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Flow Visualization and Flow Field Measurements of a _i _1

1/12 Scale Tilt Rotor Aircraft in Hover

Charles D. CoffenGraduate Research Assistant

Comell UniversityIthaca, NY

Albert R. GeorgeProfessor

Comell UniversityIthaca, NY

Hal HardingeGraduate Student

Comell UniversityIthaca, NY

Ryan StevensonUndergraduate Research Assistant

Comell UniversityIthaca, NY

ABSTRACT INTRODUCTION

This paper details the results of flow visualizationstudies and inflow velocity field measurementsperformed on a 1/12 scale model of the XV-15tilt rotor aircraft in the hover mode. The complexrecirculating flow due to the rotor-wake-bodyinteractions characteristic of tilt rotors was

studied visually usir_g neutrally buoyant soapbubbles and quantitatively using hot wireanemometry. Still and video photography wereused to record the flow patterns. Analysis of thephotos and video has provided information onthe physical dimensions of the recirculatingfountain flow and on details of the flow includingthe relative unsteadiness and turbulence

characteristics of the flow. Recirculating flowswere also observed along the length of thefuselage. Hot wire anemometry results indicatethat the wing under the rotor acts to obstruct theinflow causing a deficit in the inflow velocitiesover the inboard region of the model. Hot wireanemometry also shows that the turbulenceintensities in the inflow are much higher in therecirculating fountain re-ingestion zone.

Tilt rotor aircraft have several novel features

which affect their aerodynamic characteristics. Invarious flight modes the rotor and rotor-wake-body aerodynamics are different from eitherhelicopters or conventional aircraft. In theoperation of a tilt rotor aircraft in hover, thepresence of the wing and fuselage beneath therotor affects the aerodynamics by introducingcomplex unsteady recirculating flows.

The fundamental geometry of the tilt rotor aircraft(Figure 1 picture of aircraft) consists of prop-rotors mounted on tiltable nacelles which are

located at or near the tips of a fixed (non-tilting)wing. The prop-rotor is sufficiently large so thatthe benefits of low disk loading are gained forefficient hover flight. The prop-rotor is designedto provide the desired performance balancebetween the axial-flow critical hover requirementand the axial-flow airplane mode require-mentl,2, 3. The tilt rotor introduces a number of

unique prop-rotor/airframe aerodynamic inter-actions that must be addressed to properly

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understand the significant performance issues.In hover mode the wing and fuselage provide apartial ground plane in the near wake whichcauses the development of an inboard-boundspanwise flow over the wing and fuselagesurface. At the aircraft's longitudinal plane ofsymmetry, the opposing flows collide, producinga low energy fountain flow with upward velocitycomponents which are then re-ingested by therotors. Full analytical representation of this flowwould require a three-dimensional, time-variant,rotor/rotor, and rotor-airframe interaction model.

Some characteristics of tilt rotor flow have been

studied both experimentally and analytically.Experimental tests on large and full scale modelshave been used to study the down load on the

wing in the hover condition 4,5,6,7,8. Thisproblem is of great interest as 10%-15% of therotor lift is needed to overcome the down force

on the wing caused by the down wash flow overthe wing. All of these tests used one wing androtor and an image plane rather than a completemock up. According to the study by Felker and

Light 8 the size of the image plane can have aprofound effect on the test results. One previousexperimental study of the tilt rotor hovercondition not limited to downwash and down

loads was by Rutherford 9. In this test smokeand tuft flow visualization techniques were usedon a model consisting of a single wing and rotor.An image plane was used to simulate the tworotor-wing flow phenomena.

The study of reference (10) attempts toanalytically study the flow of a tilt rotor XV-15 inhover by numerically solving the unsteady, thinlayer compressible Navier-Stokes Equation.While the results of this study are encouraging,not all the major features of tilt rotor hover floware captured in the computation. In particular,this study included only a wing and rotor imagesystem in the calculation. Attempts at modelingtilt rotor hover flow by assuming a plane ofsymmetry and disregarding the fuselage will beshown here to be insufficient.

While a full analytical representation may bedesirable in the long term, a more expedientmethod is required for generating data needed forcurrent aerodynamic and aeroacoustic calcu-lations and design. Thus a 1/12 scale model(consisting of two rotors, wing, and fuselage) of

the XV-15 tilt rotor was built in order to qualifyand quantify the complex flows about a hoveringtilt rotor. This model enables the study of theunsteadiness and ths side to side shifting of boththe fountain flow which have not been previouslyexplored. Prior tests have relied on an imageplane to simulate the tilt rotor hover con-figuration. Past experimental tests have also notincluded the fuselage which also affects theoverall flow. Important flow phenomenon werefound to occur along and above the length of thefuselage. In addition, attempts to model the flowanalytically require accurate and completeexperimental information for comparison andvalidation. Previous tests have not been adequatein this respect.

