Table Of ContentMECHANISMS OF ACTIVE AERODYNAMIC LOAD REDUCTION ON A
ROTORCRAFT FUSELAGE WITH ROTOR EFFECTS
NormanW.Schaeffler BrianG.Allan LutherN.Jenkins Chung-ShengYao
SeniorResearchScientist SeniorResearchScientist SeniorResearchScientist SeniorResearchScientist
FlowPhysicsandControlBranch
NASALangleyResearchCenter
Hampton,Virginia,USA
ScottM.Bartram W.DerryMace
OpticsSpecialist InstrumentationSpecialist
AdvancedSensingandOpticalMeasurementBranch Sierra-Lobo
NASALangleyResearchCenter Hampton,Virginia,USA
Hampton,Virginia,USA
OliverD.Wong PhilipE.Tanner
SeniorResearchScientist ResearchScientist
U.S.ArmyAviationDevelopmentDirectorate-AFDD(AMRDEC)
JointResearchProgramOffice
NASALangleyResearchCenter
Hampton,Virginia,USA
The reduction of the aerodynamic load that acts on a generic rotorcraft fuselage by the application of active
flowcontrolwasinvestigatedinawindtunneltestconductedonanapproximately1/3-scalepoweredrotorcraft
model simulating forward flight. The aerodynamic mechanisms that make these reductions, in both the drag
and the download, possible were examined in detail through the use of the measured surface pressure distri-
butiononthefuselage,velocityfieldmeasurementsmadeinthewakedirectlybehindtherampofthefuselage
andcomputationalsimulations. ThefuselagetestedwastheROBIN-mod7,whichwasequippedwithaseriesof
eightslotslocatedontherampsectionthroughwhichflowcontrolexcitationwasintroduced. Theseslotswere
arrangedin aU-shaped patternlocated slightlydownstream ofthe baselineseparation lineand parallelto it.
Theflowcontrolexcitationtooktheformofeithersyntheticjets,alsoknownaszero-net-mass-fluxblowing,and
steadyblowing. Thesamesetofslotswereusedforbothtypesofexcitation. Thedifferencesbetweenthetwo
excitationtypesandbetweenflowcontrolexcitationfromdifferentcombinationsofslotswereexamined. The
flowcontrolisshowntoalterthesizeofthewakeanditstrajectoryrelativetotherampandthetailboomand
itisthesechangestothewakethatresultinareductionintheaerodynamicload.
Nomenclature D Fuselagedragforceinthewindaxis,lbs
F+ Reducedfrequency, fW/U
∞
ACS Fuselagecross-sectionalarea(maximum), ft2 f Excitationfrequency,Hz
Aj Jetslotexitarea, ft2 H Fuselageheight(maximum),inches
CD Fuselagedragcoefficient,D/(q∞ACS) L Fuselageliftforceinthewindaxis,lbs
CL Fuselageliftcoefficient,L/(q∞ACS) M Machnumber
Cp Pressurecoefficient,(p−ps)/q∞ m˙ Massflowrate,lbm/s
CT Rotorthrustcoefficient,T/ρ∞πR2(ΩR)2 q∞ Freestreamdynamicpressure, 21ρ∞U∞2, psi
Cµ Momentumcoefficient Re Reynoldsnumber,U∞(2RF)/ν
R Rotorradius,inches
Presented at the American Helicopter Society Technical
R Referencerotorradius,inches
Meeting on Aeromechanics Design for Vertical Lift, San F
T Rotorthrust,lbs
Francisco, CA, January 20-22, 2016. This is a work of the
U Freestreamvelocity, ft/s
U.S.Governmentandisnotsubjecttocopyrightprotectionin ∞
V Jetexitvelocity,eitherpeakorbulk, ft/s
theUnitedStatesofAmerica. j
1
W Fuselagewidth(maximum),inches weightwouldbeunaffectedandremaininbalance. Foraro-
X/R Normalizedstreamwisecoordinate torcraft, the picture is slightly more complicated. The thrust
F
Y/R Normalizedspanwisecoordinate developed by the rotor disk, again in steady level flight, is
F
Z/R Normalizedverticalcoordinate usedbothtobalancetheweightandanynegativeliftthatthe
F
α Fuselageangleofattack,degrees fuselage generates and also to balance the drag. This raises
∆C Changeinthedragcoefficientasapercentageof aquestionwhenitcomestojudgingtheeffectivenessofany
D
thebaseline dragreductionstrategy: Whatistheimpactofthedragreduc-
∆C Change in the lift coefficient as a percentage of
L tion on the download experienced by the fuselage under the
thebaseline
influenceofarotor? Sincetherotorisresponsibleforprovid-
µ Rotoradvanceratio
ν kinematicviscosity, ft2/s ingboththeliftingforcefortherotorcraftandthepropulsive
forceforforwardflight, anybenefitfromareductionindrag
Ψ rotorazimuthangle-Blade1,degrees
ρ Density,slugs/ft3 couldbeoffsetbyanincreaseindownload,effectivelylimit-
inganybenefitfromapplyingtheflowcontrol. Thus,itwould
σ Thrust-weightedrotorsolidity
be preferred that any flow control strategies identified to re-
ducethefuselagedragalsoreducethedownloadactingonthe
fuselage.
