# Development of an Experimental Capability to Produce Controlled Blade Tip/Shroud Rubs at Engine Speed

Corso Padova, Jeffrey Barton, Michael Dunn, Steve Manwaring, Gamaliel Young, Maurice Adams, Michael Adams

► To cite this version:

Corso Padova, Jeffrey Barton, Michael Dunn, Steve Manwaring, Gamaliel Young, et al., Development of an Experimental Capability to Produce Controlled Blade Tip/Shroud Rubs at Engine Speed. Journal of Turbomachinery, 2003, 127 (4), 10.1115/1.1934429. [hal-01553829](https://hal.science/hal-01553829)

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# Development of an Experimental Capability to Produce Controlled Blade Tip/Shroud Rubs at Engine Speed

An experimental capability using an in-ground spin-pit facility specifically designed to investigate aeromechanic phenomena for gas turbine engine hardware rotating at engine speed is demonstrated herein to obtain specific information related to prediction and modeling of blade-casing interactions. Experiments are designed to allow insertion of a segment of engine casing into the path of single-bladed or multiple-bladed disks. In the current facility configuration, a 90 deg sector of a representative engine casing is forced to rub the tip of a single-bladed compressor disk for a selected number of rubs with predetermined blade incursion into the casing at rotational speeds in the vicinity of 20,000 rpm.

Corso Padova

Jeffrey Barton

Michael G. Dunn

Gas Turbine Laboratory,
Ohio State University,
Columbus, OH 43235

Steve Manwaring

Gamaliel Young

GE Aircraft Engines,
Cincinnati, OH 45215

Maurice Adams, Jr.

Michael Adams

Machinery Vibrations, Inc.,
Chagrin Falls, OH 44022

## 1 Introduction

Contemporary gas turbines rely on complex blade-casing rub-in-systems to improve the tip clearance behavior during operation from initial service to maximum lifetime [1,2]. Typically a rub-in-system in the compressor section of the engine may consist of a specific circumferential area of the metal alloy case shaped to accept coatings of materials selected for in-service wear and, when required, fire shielding. A momentary eccentricity of a few tenths of 1 mm between a blade and the engine case, which may occur because of an abrupt maneuver in flight or in a hard landing, may cause the tips of a few airfoils to incur the rub-in surface.

Blade-to-case rubs can degrade the operation of an engine through the induction of high amplitude shaft vibration and severe blade/seal wear, and may lead to catastrophic failure of the engine in the worst occurrence. Precisely for these reasons, engines are already routinely tested for containment and survival of blade out events. But to enable the formulation of a comprehensive design methodology for rub-in-systems, particularly one capable to predict and take advantage of the potential offered by new and emerging rotor/blade concepts, advances in several underlying technologies are needed. Among them are: rotor dynamics under severe disturbances [3–6], forced vibrations of bladed disks [7–9], impact dynamics [10–13], and abradability of materials [14].

While the studies reported earlier and in the companion literature have provided excellent conceptual frameworks for developing the desired design methodology, there is much need for controlled experimental data obtained in experiments conducted with real engine components. The experimental capability described in this paper is intended to fill such need.

In the development of the facility and experimental method reported, considerable emphasis was placed on engine manufac-

turer input regarding the facets of the rub phenomena where results would yield the highest design-advance payoff. Characterizing blade impact dynamics at engine speed was highest on the industry list. Seminal work on blade contact forces during rubbing has been published by Ahrens et al. [12]; however, the measurements were done at a rotational speed about one order of magnitude below those typical of aircraft engines. In addition, engine hardware was not used in this earlier research activity. By contrast, characterizing the high-frequency disturbances occurring at blade first contact and at detachment during repeated rub events is important in the current research program and justifies the preference for experimenting with engine hardware rotating at design speed.

## 2 Experimental Arrangement

The arrangement of an in-ground spin-pit facility (SPF) specifically designed to investigate aeromechanic phenomena for gas turbine engine hardware rotating at engine speed is described in this section. The underground containment tank that houses both the rotating component and a high-stiffness spindle is cylindrical (1.52 m in diameter by 0.65 m high), and has 0.90 m diameter openings at the top and at the bottom. Figure 1 is a photograph of the facility with the protective top cover in place.

