Abstract:
A control mechanism is provided for moving at least two components of a gas turbine engine. The control mechanism comprises a movable actuation rod and a first linkage arrangement which includes a first bell crank connecting the actuation rod to a first component. Movement of the actuation rod produces an output motion of the first bell crank which in turn drives movement of the first component. The control mechanism further comprises a second linkage arrangement which includes a second bell crank connecting the actuation rod to a second component. Movement of the actuation rod produces an output motion of the second bell crank which in turn drives movement of the second component. The first and the second linkage arrangements are configure so that over a predetermined range of movement of the actuation rod the first component is moved which the second component is not moved.

Description:
The present invention relates to a mechanism for controlling movement of multiple variable components, and in particular, but not exclusively, controlling movement of unison rings which vary the angles of respective rows of vanes of a gas turbine engine. 
     BACKGROUND 
     Mechanical linkage arrangements incorporating bell cranks can be used to provide rotational control of annular arrays of vanes in multiple stages in a compressor of a gas turbine engine. Such an arrangement may comprise a bell crank, rotatable about a fulcrum, an actuation rod being pivotably connected to an input arm to the bell crank and a control rod being pivotably connected to an output arm of the bell crank. Movement of the actuation rod rotates the bell crank which in turn causes motion of the control rod. This motion can then be used to drive a unison ring which changes the angles of a row of vanes. Existing linkage arrangements can provide linked behaviour between the different rows of vanes. 
     SUMMARY 
     The present invention is at least partly based on the realisation that it would be desirable to introduce a degree of “lost motion” between linkage arrangements sharing the same actuation rod so that different components can be operated at different times or rates. 
     Thus, a first aspect of the invention provides a control mechanism for moving at least two components of a gas turbine engine, the control mechanism comprising: 
     a moveable actuation rod, 
     a first linkage arrangement which includes a first bell crank and which operatively connects the actuation rod to a first component of the gas turbine engine such that movement of the actuation rod produces an output motion of the first bell crank which in turn drives movement of the first component, and 
     a second linkage arrangement which includes a second bell crank and which operatively connects the actuation rod to a second component of the gas turbine engine such that movement of the actuation rod produces an output motion of the second bell crank which in turn drives movement of the second component; 
     wherein the first linkage arrangement and the second linkage arrangement are configured so that over a predetermined range of movement of the actuation rod the first component is moved by the first bell crank while the second component is not moved by the second bell crank. Typically, the first linkage arrangement and the second linkage arrangement are also configured so that over a further range of movement of the actuation rod the first component is moved by the first bell crank while the second component is moved by the second bell crank 
     Thus, advantageously, the lost motion can be produced by linkage arrangements incorporating bell cranks. Such linkage arrangements can be made mechanically reliable and relatively compact and lightweight. 
     The control mechanism may have any one or any combination of the following optional features. 
     The first linkage arrangement and the second linkage arrangement may be further configured so that over a second predetermined range of movement of the actuation rod, spaced from the first predetermined range of movement, the first component is moved by the first bell crank while the second component is not moved by the second bell crank. Thus, for example, lost motion can be produced at the beginning and at the end of the stroke of the actuation rod, while in the middle of the stroke both components are moved. 
     The second linkage arrangement may include a further bell crank operatively connected between the actuation rod and the second bell crank. The respective configurations of the second and further bell cranks and their relative positions can be used to produce the lost motion. 
     The first bell crank and the further bell crank may be provided by the same bell crank. This can reduce the number of particularly rotating parts, thereby allowing weight-savings and reliability improvements to be achieved. 
     Preferably, the first and the second components are respective unison rings. Typically, the unison rings vary the angles of respective rows of vanes of the gas turbine engine. For example, the unison ring of the first linkage arrangement can vary the angles of a row of inlet guide vanes and the unison ring of the second linkage arrangement can vary the angles of a row of stator vanes downstream of the inlet guide vanes. 
     The second linkage arrangement may also operatively connect the actuation rod to a third component of the gas turbine engine such that movement of the actuation rod produces an output motion of the second bell crank which in turn drives movement of the third component, the second linkage arrangement being configured so that over said predetermined range of movement of the actuation rod (and optionally over the second predetermined range of movement of the actuation rod) the third component is not moved by the second bell crank. Typically, the second linkage arrangement is also configured so that over the further range of movement of the actuation rod the third component is moved by the second bell crank. Thus both the second and the third components can experience lost motion. 
     Preferably, the third component is a third unison ring. Typically, the third unison ring varies the angles of a row of vanes of the gas turbine engine. For example, when the unison ring of the first linkage arrangement varies the angles of a row of inlet guide vanes and the unison ring of the second linkage arrangement varies the angles of a row of stator vanes downstream of the inlet guide vanes more, the third unison ring can vary the angles of a third row of stator vanes also downstream of the inlet guide vanes. 
