ENGINE THRUST WITH A STRAIN GAUGE AT A TRUNNION

Abstract: Determining the thrust of an engine such as a jet engine. Locations on an engine trunnion at which the most repeatable strain occurs during use are determined and a strain gauge is located. The determination of the location or locations of most repeatable strain involves applying different strains or different types of strain on the trunnion, constraining movement of the trunnion and determining thermal properties of the trunnion. FEM is used.

Description: ENG INE THRUST WITH A STRAIN GAUGE AT A TRUNNION The present invention relates to a force measurement system, in particular for measuring the force produced by an engine connected to a support by a trunnion or equivalent device. For decades there have been attempts to measure the thrust produced by jet engines and the like. However, all the attempts known to the applicant have failed. For example, the National Gas Turbine Establishment of the Ministry of Defence, in report number 78004 of May 1978 entitled "Guide to In-flight Thrust Measurement of Turbojets and Fan Engines," concluded that direct measurement of thrust inflight is not feasible. The guide suggested that a transducer coupled to the engine trunnions may permit force measurement on a ground level test bed. However, such measurement has only been possible on a test bed and for limited types of force. An indication of engine thrust during flight has been limited to estimates based on parameters such as fuel consumption, which can only provide a rough guide as to the state of operation of the engine. There are many disadvantages in not being able to measure thrust accurately. For example one disadvantage is that any vehicle with two or more jet engines normally has its engines producing different thrust levels, forcing correction to the direction of motion of the vehicle typically by means of steering the vehicle, which produces drag, increasing fuel consumption and reducing manoeuvrability. It is also not possible to determine with accuracy any change in performance of an engine. The present invention seeks to provide a method and apparatus for determining the thrust produced by an engine, for example a jet engine, a rocket motor or a turbo prop. According to an aspect of the present invention, there is provided a system for setting up a thrust measurement system for an engine as specified in claim 1. According to another aspect of the present invention, there is provided a method of setting up a thrust measurement system for an engine as specified in claim 7. The term frunnion as used herein is intended to cover any support or mount for an engine. According to another aspect of the present invention, there is provided a trunnion assembly for an engine as specified in claim 14. According to another aspect of the present invention, there is provided an engine management system as specified in claim 15. The engine management system may provide functions such as control of engine thrust, for example to equalise the thrust produced by a plurality of engines coupled to the system, and to measure engine performance, for example for determining engine ageing and/or malfunction. The invention also allows for improving the design of an engine, for example during development, as is described in further detail below. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an example of engine trunnion; Figure 2 is a cross-sectional view of a casing of the trunnion of Figure 1 subjected to deformation with a ball joint thereof constrained; Figure 3 is a cross-sectional view of a casing of the trunnion of Figure 1 subjected to deformation with a ball joint thereof free to rotate; Figure 4 is a view of the bending strain on the (Figure 1) trunnion's outer surface, when subjected to a load, with the ball joint thereof constrained; Figure 5 is a view of the bending strain on the (Figure 1) trunnion's outer surface, when subjected to a load, with the ball joint thereof free to rotate; Figure 6 is a view of the shear strain on the (Figure 1) trunnion's outer surface. when subjected to a load, with the ball joint thereof constrained; Figure 7 is a view of the shear strain on the (Figure 1) tmnnion's outer surface, when subjected to a load, with the ball joint thereof free to rotate; Figure 8 is a view of the shear- strain on the (Figure 1) trunnion's inner surface, when subjected to a load, with the ball joint thereof constrained; Figure 9 is a view of the shear strain on the (Figure 1) trunnion's inner surface, when subjected to a load, with the ball joint thereof free to rotate; and Figures 10a and 10b show a flow chart of a preferred method of fitting a strain transducer to a trunnion. Figures 1 to 9 illustrate an example of the preferred methodology of determining the most appropriate location for a strain gauge, that is the location which will give the most repeatable strain readings. In the preferred embodiment, the trunnion, or at least one component thereof, is subjected to a variety of stresses in different operating conditions. In the example given, it is envisaged that one joint of the trunnion may be able to rotate but might also be constrained. Therefore, the procedure involves testing the trunnion in both of these operating conditions. Similarly, it is envisaged that the