GB2372729A

GB2372729A - Thrust reverser arrangement with means for reducing noise

Info

Publication number: GB2372729A
Application number: GB0105341A
Authority: GB
Inventors: Barry Norman Hocking
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2001-03-03
Filing date: 2001-03-03
Publication date: 2002-09-04
Also published as: GB0105341D0

Abstract

A thrust reverser unit comprises a plurality of blocker doors 42, and an actuation mechanism comprising a sleeve 40, which can be translated to expose a cascade structure 38, is arranged in conjuction with a strut 46, to move the blocker doors 42, between a first deployed position, substantially obturating the flow of exhaust gases, and a second non-deployed position wherein the blocker doors 42, define exhaust noise reduction means. For this purpose, the blocker doors may be provided with tabs 18, which may comprise shape memory material.

Description

THRUST REVERSER ARRANGEMENT

The present invention relates generally to a ducted gas turbine engine thrust reverser unit comprising means to reduce engine exhaust noise.

Ducted gas turbine engines are designed to enable a stream of bypass air from a fan assembly to pass through a channel formed by a duct between a casing of the core engine and an outer ducting of a cowling. It is well known to provide ducted gas turbine engines with a thrust reverser unit (TRU) which when operated deflects the bypass air flow so as to assist in braking of an aircraft on which the engine is mounted. Such thrust reversing means are designed in various forms. One such prior art proposal, GB1265850, provides a plurality of blocker doors which are translatable into positions within the bypass duct and which obstruct the bypass air flow. A plurality of deflector vanes or cascades, as known in the art, are disposed within the cowling and which are uncovered when the thrust reverser is in the deployed position. Thereby, fan bypass air is exhausted from the bypass duct through the cascade, which imparts a forward component of velocity to the exhaust air thereby providing braking for the aircraft.

There is a constant desire to reduce the noise of aircraft engines and in particular the jet or exhaust noise generated by the interaction of exhaust gas streams with one another and the ambient air. The exhaust noise is generated by a turbulent shear layer formed between the gas streams. It is commonly known to provide the bypass duct and the core engine duct with noise suppressing means generally disposed to the translating sleeve of each duct, as described in GB 2,289, 921 and UK Application GB 0025727.9. These noise suppressing means generally comprise tabs or chevrons which are configured to generate

vortices. These vortices reduce the exhaust noise by promoting more effective mixing of the gas streams.

It is therefore desirable and is an object of the present invention to provide an improved gas turbine engine thrust reverser assembly which also suppresses exhaust noise.

According to a first aspect of the present invention there is provided a gas turbine engine nozzle arrangement for a flow of exhaust gases therethrough between an upstream end and a downstream end thereof, comprising a nozzle wall and a thrust reverser unit comprising an actuation mechanism and a plurality of blocker doors, the actuation mechanism arranged to move the blocker doors between a first deployed position, substantially obturating the flow of exhaust gases through the downstream end of the nozzle arrangement, and a second non-deployed position wherein the blocker doors define exhaust noise reduction means. Preferably the blocker doors are circumferentially disposed about the nozzle wall.

Preferably nozzle arrangement further comprises a core nozzle wall which is radially inward of and substantially concentric with the nozzle wall, the nozzle wall comprises a forward portion, a translating sleeve, and the thrust reverser unit which further comprises means for reversing thrust of the exhaust gases, and the actuator mechanism, the actuator drivingly connects the translating sleeve to the forward portion, the blocker doors are pivotally connected to the downstream end of the translating sleeve and a strut is pivotally connected at one end to the blocker door and at its distal end to the core nozzle wall so that, in use, the actuator drives the translating portion rearward to expose the means for reversing thrust the blocker doors are moved between the second non-deployed position and the first deployed position by the strut drawing the blocker doors radially inwardly and thereby

forcing the bypass airflow through the means for thrust reversing.

Preferably the means for reversing thrust is a cascade structure and the cascade structure is disposed substantially within the translating sleeve so that the cascade structure is not exposed to bypass air flow when the thrust reverser unit is in the second non-deployed position. Preferably, the blocker doors are disposed at the downstream end of the translating sleeve and the actuator is disposed to the forward portion and is connected to the translating portion via a rod.

Preferably the blocker doors are generally trapezoidal in shape having lateral edges which taper towards one another in the downstream direction, however, alternatively the blocker doors are generally rectangular in shape.

Alternatively the blocker doors overlap one another along a substantial portion of the lateral edges.

Alternatively the blocker doors comprise a downstream edge and trapezoidal tabs, the trapezoidal tabs being disposed to the downstream edge and define the exhaust noise reduction means.

Preferably the blocker doors comprise a downstream edge which is of a curved configuration so that in the first deployed position the curvature of the downstream edge substantially matches the curvature of and abuts to the core nozzle wall.