Vertical axis spin testing for design validation of turbomachinery components dates back to World War II applications, which sought to solve problems with military aircraft turbochargers. However, the research projects envisioned from the inception of this facility design dictated the use of a high-stiffness spindle. This requirement forced a significant departure from the common way to conduct a spin test, whereby a rotor is suspended in the heavily armored vacuum tank from a flexible shaft. That shaft arrangement allows the rotor to find its own balance axis and eliminates the need for extreme precision in mounting and balancing. Unfortunately, it is unsuitable to reproduce blade-to-case rubs that can degrade the performance of the engine through introduction of high amplitude shaft vibration and severe blade/seal wear.

Thus the first major subsystem component required for the facility is a vertical and high-stiffness spindle capable of operating
<figure><img src="image_2.png" /><figcaption>Fig. 1 General view of facility (with protective top cover)</figcaption></figure>

at 20,000 rpm for a reasonable period of time. The system utilized here incorporates hybrid bearings and an oil mist lubrication system. This component will be described in more detail later in the paper.

A second major subsystem required to investigate blade-to-case rubs is a fast-acting mechanism that allows insertion of the engine casing into the path of the bladed rotor. Such a mechanism was designed and developed to research asymmetric and full rotor clearance closure for engine stages. In single blade asymmetric clearance closure investigations, a rapid contact of the bladed rotor with its housing is simulated as would occur in an abrupt flight maneuver. In full rotor clearance closure, an extended contact of the bladed rotor with its housing is simulated as could be experienced in some flight phases due to temporary thermal imbalances in different parts of the engine. The known requirements for those types of investigations have defined the general layout of the electromechanical, pneumatic, and measurement subsystems of the facility. The fast-acting incursion mechanism is described in detail later in this paper.

In addition to the two major subsystems noted earlier, a few other facility design features are key in making the facility a versatile tool for performing research on many areas of aeromechanics, including rubs, blade excitation, and blade dampers. A modern high-efficiency air turbine is used to spin-up rotors with large moment of inertia in a few minutes. Moderate vacuum is maintained in the containment tank during operation to eliminate high air friction losses and to reduce the risk of explosion of metal dust or oil fog during an experiment. A high-speed slip ring and hollow spindle is used to conduct strain gauge signals from an instrumented airfoil to the laboratory data collection instrumentation.

## 2.1 High Stiffness Spindle.
The vertical spindle of the SPF shown in Fig. 2 is capable of operating at 20,000 rpm. It consists of a hollow shaft supported on high speed, ceramic hybrid, angular contact ball bearings. The shaft is driven from the top by a reversible air motor. The single stage engine rotor is bolted beneath the spindle assembly and is surrounded by the containment test chamber. The outer spindle housing is made from an aluminum forging. A steel sleeve is inserted into the aluminum housing and serves as the bearing housing. Figure 3 show a photo of the spindle assembly with the slip ring and the drive cover removed. The connector assembly and drive are exposed at the top of the upper housing.

The first critical speed of the spindle is higher than the maximum operating speed. The analysis of the high-stiffness bearing arrangement indicates a shaft critical speed of 23,500 rpm for the design manufacturing tolerances and assumed operating temperature effects.

## 2.2 Incursion Mechanism System.
The function of the incur-

<figure><img src="image_3.png" /><figcaption>Fig. 2 Section of spindle</figcaption></figure>

sion mechanism is to move the engine-case segment into the blade path at an exact time for a precise distance and duration, which then determines the number of blade strikes. After a few strikes, the mechanism retracts the casing out of the blade path.

The experimental configuration allows insertion of a segment of engine casing into the path of single or multiple bladed disks while the disk is operating at engine speed. In the current configuration, a 90 deg sector of a representative engine casing is forced to rub the tip of a single bladed compressor disk for a limited number of times with predetermined blade incursion into the cas-

<figure><img src="image_4.png" /><figcaption>Fig. 3 Spindle photo</figcaption></figure>
CASING SEGMENT SUPPORT RING ASSEMBLY

<figure><img src="image_5.png" /><figcaption>Fig. 4 Incursion mechanism support ring</figcaption></figure>

ing. For the facility development measurements described herein, the speed at the time of rub was on the order of 17,000 rpm. For this rotational speed, the instrumented airfoil returns to the same circumferential position in 3.8 ms, thus several contacts between blade and shroud are experienced before the eccentricity disturbance is removed. The compressor disk and blade rotate on a vertical rigid spindle that is operated by an air turbine. The casing sector is mounted on a unit based on three piezoelectric load cells and attached to a support ring that swivels in the horizontal plane. The single throw by a fast acting gas piston attached to a cam follower is coupled to the support ring and brings the casing in and out of the path of the rotating blade for a few rubs over a time interval of about 20 ms. The incursion of the rubs is prescribed by setting the initial position of the mechanism in increments of 5 µm.

