Abstract:
A method for varying the geometry of a mid-turbine frame includes detecting a strain in a mid-turbine frame with a piezoelectric material; applying a deformation voltage to the piezoelectric material as a function of the detected strain; deforming the piezoelectric material to actuate an actuation plate; and repositioning an engine casing through the actuation of the actuation plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is a divisional patent application of U.S. patent application Ser. No. 12/178,352, filed Jul. 23, 2008. All references are incorporated herein. 
     
    
     BACKGROUND 
       [0002]    Turbofans are a type of gas turbine engine commonly used in aircraft, such as jets. The turbofan generally includes high and low pressure compressors, high and low pressure turbines, high and low spool shafts, a fan, and a combustor. The high-pressure compressor (HPC) is connected to the high-pressure turbine (HPT) by the high spool rotatable shaft, and together act as a high-pressure system. Likewise, the low-pressure compressor (LPC) is connected to the low-pressure turbine (LPT) by the low spool rotatable shaft, and together act as a low-pressure system. The low spool shaft is housed within the high spool shaft and is connected to the fan such that the HPC, HPT, LPC, LPT, and high and low spool shafts are coaxially aligned. 
         [0003]    Air is drawn into the jet turbine engine by the fan and the HPC. The HPC increases the pressure of the air drawn into the system. The high-pressure air then enters the combustor, which burns fuel and emits exhaust gas. The exhaust gas flows from the combustor into the HPT where it rotates the high spool shaft to drive the HPC. After the HPT, the exhaust gas is exhausted to the LPT. The LPT uses the exhaust gas to turn the low spool shaft, which powers the LPC and the fan to continually bring air into the system. Air brought in by the fan bypasses the HPT and LPT and acts to increase the engine&#39;s thrust, driving the jet forward. 
         [0004]    In order to support the high and low pressure systems, bearings are located within the jet turbine engine to help distribute the load created by the high and low pressure systems. The bearings are connected to an engine casing that houses a mid-turbine frame located between the HPT and the LPT by bearing support structures. The bearing support structures can be, for example, bearing cones. The load from the bearing support structures are transferred to the engine casing through the mid-turbine frame. Decreasing the weight of the engine casing can significantly increase the efficiency of the jet turbine engine and the jet itself. Additionally, maintaining the sealing continuality between the HPT and the mid-turbine frame, and between the LPT and the mid-turbine frame, reduces leakage and also improves the efficiency of the jet turbine engine. 
       SUMMARY 
       [0005]    A method for varying the geometry of a mid-turbine frame includes detecting a strain in a mid-turbine frame with a piezoelectric material; applying a deformation voltage to the piezoelectric material as a function of the detected strain; deforming the piezoelectric material to actuate an actuation plate; and repositioning an engine casing through the actuation of the actuation plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a partial sectional view of an intermediate portion of a gas turbine engine. 
           [0007]      FIG. 2  is an enlarged perspective view of a mid-turbine frame assembly having an actuated variable geometry. 
           [0008]      FIG. 3  is a cross-sectional perspective view of the mid-turbine frame assembly. 
           [0009]      FIG. 4  is a block diagram of the circuitry for the mid-turbine frame assembly. 
           [0010]      FIG. 5  is a top view of a segment of the mid-turbine frame assembly. 
           [0011]      FIG. 6  is a cross-sectional perspective view of a segment of the mid-turbine frame assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  shows the turbine section of a gas turbine aircraft engine  10  about a gas turbine engine axis centerline. Gas turbine engine  10  generally includes high-pressure turbine  12 , low-pressure turbine  14  and mid-turbine frame  16 . 
         [0013]    High-pressure turbine  12  includes one or more rows of blades  18  mounted on the rim of a disk  20 . Disk  20  attaches to high spool (or high-pressure) shaft  22 , which first (or forward) bearing  24  supports. High-pressure turbine  12  exhausts hot gases into annular flow chamber  26 . 
         [0014]    Exhaust gas from high-pressure turbine  12  flows through annular flow chamber  26  and into low-pressure turbine  14 . Low-pressure turbine  14  includes a number of rows of blades  28  mounted onto disks  30 . Bolts  32  bolt disks  30  together. Blades  28  are alternated with stationary vanes  34 . One or more blades  28  may have an extension  36  of low spool (or low-pressure) shaft  38 . Second (or aft) bearing  40  supports low spool shaft  38 . 
