US3672788A

US3672788A - Variable pitch aerofoil blades

Info

Publication number: US3672788A
Application number: US34582A
Authority: US
Inventors: John Henry Ellinger
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1966-12-05
Filing date: 1970-05-04
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27
Also published as: DE1601640A1; FR1553936A

Abstract

VARIABLE PITCH AEROFOIL BLADES ARE PROVIDED WITH A GEAR DRIVE HAVING A FEEDBACK SO THAT WHEN THE MECHANISM REACHES AN EQUILIBRIUM (FEATHERED) CONDITION ANY DISTURBANCE AFFECTING THE BLADE CAUSES THE GEAR DRIVE TO ADJUST THE BLADES TO CANCEL THE EFFECT OF THE DISTRUBANCE.

Description

June 27, 1972 J. H. ELLINGER 3,672,788

VARIABLE PITCH AEROFOIL BLADES Original Filed Nov. 29, 19s? 4 Sheets-Sheet 1 85 F/Gl A Inventor Attorneys June 27, 1 972 J. H. ELLINGER 3,572,733

VARIABLE PITCH AEROFOIL BLADES Original Filed Nov. 29, 1967 4 Sheets-Sheet 2 Atlorneyg June 21,1972 7 J. H. ELLINGER 3,672,788

VARIABLE PITCH AEROFOIL BLADES Original Filed Nov. 29, 1967 4 Sheets-Sheet 3 A ttofne y J. H. ELLINGER 3,672,788

VARIABLE PITCH AEROFOIL BLADES June 27, 1-972 Original Filed Nov. 29, 1967 4 Sheets-Sheet 4 Inventor A ttorneys United States Patent ABSTRACT OF THE DISCLOSURE Variable pitch aerofoil blades are provided with a gear drive having a feedback so that when the mechanism reaches an equilibrium (feathered) condition any disturbance affecting the blade causes the gear drive to. adjust the blades to cancel the efiect of the disturbance.

This application is a continuation of co-pending United States application Ser. 'No. 686,435 filed Nov. 29, 1967 (now abandoned).

This invention relates to variable pitch aerofoil blades and more particularly to a mechanism adapted to drive such blades.

The present invention has particular but not exclusive application to variable pitch aerofoil blades forming part of the fan of a gas turbine engine.

According to the present invention variable pitch aerofoil blades comprise a plurality of aerofoils driven from a main shaft to rotate about the shaft axis and each mounted so that at least part of each said aerofoil or hydrofoil is rotatable about its longitudinal axis, a gear train drivingly connected to each said part for rotating the part about its longiutdinal axis, said gear train having at least one mode of operation in which it is adapted to rotate said parts about their longitudinal axes in such a manner that when the mechanism reaches an equilibrium position any disturbance affecting the aerofoils or hydrofoils causes a disturbance in the shaft rotation which then drives the gear train so as to cancel the effect of the disturbance.

Preferably said gear train is adapted so that the speed of response of said aerofoil blades to a disturbance may be varied.

Said variable pitch aerofoil blades may be adapted to be rigidly locked directly or indirectly to the shaft when said gear drive is inoperative.

Each said blade may be carried on a blade ring or disc, and rotation of each said blade about its longitudinal axis may be effected by one or more radial shafts which rotate in radial hearings in said disc or ring, the disc or ring being mounted for rotation with the main shift and each said radial shaft terminating in a pinion, said pinion or pinions being drivingly engaged at right angles to a common annular gear surrounding the shaft. Said gear train is preferably adapted in different modes of operation to rotate the annular gear at diflferent rotational speeds.

The invention will now be particularly described merely by way of example with reference to the accompanying drawings in which:

FIG. 1 is a partly broken away sketch of a gas turbine engine including aerofoil section blades according to the invention,

FIG. 2 is a partly sectioned perspective view of the blades and drive mechanism according to the invention, with a simplified geared drive for low windmilling speeds,

FIG. 3 is a sectional view of a further drive mechanism according to the invention used to provide a feathering drive for the aerofoil section rotor blades of the fan of a gas turbine engine for higher windmilling speeds, and

3,672,788 Patented June 27, 1972 FIG. 4 is a perspective and partly broken away view of the main portion of the drive mechanism of FIG. 3.

In FIG. 1 there is shown a gas turbine engine having an outer casing 81 within which there are mounted a compressor 82, combustion equipment (not shown) a turbine (not shown) and a final nozzle. The compressor and turbine are drivingly interconnected and as the first stage of the compressor there is mounted a fan 83'. The fan comprises a plurality of blades 84 each of which consists of an inner portion 1 and an outer portion 2 These portions are joined by a blade ring 15 which forms in effect the leading edge of the casing 81 of the main engine. The fan blades 84 are surrounded by a fan cowling 85.

