measurement of rotorcraft blade deformation using ... · taining moire fringes, which are...

149

Measurement of rotorcraft blade deformationusing Projection Moire Interferometry1

Gary A. Fleming∗ and Susan Althoff GortonNational Aeronautics and Space Administration,Langley Research Center, Hampton, VA 23681-2199,USA

Received 24 March 2000

Revised 24 March 2000

Projection Moire Interferometry (PMI) has been used to ob-tain near instantaneous, quantitative blade deformation mea-surements of a generic rotorcraft model at several test condi-tions. These laser-based measurements provide quantitative,whole field, dynamic blade deformation profiles condition-ally sampled as a function of rotor azimuth. The instanta-neous nature of the measurements permits computation of themean and unsteady blade deformation, blade bending, andtwist. The PMI method is presented, and the image process-ing steps required to obtain quantitative deformation profilesfrom PMI interferograms are described. Experimental resultsare provided which show blade bending, twist, and unsteadymotion. This initial proof-of-concept test has demonstratedthe capability of PMI to acquire accurate, full field rotorcraftblade deformation data.

Keywords: Projection Moire Interferometry, PMI, rotorcraft,helicopter, blade, deformation, deflection

1. Introduction

Rotorcraft blade dynamics are a driving factor in thedesign of low noise, low vibration helicopters. Blade

1An earlier form of this paper was presented at the 3rd Interna-tional Conference on Vibration Measurements by Laser Techniques,Ancona, Italy, June 1998. The conference was organized by theItalian Association of Laser Velocimetry. The articles were selected,edited, and reviewed by a committee chaired by: Professor EnricoPrimo Tomasini, Department of Mechanics, Faculty of Engineering,University of Ancon, Italy.

∗Corresponding author: Gary A. Fleming, NASA Langley Re-search Center, Mail Stop 493, Hampton, VA 23681-2199, USA. Tel.:+1 757 864 6664; Fax: +1 757 864 8315; E-mail: [email protected].

geometry, stiffness, and structural response significant-ly affect rotor performanceand acoustics. The develop-ment of dynamic prediction codes has been hamperedby the lack of experimental data, due in part to inad-equate instrumentation. The instrumentation requiredto measure the mean and fluctuating components ofblade deformation and attitude are subject to many re-strictions dictated by the complexities of the spinningrotor. In general, the instrumentation must be: (a)non-contacting to avoid artificially induced mechanicaland aerodynamic effects; (b) capable of acquiring near-instantaneous measurements at designated azimuth po-sitions; and (c) capable of obtaining measurements atmultiple spanwise and chordwise locations along theblade. Instrumentation satisfying these criteria andtheir application to rotorcraft blade dynamics has justbegun to emerge. As part of the recent Higher harmoniccontrol Aeroacoustic Rotor Test (HART), Dutch, Ger-man, and French researchers conducted comparativetesting of three different blade deflection methods inthe German-Dutch Wind Tunnel (DNW) [1]. Methodsevaluated were (a) conventional strain gauge, (b) TargetAttitude in Real Time (TART), and (c) projected gridmethod. Blade deflections measured using these tech-niques were in good agreement, but advantages werenoted for the projected grid method. A technique sim-ilar to the projected grid method, Projection Moire In-terferometry (PMI), has recently been applied in theNASA Langley 14-by 22-Foot Subsonic Wind Tun-nel to measure mean and fluctuating blade deformationprofiles. PMI differs from the projected grid method inthat PMI uses interferometric methods to extract quan-titative deformation data from the raw images, where-as the projected grid method uses grid line registrationand tracking. This paper describes the PMI technique,image processing steps, and its application to measur-ing azimuth dependent mean and fluctuating blade de-formation profiles. Blade deformation results obtainedfrom a generic four-bladed, Mach scaled rotor are alsopresented for several different flight conditions.

Shock and Vibration 7 (2000) 149–165ISSN 1070-9622 / $8.00 2000, IOS Press. All rights reserved

150 G.A. Fleming and S.A. Gorton / Measurement of rotorcraft blade deformation using Projection Moire Interferometry

2. Projection Moire Interferometry

Projection Moire Interferometry (PMI) is an optical-ly simple, non-contacting measurement technique usedsince the 1970’s for surface topology and shape charac-terization [2]. Widely varying applications of PMI formeasuring surface topologies and deformations haveranged from orthopedics [3], to aircraft components [4],to animals [5]. The fundamentals of PMI are wellknown [6,7], but only recently have PMI and simi-lar systems been used to measure wind tunnel modeland rotor blade deformations while under aerodynamicload. The following discussion briefly describes thefundamentals of PMI and the image processing stepsused to obtain quantitative deformation profiles.

