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Vortex Wake Geometry of a Model Tilt Rotor in Forward Flight

Gloria K. Yamauchi Wayne Johnson Alan J. Wadcockgyamauchi@mail.arc.nasa.gov wrjohnson@mail.arc.nasa.gov awadcock@mail.arc.nasa.gov

Aerospace Engineer Aerospace Engineer Principal ScientistNASA Ames Research Center NASA Ames Research Center Aerospace Computing Inc.Moffett Field, California, USA Moffett Field, California, USA Moffett Field, California, USA

Abstract

The vortex wake trajectory from one rotor of a 0.25-scale V-22 tiltrotor model was measured for four testconditions in the NASA Ames 40- by 80-Foot Wind Tunnel. Vortex wake images were acquired using a laserlight sheet and video camera. Wake trajectories were constructed by extracting vortex positions from the videoimages. Wake trajectories were also calculated using the comprehensive analysis CAMRAD II. Measured andcalculated wake geometries exhibit similar trends when advance ratio is varied at fixed thrust or when thrust isvaried at fixed advance ratio.

Notation

A rotor disk area, πR2

CT rotor thrust coefficient, thrust/(ρ(ΩR)2A)Mtip blade tip Mach number, ΩR /sound speedR blade radius

µ advance ratio, tunnel speed/ΩRΩ rotor rotational speedρ air densityΨb azimuth position of reference blade relative

to laser sheet

Introduction

The aerodynamics of a tiltrotor blade can bevastly different than a conventional helicopter blade.Compared to a helicopter blade, tiltrotor blades have ahigher built-in twist, higher solidity, and higher diskloading. These differences result in very differentblade load distributions. For this reason existinganalytical and empirical models developed forhelicopter rotors do not necessarily apply to tiltrotors.In particular, wake geometry models for helicoptersare inadequate for tiltrotor wake systems. In bothlevel flight and descent conditions where blade-vortexinteraction (BVI) occurs, tiltrotor blades can undergonegative tip loading over a substantial region of therotor disk unlike conventional helicopter blades. Thenegative tip-loading causes dual vortices, of oppositesign, to be shed from a single blade. The dual vorticesgreatly complicate the wake geometry and present achallenge to the analyst trying to model the wake.

Presented at Heli Japan 2002, AHS International Meetingon Advanced Rotorcraft Technology and Life SavingActivities, Tochigi, Japan, November 11-13, 2002.Copyright 2002 by the American Helicopter SocietyInternational, Inc. All rights reserved.

In 1998, an isolated 0.25-scale tiltrotor was testedin the Duits-Nederlandse Windtunnel Large Low-Speed Facility (Ref. 1). The model was the Tilt RotorAeroacoustic Model (TRAM). During thisaeroacoustic test, wake geometry measurements andwake velocity field measurements were acquired onthe advancing side of the rotor disk (Ref. 2). The flowmeasurements revealed, for the first time, thecomplicated geometry of the tiltrotor wake. Counter-rotating vortex pairs were clearly evident, but the ageand origin of the vortices were not determined in thelimited test time available.

Recent correlation efforts using CAMRAD II andairloads data from the TRAM rotor highlighted theinadequacies of conventional helicopter wake modelsfor predicting tiltrotor aerodynamics (Refs. 3-4). Anew wake model designed to capture the complexitiesof a tiltrotor wake was therefore developed (Ref. 5)for CAMRAD II.

In late 2000, the isolated TRAM rotor wasincorporated into a full-span tiltrotor model and testedin the NASA Ames 40- by 80-Foot Wind Tunnel.Extensive planning and preparation for wakegeometry and velocity field measurements (usingParticle Image Velocimetry (PIV)) preceded the test.Since wake age was not measured during the isolatedrotor test, the objectives of the full-span wakegeometry measurements were to identify the locationalong the blade span where the dual vortices formedand to record the location of the vortex pair as afunction of rotor azimuth. Flow visualization using alaser light sheet (LLS) would be used to achieve thisgoal. PIV would then be used to measure size andstrength of selected vortices. Test conditions wereselected to match the isolated rotor flow measurementconditions of Ref. 2. Unfortunately, the test in the 40-by 80-Foot Wind Tunnel ended prematurely due tofailure of a lubrication system and no PIV data wereobtained. Limited wake geometry measurements wereacquired, however, and these data are the subject ofthis paper.

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This paper provides a detailed description of theexperimental set-up used to acquire the wake imagesand the subsequent analysis of the images. Details ofthe CAMRAD II tiltrotor wake models are described.Comparisons between the measured and calculatedvortex locations are presented and discussed for fourtest conditions.

Facility and Model Description

The 40- by 80-Foot Wind Tunnel at NASA AmesResearch Center is a closed-circuit, single-returntunnel driven by six 40-foot diameter fans. The testsection is acoustically treated and measures 39-feethigh, 79-feet wide, and 80-feet long. The maximumspeed of the tunnel is approximately 270 knots.

NASAs Tilt Rotor Aeroacoustic Model (TRAM)is a nominal quarter-scale representation of the V-22aircraft. The TRAM can be configured as an isolatedrotor or a full-span aircraft. Each 3-bladed rotor has adiameter of 9.5 feet. The full-span configuration hasdual powered rotors with manually adjusted nacelletilt. Model measurements include blade structuralloads, blade airloads, individual rotor forces andmoments, fuselage forces and moments, and wingstatic pressures. Further details about the TRAM arereported in Refs. 6-8. Figure 1 shows the TRAMinstalled in the Ames 40- by 80-Foot Wind Tunnel.