         [0015]    Mid-turbine frame assembly  16  is located in annular flow chamber  26 , between high-pressure turbine  12  and low-pressure turbine  14 . Mid-turbine frame assembly  16  includes engine casing  42 , mid-turbine frame  44 , mounts  46 , first bearing  24  and second bearing  40 . For convenience, mid-turbine frame assembly  16  will be referred to as having a forward side  60  adjacent to high-pressure turbine  12  in engine  10  and aft side  62  adjacent to low-pressure turbine  14  in engine  10 . Mid-turbine frame assembly  16  has an actuated variable geometry that increases the efficiency of engine  10  by maintaining continuity between high pressure turbine  12  and low pressure turbine  14 , thus minimizing thermodynamic losses. Mid-turbine frame assembly  16  also has a lightweight design that transfers a minimum amount of unbalanced loads from first bearing  24  and second bearing  40  through mid-turbine frame  44  to casing  42  and mounts  46 . 
         [0016]    Engine casing  42  surrounds mid-turbine frame  44 , and protects mid-turbine frame  44  from the surroundings. Engine casing  42  functions to transfer loads from mid-turbine frame  44  to mounts  46 . 
         [0017]    Mid-turbine frame  44  is housed within engine casing  42 , and connects to engine casing  42  and first and second bearings  24  and  40 . Mid-turbine frame  44  transfers the loads from the first and second bearings  24  and  40  to engine casing  42  and mounts  46 . Mid-turbine frame  44  normalizes and equilibrates the loads from first and second bearings  24  and  40  so that a minimum amount of unbalanced load is transferred to mounts  46 . 
         [0018]    First and second bearings  24  and  40  are located at forward end  60  and aft end  62  of mid-turbine frame assembly  16 , respectively, below engine casing  42 . First and second bearings  24  and  40  support thrust loads, vertical tension, side gyroscopic loads, as well as vibratory loads from high and low spool shafts  22  and  38  located in gas turbine engine  10 . The loads supported by first bearing  24  and second bearings  40  transfer to engine casing  42  and mounts  46  through mid-turbine frame  44 . 
         [0019]      FIGS. 2 and 3  are an enlarged perspective view of mid-turbine frame assembly  16  and a cross-sectional perspective view mid-turbine frame assembly  16  respectively, and will be discussed together. Mid-turbine frame assembly  16  includes engine casing  42  and mid-turbine frame  44 . Engine casing  42  surrounds mid-turbine frame  44  as previously described. Engine casing  42  has interior surface  56  and exterior surface  58 . Dimples  54  are formed in engine casing  42  so that protrusions extend from interior surface  56  and indentions are formed in exterior surface  58 . Dimples  54  project towards but do not engage mid-turbine frame  44 . Dimples  54  stiffen engine casing  42 , eliminating the need for rails along exterior surface  58  of engine casing  42 . The elimination of rails gives engine casing  42  a lightweight and cost-effective structure. Additionally, the elimination of rails reduces the drag on engine casing  42 . 
         [0020]    Mid-turbine frame  44  generally includes torque box  47 , structural struts  48 , actuation plates  50   a ,  50   b  and actuation (or oleo) struts  52   a ,  52   b . Torque box  47  has a ring structure and is positioned between first bearing  24  and second bearing  40  and structural struts  48 . Torque box  47  takes the loads, or torque, from first and second bearing  24  and  40  and combines them prior to transferring the loads to structural struts  48  and oleo struts  52   a  and  52   b.    
         [0021]    Structural struts  48  extend between interior surface  56  of engine casing  42  and torque box  47 . First end  55  of structural strut  48  connects to torque box  47 . Second end  57  of structural strut  48  connects to interior surface  56  of engine casing  42  at the center of dimples  54 . Structural struts  48  transfer a portion of the loads from first and second bearings  24  and  40  to mounts  46 . 