2.1. PMI fundamentals

Conventional PMI relies on the projection of a gridof equispaced, parallel lines onto the surface of the ob-ject of interest. When the object in a reference con-dition is imaged by a CCD camera, the grid lines willhave a regular spatial distribution across the object sur-face. Under load, the object deforms, and the project-ed grid lines lie in different spatial locations. Subtrac-tion of digitized images acquired in the reference anddeformed conditions produces an interferogram con-taining moire fringes, which are deformation contours.This process is often referred to as projection moiredifference contouring, shown schematically in Fig. 1.The moire fringe contour interval, or fringe sensitivityconstant, is determined by the projected grid line pitchand angles of projection and observation. Figure 2shows a generic PMI configuration using symmetric il-lumination and observation. A Ronchi ruling, i.e. a bi-nary grating of parallel lines having equal spacing andthickness, is used to generate the projected grid lines.The system is sensitive to the out-of-plane componentof displacement, along the bisector of the angle (α+β).Geometric analysis leads to the relation [6]:

∆z = FSC =d

tanα + tanβ(1)

where∆z is the distance between contour planes,FSCis the fringe sensitivity constant,d is the distance be-tween the projected grid lines on the object surface, andα andβ are the angles of illumination and observation,respectively. Non-symmetricoptical configurations arepossible, but care must be taken to ensure the desireddeformation component is measured and that the con-tour intervals remain planar. The contour intervals will

remain planar and horizontal if the projector and re-ceiver are located in the same horizontal plane. Forthese cases, equation (1) remains valid for determiningthe fringe sensitivity constant. This permits configura-tions such as normal illumination and oblique observa-tion without introducing non-linearities in the contourplanes. Further derivation for more complicated non-symmetric configurations is presented in references 2and 6.

2.2. PMI image processing

The most recent advancements in PMI have occurredin the development of automated image processing rou-tines for extracting quantitative deformation profilesfrom PMI interferograms. Although the interferogramis a valuable tool for real-time visualization of the de-formed object, interferograms alone do not provide de-tailed quantitative results. Differences in image pro-cessing methodology distinguish PMI from the project-ed grid method. For the projected grid method, cen-troidal positions of each projected grid line were reg-istered and tracked to obtain rotor blade deflection da-ta. The PMI image processing steps adopt proceduresfrom interferometric analysis to obtain quantitative dis-placement profiles.

Algorithms for quantitative interpretation of PMIinterferograms are generally based on phase shifting.Phase shifting interprets deformations as changes inoptical phase. Further processing of the phase distribu-tion and division by the system wave number2π/FSCproduces the reconstructed deformation profile. Phaseshifting can be accomplished either (a) mechanically,by physically moving the Ronchi ruling and recordingsuccessive images, or (b) computationally, by math-ematically phase shifting a computer-generated refer-ence grid and interfering it with a digitized PMI expo-sure. Phase shifting by mechanical movement of theruling can introduce errors from mechanical tolerancesand requires multiple exposures of the deformed objectin a stationary position. The latter requirement pre-cludes mechanical phase shifting from rotorcraft bladedeformation studies since the blades of an operatingrotor are never stationary. Thus, computational phaseshifting is used for the current work.

The image processing steps used to obtain quanti-tative deformation profiles from digitized PMI imagesare shown in Fig. 3 for a test specimen in a laborato-ry environment. The test specimen is a 250-mm longaluminum beam, fixed at the left end, with the freeend deflected out of plane. Figure 3(a) shows a dig-

G.A. Fleming and S.A. Gorton / Measurement of rotorcraft blade deformation using Projection Moire Interferometry 151

Deformed StateReference State Deformation Contours

Fig. 1. Projection moire difference contouring.

Contour Planesd

PMI ReceiverPMI Projector

Ronchi Ruling

βα

∆z

Fig. 2. Symmetric projection moire interferometry configuration.

itized image of the beam in a deformed state. Theprojected grid lines are apparent. Perspective distor-tion caused by oblique observation of the test specimenwas present, but has been removed using bilinear warp-ing techniques [8,9]. The corrected image shown inFig. 3(b) is then computationally interfered with a refer-ence grid, Fig. 3(c). The computer generated referencegrid is a perfect sinusoid varying in intensity along therows of the image. The reference grid period is chosento correspond with the mean projected grid line periodobserved by the CCD camera. The interference processdepicted in Fig. 3(c) is performed four times, each withthe computational reference grid phase incremented by90◦. The resulting interferograms contain low frequen-cy moire fringes and high frequency artifacts from theinterference process. After low-pass filtering, only the

moire fringes remain as shown in Fig. 3(d), and thewrapped phase map in Fig. 3(e) is computed by [10]:

θ = tan−1

[I270 − I90

I0 − I180

](2)

In equation (2) above,θ is the optical phase,Ix arethe phase shifted interferograms as in Fig. 3(d), and thesubscripts represent the phase of the computer generat-ed reference grid. The phase values shown in Fig. 3(e)have been scaled such that black corresponds to 0◦

phase, while white corresponds to 359◦ phase. Finally,phase unwrapping must be performed on the phase dis-tribution to obtain the surface topology of the deformedobject. Phase unwrapping is the most difficult portionof the entire process, and routines to perform the oper-ation vary widely in scope and applicability. A com-


(a) Raw Data (b) De-warped Image (c) Interfered with Reference

(d) Low-pass Filtered (e) Wrapped Phase Map (f) Displacement Profile

Displacement, mm

-8.0 0.0

Fig. 3. PMI image processing steps.