This study provides a more thorough under-standing of the qualitative and quantitative char-acteristics of the recirculating flow, such asrelative turbulence levels, unsteadiness in theflow and the three dimensional nature of the

fountain flow. The results of this study willserve as a comparison for CFD calculations anddesign modifications.

]EXPERIMENTAL SETUP

The scale of the model was determined by thesize of the largest commercially availablepropeller, 24 inch diameter with a pitch of 8inches. Electric motors were chosen to powerthe model. No attempt was made to Mach scalethe rotor tip speed. As the rotor pitch was scaledapproximately, the ratio of tip speed to inducedvelocity will be approximately scaled. Mostfixed speed electric motors operate at either 1700or 3450 rpm. In order to obtain higher inflowvelocities (less relative error for the measuringequipment available), a motor speed of 3450 rpm

was chosen. Two 1.5 hp motors operating at3450 rpm were used. The motors wereuncoupled as coupled motors would have beendifficult to implement and maintain. Theuncoupled motors were operated at the samespeed by controlling one motor's speed with avariac and matching speeds with a stroboscope.We do not expect that the lack of phase lockingbetween the rotors will be significant as the rotortip speed is much greater than the inducedvelocity giving many tip vortices and associatedwake structures per unit axial length. The motorsspin in opposite directions so that the blades

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rotate towards the tail as they pass over the wing.The motors are located approximately 7 inchesbelow the wing, far enough below the rotors tobe out of the flow yet not so far as to make shaftwhirl a problem.

The wing and fuselage were both constructedusing a styrofoam base covered with fiberglass,fiUer and paint. The horizontal and vertical tailwere not installed in the tests reported here. Thewing is removeable and has adjustable flaps andflaperons to allow for testing of various wingconfigurations. The model is mounted on a steelframe to oppose the lift of the rotors and reducevibration. The model is supported by a woodenstructure elevated above the floor such that the

rotor plane is 60.75 inches above the ground.This corresponds to a hover height of 60.75 scalefeet. Figure (2) shows the the experimental setup including the model, electric motors and teststand. The model was operated in an area ofdimensions greater than 25 feet. The nearestsignificant objects in the room wereapproximately 10 feet away. The orientation andplacement of the model in the room was foundnot to affect the observed and measured flow.

FLOW VISUALI:_ATIQN

A very effective tool for visualizing the complexflows of the model XV-15 was a bubble

generator. The Model 3 Sage Action Incor-porated Bubble Generator combines compressedair, helium, and a soap solution to produce

neutrally-buoyant helium Filled bubbles 11. Thebubbles follow the pathlines of the flow and areable to accurately trace the intricate flow patternsof the hovering tilt rotor. Rubber tubing and analuminum tube wand were used to insert the

bubble at various locations in the flow. Duringthe experiment, the room was darkened and ahigh intensity Varian arc lamp was used toilluminate the bubbles.

Results of Still Photographs:

The flow around a hovering tilt rotor is extremelyunsteady. Still photographs were used to capturevarious features of the flow. It must be notedthat these are instantaneous visualizations. ASA

1600 f'tlm was used with an f-stop of 5.6 and0.25 to 1.0 second exposure times. Greaterexposure times produced pictures with too many

'cluttered' pathlines and smaller exposure timesdid not allow enough light. Still photographswere taken of the front and side of the model.Unless otherwise obvious, the bubbles were

inserted in the longitudinal plane of symmetry.In order to view a two dimension slice of the the

flow patterns, the light source was situatedperpendicular to the direction of the camera.

Figure (3) is a top view schematic of the modeland indicates the aircraft axis and indicates probelocations for the various experiments.

The images presented here are computer enhance-ments of the original stills which were digitizedusing a scanner. Figure (4) shows an unen-hanced photo. Figure (5) is the same photowhich has been inverted (negative image) toshow the pathlines as black streaks on a whitebackground. The contrast of the scanned imageswas improved to better def'me the bubble streaks.This process often caused a blurring of the modelwith the background. Also, light reflected off themodel caused distracting shadows and in manycases obliterated the outline of the model. In

order to reduce this annoying affect, the outlineof the model was enhanced and the background

edited to clarify the image and remove distractingshadows. In no cases were the bubble streaks or

any part of the flow embellished or manipulated.