         [0022]    Dimples  54  shorten the length of structural struts  48 . The length of structural struts  48  determines the critical buckling load of the struts; the critical buckling load varies inversely as a square of the length of the strut (i.e. a shorter strut increases the critical buckling load of the strut). The shortened length of structural struts  48  between dimple  54  and torque box  47  increases the critical buckling load and the load carrying capacity of structural struts  48 . The shortened structural struts  48  also reduce the weight of mid-turbine engine assembly  16 , thus increasing the specific fuel consumption (SFC) of engine  10 . Additionally, structural struts  48  may be hollow to further reduce the weight of engine  10 . 
         [0023]    When coupled with structural struts  48 , dimples  54  act as local stiffeners and increase the critical buckling load of engine casing  42 . The increased critical buckling load of engine casing  42  allows wall thickness T c  of engine casing  42  to be thinner while still providing the same load capacity. Thus, the weight of engine casing  42  is reduced and the SFC of engine  10  is increased. 
         [0024]    Actuation plates  50   a ,  50   b  attach around the circumference of and axially extend from torque box  47 . Forward actuation plates  50   a  attach to forward side  60  of torque box  47 , and aft actuation plates  50   b  attach to aft side  62  of torque box  47 . One forward actuation slot  61   a  is formed in each forward actuation plate  50   a , and one aft actuation slot  61   b  is formed in each aft actuation plate  50   b . In one example, actuation slots  61   a ,  61   b  are formed in the center of actuation plates  50   a ,  50   b  and extend away from torque box  47 . In another example, actuation slots  61   a ,  61   b  extend along an axis parallel to the axis of high spool shaft  22 . 
         [0025]    Oleo struts  52   a ,  52   b  extend between actuation plates  50   a ,  50   b  and engine casing  42 . Oleo struts  52   a ,  52   b  are located on opposite sides of structural strut  48 . Forward oleo strut  52   a  connects to forward side  60  of engine casing  42  and to forward actuation plate  50   a . Aft oleo strut  52   b  connects to aft side  62  of engine casing  42  and aft actuation plate  50   b . One forward oleo strut  52   a  fits within each forward actuation slot  61   a  in forward actuation plates  50   a . Similarly, one aft oleo strut  52   b  fits within each aft actuation slot  61   b  in aft actuation plates  50   b.    
         [0026]    Oleo struts  52   a ,  52   b  are active actuator struts that transfer amplified displacements caused by actuation plates  50   a ,  50   b  as local vertical forces. Movement of oleo struts  52   a ,  52   b  reposition forward side  60  of engine casing  42  and aft side  62  of engine casing  42  respectively. Because each oleo strut  52   a ,  52   b  is attached to an individually actuated actuation plate  50   a ,  50   b , each oleo strut is individually actuated. 
         [0027]    Forward and aft actuation plates  50   a  and  50   b  contain piezoelectric actuators  63   a ,  63   b  having piezoelectric material in actuation slots  61   a ,  61   b . Piezoelectric materials produce an electric energy when deformed or strained, and conversely transform electrical energy fields into mechanical deformation or strain actuation. In one example, the piezoelectric material is a naturally occurring material such as quartz, Rochelle salt or tourmaline. In another example, the piezoelectric material is a manufactured piezoelectric ceramic having enhanced piezoelectric properties, such as polycrystalline ferroelectric materials (i.e. barium titanate (BaTiO s )) and lead zirconate titanate (PZT). 
         [0028]    In response to a sensed strain, piezoelectric actuator  63   a  in actuation slot  61   a  produces voltage. This voltage is sensed and an amplified voltage is applied to piezoelectric actuator  63   a . In response to the amplified voltage, piezoelectric actuator  63   a  deforms causing actuation plate  50   a  to actuate or move. The movement of actuation plate  50   a  vertically actuates oleo strut  52   a , and repositions engine casing  42 . Piezoelectric actuator  63   b , actuation plate  50   b , and oleo strut  52   b  function in a similar manner. Thus, piezoelectric actuators  63   a ,  63   b  convert a strain in the torque box into an amplified force that repositions engine casing  42 . The strain sensed may be due to interference pressure of adjacent components, thermal growth, mechanical growth or temperature differences. 
