Overview of the Testing of a Small-Scale Proprotor

Larry A. Young Earl R. Booth, Jr.NASA Ames Research Center NASA Langley Research CenterMoffett Field, CA Hampton, VA

Gloria K. Yamauchi Gavin BothaNASA Ames Research Center NASA Ames Research CenterMoffett Field, CA Moffett Field, CA

Seth DawsonThe Boeing CompanyMesa, AZ

Summary

This paper presents an overview of results from the wind tunnel test of a 1/4-scale V-22 proprotor in theDuits-Nederlandse Windtunnel (DNW) in The Netherlands. The small-scale proprotor was tested on theisolated rotor configuration of the Tilt Rotor Aeroacoustic Model (TRAM). The test was conducted by ajoint team from NASA Ames, NASA Langley, U.S. Army Aeroflightdynamics Directorate, and TheBoeing Company. The objective of the test was to acquire a benchmark database for validatingaeroacoustic analyses. Representative examples of airloads, acoustics, structural loads, and performancedata are provided and discussed.

Nomenclature

CP Rotor power coefficient

CT Rotor thrust coefficient

FM Rotor hover figure of merit

Mtip Rotor tip Mach number

Presented at the American Helicopter Society55th Annual Forum, Montreal, Canada, May 25-27, 1999. Copyright © 1999 by the AmericanHelicopter Society, Inc. All rights reserved.

R Rotor radius

x/R non-dimensional tunnel longitudinalcoordinate, origin at hub, positivedownstream

y/R non-dimensional tunnel lateralcoordinate, origin at hub, positive onrotor advancing side

z/R non-dimensional tunnel verticalcoordinate, origin at hub, positive up

V Wind tunnel test section velocity

as Rotor shaft angle, deg, shaft vertical atzero degrees angle, positive aft

m Advance ratio, V/WR

h Proprotor efficiency, CTm/CP

y Rotor azimuth, deg.

W Rotor rotational speed, rad/s

Introduction

The successful introduction of civil tiltrotoraircraft is dependent in part on identifying andreducing, or suppressing, the noise generationmechanisms of tiltrotor aircraft proprotors. Toaccomplish these goals, a series of wind tunneltests with a new generation of tiltrotor models isrequired. The purpose of the Tilt RotorAeroacoustic Model (TRAM) experimentalprogram is to provide data necessary to validateperformance and aeroacoustic predictionmethodologies and to investigate anddemonstrate advanced civil tiltrotor technologies.The TRAM project is a key part of the NASAShort Haul (Civil Tiltrotor) (SH(CT)) program.The SH(CT) program is an element of theAviation Systems Capacity Initiative withinNASA. Reference 1 summarizes the goals andobjectives and the overall scope of the SH(CT)program.

The current scope of TRAM experimentalinvestigations is focused on the following:

1. Acquisition and documentation of acomprehensive isolated proprotoraeroacoustic database, including rotorairloads.

2. Acquisition and documentation of acomprehensive full-span tiltrotoraeroacoustic database, including rotorairloads, to enable assessment of keyinteractional aerodynamic and aeroacousticeffects by correlating isolated rotor and full-span TRAM wind tunnel data sets withadvanced analyses.

3. An advanced technology demonstrator testplatform for low-noise proprotors.

The first wind tunnel test of the TRAM projectwas an isolated rotor test in the Duits-Nederlandse Windtunnel (DNW) in TheNetherlands (Fig.1). This isolated rotor test wasthe first comprehensive aeroacoustic test for atiltrotor proprotor, including not only noise andperformance data, but airload and wakemeasurements as well. The TRAM isolated rotortest stand was installed and tested in the DNWopen-jet test section during two tunnel entries, inDecember 1997 and April-May 1998.

Fig. 1. TRAM Isolated Rotor Configuration inthe Duits-Nederlandse Windtunnel (DNW) in

The Netherlands

This paper provides an overview of the dataacquired during the TRAM DNW test. Follow-on plans for the full-span (dual-rotor, completeairframe) TRAM will be briefly discussed.

