THE DEVELOPMENT AND TESTING OF A GAS TURBINE ENGINE FOREIGN OBJECT DAMAGE (FOD) DETECTION SYSTEM

Transcription

1 Proceedings of ASME Turbo Expo 21: Power for Land, Sea and Air GT21 June 14-18, 21, Glasgow, UK GT THE DEVELOPMENT AND TESTING OF A GAS TURBINE ENGINE FOREIGN OBJECT DAMAGE (FOD) DETECTION SYSTEM D N Cardwell, K S Chana & M T Gilboy QinetiQ Ltd Cody Technology Park Farnborough Hampshire GU14 LX ABSTRACT This paper details the development of a prototype in-flight foreign object damage (FOD) detection system through various stages, resulting in a system capable of detecting objects as small as one gram (1g) mass. The system comprises an eddy current sensor based tip timing system and acoustic emissions (AE) vibration sensors controlled through a digital signal processor (DSP). QinetiQ have developed low-cost, weight-neutral contamination-immune eddy current tip timing sensors for use in engine health management. Engine tests confirmed these sensor s potential for detecting FOD events. FOD detection algorithms were developed and implemented in a prototype DSP that was built and tested on a ground based gas turbine engine. The trials showed that the prototype DSP FOD detection system could detect dynamic FOD events at full engine speed. Further work was carried out to enhance the FOD detection system, overcoming limitations in the previous system through the implementation of enhanced algorithms and its extension to accept four eddy current sensor inputs as well as a vibration signal input from an AE sensor. An algorithm that computes engine speed from the tip timing data was also implemented to alleviate the need for a separate 1/rev signal. A number of engine trials were successfully completed in order to validate the system. The speed algorithm has been successfully validated on engine trials and comparisons with a conventional optical based 1/rev showed the DSP generated 1/rev signals to be almost identical to the conventional system. Typically, the error was in the region of.3% speed. The investigations culminated in a test series designed to ascertain the system s sensitivity to foreign object impacts. These demonstrated that the system was capable of detecting objects down to one gram (1g) mass introduced at low speed into the engine intake. INTRODUCTION With the continuous drive for modern aircraft to have lower whole life cycle costs, increased monitoring of engine components during operation is required in order to detect potential failures and to avoid unnecessary down time. High cycle fatigue has a major impact on fleet availability, safety and whole life costs and yet, today, no instrumentation is available to monitor blade vibration levels on in-service engines. Detection of unusual blade vibration levels caused by damage or foreign object impact gives additional information to inform the post flight inspection, repair and replacement process. QinetiQ have developed a low-cost, weight-neutral contamination-immune eddy current tip timing sensor for engine health management [1]. This sensor, by measuring the arrival time of individual blade tips, characterizes the unsteady vibration response, and from this determines the health of compressor blades. The sensors and system have been demonstrated on uninstalled engines and rig tests. Early tests confirmed the sensor s potential for detecting FOD events [2] and showed potential for use as part of an inflight FOD detection/health management system. FOD detection algorithms were implemented in a prototype detection system which was subsequently validated on a Spey RB engine at MoD Shoeburyness in 28 [3,4]. These trials showed that the prototype DSP FOD detection system was capable of detecting dynamic FOD events at full engine speed. 1 Copyright 21 by ASME

2 FIGURE 1: PHASE 1 PROTOTYPE DSP Further work encompassed additional development of the early FOD detection system and the use of enhanced detection algorithms. The system was extended to accept four eddy current sensor inputs to increase the likelihood of capturing a dynamic FOD event, as well as a vibration signal input from an Acoustic Emission (AE) sensor to assist in detection and as a cross-reference with data from the tip timing sensors to ensure that false alarms are avoided. The new FOD detection system has the ability to evaluate accurate spool speed from the tip timing data and incorporates a storage module for the storage of time of arrival and dynamic FOD events. A number of Spey engine trials were successfully carried out at MoD Shoeburyness throughout 28 and 29 to validate the FOD detection/reporting system. This paper covers the development of the system and of a FOD delivery system designed to allow assessment of the system s ability to detect and report various small FOD items introduced at various heights into the engine intake. FOD DETECTION SYSTEM The original FOD detection system was designed around an Explorer 16 digital signal processing (DSP) development board. The board has a dspic33f 16 bit digital signal processing CPU with 256 Kbytes of on chip flash memory and 3 Kbytes of data RAM. The development board is a single chip microprocessor optimised for high speed numeric operations. The board also has up to 85 programmable digital I/O pins, serial port output, 8 indicator LEDs and an LCD display. This allows processed data to be input and output and/or displayed on the LCD screen or signal outputs via the LEDs. An algorithm developed using previously recorded raw eddy current sensor FOD data was programmed onto the chip to allow real time FOD detection from conditioned sensor output signals. FIGURE 1 shows a photograph of the DSP with descriptions of the layout. The other integral part of the FOD detection system is the sensor and its associated signal conditioning as shown in FIGURE 2. The sensor is a single coil directional active eddy current sensor, powered from and signal conditioned in a T 3 signal conditioning box. The T 3 has both raw and triggered TTL output. The system functions in the following manner. FIGURE 2: TIP TIMING SENSOR AND T 3 SIGNAL CONDITIONING BOX 2 Copyright 21 by ASME

