GB2207210A - Cleaning lance - Google Patents

Abstract

In some aircraft operating environments the high pressure turbine blades, virtually the hottest part of the jet engine, are susceptible to the formation of surface deposits which can substantially impair the cooling efficiency of the blade. In particular, these deposits tend to block the cooling air exit holes in the leading edges of surface cooled blades. A lance for cleaning the deposit from the leading edge and from in and around the exit holes has a nozzle for directing a jet of cleaning material, eg fine grit or pressurized water at the edge and into the exit holes. Clean air is also directed throughout a cleaning cycle into exit holes upstream of the cleaning jet in order to purge grit from the internal gallery communicating with the exit holes and to prevent the grit being blown back into the cooling air supply system. <IMAGE>

Description

CLEANING LANCE The invention concerns a cleaning lance and a method of its use to direct a jet of abrasive cleaning grit onto an inaccessible item, in particular, to clean the leading edge of a gas turbine blade having cooling air exit holes in the leading edge.

The invention finds particular application in cleaning, in situ, turbine blades in a gas turbine engine, especially air-cooled turbine blades of the type in which cooling air is ducted through the interior of the blade and is allowed to escape through cooling apertures formed in the wall of the blade, for example, at the leading edge. The high pressure turbine blades are amongst the most critical parts of an engine, being subject to the impingement of very hot gases. Generally, the hotter the temperature which the turbine can withstand, the greater the engine thrust that can be produced.

However, at such high temperature solid matter, dust and that sort of material, passing through the engine in the gas stream tends to precipitate on the surface of the turbine blades creating overheating problems. The material forms a hard surface coating on the blades, restricting and possibly even blocking the cooling air exit holes.

The loss of surface cooling air can result rapidly in failure of the blades if the engine performance is not restricted to lower the blade temperatures.

The solution proposed in accordance with the method of the present invention is to abrade the leading edge of turbine blades so affected with a fine grit to clear the exit holes for the cooling air without stripping down the engine. Performed as often as required the lifetime of the blades can be increased by a substantial factor. The proposed apparatus is intended to permit the layer deposited on the surface of the blades to be removed at least from around the cooling air exit holes by means of a jet of cleaning material and to prevent the cleaning material passing back into the cooling air supply ducts.

According to one aspect of the invention there is provided a cleaning lance for cleaning gas turbine blades comprising a cleaning head for discharging a first jet of cleaning material and for separately discharging a second jet of clean air, means for imparting to the head a compound movement involving a reciprocating component and a rotary component whereby the lance head may be made to follow the curve of the leading edge of a turbine blade.

Preferably the cleaning head is adapted for insertion through a port in the casing of a gas turbine engine for carrying out cleaning of the blades in situ. The cleaning material may be fine abrasive grit or high pressure water. Other cleaning materials may be substituted. Also, the supply of cleaning material and air are preferably individually controlled in particular to continue the jet of clean air after the cleaning phase of the operation has terminated in order to purge cleaning material from internal cooling passages in the turbine blade.

According to another aspect of the invention a method of cleaning a gas turbine blade in situ comprises inserting the previously mentioned cleaning lance through a port in the engine casing, locating the cleaning head a short distance in front of the blade, directing the air jet towards the blade root and directing the cleaning jet at the blade leading edge, and passing the cleaning jet along the leading edge at least once.

The method is particularly suited to cleaning air cooled turbine blades of the type having internal passages supplied with cooling air which is bled through small diameter exit holes in the blade leading edge to provide a surface cooling film.

The invention will now be described in greater detail with reference, by way of example only, to the apparatus illustrated in the accompanying drawings in which: Figure 1 shows a perspective overall view of a cleaning lance for cleaning the turbine blades of a twin spool bypass turbo-jet engine.