Description of Model and Wind TunnelFacility

A general description of the TRAM isolatedrotor configuration is found in Reference 2. Adescription of the DNW and its rotary-wing testcapability is found in Reference 3. The modelproprotor tested on the TRAM isolated rotor teststand was a 1/4-scale (9.5 ft diameter) V-22rotor. The rotor was counterclockwise rotating

(planform view over the rotor); i.e., it was aright-hand side or starboard rotor. The isolatedproprotor was tested in the 8x6m open-jet testsection of the DNW. The 1/4-scale V-22proprotor was tested at reduced tip speed of 0.63tip Mach number because of operationalconsiderations (the nominal design tip speed ofthe V-22 Osprey aircraft is Mtip = 0.71). Allairplane-mode proprotor data were acquired at0.59 tip Mach number (equivalent to that of theV-22 aircraft).

The TRAM isolated rotor test stand wascomprised of two major elements: the rotor andnacelle assembly and the motor mount assembly.The rotor and nacelle assembly was attached tothe acoustically treated isolated rotor test stand ata mechanical pivot or Ôconversion axisÕ (fig. 4).This conversion axis allowed the nacelle to bemanually rotated (in between tunnel runs) in 5degree increments from airplane to helicoptermodes. An electric motor provided power to therotor via a super-critical driveshaft. Rotor shaftangle changes were accomplished in flight withthe DNW sting, which automatically maintainedthe hub on tunnel centerline. All rotating datachannels were amplified by a Nationaal Lucht-enRuimtevaartlaboratorium (The NetherlandsNational Aerospace Laboratory, NLR) developedRotating Amplifier System (RAS) to enhancetransducer signal to noise ratios before enteringthe slipring (reference 4).

Electric Motor

High-Speed Supercritical Drive Shaft Assembly

Nacelle Assembly with 11.3:1 Gearbox

Rotating Amplifier System

1:1 Center Gear Box

Control System

Nacelle Tilt Conversion Axis & Coupling

Gimballed Rotor Hub & Pitch-Case Assembly Rotor Balance

Figure 4 -- Isolated Rotor Configuration(Planform View With Nacelle Assembly

Positioned in Airplane-mode)

A more detailed summary of the characteristicsof the TRAM isolated rotor test stand and the1/4-scale V-22 rotor is noted below:

¥ TRAM Isolated Rotor ConfigurationDescription (Fig. 4)

- Test stand is wind tunnel sting-mounted; compatible with DNW andNational Full-scale AerodynamicsComplex (NFAC) stings

- Drive train designed for nominal 300HP and 18,000 RPM motor; employs

one right-angle 1:1 gearbox and a11.3:1 gear reduction transmission

- Nacelle tilt/incidence angle (about theÔconversion axisÕ) is ground adjustable

- Six-component rotor balance andinstrumented torque coupling (withprimary and secondary measurements)

- 300-ring slip ring and rotating amplifiersystem

- Three electromechanical actuators and arise-and-fall swashplate; rotating andnonrotating scissor sub-assembly designallows full proprotor collective/cyclicranges without changing hardware

- rotor control system and consoledesigned to minimize re-riggingbetween different operating regimes

- Self-contained model utilities withinnacelle and motor-mount assemblies

- Motor-mount acoustically treated withfoam panels

- Nacelle assembly not acousticallytreated but geometrically scaled for V-22 aircraft outer mold

- Model capable of being tested to full V-22 operating envelope

- Rotor shaft interface hardware designedto easily install and test advancedproprotors on TRAM test stands

- Isolated rotor configuration is hardwarecompatible with the full-span model;hardware is shared between the two teststands

¥ Rotor Characteristics and Instrumentation(Fig. 5)

- Gimballed rotor hub with constantvelocity joint (spherical bearing andelastomeric torque links)