Introduction
Thecurrentresearcheffortisthefinalwindtunneltestofa
Foralargenumberofrotorcraft,thedesignforthefuselageis
studythatwasundertakentoexaminetheapplicationofactive
guidedbymaximizingthemissionutilityofthevehiclerather
flow control for drag and download reduction on a generic,
thanitsaerodynamicefficiency. Thiscanresultinaflowfield
nonproprietary, fuselage. The study reported here was con-
aroundthefuselagethattendstoresembletheflowfieldaround
ducted under the NASA Rotary Wing (RW) Project, now
abluff-body,withtheflowaroundtheaftendofthefuselage
the NASA Revolutionary Vertical Lift Technology (RVLT)
being dominated by massive flow separation. This leads not
Project. Ithasbeen,sinceitsinception,anintegratedexperi-
only to a large amount of pressure drag acting on the fuse-
mentalandcomputationalresearcheffortthatstartedwiththe
lage,butalsotoahigherlevelofrotor-induceddownload,or
creationofanewfuselagegeometryfollowedbysmall-scale
negativelift. Ithasbeennotedthatthecruisedragofarotor-
wind tunnel investigations of the baseline characteristics of
craftistypicallyanorderofmagnitudehigherthanthecruise
theisolatedfuselageandcorrespondingsimulations.Theinte-
dragofafixed-wingaircraftofthesamegrossweight(Ref.1)
gratedexperimentalandcomputationalapproachofthiswork
and also that, for rotorcraft operating at high forward speed,
allowed the baseline aerodynamic characteristics to be doc-
halfofthepowerdeliveredbythemainrotorisusedtoover-
umented and validated simultaneously (Ref. 5). Out of this
come the aerodynamic forces, drag and download, acting on
effort, and a related collaboration between NASA and ON-
thefuselage(Ref.2). Iftheapplicationofactiveflowcontrol
ERA, a flow control strategy was identified computationally
(AFC) could be used to deliver back aerodynamic efficiency fortheisolatedfuselageat0◦ angleofattack. Thisflowcon-
to an otherwise aerodynamically inefficient fuselage design,
trolstrategywasthenappliedcomputationallytothemorerel-
the range, speed, and/or fuel efficiency of the vehicle could
evantenvironmentofafuselageatanegativeangleofattack
be increased and its mission utility would not only be pre-
androtoroperatingathighadvanceratio(Ref.9). Thesecon-
served,butenhanced. TheworkofMartinetal.(Ref.3)and
ditionsaretypicalofrotorcraftoperatingathighspeedandare
Ben-Hamou et al. (Ref. 4) has demonstrated that active flow
theconditionsunderwhichthedraganddownloadreductions
control can be applied to achieve a reduction in drag on an
are desired. The computational effort allowed for the study
isolated fuselage. This directly led to further work in apply-
of the effect of the rotor and its wake on the fuselage drag
ingAFCtotheproblemofdragreductiononanisolatedfuse-
and download. It provided a mechanism for evaluating the
lage, specifically the work of Schaeffler et al. (Ref. 5), Woo
effectiveness of active flow control in reducing the drag and
etal.(Ref.6),ColemanandThomas(Ref.7),andLePapeet
download forces and for how different flow control actuator
al. (Ref. 8). Once the research transitioned from an isolated
parameters,suchasslotsize,frequency,andpeakjetvelocity,
fuselagetoafuselageinthepresenceofarotor,attentionalso
affectedthelevelofloadreduction. Ultimately,thecombined
neededtobepaidtothedownload,ornegativelift,developed
knowledge gained from the computations and the earlier ex-
bythefuselage, inadditiontothedrag. Thisisbecausecare
perimentalworkenabledthedesignoftheslotlayoutandflow
mustbetakentoensurethatanyflowcontrolstrategiesthatare
controlstrategythatwastestedexperimentallyinthecurrent
identifiedtoreducethefuselagedrag,donothaveanadverse
research effort. The final wind tunnel entry had three dis-
effectonthedownload.