Fig. 4. PMI projection system for rotorcraft blade deformation measurements.

prehensive review of phase unwrapping algorithms ap-pears in reference 11. The custom algorithms used hereperform unwrapping on small subsections of the imageto prevent error propagation,and contain intelligence toaccommodate noise and minor fringe discontinuities.Following phase unwrapping, the phase distribution isdivided by the system wave number2π/FSC to obtaina surface topologyof the deformed object. This processmust be performed for the object in both its deformedand reference conditions. Differencing the two result-

ing surface topologies yields the quantitative deforma-tion profile shown in Fig. 3(f). After performing theinitial software setup, the processing time required togo from the raw data presented in Fig. 3(a) to the actualdisplacement profile in Fig. 3(f) is approximately 30seconds when processed on a 166 MHz Pentium PC.Experiments using this beam to investigate PMI systemperformance yielded deformation measurements accu-rate to within 0.1 mm at resolutions better than 50µm.However, the resolution will decrease with increasing


field-of-view.

3. Application of PMI to rotorcraft testing

The design and implementation of a PMI system formeasuring rotorcraft blade deformation presents uniquechallenges beyond conventional deformation studies.The system must have a large field-of-view to permitmeasurements over a wide azimuth range, acquire con-ditionally sampled azimuth dependent data, and obtainnear-instantaneous images of the spinning rotor for de-termination of both mean and dynamic deformation. APMI system satisfying these criteria and its use for ro-torcraft blade deformation measurements is describedbelow.

3.1. Optical configuration

PMI is optically simple, requiring only two majorcomponents: a projector and receiver. The PMI projec-tion system constructed for the Langley 14-by 22-FootTunnel is shown in Fig. 4. The PMI system installationis shown in Fig. 5. A 15 W, 800 nm broadband laserdiode bar was used as the projector illumination source,providing high illumination energy at short pulse du-ration. Light emitting from the laser was collectedby a cylindrical lens and directed to back-illuminate a79 line/cm Ronchi ruling. The ruling was placed at theimage plane of a standard 50 mm F/1.4 SLR cameralens, whose focus was adjusted to sharpen the grid lineprojection in the plane of the rotor. A 75 mm diame-ter mirror, anti-reflective coated for the near infrared,was positioned in front of the projector lens to directthe projected grid pattern vertically upward to illumi-nate the underside of the rotating blades. The nomi-nal elevation of the blades was 2.2 meters above theprojection system. The projected grid line pitch at themeasurement plane was 15.75 mm.

The high power output of the laser diode permittedmeasurements to be made over a 1.2-x 1.2-meter field-of-view, covering over 50◦of rotor azimuth. Diode illu-mination levels were sufficient to capture the azimuthalrotation of the blades to within 1.4◦ at rotor speeds of2000 rpm. The broadband emission wavelength of thelaser diode rendered speckle undetectable. Althoughany non-coherent light source can be used, the use ofbroadband laser diodes provides other operational ad-vantages over other sources such as flash lamps. Theseinclude lights-on facility operation without loss of con-trast of the PMI projected grid lines, and simultane-

ous operation with other laser-based instrumentationtechniques via optical band pass filtering.

The PMI receiver was a generic RS-170 analog videocamera electronically shuttered to 1/10,000 second.The camera was fitted with a 28–70 mm zoom lens andpositioned to view the underside of the rotor blades.The laser diode was synchronized with the video cam-era to provide illumination only throughout the openshutter exposure. An IR-pass filter was installed toblock visible light from the CCD array while allow-ing collected laser light to pass. The interlaced analogvideo signal was digitized in fields to 640-x 240-x 8-bits using a PC-based frame grabber. Only one fieldper video frame was captured and used for process-ing. Following camera calibration and removal of per-spective and optical distortions, a spatial resolution of1.47 mm/pixel was realized.

3.2. Conditional sampling

Conditional sampling of the PMI receiver video sig-nal was required to obtain blade deformation measure-ments at designated rotor azimuths. To implement con-ditional sampling for PMI and other instrumentationtechniques, a custom Helicopter Rotor Azimuth Posi-tioning System (HRAPS) was integrated with the rotorshaft encoder. The shaft encoder continuously outputdigital pulses at a rate of 1024 per revolution. TheHRAPS counted each pulse, and latched the azimuthposition when triggered. If the latched azimuth posi-tion corresponded to one of the designated positionsdownloaded into a lookup table, the HRAPS set a datavalid line high. Other instruments monitored the senseof the data valid line to determine whether the datashould be saved.

To fully implement conditional sampling with PMI,the same trigger signal was provided to HRAPS and thelaser diode controller such that the HRAPS latched theshaft encoder position in coincidence with each laserpulse. Initially, video was continuously digitized into asingle frame buffer at a given frame rate. The data validline was polled after each image acquisition. If thedata valid line was low, video continued to be digitizedinto the original frame buffer, overwriting the previousdata. If the data valid line was high, video acquisitionadvanced to the next frame buffer, thus saving the validdata in host memory. This cycle continued until the de-sired number of valid azimuth dependent images wereacquired. The entire image sequence was then writtento disk.


PMI Projector

IRTS

LoweredFuselage

PMI Receiver

Fig. 5. PMI system installed in the 14-By 22-Foot Subsonic Tunnel for the Rotor Wake / Configuration Aerodynamics Test.