- Rotor hub is dynamically andkinematically similar to V-22 aircrafthubs

- 1/4-scale V-22 rotor set with bothstrain-gauged and pressure-instrumented blades

- Rotor pressure-instrumentation consistsof 150 transducers (three different typesof Kulite transducers) distributed overtwo rotor blades

- High fidelity scaling with respect to theV-22 rotor for blade/airfoil contours

- First elastic modes (flapwise,chordwise, and torsional) of bladesdynamically scaled to V-22 frequencies

- Both strain-gauged and pressure-instrumented blades have nominallyidentical mass distributions and CGlocations

- Adequate instrumentation provided toacquire a good blade/hub structural loaddata set for analytical correlation.

(a)

(b)

Fig. 5a-b TRAM 1/4-scale V-22 Rotor (a. Bladeand b. Hub)

Test Description

Two tunnel entries were conducted in the DNWwind tunnel with the TRAM isolated rotor teststand and the 1/4-scale V-22 rotor. The firsttunnel entry was in December 1997 and it wasfocused on TRAM test stand risk reduction andenvelope expansion for the 1/4-scale V-22 rotor.The second entry in April-May 1998 wasdevoted to acquiring a high quality isolated rotoraeroacoustic database for the SH(CT) program.

The German Dutch Wind Tunnel (DNW) islocated in Emmeloord in The Netherlands. It is aworld-class acoustic wind tunnel facility that hasbeen used for several important internationalrotorcraft acoustic test campaigns since itbecame operational in the late 1970Õs. NASAand the U.S. Army have previously conductedjoint helicopter acoustic tests in the DNW. Likemany of these previous tests, the U.S. Armyenabled access to the DNW facility for theTRAM isolated rotor acoustic test.

The DNW test focused mostly on low-speedhelicopter-mode test conditions. The testobjective priorities were in order of importance:detailed acoustic survey of Blade VortexInteraction (BVI) phenomena in helicopter-modedescent; broadband noise in hover and low-speedhelicopter-mode flight; parametric trend data (asa function of as, m, Mtip, and CT) on helicopter-mode acoustics; airplane-mode performance andacoustic measurements; transition flightperformance and loads measurements. Becauseof time and load limit constraints, transitionflight (-15 < a s < -75 degrees) measurementswere not made. Data was acquired to meet allother test objectives.

The TRAM isolated rotor test stand, as earliernoted, was mounted on the DNW sting. TheDNW sting is articulated to allow for not onlyangle of attack sweeps but beta/yaw angle andvertical sting translation sweeps as well. Thissting articulation capability proved to be veryuseful during the successful execution of the testprogram. The TRAM test stand was positionedinside the open-jet test section of the DNW. Theouter (outside the tunnel flow) containment ofthe test section was acoustically treated withfoam fairings. A DNW-provided acoustic

traverse was positioned underneath the model foracoustic measurements during the test. TheDNW also provided laser and associatedequipment to make laser-light-sheet flowvisualization and particle image velocimetrymeasurements during the TRAM test.

Proprotor Performance

Rotor performance measurements were madeduring the DNW test with the TRAM six-component rotor balance and an instrumented(torque and residual thrust) flex-coupling (seefigure 2).

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 0.005 0.01 0.015 0.02CT

Ho

ver

Fig

ure

o

f M

erit

Helicopter-Mode

Airplane-Mode

Fig. 2 Hover Figure of Merit (Mtip= 0.63)

The 1/4-scale V-22 TRAM figure-of-merit datacompares reasonably well with data from a large-scale proprotor hover test at NASA AmesResearch (Reference 5). No Reynolds numbercorrections have been made for the above datapoints. The two sets of data (airplane- andhelicopter-mode) reflect the two configurationsfor which hover data was taken. (Helicopter-mode is when the nacelle incidence angle withrespect to the motor-mount assembly and DNWsting axis is 75 degrees (near perpendicular) andairplane-mode is when the nacelle incidenceangle is zero degrees.) Because of bodyinterference effects from the TRAM test standmotor-mount sub-assembly, it is not surprising tonote a fairly substantial impact on hover figure

of merit for the Ôhelicopter-modeÕ versus theÔairplane-modeÕ configurations.