Preferably the translating sleeve comprises a substantially annular extension, the annular extension extends substantially in the downstream direction and partially radially outwardly surrounds the blocker doors thereby defining an aft portion of the blocker doors which is exposed to the bypass gas flow for exhaust noise reduction means when the thrust reverser unit is in the second non-deployed position.

Alternatively the aft portion comprises a trapezoidal tab, the trapezoidal tab is generally configured to form

the noise reduction means. Furthermore the aft portion comprises a triangular tab, the triangular tab is generally configured to form the noise reduction means.

Preferably the blocker doors are generally radially inwardly curved.

Alternatively the aft portion comprises a first

tangent angle between 50 and 200 and a second tangent angle between 50 and 200 and generally having a smooth transition therebetween.

Alternatively the blocker doors comprise the aft portion having a first tangent angle between 00 and 200 and a third angle between 0'and 20'and generally having a straight transition therebetween.

Preferably the nozzle arrangement is arranged to move the blocker doors between the second position and a third position where the blocker doors are aligned with the nozzle wall to reduce aerodynamic drag.

Preferably the strut subtends a first forward angle, relative to a radial line when in the second non-deployed position and in the third position the strut is angled at an angle between the first angle and the radial line.

Alternatively the strut comprises shaped memory material, which is configured to change shape in response to a change in an applied field between a first shape, where the blocker doors are disposed in the second nondeployed position and a second shape, where the blocker doors are disposed the third position.

Alternatively the blocker doors comprises shaped memory material, which is configured to change shape in response to a change in an applied field between a first shape, where the blocker doors are disposed in the second non-deployed position and a second shape, where the blocker doors are disposed the third position. Moreover, the aft portion comprises shaped memory material and the trapezoidal tab comprises shaped memory material.

Preferably the applied field is a temperature flux and the shaped memory material comprises any one of a group comprising Titanium, Manganese, Iron, Aluminium, Silicon, Nickel, Copper, Zinc, Silver, Cadmium, Indium, Tin, Lead, Thallium, Platinum.

Alternatively the applied field is an electrical signal and the shape memory material comprises an electrostrictive material selected from the group comprising Lead Zirconate Titanate, Lead Magnesium Niobate, Strontium Titanate and a polymer group including polyvinylidene fluoride.

Alternatively the blocker door comprises a fin, the fin, in use, prevents spillage of gas over at least a portion of the lateral edge of the blocker door and thereby substantially prevents a vortex being formed from that edge.

Preferably a method of operating an aircraft comprises the steps of: deploying noise reduction means prior to take-off; not deploying noise reduction means above a predetermined aircraft altitude and; deploying the noise reduction means below the predetermined aircraft altitude.

The present invention will now be described by way of example only with reference to the following figures in which: Figure 1 is a schematic section of a ducted fan gas turbine engine; Figure 2 is a more detailed schematic section of a bypass duct assembly comprising a thrust reverser unit in a non-deployed position; Figure 3 is a more detailed schematic section of a bypass duct assembly comprising a thrust reverser unit in a deployed position; Figure 4 is a schematic view of Figure 2 and shows the downstream end of the ducted fan gas turbine engine and showing blocker doors in a non-deployed position;

Figure 5 is a schematic view of Figure 2 and shows the downstream end of the ducted fan gas turbine engine and showing blocker doors having tabs; Figure 6 is a more detailed schematic section of a bypass duct assembly comprising a thrust reverser unit in a non-deployed position; Figure 6A is a detailed section of Figure 6 showing an embodiment of the blocker door assembly; Figure 6B is a detailed section of Figure 6 showing a further embodiment of the blocker door assembly; Figure 7 is a more detailed schematic section of a bypass duct assembly comprising a blocker door in an exhaust noise suppressing deployed position; Figure 8 is a forward looking view of the downstream end of the exhaust nozzle assembly; Figure 8a is a view looking along arrow C of Figure 8.

With reference to Figure 1, which is a schematic section of a ducted fan gas turbine engine 10. A ducted fan gas turbine engine 10 comprises, in axial flow series an air intake 5, a propulsive fan 2, a core engine 4 and an exhaust nozzle assembly 16 all disposed about a central engine axis 1. The core engine 4 comprises, in axial flow series, a series of compressors 6, a combustor 8, and a series of turbines 9. The direction of airflow through the engine 10, in operation, is shown by arrow A and the terms upstream and downstream used throughout this description are used with reference to this general flow direction. Air is drawn in through the air intake 5 and is compressed and accelerated by the fan 2. The air from the fan 2 is split between a core engine 4 flow and a bypass flow. The core engine 4 flow enters core engine 4, flows through the core engine compressors 6 where it is further compressed, and into the combustor 8 where it is mixed with fuel which is supplied to, and burnt within the combustor 8.

