Mark R. Woike, James W. Roeder, Christopher E. Hughes, and Timothy J. Bencic

Glenn Research Center, Cleveland, Ohio

Testing of a Microwave Blade Tip Clearance

Sensor at the NASA Glenn Research Center

NASA/TM—2009-215589

April 2009

AIAA–2009–1452
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Mark R. Woike, James W. Roeder, Christopher E. Hughes, and Timothy J. Bencic

Glenn Research Center, Cleveland, Ohio

Testing of a Microwave Blade Tip Clearance

Sensor at the NASA Glenn Research Center

NASA/TM—2009-215589

April 2009

AIAA–2009–1452

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

47th Aerospace Sciences Meeting

sponsored by the American Institute of Aeronautics and Astronautics

Orlando, Florida, January 5–8, 2009
Acknowledgments

This work has been supported by the Fundamental Aeronautics Supersonic Cruise Effi ciency Project, the NASA Aviation 

Safety Integrated Vehicle Health Management Project, the NASA Fundamental Aeronautics Subsonic Fixed Wing Project,

and the NASA Aeronautics Test Program Offi ce. The authors would like to thank David Stark, Christopher Griffi ths,

James DeBonis, and John Lekki of the NASA Glenn Research Center, and Jon Geisheimer, Thomas Holst,

David Burgess, Scott Billington, and Michael Hafner of Vibro-Meter for their support.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5285 Port Royal Road

Springfi eld, VA 22161

Available electronically at http://gltrs.grc.nasa.gov

This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2009-215589 1

Testing of a Microwave Blade Tip Clearance Sensor 

at the NASA Glenn Research Center 

Mark R. Woike, James W. Roeder, Christopher E. Hughes, and Timothy J. Bencic 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135

Abstract 

The development of new active tip clearance control and structural health monitoring schemes in 

turbine engines and other types of rotating machinery requires sensors that are highly accurate and can 

operate in a high-temperature environment. The use of a microwave sensor to acquire blade tip clearance 

and tip timing measurements is being explored at the NASA Glenn Research Center. The microwave 

blade tip clearance sensor works on principles that are very similar to a short-range radar system. The 

sensor sends a continuous microwave signal towards a target and measures the reflected signal. The phase 

difference of the reflected signal is directly proportional to the distance between the sensor and the target 

being measured. This type of sensor is beneficial in that it has the ability to operate at extremely high 

temperatures and is unaffected by contaminants that may be present in turbine engines. The use of 

microwave sensors for this application is a new concept. Techniques on calibrating the sensors along with 

installation effects are not well quantified as they are for other sensor technologies. Developing 

calibration techniques and evaluating installation effects are essential in using these sensors to make tip 

clearance and tip timing measurements. As a means of better understanding these issues, the microwave 

sensors were used on a benchtop calibration rig, a large axial vane fan, and a turbofan. Background on the 

microwave tip clearance sensor, an overview of their calibration, and the results from their use on the 

axial vane fan and the turbofan will be presented in this paper. 

I. Introduction 

The active control and minimization of the gap between the rotating turbine blades and the stationary 

case of gas turbine engines is being investigated as a means of increasing engine efficiency, reducing fuel 

consumption, reducing emissions, and increasing engine service life (Ref. 1). In addition, the ability to 

monitor the structural health of the rotating components, especially in the hot sections of turbine engines, 

is of major interest to the aerospace community in improving engine safety and reliability. Both of these 

concepts require the development of sensors that are reliable and can operate in the harsh conditions that 

exist in a turbine engine. Under the NASA Fundamental Aeronautics and the NASA Aviation Safety 

programs, microwave sensor technology is being explored at the Glenn Research Center as a means of 

making high-temperature noncontact tip clearance and tip timing measurements for use in active 

clearance control, structural health monitoring, and other close tolerance-control applications associated 

with turbine engines and rotating machinery. 

