Abstract:
A method for determining the free-field thrust of a gas turbine engine using an enclosed gas turbine engine test facility without recourse to an outdoor gas turbine engine test facility. The method includes calculating the gas turbine engine intake momentum drag and cradle drag generated and compensating for these losses. In a further embodiment, the base drag is calculated and also compensated for.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a method of determining the free-field thrust of a gas turbine engine using an enclosed gas turbine engine test facility. In particular, the present invention concerns a method for calibrating an enclosed gas turbine engine test facility to determine the free-field thrust of such an engine. Furthermore, the presents invention concerns apparatus used for calibration of an enclosed gas turbine engine test facility. 
     2. Description of Related Art 
     It is sometimes necessary to accurately measure the thrust produced by a gas turbine engine, for example during certification of an engine type or during pass-off of an individual engine. The thrust of any gas turbine engine will vary according to the ambient conditions in which it operates. It is therefore necessary to standardise the thrust produced by a gas turbine engine to a static or ‘free field’ thrust, produced on an International Standard Day. That is to say, the thrust that would be generated if the engine were operating in an undisturbed atmosphere at precisely defined temperature, pressure and density. 
     A convenient method of measuring the thrust is to use an enclosed gas turbine engine test facility. Typically, such a facility comprises an enclosure housings a thrust-measuring cradle to which the gas turbine engine is mounted. Inlets in the enclosure allow a substantially undisturbed flow of air into the engine while an exhaust outlet, also known as a detuner or augmenter, provides exit means for the hot exhaust gasses produced. 
     The enclosed test facility offers a number of benefits. Because the engine is shielded from the elements, testing can take place in consistent conditions, regardless of weather conditions. Also, careful design minimises the environmental impact of engine testing, particularly noise. However, an enclosed test facility suffers limitations. The flow of exhaust gas into the exhaust outlet, often by design, generates an airflow through the test facility. This flow, as much as three times that through the engine, protects the detuner from the exhaust gas but also generates drag. In effect a negative thrust is created that reduces the thrust of the engine by as much as ten percent. Thus the test facility must be calibrated to indicate the thrust that the engine would produce if surrounded by a static atmosphere. 
     It is common practice to calibrate an enclosed test facility by “back-to-back” tests against an open-air test facility. An open-air test facility comprises a thrust cradle to which an engine is mounted and operated, supported far enough away from external influences, such as the ground, that air around the engine remains substantially unaffected during operation. In this way, the “free field” thrust of the engine can be measured. However, the procedure is costly and time consuming as the external test facility is not as flexible as the enclosed test facility. The open-air facility is, by necessity, exposed to the elements, limiting availability. Furthermore it has a greater environmental impact and may be subject to restrictions on operating times. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems of measuring the free field thrust of a gas turbine engine using an enclosed gas turbine engine test facility, in essence providing a ‘first principles’ method for calculating a thrust correction which is used to derive the static thrust of the engine at a given operating point. Furthermore, by collecting the information over a range of operating conditions, a correction curve is produced to calibrate an indoor testbed without need to refer to an outdoor testbed. 
     According to the present invention, there is provided a method for determining the free field thrust of a gas turbine engine by use of an enclosed gas turbine engine test facility, wherein the enclosed gas turbine engine test facility comprises an enclosure having an air inlet and an exhaust outlet, there being located within the enclosure a thrust cradle having movable support means and thrust measurement means, the method including the steps of attaching the gas turbine engine to the moveable support means, operating the gas turbine propulsion engine at at least one predetermined engine operating point, measuring the thrust applied by the engine to the thrust cradle via the thrust measurement means, calculating the gas turbine engine intake momentum drag generated by airflow into the gas turbine propulsion engine intake, calculating the thrust cradle drag force generated by airflow past the moveable support means of the thrust cradle, and adding the gas turbine engine intake momentum drag and the thrust cradle drag force to the measured thrust thereby determining the free field thrust at the at least one engine operating point. 
     According to a further aspect of the present invention there is provided such a method comprising the further step of calculating the base drag generated by airflow past the gas turbine engine exhaust nozzle and adding the gas turbine engine intake momentum drag, the thrust cradle drag force and the base drag to the measured thrust thereby determining the free field thrust at the at least one engine operating point. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further detail of the invention and how it may be carried into practice will now be given with reference to the accompanying drawings in which, 
         FIG. 1  shows a cross-section of a typical enclosed gas turbine engine test facility, the thrust cradle shown being simplified to aid understanding, 