Combustion of the fuel with the compressed air from the compressors 6 generates a high energy and velocity gas

stream which exits the combustor 8 and flows downstream through the turbines 9. As the high energy gas stream flows through the turbines 9 it rotates turbine rotors extracting energy from the gas stream which is used to drive the fan 2 and compressors 6 via engine shafts 11 which drivingly connect the turbine 9 rotors with the compressors 6 and fan 2. Having flowed through the turbines 9 the high energy gas stream from the combustor 8 still has a significant amount of energy and velocity and it is exhausted, as a core exhaust stream, through the engine exhaust nozzle assembly 16 to provide propulsive thrust. The remainder of the air from, and accelerated by, the fan 2 flows within a bypass duct 28 around the core engine 4. This bypass air flow, which has been accelerated by the fan 2, flows to the exhaust nozzle assembly 16 where it is exhausted, as a bypass exhaust stream to provide further, and in fact the majority of, the useful propulsive thrust of the engine 10.

In the embodiment shown the exhaust nozzle assembly 16 comprises two generally concentric sections, namely a radially outer bypass exhaust nozzle 12 and an inner core exhaust nozzle 14. The inner exhaust nozzle 14 is defined by a generally frusto-conical core nozzle wall 15. This defines the outer extent of an annular core exhaust duct 30 through which the core engine flow is exhausted from the core engine 4. The inner extent of the core exhaust duct 30 is defined by an engine plug structure 22. The radially outer bypass exhaust nozzle 12 is defined by a generally frusto-conical nozzle wall 17 and is supported by an annular array of fan outlet guide vanes 7, which also act to straighten the fan bypass air flow. The nozzle wall 17 defines the outer extent of an annular bypass exhaust duct 28 through which the bypass fan flow is exhausted.

Referring now to Figure 2, which is a more detailed schematic section of a bypass nozzle assembly 12 comprising a thrust reverser unit (TRU) 32 in a second non-deployed

position. The TRU 32 comprises a cascade structure 38, of general configuration as known in the art, generally disposed within a translating sleeve 40 of the bypass nozzle wall 17 when the TRU 32 is in the second nondeployed position. The cascade structure 38 comprises a plurality of vanes 37 disposed therein. An actuator 34 is mounted in the forward portion 41 of the nozzle wall 17 and is drivingly connected via a rod 36 to the translating sleeve 40 at a mounting 39. The actuator 34 is a hydraulic ram actuator, but it is also possible to use other actuation means such as pneumatic or electrical types. A blocker door 42 is attached to the translating sleeve 40 of the nozzle wall 17 by a rotatable connector 44. In the second non-deployed position shown, the blocker door 42 is radially held in place by a strut 46. The strut 46 is rotatably connected to, at one end, the aft end of the blocker door 42 and at its distal end to the core nozzle wall 15. Thus in the second non-deployed position the strut 46 is substantially aligned in the radial direction.

To reduce aerodynamic losses in the bypass duct 28, the strut 46 is provided with an aerodynamic shape therefore minimising propulsive thrust losses.

Figure 3 is a more detailed schematic section of a bypass duct assembly 12 comprising a thrust reverser unit 32 in a first deployed and operational position. When operated the actuator 34 drives the rod 36 rearwards and in turn the translating sleeve 40 of the nozzle wall 17 is axially translated rearward. As the translating sleeve 40 of the nozzle wall 17 axially translates rearwards the strut 46 pivots about at its radially inner and distal end until it is generally axially aligned and in doing so draws the downstream end of the blocker door 42 radially inwardly, thereby obstructing the fan air flow passing through the bypass duct 28. As the translating sleeve 40 axially translates rearward the cascade structure 38 is exposed to the bypass fan air flow which, as the bypass

duct 28 is obstructed, is directed through the plurality of vanes 37. The vanes 37 are configured to turn the air flow indicated at B, generally radially outwardly and forwardly, thereby providing braking for the aircraft.

Figure 4 is a schematic perspective view of Figure 2 and shows the downstream end of the ducted fan gas turbine engine 10 and shows the blocker doors 42 in the second nondeployed position. In this second non-deployed position the blocker doors 42 define circumferential spaces 19 therebetween. The blocker doors 42 have lateral edges 50 and 52 and it is intended that edge 50 and edge 52 substantially abut one another when the blocker doors 42 are in the first deployed position. The blocker doors 42 also comprise a downstream edge 56, which in this preferred embodiment of the present invention is generally curved in the tangential direction, view as in Figure 4. When deployed the curved edge 56 fits or matches against the curvature of the generally frusto-conical core nozzle wall 15 and thus has a substantially similar radius of curvature.

In positioning the blocker doors 42 at the downstream periphery of the bypass nozzle assembly 12 it is particularly advantageous and intentional for the blocker doors 42 to perform an additional purpose as exhaust noise reduction means. As should be generally understood from the teachings of UK Application GB 0025727.9, which disclose generally trapezoidal tabs disposed to the downstream periphery of the core and bypass nozzle walls for exhaust noise suppression, vortices are generated and shed from the lateral edges of the trapezoidal tabs.