This paper discusses testing and evaluation that has been accomplished during the last year using a 

microwave sensor system that was developed and purchased under NASA’s Small Business Innovation 

Research (SBIR) Program from Radatec, Inc. (currently Vibro-Meter, SA). Background on the 

microwave sensor system and its recent use on a calibration rig, an axial vane fan, and a NASA turbofan 

are discussed in this paper. 
NASA/TM—2009-215589 2

II. Sensor Background and System Description 

The microwave tip clearance sensor system that is currently being used at the NASA Glenn Research 

Center was developed by Radatec, Inc. (currently Vibro-Meter, SA) through NASA’s SBIR Program 

under a Phase II contract that was awarded in 2002. Work on the sensor continued from 2004 to 2005 as 

part of NASA’s Ultra-Efficient Engine Technology (UEET) Program. In late 2006, the system was 

delivered to the Glenn Research Center as part of a SBIR Phase III commercialization contract. This 

system currently has two sensor channels along with four high-temperature probes. The microwave tip 

clearance sensor system works on principles that are similar to a short-range radar system. The tip 

clearance probe is both a transmitting and receiving antenna. The sensor emits a continuous microwave 

signal and measures the signal that is reflected off the metallic target, which in this case is a rotating 

blade. The motion of the blade phase modulates the reflected signal. This reflected signal is compared to 

an internal reference signal and the phase difference directly corresponds to the distance to the blade. 

More detailed information on the sensor’s theory of operation can be found in References 2 to 5. 

The high-temperature tip clearance probe shown in Figure 1 is approximately 14 mm in diameter and 

26 mm long. It contains the transmitting and receiving antenna and is designed to be installed in the 

casing of the engine where it can measure the radial clearance between it and the turbine blade tips. The 

probe is made of high-temperature material and is designed to withstand temperatures up to 900 °C 

uncooled, 1200 °C with cooling air. The associated sensor electronics and data acquisition system for the 

microwave tip clearance system are shown in Figure 2. This hardware contains the radiofrequency (RF) 

generator, RF detector, and all of the associated hardware required to generate, measure, and condition 

the microwave signals. It also contains a rack-mounted PC that runs the data acquisition and display 

software. It is intended that the sensor electronics be located away from the engine in an environmentally 

benign area. The probes are connected to the electronics using a microwave rated co-axial cable. Cable 

distances up to 15 m can be currently supported between the probes and the sensor electronics. The data 

acquisition computer is connected to the sensor electronics through a network switch. This computer can 

be remotely located away from the sensor electronics to an area such as a control room using a CAT5E 

connection. 

The existing microwave system operates at 5.8 GHZ and with the current probes can measure 

clearance distances up to ~25 mm (i.e., one-half the radiating wavelength). This technology has an 

ultimate goal of obtaining accuracies approaching 25 μm. This is in the process of being developed in the 

next generation of probes. More on the system’s use and observed accuracies with the current probes are 

discussed in this paper. 
NASA/TM—2009-215589 3

As indicated earlier, NASA has several programs interested in the development and use of sensors 

that can make accurate tip clearance and timing measurements in the harsh environment associated with 

gas turbine engines. The microwave probe’s ability to operate at high temperature and see through 

contaminants that are present in a turbine engine is a major discriminator between it and other 

technologies. These sensors were originally being targeted for use in an active clearance control concept 

that was being developed at the Glenn Research Center(Ref. 6). In addition, there is interest in using 

these sensors to make structural health measurements by either using tip clearance to monitor blade 

growth and wear or using tip timing to monitor variations in movements between blades. Over the last 

year a series of experiments have been undertaken at the Glenn Research Center as a means to better 

understand this technology and evaluate its capabilities. The near-term goal of the project is to evaluate 

the sensors ability to make tip clearance and timing measurements on rotating machinery. The long-term 

project goal is to use these sensors in an actual aeroengine at high-temperature conditions. This is 

currently being targeted to occur in the 2009 to 2010 timeframe. 