         FIG. 2  shows a cross-section of the enclosed gas turbine engine test facility shown in  FIG. 1  being used to test a turbofan engine, with forces and flows superimposed on the figure, 
         FIG. 3  shows measurement apparatus for use in carrying out the present invention, 
         FIG. 4  shows alternative measurement apparatus for use in carrying out the present invention, 
         FIG. 5  shows a perspective view of the thrust cradle from the enclosed gas turbine engine shown in  FIGS. 1 and 2 , with some features removed for clarity, 
         FIG. 6  shows a series of profiles and associated coefficients of drag (Cd), 
         FIGS. 7   a  and  7   b  show a more detailed view of the nozzle of the turbofan engine shown in  FIGS. 2 and 5 , illustrating measurement apparatus for use in carrying out the present invention, 
         FIG. 8  illustrates an alternative configuration of turbofan and 
         FIG. 9  illustrates alternative measurement apparatus for carrying out the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning now to the drawings,  FIG. 1  illustrates a typical enclosed gas turbine engine test facility also known as an indoor test cell or indoor testbed. The facility comprises a thrust cradle  2  to which a gas turbine engine engine can be mounted. The thrust cradle comprises a moveable support means  4  mounted to a static support structure  6  by flexure plates  8 . Attached between the moveable means  4  and static means  6  is a load cell  10 . 
     The thrust cradle  2  is housed within an enclosure  12  that provides a stable environment and shelter from the elements. An air inlet  14  is provided in the enclosure  12 , shaped to provide an undistorted flow of air into the enclosure  12 . An exhaust outlet  16 , also known as a detuner or augmenter, allows exhaust gas and entrained air to leave the enclosure  12 . 
       FIG. 2  shows the test facility of  FIG. 1  in use. A turbofan engine  18 , a type of gas turbine engine, is mounted to the thrust cradle  2 . Gas turbine propulsion engines are well understood and it is sufficient for the purposes of describing the present invention to explain that an airflow  17  is drawn into the turbofan  18  via an intake  20 . Within the engine (not shown) a portion of the air is compressed and combusted with fuel to produce a ‘hot’ exhaust stream while the remaining air is accelerated by a fan to produce a ‘cold’ exhaust stream. The resulting hot and cold gasses form the exhaust  22  which, in this case, is expelled via a common nozzle  24  to provide thrust  25 . 
     The turbofan  18  is substantially unaltered for operation on the testbed. However, an ‘airmeter’  26 , mounted to the engine intake  20 , is used to accurately measure the mass flow into the engine  18 . A flared intake  27 , attached forward of the airmeter  26 , minimises distortion of the airflow  17  into the engine  18 . A mesh ‘stone/debris guard’  28  is provided in front of the flared intake  27  and airmeter  26  to prevent ingestion of objects that could damage the engine  18  during operation. 
     A static pressure sensor  29  is located in the exit plane of the nozzle  24 , in the same horizontal plane as the engine  18  centreline. It is at least 60 cm from the walls of the enclosure  12 . 
     During operation of the engine  18 , air is drawn through the air inlet  14  into the enclosure  12  by the engine  18  and by the interaction of the engine nozzle  24  and the detuner  16 . The turbofan exhaust nozzle  24  and the detuner  16  operate as an ejector nozzle, generating a region of low static pressure at the detuner  16  which demands an auxiliary airflow  30  through the facility and into the outlet  16 , which it protects from the hot exhaust gas  22 . Furthermore, the flow  30  generated through the enclosure  2  prevents exhaust gas  22  from being drawn into the engine intake  20 , which would otherwise harm the engine  18 . It is good design for the airflow  30  generated by the nozzle  24 /detuner  16  to be at least equal to the airflow  17  required by the engine  18 . 
     As previously described, the airflow  17  into the engine  18  is accelerated and exhausted through the engine nozzle  24 . The thrust  25  generated applies a force to the moveable support means  4  which, restrained only by the flexure plates  8 , applies a force  32  to the load cell  10 . The force  32  applied to the load cell  10  is the measured thrust  25  of the engine. 
     A number of factors reduce the apparent measured thrust of the engine from the thrust that would be achieved in static conditions. The air flow  30  through the enclosure  12  generates three main sources of thrust reduction; Intake momentum drag  34 , cradle drag  36  and base drag  38 . 
     The present invention provides a ‘first principles’ method for calculating each of these sources of drag from parameters measured during engine operation. 
     Intake Momentum Drag 
     Intake momentum drag  34  arises because, with an enclosed testbed, the airflow  17  into the engine  18  is travelling faster than would be the case in static conditions, hence the work the engine  18  can apply to it is reduced. It typically generates 85-95% of the apparent thrust loss when using an indoor testbed. Intake momentum drag is calculated by measuring the velocity of the airflow  17 , ahead of the airmeter  26  inlet and multiplying it by the mass flow of the airflow  17  through the airmeter  26  according to the following formula 
         Equation     ⁢           ⁢   1   ⁢     :         
         Intake   ⁢           ⁢   momentum   ⁢           ⁢   drag   ⁢           ⁢     (       calculated   ⁢           ⁢   in   ⁢           ⁢   Kilonewtons     ,   KN     )       =         ω   inlet     ×     v   inlet       1000         
         Where;   W inlet  is the engine inlet mass air flow, measured in kilograms per second (kg/s)   V inlet  is the mean airflow velocity approaching the airmeter inlet, measured in meters per second (m/S)       

