Patent Publication Number: US-11655725-B2

Title: Active clearance control system and method for an aircraft engine

Description:
TECHNICAL FIELD 
     The application relates generally to engines and, more particularly, to active clearance control for aircraft engines. 
     BACKGROUND OF THE ART 
     Active clearance control (ACC) systems are used to control tip clearances in aircraft engines. In most existing ACC systems, a flow cooling air is directed towards a turbine case so that an appropriate tip clearance between the turbine blades and the turbine case is obtained according to engine requirements. It however remains desirable to design such ACC systems such that an increase in engine performance and efficiency can be achieved. Therefore, improvements are needed. 
     SUMMARY 
     In one aspect, there is provided a method for controlling a tip clearance between a turbine casing and turbine blade tips of an aircraft engine. The method comprises obtaining at least one operational parameter of the aircraft engine, determining, based on the at least one operational parameter, a current value of the tip clearance and a target value of the tip clearance, computing a limiting factor to be applied to the target value of the tip clearance, applying the limiting factor to the target value of the tip clearance to obtain a tip clearance demand for the aircraft engine, and controlling a tip clearance control apparatus of the aircraft engine based on a difference between the current value of the tip clearance and the tip clearance demand. 
     In another aspect, there is provided a system for controlling a tip clearance between a turbine casing and turbine blade tips of an aircraft engine. The system comprises a processing unit and a non-transitory computer readable medium having stored thereon program code executable by the processing unit for obtaining at least one operational parameter of the aircraft engine, determining, based on the at least one operational parameter, a current value of the tip clearance and a target value of the tip clearance, computing a limiting factor to be applied to the target value of the tip clearance, applying the limiting factor to the target value of the tip clearance to obtain a tip clearance demand for the aircraft engine, and controlling a tip clearance control apparatus of the aircraft engine based on a difference between the current value of the tip clearance and the tip clearance demand. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG.  1    is a schematic cross sectional view of a gas turbine engine, in accordance with an illustrative embodiment; 
         FIG.  2    is a block diagram of the controller of  FIG.  1   , in accordance with an illustrative embodiment; 
         FIG.  3    is a block diagram of an example computing device, in accordance with an illustrative embodiment; 
         FIG.  4 A  is a flow diagram of an active clearance control method for an aircraft engine, in accordance with an illustrative embodiment; and 
         FIG.  4 B  is a flow diagram of the step of  FIG.  4 A  of controlling a tip clearance control apparatus, in accordance with an illustrative embodiment. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
       FIG.  1    schematically illustrates a turbofan gas turbine engine  100  presented as a non-limiting example and incorporating an active clearance control (ACC) system as described herein. It is understood that aspects described herein may be suitable for use in other types of gas turbine engines. Engine  100  may be of a type suitable for aircraft (e.g., subsonic flight) applications. Engine  100  may comprise a housing or annular outer case  10 , annular core case  13 , low-pressure spool  12  which can include fan  14 , low-pressure compressor (LPC)  16  and low-pressure turbine (LPT)  18 , and high-pressure spool  20  which can include high-pressure compressor (HPC)  22  and high-pressure turbine (HPT)  24 . Low-pressure turbine  18  and high-pressure turbine  24  may be part of a multistage turbine section  23  of gas turbine engine  100 . Similarly, low-pressure compressor  16  and high-pressure compressor  22  may be part of a multistage compressor section  27  of gas turbine engine  100 . Annular core case  13  may surround low-pressure spool  12  and high-pressure spool  20 , and may define core gas path  25  extending therethrough. Combustor  26  may be provided in core gas path  25 . Annular bypass air duct  28  may be defined radially between annular outer case  10  and annular core case  13  for directing a bypass air flow driven by fan  14 , to pass therethrough and to be discharged to the ambient environment at an aft portion of engine  100  to produce thrust. 
     Gas turbine engine  100  may comprise an ACC system  30 . In one embodiment, the ACC system  30  is configured to control a clearance or gap (also referred to herein as “tip clearance”) between the tips of rotating blades (not shown) of the high-pressure turbine  24  and an inner diameter of turbine case  40 . During engine operation, thermal and mechanical radial deflections of the engine&#39;s components cause the tip clearance to deviate from the assembly clearance built into the engine  100 . The ACC system  30  is used to maintain minimal clearance while avoiding running the turbine blades into the turbine case  40  (a condition referred to as “rubbing” or “rubs”) over the entire flight cycle. In one embodiment, the ACC system  30  controls the tip clearance thermally by distributing relatively cool clearance control fluid to the radially outer surface (not shown) of turbine case  40 . The clearance control fluid, which may come from engine bleed sources (e.g. bleed air extracted from a compressor section of the engine  100 ), causes the turbine case  40  to displace radially inwards towards the blade tips of the high-pressure turbine  24  (i.e. to shrink or contract). The tip clearance between the inner diameter of the turbine case  40  and the turbine blade tips is thus lowered. This in turn reduces the amount of combustion gases that escape around the blade tips, thereby increasing efficiency and fuel economy of the engine  100 . By controlling the amount of clearance control fluid that is distributed to the turbine case  40  (i.e. by supplying more or less clearance control fluid thereto), the ACC system  30  can lower (i.e. close) or increase (i.e. open) the tip clearance as desired, depending on flight conditions. 