tinct phases, each of which utilized a different ramp section
One of the key differences between rotary-wing aerody- on the fuselage. The first phase of the entry was devoted to
namics and fixed-wing aerodynamics can be illustrated by a theapplicationofpressure-sensitivepaint(PSP)totheupper
simpleforcebalance.Forafixed-wingaircraftinsteadyflight, surfaceofoneoftherotorbladesandutilizedaclean,unmod-
liftbalancesweightandthrustbalancesdrag. If, throughthe ified, ramp section. Interested readers can find a description
applicationofactiveflowcontrol,thedragactingonthefixed- ofthePSPexperimentalworkinWatkinsetal.(Ref.10). The
wing aircraft, in steady level flight, was reduced, the thrust second phase of the entry was the one that is reported here,
required would be correspondingly reduced and the lift and and also in Schaeffler et al. (Ref. 11), and featured a ramp
2
section equipped with eleven slots through which flow con- agenerichelicopterandalsotobeanalyticallydefined,allow-
trol excitation could be delivered. Finally, the third phase of ingittobeeasilyreproducedforcalculations(Ref.17). The
theentrywasalsoanactiveflowcontrolexperimentthatuti- original ROBIN fuselage has been utilized in several other
lized a ramp equipped with fluidic actuators for load allevi- wind tunnel investigations (Refs. 18,19) and is widely used
ation and the results from this phase are reported in Martin intherotorcraftCFDcommunity(Refs.20,21). Toarriveat
etal.(Ref.12). Additionally,acomprehensivereportofboth theROBIN-mod7fuselage, thestandardcoefficientsthatde-
oftheactiveflowcontrolphasesoftheentrycanbefoundin finetheROBINfuselageweremodifiedtocreateanewfuse-
Ballardetal.(Ref.13). lage geometry that has a rectangular, as opposed to square,
cross-section, a well-defined ramp section, and a high tail-
boom. The fuselage calculation procedure and the modified
ExperimentalFacilityandModel
coefficientsfortheROBIN-mod7arediscussedfullyinScha-
effleretal.(Ref.5). Thisnewfuselageisrepresentativeofa
ThecurrentresearcheffortwasconductedintheNASALan-
generichelicopterfuselagewithinthe lighttomediumrange
gley14-by22-FootSubsonicTunnel(14x22). The14x22is
of civil configurations with a large aft loading cargo section,
a closed-circuit, single-return, atmospheric wind tunnel with
typicalofacommercialrotorcraftthatencountersalargefuse-
a reconfigurable test section. The test section can be oper-
lage drag at high-speed. Following the ROBIN convention,
ated in either an open or closed configuration. For the work
the fuselage length is scaled to a reference rotor radius, R ,
presentedhere, thetunnelwasoperatedintheclosedconfig- F
which results in a non-dimensional fuselage length of 2R .
uration in which the test section has the dimensions of 14.5 F
Since its introduction, the ROBIN-mod7 fuselage has been
fthigh,21.75ftwide,and50ftlongandcanachieveamax-
adoptedbyotherresearcherswithintherotorcraftcommunity
imum speed of 348 ft/s with a dynamic pressure of 144 psf.