The incursion mechanism can be broken down into three major subsystems. The structural subsystem consists of a support ring that carries the casing segment along during its motion, a linear cam device that controls the motion, and high-stiffness components anchoring all elements of the system to the containment tank. The actuation subsystem moves the cam block on a linear slide via a pneumatically operated piston and its associated controls. The third subsystem comprises the motion instrumentation and triggering electronics.

The function of the support ring is to carry the casing segment that is mounted to the load-measuring unit (LMU) through its

travel cycle into and back out of the blade path. The ring surrounds the disk and pivots on a near zero clearance cylindrical bearing. The stationary pivot point of the bearing is a hollow steel shaft surrounded by a bronze oil-impregnated sleeve. Support for this pivot is provided by a gusseted weldment that attaches to the bottom surface of the containment tank. A cam-follower pin is built into the support ring at the location 180 deg from the pivot. The pin follows a sliding cam profile to effect ring motion. Vertical ring movement is held to less than 125 µm by a set of oil impregnated bronze bearing pads held in a "C" clamp like assembly. The pads are in contact with the ring reinforcement webs. There is a second bolt-on reinforcing web that spans the lower diameter of the ring to help maintain dimensional stability under load. Figure 4 shows a schematic of the structural subsystem and identifies its main elements. Figure 5 shows a picture of the subsystem.

A slot in the cam block guides the cam-follower pin on the support ring. The block is mounted on a linear slide and both are moved thru their travel with a pneumatic piston. The slide and block travel path is along the centerline that extends from the ring follower pin to the ring pivot. A shock absorber is mounted near the end of the slide base to prevent over travel. This assembly is mounted to an adjustable and lockable mechanism that travels perpendicularly to the slide movement to provide initial static adjustment. Support for these components is a table mounted to the
<figure><img src="image_6.png" /><figcaption>Fig. 5 Incursion mechanism photo</figcaption></figure>

bottom surface of the vessel with substantial welded gussets to the vessel sides for stability. Figure 6 shows a schematic of the subsystem and identifies the main components.

The slide is of a square gib design with a 23 cm saddle (moving component) and a 51 cm base for a 28 cm total travel. Anti-

friction surfaces are Roulon® and are set for zero clearance by adjusting a line of setscrews. Rated static loads are 170 kN in the vertical plane and 18.8 kN in the horizontal plane. The intended design of this slide is for high speed, high load, and high accuracy machinery. The high load rating and close tolerances allow for accuracy and minimal overshoot in ring travel.

The cam block is machined from C1018 steel (ASTM-A108) and bolted and pinned to the slide saddle. This particular steel was chosen for the finish quality obtainable and desired for a smooth surface at the follower pin contact. The profile of the cam is shown in Fig. 7. This profile, along with the travel speed characteristics provide for incursion timing (the casing must be fully extended into the blade path in less than one revolution), and length of time that the casing is in the blade path (referred to as dwell time). Maximum incursion distance is also controlled. Then, the casing must be retracted out of the blade path and the cam block/slide saddle slowed safely to a stop. The piston actuator is double acting with a 65 mm bore capable of 4.37 kN force.

The diagram of Fig. 8 shows the pneumatic and electrical components, and pressure instrumentation for controlling and monitoring the pressurization of the actuator piston. The reservoir is positioned close to the extend Valcor® valve in order to ensure adequate volume to the piston and is pressurized with gaseous helium. The Valcor® is an axial flow fast-acting full flow valve. Actuation times are on the order of 12 ms and very repeatable. The interval timer accepts either a local or remote trigger and has user programmable times for delay time and on time. The Valcor® valve requires a high initial voltage and so has its own supply in a metal enclosure separate from the control room. Pressure readouts are used as displays for initial setup, and paralleled to oscilloscopes and the data acquisition system for performance diagnostics. The retract side of the piston is connected to a three way valve using vacuum at prefire setup, shop air for post-test retraction, and a vent to ambient to relieve retract pressure. The operation of the subsystem is described later in Sec. 3.

## 2.3 Load Measuring Unit.
The segment of engine casing against which the blade rubs is carried by a metric support that includes three load cells. Each piezoelectric load cell measures three force components. Two end cells are placed below the load line, which is located in the midplane of the engine case. The center cell is located above. The arrangement is designed to minimize the moment loads on each cell and to obtain moment information around three mutually perpendicular axes from the entire unit.