3 The eddy current sensor detects the blade passing; the signal is conditioned and triggered in the T 3 to remove the effect of variations in the tip gap which result in variations in signal amplitude. The triggered signal is then fed as a TTL output into the DSP. Differences between the ideal blade arrival times and the actual blade arrival times are found, and the corresponding blade tip deflections are calculated. The blade deflection level is displayed via an LED strip, with higher deflections resulting in more LEDs being lit. The data can also be streamed via a serial port to a computer for storage and further detailed processing after the event. In addition the DSP computes a speed signal from the tip timing data which can be output from the DSP. give an indication of the calculated blade deflections. The configuration was such that 5-6 LEDs illuminated to represent the baseline deflection level at top speed. All 8 LEDs illuminated corresponds to the maximum blade deflection expected during a FOD event. After the FOD event, when the instantaneous blade deflection level has returned to the background level, the highest light illuminated during the FOD event remains illuminated to show the maximum blade deflection caused by the FOD impact. The prototype DSP was tested during a series of ground based engine trials using a Spey RB jet engine. FIGURE 3 shows the tip timing sensors fitted into the Spey engine fan casing and FIGURE 4 the circumferential sensor positioning. A number of FOD items were released into the engine, and the DSP system was shown to capture FOD events successfully at full speed. FIGURE 5 shows a sequence recorded using a video camera of the LEDs illuminating during a FOD event in real time, along with the corresponding blade deflection plot generated subsequent to the test. FIGURE 3: TIP TIMING SENSORS FITTED IN THE SPEY FAN CASING FIGURE 5: Blade deflection plot, with sequenced photos of the prototype DSP LEDs illuminating to indicate a FOD event -1 FIGURE 4: SENSOR CIRCUMFERENTIAL LOCATION FORWARD LOOKING AFT Full-speed engine data from previous tests were used to scale the data for both base level and FOD impact deflections. The number of LEDs illuminated on the DSP was configured to Although the dynamic FOD events were successfully detected and reported during the trials, a number of limitations with the original prototype DSP design were identified. Scale factors for baseline and FOD impact blade deflections were not set optimally resulting in the baseline deflection levels being relatively high. It is likely that deflection 3 Copyright 21 by ASME

4 Traffic lights for FOD indication (blade deflection and vibration) LCD display Eddy current sensor health indicator lights Real-time bar chart display of blade deflection and vibration levels Four eddy current sensor inputs Serial port for streaming data to a computer or other storage device Power 15-3V DC Acoustic Emission sensor input Controls to configure traffic light trigger thresholds, blade number and working range FIGURE 6: SECOND GENERATION (PHASE 2) DSP UNIT levels increased due to deterioration of the first stage rotor resulting from previous FOD testing on the engine. In addition recovery times for the system to return to baseline deflection levels following a FOD event or after a reset were excessive, precluding the ability to detect multiple or closely sequential FOD events. Another significant limitation was the single channel input, which might lead to the possibility of missing some dynamic FOD events, as on occasion, the effect only lasts for a fraction of a revolution. DSP PHASE 2 In order to address the limitations with the original DSP a number of enhancements and extensions were implemented into a second generation (Phase 2) DSP. The new DSP was designed to accept four eddy current sensor inputs and an Acoustic Emission (AE) sensor input. The vibration sensor input being added to act as a cross-reference to the tip timing inputs in order to avoid false alarms. The architecture of the DSP was extended to include five separate processors, one to process each of the eddy current sensor inputs and the fifth to process the vibration input and the user inputs and front panel display. Each eddy current sensor input has a sensor health indicator LED to show if the sensor is functioning. The new DSP also features LED bar charts to represent the instantaneous blade deflection and vibration levels measured by the sensors and two sets of traffic lights, to indicate when levels have exceeded user set alarm conditions. A refined speed algorithm was also implemented in the phase 2 DSP. Time of arrival and vibration data, along with front panel and alarm data can be streamed to a solid state data storage device via a serial output for subsequent detailed data processing, alleviating the requirement for a conventional blade tip timing system (BTT). FIGURE 6 shows a photograph of the completed Phase 2 DSP showing the front of the FOD detection DSP including the indicators and setting switches. DSP SPEED SIGNAL Most tip timing systems require a separate once-per-rev signal, to reference the blade arrival time for each revolution of the spool. The Spey engine does not have a speed signal suitable as a tip timing reference; therefore, an accurate 1/rev was fitted by attaching a reflective target to the balance plate on the front of the fan disk. A signal was obtained via an optical laser probe. However, this method, while giving an accurate 1/rev, suffered from problems associated with contamination of the optical probe. Analysis of existing data suggested that extracting reliable engine speed data from tip timing data was possible, extending the capability of the casing-mounted tip timing system could then reduce the complexity of access to the core and thus improve the maintainability of the standard engine speed sensing system, whilst making the blade health management system more independent. Work was carried out at QinetiQ to develop a code to compute an artificial 1/rev and a speed algorithm was implemented in the DSP FOD detection system. The speed signal from the DSP generated 1/rev was highly consistent and stable. During the validation tests, FOD events were successfully detected by the DSP without the need for a 4 Copyright 21 by ASME