Experimental Set-Up for Flow Measurements

The large size of the test section combined withlimited optical access presents a challenge for thesuccessful implementation of optical techniques. Theplanned PIV acquisition requirements dictated thecamera and laser set-up which were also used forwake geometry measurements. If wake geometrymeasurements alone were planned, a less complexset-up would have been implemented.

In order to identify the spanwise location of thedual-vortex formation, a vertical light sheetcorresponding to a blade azimuth angle close to 90degrees on the advancing side of the left-hand rotordisk was chosen for the current study. The placementof the light sheet at the left-hand rotor was necessarygiven the limited choices for locating the laser, optics,and cameras. The laser beam was introduced into thetest section from the right-hand-side of the test section(pilot's viewpoint). Figure 2 illustrates the launch ofthe laser sheet into the test section from a speciallyconstructed wall cavity at the required streamwisestation. The laser sheet passed above the RH rotor(pilot's view) and immediately downstream of thespinner on the LH rotor in order to reach the outboardhalf of the rotor disk where the measurements weremade on the advancing blade. The white rectangle inFig. 2 indicates the camera field of view.

Laser and Laser Sheet OpticsThe laser was placed on an optical table located

on a wide external platform outside the test section onthe right-hand-side of the tunnel (pilot's view). Thelaser is a Spectra Physics PIV400 dual-oscillatorNd:Yag laser with 350 mJ/pulse at 532nmwavelength. Laser pulse duration is 9 ns. For theTRAM tip radius of 4.75 ft the desired tip Machnumber of 0.63 corresponds to a 1/rev frequency ofabout 23.58 Hz. The maximum repetition rate for thelaser is 15 Hz, therefore the rotor 1/rev pulse train wasdivided by 2 giving an acceptable laser trigger rate of11.79 Hz. The laser was tuned to operate at afrequency of 11.8 Hz ±1 Hz, thereby providingsufficient margin for day-to-day variations in thespeed of sound caused by ambient temperaturechanges.

A test section wall cavity was modified to housethe necessary LLS optics. Beam-steering mirrors andoptics for spreading the laser sheet were rail-mountedinside the wall cavity to simplify streamwiseadjustment of the laser sheet position.

Laser Sheet AlignmentThe vertical laser sheet was positioned cross-

stream at a suitable streamwise location so as to grazethe blade trailing edge with the blade at a nominalangle of 90 degrees. The procedure for locating thesheet is described next.

The TRAM model was pre-positioned with thenacelles at 85 degrees and the fuselage angle-of-attackat +11 degrees (nose up). The resulting tip-path-planewas at 6 degrees nose up. The rotor gimbal lock wasinstalled on the LH rotor and the LH rotor manuallyrotated until the primary blade was close to 90degrees blade azimuth. A vertical plane wasestablished in the test section using an industrial He-Ne laser level. This vertical plane was then adjustedto be perpendicular to the tunnel axis using acousticpanels on opposite walls of the test section asmarkers. Reference drawings for the wind tunnelindicate a maximum allowable error in wall panelplacement of ±1/4 inch. This implies a maximumerror in placement of the cross-stream laser sheet of±1/2 inch in 79 ft, or ±0.030 degrees. Using thereference marks on the tunnel wall, the laser level wasthen adjusted in the streamwise direction until thesheet grazed the blade trailing edge over the outboardhalf of the blade. Markers were placed on the tunnelwall to identify this reference plane. The laser levelwas then turned off and the Nd:Yag laser launchedinto the test section. The focused sheet (about 1 mm-thick) was positioned about 2 mm downstream of theblade trailing edge (over the outermost 50% of theblade radius). The focus was then translated outboardof the blade tip in order to produce a 4-mm thick sheetin the area of interest, satisfying the requirements forthe planned PIV measurements. The resulting LLSwas 4-mm thick, grazing the blade trailing edge, withthe LLS in the vertical plane precisely perpendicular

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to the tunnel axis. The markers on the tunnel wallidentifying the reference plane were continually usedto check for any obvious drift in the laser sheet.

CameraFigure 3 shows the installation of the LLS

camera in the camera port on the left side of thetunnel. The TRAM is seen in the background throughthe window. Cantilevered to the side of the pan-and-tilt stage is a strobe and above is the LLS camera. Thecamera is a Hitachi KP-F1U with Fujinon 75 mm lensat F4.0 with automatic gain control. The camera wascarefully positioned using the remote-control pan-and-tilt stage and focus was set manually. The twoboxes on the left inside the wall cavity are the strobepower supply sitting on top of the pan-and-tiltcontroller. All this equipment was designed to roll outon a pair of rails to simplify access. The window ismade from 1/4-inch float glass.