Figure 2 shows a sections view through the cleaning head of the lance, Figure 3 shows separate views of a cleaning lance, a substitute forked end guard, and a dummy lance, Figure 4 shows a diagrammatic sectional view of a twin-spool bypass turbo-jet engine with a cleaning lance in position for cleaning the first stage blades of the high pressure turbine, Figure 5 shows a block diagram of an electronic control system for operating the lance, and Figure 6 shows a simple flow chart illustrating the operation of the system.

The device illustrated in Figure 1 comprises a housing 10 of sheet metal having a relatively heavy gauge base portion 10a, an upstanding flange portion lOb and relatively thinner gauge side panels lOc, lOd, and lOe. Rotatably mounted in the base IOa is a lance assembly generally indicated at 12, one end of the lance extends through an aperture in the base lOa into the housing 10. The side panel lOc is attached to the base lOa and flange lOb. The remaining panels lOd and lOe form part of a side opening breech type assembly 14 which is hinged on a tube 15 also rotatably mounted in the base lOa. This breech assembly 14 is shown in the open position in Figure 1.

The lance assembly 12 comprises a metal hub member 16, in the form of a short cylinder, secured to one side of the base lOa and surrounding the lance aperture. Rotatably mounted on the hub 16 is a knurled bayonet attachment ring 18, which has bayonet lugs on the hidden face, facing away from the base 10a. A cylindrical metal boss 20 projects forwards from hub 16 and carries an elongate barrel 22 which terminates in a further cylindrical head member 24. The hub 16, boss 20 and barrel 22 are mounted co-axially with respect to each other and provide a continuous central bore through which passes a cleaning material delivery tube 26.

The head 24 also has an internal bore 28, as shown in Figure 2, through which the tube 26 passes.

Although the bore 28 is co-axial with the barrel 22 the head 24 is eccentrically positioned to accommodate an air supply passageway 30 in the resulting thicker wall portion of the head.

The air supply enters the lance assembly through a radial inlet tube 32 mounted on the hub 16 and which communicates with an internal passage in the hub. This internal passage extends axially through the wall of the hub 16 and boss 20 and leads into an air duct 34 extending alongside the barrel 22.

At the head end of the lance assembly the duct 34 passes into the passageway 30. The passageway 30 is stepped internally in the head 24 closer to the axis of the barrel 22 and supplies air to a snorkel tube 36.

The ~ snorkel 36 exhausts air in a substantially transverse direction relative to the axis of the lance assembly. The cleaning material delivery tube 26, which extends along the axis of the barrel 22 is terminated by a nozzle 38 which also directs a jet of cleaning material sideways, generally in the same direction as the air jet. Cleaning material is supplied to tube 26 through a connecting pipe 26a.

The cleaning lance carrying the tube 26 is shown separately in the perspective view of Figure 3.

For the greater portion of the length of the lance the tube 26 is supported concentrically inside a second tube 40. This lance tube 40 is sealed around the inner tube 26 at its head end, and at the opposite end is mounted in cam member 42. A raised plinth 44 on one side of the member 42 is formed with an oblique groove 46. When assembled the lance tube 40 passes through the aperture in base lOa providing communication with the interior of hub 16 and, as illustrated in Figure 1, is free to move axially towards and away from the base lOa.

A spool 48 is secured to the rear face of the cam member 42, cushioned between annular rubber dampers 50, by means of a tube nut 52. The supply tube 26a extends through the centre of nut 52, spool 48 and dampers 50 to connect with the delivery passage of tube 26. A forked member 54, which will be described below, engages the spool 48 in operation, to impart to it an axially reciprocating component of motion in a manner yet to be described. As previously mentioned the lance tube 40 passes through the barrel 22. At the head end of the barrel the tube 40 passes into the bore 28 of the cylindrical head member 24, but does not extend through it. Rollers 56 journalled in the head 24 at locations angularly spaced around the circumference of the bore 28 support the lance tube 40 towards its head end.The rollers 56, of which there may be at least three spaced around the bore, protrude slightly into the bore 28 to engage and support the tube 40. Their rotational axes are disposed transversely relative to the longitudinal axis of tube 40 thereby supporting the tube during movement in either axial direction.