Figure (5) is a head on photo of the model withthe bubbles being injected directly over theintersection of the longitudinal axis and therotor/rotor axis. This figure clearly shows therecirculating fountain and indicates the height ofthe fountain to be approximately 1/2 the radius ofthe rotor. This figure also shows an unsteadystagnation point on the fuselage where thespanwise flows intersect and erupt up betweenthe two rotors and are then re-ingested. Figure(6) emphasizes the re-ingestion part of the flowas bubbles inserted just above the unsteadystagnation point are convected up between therotors where they are entrained into the inducedflow and are re-ingested. Figure (7) indicates theextremely turbulent nature of the fountain flownear the unsteady stagnation point. Here bubblesare injected horizontally along the longitudinalaxis. The random direction of the pathlines, thesharp lateral perturbations, and the fact that thebubbles are injected at the same location in theflow and follow completely dissimilar paths, allindicate the highly unsteady and turbulent nature

of the fountain flow. Note that all of theseimagesprojectthe3-D flow ontoaplaneandthatinformation about the flow out of the plane islost. Figure(8) showsbubblesbeinginjectedoffcenterand showsthe spanwiseflow curling upbeforereachingthelongitudinalaxis. This figurealsoshowsonebubblebeingconvectedfrom thecenterof thefountaininto the interiorof therotordisk and another being convected into theoppositerotor. Presumablytheseeventsaredueto particularly large turbulent eddies beingingested.

The next four Figures (9-12) illustrate thefountain flow from the side and clearly illustratethe multi-dimensional nature of the re,circulatingfountain flow. These aspects of the flow aredifficult to explain as the images show severalrecirculation paths. Note that the model used inthis study did not include a tail wing assemblywhich may have some minor influence on thefountain flow over the rear of the fuselage.

The pathlines of the bubbles over the rear of thefuselage depend strongly on their point ofinjection. Figure (9) shows bubbles beinginjected above the tail, over the longitudinal axis,and above the rotor plane. The bubbles areswept horizontally along the longitudinal axis andare ingested into the rotors to both sides of the

upflow shown in Figure (5). The bubbles passthrough the rotor plane and travel back towardsthe tail along the fuselage. Here the bubblesrecirculate upward above the rotor plane and areeventually re-ingested. Figure (10) shows thebubbles being injected along the fuselage near thetail. This photo differs from the one above inthat the bubbles are injected below the rotorplane. The bubbles are lifted off the fuselage andare entrained in the recirculating flow along andabove the fuselage. The next Figure (11) showswhat happens when bubbles are injected over thewing, over the longitudinal axis, and below thethe rotor. This probe placement is similar to thatof Figure (6) but the photo is from a differentview. Here the bubbles are convected upbetween the rotors and are dispersed rearwardand to one or the other side of the fuselage wherethey are re-ingested. Figure (12) demonstrateshow the near laminar inflow becomes distorted

and erratic as it interracts with the recirculatingfountain flow over the fuselage. Unlike thecoherent flow structures provided by the head onphotos, these images do not indicate any distinct

flow pattern which can be said to characterize theflow above the fuselage. Most likely, the entireregion from the wing/fuselage intersectionrearward on the fuselage represents an unsteadystagnation zone. This flow is characterized bylow velocity highly turbulent flow. The flowalong the fuselage tends to be entrained upbetween the two rotors and re-ingested throughthe rotors in a cyclical process. The pathlines are

dependent on the point of origin, but it appearsthat bubbles injected anywhere above thefuselage will eventually enter this stagnation zoneand become re-ingested.

The most important conclusion that can be drawnfrom these photos is that the fountain is not aphenomenon which can be viewed or analyzed intwo dimensions. This flow is multi-directional

and includes spanwis¢ flow along the wings andunsteady flow along the fuselage. Both of thesecomponents exhibit turbulent upward flow pathswhich resuk in re-ingestion of the flow.