         [0029]    Piezoelectric actuators  63   a ,  63   b  may be configured as, for example, a unimorph piezoelectric actuator, a multilayer piezo bender or a multilayer piezoelectric actuator. Piezoelectric actuators  63   a ,  63   b  may contain a steel layer and/or a ceramic layer. The steel layer senses mechanical loads or strains, and the ceramic layer senses thermal loads. Thus, in one example where piezoelectric actuators  63   a ,  63   b  have a steel layer and a ceramic layer, oleo struts  52   a ,  52   b  are actuated based upon sensed mechanical strains and sensed thermal strains. 
         [0030]      FIG. 4  illustrates circuitry  72  used to actuate piezoelectric actuators  63   a ,  63   b . Circuit  72  includes piezoelectric actuators  63   a ,  63   b , power distribution source  74  and external power bus  76 . Piezoelectric actuators  63   a ,  63   b  are connected to power distribution source  74  by transmission lines  78   a ,  78   b  and reception lines  80   a ,  80   b . Transmission lines  78   a ,  78   b  transmit a voltage from piezoelectric actuators  63   a ,  63   b  to power distribution source  74 . Reception lines  80   a ,  80   b  transmit an amplified voltage from power distribution source  74  to piezoelectric actuators  63   a ,  63   b . Piezoelectric actuators  63   a ,  63   b  are also connected to each other and to power distribution source  74  by grounding lines  82 . 
         [0031]    Power distribution source  74  is connected to external power bus  76  by transmission line  84  and reception line  86 . External power bus  76  transfers a specified voltage to power distribution source  74  through reception line  84  based upon a signal sent by power distribution source  74  through transmission line  84 . 
         [0032]    When piezoelectric actuator  63   a  senses a strain, it produces a voltage. This voltage is transferred to power distribution source  74  through transmission line  78   a . If the voltage is above a predetermined threshold, power distribution source  74  calls on external power bus  76  to generate an amplified voltage by sending a signal through transmission line  84 . The amplified voltage is transferred through reception lines  86  and  80   a  to piezoelectric actuator  63   a  in order to actuate piezoelectric actuator  63   a . Piezoelectric actuator  63   b  functions in a similar manner. 
         [0033]    Power distribution source  74  will only distribute a voltage to piezoelectric actuators  63   a ,  63   b  if the sensed strain or produced voltage is above a predetermined threshold. For example, if the sensed strain is mechanical growth, a voltage is only applied to piezoelectric actuators  63   a ,  63   b  when a movement at least about equal to the movement expected at the redline speed (or maximum speed of high spool shaft  22 ) plus about 1% to about 5% is sensed. In this example, a voltage is not applied to piezoelectric actuators  63   a ,  63   b  when the sensed movements are less than the movement expected at the redline speed plus about 1% to about 5%. 
         [0034]    Power distribution source  74  determines by how much the voltage from piezoelectric actuators  63   a ,  63   b  should be amplified. Power distribution source  74  determines the voltage applied to piezoelectric actuators  63   a ,  63   b  through lines  80   a ,  80   b  based upon the magnitude of the voltage through lines  78   a ,  78   b  from piezoelectric actuators  63   a ,  63   b . Additionally or alternatively, power distribution source  74  may determine the voltage applied to piezoelectric actuators  63   a ,  63   b  based upon the magnitude of the voltage change from piezoelectric actuators  63   a ,  63   b  (i.e. how quickly the strain grows or changes). 
         [0035]    As described above, actuation plates  50   a ,  50   b  extend around the circumference of torque box  47 . Each actuation plate  50   a ,  50   b  contains a piezoelectric actuator  63   a ,  63   b . Each piezoelectric actuator  63   a ,  63   b  is connected to power distribution source  74 . Therefore, each piezoelectric actuator  63   a ,  63   b  may be individually actuated based on the loads or strains sensed by that particular actuator  63   a ,  63   b.    
         [0036]    Circuit  72  may be designed with redundancies in case of a failure of one of the elements. For example, circuit  72  may contain two power distribution sources  74  in the event that the primary power distribution source  74  fails. Each power distribution source  74  would be connected to each piezoelectric actuator  63   a ,  63   b  in mid-turbine frame assembly  16 . Similarly, circuit  72  may be designed with two external power buses  76 , wherein each external power bus  76  is connected to each power distribution source  74 . 