Five accelerometers are also mounted on the LMU, which is shown in Fig. 9. Three radial directions are measured at locations very close to each load cell. A triaxial block, which is mounted near the center load cell, is equipped with the remaining two accelerometers that measure one axial and one circumferential direction in the middle of the LMU.

As in the actual engine, the radius of the casing cylinder is greater than the radius of the circular path followed by the blade tips under normal operating conditions. A tight but finite clearance is maintained at the rub-in-system surface. This condition is applied to ensure that no impact of the blade will occur first at the

<figure><img src="image_7.png" /><figcaption>Fig. 6 Incursion mechanism, cam, and slide</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 7 Cam profile</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 8 Pneumatic control system</figcaption></figure>

edge of the LMU for all chosen values of eccentricity between casing and rotor in the experiments. In addition, the corners of the casing sector are beveled for added margin of safety.

## 3 Operation

During a typical experiment the rotor is spun-up to the target speed in less than 3 min. As the rotor speed approaches the target value selected for a rub to occur, the angular acceleration is gradually decreased so that the airfoil impact with the shoe occurs effectively at a constant value of rpm. A few rubs are obtained as follows. First, the casing movement toward the spinning blade tip

is initiated by the triggering system described later in this section. This results in the motion shown in Fig. 10. Also shown in the figure are the markers that indicate the passing of the blade over the LMU for duration of about 1 ms. Second, during the casing motion, the tip sweep line is crossed as depicted in the figure. After this point in time, the LMU is in the path of the spinning

<figure><img src="image_10.png" /><figcaption>Fig. 9 Load measuring unit LMU</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 10 Casing motion and incursion region</figcaption></figure>
• The measurements are best examined into five [at least] time scales: *rub set* (30 to 50 ms) and *post-rub damping* (200 ms ca.), *shoe-sweep* (1 ms ca.), *single rub* (0.1 to 0.7 ms), *return-to-rub* (3 ms ca.).

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 11 Time Scales of blade rubs</figcaption></figure>

blade and a rub is obtained as soon as the blade returns over the LMU sector. This is shown by the appearance of the blade contact marker in the figure. Except for a range of very small incursions, the rubs begin with the casing still advancing toward the spinning blade. The more severe the incursion, the more rubs occur before the casing locks into the zero-lead region shown in Fig. 10.

Third, during the time that the casing is in the maximum incursion position, up to six rubs can take place for the values of rotational speed at which experiments were conducted. In fact, due to the settings chosen in the triggering circuit, five rubs were obtained in most cases. All zero-lead incursions occur with the same nominal interference between the path of the blade tip and the arc of the casing. The deviation of the actual interference from the nominal value was monitored in repeated experiments and found to be less than ±3.8 µm.

Fourth, the continuing motion of the cam forces the casing to begin to retreat. In this phase, the rubs continue because the casing is still inside the incursion region. As before, the more severe the incursion, the more rubs occur before the casing clears the exit boundary of the incursion region.

Finally, the casing crosses the tip sweep line and the rubs end. In the experiments performed to this date, the maximum incursion imposed was for 400 µm and resulted in a total of 15 rubs.

It is important to note that the measurements obtained during each experiment should be examined according to a number of different time scales. Five scales are illustrated in Fig. 11. In order of decreasing duration they are: the time of damping of blade excitation after a set of rubs, the time spanned by an entire set of rubs, the time for one revolution, the time interval a blade spends in free oscillation after a rub and before returning for a subsequent rub, the time interval during which the blade sweeps over the LMU, and the duration of an individual rub.

It is precisely to identify during an experiment the different scales of the rub phenomena that two types of markers are recorded while operating the SPF. First contact is detected by a sensor and is used to begin data acquisition in one type of experiments (disk/blade growth experiments) as described later. In the second type of experiments (ramp-incursion experiments) that are discussed in greater detail in this paper, the shoe motion toward the spinning blade is manually initiated and is followed, after a suitable delay, by the signal to begin data acquisition. In either case, the data acquisition system is already running in sample-and-hold mode at the precise instant when the command to acquire is received, and a prerub hold time is programmed into it. Thus any of the measurement histories shows typically a prerub value for about 200 ms, the onset of the rub event, the dynamic response resulting from the impact of the airfoil with the shoe, the

<figure><img src="image_14.png" /><figcaption>Fig. 12 Triggering of case motion</figcaption></figure>

release of the airfoil immediately following detachment from the shoe, the vibrational response of the airfoil as it spins around once more after having been excited by the impact, the effect of repeated impacts, and the decay in dynamic response after the conditions causing the incursion have been removed. Typical histories last 829 ms and consist of a few to several rubs depending on the type of experiment.