Results of Video Recording:

In order to record the strong time dependence ofthe flow, video recordings of the bubble

.pathlines were also made. Bubbles were insertedm various places in the flow to show various

.pathlines. The location of insertion point is very_mportant for visualizing different flowphenomenon. One important feature of the videorecordings is the availability of slow motion andframe-by-frame advance. The downwash vel-ocities are fast enough to make it difficult for thehuman eye to see all there is to see in the video at

standard speeds. The frame-by-frame advanceshows 0.04 second intervals. Between frames,

the pathlines disappear and new pathlines appearas bubbles move in and out of the stream of light.This demonstrates the gross unsteadiness in theflow and demonstrates the 3-dimensional nature

of the flow. This is an important point becausethe recorded images show the fountain flowsuperimposed on a 2D plane. In fact, thefountain flow is omni-directional and must be

studied from all angles in order to fully appreciatethe complexity of the flow.

Another important phenomenon of the flowdocumented with video was the side-to-side

switching of the fountain. This effect is mostapparent when the model is viewed head on and

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bubblesareinjectedabovetherotor plane. Therecirculating flow is seento switch randomlyfrom sideto sidein time. The time scaleof thisphenomenonwasmeasuredto beanywherefrom0.04 secondsor less (frame advancetime) to0.75seconds.Thefluctuationis characterizedbythe fountainflow being re-ingestedmostly intoone of the two rotors. This switching indicatestheunsteadinessof thestagnationregionon andabove the fuselage. The asymmetry of thisphenomenonshowsthat modelingthe fountainflow with one rotor and an image plane isinadequate for studying the time varyingpropertiesof theflow.

HQT WIRE EXPERIMENTS

Hot wire anemometry was another techniqueused to help characterize the fountain flow.These experiments were conducted with a singlewire probe. Unless otherwise indicated, the wirewas parallel to the plane of the rotor and wasoriented parallel to the longitudinal axis. Theanemometer was connected to a Macintosh based

data acquisition system. Three experiments arereported. The first experiment was an attempt tocharacterize the unsteadiness of the flow by

examining time traces of velocity measurements.For these measurements, a sampling rate of 4000

samples per second for one second was used.Another experiment was to measure the spectrumof turbulence due to the recirculating fountain.

For this experiment the sampling rate was 30,000

samples per second. 214 evenly spaced sampleswere used to find the spectrum. The lastexperiment was an attempt to quantify the meanand rms inflow velocities over the rotor plane.The mean velocity spatial variations are affectedby the wing obstructing the rotor wake, and therms velocity spatial variations are affected by there-ingestion of the fountain turbulence. 625locations one inch above the rotor plane weremeasured. A sampling rate of 40 samples persecond for 15 seconds was used.

Time Traces:

The unsteadiness of the location and turbulent

intensity of the fountain flow is evident from timetraces of velocity at various points in the flow.Figure (13a-b) compare two 0.13 second timeseries of inflow velocity measured 2 inches fromthe tip on the rotor/rotor axis and one inch above

the rotor plane (in the fountain re-ingestion zoneof the rotor). Refer to Figure (3) for a schematicof the model and measurement locations. Basedon the results of the flow visualization study, this

location was chosen as a reference point becauseit is more or less centered in the re-ingestion area.These series show how the inflow can be either

laminar, essentially showing only the blade

passing, Figure (13a), or very turbulent, Figure(13b), due to re-ingestion of the wake. Figure(14) gives an example of the intermittency of the

phenomenon as this sample shows a nearlylaminar inflow broken by a 0.05 second burst ofturbulence followed by a return to laminar flow.A study of a 4.0 second series, Figure (15), didnot indicate any discernable pattern in the periodof intermittency. Figure (16) shows a time traceover the outboard section of the rotor plane, oneinch above the rotor and 2 inches from the tip onthe rotor/rotor axis. Here the inflow is laminarand is undisturbed by the turbulent fluctuating

recirculating fountain flow. Another interestingobservation is that the peak inflow velocity due to

the potential flow is 2 rn/s greater in Figure (16)than in Figure (13a). This result is due to thewing obstructing the flow inboard and will bediscussed in more detail below.

The height of the fountain was also studied bytaking time traces at 1, 3, and 5 inches above therotor plane at the point 2 inches inboard of therotor tip on the rotor/rotor axis (in the fountainre-ingestion zone of the rotor). Figures (17a-c)show the progression of heights. The velocityfluctuations due to the potential flow associatedwith each blade passage decreases withincreasing height. The amplitude of fluctuationsdue to the re-ingested fountain turbulencedecreases from 1 to 3 inches above the rotor

plane, but does not decrease as much between 3and 5 inches above the rotor plane. Thefluctuations are more rapid at the 1 and 3 inch

heights than the 5 inch height. This indicates thatthe smaller turbulent eddies are re-ingested at alower height and that only large scale eddies arerecirculated to greater heights above the rotorplane. This result can also be seen in the flowvisualization photos as the pathlines become lesserratic as they travel higher above the rotor plane.This information may be useful for futureattempts to ameliorate the fountain affect byreducing or eliminating its high frequencycontent.