Although the fan blades 8% have been referred to as being single blades, the outer portions are in fact displaced circumferentially from theinner portions so that they lie half-way between the inner portions 1.

It'will be understood that should any component failure lead to the necessity to shut down an engine or progressively arrest it when the aircraft in which it is mounted is in flight the fan 84 will provide drag which varies with the geometry of the blade system in the by-pass ductrIf such an occurrence has to be allowed for it is normally necessary to uprate the engines to cover this contingency leading to increased cost and weight of the installed engine. A possible alternative would be to feather the fan blades.

Thus the two principal reasons for feathering the outer fan blades are:

(a) partial feathering to reduce drag on windmilling probably due to obstruction in fuel supply to engine (b) fully feathering to arrest rotor, anticipating damage to the engine under certain conditions of engine failure.

As in these fan blades there is usually a significant twist between the root and the tip it is undesirable to feather the entire blade since the drag increase of parts of the blade which are substantially parallel to the air flow will off-set drag decrease in those parts which are at a substantial angle. Therefore in the present case only the portion 2 is feathered, the portion 1 being maintained in its original relationship.

The mechanism for feathering these outer portions should ensure that the windmilling rotor comes to rest when the blades are fully feathered, causing a significant reduction in drag.

The need to split the blades must not affect the air flow into the first stage of the compressor. The blade mounting and mechanism to support the blades should therefore be miniaturized so that they can be located in a slim annulus rotating ahead of the compress-or housing without obstructing it.

FIG. 2 illustrates a simplified configuration of the feathering mechanism and drive for a fan with low windmilling speeds. The fan blade is divided into two parts by an outer blade ring 15, the inner blade portion 1 and the outer blade portion 2. As the pitch angle of portion 1 is normally small it is not desirable to feather this part of the blade. The relatively small root area at the blade mounting would in any case virtually preclude the installation of a suitable mechanism. The outer blade portion 2 is mounted at a significant pitch angle which increases with diameter and consequently only this portion is feathered.

The inner blade portions 1 are mounted on an inner blade ring 16 which is driven at .17 from a shaft 18.

The outer blade portion 2 is subject to high centrifugal loading both along and about its longitudinal axis. The blade mounting is designed to cater for these forces.

Blade portion 2 may be made in hollow metal or a suitable high strength plastic. In the latter case the plastic is partly embedded in a metallic mounting 3 which is part of the metallic journal 4 ribbed internally at S to minimize weight. Two

axial bearings

6a, 6b and end nuts (not shown) and two radial bearings 60 and 6d transfer the forces and moments actingon the blades to the outer blade ring on corresponding surfaces at 7. The blade journal is also provided with a toothed perimeter 8 which acts as a wheel mating with two worm gears (only one shown) 9a, 9b at a high reduction ratio. r

The radial force acting on the

bearings

6a and 6b is transferred in shear through four webs 10 to the end rims 12 of the outer blade ring 15. These rims are designed to take most of the centrifugal forces acting on the blades, the blade mounting and the blade drive. The rims should be of minimum radial width to reduce aerodynamic drag as set out above. Their radial location coincides with the casing 51 which supports the static blades of the second and subsequent compressor stages.

The angle between the webs 10 equals the angle through whichthe blade portions 2 will rotate and the webs will provide suitable stops to limit the angle of rotation when high centrifugal twisting moments act on the blade portions under normal flight conditions.

The webs 10 also incorporate bearings for the worm gears 9a and 9b which are subject to high axial forces as well as normal gear separating forces.

' The tangential torque transmitted through the blade portion is resisted by four webs Illa, 11b, -11c, 11d (only one shown) interposed between surface 7 and two internal peripheral webs (not shown) reinforcing the bore of the outer blade ring. These peripheral webs transfer moments due to tangential torque on blade portions to a position midway between blade portions, at the point where the tips of the internal blade portions are fixed. As the moments cause equal and opposite radial forces at these points, only strength in shear must be provided and this is offered by the mid section of axial surface 13 (equal to the bearing width).

Axial loading on the outer blade portions due to the propulsive force provides a moment tending to twist the outer ring. This force is almost completely eliminated by a small forward inclination of the bearing 7, using the radial centrifugal force to provide a correcting moment. Thus torsion on the outer blade ring is small. Torsion is resisted by boxing in the outer ring by cover plates 13a, 13b. The H section of the outer blade ring ensures that either the inner or the outer part of the blade ring offers some torsional resistance and thus minimises torsionally induced stresses in the end rims 12 of the blade ring. Thus the outer section of the blade ring provides the outer blade portion mounting and encloses the blade feathering mechanism.

The inner part of the outer blade ring secures the inner blade portions at their tips to the outer blade ring and covers the outer blade portion end nuts mentioned above.