The lance tube 40 is turned, when loaded into barrel 22, so that the grooved side of the raised plinth 44 faces towards the flange lOb. A spherical end 58 of a plunger 60, which is mounted in the flange lOb, is engaged with the oblique groove 46 in the pl-inth whereby to act as a guide producing a limited rotation of the tube 40 in response to its axial movement.

Axial movement is imparted to the lance tube 40 by means of movement of the bifurcated member 54 which is engaged with the spool 48 when the breech type mechanism 16 is closed. The member 54 is carried by a carriage 62 which is slidably mounted on a support tube or rod 64. The rod 64 is rigidly mounted between an end bracket 66 and a flange 68.

The flange 68 is rigidly mounted on the surface of the relatively large diameter tube 15 which is rotatably mounted, towards one end, in the base 10a. The end bracket 66 is attached to the opposite end of tube 15 for movement therewith.

The carriage has a further saddle 72 which slidably engages the tube 15. The arrangement is such that the carriage 62 is able to move backwards and forwards along the tube 64 between limits defined by the end bracket 66 and the flange 68. Saddle 72 prevents the carriage 62 rotating about the rod 62.

The assembly can be swung on the rotatable mounting of the large tube 15 in the base 10a into a closed position in which the spool 48 is engaged by the upstanding forked member 54.

Longitudinal movement of carriage 62, and therefore also of member 54 and the cleaning lance 40 through engagement with spool 48, is controlled by rotation of lead screw 74. An electrically powered motor 76, having an integral rotary encoder, drives the lead screw 74 through a step-down gearbox 78 which rotates a threaded bush 80 helically engaged with the lead screw 74. The end of the lead screw inside the breech mechanism housing is engaged with a further threaded bush (not shown) which is held captive in a flange 82 formed integrally with the main part of carriage 62. The position of the carriage relative to the lead screw 74 may be adjusted by turning a knurled nut 84 attached to the lead screw. The setting of the lead screw is then locked by a second knurled nut 86 and lock nuts 88a and 88b which secure the lead screw from rotation relative to the carriage 62.Held, thus, from rotation when the motor 76 drives the bush 80 the lead screw 74 moves in an axial direction according to the direction of rotation of the bush.

The rotary encoder contained within the motor 76 determines the number of revolutions of the motor in either direction, this number is set in accordance with the range of travel required for the carriage 62. Thus, in use, the initial starting point of the carriage is set by adjustment of the lead screw setting, whereafter the amount of travel and direction of movement is controlled entirely by the rotary encoder.

In operation, the cleaning lance is inserted through a port in the engine casing providing access immediately in front of the blades to be cleaned as depicted in Figure 4. It is already known, and is common practice, to provide so-called bores copse ports through which viewing scopes can be inserted for visual inspection of internal parts of the engine, the presently described device is intended to make use of these ports. The engine diagrammatically illustrated in Figure 4 is a twin-spool bypass turbo-jet of well known type. A portion of the engine airflow is ducted around the main core of the engine through a bypass. Since the high pressure turbine stages, the blades of which the present device is intended to clean, are in the main core radially aligned ports are provided in the inner and outer engine casings.The axes of these ports are indicated by dashed lines in Figure 4. The distance between the mounting ring 18 and the head 24 is arranged to span the width of the bypass duct between the engine casings.

The length of barrel 22 is tailored to span the engine bypass duct, in the described example. For use in other locations or non-ducted bypass engines a much reduced barrel only may be required since it will then be unnecessary to carry the air and cleaning material across the duct.

A mounting plate 90 designed to be attached on the external surface of the outer casing may be provided to receive the bayonet ring 18. The device could be mounted directly on the engine if the weight load is permitted, alternatively the mounting plate 90 is useful to transfer load to strengthened mounting points. The mounting plate preferably has adjustable mounts or jacks permitting angular alignment of the lance barrel, so that the cleaning jet can be aligned with a leading edge of a blade. The position of lead screw 74 is subsequently set to align the cleaning jet radially so that the cleaning jet will impinge directly on the blade edge in the region of the exit holes and directly onto any accumulation in the holes themselves.Finally, the turbine itself is rotated to bring a first blade into register with a cleaning jet nozzle, thereafter, the degree of rotation of the turbine disc required to bring each of the remaining blades into register at the same location is precisely known.