III. Calibration Experiment 

The first experiment that was accomplished with the microwave tip clearance system was the 

calibration and evaluation of the probes using a benchtop calibration rig. Like other sensor technologies, 

measurements made using the microwave sensors were very specific to the geometry of the blade being 

measured. The sensor made an average measurement of the spot size that was cast on the blade. To obtain 

accurate and useful results, this average measurement needed to be mapped to the actual minimum 

clearance between the target blade and the sensor. This was done by calibrating the sensor against the 

blade geometry that it was intended to be used on for making tip clearance measurements. Since the use 

of microwave sensors is an emerging technology, techniques on calibration and installation effects were 

not fully quantified as they were for older technologies. The main goal of this experiment was to develop 

the infrastructure and techniques to calibrate the probes for use on experiments at the Glenn Research 

Center. In addition, the calibration rig provided a good venue to evaluate the probe’s accuracy and 

understand any effects associated with the probe’s installation. 
NASA/TM—2009-215589 4

The calibration rig setup shown in Figure 3 consists of a rotary stage and a linear stage controlled by a 

programmable motion controller. The candidate blade for the calibration was mounted on the rotary table 

where it could be operated up to 300 rpm (rotations per minute). The microwave probe was mounted on 

the linear stage where it could be moved to a desired distance from the blade tip for calibration. This 

linear table used an optical encoder with an accuracy of 4 μm for position indication. A typical calibration 

involves installing the probe and blade on the stages at a known baseline distance apart from each other 

using a precision metric gauge block. Typically, the baseline distance chosen was the largest distance for 

the calibration. Once this baseline distance was established, the setup was confirmed by moving the linear 

stage over the entire calibration range and verifying the distances between the blade and sensor using the 

precision gauge blocks at several distance settings. The metric gauge blocks used had an uncertainty of 

0.14 μm. Once the setup was confirmed the blade was operated at 200 rpm using the rotary stage, and the 

probe was moved to the desired gap distances using the linear stage and encoder. At each desired 

distance, the “measured” or uncorrected reading was recorded. This was done over the entire calibration 

range. 

The following items need to be noted when using and calibrating the microwave probes. First, the 

blade needs to be spinning in order for the microwave clearance system to detect and measure the 

clearance. The system operates on the principle of extracting the phase modulation from the reflected 

signal that is caused by the motion of the blade. To measure the clearance it needs to see a target that is 

moving. Second, the microwave probes were designed to be mounted in a metallic casing or reference 

plane. This allows the microwave field to emit properly from the face of the sensor. The current 5.8 GHZ 

sensors were designed to be recessed by 0.50 mm in an engine or fan casing. For the calibrations 

conducted in this experiment, a 265- by 425-mm metal sheet was used to simulate the fan casing for the 

probe’s installation. Third, for the calibration it is desirable to match the physical setup of the actual 

installation on the engine or test article as much as possible. This is very similar to what was done for 

other tip clearance sensor technologies. This involves matching the probe installation depth, simulating 

the engine casing as much as possible, and matching the blade angle as best that can be done. One 

additional feature that the microwave probes have is the field emitted by the probe has a polarity. This 

feature can be used as an extra adjustment in improving the quality of the signal based on the size of 

blades and overall geometry. To maximize the amount of signal that was sent out and collected from the 

blade, the face of the probe was aligned so its polarity was in line or parallel with the length of the blade 

tip that it was measuring. To minimize the amount of signal that was sent out and collected from the blade 
NASA/TM—2009-215589 5

the probe was rotated so its polarity was perpendicular to the length of the blade tip. Generally, it is 

recommended by the manufacturer that a parallel alignment be used for blades that are much thinner than 

the diameter of the probe and a perpendicular alignment be used for blades that are thicker than the 

diameter of the probe. Generally, the best response may fall somewhere between. The probe alignment is 

a tradeoff that needs to be made based on the application. As with the previous installation parameters, 

when calibrating the probes it is critical to duplicate the alignment that is used in the actual engine or test 

rig. During this calibration experiment the probe’s polarity was varied as a means of assessing and 

understanding its effect. 