5 separate 1/rev input. If the accuracy of the simulated speed signal can be shown to be comparable to that of a conventional 1/rev, this may also be used to calculate spool speed for use by other engine systems. The main disadvantage of the DSP calculated speed relative to a shaft mounted 1/rev is that the DSP speed does not lock the blade position and therefore individual blades must be identified from their position in the blade stack. FIGURE 7: DSP AND 1/REV SPEED COMPARISON Validation of the speed output was carried out during engine trials. Measurements and comparisons were carried out over complete acceleration and deceleration cycles at various rates of acceleration to assess how well the system operated over the entire engine speed range. Analysis showed that the computed speed signal compared well to the measured 1/rev, including regions of rapid change, indicating that the DSP speed signal can be used as a reliable reference in most situations, including FOD detection. The maximum error for the DSP generated speed signal was.4% of the 1/rev speed, which occurred in areas of maximum acceleration. In areas of lower acceleration or constant speed, the error was typically around.3%. FIGURE 7 shows a plot comparing the DSP generated speed signal and the balance plate mounted 1/rev. VALIDATION OF PHASE 2 DSP AND FOD CALIBRATION Enhancements to the DSP made during Phase 2 of the program were validated during a series of engine trials in late 28 and throughout 29. Initially the items used to simulate FOD were bundles of crayons. These were chosen as they cause an impact on the blades similar to that caused by items that are representative of runway FOD such as stones; however, the crayons do not cause permanent damage to the blades, making them suitable for testing. The traffic light triggering criteria were set such that the expected blade deflections caused by the introduction of FOD would trigger a FOD event on the DSP, while being at a level that would prevent spurious triggering. Trigger levels were also set for the AE sensor input in a similar manner. The ability of the DSP system to detect FOD impacts was successfully demonstrated during the engine trials. Once it had been shown that FOD impact damage was detectable using eddy current and AE sensors mounted on the casing, it was decided to expand the test matrix in order to determine if it was possible to assess the ability of the four channel DSP to detect foreign object impacts of various sizes ingested into the engine at different radial heights. The Spey engine to be used for these tests had titanium first stage fan blades with snubbers. For the trial, the snubbers were removed so as to allow the blades a more free movement and make them more representative of modern blade designs. It was decided therefore that prior to the tests an analysis of the stress distribution of the Spey engine first stage fan blade was to be performed. The stress and vibration analysis was carried out to confirm that the removal of the snubbers would not adversely affect the structural integrity of the blade, to understand the various vibration modes of the fan blade during operation and to identify locations of maximum stress to assist in determining the FOD ingestion locations. 52 MPa FIGURE 8: PREDICTED STRESS CONTOURS ON THE SPEY FAN BLADE 5 Copyright 21 by ASME