In the example being described a second electric motor, with a rotary shaft encoder based control system, is connected to drive the turbine disc mechanically through an ancilliary gearbox drive.

thereby each blade of the turbine disc can be stepped or indexed in turn past the cleaning head for a distance equal to the interblade spacing. By controlling both electric motors, ie the last mentioned motor and that in the lance assembly, from a common source arranged to synchronise their operations a complete cleaning operation may be performed automatically after initial alignment.

Assuming that the air and cleaning jet nozzles 36, 38 have been aligned with the leading edge of a turbine blade, the preferred method of cleaning a turbine blade of the type already referred to is as follows. The air nozzle is closer to the root of the blade and clean pressurised air is blown at the blade at a pressure of roughly 80 psi (about 3.5 bar). Internal cooling air for the turbine blade enters through the root of the blade and the gallery supplying surface cooling air to the exit holes along the leading edge is also fed from the root end. Thus, the reverse flow of clean air entering the gallery towards the root end through the first few exit holes sweeps cleaning material in the normal direction of air flow and out through the normal exhaust hole in the blade tip. Also, this air prevents cleaning material being blown back into the cooling air supply system.

Preferably the air snorkel 36 is left in position near the blade root during a cleaning cycle, but it may be arranged to traverse the leading edge following the path of the cleaning jet.

Cleaning material is propelled through nozzle 38 towards the row of surface cooling holes in the leading edges. When the motor 76 is powered this jet of cleaning material is made to traverse the leading edge. At least one pass is made along the edge, the exact number required to satisfactorily clean the edge and the cooling holes will vary according to the thickness and hardness of accumulations. Clean air forced into the gallery will purge the inside of the blade sweeping cleaning material in the gallery towards the tip exhaust aperture. The supply of cleaning material to pipe 26 is terminated after the required number of passes along the blade, whilst the clean air supply is maintained long enough to purge all, or virtually all, cleaning material from the interior of the blade.

In the particular example being described, to clean a blade having cooling holes of the order of 0.012" (3.00 x 10 mm) the cleaning material comprised aluminium oxide grit of particle size 27 microns (2.7 x 10 mm). Alternative materials and particles may be used, of course, as determined most suitable for each particular case. High pressure water may also be used as cleaning material with considerable effect.

As has already been mentioned the lance is mounted not directly on the engine casing but on a mounting plate which is attached to a convenient load bearing part of the engine structure. During installation and initial alignment operations it is preferred to substitute a dummy lance assembly comprising the knurled bayonet ring 18 and a solid lance barrel 92, in order to obviate damage to the tubes and nozzle 26, 30, 36, 38 of the lance proper. when the mounting plate is correctly aligned the dummy lance is removed and replaced by the cleaning lance. The complete cleaning lance assembly can be installed in two stages; firstly, the barrel 22 and air snorkel 30, 36 can be installed separately from t he lance tube 40 carrying the cleaning material delivery tube 26 and nozzle 38. In that event it is preferred to substitute the forked end guard tube 94 for the tube 40.The forked end is designed to flank the air nozzle 36 so that an air jet can be used alone.

Experience has shown that it is advantageous to locate the head end of lance tube 40 by means of the rollers 56, rather than by a sliding fit, because cleaning grit particles tend to enter the bore 28 and cause jamming of the lance. The rollers ensure that the movement of the lance tube remains smooth and free. Similarly it has been advantageous to provide the tube 64, which support the driving carriage 62, with a helical groove 96 ,so that the grit does not accumulate in the sliding bearing in the carriage. A plain bush may therefore be used in the carriage 62.