For this experiment, calibration was done against two different blade geometries. The first geometry 

that was tested was a relatively thin first-stage compressor blade. This blade shown in Figure 4 is 

~105 mm long, ~6 mm thick at its widest point, and has a chord length of ~37 mm. This blade was chosen 

to see how well the probes would calibrate against a blade’s geometry that was smaller than the diameter 

of the probe. The second geometry tested was a flat box. This box, shown mounted on the rotary stage in 

Figure 3, was ~26 mm thick, ~76 mm high, and ~107 mm long. It was chosen to simulate a blade that was 

thicker than the diameter of the probes. In addition, it mimicked the thickness of the blades of an axial 

vane fan that the probes were used on to measure tip clearances. The results of the experiment on the axial 

vane fan will be discussed in section IV of this paper. 

The calibration curve for a microwave probe using the thin compressor blade is shown in Figure 5. 

The y-axis is the “measured” or uncorrected clearance reading that was acquired from the probe. The xaxis is the actual clearance. It represents the distance between the simulated fan casing, with sensor, and 

the tip of the blade. The calibration was done over a range of 0.5 to 13.0 mm. The resulting curve is a 

third-order polynomial and has fairly good sensitivity at clearances above 2 mm. At clearances lower than 

2 mm, the response flattens out and the sensor becomes less sensitive. This effect is believed to be caused 

by the near-field properties of the probe’s antenna and its interaction with the geometry of the relatively 

thin blade. The curve observed in Figure 5 was a typical response when using the probes on a thin blade 

when compared to the probe's diameter. For this calibration the sensor’s polarity was aligned to be 

parallel with the length of the blade tip to maximize the amount of energy reflected from the blade. 

Figure 6 shows the calibration curve for the same probe against the simulated thick blade. This 

calibration was undertaken as a means of capturing the necessary correction data for measuring the tip 

clearances on a large axial vane located at the NASA Glenn Research Center. As in the previous case, the 
NASA/TM—2009-215589 6

y-axis is the “measured” or uncorrected clearance reading that was acquired from the probe. The x-axis is 

the actual clearance distance between the simulated fan casing, with sensor, and the tip of the blade. The 

calibration was done over a range of 1.0 to 13.0 mm. The resulting curve is a second-order polynomial 

and has a relatively constant sensitivity over the entire range. For this calibration the sensor’s polarity was 

aligned perpendicular to the length of the blade tip. A similar shaped curve, with slightly different values, 

was observed for the calibration where the probe’s polarity was aligned parallel to the thick blade’s tip. 

The curve reported in Figure 6 was a typical response when using the probes against a blade whose 

thickness was of the same order of magnitude or greater than the diameter of the probe. As a means of 

evaluating the overall accuracy and repeatability associated with the calibrations on the thick blade, both 

the probe and blade were reset and the experiments were repeated across the entire range with the 

clearance corrections applied to the measurements. The worst case error that was observed between the 
NASA/TM—2009-215589 7

actual distance and the corrected distance measured by the sensor in these verification tests was 

~0.15 mm. It is believed that a portion of this error can be attributed to inaccuracies and biases associated 

with the calibration setup. In subsequent calibrations on a turbofan blade, improvements in the calibration 

setup resulted in errors that were much smaller. This calibration is presented in section V of this paper. In 

summary, the calibrations against the thin blade and the simulated thick blade were primarily undertaken 

as a means to better understand the probes and develop techniques for their calibration. These objectives 

were achieved and the probes were successfully calibrated against two blade geometries. 

IV. Axial Vane Fan Experiment 

The second experiment was to use a sensor to make tip clearance measurements on a large axial vane 

fan located at the NASA Glenn Research Center’s 10- by 10-Foot Supersonic Wind Tunnel (10×10 SWT) 

Facility. The purpose of this experiment was to acquire experience in using the sensors on an actual piece 

of rotating machinery. It was also intended as a means to evaluate how well the calibrations accomplished 

in the laboratory transferred into an actual use in the field. The axial vane fan that was used is shown in 

Figure 7. It is part of the 10×10 SWT's infrastructure and is located in the tunnel’s Air Dryer Building

(Ref. 7). It is one of eight fans that are used to heat, dry, and cool the wind tunnel’s alumina beds. The fan 

is ~1.8 m in diameter and has 16 blades. The blades are ~362 mm long, with a chord length of ~267 mm 

and a maximum thickness of ~26 mm at the midchord position. Provisions were made for the installation 

of two probes 90° apart in the fan’s casing at the midchord position of the blades. A calibrated microwave 

tip clearance probe was installed in one of the locations. A picture of a probe installed in the fan is shown 

in Figure 8. 