Similarly, exhaust noise suppressing vortices are generated and shed from the lateral edges 50,52 of the blocker doors 42. These vortices serve to assist in the mixing of shear layers which form between exhaust gas streams. Exhaust or jet noise is generated by the turbulence in the shear layers and it is advantageous to provide additional mixing

vortices to improve the mixing in the shear layers and thereby reduce noise.

Figure 5 is a schematic perspective view of Figure 2 and shows the downstream end of the ducted fan gas turbine engine 10 and shows the blocker doors 42 in the second nondeployed position. In this embodiment of the present invention the blocker doors 42 are configured so that lateral edges 50 and 52 are substantially adjacent with one another and thus the spaces 19 (shown in Figure 4) are substantially filled. Alternatively, the lateral edges 50 and 52 may overlap one another thereby completely fill the space 19. In this embodiment it is advantageous to dispose tabs 18 to the downstream edge 54 of the blocker doors 42.

The tabs 18 are generally trapezoidal in shape with their lateral edges tapering toward one another in the downstream direction. Although one trapezoidal tab 18 is shown on each blocker door 42 more than one tab 18 may be used. The trapezoidal tabs 18 are substantially similar in configuration as those specified in UK Application GB 0025727.9 and are intended to have the same technical advantages thereof. Furthermore, when the blocker door 42 are in the first deployed position the tabs 18 form part of the bypass duct 28 obstruction. The tabs 18 define circumferential gaps 20, which are necessary in the formation of the noise suppressing vortices when the blocker doors 42 are in the second non-deployed position.

When the TRU 32 is in the first deployed position the gaps 20 allow some bypass fan air to pass through the blocker doors 42, however, this is advantageous in preventing the propulsive fan 2 from stalling thereby improving the operability of the fan 2.

In a further embodiment described herein with reference to Figure 5 deployable noise reduction means or

tabs, as described in a new UK Patent Application by the

present Applicant and having the same filing date, as the & 0 < 05-) present application and having Agents reference DY28WN7, may

be disposed to the downstream edge 54 of the blocker doors 42. Briefly, these noise reduction means relate to deployable tabs 18 comprising shape memory material and which may be independently deployed as noise reduction means from the operation of the blocker doors 42. Thus in this embodiment of the present invention the teachings of the co-pending UK patent Application should also be considered by the skilled reader to be part of the present invention.

Figure 6 is a more detailed schematic section of a bypass duct assembly 12 comprising a thrust reverser unit 32 in the second non-deployed position. The blocker doors 42 are configured substantially as shown in Figure 4 and operate in similar fashion as described hereinbefore with reference to Figures 2 and 3. However, in this embodiment the translating sleeve 40 of the bypass nozzle wall 12 comprises an extension 43 which is substantially annular. The extension 43 generally extends axially rearward of the translating sleeve 40 and surrounds radially outward a portion of the blocker door 42. In the second non-deployed position the blocker doors 42 extend axially rearward of the extension 43 and thereby an aft portion 22 of the blocker doors 42 is defined and which is exposed to the bypass gas stream. In the second non-deployed position the aft portion 22 is defined as a generally trapezoidal shape. Thus in this position noise suppressing vortices are generated and shed by the remaining and exposed lateral edges 50,52 of the aft portion. As well as partly defining the trapezoidal portion 22 the extension 43 is intended to provide a smooth aerodynamic radially outer air-washed surface of the bypass nozzle 12. It is intended that this trapezoidal portion 22 is similar in configuration to the trapezoidal tabs disclosed in UK Application GB 0025727.9 and it is intended to have the same technical advantages thereof particularly with respect to exhaust noise reduction means. Similar to the

embodiment of the present invention described with reference to Figure 5 this embodiment, the trapezoidal portion 22 may comprise deployable tabs as disclosed in the co-pending UK Patent Application by the present Applicant

and having the same filing date as the present application 0 < < -3 < p7. and having Agents reference DY28WM7.

W4

Figure 6A is a detailed section of Figure 6 showing an embodiment of the blocker door assembly and in particular the configuration of the trapezoidal portion 22. In this embodiment of the present invention the trapezoidal portions 22 are generally continuous with the profile of the translating sleeve 40 of the bypass nozzle 12. It should be understood that the bypass nozzle 12 is generally frusto-conical and in particular the annular nozzle wall 17 is radially inwardly arcuate toward its downstream end. It is known for conventional bypass duct walls to subtend a radially inward tangential angle of approximately 15 , relative to the engine central axis, at their downstream edge. It is intended that the blocker doors 42 follow the general curvature of the bypass nozzle 12 and thus in this embodiment the downstream edge 54 of the trapezoidal portions 22 has a radially inward tangential angle of approximately 150 with respect to the central engine axis 1.