A complete cleaning equipment for use on a gas turbine engine comprises a cleaning lance, as described above, video inspection equipment, also adapted for insertion through a borescope port, an abrasive powder supply unit, a compressed air supply, an electric motor for indexing the engine, in addition to the motor already described which powers the lance, and a control unit illustrated in Fig 5. The control unit is required to generate output signals to control rotation of both electric motors for engine rotation and lance traverse and further outputs for controlling the supply of cleaning powder and purge air.

The basis of the control unit is a 16 bit microprocessor 100 and a 10MHz clock pulse generator, generally indicated at 102. The main control program is stored in a programmable read only memory (PROM) 104, a second memory comprising random access memory (RAM) 106 is used to store certain predetermined transient data or constants.

For example, RAM 106 is used to store the number of motor rotations required for one traverse of the cleaning lance, the speed of traverse, the number of cleaning passes for both cleaning powder and purge air, the total number of blades to be cleaned in one complete engine cleaning cycle, the number of motor rotations required to index the engine from one blade rotation to the next, and the speed of rotation of the engine, this list is not exhaustive. Also connected with the microprocessor 100 is a power-up reset circuit 108, a programmable clock generator 110 acting as a non-vectored interrupt generator, an address latch 112 and a data buffer 114, the circuits are interconnected via a multiplex bus 115.

An interface module 116 is also provided, this is based on a programmable input/output chip which includes three digital counters (not shown separately). The module 116 is interconnected with the mircroprocessor 100 to receive control signals therefrom directly and address and data via the address latch 112 and data buffer 114. An output from the 1MHz clock generator 110 is also connected via a divide-by-four circuit 118 to provide a 250 KHz clock signal input for the three counters.

The interface module 116 provides outputs to a blast control unit 120, which controls the supply of abrasive powder and compressed air, and to the power drive circuits of two DC motors 76 and 122 which drive the cleaning lance in its traverse motion and rotate the engine respectively. Forward and reverse motion signals are provided in each case. The motor 76 is housed in the lance unit as described above in connection with Fig 1. The second motor 122 is temporarily mounted on an engine ancilliary unit to drive a power take-off shaft through a bevel gear and step-down gearbox (not shown). Both motors are fitted with optical encoders 124 and 126 respectively which produce a fixed number of pulses per motor revolution, typically 60, and these are fed back to the interface module 116 through squaring circuits 128 and 130.These square wave inputs are connected to timer/counters in the interface module.

In operation, one or the other of the counters provides a driving output, in the form of a variable mark-to-space ratio signal, to power drive circuit, 132 or 133 as required, which converts the signal to a DC voltage across the motor winding As the motor rotates the corresponding optical encoder, 124 or 126, generates an output comprising a string of pulses which are counted by a second counter. The target count number is stored in RAM 106. There are only three counters provided on the interface chip 116, but, as the two motors 76 and 122 are never operated simultaneously, one of the counters in able to be shared by both motor control control loops.

The electronic control for the cleaning lance is housed in a separate unit which has a front panel which is used for mounting some manual controls, monitor lamps 134, display panel 135. A keypad 136 is also provided separately connected to chip 116.

The manual controls comprise a plurality of switches, generally indicated at 138, and a potentiometer 140. The potentiometer is used to provide a reference voltage to set the traverse speed of the cleaning lance, which voltage is connected to one input of a comparator 142. The other comparator input is connected to a digital to analogue convertor 144. The converter and comparator are used to read the setting of the speed potentiomenter by use of a software algorithm which emulates a successive approximation analogue to digital convertor. The traverse mechanism may be provided with a microswitch to indicate one limit of movement, this may be arranged to reset the traverse motor counter. Alternatively, a manual reset facility may be provided in which case the mechanism is "zeroed" during an initial setting-up procedure.The front panel switches indicated at 138 include a manual reset button, a start/stop sequence switch and further switches for manually controlling energisation of the motors 76 and 122 for initial setting-up and a manual override to control the supply of abrasive powder to the cleaning head.