Data was acquired over several operations of the vane fan varying the configuration and setup of the 

sensors. Data was taken with the probe’s polarity aligned both parallel and perpendicular to the length of 

the blade tip in order to assess the effect of the sensor’s alignment on the measurement. In addition, data 

was taken in the synchronous, asynchronous, corrected, and uncorrected data modes as a means to better 

understand the system’s settings and sensor capabilities. Only a summary of the results of the test runs for 

this experiment will be included in this paper. 

A time history of the average and minimum clearances measured for an operation of the vane fan is 

shown in Figure 9. For this particular test run the probe was installed so its polarity was aligned 

perpendicular to the length of the blade tips. The data was acquired at a fixed sampling rate of 400 KHZ 
NASA/TM—2009-215589 8

over an 8-min period of the fan operating at a steady-state condition of 1200 rpm. The average blade tip 

clearance for the 16 blades over this time period was 2.84 mm. The minimum clearance detected over this 

period was 2.17 mm with average minimum running at 2.55 mm. These measurements were consistent 

with the expected operation of the vane fan and are in line with the clearance measurements that were 

made statically. For comparison, the measurements that were made statically at this probe’s location 

yielded a 16-blade average tip clearance of 3.33 mm and a minimum blade tip clearance of 2.87 mm. The 

approximate half a millimeter difference between the static and running clearance measurements was due 

to blades locking in their seats during fan operation. This effect is hard to duplicate when making static 

measurements. Other test runs were accomplished by varying the probe’s polarity (parallel to the blade 

tips) and acquiring data using different acquisition modes (fixed sample rate vs. synchronous sample 

rate). The results of these runs were similar in magnitude to the results shown below. However, it was 
NASA/TM—2009-215589 9

observed that for the fan’s “thick” blades, a cleaner, less noisy signal was obtained with the probe's 

polarity aligned in perpendicular to the length of the blade tip. 

Regarding the sensors’ ability to detect individual blade clearances, Figure 10 shows the individual 

clearances that were measured for one revolution of the fan. The x-axis displays the blade number and the 

y-axis displays the measured tip clearances in millimeters. For this particular revolution the measured 

clearances ranged from 2.34 to 3.37 mm, with the variation in clearances between blades being on the 

order of tenths of millimeters. As previously indicated these clearances are in line with the expected 

operation of the vane fan. 

This experiment represented the first time that the microwave tip clearance probes were used at the 

NASA Glenn Research Center to acquire tip clearance data on actual rotating machinery. While this 

experiment was more of a qualitative study, its baseline objectives were successfully accomplished. 

Experience was gained on using the microwave probes and the system was successful in acquiring tip 

clearance data on the axial vane fan. In addition, the calibrations that were accomplished in the laboratory 

on the simulated geometry transferred well into actual use in the field. One limitation of the experiment 

was that a reference sensor was not installed and used to compare the readings acquired from the 

microwave sensors. This weakness was acknowledged and there are future plans to install the microwave 

sensors in this axial vane fan or another test rig along with another type of clearance sensor (capacitive, 

eddy current, etc.) as a means of acquiring comparison data. 