In this way the aft portion 22 is exposed to the bypass gas stream and is therefore suitably configured to generate and shed exhaust noise reducing vortices. Although an angle of approximately 15'is a preferred embodiment any angle between approximately 50 and 250 would provide a reduction in exhaust noise.

It is further advantageous to increase the rate of curvature of the trapezoidal portion 22. This rate of change of curvature is defined by a first tangent line R at the downstream end of extension 43, having a first tangential angle g, relative to the line P which is parallel to the central engine axis 1. A preferred tangential angle 9 is between 0'and 20'. A second tangential angle is defined by the angle between the first tangent R and a second tangent S which is tangential to the downstream end of the trapezoidal portion 22. A preferred tangential angle P is between 0'and 20'. This increase in the rate of curvature helps to increase the air flow spillage over the lateral edges of the trapezoidal portions 22 thereby increasing the strength of the noise reducing vortices. This is equally so for other embodiments of the present invention described herein where the blocker doors 42 form the noise reduction means.

Figure 6B is a detailed section of Figure 6 showing a further embodiment of the blocker door assembly 42 and in particular the configuration of the trapezoidal portion 22.

In this embodiment of the present invention the trapezoidal portions 22 are generally continuous with the profile of the translating sleeve 40 of the bypass nozzle 12 except in that the trapezoidal portion 22 is substantially straight and inwardly angled relative to the remainder of the blocker door 42. The first tangent line R at the downstream end of extension 43 has a first tangential angle dz relative to the line P which is parallel to the central engine axis 1. A preferred tangential angle & is 15 , although any angle between 50 and 200 is beneficial. A third angle is defined by the angle between the first tangent R and an extended line T which is extended from the downstream end of the trapezoidal portion 22. A preferred tangential angle P is 100 although any angle between 00 and 20'is beneficial for noise reduction purposes.

It should be generally understood that the embodiments described with reference to Figures 6A and 6B posses similar technical benefits of the inclined tabs which are described in UK Application GB 0025727.9 and it is intended that the trapezoidal portions 22 follow the teachings of that application.

Figure 7 is a more detailed schematic section of a nozzle arrangement 16 comprising blocker doors 42 in the

second deployed position for exhaust noise suppression purposes. This embodiment relates to nozzle arrangement which is configured to move the blocker doors between the second position and a third position where the blocker doors 42 are aerodynamically stowed and substantially not exposed to the gas stream and therefore do not act as noise suppressing means. In this embodiment the third position is intended to be used at aircraft cruise or where the aircraft is above a predetermined altitude, where noise reducing means are not required thereby reducing performance losses.

It is a constant and important safety concern that thrust reverser units 32 are not inadvertently deployed during flight, for obvious reasons, and this embodiment of the present invention makes use of a current design of safety mechanism (not shown) which is intended to prevent the inappropriate deployment of the TRU 32. Such a safety mechanism can be a simple spring loaded pin which engages a complimentary slot. The pin and slot are disposed on the forward portion 41 and the translating sleeve 40 respectively or alternatively the translating sleeve 40 and the forward portion 41 respectively. The pin may be disengaged from the slot by independent means so as to allow the actuator 34 to axially translate the translating sleeve 40 in order to deploy the TRU 32 when desired. Whilst engaged, the slot and pin arrangement is configured to allow the translating sleeve 40 to be moved between the second position and the third position by the actuator 34. In this way failure of the actuator 34 does not lead to inadvertent deployment of the TRU 32 as the safety mechanism remains engaged.

Referring again to Figure 7 which shows the TRU 32 in the second position where the forward portion 41 and the translating sleeve 40 substantially abut one another and the gap 35 (see figure 6), thereby being closed. The actuator 34, in closing the gap 35, moves the translating

sleeve 40 between the second position and the third position thereby the blocker door 42 is drawn radially inwardly by the strut 46. The arrangement of the strut 46 is such that in the third position (similar to that shown in Figure 6) the strut 46 is substantially radially aligned and in the second position (Figure 7) the strut 46 subtends a first forward angle 47. In this way the blocker door 46 is drawn radially inwardly when the translating sleeve 40 is moved forward between the third and second positions by the strut 46.

Alternatively this embodiment is also suitable for reducing the cross sectional area of flow of the bypass duct 28 while at cruise. In reducing the cross sectional flow area the bypass exhaust gases are accelerated and exhaust the nozzle 12 at a greater velocity. In this way the difference between aircraft air speed and the velocity of the exhaust gases is increased and either a greater aircraft velocity may be achieved or a reduction in specific fuel consumption for a given air speed is made.