     In one embodiment, the ACC system  30  may be deactivated when a controller  38  of engine  100  senses that the engine  100  is undergoing sudden transient operation (e.g., fast deceleration or acceleration). In this manner, the high-pressure turbine  24  may be protected from rubs. As such, the ACC system  30  may be used mostly during long cruise segments where the engine  100  is most stable. 
     In one embodiment, the ACC system  30  may comprise a transfer conduit  32  in fluid communication with core gas path  25  at a location, for example, of a compressor section  27  of engine  100 . In some embodiments, the location can correspond to an axial location of a compressor boost stage of engine  100 . In some embodiments, the location can correspond to an axial location of low-pressure compressor  16 . In some embodiments, the location can correspond to an axial location downstream of low-pressure compressor  16 . In some embodiments, the location can correspond to an axial location of high-pressure compressor  22 . In some embodiments, the location can correspond to an axial location upstream of high-pressure compressor  22 . In some embodiments, the location can correspond to an intermediate pressure location within the compressor section of engine  100  such as, for example, an axial location between low-pressure compressor  16  and high-pressure compressor  22 . Accordingly, transfer conduit  32  may be configured to receive bleed air from the compressor section  27  of engine  100 . 
     It is understood that transfer conduit  32  may be coupled to receive clearance control fluid (e.g., compressor bleed air) from one or more different sources depending on the temperature and flow requirements to achieve the desired tip clearance control. For example, in some embodiments, transfer conduit  32  may be configured to receive bypass air from bypass duct  28 . In some embodiments, transfer duct  32  may be configured to receive a mixture of bypass air and pressurized bleed air extracted from compressor section  27  to produce clearance control fluid of a desired temperature and flow rate. 
     ACC system  30  may comprise one or more tip clearance control apparatus (referred hereinafter in the singular) including, in one embodiment, a flow regulator  34  in fluid communication with the turbine case  40  via one or more manifolds  36  (referred hereinafter in the singular). The flow regulator  34  is configured to control the flow of clearance control fluid (e.g., compressor bleed air) from transfer conduit  32  to the manifold  36 , to in turn control the flow of clearance control fluid towards the turbine case  40  for controlling a radial displacement thereof. In one embodiment, the flow regulator  34  is a valve (also referred to herein as a “clearance control valve”). Flow regulator  34  may be actively controllable via controller  38  of engine  100 , such as an electronic engine controller (EEC) for example. More specifically, the flow regulator  34  is configured to be actuated between at least one open position and at least one closed position in order to control the amount of clearance control fluid that is distributed to the turbine case  40  for adjusting the tip clearance. For example, when the flow regulator  34  is opened, the flow of clearance control fluid causes a decrease in the tip clearance. The reduction in (or closing of) the tip clearance may be desirable when the engine is decelerated (e.g., during landing approach), which results in a rapid increase in the tip clearance due to thermal and mechanical radial deflections of the engine components, particularly of the high-pressure turbine  24  components and case  40 . Conversely, when the flow regulator  34  is closed, the flow of clearance control fluid causes an increase in the tip clearance. The increase in (or opening of) the tip clearance may be desirable in conditions, such as during takeoff, where the tip clearance is rapidly diminished as the speed of the engine  100  is increased. 
     It should be understood that the flow regulator  34  may be actuated via controller  38  to one or more positions. For example, the flow regulator  34  may be actuated to a fully closed position (i.e. a position in which no clearance control fluid passes through), one or more partially open positions so as to control or modulate the amount of clearance control fluid that passes through the flow regulator  34 , and a fully open position (i.e. a position in which the maximum amount of clearance control fluid possible passes through the flow regulator  34 ). 
     In some embodiments, flow regulator  34  may be configured to controllably direct, via clearance control conduit  42 , at least some of the clearance control fluid (delivered via transfer conduit  32 ) towards turbine case  40  (and manifold  36 ) of turbine section  23 . In some embodiments, the flow regulator  34  may also controllably direct at least some of the clearance control fluid being delivered via transfer conduit  32  towards bypass duct  28 . The amount of clearance control fluid directed towards turbine case  40  (and manifold  36 ) via clearance control conduit  42  is controlled by controller  38 , by way of flow regulator  34 , based on the requirements for tip clearance control. Manifold  36  may be of any suitable type and may be disposed in turbine section  23  of engine  100 . The manifold  36  may be configured to receive at least some of the clearance control fluid (provided via clearance control conduit  42 ) and to direct the clearance control fluid on an outer surface of the turbine case  40  to cause the diameter of the turbine case  40  to shrink, thereby reducing (i.e. closing) the tip clearance. 