3.3. Calibration procedures

Prior experience has shown the largest systematicerror source in PMI systems of this type is the abilityto accurately determine the illumination and observa-tion anglesα andβ for calculation of theFSC [seeEquation (1) and Fig. 2]. Rather than calculate the FSCbased on measured values ofα andβ, the FSC wasdeduced directly by performing in-situ calibrations ofthe PMI system. The PMI system calibration was per-formed using a flat calibration plate installed parallelto the test section floor at the height of the rotor hub. A1.2-x 1.2-meter aluminum honeycomb panel was usedfor the calibration plate. This material provided suffi-cient rigidity to prevent any sag through the middle ofthe panel when mounted horizontally. A single bladewas removed from the four-bladed rotor during calibra-tion to allow the plate to be positioned within the PMIsystem region-of-interest. The plate was situated suchthat one edge was as close to the hub center as possible.

Both sides of the calibration plate were painted flatwhite. One side, referred to as the dot card side, hada square array of black dots (approximately 30-x 30dots) painted on 63.5 mm centers. The PMI systemcalibration was performed by orienting the calibrationplate such that the dot card side faced the tunnel testsection floor, making the dots visible to the PMI sys-tem video camera. The dot card side was illuminatedwith the PMI projector system with the Ronchi rulingremoved. Several images of the dot card were acquiredand averaged. The average dot card image was thenused to determine image warping coefficients required

for the removal of perspective and optical distortionsassociated with the PMI video camera and lens. Af-ter the dot card images were obtained, the card wasflipped so the remaining flat white side was visible tothe PMI system video camera. The Ronchi ruling wasreinstalled in the projection system, and grid lines wereprojected onto the flat white surface. The target wasthen pitched to known angles, and images of the gridlines projected onto the target were acquired at eachknown pitch angle. The image sets acquired at eachpitch angle were averaged,and the average images wereused to calculate the PMI systemFSC. TheFSC forthe PMI system used in this test was determined to be19.0 mm/fringe.

Images of the calibration plate (flat white side) at0◦ pitch angle were also used as reference conditionimages for PMI data analysis, defining the plane of ze-ro blade deformation. When acquiring PMI measure-ments of stationary objects, reference images of the ob-ject in a non-loaded state are acquired and used as a ze-ro deformation reference. However, the continuouslyvariable positions of the rotor blades make it difficultto establish a reference state for rotorcraft. Referenceimages of the blades at designated azimuth positionscould potentially be obtained by positioning and lev-eling the blades within the plane of the rotor. How-ever, at flight conditions, the blades exhibit dynamiclead-lag and can be either azimuthally leading or lag-ging the azimuth location indicated by the rotor shaftencoder. This can inhibit reference/deformed imageoverlay and significantly complicate image processing.The flat plate approach of defining a reference plane


provides reference data for all azimuth locations withinthe PMI system field-of-view, eliminating concerns ofreference/deformed image overlay. Thus, subsequentmeasures of rotor blade deformation were obtained rel-ative to this horizontal plane.

4. Experiment

The primary purpose of the Rotor Wake/ConfigurationAerodynamics Test (RW/CAT) was to investigate theeffect of the fuselage on the rotor wake geometry.This required measuring the independent and combinedwakes of the rotor and fuselage to isolate nonlinear in-teractions. The Isolated Rotor Test System (IRTS) wasconstructed since it is impossible to investigate isolat-ed rotor and fuselage effects using models incorporat-ing the rotor and fuselage about a single drive system.The IRTS system was post-mounted from the ceilingof the tunnel and permitted rotor operation either aloneor with a fuselage supported underneath it. These con-figurations are shown in Figs 6(a–b). The fuselage waslowered to the test section floor when the rotor wastested in the isolated configuration. The rotor systemconsisted of a Mach-scaled, four bladed, fully articu-lated rotor with a rotor radiusR = 860 mm. These re-search blades were not aeroelastically scaled, but rathervery stiff in flapwise (bending along the blade length),chordwise (bending along the blade width), and tor-sional (twisting along the blade length) directions. Theblades had a rectangular planform,−8 degrees of lineartwist, and a NACA 0012 airfoil along the span. Theblades and their properties are documented in refer-ence 12 and a detailed description of the hub can befound in reference 13.

Conventional laser velocimetry (LV) and DopplerGlobal Velocimetry (DGV) [14] measurements of flow-field velocity were acquired to map the rotor wake ge-ometry. The rotor wake geometry and vortex strengthare largely affected by blade position and deformation.Therefore, a complete database of wake geometry mustinclude blade position and deformation data. To mea-sure the blade deformation profiles, the PMI systemdescribed in Section 3 was installed in the floor of thefacility as shown in Fig. 5. The PMI projector was posi-tioned at approximately 50% rotor radius on the blade-advancing side of the helicopter, projecting verticallyto illuminate the underside of the rotating blades. A 1-x 1-meter floor opening near tunnel centerline (Fig. 5)provided ample optical access for the PMI projector.This opening also housed the third component of the LV

system used for aerodynamic velocity measurements.No modifications were made to the existing LV hard-ware for the incorporation of the PMI system. ThePMI receiver camera was mounted at the same stream-wise location and height as the projector, 2.3 metersoutboard perpendicular to tunnel centerline. A singlefloor panel was removed to accommodate the cameraand provide optical access. The PMI and DGV systemswere synchronized and operated simultaneously to pro-vide near instantaneous blade deformation / wake ve-locity correlations for several flight conditions. Unifi-cation of the two instruments required the synchroniza-tion of 9 video cameras, 5 data acquisition computers,and two lasers. This represents the first known attemptat using unified global instrumentation techniques tomeasure dynamic blade deformation and aerodynamicreaction [15].