6 The Stress analysis was conducted taking into account both the centrifugal and pressure loads. This required aerodynamic analysis of the blades to provide boundary conditions for finite element (FE) analysis. These aerodynamic calculations were performed using the QinetiQ TRANSCode program (CFD). The stress analysis was performed using the commercial tool, Abaqus. The blade was meshed with partial snubbers to model as accurately as possible the actual blades. Preliminary analyses were performed for the static case, at 9,2 rpm (with centrifugal loading but not pressure loading) and at 8, rpm (with centrifugal loading and with and without pressure loading). The vibration frequencies and modes were extracted and the stress patterns examined. The results of the analysis indicated that the blade structural integrity would not be compromised to an unsafe level. FIGURE 8 shows the predicted stress contours on the blade surface and the location of the maximum stress. In order to increase the test matrix for the assessment of the sensitivity of the system to FOD impacts, it was necessary to design a delivery system capable of releasing the FOD items into the intake of the engine at the desired locations. To keep the test matrix simple and achievable within the available test slot, three ingestion locations were chosen. These were at 1%, 5% and 9% radial height on the first stage fan. The delivery system was constructed from box section steel, with a non structural aerodynamic wing on the leading edge to minimize the disruption to the air flow. A box section chute attached to the bar at the chosen ingestion location caught and deflected the FOD item into the front of the engine at the desired height. Up to 19 FOD items were stored in a remotely controlled rotatable magazine which was able to drop FOD items into the delivery bar and from there into the engine just upstream of the fan inlet guide vanes. FIGURE 9 shows two views of the delivery bar mounted in the engine intake duct. FIGURE 9: FOD ITEM RELEASE BAR die. Tin fishing shot of 1.6g and.8g was also used. Examples of the FOD items and the die used to make the plastic wax spheres are shown in FIGURE 1. During testing, approximately 2 items were released into the engine, the vast majority while at full engine speed. FIGURE 1: FOD ITEMS USED DURING THE TESTS RESULTS In order to verify the FOD strikes and eliminate as far as possible spurious triggers of the FOD detection system, the time of arrival data were recorded on a standard tip timing system in addition to being fed to the DSP modules and recorded from these on to solid state storage modules. The response of the DSP modules was also recorded using a standard digital video camera and approximately one third of the actual FOD drops were recorded using high speed digital video. The high speed video camera was mounted outside the engine intake duct and the ingestion of the FOD was viewed through a Perspex window mounted in the side of the duct. The camera was a Phantom high speed video camera and the images were recorded in black and white at a rate of 7259 frames per second. For a particular FOD drop, data from all sources was cross referenced to eliminate any spurious triggers on the FOD detection system. Although the alarm levels were set to avoid any spurious triggers, in approximately 2% of recorded fod strikes it was not possible to cross reference the strike with other recorded data and so these FOD strikes were eliminated from the results as potentially spurious. The items used to simulate FOD were selected and designed to provide consistent results for a particular mass/material and to be non damaging. The items were made from two materials and five different masses ranging from.8g to 4.g. The materials used were a prototyping plastic wax with a density of 94kg/m 3 and tin (density 734kg/m 3 ). The plastic wax was molded into spheres of 4.g, 2.g and 1.g using a 6 Copyright 21 by ASME

7 4 QinetiQ-Esterline Aerospace Standard sensors 4 QinetiQ-Monitran Industrial Standard sensors FIGURE 11: DSP FOD STRIKE DETECTION FIGURE 11 shows a single frame from the low speed video of the DSP modules at the moment of the FOD strike detection. In this case, the item is a 4.g plastic sphere released at 5% radial height. Each module is fed data from four separate sensors mounted over the first stage fan. Those fed to the module on the left are QinetiQ/Esterline * prototype aerospace standard sensors and those fed to the module on the right QinetiQ/Monitran industrial standard production sensors. FIGURE 12 shows data recorded on the standard blade tip timing system for the same FOD drop. The plot shows the rev to rev deflection for each blade and the FOD strike is clearly visible on blade number 8, highlighted in red on the plot. FIGURE 13 shows three single frames following the progression of the FOD item from its emergence from the delivery chute to the moment of impact for the same FOD drop. The ball maintains its release radial height while travelling towards the fan. In this case the ball does not impact the inlet guide vane (IGV) nor is it substantially deflected circumferentially by the air flow. This was the case for the vast majority of the drops; however some were deflected after hitting the IGV and for releases at the casing the FOD item was often deflected circumferentially by the inlet airflow. Although two DSP modules were used during the tests, the results presented below are from only one module taking signals from four tip timing sensors and one vibration sensor. They are not an amalgamation of the data from both modules. FIGURE 12: FOD STRIKE RECORDED IN STANDARD TIP TIMING DATA FIGURE 14 shows the results for different FOD items dropped at the 1% radial height. The upper plot represents the strikes as a percentage, while the lower shows the actual number of items dropped. Two levels of alarm were set in the DSP module, amber and red. The amber alarm was set to trigger on deflections approximately 1% above the background level deflection, high enough to avoid spurious triggering, while the red alarm was set to trigger on deflections approximately 6% above the background level deflection. * Esterline Advanced Sensors, Farnborough UK Monitran Technology Ltd, Penn, Bucks UK 7 Copyright 21 by ASME