     Although illustrated as a turbofan engine, the engine  100  may alternatively be another type of engine, for example a turboshaft engine, also generally comprising in serial flow communication a compressor section, a combustor, and a turbine section, and a fan through which ambient air is propelled. A turboprop engine may also apply. In addition, although the engine  100  is described herein for flight applications, it should be understood that other uses, such as industrial or the like, may apply. 
     Referring now to  FIG.  2    in addition to  FIG.  1   , the controller  38  used to perform active clearance control (ACC) for an aircraft engine, such as gas turbine engine  100  of  FIG.  1   , will now be described in accordance with one embodiment. As will be described further below, the controller  38  may be configured to enable so-called “optimal” or “peak” operation of the ACC system  30  in regimes where such operation of the ACC system  30  at optimal operation may be detrimental to overall engine performance. As used herein, optimal operation of the ACC system  30  refers to operating the ACC system  30  according to an ACC control schedule that maximizes the operating efficiency of the high-pressure turbine  24  by closing tip clearances past a deviation (or distortion) of the turbine case  40  from a circular cross-section, also known as “out-of-roundness”. In some embodiments, optimal operation of the ACC system  30  is achieved with the tip clearance control apparatus (e.g., the flow regulator  34 ) in a maximum open position. It should however be understood that optimal operation of the ACC system  30  may be achieved with the with the tip clearance control apparatus being brought to any other suitable position. 
     When the ACC system  30  is designed to maximize the operating efficiency of the high-pressure turbine  24 , the increased core shaft speed (N2) of the engine  100  that results from the increased HPT efficiency may cause the operation of the high-pressure compressor  22  to deviate from its peak efficiency. This is due to the fact that the high-pressure compressor  22  and the high-pressure turbine  24  are operatively coupled to the same shaft (i.e. high-pressure spool  20 ). As a result, the overall performance of the engine  100  can worsen, with an increase in inter-turbine temperature (ITT) (i.e. an ITT degradation) being exhibited. To overcome this problem, it is proposed herein to apply a limit on the tip clearance value being targeted by the ACC control schedule (also referred to herein as the “ACC control schedule target” or the “target value of the tip clearance”) in order to ensure that engine performance does not worsen as a result of application of the ACC control schedule target. This may in turn improve engine performance. 
     The controller  38  illustratively comprises an input unit  202 , a limiting factor computation unit  204 , a tip clearance demand computation unit  206 , a tip clearance controlling unit  208 , and an output unit  210 . The input unit  202  is configured to obtain one or more measurements of one or more operational parameters of the engine  100 . The operational parameter(s) being measured include, but are not limited to, one or more of ambient air pressure, ambient air temperature, engine velocity, an exhaust gas temperature, an engine inlet temperature, a compressor pressure, a compressor temperature, a shaft speed, and fuel consumption of the engine  100 . In some embodiment, the input unit  202  may derive additional parameters from other measurements acquired throughout the engine, the additional parameters including, but not limited to, engine inlet pressure, turbine pressure, mass flow, and thrust. One or more sensing devices (not shown) positioned throughout the engine  100  may be used to acquire the measurement(s) of the operational parameter(s) and provide the measurement(s) to the controller  38  using any suitable communications means. The measurement(s) (and, in some embodiments, the additional parameters derived form the measurement(s)) are then received at the input unit  202  and used by the limiting factor computation unit  204  to determine, based on the operational parameter(s), a current value of the tip clearance and a target value of the tip clearance, and compute a limiting factor to be applied to the target value of the tip clearance in order to enable the ACC system  30  to maximize the efficiency of the high-pressure turbine  24  while maintaining or improving engine performance (i.e. while limiting the engine&#39;s core shaft speed to acceptable operating conditions). 
     As will be discussed further below, the limiting factor may be computed by the limiting factor computation unit  204  as a function of a corrected speed of the engine  100 . It should however be understood that, in other embodiments, the limiting factor may be computed as a function of other suitable engine parameters. In one embodiment, these other parameters (referred to herein as operating parameters of the high-pressure compressor  22 ) may define operation of the high-pressure compressor  22  and may include, but are not limited to, the pressure in (or a pressure difference across) the engine&#39;s high-pressure compressor  22  and a corrected airflow entering the high-pressure compressor  22  (e.g., corrected by the engine&#39;s inlet temperature or pressure). For example, a pressure ratio between a pressure P3 taken at the exit of axial compressor and the entrance of the centrifugal compressor (i.e. at engine station 3, not shown) and a pressure P25 taken at engine station 2.5 (see  FIG.  1   ) may be used. In yet other embodiments, the limiting factor may be computed based on other engine parameters indicative of a performance (or deterioration) of the engine  100 , these other parameters including, but not being limited to, an ITT of the engine  100  and a fuel flow to the engine  100 . 
     In one embodiment, in order to ensure a gradual transition in the ACC control schedule (from no application of the limiting factor to full application thereof), the limiting factor computation unit  204  is configured to compute a blending factor to be applied to the target value of the tip clearance. The blending factor may be computed as follows: 
     