LLS Camera CalibrationThe camera calibration target consisted of a 9 x

9 rectangular grid with mesh size 4 inches vertical by8 inches radial. The 3 ft x 6 ft target was large enoughto span from inside the blade root to outside the bladetip. The target was suspended vertically from anairstand and aligned with the laser sheet so that thelaser sheet washed over the front face of thecalibration image. Once in position, the laser wasturned off and a short video sequence acquired fromthe LLS camera. A group of 250 video frames wasthen averaged to produce a single image that was usedto convert image space to physical space. Gridlocations determined from the averaged image weretabulated and curve fit to the known physical gridcoordinates in the following manner:

Y = f(xp, yp)

Z = g(xp, yp)

where Y and Z represent physical space coordinates(inches) and xp and yp represent image space (pixels).A bi-quadratic curve fit was performed resulting in astandard deviation for the residual errors in the Y-direction of 0.06 inch. The equivalent value for the Z-direction was 0.02 inch. Typical scale in the cameraimage was 17 pixels/inch in the vertical direction and8.5 pixels/inch in the horizontal direction. Maximumuncertainty in identifying the grid location in thecalibration image was ±1 pixel. This corresponds to∆Y = 0.12 inch and ∆Z = 0.06 inch. The errors in thecamera calibration were therefore consistent with theestimated uncertainty in the grid point locations.

The gimbaled hub presented a problem whentrying to use the stationary rotor tip as the coordinateorigin. To simplify matters, the "on condition" bladetip location was used as the reference origin.

Flow SeedingSuccessful LLS flow visualization requires

highly non-uniform seed distribution in the area ofinterest. For this test, the rotor tip path plane wasapproximately 2.5 ft above the tunnel centerline. Inorder to deliver the seed at the desired height, themost economical (in terms of cost and effort) solutionwas to position a 70-foot work stand downstream ofthe fan drive. The work stand was a significantdistance upstream from the test section. Two Coronamineral oil seeders were mounted on the work stand.By varying the height of the work stand, adequateadjustment of the smoke release point was obtained.During the flow visualization run, the tunnel airexchange door was open 100% (10% air exchange) inorder to limit the background level of the smoke,thereby maintaining maximum contrast in the LLSimages between seeded and unseeded flow.

Data AcquisitionThe primary data acquisition system for the flow

visualization data was a Sony DHR-1000 digital videocassette recorder with internal time code generator(HH:MM:SS:FF). As backup, the analog video signalwas also routed to a JVC S-VHS VCR model HR-S4600U. The video was simultaneously displayed ona video monitor for real-time display and control ofthe smoke injection.

The flow visualization study was planned tooccur in two stages. During the first stage, each of the4 test conditions was documented on video in "slipsynch" mode. The laser was triggered at a fixedfrequency slightly different from the rotor rpm(actually rotor rpm/2) which caused the rotor blade toslowly precess through the video frame. During motortesting prior to tunnel entry the feedback loop on themotor had been optimized to provide a perfectlysteady rotor rpm. Unfortunately, during the windtunnel test (presumably with a different operatingpoint for the motor) this was not the case despiteadjustments to the feedback loop. As a result of rotorrpm variations, it was not possible to assign vortexage to any single video frame apart from those frameswhere the blade trailing edge lies in the laser sheet.This does not prevent the documentation of the vortextrajectory, however, and this was the main goal. Fourtest conditions were documented in slip-synch mode(two thrust levels each at µ = 0.10 and µ = 0.15) overa period of about 20 minutes. The second stage of theflow visualization study called for additionaldocumentation of the wake geometry from acquisitionof video segments at constant blade azimuth withrespect to the laser sheet, in increments of 15 degreesof blade motion. Unfortunately, model failurecurtailed the run at the conclusion of the slip-synchportion of the study and no second stagemeasurements were acquired.

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Post-Test Data ProcessingThe analog video signal was a standard 30-

frames/sec (60 interlaced fields) black and whitevideo stream. With the laser pulse rate of 11.79 Hz,only about 12 of the 30 video frames in each secondcontained useful information. The other 18 imageswere blank. Four to six minutes of video stream wererecorded for each of the four test conditions. All LLSflow visualization images were transferred tocomputer hard disk for subsequent analysis. All blankframes were removed for efficiency in data storage.Video playback and analysis were performed usingNIH Image, a public domain image-processingprogram developed by the National Institutes ofHealth (http://rsb.info.nih.gov/nih-image/).

The primary goal of the data processing was todetermine the coordinates of the vortices in the planeof the laser sheet. This was accomplished using thefollowing procedure for each test condition. First, thevideo frames were repeatedly viewed in succession asa movie to identify the vortical structures anddetermine the general trajectory of the vortices. Next,the vortex location in pixel space and the vortexrotation direction were tabulated frame by frame. Fora given frame, there may be as many as 3-4identifiable vortices or there may be none. Afterprocessing about 2 minutes of video, representingapproximately 1440 frames, the data were plotted todetermine whether there were enough data points todefine a vortex trajectory. Additional frames wereprocessed until a well-defined trajectory wasobtained. The next step was to assign an azimuthposition Ψb (nondimensional time) to each vortex.This required another pass through the data. Thereference time Ψ b is the azimuth angle of thereference blade corresponding to the identifiedintersection of the wake with the laser light sheet. Forthe calculated wake geometry, this time (and indeedthe actual wake age) is available for all intersections.For the measured wake geometry, Ψb can only beassigned for those frames where the blade trailingedge was coincident with the laser sheet, hence onlywhen Ψb is a multiple of 120 degrees. Note that bladeazimuth is defined by the location of the pitch axisand not the blade trailing edge, and so for Ψb =0 theblade azimuth is close to 90 degrees but notnecessarily equal to 90 degrees.