The outer blade ring is highly stressed to lighten it as much as possible. Consequently it will suffer considerable radial deformation at full speed. The deformation of the inner blade ring 16 at the point of attachment 17 to the drive 18 should be small and equivalent to about one third of the stress in the outer blade ring. This will be very diflicult to ensure if the inner blade portions II are rigidly attached to the outer blade ring 15. The method of feathering the outer blade portions also demands that the inner blade portions should be displaced by half a blade pitch relative to the outer blade portions. A rigid connection between the inner blade portions and the outer blade ring would cause the outer blade ring tobecome distorted (wave shaped) along the perimeter, causing high stresses both in the outer blade ring and along the inner blade portions. This would greatly impede the design of the outer blade ring and stress the inner blade portions excessively as they are subject to a high tangential bending moment as well as centrifugal loading due to their own weight. A simple device which relies on sliding contact between the inner blade portion tips and the outer blade ring would be subject to severe fretting.

The elastic element designed to minimize radial loading on the outer blade ring and the inner blade portions respecti'vely should primarily have only one degree of freedom thus providing a rigid tangential drive between the inner blade portions and the outer blade ring. Suflicient torsional stiffness, will limit undue fore and aft oscillation of the outer blades whilst restraining all axial movement. The boxed-in spring flexure 14 satisfies this requirement especially as it will be torsionally weaker than the outer blade ring 15 and thus reduce torsionally induced stresses in the ring.

It will be seen that the tip of the inner blade portion 1 forms one side of an axially extending box section, the opposite side of which is rigidly attached to; the ring 15. The two remaining sides are slightly flexible and allow controlled relative movement between the blade portions 1 and ring 15 in substantially only the radial direction. Thus since the two flexible sides cannot lengthen or shorten any relative movement must be in the direction of the other two sides, that is the radial direction.

It will be understood that although a single spring extending circumferentially and rigidly attached to both the inner blade portion tips and the outer blade ring would give the ring controlled freedom of radial motion, this arrangement would cause undesirable bending loads on the inner blade portion tips. With the illustrated arrangement the inner blade portion tips move preferentially in a radial direction with respect to the outer blade ring.

The inner blades 1 and the inner blade. ring 16 are preferably made in metal to ease manufacture and assembly although a plastic construction could eventually be used to lighten the structure. By fitting the outer blade portions 2 into the outer blade. ring .prior to fitting the inner blade portions to the outer blade ring, assembly of the inner blade ring 16 is facilitated; the inner blades are finally held in a dovetail and if necessary shrunk into position.

The inner blade ring 16 is designed to minimise the radial movement of spring flexures 14 to minimize their length on one hand and to minimize axial stress at the interface 17 where drive 18 is attached to inner blade ring 16.

The radial surfaces of ring 16 are therefore tapered to olfer an approximate root stress of A; that in the outer blade ring and a circumferential stress /2. that in the outer blade ring. The inner blade ring is thus subject to critical stressing.

Drive 18 is stressed to take driving torque, aerodynamic loading, gyroscopic forces and radial expansion which can cause significant deformation. The peripheral spline 19 and bearing surface 20 are therefore not critically dimensioned.

Details of the transmission, which is outlined below, are given in our US. Pat. 3,467,198.

If the windmilling speed of the fan is very low the following simple gear transmission will be suitable, because the approximately tenfold increase in the speed of shaft 24 over hub 18 can be tolerated.

The outer blade portion 2 is rotated via toothed wheel 8, worm gears 9a and 9b and shafts 21a and 2117 (only one shown) which are connected flexibly to the

wheels

22a and 22b. Both wheels are rotated in opposite sense by a central worm 23 whose drive rotates two blade portions on each side of the worm. Thus 24 outer blade portions would require 6 worm gears 23. The reduction ratio between worms 9 and wheel 8 is a compromise between the following extremes; it should be sufliciently low to ensure reasonably fast blade feathering whilst adequately reducing the torque due to the bearing friction and centrifugal twisting moment on windmilling to a low level when a self locking device is provided and the weight of flexible couplings and the shafts 21 in the high gravity field becomes acceptable.

This will ensure that a relatively low torque is transmitted through the small flexible couplings between adjacent blades linked by a common shaft and to make the system less sensitive to inevitable machining errors.

The reduction ratio between wheels 22a and worm 23 is again a compromise between a minimum reduction for fast feathering and torque which permits a shaft drive 24 of adequate elasticity to be guided through the inner blades, which deflect significantly under torque at full speed.

Intermediate bearings may support the drive which terminates in pinion 26. The pinion is mounted in an internal bearing 27 which is adequately connected at 28 with bearing 29 of bevel wheel 30 through a sleeve which is independently mounted for inner blade ring 16 and thus is not susceptible to the high centrifugal loads acting on it.