It is important to relate the above sequence of events to rub occurrences in flight. It is clear that in the experiments just like in flight, for a given incursion depth or severity of a rub event, the rate of incursion in each strike will be positive, null or negative during different phases of the set of rubs that makes up an event. By comparing measurements obtained at different experimental conditions, the effects of incursion depth and rate of incursion can be separated. In the middle rub of Fig. 11, one can see the blade detaching and reattaching during a single strike interval.

Furthermore, the effects of rub repetition can be identified. For the example shown in Fig. 11, the first rub is clearly brief and is followed by rubs of equal duration (250 µs) later. Although not a full engagement, the 1st rub is important because it imparts to the airfoil an initial vibratory motion. In addition, in zero-lead rubs, the deviation of the actual interference from the nominal value was monitored in repeated experiments and found to be less than ±3.8 µm (±1.5 mil). Rub Nos. 2 and 3 are equal in duration and incursion depth. However, as the blade comes around after each rub it is excited and one would intuitively expect that rub Nos. 2 and 3 will be different in blade vibratory response and in tip loads during contact to the casing. The extent to which this was found to be true is discussed in Sec. 4 later.

The actuation of the incursion mechanism system is obtained as follows (reference should be made to Fig. 8 earlier). Initially, the air piston is retracted with shop air, vented, and the retract side brought to a vacuum. Both extend side Valcor® valves are closed and the reservoir is charged with helium. The interval times are set and the timer is armed and ready for either a manual or remote trigger. When a trigger is received the timer waits for the delay time then fires the Valcor® valves through its power supply for a set-on time. The piston then pushes the cam block through its travel.

Ring and attached LMU, casing segment initial position, and casing actuated motion are measured with a capacitance probe referred to as the casing stand off (CSO) sensor. Lion Precision manufactures the probe and associated electronics. This system has a certified resolution of 0.36 µm, a range ±1 mm, and a bandwidth of 16,800 Hz. The output is in volts. Cam block motion is measured with a LVDT manufactured by UniMeasure. It has a 25.4 cm stroke and has an in-house calibration of 0.3%. Several combinations of cam profile, helium pressure, and valve timing were tried during a total of 51 trial runs. The last of these runs is plotted in Fig. 12.

In disk/blade growth experiments, rubs of progressively in-
<figure><img src="image_15.png" /><figcaption>Fig. 13 Blade contact trigger circuit</figcaption></figure>

creasing severity were obtained by using the expansion of the rotating component under increasing rotational speed as the mechanism driving the interference relative to a stationary casing. The total number of rubs obtained in these experiments was greater than the number of rubs in the ramp-incursion case by at least one order of magnitude. The focus of this class of experiments is on the vibratory response of the blade when undergoing a multitude of excitation cycles.

The trigger to start both the data acquisition and incursion sys-
tems must be precisely controlled. Two different triggering
schemes were used. One for the disk/blade growth experiments
where the casing segment is stationary and the blade grazes it as a
result of tip growth. The other for the ramp-incursion experiments
where the casing is suddenly brought into the blade path. The
high-speed data acquisition has a selectable window in which to
collect data and record the rub events, therefore it must trigger at
the appropriate moment. The incursion mechanism is stationary
for the disk/blade growth experiments, requiring no trigger. For
the ramp-incursion experiments it must be precisely timed.

For the disk/blade growth experiments, a voltage signal needed
for the trigger to start the acquisition is produced when the blade
tip first touches the casing segment. This is accomplished by set-
ting the rotor to 5 V and the casing to ground. The blade tip
touching the shoe completes a circuit and triggers the data system.
Figure 13 shows a simplified diagram.