       FIG. 3  shows a view on A—A of FIG.  2 . Apparatus is shown which is used to measure the velocity of the airflow  17  approaching the airmeter where the airmeter  26  throat diameter  38  is less than 1.8 m. The apparatus comprises a cruciform array  40  to which are mounted five shrouded anemometers  42 , 44 , 46 , 48 , 50 . The array  40  is preferably constructed of cylindrical tubing, no more than 50 mm in diameter. The array  40  is located in a vertical plane, parallel the airmeter  26  inlet plane, positioned at a distance between two and three times the airmeter internal throat diameter  38 , upstream of the inlet. A first anemometer  42  is located at the centre of the array, on the centreline of the intake  20 . The remaining four anemometers  44 , 46 , 48 , 50  are located on a pitch circle diameter which diameter  52  is 150% the airmeter internal throat diameter  38 , coaxial with the engine centreline. The anemometers are equi-angularly spaced apart. 
       FIG. 4  shows apparatus used to calculate the approach velocity of the airflow upstream of the airmeter where the airmeter diameter is greater than 1.8 m. The apparatus is essentially the same as for  FIG. 3 , and like items carry like reference numbers, however, a second group of 4 anemometers  54 , 56 , 58 , 60  is arranged on a pitch circle diameter  62  which diameter is 75% that of the airmeter internal throat diameter  38 , again located coaxial with the intake centreline. The anemometers are equi-angularly spaced apart. 
     During calibration/operation of the testbed the airflow velocities measured by the anemometers are averaged to give a mean approach velocity in the plane of the anemometers. At the same time, the mass of the airflow  17  into the engine  18  is measured by the airmeter. The airflow  17  mass W inlet , and mean approach flow velocity v inlet  are used in equation 1 to derive the intake momentum drag, referred against a known operating condition such as engine fan speed. 
     It should be emphasised that the calculation of intake momentum drag relates to a configuration wherein the engine  18 , airmeter  26  and stone/debris guard  28  are attached to the moveable support means. If this is not the case and the stone/debris guard  28  is attached to a static structure, pressure loss through the debris guard  28  will reduce the inlet momentum drag  34  and hence increase the measured thrust of the engine  18 . If using a grounded stone/debris guard, its removal is recommended and back-to-back testing should be carried out to account for fixed debris guard effects. 
     Cradle Drag 
     Cradle drag  36  is that force exerted upon the moveable support means  4  of the thrust cradle  2  by the airflow  30  through the testbed enclosure  12 . This will be better understood if reference is made to  FIG. 5  which shows a perspective view of the moveable support means of a thrust cradle with the stone/debris guard  28  removed for clarity. The movable support means  4  comprises a number of discrete structures  64 , 66 . These include the mounting cradle  64  and test equipment  66  that is mounted to the moveable support means  4 . During an engine test, the airflow  30  through the enclosure  12  creates a drag force on each structure  64 , 66  which, combined, form the cradle drag  36 . The size of each drag force will depend, principally, on three factors; the area of the component  64 , 66  that lies normal to the airflow through the test facility, (also called the frontal blockage area, represented by the shaded area  68 ), the velocity of the airflow past the component, and the way in which the profile of the component interacts with the flow past it, quantified by a coefficient of drag (Cd). 
     The following equation is used to calculate the force exerted on a single component
 