It will be appreciated that this construction enables pinions 26 to mate with bevel wheel 30 without being affected substantially by deformation of the inner blade ring 16. The pinions 26 are thus enabled to axially support shaft 24.

A simplified form of geared drive will now be outlined only suitable for fans with low windmilling speeds owing to the high speed increase caused between bevel wheel 30 and pinion 26. For higher windmilling speeds a gear box is described in greater detail below with reference to FIGS. 3 and In FIG. 2 the bevel wheel carries two internal bearing surfaces 31 and 32 and a long spline 33. This spline is engaged by slide 34 whose short internal spline 35 engages with splines 19 on the drive 18 or alternatively the slide engages conical surface 36 on the mating surface 37 of the static housing 38. In each case, the spline 39 on the periphery of slide 34 will engage the internal splines 33 of bevel Wheel 30. The slide is centered on bearing 40 which is in continuous operation whilst the fan blades rotate. Slide 34 is moved axially by a series of hydraulic cylinders 41 fitted to the static mounting 38. Thus flexible pipe connections are avoided. It will be possible to use an electric actuator in place of the cylinder 41.

During normal flight of an aircraft powered by the engine 49, slide 34 drive bevel wheel 30 at the same speed as drive 18 by engagement between

splines

19, 35, 39 and 33. Thus there is no relative movement of pinion 26 about its own axis and the entire mechanism except bearing 40 and worm gear 9 is at rest and unloaded. It will be appreciated that axial centrifugal loads acting on the outer blade portions will cause considerable frictional force on the small relatively high friction bearing 6b. Our Pat. 3,467,- 198 shows how this friction force is used to advantage by a specific detail design which offers relatively low worm and worm bearing loading at full speed when the centrifugal twisting moment is a maximum by partially locking the blade root. Thus minimum component sizes and optimum selection of materials for wear and fretting is ensured.

When the engine windmills and the rotor is to be arrested, hydraulic cylinders 41 are activated and the slide 34 is moved axially to engage on

conical surfaces

36 and 37 respectively. As the slide is thus arrested bevel wheel 30 is arrested and pinion 26 will now rotate about its own axis actuating the drive and feathering the outer blades. The rotor will therefore be subject to progressively less aerodynamic force tending to windmill the fan. When the blades have turned to a position where the air load is equal on both sides of the blade the rotor comes to rest. If, due to inertia the blades are turned beyond the equilibrium position then the rotor will reverse its direction of rotation clue to air loading on the reverse surfaces of the blades, Some slip is then anticipated between

conical surfaces

36 and 37. Thus the gear train has its own feed back system and requires no attention by the operator or complex monitoring equipment. This dnive will be seen to be an exceptionally light epicyclic system which has low mechanical efliciency. This will make it relatively insensitive to hunting, whilst avoiding high machining tolerances.

To restore the blades to their original setting after engine maintenance the rotor is turned in the reverse direction by an external drive whilst the

conical surfaces

36 and 37 are lightly engaged so that there will be some slip on the interface when the blades have reached the fully feathered position.

It will be appreciated that if a modified slide 34 is rotated fractionally either faster or slower than drive 18 the drive will be able to feather in either direction in flight as discussed later.

If the windmilling speed of the fan is more than 300 r.p.m. the increase in speed between the bevel wheel 30 and pinions 26 on engaging the drive is too high and a two speed geared drive must be introduced. This gear box is shown in FIGS. 3 and 4.

In FIG. 3 the wheels are driven from a worm 111 which are items 22 and 23 in FIG. 2. This worm comprises an outer portion mounted on bearings from a blade ring 112 and driven by diaphragm 113 which is splined to a radially extending drive shaft 114. The drive shaft 114 extends through an inner fixed portion 115 of the fan blades to the blade ring 112. The inner portion 115 is supported from an inner blade ring 116 and the shafts '114 extend through apertures 117 in this inner blade ring.

At their innermost extremities the shafts 114 are connected to bevel pinions 118. The shafts are splined to these pinions and the shafts are retained by means of lock nuts 119. Each bevel pinion 118 is adjustably mounted in two axial bearings 120a and 12%, which transfer the centrifu-gal load acting on the drive shaft 114 on to the bearing mountings. Each bevel pinion is radially mounted on two bearings 121a and 121b.

The pinions 118 are mounted at a fixed radius about the shaft 122 of the engine, this shaft being the main drive shaft for the fan blades. The shaft 122 is of large diameter and is hollow, being shown broken down the middle in the drawing for convenience. The shaft 122 carries drive from the turbine of the engine to the inner blade ring 116 and hence to the inner blade portions 115, the outer blade ring 1 12 and the remainder of the fan blades. The shaft 122 is attached through a flange 123 to a second part of the shaft 124 which runs in a bearing 125 which is mounted in a static blade ring 126.