(Refs.22–24),particularlythoseexploringtheapplicationof
The airflow in the test section is produced by a 40 ft diam-
activeflowcontrolonrotorcraftfuselages(Refs.6,7,25). For
eter, 9-bladed fan driven by a 12,000 hp main drive. Addi-
the ROBIN-mod7 fuselage tested in 14x22, the size is based
tionalinformationaboutthe14x22canbefoundinGentryet
upon a reference rotor radius of 61.9655 inches, with a total
al.(Ref.14).
fuselagelengthof123.931inches(X/R =2.0). Inreference
F
The model utilized in this research effort was comprised toamediumcivilrotorcraft, itcanbeconsideredanapprox-
oftheGeneralRotorModelSystem(GRMS)andtheROBIN- imately1/3-scalemodel. Thefuselagegeometry anddimen-
mod7(ROtorBodyINteraction)fuselage. TheGRMSisaro- sions can be seen in Fig. 1. It is worth noting that the rotor
tordrivesystemthatispoweredbytwo,75hp,water-cooled system utilized in the current work is oversized for the fuse-
electricmotorsthatdrivea5.47:1transmission.Therotorsys- lage by about 7% and, due to the sting mounting utilized in
temdrivenbytheGRMSconsistedofafully-articulatedrotor the current research effort, the tail cap, which closes off the
hubandasetoffourrotorblades,whichutilizeGovernment tailboom, isnotused. Thisshortenstheoveralllengthofthe
RC-seriesairfoils. Therotorsystemhadadiameterof11.08 fuselageto105inches(X/R =1.694).
F
feetandthecuffofoneoftherotorbladeswasinstrumented
For this research effort, the ROBIN-mod7 fuselage and
to measure blade pitch, lead-lag, and flapping angles. De-
theGRMSweremountedtoastingadapterandlongcannon,
tailedpropertiesfortherotorsystemcanbefoundinWonget
which,inturn,wasmountedtothemastofoneofthe14x22
al.(Ref.15).
facilitymodelcartsinstalledintheaftbayof14x22. Dueto
The GRMS is equipped with two independent six- thelengthofthelongcannon, GRMSandtheROBIN-mod7
component strain gage force and moment balances, one for fuselage end up positioned over a filler cart, which was in-
measuring the loads acting on the fuselage and one for mea- stalled in the front bay of 14x22. The mast on the aft cart
suringtheloadsactingontherotor. Thereareaccelerometers providesbothpitchandelevationcontrol,enablingthemodel
mounted within the GRMS to monitor the operational loads tobepositionedatarangeoffuselageanglesofattackwhile
actingonthecompletesystemduringtesting. Thereisalsoa maintaining a constant height of the rotor hub, nominally
rotorshaftencoder,whichprovidedboth1/Revand1024/Rev 87 inches, in the test section. The model was only tested at
timing signals. These timing signals were used for synchro- 0◦ yaw. Thecompletemodelinstallationinthe14x22canbe
nizingthedataacquisitionandthetimingoftheflowcontrol seeninFig.1(e).
to the azimuthal position of the rotor. The rotor system was
Eightoftheflowcontrolslotswerelaidoutsymmetrically
operated at a rotational speed of 1150 RPM, which resulted
withrespecttothefuselageverticalcenterlineandascloseto
in a hover rotor tip Mach number of 0.58. The rotor system
a continuous U-shape, that the constraints of packaging ac-
wasdesignedwithanominalshafttiltangleof-3◦ relativeto
tuators on the inside of the fuselage shell would allow. This
the fuselage. Additional details regarding the GRMS can be
arrangementcanbeseenschematicallyinFig.1. Onthefuse-
foundinWilson(Ref.16).