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Velocity versus time traces were also used tostudy the fountain flow above the fuselage.Figure (18a-e) show series at five locations onthe longitudinal axis one inch above the rotorplane. These Figures show how the flow isrecirculated preferentially to the rear of theaircraft. The velocity fluctuations fall offdrastically as the probe is moved forward of thewing while remaining fairly constant as the probeis moved rearward. Figure (18e) is of specialinterest as it shows large rapid fluctuations onerotor radius behind the wing/fuselageintersection. This shows that the longitudinalrecirculating flow is not a minor or secondaryaffect and should be studied further as it mayeffect many operational characteristics such ashover performance, stability, and interior cabinnoise.

Time traces were also generated for the case ofthe hot wire centered between the two rotors over

the wing and one inch above the rotor plane withone and both rotors spinning. A comparison ofthese two half second traces (Figure (19a-b))shows how the two rotors and wing fuselagecreate a highly turbulent recirculating flow. Thetrace with one rotor running, Figure (19a) showsvariations corresponding to the blade passage andsome very mild disturbances of magnitude lessthan 1 meter/second due to ambient room

turbulence. With both rotors spinning, Figure(19b) the fluctuations have magnitudes as greatas 14 m/s which are due to the unsteady turbulentrecirculatJng flow.

Turbulence Spectra:

The time traces described above are meant to

provide a more qualitative view of the relativemagnitudes and intermittency of the flowcomponents measured by a single hot wire.Another experiment was to quantify the nature ofthe fountain turbulence. The spectrum ofvelocity squared was calculated from a stream ofvelocity measurements from a single hot wire.Figure (20) compares the power spectrum ofinflow velocities measured by a single wire probe1 inch above the rotor plane and 2 inches fromthe tip on the rotor/rotor axis in the re-ingestionzone. The top curve shows the turbulence due to

the re-ingestion of the fountain flow. The lowercurve is the power spectrum of the ambientinflow turbulence which was found by taking

measurements with only one rotor spinning andthe model removed from the experiment. Bothspectra show the blade passing harmonics atinteger multiples of approximately 113 Hz. Thisplot shows that there is 5 to 10 times moreenergy per unit frequency in the fountainturbulence than in the room turbulence for

frequencies between 10Hz and 2kHz. This plotalso shows that the spectral energy for the twocases becomes the same at frequencies higher

than 10kHz. The greater low frequency contentof the fountain inflow may be due to theintermittency discussed above while the increasedhigh frequency velocity fluctuations are due tothe re-ingestion of the turbulent recirculatingfountain flow.

Mean and rms Inflow Velocities:

Mean and rms velocities were also measured on

an evenly spaced 25" by 25" square grid 1 inchabove the rotor plane. Three grids of data weretaken for the three Cartesian orientations of the

hot wire. Reduction of this data was complicatedby several factors. A fully three dimensionalturbulent flow cannot be quantifiedusing a singlewire hot wire unless some assumptions are madeabout the relative magnitudes of the velocitycomponents. Also, the grid of hot wire measure-ments would have been better oriented in a

cylindrical coordinate system as this would bestfit the geometry of a hovering rotor. A Cartesian

system was used as the only available traversewas not appropriate for moving the probe withrespect to radius and azimuthal angle. The datathen had to be reduced in cylindrical coordinates.In order to do this, the theta component of meanvelocity was assumed to be negligible comparedto the radial and axial mean velocities. Another

complicating factor was that the data was takenwith a single wire probe which meant that datapoints had to be measured separately for each hotwire orientation. This leaves the possibility forerror due to inexact probe placement for each setof measurements. Another source of error is that

the rms values of velocity are sometimes morethan 20% of the mean velocity which implies thatKing's Law is less accurate. Future measure-ments will be conducted with an x-wire.

Figure (21) is a contour plot showing the axialcomponent of mean velocity over the rotor plane.Clearly there is a deficit in the inflow velocity

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over the wing. The deficit is recognizeable by

lighter shading over the wing and lower valuedcontours. As has been hypothesized before 12,13this deficit is due to the wing obstructing the flowbeneath the rotor. While these results are not

accurate enough to precisely model the fountainflow, they are conclusive in showing thisimportant phenomenon of tilt rotor hover flow.