Uncertainty in Extracting Vortex LocationThe best indication of the vortex center was

provided by a particle void. The presence of a particlevoid provided an objective measurement of the vortexcenter with maximum error of ±0.5 pixel (∆Y = 0.06inch and ∆Z = 0.03 inch). If no particle void wasevident, the measurement of vortex location becamehighly subjective and measurement uncertaintyincreased correspondingly. Without a particle void wewere forced to identify the apparent center of any"circular" smoke mass as the vortex center. In generalthe smoke feature had an elliptic cross-section due to

oblique camera observation angle and/or vortexfilament angle with respect to the plane of the LLS.On occasion, internal features were used to identifythe vortex center (either reduced or increased smokedensity) leading us to identify some point other thanthe smoke centroid as the vortex center. There wereno definitive rules and the measurement could behighly subjective.

In general, the size of the swirling smoke massused to define the vortex grows as the vortex ages. Ifthe center of this rotating smoke mass was identifiedas the vortex center then the uncertainty in themeasurement of vortex location increased linearlywith the size of the smoke mass. Each individualmeasurement had a high degree of uncertainty.However, by analyzing many video frames, thenumber of vortex location measurements wasincreased sufficiently to define the mean vortextrajectory with high confidence. A reasonableestimate for the maximum uncertainty in determiningthe vortex center was 1/2 the radius of the smokemass. Based on the size of the typical vortex swirlpattern used to determine the vortex center, themaximum uncertainty in either radial or verticallocation was about ± 1 inch. This was also the typicalscatter found in the observations. The scatter in thevortex trajectory data, therefore, may be due to vortexwander or due to the uncertainty in locating the vortexcenter when no particle void was present. Assuming anormal distribution of the vortex location within thesmoke mass gives a standard deviation of about 1/3inch in either direction. The assumption that thevortex center can be identified by the "centroid" ofsome mass of swirling smoke could be erroneous,however. There may be a deterministic error presentin such a determination of vortex location. All weknow with certainty was that the vortex center waslocated somewhere within the smoke mass. Figure 4shows a representative video frame. The location ofthe blade trailing edge when passing through the sheetis identified along with the location of the tip trailingedge. In this particular frame, three vortices areidentified. Other vortical structures may be present inthe image, but unless the structure is consistentlypresent and clearly swirling, the structure wasexcluded as a vortex.

Analysis

The rotorcraft comprehensive analysisCAMRAD II was used to calculate wake geometriesfor the four conditions for which measured wakegeometry is available. CAMRAD II is anaeromechanical analysis of helicopters and rotorcraftthat incorporates a combination of advancedtechnologies, including multibody dynamics,nonlinear finite elements, and rotorcraftaerodynamics. The trim task finds the equilibriumsolution (constant or periodic) for a steady stateoperating condition, in this case a single rotor

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operating in a wind tunnel. Calculations for a singlerotor, rather than a dual rotor system, wereappropriate for this study since the region of interestwas the near wake which is assumed to be unaffectedby the presence of the other rotor. Also,computational time was greatly reduced by modelingthree blades rather than six. For wind tunneloperation, the thrust and flapping (longitudinal andlateral gimbal tilt) were trimmed to measured values.The aerodynamic model includes a free wake analysisto calculate the rotor nonuniform induced-velocities.CAMRAD II is described in Refs. 9 to 13.

Tiltrotor Wake ModelThe CAMRAD II rotor wake analysis uses

second-order lifting line theory, with the general freewake geometry calculation described in Refs. 12 and13. Two tiltrotor wake models are described in Refs. 3and 4, characterized as the rolled-up and the multiple-trailer models.

The rolled-up model uses a dual-peakrepresentation to accommodate negative tip loading.The tip vortex, with a core radius of 20% mean chord,is defined by the negative tip-loading. The positivetrailed vorticity inboard of the negative tip loadingrolls up with a core radius of 30% mean chord. Thereis partial entrainment of the trailed vorticity into thetip vortex. The dual-peak model is only used atazimuths where the negative loading extends inboardto at least 0.945R.

The multiple-trailer model has a discrete trailedvortex line emanating from each blade aerodynamicpanel edge. Although the multiple-trailer modelprovides good correlation of calculated and measuredairloads of an isolated tiltrotor, the performancecorrelation was unsatisfactory. The multiple-trailermodel was therefore modified to incorporate asimulation of the tip vortex formation process (Ref.5). In the modified model, known as the multiple-trailer with consolidation, the trailed vortex lines atthe panel edges are combined into rolled-up vorticesusing the trailed vorticity moment to scale the rate ofroll-up. All the vorticity in adjacent vortex lineshaving the same sign eventually rolls up into a singlevortex located at the centroid of the vorticitydistribution. Reference 5 showed that adding theconsolidation feature improved the performancecorrelation while maintaining good airloadscorrelation. The consolidation feature was thereforeused with the multiple-trailer model in this study.

Results

Measured and calculated wake geometries arepresented for the four test conditions provided inTable 1. All data were acquired for a rotor tip pathplane angle of 6 deg (shaft tilted aft) andapproximately zero 1/rev flapping. The experimentalresults are presented first, followed by the CAMRAD

II calculations. Comparisons between the measuredand calculated wake geometries are then discussed.

Table 1. Test ConditionsCase CT µ Mtip

A 0.0087 0.150 0.630B 0.0125 0.150 0.630C 0.0087 0.099 0.629D 0.0127 0.099 0.630

Experimental ResultsThe following points must be kept in mind as the

data are discussed:

1. The left-hand rotor under investigation rotates ina CW (clockwise) sense when viewed fromabove.

2 . The laser light sheet is at a nominal bladeazimuth of 90 degrees, on the advancing side ofthe rotor disk.