lage, this U-shaped group of slots is aligned 23◦ from the
The ROBIN-mod7 fuselage was developed for the initial vertical,whichisthesameorientationas,andslightlydown-
small-scale testing that preceded the current research effort stream of, the flow separation line of the baseline configu-
(Ref. 5) and is a variation of the ROBIN fuselage geome- ration. The slot height was 0.020 inches and the slots were
try, which was developed at NASA Langley in the 1970s. orientedata25◦ angletothelocalsurface. Eachoftheslots
The original ROBIN was developed to be representative of wasassignedanumber. Whenlookingupstreamfrombehind
3
themodel,whichistheviewasshowninFig.1(f),slot1was Forthesteadyblowingportionofthiseffort, themomen-
theuppermostslotontheleft,orport,side.Slots2and3were tum coefficient, C , was defined based upon the total mass
µ
alongtheportsideandslot4wastheslotalongthebottomof flow,asmeasuredbyaninlineflowmeterupstreamofthefour
thefuselageandtheslotclosesttothefuselageverticalcenter- valvesthatcontrolwhichoftheslotpairsareactive. Thejet
lineontheportside. Thispatterncontinuedonthestarboard velocity was computed based on the mass flow and the slot
sideofthefuselagewithslot5beingonthebottomandclosest areaandthus,representsthebulkvelocityofthejet. Thetotal
tothecenterline,slots6and7beingonthestarboardside,and andcavitypressuresmeasuredintheplenumofeachslotwere
slot 8 being the uppermost slot on the starboard side. Three usedduringtheexperimenttobalancethemassflow.Themo-
additional slots were located upstream of the primary eight mentumcoefficientwascomputedasfollows:
slots, slots 9, 10, and 11, and these are visible in Fig. 1(f).
m˙V
Flowcontrolcasesinvolvingtheseslotswillnotbediscussed j
C = . (2)
here. µ qACS
During the course of the experiment, each of the slots
Throughout,C canalternativelybepresentedasapurenum-
µ
wereconnectedtoeitherapressurizedplenumthatdelivered
berorapercentage,asinC =0.025orC =2.50%.
µ µ
asteadyblowingexcitationthroughtheslotortoasynthetic
jet actuator that delivered an oscillatory suction-blowing ex-
citation through the slot, also known as zero-net-mass-flux StereoLargeField-of-ViewPIVSystem
(ZNMF) blowing. Each of the eleven synthetic jet actuators
haditsowndrivesignalandamplifiertoallowthephasingand Toinvestigateanddocumenttheeffectofdifferentactiveflow
amplitudeoftheexcitationtobeindependentlycontrolledon control configurations on the fuselage wake, off-body mea-
a slot-by-slot basis. The 1/Rev and 1024/Rev timing signals surements were made using a Large Field-of-View PIV Sys-
from the GRMS were utilized to synchronize the phasing of tem (LFPIV). LFPIV has been used in the 14x22 for several
thesyntheticjetactuatorsandthedataacquisitionwithrespect yearsandhasproventobeavalidandefficienttooltodocu-
to the rotor. When configured for the synthetic jet actuators, mentwakeflowsforbothfixed-wingandrotary-wingconfig-
thetemperatureofeachofthedriversandthecavitypressure urations.
of each of the cavities were monitored and recorded. When
ThePIVsystemwascomposedoftwo1.5JouleNd-YAG
configuredforsteadyblowing,thecavitypressurewithineach
lasersandtwo,11Megapixel,camerasequippedwith210mm
of the pressurized plenums and the total pressure at the inlet
focallengthlenses. Sincethetestsectionwasclosedforthis
of each plenum was measured by an Electronically Scanned
test(i.e., wallsandceilingdown), opticalaccesswaslimited
Pressure (ESP) module mounted within the fuselage. Only
to windows in the tunnel sidewalls. On the south side of the
four control valves were used in the steady blowing portion
tunnel, the laser beam passed through a set of sheet-forming
of the test. Each valve supplied air to two plenums enabling
optics to create a lightsheet that was then projected through
theslotstobeactivatedinpairssymmetricwithrespecttothe
oneofthesidewallwindowstothemeasurementplanedown-
fuselagecenterline.
stream of the fuselage ramp. The lightsheet was inclined 10
Forbothsyntheticjetexcitationandsteadyblowing,differ-
degreesrelativetoverticalinordertomakethemeasurement
ent configurations of active and non-active slots were tested.
plane nearly parallel to the ramp when the model was set at
Two of these configurations were given names. For the first
negativeanglesofincidenceasdepictedinFig.2,whichalso
configuration, referred to as the U-configuration, all eight of
shows the relationship between the measurement plane and
the slots were active. The second configuration, referred to
thewidthofthefuselage.Atthenominaltestangles,themea-
asUx45-configuration,modifiestheU-configurationsuchthat
surementplaneintersectedthetailboomatX/R =1.31.