trunnion may be subjected, in this example, to bending and shear strains, so the procedure considers both of these types of strain. The types of strain which are tested and the operating conditions are chosen to suit the particular application and can even vary in dependence upon the actual component or components of the πτmnion on which it is desired to place a strain gauge. With respect to this last point, this methodology allows a strain gauge to be placed on any component of the trunnion, the aim being to identify one component which produces a repeatable and measurable strain for all stress and operating conditions. It is envisaged that the most suitable component(s) will be a principal stress bearing component, although this is not necessarily the case. In the example shown in Figures 1 to 9, both inner and outer surfaces of the trunnion casing are tested and found to provide a suitable location for a strain gauge. The casing is normally a suitable component because it has a reasonable surface area for holding a strain gauge and can provide a suitable path for the leads of the strain gauge. Referring to Figure 1, the example of trunnion 10 shown is typical of the type used for securing a jet engine to a fuselage. The trunnion 10 includes a first fixed end 12 which is secured either to the engine or to the fuselage and a second rotatable end 14 which fits into a ball socket (not shown) of the other of the engine and fuselage. As will be apparent to the skilled person, such a trunnion is typically made from a plurality of components which perform various functions, all well known in the art. One or more of the components of the trunnion is specifically designed to take the load between the engine and the fuselage. Figures 2 and 3 show an example of deformation of trunnion 10 when a lateral load analogous to that produced during operation of the engine is applied between the engine and the fuselage via the trunnion 10. In Figure 2, the ball joint 14 is constrained from rotation. It has been found during experimentation that there are situations when the ball joint 14 may become constrained in this manner. Figure 3, on the other hand, shows the deformation of the trunnion when the ball joint 14 is free to rotate. Figures 4 and 5 are strain graphs showing the bending strain of the trunnion under the deformation conditions of Figures 2 and 3 respectively. In Figure 4, there is no localised area of bending strain, while in Figure 5 there can be seen two areas of bending strain on the outer surface of the trunnion 10. Therefore, this test shows that there is no location on the casing of the trunnion where the same or a similar bending strain occurs in the two operating conditions considered in this test. Figures 6 and 7, on the other hand, are strain graphs showing the shear strain at the outer surface of the casing of the trunnion under the deformation conditions of Figures 2 and 3 respectively. In Figure 6 there is an annulus of shear strain extending around the neck portion 16 of the casing which bulges towards its fixed end 12. In the condition where the ball joint is free to rotate, as depicted in Figure 7, there is also a strain annulus around the neck portion 16; however, this annulus bulges towards the ball joint 14 of the trunnion casing. A consideration of Figures 6 and 7 will show that there is no position on the outer surface of the casing where the same or a similar shear strain occurs in the two operating conditions of the trunnion which are being studied. Figures 7 and 8 show the internal wall of the casing of the trunnion 10 when subjected to a shear strain under the operating conditions shown in Figures 2 and 3 respectively. As can be seen in Figures 8 and 9, in both cases there is an oval area of strain at the neck 16 of the casing. In both cases, the level of strain is similar. Therefore, the internal surface of the casing of the trunnion 10 in this example does provide a suitable location for a strain gauge. A preferred embodiment of process for locating a strain gauge on a trunnion is shown the flow chart of Figures 10a and 10b, which uses the above-mentioned methodology. This embodiment uses computer modelling of the trunnion components to facilitate the analysis. Step 20 (Figure 10a) involves obtaining drawings of the relevant components of an existing trunnion to be analysed for use in developing the computer model. At step 22, an FE model of the load-bearing structure is set up. In this example, the analysis is restricted to the main load-bearing components, as these are likely to provide a suitable location for a strain gauge. However, as explained above, any component of the tn nion may be suitable, so this step could set up an FE model of any component which it is desired to analyse, at the same time, before or after analysis of the main load bearing components. For example, if there is a component which can easily support a strain gauge and has ready provision for the leads of the strain gauge, this component could be tested first. At step 24, a load is applied to the model(s) with different constraint conditions. Surface strains resulting from this load under the different constraint conditions are then analysed at step 26. Steps 24 and 26 are repeated if necessary for different types of load, in a manner