       
         
           
             
               
                 
                   
                     b 
                     f 
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             0 
                             , 
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                                   
                               Engine_Param 
                             
                             &lt; 
                             X 
                           
                         
                       
                       
                         
                           
                             
                               Engine_Param 
                               - 
                               X 
                             
                             
                               
                                 ( 
                                 
                                   X 
                                   + 
                                   Y 
                                 
                                 ) 
                               
                               - 
                               X 
                             
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                                   
                               X 
                             
                             ≤ 
                             Engine_Param 
                             ≤ 
                             
                               X 
                               + 
                               Y 
                             
                           
                         
                       
                       
                         
                           
                             1 
                             , 
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                                   
                               Engine_Param 
                             
                             &gt; 
                             
                               X 
                               + 
                               Y 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where b f  is the blending factor, Engine_Param is an engine parameter (e.g., corrected speed) which is related to operation of the high-pressure compressor  22  (i.e. an operating parameters of the high-pressure compressor  22 ) and/or is indicative of degradation of performance of the engine  100 , X is a first engine parameter (e.g., corrected speed) threshold, and X+Y is a second engine parameter (e.g., corrected speed) threshold. 
     The values of the first and second engine parameter thresholds may vary depending on engine configuration. In one embodiment, the values of the first and second engine parameter thresholds are determined based on engine performance simulations across the entire flight envelope of the aircraft. The first engine parameter threshold represents a value of the engine parameter, which when reached, triggers application of the limiting factor to the target value of the tip clearance. In other words, the controller  38  does not apply the limiting factor (i.e. the blending factor is set to zero (0)) when the value of the parameter of the engine  100  is below the first engine parameter threshold. The second engine parameter threshold corresponds to the engine parameter value at which optimal operation of the ACC system  30  begins to degrade the engine&#39;s performance and the ITT improvement is negligible (e.g., substantially equal to zero (0)). When the value of the engine parameter is above the second engine parameter threshold, the blending factor is fully applied to the target value of the tip clearance (i.e. the blending factor is set to one (1)). When the value of the engine parameter is within the first and second engine parameter thresholds, the blending factor is set to a value between zero (0) and one (1), the value of the blending factor being calculated linearly as a function of the engine parameter. 
     As previously noted, the limiting factor, and more specifically the blending factor, may be computed as a function of a corrected speed (Ncorr) of the engine  100  as follows: 
     