V. NASA Turbofan Experiment 

The third experiment was to use the microwave sensors to obtain tip clearance and timing data on a 

NASA turbofan test article that was tested in the Glenn Research Center’s 9- by 15-Foot Low-Speed 

Wind Tunnel (9×15 LSWT) Facility (Ref. 8). The purpose of this experiment was to evaluate the sensor’s 

ability to acquire data on aeroengine size test article and blades. The turbofan has 18 blades and is shown 

in Figure 11. It is being used to develop and evaluate noise reduction concepts under NASA’s Subsonic 

Fixed Wing Program. Two probes were installed 90° apart in the turbofan’s casing at the midchord 

position of the blades. The first probe was installed at the 90° position, which is on the left-hand side of 

the casing looking downstream at the fan. The second probe was installed at the 180° position, which is 

on the bottom of the casing looking downstream at the fan. 
NASA/TM—2009-215589 10

This application was a little different than the Axial Vane Fan Experiment presented in the previous 

section of this paper. First, the blades were made out of a nonferrous composite material. The tips of these 

blades were nickel coated to allow measurement by the microwave sensors. Second, the turbofan’s casing 

was lined with a nonferrous abradable material so the fan blades would not be damaged in the event of a 

blade rub during operation. However, for the sensors to work properly they needed to be installed in a 

metallic casing to allow the microwave field to properly propagate from it. To accomplish this, the 

sensors were recessed in the nonferrous rub strip at a distance greater than normally done, in order to use 

the metallic part of the casing as the required reference plane. This also had the benefit of moving the 

sensors back to a much safer location in the event that a deep blade rub was experienced. For this 

experiment, holes were drilled all the way through the casing including the nonferrous liner in order to 

facilitate the probe installation and the collection of physical distance measurements. However, in 

hindsight, since the sensors have the ability to see through a nonferrous material, a better idea would have 

been to only drill far enough to let the probes be installed in the metallic part of the liner and to leave the 

nonferrous rub strip intact. This would have resulted in a clean and intact flow path, allowing clearance 

measurements to be made during all phases of the test program. It is planned to pursue this type of 

installation for the next test entry of the NASA turbofan. 

In order to make the tip clearance measurements on the turbofan a series of calibrations were carried 

out to map the actual clearance to the uncorrected clearance measured by the sensor. The calibration was 

done using the previously described calibration rig with an actual blade and the probes installed in a 

section of the fan casing. This setup allowed the calibration to be done in a configuration that was as close 

as possible to the actual use on the turbofan. It also allowed the offset due to the probe being recessed in 

the nonferrous rub strip to be included in the calibration. For this application the sensor’s polarity was 

aligned to be parallel with the length of the blade tip to maximize the amount of energy reflected from the 

blade. The calibrations were accomplished over a range from 1.0 to 3.0 mm between the tip of the blade 

and the fan casing. The calibration curve for one of the probes is shown in Figure 12. The y-axis is the 

“measured” or uncorrected clearance reading that was acquired from the probe. The x-axis is the actual 

clearance distance between the fan casing and the tip of the blade. The resulting curve is a second-order 

polynomial and shows good sensitivity over the range of calibration. One issue involved with the 

calibration is that the clearances on the turbofan were expected to be a little under a millimeter. Due to the 

particular geometry of this setup obtaining a repeatable calibration of distances under a millimeter 
NASA/TM—2009-215589 11

between the fan casing and blade was not possible. However, since the probes had a second-order 

response in the targeted calibration range, it was determined that accurate results could be obtained in 

projecting the curve fit for clearances that were a little under the calibrated range. This assumption held 

true in previous calibrations. As a means of evaluating the calibration and its repeatability, a verification 

test was conducted by resetting the probe and repeating the measurements across the entire range with the 

clearance correction curve being applied to the measurements. The largest difference that was observed 

between the actual clearance and the corrected clearance in this verification was ~0.05 mm. This was a 

major improvement when compared to the errors observed in previous calibrations. 

It needs to be acknowledged that these calibrations were accomplished after the probes were used on 

the turbofan. This was done because the calibrations accomplished prior to the test entry were not done 

with the same probe alignment and system settings that were used when acquiring data from the turbofan. 

In addition, prior to testing, it was noticed that one of the probes was not providing a strong signal and 

was replaced with a spare probe that had not been calibrated for this application. The clearance data 

acquired from the turbofan was reprocessed using the correction curves generated during the post 

calibration. Normally, a pretest calibration is sufficient if the settings and geometry used in the calibration 

are consistent with the actual use on the fan or engine. 