Figure (22) is a plot of the rms velocity over therotor plane. This contour plot uses values ob-tained from three orthogonal sets of hot wire data

and plots the values of _Vf '2 + Ve'2 + Vz '2.No attempt was made to reduce this data tocomponent form for the following reason; Whilethe turbulence is most likely isotropic at higherfrequencies, the data was measured 1" above therotor plane and therefore it also reflects thevelocity fluctuations due to the potential flowfield (blade passage). Thus we are unable to

present the rms velocities by component as weare unable to precisely separate the potential flowvelocity fluctuations from the velocity fluc-tuations due to the inflow turbulence. Qualita-

tively the existence of the potential flow is not aproblem as the turbulent velocity fluctuations inthe re-ingestion zone are signiIicandy greater thanthose caused by the potential flow field. This isapparent from Figure (22) which shows the re-ingestion zone as a region of high rms velocities(darker shading) on the in-board section of therotor. These high values are a result of theturbulent fountain flow recirculating into theinflow where it is measured as velocity

fluctuations by the hot wire. This plot alsoshows higher rms values towards the rear of thegrid which indicates that the flow is re-ingestedin the rearward side of the rotor/rotor axis. This

region of high rms velocities corresponds to there-ingestion region observed in the flowvisualization study.

Thus, this contour plot of rms velocities shows

the high levels of turbulence in the re-ingestionzone of the rotor due to the recirculating fountain.These rms velocities are significantly higher thanthe rms velocities due to the potential flow.However, the existence of the potential flow fieldmasks the ambient inflow turbulence outside of

the re-ingestion zone. Although this obscures therelative magnitudes of the re-ingested turbulenceand the ambient turbulence, the significance ofthe the fountain flow being re-ingested is readily

apparent from the high rrns velocities over theinboard section of the wing.

SUMMARY AND CONCLUSIONS

. The recirculating fountain has been studiedand recorded in photos and video. This flowphenomenon is easily identified as theopposing spanwise flows meet at an unsteadyseparation point on the fuselage and eruptover the rotor plane and are re-ingested.

. Side views of the model indicate that

unsteady recirculating flow exists along andabove the length of the fuselage. This aspectof the flow was previously unidentified andits characteristics warrant future study.

. The fountain flow is an multi-directional

phenomenon and efforts to study or modelthe flow in two dimensions will not captureall the important aspects of the flow.

. The recirculating fountain is strongly timedependent as it tends to randomly shift aboutthe aircraft's longitudinal axis. This causesin intermittency effects which cannot bestudied by replacing one side of the modelwith an image plane.

. The unsteadiness and intermittency of thefountain flow were observed with velocitytime traces obtained using hotwire

anemometry. The time traces indicated muchmore turbulent flow over the rear of theaircraft than over the nose.

. Turbulence spectra indicate that inflowturbulence in the re-ingestion zone over therotor contains 5 - 10 time as much energy perunit frequency over a broad range offrequencies than is in the basic inflowturbulence of a single rotor with no fuselageor wing.

. Contour plots of mean and rms velocitieswere generated at a height of one inch abovethe rotor plane. These contours indicate avelocity deficit in the axial component of theinflow velocity in the region over the wing.This deficit is due to the flow obstruction

caused by the wing. The contour plot of rmsvelocity shows high values over part of the

rotor. These high values are due to thehighly turbulent flow being re-ingested.Both the inflow velocity deficit region andhighly turbulent inflow velocity region areclearlyobservedon thecontourplots.

. The results of this study are of a semi-

quantitative nature. The Reynolds numbersare lower than for the XV-15 aircraft and the

rotor blade geometries arc not scaled

precisely. Some moderate assumptions wereneeded to reduce the hot wire velocitymeasurements. Future studies axe plannedunder more rigorous conditions in order to

study the details of the flow, further quantifyit, and look for methods of altering the flow.

AC KNOWLEDGEMENTS

The authors would like to acknowledge theadvice and technical support of Edward DuU and

Todd Ringlet" who are graduate students in theAero and Acoustic Research Group at Cornell.We also wish to acknowledge the experimentaland machine work of undergraduates DarrenHalford, Paul Hamill, and John Sham. The hotwire measurements were obtained with a data

aquisition system implemented by Edward Dulland programmed by Mitchell Radisher. Spectralresults were obtained using ALDAS, a NASA

Ames code prepared by Mike Watts. The flowvisualization photographs were digitized andenhanced with the help of Theresa Howely,Illustrator and Technical Specialist. This

research was supported by NASA Grant NAG 2-554, the NASA-Cornell Space Grant Program,and the Cornell College of Engineering

Undergraduate Research Program supported byMr. James Moore.