       
         
           
             
               
                 
                   
                     b 
                     f 
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             0 
                             , 
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                                   
                               Ncorr 
                             
                             &lt; 
                             
                               X 
                               ⁢ 
                                   
                               rpm 
                             
                           
                         
                       
                       
                         
                           
                             
                               Ncorr 
                               - 
                               X 
                             
                             
                               
                                 ( 
                                 
                                   X 
                                   + 
                                   Y 
                                 
                                 ) 
                               
                               - 
                               X 
                             
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                                   
                               X 
                               ⁢ 
                                   
                               rpm 
                             
                             ≤ 
                             Ncorr 
                             ≤ 
                             
                               X 
                               + 
                               
                                 Y 
                                 ⁢ 
                                     
                                 rpm 
                               
                             
                           
                         
                       
                       
                         
                           
                             1 
                             , 
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                                   
                               Ncorr 
                             
                             &gt; 
                             
                               X 
                               + 
                               
                                 Y 
                                 ⁢ 
                                     
                                 rpm 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where Ncorr is the engine&#39;s corrected speed, X is a first corrected speed threshold, and X+Y is a second corrected speed threshold. 
     It should however be understood that, in other embodiments, the blending factor may be based on the pressure ratio across the high-pressure compressor  22 , a corrected airflow entering the high-pressure compressor  22 , an ITT of the engine  100 , or a fuel flow to the engine  100 . 
     The pressure ratio across the high-pressure compressor  22  may be computed as follows: 
     
       
         
           
             
               
                 
                   
                     P 
                     ⁢ 
                     R 
                   
                   = 
                   
                     
                       P 
                       ⁢ 
                       3 
                       ⁢ 
                       Q 
                       ⁢ 
                       2 
                       ⁢ 
                       5 
                     
                     = 
                     
                       
                         P 
                         ⁢ 
                         3 
                       
                       
                         P 
                           
                         2.5 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     where PR is the pressure ratio, P3 is the total pressure at the exit of the high-pressure compressor  22  (typically at engine station 3), and P2.5 is the total pressure at the entrance of the high-pressure compressor  22  (typically at engine station 2.5). 
     The corrected airflow entering the high-pressure compressor  22  may be computed as follows: 
     
       
         
           
             
               
                 
                   Wcorr 
                   = 
                   
                     
                       W 
                       ⁢ 
                       
                         2 
                         . 
                         5 
                       
                       ⁢ 
                       
                         
                           
                             T 
                               
                             2.5 
                           
                           
                             T 
                             STD 
                           
                         
                       
                     
                     
                       
                         P 
                           
                         2.5 
                       
                       
                         P 
                         STD 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where Wcorr is the corrected airflow, W2.5 is the mass flow rate of fluid entering the high-pressure compressor 22, T2.5 and P2.5 are the total temperature and total pressure at the entrance of the high-pressure compressor  22 , respectively, and T STD  &amp; P STD  are the standard (sea level static) ambient temperature and pressure, respectively. 
     In addition, although the blending factor is described herein above as being computed linearly, it should be understood that the limiting factor computation unit  204  may be configured to compute the blending factor using any suitable approach other than a linear approach. For example, additional thresholds (other than X and X+Y described above) may be defined and curve fitting using functions including, but not limited to, higher order polynomial functions using linear regression, may then be used to obtain the blending factor. Alternatively, each threshold may be connected using a piecemeal linear function in order to compute the blending factor. 
     In one embodiment, the corrected speed is a corrected shaft speed of the engine  100 . More specifically, the engine&#39;s core shaft speed is corrected to the total temperature of the air entering the low-pressure compressor  16  at a leading edge of the fan  14 , also referred to herein as the engine&#39;s inlet temperature taken at engine station 2 (see  FIG.  1   ). The limiting factor computation unit  204  may therefore compute the corrected speed as follows: 
     