8 1% 8% 6% 4% 2% 25 % 4. g P 2. g P 1. g P 1.6 g T.8 g T Number amber Number red 5 4. g P 2. g P 1. g P 1.6 g T.8 g T FIGURE 14: DETECTION RATE FOR OBJECTS RELEASED AT THE HUB BY PERCENTAGE AND NUMBER OF OBJECTS FIGURE 13: HIGH SPEED FOOTAGE OF FOD INGESTION The plot shows that approximately 4% of the 4.g plastic spheres triggered red alarms against 15% of the 2.g spheres, however, the percentage of total detections is approximately the same at around 65%. None of the 1.g plastic spheres were detected at the hub, although only a small number (4) were dropped. The percentage detection for the 1.6g tin shot was 1% with nearly 8% recording red alarms. Due to time pressures no.8g tin shot were dropped at the hub. It is highly likely that the lighter fod items, especially the 1.g plastic spheres, are influenced to a large extent by the air flow and so are able to travel through the blade passage without seriously impacting the blade surface. That the 1.6g tin shot were all detected at the hub reinforces this idea as the weight to surface area ratio is much greater than for the plastic spheres and therefore the tin shot is not influenced by the airflow to the same extent. FIGURE 15 shows the same plot for items released at mid height. A similar pattern emerges as at the hub. Considering the plastic spheres, approximately 6% of the 4.g balls trigger red alarms, while only 2% of the 2.g and none of the 1.g balls trigger red alarms. The total number of detections also falls off from 8% to 6% to 4%. This is to be expected as the mass of the objects is halved each time. Viewing the result for the tin shot, it is noticeable that the detection rate rises again to close to 8%, with 3% of the 1.6g tin shot triggering red. None of the.8g tin shot triggered red alarms; however the overall detection rate remains high at nearly 7%. 8 Copyright 21 by ASME

9 1% 9% 8% 7% 6% 5% 4% 3% 2% 1% % 2 4. g P 2. g P 1. g P 1.6 g T.8 g T schematic of the intended flight hardware. Eddy current sensors mounted over the engine first stage fan are powered and conditioned by the driving electronics. These send the conditioned signals to the DSP FOD detector, in which the signals are processed to time of arrival and checked aginst the alarm parameters. The time of arrival data, configuration and alarm data are subsequently saved onto solid state storage devices for later examination. Design modifications to the engine and aircraft for the sensor mounting and cable runs are currently being undertaken by Rolls-Royce and QinetiQ Boscombe Down. 18 1% g P 2. g P 1. g P 1.6 g T.8 g T Number amber Number red FIGURE 15: DETECTION RATE FOR OBJECTS RELEASED AT MID HEIGHT BY PERCENTAGE AND NUMBER OF OBJECTS FIGURE 16 shows data for releases at the casing. The results show a very similar pattern to that found at mid height, although the percentage of red alarms is lower in all cases where they occur (4.g and 2.g plastic and 1.6g tin). The overall detection rate is similar for the 4.g and 2.g plastic spheres, but for 1.g plastic and for both 1.6g and.8g tin shot the overall detection rate is lower. FIGURE 17 shows the distribution of detected impacts for the 4.g plastic spheres. The percentage of impacts triggering a red alarm is highest at mid height at almost 6%, falling to 4% at the hub and casing. The overall detection rate is high, with over 8% of drops being detected at mid height and casing, falling to 6% at the hub. For the 2.g spheres, FIGURE 18, the pattern is similar with the greatest number of red detections at mid height, although the percentages are lower, approximately one third. FIGURE 19 to FIGURE 21 show the plots for the rest of the FOD items. It is interesting to note that the detection rate for the 1.6g tin shot, FIGURE 2, is almost as high overall as the 4.g plastic balls, although in this case, detections are higher at the hub. This result is likely to be linked to the increased density and hardness of the tin shot % 8% 7% 6% 5% 4% 3% 2% 1% % 4. g P 2. g P 1. g P 1.6 g T.8 g T 4. g P 2. g P 1. g P 1.6 g T.8 g T Number amber Number red FIGURE 16: DETECTION RATE FOR OBJECTS RELEASED AT THE CASING BY PERCENTAGE AND NUMBER OF OBJECTS FUTURE WORK The FOD detection system has now reached the point in its development where it is ready for preliminary flight trial testing. A flight trial is planned and due to take place in 21 with tip timing sensors mounted over the first stage fan of an RB199 engine fitted to a Tornado. FIGURE 22 shows a 9 Copyright 21 by ASME