Coaxial with the shaft 122 there is a bevel gear wheel 127 having a sleeve-like axial extension 128 forming a drive sleeve. The extension 128 and the bevel 127 are mounted on radial bearings 129a and 1291). These bearings are widely separated and hence allow for the large diam eter of the bevel wheel 127. Axially adjustable bearings 130a and 130 h control the extent of the engagement between the bevel wheel 127 and the pinions 118. The bearings 130a and 1311b lie between the bevel wheel 127 and a locking ring 131 screwed to the bevel 127 and a threaded ring 132 which is screwed to a pinion carrier 109. A supplementary radial bearing 1300 between the bevel 127 and pinions i118 ensures concentricity of engagement between the bevel and the pinions.

The

bearings

129a and 12% are carried from a tubular member 133' which surrounds and is attached at one extrernity to the shaft :122, and it will be appreciated that the tubular member 133 is not subject to the torque loads which affect the shaft 122.

At the end of the sleeve-like extension 128 of the bevel 127 there are a pair of rings of gear teeth 134 and 135. These gears are external of the sleeve. Surrounding the sleeve-like extension 128 of the bevel 127 there is an eccentric sleeve 136. This sleeve is mounted in bearings 137a and 137b on a tubular extension 138 from the static blade ring 126 so that its axis of rotation is parallel to the axis of rotation of the shaft 122 although the sleeve is eccentric from the shaft and hence from the bevel 127. Cut on the inside of the eccentric sleeve 136 is a ring of internal gears 13 9 which are adapted to mesh with either of the rings 134 and on the extension 128 of the bevel 127. A further gear ring 140 is cut on the extremity of the eccentric sleeve 136 at a different radius to the ring 139. The ring 140 is adapted to mesh with a ring of gears 141 on the tubular member 133, and the positions of the rings 139 and 140 are so chosen with respect to the rings 141 and 134 that when the ring 140 meshes with the ring 141 the ring 139 also meshes with the ring 134.

On a chamfer between the end face of the eccentric sleeve 136 and its outer periphery there is mounted a clutch face 142 which is shaped to act with a second clutch face 143 mounted from the static blade ring 126. The clutch faces 142 and 143 engage one another when the eccentric sleeve 136 has been moved axially, sufficiently to engage the gear ring 139 with the gear ring 135. In order to move the eccentric sleeve axially there are provided a number of hydraulic rams 144 which operate through rods 145 on to spherical bolts 146 which engage in a groove 147. The rods 145 are held peripherally against cantilever loading by a spacer ring 148. The ring 148 is located against circumferential movement by static cantilever 158 which projects through a fitted aperture in the ring.

In order to provide drive between the shaft 122 and the bevel wheel 127 there is provided a ring 149 having a set of splines 150 which engage with external splines 151 on the tubular member 133. The ring 149 is carried in bearings 152a and 152b from the inner surface of the cylindrical extension 128 of the bevel wheel 127. There are four radial projections from the ring 149 which pass completely through axially extending slots in the cylindrical extension 128 and engage with these slots; these are shown at 153. The four extensions '153 carry commutator rings 154 and 155 outside the cylindrical extension 128 and coaxial with it, these commutator rings carrying axial bearing surfaces. The

rings

154 and 155 engage at one side in a groove 156 formed in the eccentric sleeve 136 so that the commutator rings 154 and 155, the projections 153 and the ring 149 together move axially with the eccentric sleeve 136.

In order to prevent the ring '149 skewing on axial movement and hence jamming because of its being moved by application of a force only at one side of its periphery, a segment extension 157 is provided which seats in the groove 156 at the side distant from the engagement between the groove 156 and the commutator rings 154 and 155. The extension piece 157 is mounted on a static centilever 158 carried from the static blade ring 126, and at its radially inner extremity it engages with the commutator rings 154 and 155.

In this way it is arranged that the

rings

154 and 155 are moved axially by forces acting on them at opposite ends of a diameter and hence they do not tend to skew and jam. It will be seen that when the eccentric sleeve 136 moves axially so as to engage the sets of teeth 139, 134; 140, 141, the splines 150, 151 become disengaged hence disengaging the drive between the shaft 122 and the bevel Wheel 127.

The gear system described has three modes of operation and these are described below. During the normal mode of operation, the eccentric sleeve 136 is in the ax al position shown in FIG. 3 of the accompanying drawings. In this condition the shaft 122 drives the inner blade ring 116 and also drives the bevel wheel 127 by way of the splines 150, 151 the ring 149 and the projections 153 from the ring 149 which engage with the axial slots on the cylindrical extension 128 of the bevel wheel 127. The apertures 117 in the inner blade ring 116 form in effect the cage bearings for the equivalent epicyclic drive. It will be seen that in this condition the pinions 118 and the bevel wheel 127 rotate at the same angular velocity and therefore the shafts 114 will not rotate about their axes and will not attempt to drive the fan blades about their axes.

