed for the test. By listening for a tonal change, the tap tester can determine the sub-surface structural integrity of the blade. The tonal changes may be voids in the lockfoam filler and/or disbonded areas, such as separation of the shell to a lockfoam bond. The tap tester should be able to hear in the frequency range of 3000 hz. to 8000 hz. at 30 decibels (db) or lower on the better ear. Tap testers should have their hearing checked annually. The outside surface of the blade is struck with a light uniform force in a rhythmical tempo. Tonal changes of the striking hammer may indicate sub-surface defect.

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Fig 17.110

Fig 17.111

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CATEGORY B1– MECHANICAL COMPOSITE BLADE REPAIRS

NOTE: The shell spar bond line can be located by tap testing with the tap testing hammer.

All handling and cutting of glass cloth and laminating of glass cloth and resins should be carried out in a controlled atmosphere of relative humidity and temperature as follows:

CAUTION: IN THE PROCESS OF REMOVING DAMAGED LAMINATES DO NOT REMOVE THE ADHESIVES OVER THE SPAR SO THAT THE BLADE SPAR IS EXPOSED. SHOCK LOAD CHECK When an engine has been subjected to a shock load, for example, during a heavy landing, or if the propeller is struck by a Foreign Object, the propeller shaft must be checked for concentricity by attaching a DTI to a bar that is bolted to the engine casing (Figure 17.113). With a weight attached to the end of the shaft and a DTI in contact with the front parallel portion set to zero, the shaft is rotated through 360º and the indicator movement is observed. The maximum permissible eccentricity will be stated in the appropriate maintenance manual.

A clean facility protected from dust, wind, rain, fog, cold, direct sunlight and other similar environmental factors should be used. Do not lay up glass cloth and laminates with resins and adhesives in temperatures below 35ºF.Glass cloth and bonding adhesives should be sealed in plastic bags, package laminating resins and sheath bonding adhesive in sealed containers. Local repair of damage in the shell laminate is normally permissible provided that the damage is confined within an area bonded by a line 0.50 inch minimum from the nickel sheath edge on the leading edge. (See Figure 17.112). The numbers of repairs is not normally limited, provided that each repair does not exceed 40 square inches. Laminate repair in the heater area may be performed after removal of the heater.

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Fig 17.113

Fig 17.112

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CATEGORY B1– MECHANICAL OVERSPEEDING Propellers may occasionally exceed their normal maximum rotational speed, and be subjected to centrifugal forces in excess of those for which they were designed. With variable-pitch propellers, overspeeding will normally only occur following failure of the control system, but with fixed-pitch propellers the maximum engine speed may easily be exceeded during manoeuvres if the engine speed indicator is not carefully monitored. The extent of the checks that must be carried out following overspeeding, will depend on the margin by which the normal maximum rev/min have been exceeded, and on any particular instructions contained in the approved Maintenance Manual. No special checks are normally required following overspeeding normal maximum rev/min, but it may be recommended that the track of the propeller is checked. If the propeller has been overspeeding the normal maximum rev/min, for a period in excess of any specified time limit, it should be removed for inspection. All blades should be carefully inspected for material failure, using a penetrant dye process. Blade bearings should be crack tested, and the rolling elements and raceways should be inspected for brinelling (i.e. indentation). The hub and counterweights should be inspected for cracks and distortion, and particular attention should be paid to the blade mounting threads and spigots. If the overspeeding has been excessive, the propeller should be returned to the manufacturer for investigation.

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Engine running time should be kept to a minimum consistent with satisfactory completion of the checks, and a careful watch should be kept on engine temperatures to avoid overheating. With turbine engines, changes to operating conditions should be carried out slowly, to avoid rapid engine temperature changes, and to conserve engine life.

POST INSTALLATION PROPELLER TESTING After installation of a propeller, the engine must be ground run in order to check the propeller for correct function and operation. Aircraft propeller installations vary considerably, and no set testing procedure would be satisfactory for all aircraft. It is imperative, therefore, that any particular installation should be tested in accordance with the approved AMM procedure, which will normally include the following general requirements:

When all checks have been successfully carried out, the engine should be stopped, and a thorough inspection of all propeller system components should be carried out, checking for security, chafing of pipes and cables, and signs of oil leaks.

The engine should normally be fully cowled, and the aircraft should be facing into wind before starting an engine run. It is sometimes recommended that the pitch change cylinder should be primed with oil before starting, by operation of the feathering pump.

Figure 17.114 shows the danger areas when operating the engines.

The safety precautions appropriate to engine ground running should be taken, the controls should be set as required, and the engine should be started. As soon as the engine is operating satisfactorily, and before using high power, the propeller should be exercised in the manner specified in the Maintenance Manual, to establish that the pitch change mechanism is operating. The checks specified in the Maintenance Manual to confirm satisfactory operation of the propeller system, including constant speed operation, feathering, operating of the propeller pitch change throughout its range, synchronisation with other propellers on the aircraft, and operation of associated warning and indicating systems, should be carried out.

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Fig 17.114

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CATEGORY B1– MECHANICAL Student Notes:

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