The triggering circuitry for the ramp-incursion experiments is fairly involved. The object is to get the casing sector to move out and set in place during the time interval when the blade cannot interfere with the casing surface. This time interval spans several blade revolutions, as can be seen from Fig. 10. Using a selectable time delay from a trigger that is related to instantaneous blade position, the object is achieved by delay increments equal to a fraction of one revolution. An optical sensor placed to sense blade passage at the leading edge of the shoe supplies the trigger signal. The incursion system timer can resolve to a millisecond, which is too coarse for this application, so a delay generator capable of resolving 0.1 ms is placed in front. A manual trigger-enable switch disables the sensor trigger until the rotor set speed is reached. The data acquisition system is triggered with the incursion system timer so that casing movement can be monitored during the experiment and correct position relative to blade location can be verified. Figure 14 shows a simplified diagram.

The facility instrumentation is conveniently grouped into two
categories: rotating sensors and stationary sensors.

The rotating sensors consist of miniature dynamic strain gauges arranged on the suction and pressure side of the blade. Excitation power and signal response are carried by electrical wires routed through the hollow shaft of the spindle to the rotating terminal of a slip ring unit. The stationary terminal of the unit is directly patched into the high-speed data acquisition system. In the present experiments all strain gauges were operated in constant current mode.

The primary stationary sensors consist of the three piezoelectric
load cells that are part of the LMU and of the five piezoelectric
accelerometers, also mounted on the LMU.

4 Results and Discussion

The measurements obtained simultaneously from load cells, ac-
celerometers, and strain gauges during nearly a second of data
acquisition time, when combined, offer a detailed description of
the rub phenomena. The purpose of this paper is to describe the
facility design and operation and the measurements later are used
merely to illustrate the capabilities of the facility. Extensive re-
sults from specific applications and research projects will be the
subject of subsequent papers. Load cell and strain gauge measure-
ments are discussed briefly later. The measurements of accelera-
tion are qualitatively very similar to those obtained from the load
cells.

Typical measurement traces are shown in Fig. 15 and 16. In
these figures, the initial prerub level of the sensor, the response
during about 10 ms of multiple rubs, and the postrub sensor re-
sponse are easily identified.

The three load cell measurements from top to bottom of Fig. 15 are for the first cell, center cell, and last cell passed by the blade during each revolution. Note how quickly after the rub interval the measured loads return to the prerub level. The two typical strain gauge measurements from top to bottom of Fig. 16 are for a gauge located at the tip and for a gauge located at the root of the blade, respectively. Note that the tip gauge indicates localized disturbances during the rubs, a temperature-induced displacement of the baseline, and no postrub oscillations. In contrast, the root gauge shows oscillations building up from rub to rub followed by an exponential decay after the last rub.
<figure><img src="image_16.png" /><figcaption>Fig. 14 Optical sensor trigger circuit</figcaption></figure>

As the blade rubs on the engine-case segment attached to the LMU, blade tip loads are distributed to the three LMU load cells via the rigid transfer bar. Thus the LMU acts as a mechanical filter on the tip loads. A transfer frequency response function (FRF) algorithm is needed in relating engine-casing loads to load cell measurements. In brief, the theory for the FRF algorithm is based on characterizing the LMU dynamics by impulse vibration testing, whereby the output response at location $x_0$ is given by

$$u(x_0,t) = H_{oi}(\omega)Fie^{j\omega t}$$

where $H_{oi}(\omega)$ is the output FRF at location o per unit force applied at location i.

In order to implement the FRF algorithm we proceeded as follows. Prior to conducting rub experiments, input loads from an impulse hammer were applied at three locations, in three direc-

tions for each location. At the same time, nine response measurements were recorded from the load cells. The elements of the matrix $H_{oi}(\omega)$ were computed as ratios of measured outputs to known inputs in the frequency domain. Then the inverse of the output FRF matrix was computed and stored for later use in conjunction with rub measurements.

During a rub experiment, nine response measurements are recorded from an unknown force at the rub surface. It is precisely this resultant force that is computed from the known instantaneous blade position and from the inverse of the output FRF operating on the load cell measurements transformed to the frequency domain. The computation earlier is then repeated for a discrete set of blade positions during the rub.

At present, issues associated with the detailed requirements of impulse vibration testing have restricted the application of the FRF algorithm to two components of the blade tip load, the radial and the circumferential component. However, finite element structural modeling is in progress to remove such restriction. In the

<figure><img src="image_17.png" /><figcaption>Fig. 15 Typical load cell measurements</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Fig. 16 Typical strain gauge measurements</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Fig. 17 Strain gauge measurements for shoe-sweep time scale</figcaption></figure>

mean time, even with the restriction to two components, interest-
ing comparisons of tip loads have been obtained and will be re-
ported in the near future.