3. The rotor blade height is rising for the first 90degrees of blade motion after passing through thelaser sheet.

4. Each vortex wake is viewed from behind, lookingupstream.

5. The coordinate system for the data plots is basedon the location of the trailing edge of the blade atthe blade tip as the blade trailing edge passesthrough the laser sheet.

6. Positive lift on the blade produces a vortex withclockwise (CW) rotation. Negative blade liftproduces a vortex with counter-clockwise (CCW)rotation.

The measured and calculated data are presentedin terms of intersections of the trailed vortices withthe laser light sheet. Each figure shows theintersections for many time steps. The intersectionsare identified by the azimuth position Ψ b

(nondimensional time) of the reference blade relativethe laser light sheet. So Ψb=0 corresponds to the bladejust ahead of the sheet, while Ψ b=120 deg has theblade 1/3 revolution later and the following blade justahead of the sheet. A trailed wake bundle that rolls upat radial station r and intersects the sheet with age φ,has been convected longitudinally by a distanceapproximately rsinφ=µΨb; hence the age of an

intersection is φ = sin1(µΨb/r). For example, whenΨb=120 deg a trailed vortex filament from the tip(r=1) that intersects the sheet is only φ=12 or 18 degold (that is, created then the blade is 12 or 18 degforward of the sheet) for µ=0.10 or 0.15, respectively.A trailed vortex filament created at the tip (r=1) whenthe blade is 45 deg ahead of the sheet will intersectthe sheet after about 1.10 or 0.75 revolutions (Ψb=405or 270 deg) for µ=0.10 or 0.15, respectively. Therotation of the blades creates trailed vortex lines thatare skewed in the horizontal plane, hence as they areconvected aft their intersections with the laser light

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sheet move laterally. The vertical distortion of thetrailed lines produces vertical motion of theintersections. Thus the lateral and vertical motion (asΨb increases) of the intersections at the laser lightsheet reflect the skewed geometry of the lines beingconvected downstream past the sheet.

For a tip path plane angle of 6 deg, ignoringcyclic pitch, the blade tip was 5.96 inches higher atthe front of the rotor disk (blade azimuth = 180 deg)than at an azimuth of 90 degrees (laser sheet location).One degree of cyclic pitch corresponds to a verticalmotion of the blade trailing edge of about 0.072inches. Neglecting dynamic effects, blade motion dueto cyclic pitch should have a very small effect on thevortex location.

Figures 5-8 present the measured vortexlocations and Figs. 9-12 are representative images foreach of the four test conditions. The origin for theplots in Figures 5-8 is indicated on the images of Figs.9-12. A discussion of each test condition follows.

Case A: CT = 0.0087, µ = 0.150. Measurements ofvortex trajectory for test condition A are shown inFig. 5. The lack of data close to the origin indicates apossible slow roll-up of what is clearly seen to be aCCW vortex associated with negative blade loading atthe blade tip. A corresponding wake image isprovided in Fig. 9. The trajectory of this vortex canbe followed with high confidence (confirmed by thelow degree of scatter) for a complete rotor revolution.Also visible in Fig. 5 is a cluster of points showingthe trajectory of a CW vortex assumed to developfrom an inboard station. The roll up of the CW vortexoccurs for Ψb >120 degrees. Vortices that arise fromradial stations other than the blade tip are difficult toidentify with a particular blade. The upright"mushroom" flow pattern identifiable in the flowvisualization image of Fig. 9 is consistent with anoutboard CCW vortex (negative blade loading at theblade tip) and an inboard CW vortex (positive bladeloading inboard). These flow patterns were alsoreported in Ref. 2.

Case B: CT = 0.0125, µ = 0.150. Based on airloadsmeasurements reported in Ref. 2, we know thatnegative tip loading for this condition is not asprevalent over the azimuth compared with the lowerthrust condition of Case A. The blade tip at 90 degazimuth for Case B is positively loaded (compared tothe negative tip loading of Case A) as shown by theCW vortex shed from the tip in Fig. 6. For Ψb < 120deg, the CW tip vortex first appears at the blade tiponly to disappear from view and reappear slightlyinboard, suggesting the positive blade loading hasmoved inboard from the tip. At about Ψb =240 deg, anupright mushroom pattern associated with a CCWvortex originating at the blade tip and a CW vortexoriginating inboard becomes visible. The wakeimages for this condition (Fig. 10) show a mushroompattern that convects in an upright position. As a

result, there is little difference in height between theCW and CCW vortex trajectories of Fig. 6. Case A, incomparison, shows the CCW vortex trajectory was atleast 2.5 times higher than the CW vortex trajectory.Although hundreds of images were reviewed for thiscase, there is a CW vortex which could not beassigned a value of Ψb. This CW vortex is locatedabout mid-span and slightly above the blade (Fig. 6).One explanation for this CW vortex is that the bladespanwise loading produces a counter-rotating vortexpair near the tip plus a CW vortex further inboard.The wake of Case B is more chaotic than that of CaseA, which results in a higher degree of scatter in Fig. 6than Fig. 5.