Blade tip clearance and tip timing data was acquired for several test runs of the NASA turbofan. Data 

was acquired at a variable sampling rate that was synchronized to the fan's speed. Each measurement 

consisted of two revolutions of data with 10 000 samples taken per revolution. Good clearance data was 

acquired in this experiment. Figure 13 shows the 18-blade average tip clearance measurements and the 

fan speed for one of the test runs. From these charts it is observed that the average tip clearance decreases 

as the fan speed is increased. This result is expected and is mostly due to the growth of the composite 

blades as the fan operates at higher speeds. Probe #1 measured a decrease of 0.22 mm and probe #2 

measured a decrease of 0.06 mm as the speed increased to 8875 rpm. The change in clearance observed in 

this experiment was within the range predicted for these blades. In addition, the change in tip clearances 

measured by the microwave sensors were very similar to previously observed values. In a previous test 

entry using capacitive clearance sensors, changes in tip clearances up to 0.22 mm were observed when the 

turbofan was operated over the same speed range. 
NASA/TM—2009-215589 12

An additional means of viewing the data is shown in the polar plots of Figures 14 and 15. These plots 

show the individual blade clearances measured by each of the probes for several fan speeds. As noted 

previously, these plots show that the individual blade tip clearances decrease as the fan speed is increased. 

A more noticeable change in the individual clearances can be observed in the polar plot in Figure 14 for 

probe #1. This clearly shows the decrease in the individual blade tip clearances as the fan is operated at 

higher speeds. A less noticeable change is observed in the polar plot in Figure 15 for probe #2. It still 

shows the overall trend of the tip clearance decreasing as speed is increased, but it is just not at the same 

magnitude as observed by the first probe. In reality, the small change in displacement observed by the 

probe at this location during the run is approaching the measurable limit for using this size of probe on 

this blade. However, even with this observation, these plots do show that the microwave clearance probes 

are sensitive enough and have sufficient resolution to pick up the minor variations that exist in the 

individual blade tip clearances. 

One anomaly that was observed in the data of Figure 13 is the difference in the absolute clearance 

values measured by each of the probes. The average blade tip clearance measurement made at static 

conditions for probes #1 and #2 was 0.86 and 0.91 mm, respectively. Assuming that the blades do not 

experience much growth at low speed, the clearances measured at these speeds should be close to the 

values measured statically. Using this assumption, it is observed in Figure 13 that probe #1 was reading 

~0.14 mm high and probe #2 was reading ~0.13 mm low in absolute clearance values. It is believed that 

this zero shift in measured clearances is due to dissimilarities between the probes’ installation on the 

turbofan and the installation used for the calibration. Any differences in probe alignment and installation 

depth will affect the accuracy of the measurement. Improved methods will be used in future experiments 

to better control these parameters so the installation used in the calibration more closely matches the 

installation used on the test article. It should be noted that the tip timing data acquired in this experiment 

is still being processed and evaluated. It is planned to present this data in future reports. 

In summary, this experiment has successfully demonstrated the microwave clearance probe’s ability 

to make measurements on aeroengine size hardware. The blade tip clearances measured by the probes 

were consistent with results previously observed for this turbofan. The experiment also showed that the 

probes can be used to make measurements on an application that is not typical for their designed use. The 

sensors were able to make measurements on blades made of composite materials by coating the blade tips 

with nickel. In addition, the sensor’s ability to see through the nonferrous portion of the rub strip allowed 

it to be recessed further in the fan’s casing than would have normally been done. This has a positive 

ramification for future experiments where it will be attempted to keep the nonferrous portion of the rub 
NASA/TM—2009-215589 13
NASA/TM—2009-215589 14

strip intact allowing these sensors to make tip clearance measurements on this fan over its entire range of 

testing without affecting the fan’s acoustic or aero performance. It is also planned to make improvements 

to the calibration and installation processes so the installation used in the calibration more closely 

matches the actual use on an engine or test article. 