With reference to Figure 8 and 8A, which show a forward looking view of the downstream end of the exhaust nozzle assembly 16 and a view looking along arrow C respectively. It should be appreciated that the blocker doors 42 are substantially larger than the noise reduction tabs of the UK Application GB 0025727.9. Thus if it is required to reduce the effective size of the blocker doors 42 for the purposes of optimising the use of blocker doors 42 for noise reduction purposes then fins 49 may be provided. As described in more detail in a co-pending UK patent application of the present assignee and dated having the same filing date as the present invention, the fins 49 prevent the spillage of high static pressure bypass air into the lower static pressure ambient air thereby preventing or substantially preventing a vortex being generated at the edge 50,52. In this preferred embodiment the fins 49 extend substantially axially, when the blocker

doors 42 are in their non-deployed position, until they intercept edges 50 and 52 at a predetermined position, thereby defining edge portions 51 and 53 respectively.

Thus in this way the effective axial length of the edges 50 and 52 for generating noise suppressing vortices may be readily altered and optimised. The fins 49 also extend substantially radially inwards from the blocker door 42.

The radial height of the fins 49 is dependant on the boundary layer thickness of the air flowing over the blocker doors 42 and having a component of velocity in the circumferential direction, towards the edges 50,52. It is believed that this boundary layer thickness is in the region of 5-15 millimetres and thus the effectiveness of the fins 49 will depend on its height relative to this boundary layer thickness. Furthermore the fins provide increased stiffness to the blocker door.

Another embodiment of the present invention is described with further reference to figures 2 and 3. It is an object of the present invention to provide both the means for the deployment of the blocker doors 42 and the means to suppress jet engine noise. In Figures 2 and 3 the noise reduction means is provided by the blocker doors 42 being radially inwardly angled. However, this inward angle imposes aerodynamic penalties during cruise and it is therefore a further aspect of this embodiment to reduce this performance loss at cruise. Thus in this embodiment the strut 46 comprises a shape memory material (SMM). The shape and in particular the length of the strut 46 is dependant on a field or flux change. Suitable fields to control the shape of the strut comprise temperature, electrical and magnetic fields.

For a temperature dependant SMM strut 46 a suitable material may be chosen from any, or any combination of the following materials; Titanium, Manganese, Iron, Aluminium, Silicon, Nickel, Copper, Zinc, Silver, Cadmium, Indium, Tin, Lead, Thallium, Platinum, polymers.

In the preferred embodiment the SMM strut 46 is configured so that below a predetermined temperature it assumes a first length and above the predetermined temperature the SMM strut assumes a second length. Essentially the SMM strut operates as an actuator mechanism for deploying the blocker doors 42 as noise reduction means. This predetermined temperature is known as the switch temperature and for the present invention a suitable switch temperature would be between the temperatures generally experienced at either take-off or landing and that experienced at cruise. It is generally understood that ambient temperature decreases with an increase in altitude and it is this temperature change that this first embodiment of the present invention seeks to utilise. Although the fan 2 increases the ambient temperature of the bypass gas flow, typically high altitude cruise

temperatures may be between minus 25 C to minus 40 C and ground temperatures between minus 15 C to plus 40oC. For instance a suitable switch temperature or range of temperatures would therefore be minus 15 Oc to minus 25 Oc although it should be borne in mind that these ambient temperatures will be increased due to the compression of the air by the fan 2. In this embodiment the second length is shorter than the first length. Thus at take-off and landing the SMM strut 46 assumes the second position, thereby deploying the blocker doors 46 as noise reduction means, and at cruise the SMM strut 46 assumes the first position holding the blocker doors 42 in an aerodynamically aligned position.

Alternative to the thermally responsive SMM material the struts 46 may comprise a shape memory electrostrictive material. The electrostrictive material may typically comprise lead zirconate titanate, lead magnesium niobate or strontium titanate. Alternatively, the electrostrictive material may be in the form of a polymer such as polyvinylidene fluoride. Furthermore, magnetostrictive

materials may be used having similar properties to electrostrictive material. Suitable magnetostrictive materials include Titanium, Manganese, Iron, Aluminium, Silicon, Nickel, Copper, Zinc, Silver, Cadmium, Indium, Tin, Lead, Thallium, Platinum. Although the amount of magnetostriction is usually small, it has been shown (Clark, A. E.,"Magnetostrictive rare earth-Fez compounds", Ferromagnetic Materials, Vol. 1, Ch. 7, North Holland Publishing Co. , 1980) that considerable magnetostriction in an alloy of Terbium, Dysprosium and Iron, which is commercially known as Terfenol-DTM, is possible. Terfenol DTM comprises approximately 30% Terbium and 70% Dysprosium and also traces of Iron.