10 1% 1% 9% 8% 8% 7% 6% 4% 6% 5% 4% 3% 2% 2% 1% % % FIGURE 17: DETECTION RATE FOR 4.g PLASTIC OBJECTS BY PERCENTAGE AND NUMBER OF OBJECTS FIGURE 19: DETECTION RATE FOR 1.g PLASTIC OBJECTS BY PERCENTAGE AND NUMBER OF OBJECTS 1% 9% 8% 7% 1% 9% 8% 7% 6% 5% 4% 6% 5% 4% 3% 3% 2% 2% 1% 1% % % FIGURE 18: DETECTION RATE FOR 2.g PLASTIC OBJECTS BY PERCENTAGE AND NUMBER OF OBJECTS 2 FIGURE 2: DETECTION RATE FOR 1.6g TIN OBJECTS BY PERCENTAGE AND NUMBER OF OBJECTS 1 Copyright 21 by ASME

11 1% 9% 8% 7% 6% 5% 4% 3% 2% 1% % FIGURE 21: DETECTION RATE FOR.8g TIN OBJECTS BY PERCENTAGE AND NUMBER OF OBJECTS Data storage SSM, 4 off CONCLUSIONS QinetiQ have successfully developed a prototype FOD detection system resulting in a system capable of detecting items down to less than one gram mass. The system is built around an eddy current sensor based tip timing system and acoustic emissions vibration sensors controlled through a digital signal processor. The development process has undergone two distinct phases. Firstly the development of a single channel system which displayed only the instantaneous measured blade deflection and calculated and output the spool speed. Further enhancement was carried out in the second stage of the development in order to overcome limitations in the early system. This was achieved through the implementation of enhanced algorithms and the extension of the system to accept four eddy current sensor inputs and a vibration signal input from an acoustic emissions sensor. An enhanced speed algorithm was also implemented. A number of engine trials were successfully completed throughout 28/29 to validate the system. The speed algorithm was successfully tested and comparisons with a conventional optical based 1/rev showed the DSP generated 1/rev signal to be almost identical to the conventional system. Typically, the error was in the region of.3% speed. The investigations culminated in a test series designed to ascertain the systems sensitivity to foreign object impacts. The testing was successfully carried out by introducing foreign objects into the front of the engine at three radial heights and using two different materials at five different masses ranging from.8g to 4.g. Results showed that the system was capable of detecting up to 8% of the heavier objects and a significant number of the lighter items down to less than one gram mass. These validation trials have shown that the sensitivity of the system is high and that its ability to detect these types of non-damaging foreign objects is more than sufficient for use in an aircraft foreign object damage detection and reporting system. DSP Fan Module Sensor driver and conditioning electronics TTL SIGNAL Eddy current sensor, 4-6 off Power RAW SIGNAL TIP TIMING DATA (DIGITAL SIGNAL) Power ACKNOWLEDGMENTS The Authors acknowledge gratefully the assistance of everybody involved in the work covered in this paper, and in particular the following: Al Curran, CASD/PSI UK MOD, who funded the work; Trevor Smith and Malcolm Knight of QinetiQ Shoeburyness; Donald Lyon and Huw Finney of Monitran Technology; and Esterline Advanced Sensors, Farnborough. Power FIGURE 22: TORNADO FLIGHT TRIAL SETUP REFERENCES 1. Cardwell D, Chana K and Russhard P The Use of Eddy Current Sensors for the Measurement of Rotor Blade Tip Timing Sensor Development and Engine Testing, GT , Chana K and Cardwell D The Use of Eddy Current Sensor Based Blade Tip Timing for FOD Detection, GT , Copyright 21 by ASME

12 3. Gilboy M, Cardwell D and Chana K, Initial Assessment of a Prototype Real-Time Foreign Object Damage Detection System Using Blade Tip Timing, QinetiQ Report QINETIQ/8/1689, July Gilboy M, Cardwell D and Chana K, Interim Report on Advanced Sensors for Blade Health Management (STAS_1), QinetiQ Report QINETIQ/9/848, March Copyright 21 by ASME