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 0.001 0.002 0.003 0.004 0.005 0.006

CT

Pro

pro

tor

Eff

icie

ncy

Mu = 0.325Mu = 0.350Mu = 0.375

Figure 3 Ð Airplane-Mode, Low-Speed Cruise,Proprotor Efficiency (Mtip= 0.59)

Figure 3 summarizes low-speed airplane-modeproprotor efficiency data acquired during thetest. A clean spinner fairing aerodynamic tarewas applied to the data in figure 3. Thisperformance data was acquired at the V-22aircraftÕs airplane-mode tip mach number(Mtip=0.59) and for m=0.325, 0.35, and 0.375.The data was acquired for the maximumpractical open-jet tunnel velocity, where the testsection flow field was still reasonably steady.Both primary and secondary rotor balancemeasurements are shown in the figure. Theproprotor efficiency trends measured arecomparable to performance data from previoustests (reference 6). The 1/4-scale V-22 TRAMDNW test results, though, will greatly augmentthis extremely limited airplane-mode cruiseperformance data set in the literature.

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.008 0.01 0.012 0.014

CT

CP

ASHAFT= -10 deg.-6 deg.0 deg.+6 deg.ASHAFT= +12 deg.

Fig. 4 Helicopter-Mode Power Polar (Mtip=0.63)

Figure 4 shows a power polar in helicopter-modeforward-flight for an advance ratio of m=0.15 andshaft angles ranging from as= -10 to +12degrees. This is only a small subset of the dataacquired during the DNW test. Hub/spinneraerodynamic tares were applied to the figure 4performance data. The 1/4-scale V-22 data isconsistent with full-scale XV-15 helicopter-mode isolated rotor test results at NASA AmesResearch Center (Reference 7); power polartrends are in general agreement, given thedifferences between rotor solidity and scalebetween the two tests. However, the 1/4-scaleV-22 performance data is far morecomprehensive in terms of the CT and rotor shaftangle sweeps performed.

Rotor Structural Loads and TrimmedControl Settings

The TRAM rotor had a strain-gaged blade forsafety-of-flight monitoring and blade structuralload measurement. Rotor trim (for zero gimbalangle) control settings as a function of rotor shaftangles and tunnel speed were also measured.

Figure 5 shows representative data for the rotorcontrol trim settings for cyclic pitch for the 1/4-scale V-22 rotor in helicopter-mode flight. Therotor was trimmed for zero one-per-rev gimbalangle. The cyclic pitch measurements werederived from the nonrotating control actuatorstroke positions, given the kinematic controlsystem equations. The general trend for the 1/4-scale cyclic pitch trim settings is consistent withprevious test results for proprotors in low-speedhelicopter-mode forward-flight (reference 7).

-8

-6

-4

-2

0

2

4

6

-15 -5 5 15

Rotor Shaft Angle, Deg.

Cyc

lic

Pit

ch,

Deg

.

Lateral Cyclic Pitch

Longitudinal Cyclic Pitch

Figure 5 Ð Rotor Cyclic Pitch Trim Settings(m=0.15 and CT=0.009)

Figure 6a-d is a sample set of contour plots forthe rotor flap bending loads. The redistributionof the flap bending moments across the rotordisk can be seen as the tunnel advance ratioincreases from m = 0.125 (Fig. 6a) to 0.20 (Fig.6d). Other structural load data were acquiredduring the DNW test, including blade chordbending moment and torsional loads andpitchlink, pitch-case, and flexbeam loads. Thedata will be used to validate a new generation ofcomprehensive aeromechanics analysis codes.

Bending Moment

Positive

Negative

(a) m=0.125

(b) m=0.15

(c) m=0.175

(d) m=0.20

Fig. 6a-d -- Flap Bending Moments forHelicopter-Mode Flight (CT=0.013 and as= -2

deg.)