     A further aspect of the invention provides a gas turbine engine having the control mechanism of the first aspect, optionally including any one or any combination of the optional features of the control mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  shows schematically a gas turbine engine; 
         FIG. 2  shows schematically a detailed cut-away view of the intermediate pressure compressor of a gas turbine engine; 
         FIG. 3  shows schematically a control mechanism according to a first embodiment of the present invention for operating intermediate compressor unison rings; 
         FIG. 4  shows schematically a control mechanism according to a second embodiment of the present invention for operating intermediate compressor unison rings, the actuation rod of the mechanism being at the start of its stroke; 
         FIG. 5  shows schematically the control mechanism of  FIG. 4  with the actuation rod at the end of its stroke; 
         FIG. 5A  shows an alternative configuration of the control mechanism; and 
         FIG. 6  shows a plot of measured intermediate pressure compressor (IPC) adiabatic efficiency against mass flow (in pounds/second) of the IPC inlet flow for a gas turbine engine. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows schematically a gas turbine engine generally indicated at  10  and having a principal and rotational axis X-X. The engine  10  comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , an intermediate pressure turbine  17 , a low-pressure turbine  18  and a core exhaust nozzle  19 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  11  and a bypass duct  22  which defines a bypass exhaust nozzle  20 . 
     The gas turbine engine  10  works in the conventional manner so that air entering the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow into the intermediate pressure compressor  13  and a second air flow which passes through the bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  13  compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  16 ,  17 ,  18  before being exhausted through the core nozzle  19  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  16 ,  17 ,  18  respectively drive the high and intermediate pressure compressors  14 ,  13  and the fan  12  by suitable interconnecting shafts. 
       FIG. 2  shows detail of an intermediate pressure compressor  13 . A row of variable inlet guide vanes (VIGVs)  24  is positioned before the first row of compressor blades  25  of the intermediate pressure compressor. A first row of variable stator vanes (VSV 1 s)  26  is then positioned behind the first row of compressor blades and before the second row of blades  27 , and a second row of variable stator vanes (VSV 2 s)  28  is positioned behind the second row of blades. When the compressor speed reduces, the axial velocity of the inlet air becomes low relative to the blade speeds. This increases the incidence of the air onto the blades. If the incidence increases too far aerodynamic stall can occur. To overcome this problem, the VIGVs and the VSVs can rotate about their axes, changing the angles of the vanes and thereby altering the incidence of the airflow onto the blades. 
     The angles of the VIGVs or VSVs of a row are changed by circumferentially rotating a respective unison ring  29 , each VIGV or VSV being joined to the unison ring by a respective lever  30 . Depending on compressor and engine performance, the unison ring is rotated under the control of an actuator, a linkage arrangement operatively connecting the actuator to the unison ring. 
     Thus, variable vanes can be used where high pressure ratios are required across a single compressor (e.g. intermediate  13  and/or high  14 ). As a compressor speed is reduced from its optimal design value the variable vanes are progressively closed to maintain an acceptable gas angle onto the downstream rotor blades. This prevents the compressor  13 ,  14  from surging, which can result in a loss of engine thrust and damage to turbomachinery. 
     Existing linkage arrangements provide fixed relationship rotation of each row of vanes  24 ,  26 ,  28 . However, it is desirable to further control the gas flow to further improve the capability of the compressor to prevent surge, whilst also increasing the flow capacity and efficiency of the compressor. 
       FIG. 3  shows a control mechanism according to a first embodiment of the present invention for operating intermediate compressor unison rings. The mechanism comprises a hydraulic actuator  30  (typically using engine fuel as the operating fluid) which drives an actuation rod in its length direction. The distal end of the actuation rod is connected to an input arm of a first bell crank  32 . A VIGV control rod  33  extends from an output arm of the bell crank to the VIGV unison ring  34 . The bell crank  32  and VIGV control rod  33  define a first linkage arrangement. 
     In addition, VSV control rods  35 ,  36  extend from respective output arms of a second bell crank  37 , VSV 1  control rod  35  connecting to the VSV 1  unison ring  38  and VSV 2  control rod  35  connecting to the VSV 2  unison ring  39  To rotate the second bell crank, a third bell crank  40  is operatively connected between the actuation rod  31  and the second bell crank. A first intermediate control rod  41  is pivotally connected to the proximal end of the actuation rod and extends laterally therefrom to join to an input arm of the third bell crank. A second intermediate control rod  42  then connects an output arm of the third bell crank to an input arm of the second bell crank. The second and third bell cranks  37 ,  40 , first and second intermediate control rods  41 ,  42 , and VSV control rods  35 ,  36  define a second linkage arrangement. 