A typical data point was acquired using the follow-ing procedure. After daily calibrations and model in-spections, the tunnel test section was secured and therotor was operated in hover. Tunnel speed was thenincreased to the desired operating condition as modelparameters were set simultaneously. Upon reachingsteady state, data were acquired by the tunnel static da-ta system, DGV data acquisition system, and PMI dataacquisition system. The DGV and PMI systems ac-quired instantaneous, conditionally sampled azimuth-dependent data for blade #1 azimuth positions of 0◦

to 90◦ in 10◦ increments. The azimuth (Ψ) and bladenumber convention is shown in Fig. 7. These azimuthswere chosen to maximize the amount of unsteady bladedeformation data obtained while minimizing data stor-age requirements. PMI measurements were obtainedfor two different blades: blade #4 forΨ = 90◦ to 110◦

(Ψ = 0◦ to 20◦ for blade #1), and blade #1 forΨ = 70◦

to 90◦. PMI data for blade #1 azimuth positions of30◦

to 60◦ were not processed since the blades, in major-ity, were out of the PMI receiver field-of-view. Ap-proximately 30 images were acquired at each azimuthfor each flight condition. Figure 8 shows an examplePMI data image viewing the underside of the blade andidentifies the blade geometry. Note the image is rotated90◦, with the tunnel ceiling corresponding to the rightside of the image. The projected grid lines requiredto form moire fringes are readily apparent. Creationof the moire patterns and determination of the quanti-tative deformation profiles is performed computation-ally. The PMI data were processed off-line followingcompletion of the image acquisition.


(a)

(b)

Fig. 6. (a) Isolated Rotor Test System, IRTS, and (b) IRTS+ fuselage.

Table 1Rotor and tunnel conditions corresponding to the seven flight conditions addressed in this paper. Rotorspeed was 2000 rpm for all cases

Condition Dynamic Free Stream Rotor Rotor Shaft Rotor ThrustPressure Q (Pa) Velocity (m/s) Advance Ratio Angle (degrees) Coefficient

418 47.9 9.1 0.05 0 0.0064419 47.9 9.1 0.05 0 0.0040423 47.9 9.1 0.05 0 0.0080424 478.7 27.1 0.15 0 0.0064425 478.7 27.1 0.15 −3 0.0064430 478.7 27.1 0.15 −3 0.0080431 1100.1 41.6 0.23 −3 0.0064

5. Results

PMI measurements were acquired for a variety oftest conditions, both with the isolated rotor and withthe fuselage in place. Table 1 contains tunnel velocityand rotor parameters for the seven flight conditions dis-cussed here. For brevity of discussion, the phrase “con-

dition 418” will be abbreviated C418 and likewise forthe other conditions. The range of forward speeds from9.1 m/s to 41.6 m/s covers the helicopter flight envelopefrom very low speed operations to moderate forwardspeeds, while the thrust coefficient range from 0.004to 0.008 simulates thrust levels from 1g to maneuverthrust values. The rotor advance ratio, defined as the


Blade 1

Blade2

Blade 3

PMI MeasurementArea

Blade 4

Ψ =0

Ψ =90Ψ =270

Ψ =180

V

Fig. 7. Rotor azimuth and blade numbering convention. The PMI measurement area is also illustrated.

Leading Edge

TipRoot

Trailing Edge

TunnelCeiling

Flow Direction

Fig. 8. Rotor blade geometry within PMI images.

ratio of the free stream tunnel velocity to the blade tipvelocity, has been calculated based on a constant rotorspeed of 2000 rpm. Figure 9 presents quantitative rotorblade deformation profiles for blade #4 atΨ = 0◦ atthe seven different test conditions summarized in Ta-ble 1. The fuselage was in the lowered position for thedata shown in Fig. 9. The reason for the approximate4◦ offset in the blade rotation direction remains to befully explained, but could be the combined result of (a)blade lead-lag, (b) shaft encoder offset errors, or (c)PMI receiver viewing angle. This 4◦ offset is consis-

tent among other azimuth locations measured. The de-formation profiles on the left side of the chart are mea-sured with respect to a horizontal plane aligned verti-cally with the center of rotation of the rotor hub. Pro-files on the right side of the chart indicate the change inblade deformation as a function of test condition withrespect to C418. Changes in blade shape can be clearlyobserved. Figure 9 also illustrates the ability of thePMI technique to measure deformation over the entireblade.


FlightCondition Blade Deformation Profile Blade Shape Change

from Condition 418

418

419

423

424

425

430

431

-5.0 35.0

Blade Deformation, mm Blade Shape Change, mm

-10.0 0 +10.0

500 mm

Fig. 9. PMI measured blade deflection profiles for seven different flight conditions, blade #4,Ψ = 90◦.

Blade Span, r/R

Def

lect

ion,

mm

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10

5

10

15

20

25

30

35ABC

C425, Advance Ratio = 0.15 (B)

C418, Advance Ratio = 0.05 (A)

C431, Advance Ratio = 0.23 (C)

Fig. 10. Effect of rotor advance ratio on blade deflection for blade #4, 25% chord,Ψ = 90◦.