Case C: CT = 0.0087, µ = 0.099. Figure 7 shows themeasured vortex trajectories for Case C and Fig. 11shows a typical wake pattern. The trajectory of theCCW vortex shed from the blade tip is very similar toCase A. More noticeable, however, is the completelack of evidence of any rolled-up vortex immediatelyfollowing blade passage through the laser sheet.Based on a comparison with Case A, the firstappearance of a vortex is probably well after firstblade passage. This lack of information implies thatthe trailed vorticity at the blade tip is slow to roll up,or is weak, or both. Scatter in the data is quite lowindicating a high degree of repeatability. Clockwisevortices that are shed inboard from the blade tipappear to congregate in separate groups beneath thepath of the CCW tip vortex. The mushroom flowpattern (Fig. 11) is initially upright for 120 deg < Ψb <240 deg at which point the pattern rotates into avertical (CCW vortex above CW vortex) orientationas the vortices age further.

Case D: CT = 0.0127, µ = 0.099. For Case D, there islittle negative loading at the blade tip over theazimuth, as shown by the near absence of negative(CCW) vortices. Figure 8 shows the wake trajectoryfor this case and Fig. 12 shows a representative wakepattern. This wake pattern is typical of a helicopterblade loading, that is, the overwhelming presence ofpositive CW vortices.

As the rotor blade passes through the LLS astrong CW vortex is seen to originate slightly inboardof the blade tip. The vortex center is easilydetermined from a visible particle void (Fig. 12). Asthe blade moves upstream of the laser sheet, thetrailed vortex moves rapidly higher as it moves slowlyinboard. Sometime before the following bladereaches the laser sheet, a weak secondary vortex canbe discerned outboard and above the primary tipvortex. This CCW vortex is assumed to be a bi-product of a negative tip loading that has occurred asthe blade moved upstream of the 90 deg azimuthposition. This vortex is claimed to be weak becausethe only evidence of its existence is the particle-voidin the laser sheet and because no direct evidence forthe sense of rotation of this vortex is provided by any

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adjacent smoke. The local flow is totally dominatedby the primary CW vortex. The existence of thesecondary vortex is apparently short-lived. By thetime the following blade reaches the laser sheet thesecondary vortex has disappeared. At large vortexage, the primary vortex descends as it moves inboardtowards the blade root.

Figure 8 shows little scatter in the data,indicating the high level of confidence in themeasurement and the high degree of repeatability inthe vortex trajectory from rev-to-rev. Note the highconfidence in the vortex trajectory up to first bladepassage. In general, this is where the particle voiddisappeared and the vortex center became moredifficult to identify with complete objectivity.Increased scatter beyond first blade passage isattributed to a reduced confidence in themeasurement, but could also be due to an increasedvariability in the trajectory. The path of this vortex isconsiderably lower than that observed for Cases A-C.

Calculated ResultsIn CAMRAD II, the rotor was trimmed to the

measured CT and zero 1/rev flapping angle. Themultiple-trailer wake model with consolidation wasused. The consolidation model uses the compressionform with a linear dependence of the rollup fractionon wake age (Ref. 5). The assumed vortex core radiushas a constant value of 80% of the mean chord. Tworotor revolutions of wake were simulated in theanalysis. The intersections of vortex filaments with aplane at the laser light sheet location were extractedfrom the calculations for comparison with themeasurements. Figure 13 shows the calculated wakegeometry for Case A. The spanwise circulationdistribution defined the sign of each of the sixteenfilaments released from the edges of the aerodynamicpanels of the blade. For this test condition, theadvancing blade tip was negatively loaded producinga CCW tip vortex. The inboard filaments eventuallyroll up into a positive (CW) vortex located at thecentroid of the inboard filaments. The roll up waschosen to occur at approximately Ψb=120 degrees,which resulted in good airloads correlation in Ref. 5.The roll up age was scaled with rG

2/G, where G is thetotal trailed strength and rG is the second moment ofthe vorticity (Ref. 5). The velocity field in thesimulated plane of the light sheet was calculated overa grid of 4 ft x 6 ft in increments of 0.02 ft in bothdirections (60,501 grid points). Figure 14 shows thecalculated in-plane velocity and vorticity fields forCase A at Ψb=120 degrees. The inboard vortices havesignificantly higher vorticity magnitudes than thevortices near the tip.

The calculated wake trajectories for Cases A-Dare plotted in Figs. 15-18 for 15 degree increments inblade position. The CCW tip vortex and the CWprimary (inboard) vortex are seen for Cases A-C.Evidence of the root vortex is also shown. The near-wake is represented by individual filaments which

eventually roll up into the primary vortex (recall Fig.13).

Comparing Cases A and C (Figs. 15 and 17), thereduction in advance ratio produces a steepertrajectory for both the tip and inboard vortices. Thisresult is reasonable since at higher speeds the wake isforced closer to the rotor tip path plane. ComparingFigs. 15 and 16, increasing thrust at advance ratio of0.15 reduces the blade azimuth range of negativeloading. The CCW vortex trajectory merges with theCW trajectory close to mid-span and from that point,only the CW trajectory exists. At an advance ratio of0.10, increasing thrust essentially eliminates negativeloading at the tip (compare Figs. 17 and 18) as shownby the lack of CCW vortices near the origin in Fig.18. Decreasing advance ratio at CT =0.013 alsoeliminates the negative loading (compare Figs. 16 and18). Since Case D represents a conventional bladeloading condition, the tip vortex roll-up model wasalso used as shown in Fig. 18. Using the tip vortexroll-up model forces a CW vortex to form at the bladetip.