The display 135 comprises a four digit indicator panel connected through a display logic block 145 to receive a number for the microprocessor 100 corresponding to the number of blades cleaned in an operation. The number is incremented by one each time the engine is indexed around by one blade. A greater or lesser number of display digits may be provided according to number of blades to be cleaned in one operation.

The lamps 132 provide a visual indication of each stage of a cleaning operation. In the embodiment being described there is a lamp illuminated to indicate engine rotation, cleaning head traversal in each direction either radially inwards or outwards, abrasive powder supply and, finally, when an automatic sequence is in operation.

The keypad 136 is also connected to the interface chip 116 and enables certain of the constants stored in the RAM 106 to be modified or changed.

For example, the constant defining of the length of a blade, the total number of blades and the spacing between them may be changed. Other of the stored constants may also be changed. Operation of the cleaning lance, with particular regard to an automatic sequence will be readily understood with reference to Fig 6. On switching-on the microprocessor program immediately jumps to a "power-up and initialisation" sub-routine in which certain program pointers, parameters and outputs are set to predetermined states or values.

Self-test and diagnostic routines are also implemented to ensure the control system is fault free. Some constants are stored in the RAM 106, in the particular example being described, this is a non-volatile memory and stored values are retained even in the absence of power. The keypad 136 may be used to modify or change the values of these stored constants by direct access to the memory locations at which the constants are stored. Using the keypad a memory address is first entered and then the new value to be stored is punched in. In this mode the address and new constant value are displayed on the four-digit display on the front panel.

After the initialisation phase is completed the program checks whether manual or automatic operation has been selected, see Fig 6. If manual control has been selected the software follows the program steps on the left side of the drawings. If none of the controls or external input facilities, eg the keypad, are in use the program runs in a so-called idle loop in which, in a re-circulating sequence, it continuously rechecks the setting or status of all control switches and the keypad.

When one of these is changed the program enters an appropriate sub-routine in some cases until the control function is cancelled and in other cases,such as powder supply and engine rotation, for a predetermined brief period.

The manual control functions available are: "Powder On" which initiates the supply of abrasive cleaning powder or grit to the cleaning head, a purge supply of compressed air to the cleaning jet nozzle is maintained when cleaning powder supply is discontinued, also air is supplied to the second nozzle continuously; "Traverse" which operates the traverse motor 76 to sweep the cleaning jet along the leading edge of a blade; "Traverse Inching" which allows the cleaning head to be positioned radially, using motor 76, this position can be stored in memory 106 as a basic reference position.

"Next Blade" which steps or indexes the engine around through a predetermined angle by energising the rotate motor 122; "Inch or Rotate Back/Forward" is intended primarily for setting the first blade in position by motor 122 prior to commencement of an automatic sequence, the motor position may also be stored in RAM 106 as a basic reference position, and "Continuous Rotate" which rotates the engine, again by energising motor 122, at a constant speed.

The automatic sequence loop is shown to the right of Fig 6. The basic reference position of the cleaning nozzle, the length of a traverse movement, engine movement to the next blade position use values stored in RAM 106. The program follows the illustrated sequence, circulating around the nested loops as many times as required. The purge pass mentioned following the traversal of the cleaning lance refers to a similar movement of the cleaning nozzle without abrasive powder and delivering clean compressed air only. During this movement the second non-traversing nozzle also maintains its continuous delivery of clean air towards the root of the blade.

The automatic sequence is repeated for each of a predetermined number of blades. At completion of each cycle the blade counter is incremented by one and the engine stepped around to the next blade position. When the blade counter number equals the number of blades stored in memory the program quits the automatic sequence and returns to the idle loop.

Claims (26)

CLATMS

1. A cleaning lance for cleaning gas turbine blades comprising a cleaning head having first means for discharging a jet of cleaning material and second means for discharging a jet of clean air, means for imparting to the cleaning jet a compound movement involving a reciprocating component and a rotatable component whereby the cleaning jet may be made to follow the curve of the leading edge of a turbine blade.