F
thetwocenterslots,slots4and5,werenotactive.
For this test, a stereoscopic PIV configuration was used
The definition of the momentum coefficient,C , was dif-
µ to measure three components of velocity. Cameras were po-
ferent for each of the flow control excitations. For the syn-
sitioned in window cavities on opposite sides of the tunnel,
theticjetportionofthiseffort,themomentumcoefficientwas
which placed one camera in back scatter and one camera in
definedbaseduponthepeakjetvelocityatthejetexitandwas
forward scatter. The working distance for each camera was
computedusingEq.1:
nearly 4.2 meters (13.78 feet). This distance, coupled with
Σ(ρ V 2A ) thecameraangleandsensorsizeresultedinafieldofviewof
j j j
C = . (1)
µ qA approximately 838 mm by 481 mm (width x height). Based
CS
on the camera sensor size, 4008 pixels by 2680 pixels, the
Duetothepackagingoftheactuatorswithinthefuselageshell
magnificationwasestimatedtobe0.216mm/pixel.
andthegeometryoftherampregion,theslotlengthsforeach
of the slots were not exactly the same. However, the slot Theflowwasseededusingamineraloilbasedmixtureand
lengths were symmetric with respect to the centerline of the a commercial fog machine, which produced polydispersed
fuselage. Theactualslotlengthswereusedincalculatingthe particles ranging in diameter from 0.25 microns to 1.5 mi-
jetareas,alongwiththeslotheightof0.020inches. Theden- crons. Theparticleswereinjectedintotheflowattherearof
sity,ρ ,wasassumedtobethesameasthedensityoftheair thetestsectionnearthediffuseranddistributeduniformlyby
j
inthetestsection. thefanbeforeenteringthetestsection.
4
(a) Sideview-non-dimensional (b) Rearview-non-dimensional
(c) Sideview-14x22dimensions (d) Rearview-14x22dimensions
(e) The ROBIN-mod7 model mounted on the GRMS and (f) Rearviewofthemodelhighlighting
thelongcannonsting. theactuatorlayout.
Fig.1.TheROBIN-mod7fuselage: Dimensionsandasamodelinstalledinthe14x22SubsonicTunnel.
5
(a) Streamwise (b) Spanwise
Fig.2.TheROBIN-mod7modelasaschematicshowingthelocationandorientationofthePIVmeasurementplanein
thestreamwiseandthespanwisedirections. Themodelisillustratedatanangleofattackof-6◦.
Foreachactiveflowcontrolconfiguration, aminimumof The rotor blades are modeled as nonelastic blades with
100 images were acquired synchronized with a pre-set az- flapping and lead/lag motions. This loose coupling was
imuth angle of the rotor. These images were then processed performed every half revolution of the rotor. A complete
using an interrogation area of 64 pixels by 64 pixels with an description and analysis of all of the post-test CFD can be
overlap of 50%. This corresponds to a spatial resolution of foundinAllanetal.(Ref.31).
13.8mmby13.8mm. Theuncertaintyintheaveragedveloc-
A key observation from the computational analysis in-
itiesisestimatedtobe1m/s.
volvesthetrajectoryofthewakeforboththebaselineandcon-
trolcasesandtheoriginoftheadditionalaerodynamicloads
ForwardFlightatanAdvanceRatioof0.25 actingonthefuselageinthepresenceoftherotor. Iso-surface
contours of vorticity magnitude are presented in Fig. 3 from
As stated in the Introduction, the current research effort has theOVERFLOWsimulationsusingtheSAturbulencemodel.