         [0037]      FIG. 5  is a top view of a segment of mid-turbine frame assembly  16  wherein engine casing  42  has been removed for clarity. Oleo struts  52   a ,  52   b  are located in actuation slots  61   a ,  61   b , which contain piezoelectric actuators  63   a ,  63   b . Oleo struts  52   a ,  52   b  are located on opposite sides of structural struts  48 . As shown, oleo struts  52   a ,  52   b  and structural strut  48  are located within vane envelope  53 . Oleo struts  52   a ,  52   b  are maintained about parallel to each other. In another example, forward strut  52   a  is maintained within about 2 degrees of being parallel with aft oleo strut  52   b.    
         [0038]    As shown in  FIG. 5 , piezoelectric actuators  63   a ,  63   b  in actuation slots  61   a ,  61   b  develop axial, transverse and shear strain actuation in mid-turbine frame  44  in response to a sensed strain. As discussed above, bearings  24 ,  40  introduce horizontal, vertical and off-axis loads into torque box  42 . For example, horizontal or off-axis loads may be introduced by bearings  24 ,  40  due to the dynamics of shafts  22  and  38  or thermal differences. The strain actuation of piezoelectric actuators  63   a ,  63   b  in actuation slots  61   a ,  61   b  causes self-equilibration of engine casing  42 , and results in a minimum amount of unbalanced loads being transferred to mounts  46 . 
         [0039]    When piezoelectric actuator  63   a  in actuation slots  61   a  senses a strain in torque box  47 , an amplified voltage is applied to piezoelectric actuator  63   a . The applied voltage causes piezoelectric actuator  63   a  to deform and vibrate. The vibrations from piezoelectric actuator  63   a  create axial, transverse and shear strain actuations that equilibrate the in-plane loads locally around oleo strut  52   a . Additionally, piezoelectric actuator  63   a  deforms and causes vertical motion of oleo strut  52   a  to equalize the vertical loads. Piezoelectric actuator  63   b  function in a similar manner. Piezoelectric actuators  63   a ,  63   b  may cause strain actuation because of a sensed thermal growth, mechanical vibration, such as a vibration due to sharp rotation, or interference growth. 
         [0040]      FIG. 6  is cross-sectional perspective view of a segment of mid-turbine frame assembly  16  having mid-turbine frame  44  and engine casing  42 . Engine casing  42  surrounds mid-turbine frame  44 . Dimple  54  formed in engine casing  42  creates a protrusion extending from internal surface  56  of engine casing  42  and an intention in exterior surface  58  of engine casing  42 . 
         [0041]    High spool shaft  22  transfers loads to first bearing  24 , and low spool shaft  38  transfers loads to second bearing  40 . First and second bearings  24  and  40  introduce the loads into torque box  47  of mid-turbine frame  44 , where the loads are combined and equilibrated. 
         [0042]    Structural strut  48  transfers loads from torque box  47  to engine casing  42 . First end  55  of structural strut  48  attaches to torque box  47  and second end  57  of structural strut  48  attaches to engine casing  42  at dimple  54 . In one embodiment, dimples  54  and structural struts  48  are equal in number such that each structural strut  48  connects to engine casing  42  at a different dimple  54 . In another embodiment, structural struts  48  are elliptical in shape and are sized to take a load and transfer it in a vertical direction toward engine casing  42 . In another embodiment, nine structural struts are positioned approximately forty degrees apart from one another along the circumference of torque box  47 . In another embodiment, twelve total structural struts are positioned approximately thirty degrees apart from one another along the circumference of torque box  47 . 
         [0043]    Actuation plates  50   a  and  50   b  attach to torque box  47 . Forward actuation plate  50   a  attaches on forward side  60  of torque box  47 , and aft actuation plate  50   b  attaches on aft side  62  of torque box  47 . 
         [0044]    Oleo struts  52   a ,  52   b  extend between engine casing  42  and actuation plates  50   a ,  50   b , respectively. Forward oleo struts  52   a  and aft oleo struts  52   b  are located on opposite sides of structural strut  48 . Oleo struts  52   a ,  52   b  extend through engine casing  42  so that when oleo struts  52   a ,  52   b  are actuated by actuation plates  50   a ,  50   b , engine casing  42  is repositioned. One forward oleo strut  52   a  is placed on the forward side of each structural strut  48  and one aft oleo strut  52   b  is placed on the aft side of each structural strut  48  so that oleo struts  52   a ,  52   b  are on opposite sides of structural struts  48 . 