REFERENCES

, Rosenstein, H., and Clark, R. (1986)."Aerodynamic Development of the V-22 TiltRotor", AIAA Aircraft Systems, Design andTechnology Meeting, October 1983, PaperNo. AIAA-86-2678.

. McVeigh, M. A., Rosenstein,H., andMcHugh, F. J. (1983). "AerodynamicDesign of the XV-15 Advanced CompositieTilt Rotor Blade", 39th Annual Forum of theAmerican Helicopter Society, May 1983.

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Paisley, D. J. (1987). "R o to rAerodynamics Optimization for High SpeedTiltrotors", 43rd Annual Forum of the

American Helicopter Society, May 1987.

McCroskey, W.J., Spalart, Ph., Laub,G.H., Maisel, M.D., and Maskew, B."Airloads on Bluff Bodies, With Applicationto the Rotor-Induced Downloads on Tiltrotor

Aircraft", Paper No. 11, Ninth EuroppeanRotorcraft Forum, September 13-15, 1983.

Felker, F. F., Maisel, M. D., Betzina, M.D., "Full-Scale Tiltrotor Hover

Performance", Proceedings of the 41stAnnual Forum of the American Helicopter

Society, Fort Worth, Texas, May 1985.

McVeigh, M. A., "The V-22 Tiltrotor Large-Scale Rotor Performance�Wing DownloadTest and Comparison with Theory",Proceedings of the 1 lth European RotorcraftForum, London, England, September 1985.

Clark, D. R., and McVeigh, M. A.,"Analysis of the Wake Dynamics of aTypical Tiltrotor Configuration in TransitionFlight", Proceedings of the 1 lth EuropeanRotorcraft Forum, London, England,

September 1985.

Felker, F. F. and Light, J. S., "Rowr/WingAerodynamic Interactions in Hover",Proceedings of the 42nd Annual Forum ofthe American Helicopter Society,Washington, D. C., June 2-4, 1986.

Rutherford, J. W., and Morse, H. A.(1985). The Tilt Rotor DownloadInvestigation, Videotape, U. S. ArmyAeroflightdynamics Laboratory, 7 x 10Wind Tunnel, Ames Research Center, July1985.

Fejtek, I. and Roberts, L., "Navier-StokesComputation of Wing�Rotor Interaction for aTiltrotor in Hover", AIAA Paper 91-0707,Proceedings of the 29th Aerospace SciencesMeeting, Reno, Nevada, January 7-10,1991.

¢r

Figur_ 1. Hover and forward flight configurations for the tilt rotor aircraft.

{;,_!i,:,_;..,_., ;_:_,.;.r ;S ORIGINAL PA<]_

0i: p,<><).l _U_L_( BLACK AND WHITE P_-t(JTOG_AP_

11.

12.

13

Ordway, D., SAI Bubble Generator:Description and Operating Instructions. SageAction Inc., Ithaca, New York, March,1971.

George, Albert. R., Smith, Charles A.,Maisel, Martin D., and Brieger, John T.(1989). "Tilt Rotor AircraftAeroaacoustics", Proceedings of the 45thAnnual Forum & Technology Display of theAmerican Helicopter Society, Boston,Massachusetts, May 22-24, 1989.

Coffen, C. D., George, A. R., (1990)."Analysis and Prediction of Tilt Rotor HoverNoise". Proceedings of the 46th AnnualForum & Technology Display of theAmerican Helicopter Society, WashingtonD.C., 1989.

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Figure 2. Photo of the 1/12 scale tilt rotor showing model, motors and mounting.

ORIGINAL. PAGE _S

OF POOR QUALrTY

Outboard

Rotor/Rotor Axis Inboa

18c

Fig. 18a

Fig. 18d

Fig. 18a

Longitudinal Axis

L-

Fig. 16

Figure 3. Schematic of model top view showing probe placements, axes, and approximatere-ingestion zones. (Actual. model has 2 blades and no tail for these tests).

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20.00

m/s

18.00

16.00

14.00

12.00

I0.00

8.0¢

6.0C

4.00

2.00

0.00

3.800000 3.82_,ooo 3.a5_ooo 3.B7_ooo 3.go_ooo

Time, seconds

3.930000

Figure 13a. T'ane trace of inflow velocity in the fountain m-ingestion zone showingesscntiaUy laminar flow. Data taken with the hot wire paraUcl to the longitudinal axis,inboard, 2 inches from the rotor tip on the rotor/rotor axis, and 1 inch above the rotorplane.