Drag calculated in Kilonewtons ( KN )=Δ P×A×Cd   Equation 2 
         Where ΔP=the pressure loading, measured in Kilopascals (KPa)   A=the frontal blockage area, measured in meters squared (m 2 )   Cd=the coefficient of drag of the component, (non-dimensional)       

     The pressure loading is calculated from the following equation; 
         Equation      ⁢   3   ⁢     :         
         Δ   ⁢           ⁢   P     =     P   ×     [     1   -     (     1     1   +     (     6.0449   ×     10     -   6       ×     V   2       )         )       ]           
         Where P=cell static pressure, measured in Kilopascals (KPa)   V=airflow velocity, measured in meters per second (m/s)       

     The above equation is derived from Bernoulli&#39;s equation, 
         Equation      ⁢           ⁢   4   ⁢     :         
         Dynamic   ⁢           ⁢   pressure     =       1   2     ⁢   ρ   ⁢           ⁢     V   2           
         with ρ, the air density, assumed to take the value for an ISA (International Standard Atmosphere) day. This eliminates the need to measure and calculate local cell static temperatures. Any loss in accuracy is of 2 nd  order significance, estimated as ±0.01% thrust change for ±20 K change in ambient temperature. However, if preferred, Bernoulli&#39;s equation can be used.       

     The cell static pressure P, is measured by the static pressure sensor  29 . 
     The velocity, V of the airflow  30  through the enclosure  12  is measured by shrouded anemometer. In a preferred embodiment of the present invention, the flow  30  around the moveable support means is measured by up to 10 shrouded anemometers  69 , located adjacent (approximately 0.1 m away from) the frontal blockage areas  68 , evenly spread about the moveable support means  4 . It is important that the flow velocity V  30  is measured such that a good indication of the flow field about the moveable support  4  means is given. Care should be taken that the anemometers  69  are not positioned in regions of disturbed flow, ie in the wake of blockages. 
     Where the airflow velocity adjacent the support means show a velocity distribution within ±30% of mean (V mean ), then the calculation for pressure loading is performed with the mean velocity V mean  to provide a mean pressure loading ΔP mean . 
     The blockage area  68  of the moveable support means  4  comprises the airflow-facing geometrical area of all moving elements of the mounting cradle  64  and of the rigidly attached obstructions  66 . Care must be exercised when measuring the area that regions shielded by an upstream obstruction are not included in the total blockage area. The shielded region behind a component is taken to extend to a length, L wake , where
 
 L   wake (meters)=5 ×√{square root over (A     upstream     )}   Equation 5 
         Where A upstream =the blockage area of the obstructing component measured in meters squared (m 2 ).       