envisaged in Figures 4 to 9. Then, at step 28, the process decides the optimal location for one or more strain gauges on the basis of the results given in step 26. At step 30 the process analyses the thermal properties of the trunnion to optimise compensation of the strain gauge positions or, in the extreme case, to determine if the location is unsuitable. The precise steps for this will be apparent to the skilled person. Step 32 determines whether it is necessary to make any mechanical modifications to the trunnion and in particular to the component(s) identified as being the most suitable to support a strain gauge. Typical modifications may be to produce a suitable housing or recess for the strain gauge itself and for its leads in order not to foul any other component of the trunnion in use. At step 34, the process in effect repeats steps 24, 26 and 30 to deteirnine whether the modifications will affect the operating characteristics of the trunnion. Steps 36 el seq. are carried out on a sample trunnion. At step 36 a trunnion is modified as determined for the model in step 32 and the strain gauge(s) fitted. The trunnion is then re-assembled in step 38 and is calibrated with its own instrumentation (for example, a strain gauge reader of conventional type). The instrumentation is calibrated at step 40 to give a nominal output. At step 42 (Figure 10b) the trunnion is fitted to its engine, which is then fitted to a vehicle, for example to a fuselage or test rig, at step 44 and a tie-down test is then performed. The engine is then ready to be operated and its thrust measured by means of the strain gauge(s). It will be apparent that in some instances more than one strain gauge may be needed to measure thrust for all the envisaged load and operating conditions. In this case, the system will indicate the appropriate locations for the gauges. The primary use of the thrust measure is well documented as desires in the art. For this puipose, it is envisaged a vehicle will be provided with an engine management system which will have as one input the reading(s) from the strain gauge(s) and as one output a reading of thrust. In addition, other inputs can include fuel consumption, engine temperature and so on. The measure of thrust can be used to provide precise control of the engine by control of fuelling to the engine and precise monitoring of the engine, for example to determine engine malfunction and the like. It is also possible to produce an indication of change in engine operating conditions over time, for example loss of efficiency by a comparison of change in thrust produced for a particular fuelling level. The engine management system can then produce, for example, an indication of need to service or replace the engine. For a vehicle which has a plurality of engines, control of the vehicle via the engines becomes possible. A first use is in balancing the thrust produced by the engines. As no two engines perform in an identical manner, at present a vehicle such as an aircraft is operated with the engines typically performing unequally. Differences in engine thrust are generally corrected by rudder adjustments, which generate drag and thus reduce the effective power of the engines. This results both in a reduced carrying capacity for the aircraft and increased fuel consumption, the latter also leading to reduced operating range. Balancing of the engines by adjusting fuelling thereto on the basis of the thrust measurements not only avoids having to make adjustments via increased drag, thereby avoiding these disadvantages, but also increases "turn-around" due to increased air speed and reduced drag. Moreover, the outputs of the engines can be adjusted in dependence upon the actual needs of the aircraft, so that, for example, increased thrust can be generated for take-off, thrust reduced for descent, differential thrust between the engines can be produced to assist in turning; all with reduced drag from reduced use of the rudders. It will be apparent that use of differential thrust between the engines and appropriate use of the rudders can produce very effective turning of an aircraft. There is another important area of use of this invention in addition to the advantages mentioned above. It is envisaged that an engine development system can be devised in which modifications to an engine can be assessed for their impact on engine performance. For example, different intake, nozzle and cowl designs can be tested for their effect on engine thrust Moreover, operational economy, service schedules and maximum thrust ratings can be determined by a prototype engine fitted with thrust sensing as provided for herein. Reheat performance and thrust performance with "Wind-On" and Wind-Off can also be determined. Craft manufacturers can also adapt their development programmes, for example direct thrust information can provide input into a "prototype" drag and assessment programme, into the "craft model" stress assessment and life expectancy and can also be used to validate and compare actual engine/craft performance against the prediction model(s) used in development. The disclosures in British patent application no. 9813744.1, from which this application claims priority, and in the abstract accompanying this application as incorporated herein by reference.