These are conditions of low transmitted torque and the sole purpose of the projections 153 is to prevent creep of bevel wheel 127 relative to inner blade ring 116.

It will be noticed that in this condition there is no relative movement about any of the bearings of the system except the bearing 125 and hence no wear occurs in the normal condition of the drive. As the eccentric sleeve 136 is not driven by any of the sets of gears it remains static, the slight frictional force experienced on the surfaces of the groove 156 being balanced by frictional forces on the spherical bolts 146. Thus there is no bearing wear or loss of engine power in the normal condition of the drive.

If the engine should fail, then the fan blades will require to be feathered so that the large fan can windmill. Once such a failure occurs there will no longer be any substantial torque or centrifugal loading on the inner portions of the blades and the blades and the shafts 114 will straighten. The drive will then be able to transmit substantial torque through the diaphragm 113 on the Worm 111. Also the engagement between the bevel wheel 127 and the pinions 118 is then devoid of significant deformation. Since the worms which drive the outer portions of the blades do not allow any feedback of torque, the entire gear system is unloaded.

Once the engine has failed a control system (not shown) will cause the rams 144 to push the eccentric sleeve 136 axially against the action of a synchromesh baulking device (not shown) in the eccentric sleeve. This device could take the form of a ring of spring loaded balls retained in radial drillings in the eccentric sleeve such that once the sleeve increases to a certain speed the balls retract into the eccentric sleeve and allow free axial movement of the sleeve. The sleeve is then allowed to slide axially until the sets of teeth 139' and 134; and 141 engage, simultaneously causing the splines and 151 to disengage.

Thus in this condition (which is the condition depicted in FIG. 4) there is no longer a direct drive between the shaft 122 and the bevel wheel 127. Instead of this the shaft 122 drives the eccentric sleeve 136 by way of the meshing teeth 140 and 141 while the eccentric sleeve 136 drives the bevel Wheel 127 by way of the meshing teeth 139 and 134. Since the rings of teeth 139 and 140 are on a different radius the bevel wheel 127 will now be driven at a slightly different speed from that of the shaft 122 and hence from that of the inner blade ring 116. This will result in there being an epicyclic drive to the pinions 118 which will start to feather the fan blades at a rate which is a proportion of the relative speed between the shaft 122 and bevel 127. Thus as the blades begin to feather, the shaft will start to slow down and the rate of feathering will hence progressively decrease.

It will be noted that the shaft 122 is shown broken down the middle and in fact this shaft is of considerable diameter. Thus the difference in radii between the sets of teeth 139 and 140 is only a very small proportion of the overall diameter of these sets of teeth, and hence the drive to the pinions is arranged to be quite slow.

When the shaft 122 has reduced in speed to about of the normal speed, the rams 144 are again energised and the eccentric sleeve 136 again moved axially so as to disengage the sets of

teeth

140 and 141 and 139 and 134 while engaging the teeth 139 and 135 and the clutch faces 142 and 143. By the gradual engagement of the clutch faces 142 and 143 the eccentric sleeve 136 is progressively arrested hence also arresting the bevel wheel 127 by way of the sets of teeth 139 and 135.

Thus the pinions will finally be subjected to an epicyclic drive between the stationary bevel wheel 127 and the inner bearing ring 116 driven from the shaft 122. The pinions 118 will temporarily increase their speed considerably and feather the fan blades more rapidly. Eventually either the inner blade ring will come to rest because the blades have come to rest or the blades will feather excessively (due to inertia of the outer blade ring) when the blade ring will reverse rotation and oscillate until it has hunted to rest. Any change in aircraft speed will tend to upset the equilibrium condition of the blade setting and will cause the blades to rotate the shaft 122 hence providing a feedback of an epicyclic drive to the pinions 118 which will vary the angle of the blades in a sense to bring the system back to equilibrium.

In the equilibrium condition all parts of the system are at rest and there is no torque on the system, hence there is no wear taking place. r

In order to assemble'the gear train the sequence of operations must be as follows; a first sub assembly comprises the six pinions 118 which are mounted in a casing 109 and adjusted to their radial and axial position on the bearings 120 and 121. The casing 109 with the pinions attached is then mounted into the inner blade ring 116 and locked circumferentially bya number of sleeves 160 which are screwed through the apertures 117 by inserting a screw driven through the bearing 161 of the drive shafts 114. The inner'blad'e portions 115 are then assembled to the inner blade ring 116 and the outer blade ring 112 fitted over the portions 115. The drive shafts 114 are then pushed radiallyinwards and secured in place through the splined diaphragm disc 162 by an end nut 119.The intermediate shaft bearing 161rests in the inner blade ring 116, and the worm 111 and the wheels 110 of the outer blade ring can be assembled prior to fitting splined diaphragm 113; I

i The remainder of the fan blades can next be fitted.