beencarriedoutsinceitsinceptionasacombinedexperimen- These iso-surface contours are for a single vorticity magni-
talandcomputationaleffort. Inadditiontothecomputational tude value and are shaded by the local pressure coefficient.
investigations conducted before the mid-scale testing began, Therotorazimuthangleofthedatapresentedhereis0◦,cor-
whicharesummarizedbyAllanetal.(Ref.9),aseriesofcom- responding to a rotor blade directly over the tail, or equiva-
putationalfluiddynamics(CFD)simulationswereconducted lentlythenose. Forthebaselinecase, Fig.3(a), thevorticity
after the completion of the experimental efforts. These were iso-surfaces indicate that as the flow rounds the shoulder of
conducted to simulate specific cases as close to the experi- therampitremainsattachedforashortdistanceuptheramp
mentalconditionsaspossibleandtoprovideanassessmentof andthenseparates. Whiletheflowisseparated,itstillfollows
theperformanceoftwoturbulencemodels. Theresultswere the general shape of the ramp until it is turned by the tail-
comparedtotheexperimentaldataandalsousedtoyieldad- boom. This description is reinforced by looking at the mea-
ditionalinsightintothemechanismsinvolvedintheobserved sured centerline surface pressure distribution, which is pre-
draganddownloadreductions. Allofthesesimulationswere sentedinFig.4. Herethecenterlinepressuresforthebaseline
carriedoutatanadvanceratioof0.25,afuselageangleofat- case indicate that the flow remains attached as it rounds the
tackof-6◦andathrustconditionofCT/σ =0.075.Thesteady ramp shoulder, separates shortly thereafter, and then follows
blowing excitation proved more robust than the synthetic jet thegeneralshapeoftherampratherthanremaininginamore
excitationatthisadvanceratio,soonlysteadyblowingcases streamwisedirection. ForFig.4,andtheothermeasuredcen-
wereselectedforsimulation. terline surface pressure figures to follow, the symbols on the
solid lines corresponds to pressure taps on the fuselage cen-
The numerical simulations were conducted using
terline. The dashed line is included to give an indication of
OVERFLOW solving the compressible Reynolds-averaged
thestrengthofthesuctionpeakattherampshoulderandthe
Navier-Stokes (RANS) equations. The Spalart Allmaras
singlesymboloneachoftheselinesisanaveragedvaluefrom
(SA) (Ref. 26) and Shear Stress Transport (SST) (Ref. 27)
twopressuretapssymmetricallyoffsetfromthecenterlinein
turbulence models were used for the numerical simulations
thespanwisedirection(Y/R =±0.051).
with a Rotational/Curvature Correction (RCC) model, as F
implemented in OVERFLOW (Ref. 28). The time-accurate Forbothofthesteadyblowingcontrolcases,Figs.3(b)and
rotor/fuselage simulations used a dual-time stepping method 3(c),thevorticityiso-surfacesindicatethatthewakemovesin
withtimestepsequivalentto0.125◦rotorrevolutionswith40 more of a streamwise direction then does the baseline case.
sub iterations per time step. The rotor dynamics were simu- The wake does not follow the geometry of the ramp and it
lated using a loose coupling between OVERFLOW and the stays further away from the tailboom. This is also seen in
comprehensive rotorcraft code, CAMRAD-II (Refs. 29,30). centerline surface pressures in Fig. 4. Both of the control
6
-0.5 Baseline 0.4
Ux45
U
-0.25
0.2
(a) Baseline CP 0 Z/RF
0
0.25
-0.2
0.5
0.8 1 1.2 1.4
X/R
F
Fig. 4. Measured surface pressure distribution over the
(b) U-configuration
centerlineoffuselagerampsection. AFC=SteadyBlow-
ingatC =2.07%fortheU-configurationandC =1.67%
µ µ
fortheUx45-configuration(VR=2.6forboth),µ =0.250,
C /σ =0.075,α =−6◦. Theblacklinesarethefuselage
T
geometry.
thepredictedaerodynamicloads,Fig.5(b). Theyaretheori-
ginoftheadditionalaerodynamicloadingthatthepresenceof
therotorgeneratesonthefuselage.