       
         
           
             
               
                 
                   Ncorr 
                   = 
                   
                     
                       N 
                       ⁢ 
                       2 
                       ⁢ 
                       R 
                       ⁢ 
                       2 
                     
                     = 
                     
                       
                         N 
                         ⁢ 
                         2 
                       
                       
                         
                           
                             T 
                             ⁢ 
                             2 
                           
                           
                             T 
                             STD 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where N2R2 is the corrected shaft speed, N2 is the engine&#39;s core shaft speed (i.e. the core shaft speed of the high-pressure compressor  22  and the high-pressure turbine  24 ), T2 is the engine&#39;s inlet total temperature taken at engine station 2 measured in Rankine, and T STD  is a standard (i.e. sea level static) air temperature. In one embodiment, the standard air temperature is 518.67 Rankine. As used herein, the term “total temperature” (e.g., of a moving fluid) refers to the temperature that would be measured if the moving fluid flow were brought to rest without any losses, as opposed to “static temperature” which refers to the temperature as if measured with the moving fluid flow. 
     In another embodiment, the corrected speed is a corrected shaft speed of the engine  100 , where the engine&#39;s core shaft speed is corrected to the total temperature of the air entering the high-pressure compressor  22 , also referred to herein as the engine&#39;s inlet temperature taken at engine station 2.5. The limiting factor computation unit  204  may therefore compute the corrected speed as follows: 
     
       
         
           
             
               
                 
                   Ncorr 
                   = 
                   
                     
                       N 
                       ⁢ 
                       2 
                       ⁢ 
                       R 
                       ⁢ 
                       25 
                     
                     = 
                     
                       
                         N 
                         ⁢ 
                         2 
                       
                       
                         
                           
                             T 
                             ⁢ 
                             2 
                             ⁢ 
                             5 
                           
                           
                             T 
                             STD 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     where N2R25 is the corrected shaft speed and T25 is the inlet temperature of the high-pressure compressor  22  taken at engine station 2.5. 
     In yet another embodiment, the corrected speed is a corrected fan speed of the engine  100 , where the engine&#39;s fan speed is corrected to the engine&#39;s inlet temperature (taken at engine station 2). The limiting factor computation unit  204  may therefore compute the corrected speed as follows: 
     
       
         
           
             
               
                 
                   Ncorr 
                   = 
                   
                     
                       N 
                       ⁢ 
                       1 
                       ⁢ 
                       R 
                       ⁢ 
                       2 
                     
                     = 
                     
                       
                         N 
                         ⁢ 
                         1 
                       
                       
                         
                           
                             T 
                             ⁢ 
                             2 
                           
                           
                             T 
                             STD 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     where N1R2 is the corrected fan speed and Ni is the engine&#39;s fan speed. 
     Once the blending factor is computed, the tip clearance demand computation unit  206  is then configured to apply the limiting factor (computed by the limiting factor computation unit  204 ) to the target value of the tip clearance in order to obtain a tip clearance demand that is output by the controller  38  and used to control the tip clearance control apparatus (e.g., to control the clearance control valve  34 ). This can be achieved by computing the tip clearance demand as follows:
 
 ACC   dmd=( 1− b   f )*ACC schedule   +b   f *(ACC schedule +ACC offset )  (8)
 