The second sub assembly comprises the bevel wheel 127 into which the ring'149 is fitted internally (it may be necessary to mount the rear bearing 12% in a ring which is detachable to facilitate this assembly). The pro jections 153 are assembled'and locked by the

rings

154 and 155. The bevel. wheel casing bearing assembly comprising the ring 132 and the locking ring 131 and various intermediate bearings is assembled and given an adequate preloading. The segmental extension 157 is mounted on the

rings

154 and 155 and the annulus 136 slid over the assembly and locked axially by a locking ring 163 which also defines the space 156. The spherical bolts 146 and the rods 145 are next fitted into place and the spacing ring ring 148 is balanced in position. Finally. the splined sleeve 133 is inserted into the assembly.

The third sub assembly consists of a perforated housing 138 which mounts the six rams 144 and which is to be mounted on the static blade ring 126 with various hydraulic pipes installed. The segmental cantilever 158 is attached to the housing 138.

The first sub assembly is now offered to the second sub assembly and the tubular member 133 is mounted on the shaft 122. Finally the bevel wheel 127 is engaged with the bevel pinions 118 and is adjusted and locked by the ring 132.

The entire assembly with the rods fully aligned is now offered to the static structure and each rod guided into its corresponding ram through perforations in the housing 138. By suitable axial adjustment of the spherical nuts 146 (for assembly only) each ram can be assembled in turn, to facilitate assembly.

Finally the shaft portion 122 is bolted to the shaft portion 124 through a flange 123, which is bolted through the bore of the shaft 122, and the assembly is then complete.

It will be appreciated that the particular application of the invention described above uses the gear train to ensure that the pinions 118 do not have to rotate at an excessive speed when the fan blades are being feathered. This is achieved by using the intermediate ratio of the gear train to perform the initial part of the feathering, while the third ratio is only used when the speed of the shaft 122 has dropped sufficiently to ensure that the pinions 118 are not driven at an excessive rotational speed.

Although the variable speed gear train described has been entirely mechanical and relied upon the engagement of gear wheels it would be possible to use instead a clutch Whose degree of slip could be precisely controlled. Such a clutch might well make use of a magnetic friction material which provides a controlled degree of coupling ac- 10 cording to the magnetic field imposed upon it. An electrical control system would best be used for a device.

The invention could be used for other applications than those described; thus other types of blades such a hydrofoils could be arranged to be feathered by the drive by the invention.

In some applications of the invention it may be that the stationary condition of the fan will not be the condition of least drag and under these conditions it may be required to halt rotation of the fan blades before they have fully feathered. In this case a system could be used in which arevolution counter of some kind is connected to the concentric sleeve 136. This revolution counter would be arranged to count the number of revolutions of the sleeve 136 hence indirectly counting the number of revolutions of the bevel pinions 118 and hence registering the angle of rotation of the fan blades about their longest axis. This revolution counter would be connected to the hydraulic system for the rams 144 so that when the fan blades have turned to the required minimum drag position the rams operate the concentric sleeve 136 to reengage the splines 150, 151 and hence re-establish drive between the shafts 122 and the inner blade ring 116.

If it is required to completely feather the blades, the system would be arranged that the rams could be operated a second time so as to go through the cycle outlined in column 8, lines 41 to 75, and column 9,

lines

1 and 2, and eventually totally feather the :blades.

It will be appreciated that in the embodiment described above the bevel wheel 127 rotates slower than the shaft 122. If it were arranged that the bevel wheel rotated faster than the shaft, the epicyclic drive to the pinions 118 would be reversed and such a system might prove valuable in enabling the pitch of the fan blades to be varied in either direction to suit differing aerodynamic conditions of the fan. Such a reversal could be achieved by having a further set of teeth similar to 134 and 135 on the extension 128, this set of teeth having a different number of teeth from those on the ring 134.

In the present instance, the ratio between the shaft 122 and the extension 128 is very close to 1:1. This leads to the possibility of providing the reversal of drive by altering the number of teeth on the ring 139 and thus causing the reversal of the direction of drive of the pinions. In fact an increase of a single tooth in the number of teeth in the ring 139 would effect this reversal. Therefore by arranging that the ring 139 has a variable number of teeth complete-control of the angle of the fan blades can be achieved, with pitch variation in both directions being provided.