Pixels corresponding to the 25% chord location, de-fined as a line running parallel to the blade leading edge

at 25% of the blade width, have been extracted from thePMI processed data and plotted as a function of span in


Blade Span, r/R

Def

lect

ion,

mm

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10

5

10

15

20

25

30

35A

B

C419, Thrust Coefficient = 0.0040 (B)

C418, Thrust Coefficient = 0.0064 (A)

Fig. 11. Effect of rotor thrust coefficient on blade deflection for blade #4, 25% chord,Ψ = 90◦.

Blade Span, r/R

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Def

lect

ion,

mm

0

5

10

15

20

25

30

35A, B

C, D

C418, fuselage down (A)

(B)

(C)

(D)

C418, fuselage up

C425, fuselage down

C425, fuselage up

Fig. 12. Fuselage effects on mean blade deflection for blade #4, 25% chord,Ψ = 90◦.

Figs 10 and 11. The blade span is expressed in terms ofthe normalized rotor radius,r/R , whereR = 860 mm.These figures further illustrate the quantitative effectsof flight condition on the blade deformation profile.The PMI data have been anchored to a value of 0 mm ata point 23% rotor radius at 25% chord, corresponding

to the edge of the blade / hub mechanical linkage knownas the rotor cuff attachment. This is necessary since thePMI system measures relative surface displacements inits current configuration. The data shown represent themean blade profiles for blade #4 atΨ = 90◦, loweredfuselage. As advance ratio increases from 0.05 (C418)


A

C

D

B

Blade Span, r/R

Def

lect

ion

RM

S F

luct

uati

on, m

m

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

PMI SystemNoise Floor

C418, fuselage down (A)

(B)

(C)

(D)

C418, fuselage up

C425, fuselage down

C425, fuselage up

Fig. 13. Fuselage effects on blade dynamic deflection for blade #4, 25% chord,Ψ = 90◦.

Blade Span, r/R

Def

lect

ion,

mm

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10

5

10

15

20

25

30

35BA

DC

C418, Blade 4 (B)

C418, Blade 1 (A)

C425, Blade 4 (D)

C425, Blade 1 (C)

Fig. 14. Blade-to-blade comparison, mean deflection profiles at 25% chord,Ψ = 90◦.

to 0.23 (C431) in Fig. 10, the deflection of the blade de-creases from the zero reference point by approximately4 mm at the tip. The flight condition change is a changein forward speed for a fixed thrust condition of the rotorsystem. Each blade is effectively working harder at thetwo lower flight conditions, as the speeds are below the

minimum power speed. Because the rotor hub is fullyarticulated, the inboard flapping hinge allows the bladetips to move vertically upward from the plane of therotor, or cone, and come to an equilibrium position withrespect to the lift and centrifugal forces on the blade.For the lower flight speeds, the blades are coned higher


Blade Span, r/R

Def

lect

ion

RM

S F

luct

uati

on, m

m

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

A

D

C

B

PMI System Noise Floor

C418, Blade 4 (B)

C418, Blade 1 (A)

C425, Blade 4 (D)

C425, Blade 1 (C)

Fig. 15. Blade-to-blade comparison, dynamic blade deflection at 25% chord,Ψ = 90◦.

Blade Span, r/R

Bla

de T

wis

t, d

egre

es

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

C418Regression

Fig. 16. Spanwise blade twist distribution, blade #4,Ψ = 90◦. Regression slope= 0.0102 deg./mm.

and deflected greater with respect to the hinge. In eachcase, the rotor blade is shown to be deflecting linearlyalong the spanwise direction, indicating little flapwisebending other than the steady-state coning deflection.For these flight conditions, the PMI data shows trends

which seem to be reasonable and correct.The effect of varying the rotor thrust or blade load-

ing for a constant forward flight speed is presented inFig. 11. In this figure, C419 is a thrust level well belowa normal scaled operating condition, and C418 increas-


70 80 90 100 1100

5

10

15

20

25

30

35

40

45

C418

C419

C425

C425, Theory

Rotor Azimuth, degrees

Bla

de T

ip D

efle

ctio

n, m

m

Fig. 17. Azimuth dependent mean blade tip deflection,r/R = 1.0. Rotor shaft angle for C425 was set to−3 degrees.

Blade 1

Blade 1

Blade 4/1

Blade 4

Blade 4

(a) Dewarped PMI Data (b) Blade Deflection Profiles

500 mm

Ψ = 90

Ψ = 80

Ψ = 70

Ψ = 100

Ψ = 110

-5.0 40.0

Blade Deformation, mm

Fig. 18. Azimuth dependent PMI data and measured blade deformation profiles, C425, fuselage down. Rotor shaft angle was set to−3 degrees.

es thrust to a nominal scaled thrust. In Fig. 11 it canbe clearly seen that blade 4 deflects by approximately10 mm in the change from C419 to C418. As in theanalysis for Fig. 10, this change is related to the bal-ance of forces around the flapping hinge, and the flightcondition for C419 is such that the low rotor lift doesnot create very much steady state coning. As thrust

is increased from C419 to C418, the blade loading forthe low speed flight condition increases and the bladescone upwards. PMI measures a deflection which corre-sponds to a coning change of about 0.5◦. Calculationsof the expected steady state coning change between thetwo conditions is 0.5◦, indicating that PMI is measuringthe correct deflection.