VI. Conclusion 

The series of experiments undertaken over the last year have demonstrated the microwave tip 

clearance sensor’s ability to make reliable tip clearance measurements. The microwave probes were 

successfully used to acquire tip clearance data on an axial vane fan and a turbofan. In addition, a 

calibration rig was set up and the microwave probes were evaluated against several different blade 

geometries. Further work will be done to improve the techniques used to calibrate and install the probes 

so critical parameters such as installation depth and alignment can be more closely controlled in order to 

maximize the accuracy of the measurements. In summary, testing to date has been successful and has 

shown that this technology is viable in making blade tip clearance and timing measurements. More testing 

will be undertaken in 2009 to further evaluate the microwave tip clearance sensor’s capabilities. The 

ultimate goal of the project is to use the microwave probes on an actual aeroengine in a high-temperature 

environment. This is being planned for the 2009 to 2010 time period. 

References 

1. Lattime, Scott B.; and Steinetz, Bruce M.: Turbine Engine Clearance Control Systems: Current 

Practices and Future Directions. NASA/TM⎯2002-211794 (AIAA−2002−3790), 2002. 

2. Holst, Thomas A., et al.: Development of an Optical-Electromagnetic Model of a Microwave Blade 

Tip Sensor. AIAA−2005−4377, 2005. 

3. Geisheimer, Jonathan L., et al.: Performance Testing of a Microwave Tip Clearance Sensor. 

AIAA−2005−3987, 2005. 

4. Geisheimer, Jonathan L.; Billington, Scott A; and Burgess, David W.: A Microwave Blade Tip 

Clearance Sensor for Active Clearance Control Applications. AIAA−2004−3720, 2004. 

5. Holst, Thomas Arthur: Analysis of Spatial Filtering in Phase-Based Microwave Measurements of 

Turbine Blade Tips. Master’s Thesis, Georgia Institute of Technology, May 2005. 

6. Lattime, Scott B.; Steinetz, Bruce M.; and Robbie, Malcolm G.: Test Rig for Evaluating Active 

Turbine Blade Tip Clearance Control Concepts. NASA/TM⎯2003-212533 (AIAA−2003−4700), 

2003. 

7. Soeder, Ronald H., et al.: User Manual for NASA Glenn 10- by 10-Foot Supersonic Wind Tunnel. 

NASA/TM⎯2004-212697, 2004. 

8. Soeder, Ronald H.: NASA Lewis 9- by 15-Foot Low-Speed Wind Tunnel User Manual. NASA 

TM−106247, 1993. 
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Testing of a Microwave Blade Tip Clearance Sensor at the NASA Glenn Research Center 

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14. ABSTRACT

The development of new active tip clearance control and structural health monitoring schemes in turbine engines and other types of rotating 

machinery requires sensors that are highly accurate and can operate in a high-temperature environment. The use of a microwave sensor to 

acquire blade tip clearance and tip timing measurements is being explored at the NASA Glenn Research Center. The microwave blade tip 

clearance sensor works on principles that are very similar to a short-range radar system. The sensor sends a continuous microwave signal 

towards a target and measures the reflected signal. The phase difference of the reflected signal is directly proportional to the distance 

between the sensor and the target being measured. This type of sensor is beneficial in that it has the ability to operate at extremely high 

temperatures and is unaffected by contaminants that may be present in turbine engines. The use of microwave sensors for this application is 

a new concept. Techniques on calibrating the sensors along with installation effects are not well quantified as they are for other sensor 

technologies. Developing calibration techniques and evaluating installation effects are essential in using these sensors to make tip clearance 

and tip timing measurements. As a means of better understanding these issues, the microwave sensors were used on a benchtop calibration 

rig, a large axial vane fan, and a turbofan. Background on the microwave tip clearance sensor, an overview of their calibration, and the 

results from their use on the axial vane fan and the turbofan will be presented in this paper.

15. SUBJECT TERMS

Microwave sensors; Instrumentation; Blade tip clearances; High temperature environments; Gas 

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