The struts 46 are connected via a simple electrical circuit so that an electrical current may be supplied through the strut 46. Thereby, on application of an electric current, the strut 46 changes length between the first to the second length. Removal of the electric current returns the strut 46 between the second position to the first. It is preferred that the electric current is applied by a control switch which itself is dependant on an engine or aircraft condition or mode of operation. For example and which is also a preferred embodiment, the control of the electrical current is dependant on the altitude of the aircraft. Thus below a predetermined altitude electric current is supplied to the strut 46 and thus the blocker doors 42 are drawn radially inwardly where the doors 42 perform as noise reduction means. Above the predetermined altitude the electric current is switched off and the doors 42 are then driven into aerodynamic alignment by the relaxing electrostrictive strut 46.

It should also be understood to the skilled reader that the temperature controlled shape memory material herein described may also be activated by electrical heating elements disposed to or within the shape memory material. Thus the temperature sensitive shape memory

material may be controlled by aircraft systems such as an altimeter.

It should also be obvious to one skilled in the art that other shapes of the downstream edge 56 of the blocker doors 42 may be used. For instance the downstream edge 56 may comprise triangular serrations.

With reference to all the noise reduction means described herein, a further advantage of the present invention is that the degree to which the blocker doors 42 are extended into the gas stream may be optimised easily during engine testing and noise reduction evaluation. Furthermore the blocker doors 42 may be deployed as noise reduction means to varying extents during the flight cycle of the host aircraft and thereby attenuate different noise frequencies. It has been found that, in general, increasing the radially inward angle of the noise reduction means increases the high frequency noise created by the noise reduction means themselves. Thus evaluating the correct geometry for the noise reduction means is a balance between reducing existing exhaust noise and introducing noise associated to the exhaust reduction means.

A further embodiment of the present invention is provided by arranging certain blocker doors 42 to deploy radially inwardly and other tabs to deploy radially outwardly. Preferably, alternate blocker doors 42 are arranged to deploy inwardly and outwardly around the entire periphery of the nozzle 15. The benefits of this arrangement are described in EP0984152 A2 and will therefore not be reiterated herein.

Furthermore in yet another embodiment of the invention a bypass exhaust nozzle using blocker doors 42 as described hereinbefore can be used in conjunction with prior art noise reduction tabs or a conventional forced lobed type core exhaust nozzle/mixer.

Although the invention has been described and shown with reference to a short cowl type engine arrangement in

which the bypass duct 28 and bypass exhaust nozzle 12 terminate upstream of the core exhaust duct 30 and nozzle 14, the invention may also be applied, in other embodiments, to long cowl type engine arrangements in which the bypass duct 28 and bypass exhaust nozzle 12 terminate downstream of the core exhaust duct 20 and nozzle 14. The invention however is particularly beneficial to short cowl arrangements since with such arrangements conventional noise suppression treatments of the exhaust are not practical in particular where high by pass ratios are also used.

With reference to all the embodiments hereinbefore described that a preferred method of operating an aircraft having a gas turbine engine comprising an exhaust nozzle arrangement, comprises the steps of: deploying noise reduction means prior to take-off; not deploying noise reduction means above a predetermined aircraft altitude and; deploying the noise reduction means below the predetermined aircraft altitude. The predetermined aircraft altitude is set by perceived exhaust noise levels, commonly those on the ground, and the requirement to be within those noise levels which are subject to Aviation Authority and other restrictions.

Claims

Claims 1. A gas turbine engine nozzle arrangement for a flow of exhaust gases therethrough between an upstream end and a downstream end thereof, comprising a nozzle wall and a thrust reverser unit comprising an actuation mechanism and a plurality of blocker doors, the actuation mechanism arranged to move the blocker doors between a first deployed position, substantially obturating the flow of exhaust gases through the downstream end of the nozzle arrangement, and a second non-deployed position wherein the blocker doors define exhaust noise reduction means.
2. A gas turbine engine nozzle arrangement as claimed in claim 1 wherein the blocker doors are circumferentially disposed about the nozzle wall.
3. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-2 wherein the arrangement further comprises a core nozzle wall which is radially inward of and substantially concentric with the nozzle wall, the nozzle wall comprises a forward portion, a translating sleeve, and the thrust reverser unit which further comprises means for reversing thrust of the exhaust gases, and an actuator, the actuator drivingly connects the translating sleeve to the forward portion, the blocker doors are pivotally connected to the downstream end of the translating sleeve and a strut is pivotally connected at one end to the blocker door and at its distal end to the core nozzle wall so that, in use, the actuator drives the translating portion rearward to expose the means for reversing thrust the blocker doors are moved between the second non-deployed position and the first deployed position by the strut drawing the blocker doors radially inwardly and thereby forcing the bypass airflow through the means for thrust reversing.
4. A gas turbine engine nozzle arrangement as claimed in claim 3 wherein the means for reversing thrust is a cascade