Acoustics

Among the most crucial information acquiredduring the TRAM DNW isolated rotor test wasthe acoustic data from the 1/4-scale V-22 rotor.The TRAM test stand and DNW acoustictraverse are shown in figure 7.

Fig. 7 TRAM Isolated Rotor Configuration andthe DNW Acoustic Traverse

Acoustic data were acquired using a combinationof in-flow traversing and out-of-flow fixedmicrophones. Thirteen microphones wereequally spaced from -1.86 y/R to 1.86 y/R on atraversing microphone wing (also shown infigure 7). In addition, two microphones wereplaced outside the test section flow, one abovethe model and another located adjacent to thehub on the advancing side of the rotor. Piston-phone calibrations, background noisemeasurements, and installed model reflectiontests were performed.

For each rotor test condition, the rotor hub heightwas maintained constant while the hublongitudinal location was allowed to change withshaft angle. The microphone wing was traversedalong a plane 1.73 z/R beneath the center of therotor hub from -2.76 x/R to 2.76 x/R, centered onthe actual rotor hub x-location for the testcondition. At seventeen equally spaced locations,the traversing microphone wing motion wasstopped and data was acquired.

Figure 8 is a representative contour plot of theBlade Vortex Interaction Sound Pressure Level(BVISPL) acoustic survey measurements inhelicopter-mode operation. Prominent in figure8 is the BVI Ôhot spotÕ on the advancing-side ofthe rotor.

Fig. 8 -- Acoustic Survey of Proprotor inHelicopter-Mode (BVI Descent Condition)

The general characteristic of the 1/4-scale V-22acoustic survey is similar to 0.15-scale JVX rotorresults reported in reference 8 and full-scale XV-15 results in reference 9. However, the 1/4-scaleV-22 data from the DNW test is unique in itsscope (in terms of test envelope and acousticparametric trends measured), its quality (withrespective to the tunnel flow and acousticcharacteristics), and its comprehensiveness (withrespect to the number and types of aeroacoustic,aeromechanic, and rotor wake measurementsmade). The acoustic results from the 1/4-scaleV-22 DNW test will improve the understandingand reduction of tiltrotor BVI noise which isimportant for civil tiltrotor passenger andcommunity acceptance.

Acoustic data were acquired for a range ofadvance ratios (m=0.125, 0.15, 0.175, 0.2), rotorshaft angles (as= -14 to +12 degrees), and thrustsweeps (CT=0.009 to 0.014) were investigated.A detailed and comprehensive discussion of the1/4-scale V-22 TRAM acoustic results will befound in reference 10. One example of theBVISPL acoustic trend with increasing m, forconstant as and CT, is presented in figure 9. Aswould be expected, the maximum noise levelsincrease with increasing advance ratio.

Figure 9 -- BVISPL directivity trend withincreasing m for CT = 0.013.

Airloads

Airloads data for proprotors is extremely limited.Reference 11 reported on experimentalmeasurements for a generic tiltrotorconfiguration in hover. Several CFD studieshave been conducted for hovering tiltrotors (forexample, reference 12) to understand the flowmechanisms underlying high thrust conditions.Prior to the TRAM DNW test, there existed noairloads data for proprotor forward-flightoperating conditions (either in helicopter- orairplane-modes).

Figure 10a-b is a sample set of sectional pressurecoefficient data for the 1/4-scale V-22 rotor inlow-speed helicopter-mode forward-flight. Thedifference between advancing and retreating siderotor pressure coefficient distributions isdemonstrated by comparing figures 10a and 10b.The pressure coefficients arenondimensionalized by the local velocity. Theindividual pressure coefficient distributions infigure 10a-b are all scaled the same.

Fig. 10a -- Helicopter-Mode Forward-FlightSectional Pressure Coefficient Distributions

(y=90 deg., m=0.15, CT=0.009, & as= +2 deg.)