     The linkage arrangements provide a system whereby the VIGVS can be initially closing without any significant movement of the VSVs. For example, movement of the actuation rod  31  at the position shown in  FIG. 3  will produce little or no movement of the third bell crank  40 . However, when the actuation rod has extended so that the connection between the rod and the first intermediate control rod  41  is further to the left, movement of the actuation rod will cause the VSVs to close together with the VIGV. Thus the linkage arrangements provide an initial predetermined range of movement of the actuation rod over which VIGV unison is rotated while the VSV unison rings are not rotated. This variation in movement between the vanes can provide an efficiency benefit over known systems. 
       FIG. 4  shows a control mechanism according to a second embodiment of the present invention for operating intermediate compressor unison rings. For ease of comparison, the linkage arrangement components of the second embodiment are drawn with thick lines and overlaid on the control mechanism of the first embodiment. The hydraulic actuator  30 , actuation rod  31 , VIGV control rod  33 , VSV control rods  35 ,  36 , and unison rings  34 ,  38 ,  39  are unchanged as between the two embodiments. 
     In the second embodiment, the first and third bell cranks of the first embodiments are replaced by a single first bell crank  43  having two output arms. One output arm connects to the end of VIGV control rod  33  and thence to the VIGV unison ring  34 . An intermediate control rod  44  extends from the other output arm to attach to an arm of second bell crank  45 . VSV control rods  35 ,  36  extend from the second bell crank  45  and thence to the VSV unison rings  38 ,  39  in a similar manner to the first embodiment. The first bell crank  43  and VIGV control rod  33  define a first linkage arrangement, and the first bell crank  43 , intermediate control rod  44 , second bell crank  45  and VSV control rods  35 ,  36  define a second linkage arrangement. 
     The control system of the second embodiment has one less bell crank and one less control rod compared to the system of the first embodiment. Also, side loads on the actuation rod  31  produced by the first intermediate control rod  41  can be avoided. 
     At the start of the stroke of the actuation rod  31 , the intermediate control rod  44  is aligned with the fulcrum  46  of the first bell crank  43  to achieve lost motion at the second bell crank  45 . As the actuation rod extends and the first bell crank rotates clockwise, the intermediate control rod initially turns with limited effect on the second bell crank. However, after a given rotation the crank starts to pull on the second bell crank, rotating it anti-clockwise and driving the VSV control rods  35 ,  36 . The amount of lost motion can be controlled by adjusting the initial angular position of the first bell crank  43 . 
       FIG. 5  shows the control mechanism of the second embodiment at the end of the stroke of the actuation rod  31 . The intermediate control rod  44  leading to the second bell crank  45  is being pulled at right angles to the first bell crank fulcrum  46 . This means that the rod speed and the speed of actuation of the second bell crank are at a maximum. 
     It is possible, by changing the configuration of the second bell crank  45 , to adjust the relative VSV 1  and VSV 2  opening and closing speeds. In  FIG. 5 , for example, the VSV 1  control rod  35  has swung just over the right angle to the fulcrum  47  of the second bell crank, so that its motion is gently accelerating with respect to rotation of the second bell crank, then fairly linear. The VSV 2  control rod  36 , on the other hand, is gently decelerating. 
       FIG. 5A  is an alternative configuration for driving the VSV 2  control rod  36 . By increased cranking of the bell crank, lost motion at control rod  36  at the end of the stroke of the actuation rod  31  can be achieved. Mean speed can be maintained by increasing the length of the arm. In a similar fashion, lost motion at VSV 1  control rod  35  at the end of the stroke of the actuation rod  31  can also be achieved. 
       FIG. 6  shows a plot of measured intermediate pressure compressor (IPC) adiabatic efficiency against flow (in pounds/second) of the IPC inlet flow for a typical gas turbine engine. The upper curve  101  shows the pattern of efficiencies that are achieved when the VIGV, VSV 1  and VSV 2  rows are independently actuated. The lower curve  105  shows the pattern of the pattern of efficiencies that are achieved when the VIGV, VSV 1  and VSV 2  rows are actuated in a conventional ganged arrangement (i.e. without lost motion). The middle curve  103  shows the effect of actuating the VIGV row semi-independently of the VSV 1  and VSV 2  rows, i.e. so that when reducing from a mid flow to a low flow regime initially all the rows close but then only the VIGV row closes, and when increasing from a mid flow to a high flow regime initially all the rows open but then only the VIGV row opens. This semi-independent operation of the VIGV row from the VSV 1  and VSV 2  rows is achievable using control mechanisms according to the present invention. 
     Advantageously, although the efficiencies are lower than can be achieved when the VIGV, VSV 1  and VSV 2  rows are independently actuated, a significant improvement relative to conventional ganged arrangements is achieved. Further the improvement can be obtained without introducing overly complex mechanical linkages. Also the control mechanisms of the present invention require little headroom, making them suitable for use where in situations where there are space restrictions. A further advantage of the control mechanisms is that they can be used in situations where there is a high hale angle (i.e. a high angle of taper of the compressor). 
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.