The results shown in Fig. 12 show the effects ofthe fuselage on blade deflection. Moving the fuselageup and down beneath the rotor system only slightlychanged the cyclic control settings of the rotor system,where the integrated rotor thrust and forward speedwere held constant. Although the blades were free toadjust their position relative to the flapping hinge, thefuselage effects are mostly upwash effects fore and afton the rotor disk [16], and not much change in de-flection is observed at the 90◦ azimuth measurementlocation. The deflection of the blade relative to thereference point was constant for conditions with ei-ther the isolated rotor or the rotor/fuselage combina-tion. Fuselage effects on blade unsteadiness are shownin Fig. 13 for C418 and C425. The PMI system noisefloor is also plotted in Fig. 13, determined by calculat-ing single pixel statistics from an ensemble of imagesof the aluminum honeycomb calibration panel. Thefact that the revolution-to-revolution RMS deflectionfluctuations are above the PMI system noise floor in-dicates the fluctuations are blade induced and not aninstrumentation artifact. For the very low speed flightcondition C418, it is evident that the flow environmentis turbulent and thus causes blade #4 to fluctuate upto 1 mm when the fuselage is in the lowered position.The fluctuation decreases as the fuselage is raised be-neath the rotor. The premise for this phenomenon isthat the flow over the fuselage is locally accelerated,resulting in a wake structure near the blades that con-tains a significant vertical velocity component. Thisvelocity structure may generate an integrated force onthe blades which tends to dampen any revolution-to-revolution unsteadiness. Further experimentation is re-quired to verify this hypothesis.

The indexing of the rotor blades and acquisition trig-ger cycles enabled data to be obtained for both blades1 and 4 at 90◦ azimuth. This allows the rather inter-esting blade-to-blade comparison shown in Figs 14 and15. Figure 14 shows that although the blades were ofthe same design, blade 1 consistently flew higher thanblade 4. There are several possible explanations. Whentesting rotor models, the blades are usually tracked ina hover condition; this means the blades are adjustedsuch that the blade tips fly within the same plane, ortrack. For this model, the blades are usually consideredwithin the tracking tolerance if the tips are trackingwithin 5% chord of each other. This equates to 5 mmfor this blade set. The difference in the blade deflec-tion measured by the PMI system for the blades is lessthan the 5 mm tracking tolerance, so a probable ex-planation for the blade-to-blade difference is the rotor

track tolerance was greater than the resolution of thePMI measurements. Figure 15 shows the revolution-to-revolution RMS fluctuations between blades 1 and 4atΨ = 90◦. The difference in RMS fluctuation is prob-ably related to manufacturing differences in the bladecomposite layup structure. Each of the blades experi-ences the high fluctuations for C418 and the fluctua-tions decrease for the higher speed C425. Figure 14 hasshown the mean deflection is approximately the samefor the blades, and the flowfield each blade is experi-encing should be almost exactly the same since thesedata were acquired in the same data set. The differ-ence in the RMS fluctuation can only be explained byblade-to-blade differences which the PMI technique issensitive enough to detect.

The PMI deflection data were interrogated for thedisplacement of the leading and trailing edge of theblade chord along the span of the blade. These deflec-tions were converted into a chordwise angle, or twist,at each spanwise station. Figure 16 shows the mea-sured twist along the span of the blade. The bladeswere designed and fabricated with a built-in linear twistof 8 degrees nose down referenced from the center ofrotation to the tip. Figure 16 shows that the PMI mea-surements were able to capture the twist of the bladeswithin 1 degree which is considered good. The PMImeasurements are hampered in this area by the smallchord of the blades. The measurement error and fluc-tuations are exaggerated in the twist computation sincea small change in deflection will have a large change ineffective angle over a small blade chord. PMI was ableto distinguish the changes in collective angle settingof the blades for other flight conditions and the PMImeasurements appeared to correlate well with the rotorcontrol system measurements of changes in rotor bladepitch.

The azimuth-dependent nature of blade deflectionis shown in Fig. 17. Blade tip deflection data atr/R = 1.0 is plotted versus azimuth for C418, C419,and C425, fuselage down. While tip deflections forC418 and C419 remain relatively constant as a functionof azimuth, C425 exhibits a downward trend attributedto the−3◦ rotor shaft pitch setting for this flight con-dition. A theoretical prediction of blade tip positionfor C425 based on simple geometry is also shown inFig. 17. The PMI data matches the theoretical tip de-flection profile reasonably well. Differences betweenthe measured and predicted values are likely due tothe simplicity of the theoretical model, which does notaccount for rotor dynamics or flow field complexities.The blade deflection azimuth dependencecaused by the


−3◦ rotor shaft pitch angle of C425 is clearly demon-strated in the PMI measured blade deflection profilesshown in Fig. 18. An ensemble of conditionally sam-pled, azimuth dependent PMI data were assembled toconstruct the figure. Figure 18a shows dewarped PMIdata images for each conditionally sampled azimuthlocation, and the resulting quantitative displacementprofiles have been overlayed in Fig. 18(b).