structure and the cascade structure is disposed substantially within the translating sleeve so that the cascade structure is not exposed to bypass air flow when the thrust reverser unit is in the second non-deployed position.
5. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-4 wherein the blocker doors are disposed at the downstream end of the translating sleeve.
6. A gas turbine engine nozzle arrangement as claimed in any one of claims 3-5 wherein the actuator is disposed to the forward portion and is connected to the translating portion via a rod.
7. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-6 wherein the blocker doors are generally trapezoidal in shape having lateral edges which taper towards one another in the downstream direction.
8. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-6 wherein the blocker doors are generally rectangular in shape.
9. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-8 wherein the blocker doors overlap one another along a substantial portion of the lateral edges.
10. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-9 wherein the blocker doors comprise a downstream edge and trapezoidal tabs, the trapezoidal tabs being disposed to the downstream edge and define the exhaust noise reduction means.
11. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-10 wherein the blocker doors comprise a downstream edge which is of a curved configuration so that in the first deployed position the curvature of the downstream edge substantially matches the curvature of and abuts to the core nozzle wall.
12. A gas turbine engine nozzle arrangement as claimed in any one of claims 3-11 wherein the translating sleeve comprises a substantially annular extension, the annular

extension extends substantially in the downstream direction and partially radially outwardly surrounds the blocker doors thereby defining an aft portion of the blocker doors which is exposed to the bypass gas flow for exhaust noise reduction means when the thrust reverser unit is in the second non-deployed position.
13. A gas turbine engine nozzle arrangement as claimed in claim 12 wherein the aft portion comprises a trapezoidal tab, the trapezoidal tab is generally configured to form the noise reduction means.
14. A gas turbine engine nozzle arrangement as claimed in claim 12 wherein the aft portion comprises a triangular tab, the triangular tab is generally configured to form the noise reduction means.
15. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-14 wherein the blocker doors are generally radially inwardly curved.
16. A gas turbine engine nozzle arrangement as claimed in any one of claims 12-15 wherein the aft portion comprises a

first tangent angle between 5'and 200 and a second tangent angle between 5'and 200 and generally having a smooth transition therebetween.
17. A gas turbine engine nozzle arrangement as claimed in any one of claims 12-16 wherein the blocker doors comprise the aft portion having a first tangent angle between 00 and 200 and a third angle between 00 and 200 and generally having a straight transition therebetween.
18. A gas turbine engine nozzle arrangement as claimed in any one of claims 1-17 wherein the nozzle arrangement is arranged to move the blocker doors between the second position and a third position where the blocker doors are aligned with the nozzle wall to reduce aerodynamic drag.
19. A gas turbine engine nozzle arrangement as claimed in claim 18 wherein the strut subtends a first forward angle, relative to a radial line when in the second non-deployed

position and in the third position the strut is angled at an angle between the first angle and the radial line.
20. A gas turbine engine nozzle arrangement as claimed in claim 18 wherein the strut comprises shaped memory material, which is configured to change shape in response to a change in an applied field between a first shape, where the blocker doors are disposed in the second nondeployed position and a second shape, where the blocker doors are disposed the third position.
21. A gas turbine engine nozzle arrangement as claimed in claim 18 wherein the blocker doors comprises shaped memory material, which is configured to change shape in response to a change in an applied field between a first shape, where the blocker doors are disposed in the second nondeployed position and a second shape, where the blocker doors are disposed the third position.
22. A gas turbine engine nozzle arrangement as claimed in claim 21 wherein the aft portion comprises shaped memory material.
23. A gas turbine engine nozzle arrangement as claimed in claim 21 wherein the trapezoidal tab comprises shaped memory material.
24. A gas turbine engine nozzle arrangement as claimed in any one of claims 21-23 wherein the applied field is a temperature flux and the shaped memory material comprises any one of a group comprising Titanium, Manganese, Iron, Aluminium, Silicon, Nickel, Copper, Zinc, Silver, Cadmium, Indium, Tin, Lead, Thallium, Platinum.
25. A gas turbine engine nozzle arrangement as claimed in any one of claims 21-23 wherein the applied field is an electrical signal and the shape memory material comprises an electrostrictive material selected from the group comprising Lead Zirconate Titanate, Lead Magnesium Niobate, Strontium Titanate and a polymer group including polyvinylidene fluoride.
26. A gas turbine engine exhaust nozzle arrangement as claimed in any one of claims 1-25 wherein the blocker door comprises a fin, the fin, in use, prevents spillage of gas over at least a portion of the lateral edge of the blocker door and thereby substantially prevents a vortex being formed from that edge.
27. A method of operating an aircraft having a gas turbine engine comprising an exhaust nozzle arrangement as claimed in any preceding claim wherein the method comprises the steps of: deploying noise reduction means prior to takeoff; not deploying noise reduction means above a predetermined aircraft altitude and; deploying the noise reduction means below the predetermined aircraft altitude.
28. A gas turbine engine exhaust nozzle arrangement as hereinbefore described and with reference to figures 1 to 8.
29. A ducted fan gas turbine engine as hereinbefore described and with reference to figures 1 to 18.