     where ACC dmd  is the tip clearance demand, ACC schedule  is the target value of the tip clearance (which may be a function of altitude, N2, etc.), and ACC offset  is an offset value that is applied in the ACC control schedule to ensure that the ACC system  30  does not cause a degradation in the engine&#39;s performance. For example, implementation of the offset as per equation (8) may involve shutting down the ACC system  30  or operating the engine  100  at partial power. The offset value may be predetermined and retrieved from a memory or other suitable storage accessible to the controller  38 . The offset value may alternatively be computed by the controller  38  as a function of parameters of the engine  100  (e.g., based on the measurement(s) of the engine&#39;s operational parameters). 
     The tip clearance controlling unit  208  is then configured to control the tip clearance control apparatus based on a difference between the current value of the tip clearance and the tip clearance demand (computed by the tip clearance demand computation unit  206 ). For this purpose, the tip clearance controlling unit  208  is configured to compare the current value of the tip clearance to the tip clearance demand. When the tip clearance controlling unit  208  determines that the current value of the tip clearance is above the tip clearance demand, the tip clearance controlling unit  208  generates at least one control signal comprising one or more instructions to cause the flow regulator or clearance control valve  34  to open for lowering (i.e. closing) the tip clearance. When the tip clearance controlling unit  208  determines that the current value of the tip clearance is below the tip clearance demand, the tip clearance controlling unit  208  generates at least one control signal to cause the clearance control valve  34  to close for increasing (i.e. opening) the tip clearance. The at least one control signal generated by the tip clearance controlling unit  208  is then sent to the output unit  210  for transmission (using any suitable communication means) to the clearance control valve  34 . 
     With reference to  FIG.  3   , an example of a computing device  300  is illustrated. For simplicity only one computing device  300  is shown but the system may include more computing devices  300  operable to exchange data. The computing devices  300  may be the same or different types of devices. The controller (reference  38  in  FIG.  1    and  FIG.  2   ) may be implemented with one or more computing devices  300 . Note that the controller  38  can be implemented as part of a full-authority digital engine controls (FADEC) or other similar device, including EEC, engine control unit (ECU), electronic propeller control, propeller control unit, and the like. In some embodiments, the controller  38  is implemented as a Flight Data Acquisition Storage and Transmission system, such as a FAST™ system. The controller  38  may be implemented in part in the FAST™ system and in part in the EEC. Other embodiments may also apply. 
     The computing device  300  comprises a processing unit  302  and a memory  304  which has stored therein computer-executable instructions  306 . The processing unit  302  may comprise any suitable devices configured to implement the method  400  described herein below with reference to  FIG.  4    such that instructions  306 , when executed by the computing device  300  or other programmable apparatus, may cause the functions/acts/steps performed as part of the method  400  as described herein to be executed. The processing unit  302  may comprise, for example, any type of general-purpose microprocessor or microcontroller, a digital signal processing (DSP) processor, a central processing unit (CPU), an integrated circuit, a field programmable gate array (FPGA), a reconfigurable processor, other suitably programmed or programmable logic circuits, or any combination thereof. 
     The memory  304  may comprise any suitable known or other machine-readable storage medium. The memory  304  may comprise non-transitory computer readable storage medium, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. The memory  304  may include a suitable combination of any type of computer memory that is located either internally or externally to device, for example random-access memory (RAM), read-only memory (ROM), compact disc read-only memory (CDROM), electro-optical memory, magneto-optical memory, erasable programmable read-only memory (EPROM), and electrically-erasable programmable read-only memory (EEPROM), Ferroelectric RAM (FRAM) or the like. Memory  304  may comprise any storage means (e.g., devices) suitable for retrievably storing machine-readable instructions  306  executable by processing unit  302 . 
     The methods and systems for active clearance control described herein may be implemented in a high level procedural or object oriented programming or scripting language, or a combination thereof, to communicate with or assist in the operation of a computer system, for example the computing device  300 . Alternatively, the methods and systems for active clearance control may be implemented in assembly or machine language. The language may be a compiled or interpreted language. Program code for implementing the methods and systems for active clearance control may be stored on a storage media or a device, for example a ROM, a magnetic disk, an optical disc, a flash drive, or any other suitable storage media or device. The program code may be readable by a general or special-purpose programmable computer for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. Embodiments of the methods and systems for active clearance control may also be considered to be implemented by way of a non-transitory computer-readable storage medium having a computer program stored thereon. The computer program may comprise computer-readable instructions which cause a computer, or more specifically the processing unit  302  of the computing device  300 , to operate in a specific and predefined manner to perform the functions described herein, for example those described in the method  400 . 