20.00

m/s

IB.OOJ

16.00]

14.001

10.00

8.00

6.00

4.00]

2-°°10.00_

I

1.02S000 1.051000 1.07"_000 I. 103000 I. 12_)000 I. ! 55000

Time, slconcls

Figure 13b. T'_tne mace of inflow velocity.in the fountain re-ingestion zone showingturbulent flow. Data taken with the hot wn'e parallel to the longitudinal axis, inboard, 2inches from the rotor tip on the rotor/rotor axis, and I inch above the rotor plane.

m/s

20.00

16"00 t

16.ooI1400

12.00

I0,00

8.00

6.00

4.00

2.00

0,00

0.729750 o.'Ts_7so o.76'_7so o.eo'_7so o.as_TSO o.mTso

Time, seconds

Figure 14. Tune trace of inflow velocity in the fountain re-ingestion zone showinginmrmittcntly laminar and turbulent flow. Dam taken with the hot wire parallel to thelongitudinal axis, inboard, 2 inches from the rotor tip on the rotor/rotor axis, and I inchabove the rotor plane.

rn/s

3.|10000. 3.111000

Figure 15. 4 sccond doa= o"ac= of inflow velociw in the fountain re-ingcslion zone. Dam

taken with the hot wire parallel to the longitudinal axis, inboard, 2 inches from the rowr tipon the rowr/rowr axis, and I inch above the rotor planc.

mls

20.00

18"00 f

16"001

14.00J

12.00

10.00

8.00

6.00

4"00 t

2"001

0.00_

\\\\

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0.000000 0.026000 0.052000 0.078000 0.104000

Time, seconds

0.130000

Figure 16. Tmae trace of inflow velocity over the outboard part of the rotor plane showinglaminar flow unaffected by the fountain. Data taken with the hot wire parallel to thelongitudinalaxis,outboard,2 inchesfrom therotortipon therotor/rotoraxis,and 1 inchabove the rotor _lane.

mls

20. O0

18.0©

I 0.00

14.0©

10.01

8.0_

6.00

4.00

2.O0

0.00!

0.029500 0.081500 0.Ia._500 0.16_500 0.2a_500

Time, seconds

0.289! ;00

Figure17a. I inchabove therotor plane.

Figures17a-c. T'tmetracesofinflowvelocity in thefountainre-ingestionzone atdifferentheightsabove therotor plane.Dam takenwiththehotwireparalleltothelongitudinalaxis,inboard,and 2 inchesfrom therotortipon therotor/rotoraxis.

20.O0

0.000000

m

0.208000

t1.040000

Figure 18c. Hot wire on the rotor/rotor axis.

m/s

20.00.

18.00

16. O©

14.00

12.00

10.00

8.00.

6.00

4.00

2.00

O.O0

0.000000 0.208000 o.4,_ooo o.o2_ooo o.._ooo

Time, seconds

1.040000

Figure 18d. Hot wire 8 inches rearward of the rotor/rotor axis.

Figures 18a-e. Time traces of velocity at locations along the longitudinal axis, 1 inch abovethe rotorplane. Hot wire orienmd paralleltothe longitudinalaxis.

20.00

18.00

10.00

14.00

12.00

10.00

8.00

6.00

m/s

4.00

2.00

0.00

0.000000 0.208000 o.41_ooo o.6,_ooo o.,_,ooo I.o4ooooTime, seconds

Figure 18c. Hot wire 12 inches _arwa.,d of the rotor/rotor axis.

Figures 18a-¢. Time traces of velocity at locations along the longitudinal axis, I inch abovethc rotor plane. Hot wire oriented parallel to the longitudinal axis.

20.00

111.00

16.00

14.00

12.00

m/s lO.OO

8.00

8.00

4.00

2.00

0.00

, , ,0.000000 0.104000 0.208000 0.312000 0.41 000

Time, seconds

Figure 19a. One rotor spinning showing very small velocity fluctuations.

0.520000

Figures 19a-b. Time traces of velocity showing the effect of one and two rotors spinning.Hot wire 1 inch above the rotor plane on the inmrscction of the mtorRotor and longitudin:daxis.

I

20.00

14'001

_,,ii0of_nii

"°°1_'I V' 1,,olOoO_o,, , ,0.479000 0.583000

I0.8950000.607000 0.791000

0.99V000