         [0045]    In use, in order to reposition oleo struts  52   a ,  52   b , piezoelectric actuators  63   a ,  63   b  in actuation plates  50   a ,  50   b  measure or detect strains due to thermal strain growth, interference pressure, or mechanical strain. Actuation plates  50   a ,  50   b  reflect the detected strain actuation as an amplified vertical force supplied to oleo struts  52   a ,  52   b . Oleo struts  52   a ,  52   b  transmit the amplified force to engine casing  42 , which is repositioned. 
         [0046]    Forward flange  68  extends from engine casing  42  on forward side  60  of engine casing  42 , and aft flange  70  extends from engine casing  42  on aft side  62  of engine casing  42 . Forward and aft flanges  68  and  70  connect mid-turbine frame assembly  16  to high-pressure turbine  12  and low-pressure turbine  14  respectively. 
         [0047]      FIG. 6  illustrates how the actuation of oleo struts  52   a ,  52   b  balances and equalizes the vertical loads from bearings  24 ,  40  prior to transferring them to mounts  46 . In response to a sensed force, piezoelectric actuators  63   a ,  63   b  in actuation slot  61   a  deform to amplify the force in the vertical direction. The deformation moves forward oleo strut  52   a  illustrated by force F r1 , causing small motions in engine casing  42 . Engine casing  42  transfers the motions to aft oleo strut  52   b  illustrated by force F r2  so that engine casing  42  follows an arching pattern as illustrated by arrow A and the vertical forces are balanced. 
         [0048]    The self-equilibrating case motion caused by the actuation of oleo struts  52   a ,  52   b  is small to ensure structural stability, and adjusts engine casing  42  to maintain compatibility with high-pressure turbine  12  and low-pressure turbine  14 . Oleo struts  52   a ,  52   b  ensure that the case distortions induced by high-pressure turbine  12  and low pressure-turbine  14  are small. 
         [0049]    As a whole, mid-turbine frame assembly  16  brings all loads introduced into the torque box  47  by first bearing  24  and second bearing  40  in all coordinate directions into equilibrium. Actuation plates  50   a ,  50   b  equilibrate the horizontal and off-axis loads while oleo struts  52   a ,  52   b  equilibrate the vertical loads. For example, piezoelectric actuators  63   a ,  63   b  in forward actuation plate  50   a  create strain actuation equal and opposite in direction to the horizontal and off-axis loads introduced by high spool shaft  22 . In this way, forward actuation plate  50   a  equilibrates the horizontal and off-axis loads. Aft actuation plate  50   b  works in a similar manner. 
         [0050]    Additionally, the actuation of forward oleo strut  52   a  equilibrates the vertical load introduced into the torque box  47  using engine casing  42  and aft oleo strut  52   b . Piezoelectric actuators  63   a ,  63   b  in forward actuation plate  50   a  amplify the vertical loads and create actuation force F r1  which actuates forward oleo strut  52   a . The actuation of forward oleo strut  52   a  transfers the vertical load through engine casing  42  to aft oleo strut  52   b . Forward oleo strut  52   a  reacts to vertical loads only. Aft oleo strut  52   b  works in a similar manner, transferring loads to forward oleo strut  52   a  through engine casing  42 . 
         [0051]    High spool shaft  22  and low spool shaft  38  rotate at different angular speeds, and introduce different loads into mid-turbine frame  44  and torque box  47 . The load difference between high and low spool shafts  22  and  38  may cause distortion of engine casing  42 , such as ovalization. To prevent distortion of engine casing  42 , mid-turbine frame  44  and oleo struts  52   a ,  52   b  equilibrate the loads before transferring them to engine casing  42 . It is important to maintain the circular geometry of engine casing  42  to prevent rotating engine casing  42  from interfering with surrounding stationary structures. 