The strain gauge results at different airfoil locations are com-
bined to extract modal information. Interpretation of the results is
obtained after several steps of data reduction. The two major com-
ponents of the data reduction technique are (a) a series of decom-
positions of the traces into time segments, and (b) spectral analy-
sis of each time segment. The spectral analysis component of the
data reduction technique applied conventional FFT processing ap-
plied to time segments as appropriate.

Much of the paper to this point has been devoted to describing
the facility design and operation, but Figs. 10 and 11 given earlier
and Fig. 17 given later illustrate an example of measurement of
transient dynamics for the blade and casing vibroimpact response
that is important to the industry. The reader attention is to be
focused here on the shoe-sweep time scale to compare one of the
zero-lead rubs, No. 3, to the first rub (see Fig. 11). Figure 17
shows the measurements again for the root gauge and the tip
gauge. It is noticeable that the traces show distinct spikes at the
beginning and end of the most perturbed region. The time interval
between these two spikes correlates well in all experiments with
the indications of electrical contact shown earlier as the blade-
contact marker, so the first spike is taken as indicating an attach-
ment stress wave and the last spike a detachment stress wave. In
this case the blade contact lasted about 200 µs.

Looking first at the root gauge, the first rub is not a full engage-
ment rub. The centrifugal load has been subtracted out and we see
that relative stress is zero up to first attachment. Then there is
some high frequency content probably related to a changing area
of contact as the airfoil flexes and twists under load, and after a
low frequency oscillation gradually develops. It is a first flex fre-
quency that persists also at later times. The detachment spike is
visible. Notice that it is a bit early relative to the companion event
in the third rub. Again, this first rub is not a full engagement
strike.

Now let us follow the third rub of the root gauge. The blade
comes around and is excited in the first flex mode shape. It at-
taches first and it results in a history more suggestive of an
increasing-decreasing contact area. Then it detaches and resumes
the first flex motion.

Looking now at the tip gauge, the following occurs: the start of
the first rub contact is clearly marked by a first positive spike
followed by smaller disturbances because of varying contact area
and local blade detachment. A distinct negative spike marks final
blade detachment. In rub No. 3 the spike marking the initial blade
impact is stronger and is followed by a more uniform load during

contact consistent with a deeper incursion. The detachment spike
again ends the strike at about the same level encountered during
rub No. 1.

It must be stressed that these are the durations involved in a real
engine rub. It is necessary to understand the physics under these
conditions, not at some longer duration that is easier to obtain
experimentally. The detailed measurements provided in this paper
have illustrated that the excitation state of the airfoil on the second
and subsequent encounters with the shroud is very different from
the initial encounter. This finding has resulted in re-evaluation of
the modeling commonly used in design tools used for predicting
the blade/shroud incursion interactions.

5 Conclusions

An in-ground spin-pit facility specifically designed to investi-
gate aeromechanic phenomena for gas turbine engine hardware
rotating at engine speed has been utilized successfully to obtain
specific information related to prediction and modeling of blade-
casing interactions as reported herein.

The results presented in this paper demonstrate that sudden in-
cursions of varying severity can be controlled to increments of
5 µm. Rubs can be repeated for detailed investigation of tip loads
and vibratory stresses. In addition, the amplification of blade os-
cillations under successive rubs can be quantified. In the future,
refinements in tip load measurements will be obtained from finite
elements modeling and the effect on rub forces from varying in-
cursion rate will be investigated.

The purpose of this paper was to describe the development of
the experimental capability along with results that demonstrate the
operability of the facility and the importance of these results to the
engine designer. It is interesting to note that a blade is capable of
sustaining incursions that are more significant than might have
been initially thought. The dynamics of high-frequency distur-
bances occurring at blade first contact and at detachment can be
characterized relative to low-frequency oscillations in repeated
rubs. An indication of tip loads can also be deduced from the
measurements.

In combination with refined design methods and refined mod-
eling of frictional forces, the greater understanding of the phe-
nomena that affect blade-casing interactions is expected to result
in savings in gas turbine maintenance costs applicable not only in
propulsion engines but for stationary advanced power systems as
well. Incursion measurements obtained using actual engine hard-
ware at engine rotational speed have demonstrated the need for
improved modeling of the event.

Acknowledgment

The work described in this paper was performed as part of the
General Electric University Strategic Alliance program at The
Ohio State University. The assistance of GE personnel in all
stages of the project is greatly appreciated.

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