6. Accuracy and resolution

This experiment was a first attempt at using PMI formeasuring dynamic rotor blade deflections. Since thistype of data has never been acquired for the hub andblades used in this test, there is no standard to compareagainst for determination of absolute accuracy. Previ-ous laboratory experience has shown the largest sys-tematic error in PMI measurements lies in the abilityto accurately determine the angles of illumination andobservation. These errors were minimized by perform-ing in-situ calibration of the instrument to determinethe fringe sensitivity constant [FSC, equation (1)] asdescribed in Section 3.3. Calibrations of the fringe sen-sitivity constant agreed to within 3%, establishing thebest case absolute accuracy of the measurements. Us-ing data from the in-situ calibrations, the PMI systemnoise floor was determined to be 0.08 mm based onsingle pixel statistics. This noise floor defines the in-strument’s minimum deformation resolution. Most ofthe data exhibits slight undulations along the spanwiseprofiles. These are most likely image processing arti-facts, as the blades are too stiff to develop higher orderbending modes at the flight conditions tested. Work iscontinuing to diagnose the causes of these artifacts.

7. Conclusions

This initial proof-of-concept test has demonstratedthe capability of Projection Moire Interferometry (PMI)to acquire accurate, full field rotorcraft blade deforma-tion data. These are believed to be the first measure-ments of their kind providingnear-instantaneous, quan-titative, whole field blade deformation profiles condi-tionally sampled as a function of rotor azimuth. Thenear-instantaneous nature of the measurements permit-ted examination of both mean and time varying statis-tics such as blade unsteadiness. The results obtainedshow changes in blade deflection, twist, and dynam-ic behavior with varying operating conditions. Mea-

surement accuracy is believed to be within 3%, with0.08 mm deformation resolution. The simultaneousapplication of PMI and Doppler Global Velocimetry(DGV) was successfully performed and represents thefirst attempt at using unified global instrumentationtechniques to measure dynamic blade deformation andaerodynamic reaction. PMI development will continuefor both independent operation and operation as part ofa unified instrumentation system to measure the rotorwake and blade deformation in large scale wind tunnels.

Acknowledgements

The authors appreciate the support of the membersof the Advanced Measurement & Diagnostics Branch,the Rotorcraft Aerodynamics Group, and the 14- by22-Foot Subsonic Tunnel staff.

References

[1] E. Mercker, K. Pengel, R. Kube, B. Van der Wall, A. Boutierand F. Micheli, On the Blade Deformation Measured at aScaled Helicopter Rotor, in:Proceedings on AeromechanicsTechnology and Product Design, published by the AmericanHelicopter Society, Alexandria, Virginia, 1995.

[2] K. Patorski,Handbook of the Moire Fringe Technique, Else-vier Science Publishers, Amsterdam, 1993, vii–xi.

[3] H. Kurz and O. Leder, PC-based moire for field studies on thehuman body surface,Proc. SPIE 1395 (1990), 1066–1073.

[4] B.C. Dykes, Analysis of Displacements in Large Plates by theGrid-Shadow Moire Technique, in:Experimental Stress Anal-ysis and its Influence on Design, M.L. Meyer, ed., publishedby the Institution of Mechanical Engineers, London, 1970,pp. 125–134.

[5] C.A. Miles and B.S. Speight, Recording the shape of animalsby a moire method,Journal of Physics E: Scientific Instru-ments 8 (1975), 773–776.

[6] L. Pirodda, Shadow and projection moire techniques for abso-lute or relative mapping of surface shapes,Optical Engineer-ing 21(4) (1982), 640–649.

[7] Selected Papers on Optical Moire and Applications, G. In-debetouw and R. Czarnek, ed.,SPIE Milestone Series MS 64(1992).

[8] D. Phillips, Image Processing in C, Part 14: Warping andMorphing,C/C++ Users Journal 13(10) (1995), 55–68.

[9] J.F. Meyers, Doppler Global Velocimetry – The Next Gener-ation? paper AIAA-92-3897,AIAA 17th Aerospace GroundTesting Conference, Nashville, TN, July 6–8, 1992.

[10] K. Patorski, Handbook of the Moire Fringe Technique, Else-vier Science Publishers, Amsterdam, 1993, pp. 372–373.

[11] D.W. Robinson, Phase Unwrapping Methods, in:Interfero-gram Analysis: Digital Fringe Pattern Measurement Tech-niques, D.W. Robinson and G.T. Reid, eds., published by theInstitute of Physics, Bristol, 1993, pp. 194–229.

[12] J.D. Berry and M.J. May, Flapping Inertia for Selected RotorBlades, NASA TM 104125, AVSCOM TR 91-B-019, Novem-ber, 1991.


[13] A.E. Phelps III and J.D. Berry, Description of the US ArmySmall-Scale 2-Meter Rotor Test System, NASA TM 87762,AVSCOM TM-86-B-4, February, 1987.

[14] J.F. Meyers, Development of Doppler Global Velocimetry asa Flow Diagnostics Tool, Measurement in Fluids and Com-bustion Systems,Special Issue – Measurement Science andTechnology 6(6) (1995), 769–783.

[15] J.F. Meyers, J.W. Lee, M.T. Fletcher and B. South, Hardening

Doppler Global Velocimetry Systems for Large Wind TunnelApplications, paper AIAA-98-2606, 20th AIAA AdvancedMeasurement and Ground Testing Technology Conference,Albuquerque, NM, June 15–18, 1998.

[16] J.D. Berry and S.L. Althoff, Computing Induced Velocity Per-turbations Due to a Helicopter Fuselage in a Free Stream,NASA TM 4113, AVSCOM TR 89-B-001, June, 1989.

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