Fig. 10b -- Helicopter-Mode Forward-FlightSectional Pressure Coefficient Distributions

(y=270 deg., m=0.15, CT=0.009, & as= +2 deg.)

Figure 10a is the radial pressure coefficientdistribution for the rotor advancing-side (y=90deg.) and figure 10b is the distribution for therotor retreating side (y=270 deg.). Theretreating side pressure coefficients aresignificantly greater in magnitude than theadvancing side coefficients Ð as would beexpected for a trimmed rotor in forward-flight.

Figure 10a-b represents a very limited sample ofthe literally gigabytes of airloads data acquiredduring the 1/4-scale V-22 isolated rotor test atthe DNW. The airloads data will enable newinsights into tiltrotor noise mechanisms, will bean important validation data set for a newgeneration of aeroacoustic prediction tools, andwill hopefully inspire new noise reductionstrategies for tiltrotor aircraft.

Wake Flow Visualization andMeasurements

Both laser-light-sheet (LLS) flow visualizationand vortex trajectory measurements were madefor the TRAM 1/4-scale V-22 rotor, as well asparticle image velocimetry (PIV) vortex velocitymeasurements. Figure 11 is a representativelaser-light sheet flow visualization pictureacquired during the DNW test. The rotor bladeseen in figure 11 is at y=45 degrees and thevisible vortices in the figure are on theadvancing-side of the rotor.

Fig. 11 -- Laser Light Sheet Flow Visualizationof Trailed Tip Vortices

Acquisition of a series of laser-light sheetpictures enabled the definition of the three-dimensional vortex filament trajectories for the1/4-scale V-22 rotor on the advancing-side of therotor (fig. 12). Both clockwise andcounterclockwise vortices were observed in thelaser-light sheet pictures. The rotation directionof the observed advancing-side vortices is notedin figure 12.

-500

0

500

1000

1500-500 0 500 1000 1500

x (m

m)

y (mm)

¥ CCWo CW

V

blade

hub

Fig. 12 Ð Planform View of Vortex FilamentTrajectories (Low Thrust, Moderate Positive

Rotor Shaft Angle)

Reference 13 presents more detailed findingsfrom the DNW test with respect to the LLSimages, the vortex trajectory, and PIV results.One important observation made during theDNW test was the successful imaging of dualrotor vortices being trailed on the advancing sideof the proprotor in BVI descent conditions.

Future Plans

Preparations are underway to conduct a test of1/4-scale V-22 proprotors on the Full-SpanTRAM test stand (figure 13) in the NationalFull-scale Aerodynamics Complex (NFAC) 40-by-80 Foot Wind Tunnel at NASA AmesResearch Center.

Fig. 13 -- Full-Span TRAM

Comparison of 1/4-scale V-22 test results fromthe DNW isolated rotor test and the upcomingNFAC full-span TRAM test will enableassessment of interactional aerodynamic andacoustic effects.

Conclusion

NASA and the U.S. Army have made a majorinfrastructure investment in tiltrotor testtechnology through the continuing developmentof the TRAM. This investment has begun topayoff through acquisition of fundamentalaeroacoustic and aeromechanics data from a 1/4-scale V-22 isolated rotor tested in the DNW onthe TRAM isolated rotor configuration. TheDNW data will enable substantial improvementsin the predictive capability for tiltrotor aircraft.This paper has presented an overview of thescope of the data acquired during thisexperimental investigation. Continuation of in-depth tiltrotor experimental investigations willproceed with tests on a full-span (dual-rotor andcomplete airframe) TRAM configuration.