         [0052]    The actuation of oleo struts  52   a ,  52   b  also maintains continuity between high pressure turbine  12  and low pressure turbine  14 . Mid-turbine frame assembly  16  is located between high-pressure turbine  12  and low-pressure turbine  14 . High-pressure turbine  12  operates at a higher temperature than low-pressure turbine  14 . Therefore, high-pressure turbine  12  and low-pressure turbine  14  have different thermal growth rates. 
         [0053]    To maintain conformity between forward side  60  of mid-turbine frame assembly  16  and high-pressure turbine  12 , and between aft side  62  of mid-turbine frame assembly  16  and low-pressure turbine  14 , forward and aft sides  60  and  62  of engine casing  42  must be able to be independently adjusted. Forward oleo struts  52   a  allow the diameter on forward side  60  of engine casing  42  to be adjusted independent of the diameter of aft side  62  of engine casing  43 . Aft oleo struts  52   b  function in a similar way. Because forward side  60  of engine casing  42  is impacted by a uniform thermo-mechanical field, each forward oleo strut  52   a  is similarly actuated. Thus preventing ovalization of engine casing  42  on forward side  60 . Similarly, aft side  62  of engine casing  42  is impacted by a uniform thermo-mechanical field so that each individual aft oleo strut  52   b  is similarly actuated to prevent ovalization of engine casing  42 . Oleo struts  52   a ,  52   b  reposition engine casing  42  to minimize leaks, ensure sealing continuity between high and low pressure turbines  12  and  14 , and ensure appropriate case conformity between structures, thus minimizing thermodynamic losses and improving the efficiency of engine  10 . 
         [0054]    Forward oleo strut  52   a  and aft oleo strut  52   b  should be about parallel. However, because of the load differential between high spool shaft  22  and low spool shaft  38 , oleo struts  52   a ,  52   b  do not have the same deflection. Oleo struts  52   a ,  52   b  should be actuated so that they are maintained within about two degrees of parallel to each other. 
         [0055]    Embedded mounts  64  are embedded within dimples  54  and eliminate the need for rails on exterior surface  58  of engine casing  42 . Because embedded mounts  64  are embedded within dimples  54 , embedded mounts  64  enable localized load paths directly to engine casing  42 , and provide efficient load and stress distribution in combination with the stiffening effect of dimples  54  on engine casing  42 . Embedded mounts  64  also serve to connect engine casing  42  to mounts  46  (shown in  FIG. 1 ). Embedded mounts  64  have a height at least equal to the height of dimples  54  to ensure that embedded mounts  64  can adequately act as load transfer means to mounts  46 . One embedded mount  64  is necessary at every location where mounts  46  come in contact with engine casing  42 , which depends upon the external architecture of engine casing  42 . In one example, engine casing  42  has at least three embedded mounts  64 . In another example, every dimple  54  contains at least one embedded mount  64 . 
         [0056]    Rails are no longer necessary to ensure stiffness of engine casing  42  because engine casing  42  changes geometry based on strains developed due to thermo-mechanical effects. The actuated geometry of assembly  16  also postpones case ovalization and case buckling. The geometry changes occur uniformly along the circumference of forward side  60  and aft side  62  of engine casing  42 , ensuring circular case geometry and maintaining continuity with the adjacent high pressure turbine  12  and low pressure turbine  14 . 
         [0057]    Mid-turbine frame assembly  16  has a lightweight and cost effective structure. As explained above, the structure of mid-turbine frame assembly  16  eliminates the need for rails. Additionally, dimples  54 , together with structural struts  48  and oleo struts  52   a ,  52   b  increase the load carrying capacity of engine casing  42 , allowing the thickness of engine casing  42  to be reduced. Further, the presence of oleo struts  52   a  and  52   b  allows thickness of structural struts  48  to be reduced while maintaining the same critical buckling loads for assembly  16  because oleo struts  52   a  and  52   b  transfer a portion of the load. In one example, mid-turbine frame assembly  16  has a weight reduction of about 10% to about 12%. In another example, mid-turbine frame assembly  16  weighs about 160 pounds. The weight reduction positively impacts weight related performance metrics such as specific fuel consumption (SFC). 
         [0058]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, although the present invention has been described as having a dimpled engine casing, any engine casing design may be used in the mid-turbine frame assembly.