Correlation ResultsFigures 19-22 overlay the measured and

calculated wake trajectories for Cases A-D. Figure 19shows that the calculated trajectory for the CCWvortex in Case A matches the measured trajectory inshape but is slightly lower in height. The CW vortextrajectory is not well matched. Note that the measuredtrajectory indicates roll up does not occur beforeΨb<120 deg, confirming the adequacy of theassumption for the initiation of vortex roll up age usedby the analysis. For Case B (Fig. 20), the calculationspredict negative tip loading contrary to the measuredinitial positive tip loading. The calculated CW vortextrajectory passes through the cluster of pointsattributed to the vortex of unknown wake wage, butdoes not predict a second CW trajectory near theCCW trajectory as depicted in the data. The CCWvortex trajectory is captured in shape for Case C, butthe calculated peak height above the above the bladeis 30% too high compared with the measurements(Fig. 21). The calculated CW vortex trajectory is toolow, but otherwise does a reasonable job of capturingthe trajectory shape and the age at which roll upoccurs. For Case D (Fig. 22), the roll-up modelcaptures the initial trajectory of the tip vortex, but thepeak height above the blade is 40% lower thanmeasured and the calculated radial convection ismuch slower than the measured trajectory. For thiscase, the tip vortex roll-up model provides bettercorrelation than the multiple trailer model, but neitheris adequate for capturing the measured trajectory.

Both the calculated and measured waketrajectories exhibit similar trends when advance ratiois varied at fixed thrust or when thrust is varied atfixed advance ratio. The agreement is encouragingand warrants further study.

T213-4-8

Conclusions

The vortex wake trajectory from one rotor of a0.25-scale V-22 tiltrotor was measured for four testconditions. Vortex wake images were acquired usinglaser light sheet flow visualization. Wake trajectorieswere constructed by extracting vortex positions fromthe video images. Wake trajectories were alsocalculated using CAMRAD II. Measured andcalculated wake geometries were compared.Conclusions from this investigation are listed below.

1 . For conventional (positive) tip loading orconditions where the blade tip loading is negativeover the second quadrant of the disk, assigning areference time to the vortices in the images isrelatively straightforward.

2 . Both the calculated and measured waketrajectories exhibit similar trends when advanceratio is varied at fixed thrust or when thrust isvaried at fixed advance ratio.

3 . The multiple-trailer model with consolidationprovides good correlation with the measuredwake trajectory of the tip vortex for CT=0.087.The inboard vortex at this condition was not aswell matched. At higher thrust, the agreementwas poor.

4 . Neither the tip vortex roll-up model nor themultiple-trailer model provided a good simulationof the wake trajectory of a tiltrotor blade withconventional (positive) tip loading.

References

1. Young, L.A., Booth, E.R., Jr., Yamauchi, G.K.,Botha, G., and Dawson, S., "Overview of theTesting of a Small-Scale Proprotor," AmericanHelicopter Society 55th Annual Forum, Montreal,Canada, May 1999.

2 . Yamauchi, G. K., Burley, C. L., Mercker, E.,Pengel, K., and JanakiRam, R. D., "FlowMeasurements of an Isolated Model Tilt Rotor,"American Helicopter Society 55th AnnualForum, Montreal, Canada, May 1999.

3 . Johnson, W., "Calculation of Tilt RotorAeroacoust ic Model (TRAM DNW)Performance, Airloads, and Structural Loads,"American Helicopter Society AeromechanicsSpecialists' Meeting, Atlanta, Georgia, November2000.

4 . Johnson, W., "Calculation of the AerodynamicBehavior of the Tilt Rotor Aeroacoustic Model(TRAM) in the DNW," American HelicopterSociety 57th Annual Forum, Washington, D.C.,May 2001.

5 . Johnson, W., "Influence of Wake Models onCalculated Tiltrotor Aerodynamics," AmericanHelicopter Society Aerodynamics, Acoustics, andTest and Evaluation Specialists Conference, SanFrancisco, California, January 2002.

6 . Young, L.A., "Tilt Rotor Aeroacoustic Model(TRAM): A New Rotorcraft Research Facility,"Heli Japan 98: American Helicopter SocietyMeeting on Advanced Rotorcraft Technology andDisaster Relief, Nagarafukumitsu, Gifu, Japan,April 1998.

7 . Johnson, J.L. and Young, L.A., "Tilt RotorAeroacoustic Model Project," Confederation ofEuropean Aerospace Societies (CEAS), Forumon Aeroacoustics of Rotorcraft and Propellers,Rome, Italy, June 1999.

8. McCluer, M. S. and Johnson, J. L., "Full-SpanTiltrotor Aeroacoustic Model (FS TRAM):Overview and Initial Testing," AmericanHelicopter Society Aerodynamics, Acoustics, andTest and Evaluation Specialists Conference, SanFrancisco, California, January 2002.

9 . Johnson, W., "CAMRAD II, ComprehensiveAnalytical Model of Rotorcraft Aerodynamicsand Dynamics," Johnson Aeronautics, Palo Alto,California, 1992-2002.

10. Johnson, W., "Technology Drivers in theDevelopment of CAMRAD II," AmericanHelicopter Society Aeromechanics SpecialistsConference, San Francisco, California, January1994.