Acknowledgements

The experimental results in this paper werederived from research performed under theauspices of the Tilt Rotor Aeroacoustic Model(TRAM) project and the NASA Short Haul CivilTiltrotor program (SH(CT)). The TRAM andSH(CT) programs are led at NASA AmesResearch Center by the Army/NASA RotorcraftDivision and Advanced Tiltrotor TechnologyProject Office, respectively. Other major fundingpartners and research participants in theexperimental research effort were the U.S. ArmyAeroflightdynamics Directorate (AFFD) locatedat Ames, NASA Langley Research Center FluidMechanics and Acoustics Division, and TheBoeing Company (Mesa, Arizona). In addition,the outstanding support provided by the Duits-Nederlandse Windtunnel staff during theexecution of the wind tunnel test was critical tothe success of the test.

The TRAM DNW test team also extends theirthanks to the staff of the Dutch Nationaal Lucht-en Ruimtevaartlaboratorium (NLR) and U.S.Army European Research Office (ERO) for theirtechnical and programmatic contributions to theTRAM development and the DNW wind tunnelaccess, respectively.

References

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2. Young, L.A., ÒTilt Rotor AeroacousticModel (TRAM): A New RotorcraftResearch Facility,Ó American HelicopterSociety (AHS) International SpecialistÕsMeeting on Advanced RotorcraftTechnology and Disaster Relief, Gifu,Japan, April 1998.

3. Van Ditshuizen, J.C.A., ÒHelicopter ModelNoise Testing at DNW -- Status andProspects,Ó Thirteenth European RotorcraftForum, Arles, France, September 1987.

4. Versteeg, M.H.J.B. and Slot, H.,ÒMiniature Rotating Amplifier System forWindtunnel Application Packs 256 Pre-Conditioning Channels in 187 Cubic Inch,ÓSeventeenth International Congress onInstrumentation in Aerospace SimulationFacilities (ICIASF), Naval PostgraduateSchool, Monterey, CA, September 1997.

5. Felker, F,F. Signor, D, Young, L.A., andBetzina, M., ÒPerformance and Loads DataFrom a Hover Test of a 0.658-Scale V-22Rotor and Wing,Ó NASA TM 89419, April1987.

6. Edenborough, H.K., Gaffey, T.M., andWeiberg, J.A., ÒAnalyses and TestsConfirm Design of Proprotor Aircraft,ÓAIAA Conference Paper # 72-803, AIAA4Õth Aircraft Design, Flight Test, andOperations Meeting, Los Angles, CA,August 1972.

7. Light, J.S., ÒResults from an XV-15 RotorTest in the National Full-ScaleAerodynamics Complex,Ó the AmericanHelicopter Society Fifty-third AnnualForum, Virginia Beach, VA, April 1997.

8. Marcolini, M. A., Conner, D. A., Breiger, J.T., Becker, L. E., and Smith, C. D., "NoiseCharacteristics of a Model Tiltrotor,"presented at the AHS 51st Annual Forum,Fort Worth, TX, May 1995.

9. Kitaplioglu, C., McCluer, M., and Acree, Jr.,C.W., ÒComparison of XV-15 Full-ScaleWind Tunnel and In-Flight Blade-VortexInteraction Noise,Ó American HelicopterSociety Fifty-third Annual Forum, VirginiaBeach, VA, April 1997.

10. Booth, E.R., McCluer, M., and Tadghighi,H., ÒAcoustic Characteristics of a ModelIsolated Tiltrotor in DNW,Ó presented at theAmerican Helicopter Society 55th Forum,May 1999.

11. Tung, C. and Branum, L., ÒModel Tilt-Rotor Hover Performance and SurfacePressure Measurements,Ó Forty-SixthAnnual Forum of the American HelicopterSociety, Washington D.C., May 1990.

12. Yamauchi, G.K. and Johnson, W., ÒFlowField Analysis of a Model Prop Rotor inHover,Ó Twenty-first European RotorcraftForum, Saint-Petersburg, Russia, August1995.

13. Yamauchi, G. K., Burley, C. L., Merker, E.,Pengal, K., and JanikiRam, R., "FlowMeasurements of an Isolated Model TiltRotor," presented at the AmericanHelicopter Society 55th Forum, May 1999.