2. A cleaning lance as claimed in claim 1 wherein the cleaning head is adapted for insertion through a port in the casing of a gas turbine engine.

3. A cleaning lance as claimed in claim 2 wherein the cleaning head comprises an elongate structure of axial length sufficient to bridge the bypass duct of a bypass engine.

4. A cleaning lance as claimed in any preceding claim comprising a first nozzle for discharging the cleaning material in a direction perpendicular to its longitudinal axis and a second nozzle for discharging the air jet.

5. A cleaning lance as claimed in claim 4 wherein the first nozzle is movable relative to the second nozzle.

6. A cleaning lance as claimed in claim 5 wherein the second nozzle is positioned outboard of the first nozzle so as to provide a jet of clean air at one side of the cleaning jet.

7. A cleaning lance as claimed in any preceding claim further comprising separate longitudinal ducts for supplying cleaning material and air from sources outside the engine to the cleaning head.

8. A cleaning lance as claimed in claim 7 wherein the first nozzle attached to distal end of a turbine constituting the cleaning material supply duct are dismountable from the lance.

9. A cleaning lance as claimed in claim 8 wherein the supply tube is supported towards its distal end inside a collar adapted for location in a port in an engine casing.

10. A cleaning lance. as claimed in claim 9 wherein the collar is carried at one end of an elongated support tube and the supply tube passes concentrically through the support tube and collar and is supported in the collar by a plurality of rollers spaced apart around the circumference of the collar.

11. A cleaning lance as claimed in claim 9 or 10 wherein the second nozzle is carried at the end of a further tube connected to the air supply duct passing through the wall of the collar and lying adjacent the tube leading to the first nozzle.

12. A cleaning lance as claimed in claim 11 wherein the dismountable cleaning material supply tube and first nozzle may be replaced by a dummy tube which has a bifurcated end forming two side members which lie protectively on either side of the second nozzle.

13. A cleaning lance as claimed in any preceding claim wherein the reciprocating component of the compound movement of the cleaning jet is produced by means of a rotary drive and means for converting rotary motion to longitudinal motion.

14. A cleaning lance as claimed in claim 13 wherein the means for converting rotary motion to longitudinal motion comprises a helical cam and engaged therewith a carriage restrained for longitudinal movement only.

15. A cleaning lance as claimed in claim 14 further comprising means for adjusting the relative position of the carriage and the cam.

16. A cleaning lance as claimed in any one of claim 13 to 15 wherein the rotary drive comprises a rotary electrical motor having a reversible drive controller.

17. A cleaning lance as claimed in claim 16 wherein the reversible drive controller comprises a rotary position encoder and means responsive to the encoder output to reverse the motor drive at predetermined positions.

18. A cleaning lance as claimed in any of claims 13 to 17 further comprising means for superimposing a rotary motion on the reciprocating motion of the cleaning jet.

19. A cleaning lance as claimed in claim 18 wherein the means for superimposing the rotary motion comprises a cam follower engaged with a cam formation angularly set across the longitudinal axis of the reciprocating motion.

20. A cleaning lance as claimed in any preceding claim wherein the cleaning material comprises a fine abrasive grit.

21. A cleaning lance as claimed in any of claims 1 to 20 wherein the cleaning material comprises high pressure water.

22. A cleaning lance substantially as described with reference to the accompanying drawings.

23. A method of cleaning a gas turbine blade in situ comprising inserting through the casing of the engine a lance substantially as claimed and positioning the cleaning head a short distance in front of the blade, directing a jet of air towards the blade root and a jet of cleaning material at the blade leading edge, and scanning the length of the blade with the cleaning jet in at least one pass.

24. A method according to claim 23 comprising the further step of maintaining the air jet longer than the cleaning jet.

25. A method according to claim 23 or 24 further comprising repeating a cleaning operation for each blade of a turbine disc and stepping the disc around for a distance equal to the interblade -spacing between each cleaning operation.

26. A method of cleaning a gas turbine blade in situ substantially as hereinbefore described.