     The Cd of each component  64 , 66  is derived by comparing the profile of a component with the range of silhouettes given in  FIG. 6  of the illustrations. Once an appropriate Cd is selected it is multiplied by the area of the components  64 . 
     Where the flow velocity within the test cell about the cradle is within the given tolerance of +/−30%, the cradle drag is calculated according to the following adaptation of equation 2;
 
Δp mean ×A total ×Cd mean   Equation 6 
         Where A total =the total unobstructed blockage area of the moveable support means.       

     If flow within the test cell about the cradle lies outside the given tolerance of +/−30%, cradle drag is calculated by dividing the moveable support means into a number of components and summing the drag of each. An individual drag force is calculated for each component by measuring the air velocity adjacent each component, calculating the blockage area of each component and deriving the Cd of each component. The cradle drag is calculated according to the following equation: 
         Equation     ⁢           ⁢   7   ⁢     :         
         ∑   x   l     ⁢           ⁢     (     Δ   ⁢           ⁢     p   n     ×     A   n     ×     Cd   n       )         
         Where 1 to x=the number of components comprising the moveable support means  4  and with the pressure loading ΔP n  calculated for each component according to the following equation; 
       Equation   ⁢           ⁢   8   ⁢     :         
         Δ   ⁢           ⁢     P   n       =     P   ×     [     1   -     (     1     1   +     (     6.0449   ×     10     -   6       ×     V   n   2       )         )       ]           
   where V n  is the velocity adjacent the component, measured in meters per second (m/s)       

     Base Drag 
     Base drag  38  is caused by the airflow  30  through the enclosure passing over the converging nozzle  24  of the engine  18 . As the airflow  30  accelerates, the static pressure in the region falls generating a force that opposes engine thrust  25 . 
       FIG. 7  shows a more detailed view of the nozzle  24  of the engine  18  of FIG.  2 .  FIG. 7   a  shows a side view of the nozzle  24 .  FIG. 7T  shows a view on the nozzle  24  exit. 
     The nozzle  24  converges from an upstream diameter  71  to an exit diameter  72 . Three static pressure sensors, oriented perpendicular to the exterior surface of the nozzle  24 , are equispaced along the convergent nozzle  24  at the 12 o&#39;clock position: A first ‘upstream’ sensor  73  is located at the most forward part of the converging nozzle  24 , henceforth the ‘upstream plane’  74 . A second sensor  75  is located in the mid-plane  76  of the convergent nozzle  24 , equidistant between the upstream sensor  73  and the exit plane  77  of the nozzle  24 . A third ‘exit-plane’ static pressure sensor  78  is one a group of four exit plane static pressure sensors  78 , 79 , 80 , 81  circumferentially equispaced about the external surface of the nozzle  24  in the exit plane  77 , again located perpendicular to the nozzle surface  24 . These are located at the 12 o&#39;clock position, 3 o&#39;clock position, 6 o&#39;clock position and 9 o&#39;clock position. 
     In the event that this configuration is impractical, such as engines mounted from overhead pylons in the 12 o&#39;clock position (not shown) the configuration can be angularly advanced, for example by forty five degrees so that the sequence begins at the ‘half-past-one’ position. 
     Base drag is calculated from the following equation:
 
Base drag=( A   nozzle entry   −A   nozzle exit )×( P−P   s nozzle )  Equation 9 
         Where A nozzle entry =the nozzle area at the upstream end of the convergent nozzle, measured in meters squared (m 2 ),   A nozzle exit =the nozzle area at the downstream end of the convergent nozzle, measured in meters squared (m 2 ),   P=the static pressure in the test cell, measured in Kilopascals (Kpa) and   P s nozzle =the static pressure adjacent the nozzle, measured in Kilopascals (Kpa).       