Claims: CLAIMS 1. Apparatus for setting up a thrust measurement system for an engine including means for determining the location or locations where repeatable or the most repeatable strain occurs in a trunnion supporting the engine and means for locating at least one strain gauge substantially at said location. 2. Apparatus according to claim 1, including means for subjecting the trunnion to a plurality of different stiesses, and means for determining the strain produced on the trunnion by each stress and for determining therefrom an area of common or similar strain. 3. Apparatus according to claim 2, wherein the stress subjecting means is operable to provide a plurality of different loads or different types of load. 4. Apparatus according to any preceding claim, including means operable to subject the trunnion to different constraint or other operating conditions during the determination of the location or locations where repeatable or the most repeatable strain occurs. 5. Apparatus according to any preceding claim, wherein including means to carry out the determination of the location or locations where repeatable or the most repeatable strain occurs on a model of the trunnion. 6. Apparatus according to any preceding claim, including means for establishing thermal properties of the trunnion, the determining means being operable to determine the location for the strain gauge or gauges on the basis of the location of repeatable or most repeatable strain and of the determined thermal properties. 7. A method of setting up a thrust measurement system for an engine including the steps of deteimining the location or locations where repeatable or the most repeatable strain occurs in a trunnion supporting the engine and locating at least one strain gauge substantially at said location. 8. A method according to claim 7, including subjecting the trunnion to a plurality of different stresses, determining the strain produced on the trunnion by each stress and determining therefrom an area of common or similar strain. 9. A method according to claim 8, wherein subjecting the trunnion to a plurality of different stiesses is carried out by different loads or different types of load applied to the trunnion. 10. A method according to any one of claims 7 to 9, including the step of subjecting the trunnion to different constraint or other operating conditions during the determination of the location or locations where repeatable or the most repeatable strain occurs. 11. A method according to any one of claims 7 to 10, including the step of carrying out the determination of the location or locations where repeatable or the most repeatable strain occurs on a model of the trunnion. 12. A method according to claim 11, wherein the model is a computer model. 13. A method according to any one of claims 7 to 12, including the step of establishing the thermal properties of the trunnion are determined, the location for the strain gauge or gauges being determined on the basis of the location of repeatable or most repeatable strain and of the determined thermal properties. 14. A trunnion assembly for an engine including at least one strain gauge positioned at a location where repeatable or the most repeatable strain occurs during use of the trunnion. 15. An engine management system including an engine thrust measurement system and means for controlling and/or determining the operation of one or more engines on the basis of thrust measurements obtained from the thrust measurement system. 16. An engine management system according to claim 15, including means operable to control engine thrust. 17. An engine management system according to claim 16, wherein the control means is operable substantially to equalise the thrust produced by a plurality of engines coupled to the system. 18. An engine control system according to claim 15, 16 or 17, including means operable to measure engine performance. 19. An engine control system according to claim 18, wherein the performance measuring means is operable to determine engine ageing and/or malfunction.

Inventors: KNOTT ANDREW JOHN, KNOTT, ANDREW, JOHN
Applicants: KNOTT ANDREW JOHN, SECRETARY TRADE IND BRIT, KNOTT, ANDREW, JOHN, THE SECRETARY OF STATE FOR TRADE AND INDUSTRY
Application year: 1999
Main classification: G01L5
IPC codes: F01D25, F02C7, G01L5