     Computer-executable instructions may be in many forms, including program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. Typically the functionality of the program modules may be combined or distributed as desired in various embodiments. 
     Referring now to  FIG.  4 A  and  FIG.  4 B , an active clearance control method  400  for an aircraft engine, such as gas turbine engine  100  of  FIG.  1   , will now be described in accordance with one embodiment. The method  400  comprises obtaining, at step  402 , at least one operational parameter of the aircraft engine. As described herein above with reference to  FIG.  2   , step  402  illustratively comprises obtaining operating engine parameter(s) acquired using one or more sensing devices associated with the aircraft engine. The next step  404  involves determining, based on the operational parameter(s), a current value of the tip clearance and a target value of the tip clearance. The next step  406  involves computing a limiting factor to be applied to the target value of the tip clearance in order to enable optimal operation of the ACC control system (reference  30  in  FIG.  1   ) and maximize HPT efficiency while improving engine performance, as discussed herein above. Step  408  then involves applying the limiting factor to the target value of the tip clearance to obtain a tip clearance demand. In one embodiment, steps  406  and  408  involve computing one or more of equations (1) to (8) described herein above. The method  400  then flows to step  410  which involves controlling a tip clearance control apparatus (e.g., clearance control valve described herein above with reference to  FIG.  1   ), based on a difference between the current value of the tip clearance and the tip clearance demand. 
     As illustrated in  FIG.  4 B , step  410  illustratively comprises comparing, at step  412 , the current value of the tip clearance (determined at step  404 ) to the tip clearance demand (obtained at step  408 ). The next step  414  is to assess whether the current value of the tip clearance is above the tip clearance demand. If this is the case, the clearance control valve is opened at step  416  to lower the tip clearance. Otherwise, when it is determined that the current value of the tip clearance is not above the tip clearance demand, the next step  418  is to assess whether the current value of the tip clearance is below the tip clearance demand. If this is the case, the clearance control valve is closed at step  420  to increase the tip clearance. Otherwise, if it is determined that the current value of the tip clearance is neither above nor below the tip clearance demand (meaning that the current value of the tip clearance is substantially equal to the tip clearance demand), the method  400  flows back to step  402  of obtaining at least one operational parameter of the aircraft engine. In order to open or close the clearance control valve, one or more control signals may be generated (at step  416  or  420 ) and output to the clearance control valve to cause the clearance control valve to be actuated to the open or closed position, as discussed herein above. Once the clearance control valve is opened or closed, the method  400  flows back to step  402  of obtaining at least one operational parameter of the aircraft engine. 
     The embodiments described herein are implemented by physical computer hardware, including computing devices, servers, receivers, transmitters, processors, memory, displays, and networks. The embodiments described herein provide useful physical machines and particularly configured computer hardware arrangements. The embodiments described herein are directed to electronic machines and methods implemented by electronic machines adapted for processing and transforming electromagnetic signals which represent various types of information. The embodiments described herein pervasively and integrally relate to machines, and their uses; and the embodiments described herein have no meaning or practical applicability outside their use with computer hardware, machines, and various hardware components. Substituting the physical hardware particularly configured to implement various acts for non-physical hardware, using mental steps for example, may substantially affect the way the embodiments work. Such computer hardware limitations are clearly essential elements of the embodiments described herein, and they cannot be omitted or substituted for mental means without having a material effect on the operation and structure of the embodiments described herein. The computer hardware is essential to implement the various embodiments described herein and is not merely used to perform steps expeditiously and in an efficient manner. 
     The term “connected” or “coupled to” may include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). 
     The technical solution of embodiments may be in the form of a software product. The software product may be stored in a non-volatile or non-transitory storage medium, which can be a compact disk read-only memory (CD-ROM), a USB flash disk, or a removable hard disk. The software product includes a number of instructions that enable a computer device (personal computer, server, or network device) to execute the methods provided by the embodiments. 
     The embodiments described in this document provide non-limiting examples of possible implementations of the present technology. Upon review of the present disclosure, a person of ordinary skill in the art will recognize that changes may be made to the embodiments described herein without departing from the scope of the present technology. Yet further modifications could be implemented by a person of ordinary skill in the art in view of the present disclosure, which modifications would be within the scope of the present technology.