What is claimed is:

1. A variable pitch aerofoil assembly comprising:

a plurality of aerofoil blades,

a main drive shaft means having said plurality of aerofoil blades operatively attached thereto so that the blades can be rotated about the axis of said main drive shaft means,

drive means for rotating the aerofoil blades about their own axes, said drive means having a variable ratio gear train means providing a plurality of modes of operation so that the speed of response of the aerofoil blades to a disturbance may be varied, said variable ratio gear train means including (a) pinion means operatively associated with each of said aerofoil blades and constrained to rotate with rotation of said main drive shaft means, (b) a bevel gear wheel means carried coaxially with said main drive shaft for meshing with said pinion means, (c) interconnection means for normally interconnecting said main drive shaft means with said bevel gear wheel means so that the bevel gear wheel means is driven normally at the same speed as is the main drive shaft means, and ((1) means for varying the rotational speed of said interconnection means with respect to said main drive shaft means, and

arresting means for arresting said bevel gear wheel means during emergency conditions. 1

2. The assembly of claim 1 in which there is a blade ring which carries said plurality of aerofoil blades and a plurality of radial shafts extending to said ring and drivingly connected to said blades for rotation thereof about their longitudinal axes, said ring being mounted for rotation with said main shaft and each said radial shaft terminating in one of said pinion means, and said bevel gear means being in the form of an annular gear wheel surrounding said main shaft and engaging with said pinion means, the arrangement forming in operation a right angle epicyclic drive.

3. The assembly of claim 2 in which said drive means includes a controlled slip arrangement whereby said annular gear wheel is driven ,from the main drive shaft means. a

4. The assembly of claim 3 in which none of the com; ponents of said controlled slip arrangement rotate about their own axis at a speed significantly in excess of the rotational speed of the main drive shaft means.

5. The assembly of claim 4 in which there is an axially movable sleeve adapted to engage externally with said annular gearwheel, and external splines on said main drive shaft means adapted to engage with internal splines on said sleeve so that said main drive shaft means and the annular gear wheel may be caused to rotate together without slip thus locking the variable pitch blades.

6. The assembly of claim 5 in which said arresting means comprises a clutch face carried from said sleeve and adapted to engage with a clutch face carried from static structure to partially or wholly arrest the sleeve, thereby partially or wholly arresting the annular ,g'ear wheel and forming an epicyclic drive. I

7. The assembly of claim 6 in whic hthere are external splines on said sleeve and internal splines on said annular gear wheel which are adapted to engage whereby said sleeve engages with said annular gear wheel.

8. The assembly of claim 6 in which there are one or more hydraulic pistons and cylinders attached to said static structure and adapted to move and clamp said sleeve axially.

9. The assembly of claim 5 in which there is an internally geared annulus surrounding said main drive shaft means and eccentric form it, a tubular extension from said annular gear wheel having slots therein, and radial arms extending from said sleeve to said eccentric annulus and passing through the slots in said tubular extension.

10. The assembly of claim 9 wherein said arresting means includes a clutch face carried bysaid eccentric annulus and which is adapted to engage witha clutch face carried by static structure to arrest or retard the eccentric annulus, there being at least onehydr aulic piston and cylinder mounted from the static structure and adapted to move said annulus axially to effect engagement and, disengagement of said clutch faces, a

11. The assemblyof claim 10 in which there is a diaphragm which engages with a cantilever st'rut, said diaphragm engaging with said pistons'to arrest torque acting on them, said cantilever strut also engaging with said radial arms at a radius of maximum eccentricity of said annulus. v r I 12. The assembly of claim 9 in which there is an ex,- ternal gear carried from said shaft and an internal annulus gear, on said eccentric annulus and engaging'with said external gear, said eccentric annulus also carrying a further internal gear and there being two equal external gears carried from said tubular extension and adapted to engage with said further internal gear. a

13. The assembly of claim 12 in which said external gear carried from said shaft is of'slightly less diameter than its mating gear on said eccentric annulus and in which saidfu'rther. gear carried from said eccentric annulus is of equal diameter to said first gear, whereby'said annular gear wheel may be rotated at a slightly different speed from said shaft. I l e l l QReferen'ces Cited UNITED STATES PATENTS 2,423,400 7/1947 Nichols 416 x 2,480,468 8/1949 Hoinville t 416-151 2,696,888 12/1954 Chillson et al 416 52 3,422,625 1/1969 Harris 416--152 X 3,467,198 9/1969 Ellinger 416 160 x 3,468,473 9/1969 Davies et a1 416-193 UX FOREIGN PATENTS 976,922 11/1950 France 416 152 984,485 1 2 /1951 France 416--152 EVERETTE A. POWELL, 1a., Primary Examiner US. 01. X.R. 11 -152, 160