11. Johnson, W., "Rotorcraft AeromechanicsApplications of a Comprehensive Analysis," HeliJapan 98: American Helicopter Society Meetingon Advanced Rotorcraft Technology and DisasterRelief, Gifu, Japan, April 1998.

12. Johnson, W., "A General Free Wake GeometryCalculation for Wings and Rotors," AmericanHelicopter Society 51st Annual Forum, FortWorth, Texas, May 1995.

13. Johnson, W., "Rotorcraft Aerodynamics Modelsfor a Comprehensive Analysis," AmericanHelicopter Society 54th Annual Forum,Washington, D.C., May 1998.

T213-4-9

Figure 1. Full-span TRAM installed in NASA Ames 40- by 80-Foot Wind Tunnel. Acoustic traverse in

foreground.

Figure 2. LLS projection across 40- by 80-Foot Wind Tunnel test section. White rectangle indicates camera

field of view. View looking downstream.

Laser andoptics behindwall

LLS cameraport

T213-4-10

Figure 3. View of model from behind LLS camera.

Figure 4. Sample LLS image. View from behind advancing side of left rotor. Coordinate system origin at bladetip trailing edge. Blade rotation is into paper.

Vertical distance above bladetip trailing edge

Radial distance inboard from tip

Counter- clockwise vortex

Clockwisevortex

Approximate outline of bladetrailing edge when passingthrough sheet

Strobe powersupply

Pan-and-tiltcontroller

LLS camera

Strobe

Pan-and-tilt stage

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inch

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Radial distance inboard from tip (inches)

Ψb=120 deg

Ψb=240 deg

Figure 5. Case A. CT =0.0087, µ=0.150.Measurements.

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B2-CW

B3-CW

B1-CCW

B2-CCW

B3-CCW

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tip (

inch

es)

Radial distance inboard from tip (inches)

Ψb=120 deg

Ψb=240 deg

Ψb=360 deg

Ψb=360 deg

Ψb=480 deg

Figure 7. Case C. CT =0.0087, µ=0.099.Measurements.

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B1-CCWB2-CCWB3-CCW

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Radial distance inboard from tip (inches)

CW (unknown wake age)

Figure 6. Case B. CT =0.0125, µ=0.150.Measurements.

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B1-CW

B2-CW

B3-CW

B1-CCW

B2-CCW

B3-CCW

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tip (

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es)

Radial distance inboard from tip (inches)

Ψb=120 deg

Ψb=240 deg

Ψb=360 deg

Figure 8. Case D. CT =0.0127, µ=0.099.Measurements.

T213-4-12

Figure 9. Case A. CT =0.0087, µ=0.150. View from behind left-hand rotor on advancing side.

Figure 10. Case B. CT =0.0125, µ=0.150. View from behind left-hand rotor on advancing side.

Vertical distance above blade tip

Radial distance inboard from tipbl d i

Vertical distance above blade tipRadial distance inboard from tip

Clockwisevortex

Counter-clockwisevortex

Counter-clockwisevortex

Clockwisevortex

T213-4-13

Figure 11. Case C. CT =0.0087, µ=0.099. View from behind left-hand rotor on advancing side.

Figure 12. Case D. CT =0.0127, µ=0.099. View from behind left-hand rotor on advancing side.

Vertical distance above blade tipRadial distance inboard from tip

Vertical distance above blade tip

Blade entering laser sheetVertical distance above blade tip

Clockwisevortex

Particle void

Radial distance inboard from tip

Counter-clockwisevortex

Clockwisevortex

T213-4-14

Figure 13. Case A. CT =0.0087, µ=0.150. Calculated vortex filament intersections with plane of laser light sheet.

Figure 14. Case A. CT =0.0087, µ=0.150. Blade position=120 degrees. Every 8th vector shown. Velocity andvorticity field in simulated laser light sheet plane.

Negative (CCW) tipvortex

Positive (CW)inboard vortexresulting fromcombined trailers

Intersection offilaments and lightsheet

bound vortex

root vortex

positivevorticity

negativevorticity

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Radial distance inboard from tip (inches)

Ψb=120 deg

Ψb=240 deg

Ψb=360 deg

Near wake filament roll-up

Root vortices

Figure 15. Case A. CT=0.0087, µ=0.150. Calculations.

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Radial distance inboard from tip (inches)

Ψb=120 deg

Ψb=240 deg

Ψb=360 deg

Figure 17. Case C. CT=0.0087, µ=0.099. Calculations.

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Radial distance inboard from tip (inches)

Ψb=120 deg

Ψb=240 deg

Figure 16. Case B. CT=0.0125, µ=0.150. Calculations.

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tip (

inch

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Radial distance inboard from tip (inches)

Ψb=240 deg

Ψb=360 deg

Tip vortex roll-up model

Figure 18. Case D. CT=0.0127, µ=0.099. Calculations.

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Radial distance inboard from tip (inches)

Figure 19. Case A. CT =0.0087, µ=0.150. Correlation.

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Radial distance inboard from tip (inches)

Figure 21. Case C. CT =0.0087, µ=0.099. Correlation.

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Radial distance inboard from tip (inches)

CW (unknown wake age)

Figure 20. Case B. CT =0.0125, µ=0.150. Correlation.

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B1-CWB2-CWB3-CWB1-CCWB2-CCWB3-CCWCW-calcCCW-calc

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tip (

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Radial distance inboard from tip (inches)

tip vortexroll up model

Figure 22. Case D. CT =0.0127, µ=0.099. Correlation.