     The static pressure is measured by the same static pressure sensor  29  as used in the calculation for cradle drag. The static pressure adjacent the nozzle, P s nozzle  is measured by the apparatus shown in FIG.  7 . 
     P s nozzle  is the average of the first pressure sensor  73 , the second pressure sensor  75  and the mean (P exit     mean   ) of the exit plane sensors  78 , 79 , 80 , 81  according to the following equation: 
         Equation     ⁢           ⁢   10   ⁢     :         
         P   snozzle     =         P   upstream     +     P     mid   -   nozzle       +     P     exit   mean         3         
 
     The overall calculated thrust correction is the sum of the three components, noting that on a well designed test facility, inlet momentum drag will account for approx 85-95% of this total value. In these circumstances, a typical thrust correction component for an indoor test facility is likely to be ideally in the region of 1.0%-5.0% of gross thrust. 
     In a preferred embodiment of the present invention, the engine  18  is accelerated and decelerated to produce a pair of thrust curves, a ‘long’ performance curve and a ‘short’ performance curve. The long curve consists, typically, of 13 points taken in the following order:—85% (engine speed), 91%, 97%, 100%, 94%, 88%, 80%, 75%, Idle, 40%, 50%, 60%, 70%. The short curve consists of the first 6 points of the long curve, 85%, 91%, 97%, 100%, 94%, and 88%. The engine is first stabilised at 85% engine speed for approximately 10 minutes. The long performance curve is then carried out; the engine being held at each operating point for 5 minutes before two steady-state scans are acquired of those parameters being measured. Preferably, the engine is then shut down for two hours. The short performance curve is then run again with the engine being held at each operating point for 5 minutes before two steady-state scans are acquired of those parameters being measured. 
     From the parameters measured, a total overall thrust correction is calculated for each point on the long and short performance curves. From this information, a total aerodynamic thrust correction characteristic is created for use in a performance analysis or pass-off programme. This correction is applied against a parameter such a fan speed, HP spool speed, inlet airflow, inlet flow function etc. 
     It will be understood that the above description is one example of how the present invention may be put into practices and is not intended to limit the scope of the present invention. 
     It should be emphasised that this derivation of base drag is for use where there is a common exhaust nozzle  24  for both hot and cold gas streams. In the case of turbofans with separate nozzles for the hot and cold gas streams, the base drag term will be reduced. This will be better understood if reference is made to  FIG. 8  which shows g dual nozzle turbofan. The turbofan  84  has a cold annular nozzle  86  surrounding a circular hot nozzle  88 . During operation, the cold exhaust stream  90  surrounds the hot exhaust gas  92  issuing from the hot nozzle  88 . Because the cold exhaust stream  90  shields the hot nozzle  88  from the gas flow  30  through the testbed, there is no significant base drag  38  on the hot nozzle  88  although the gas flow  30  through the testbed will impinge on the cold nozzle  86 . Because the cold nozzle is further upstream from the detuner  16  and the flow  30  velocity lower, the base drag term is significantly reduced when compared with a common nozzle  24 . It may therefore be possible, subject to analysis, to exclude the term from the overall thrust correction. Nevertheless, if the term is included, it will be necessary to consider the cold nozzle  86  only. 
     Furthermore, the apparatus used to measure flow  17  velocity upstream of the airmeter is one example of how the airflow  17  can be measured. It will be understood this is not intended to limit this to only the apparatus described, for example, where there is suitable structure (not shown) within the test cell upstream of the engine  18 , it may be preferable to mount the anemometers ( 42 .. 60 ) directly to the structure, eliminating the need for the cruciform  40 . 
     In another embodiment, shown in  FIG. 9 , the cruciform  40  is replaced with a vertically traversing boom  94  extends across the test cell  12  at a distance from the front face of the airmeter  95  of at least twice the internal diameter of the airmeter  95 . The traversing boom  94  is equipped with three or five anemometers  96  depending upon the internal diameter of the airmeter. In operation, the boom  94  is traversed vertically and measurements taken at points corresponding to the measurement points achieved by the fixed points of the cruciform apparatus shown in  FIGS. 4 and 5 . This traversing device  94  enables the measurement of a greater array of velocity points that enables full contour profile plotting of test cell airflow velocities. 
     In a further alternative, the airflow through the facility can be derived mathematically using computational fluid dynamics (CFD) (not shown) and boundary conditions from anemometers positioned about the thrust cradle and airmeter  26  (not shown). The use of the traversing boom  94  to enable full contour plotting of test cell airflow enhances the CFD process.