(c) Ux45-configuration Building upon the insight gained from the computational
results, consider the velocity field on a plane immediately
downstream of the fuselage ramp as measured by the large-
field-of-view stereo PIV system. In Fig. 6, a representative
Fig.3.Vorticitymagnitudeiso-surfacecontoursshadedby exampleofthecompletemeasuredvelocityfieldispresented
Cpforthebaseline,U-,andUx45-configurationsfromthe fromaviewpointlookingupstreamattherampsection. Itis
CFDsimulationsusingtheSAturbulencemodel,atarotor presented in horizontal and vertical coordinates as measured
azimuth angle of 0◦. AFC = Steady Blowing at VR = 2.6, within the PIV measurement plane and nondimensionalized
µ =0.25,CT/σ =0.075,andα =−6◦ bythereferencerotorradiusforthefuselage. ThestereoPIV
measurementsresultinavectorfieldwiderthanthewidthof
cases have a reduced suction peak indicating an earlier sep- thefuselage, whichis±0.164inthesamecoordinates. The
aration than the baseline case with the Ux45-configuration black semi-circle represents the size and location of the tail-
showing less of a tendency to follow the shape of the ramp. boom, relative to the measurement plane. The vectors plot-
For these control cases, as throughout, when comparing the ted are the in-plane components relative to the measurement
U-configurationtotheUx45-configuration,theratioofthejet plane.Thereareamaximumof114vectorshorizontallyanda
velocity to the freestream velocity, the velocity ratio VR, is maximumof52vectorsvertically. Inthenondimensionalized
heldthesame. Forthecaseshere, theVR=2.6corresponds coordinates,thevectorspacingis0.00877.
to aC = 2.07% for the U-configuration andC = 1.67% for
µ µ
In order to make the vector field easier to visualize, from
theUx45-configuration,experimentally.
this point forward, a convention will be adopted where ev-
InFig.5(a),theexperimentallymeasuredsurfacepressure ery other vector horizontally is skipped and only every third
on the upper portion of the nose of the fuselage (X/R = vector vertically is presented. This allows the vectors to be
F
0.200,Y/R =0.016,Z/R =0.145)versustherotorazimuth displayed with a longer length, thus allowing the overall in-
F F
angle is presented. The high values of pressure as the rotor plane pattern of the flow field to be visualized. The out-of-
blades pass over the nose are clearly seen and correspond to planecomponentofthevelocitywillberepresentedbycolor
the high pressure level seen on the nose in Fig. 3, which is contours. Thecontoursfromthepartoftheplanewheremea-
shadedred. Thesehighpressureloadingsasthebladepasses surementswerenottakenaresuppressedbynotshowingcon-
over the nose generate forces that push the nose down, in- tours below a value ofU /U =0.07. An additional con-
PIV ∞
creasing the download, and push the nose aft, increasing the vention was adopted that the width of the wake will be in-
drag. Thesebladepassageeventscorrespondtothepeaksin dicated by only presenting contours of the out-of-plane ve-
thedragandthemaximumnegativevaluesfortheliftseenin locity, which are less than 0.9 times the freestream velocity
7
(U /U ≤0.90). changing.ForeachoftheC valuesplotted,anadditionalpair
PIV ∞ µ
of slots is activated. This is being done in sequence starting
Following these conventions, the velocity fields for the
withtheupperpair,slots1and8,andcontinuinguntilallthe
baseline,U-andUx45-configurationsarepresentedinFig.7.
slotpairsareactive. Pair1(P1)wouldbeslots1and8,pair2
TheacquisitionofthePIVdatawassynchronizedwiththero-
(P2)wouldbeslots2and7,pair3wouldbeslots3and6,and
tor through the use of the 1/Rev pulse generated by GRMS
pair4wouldbeslots4and5. ThesecondtothelastC value
when the leading edge of blade 1 passes over the tail and µ
this position is a rotor azimuth angle of 0◦. The PIV data plotted would correspond to the Ux45-configuration and the
presented here is for two different rotor azimuth angles, 23◦, finalvalueplottedwouldcorrespondtotheU-configuration.
whichwillbethestandardangleunlessotherwisenoted,and The performance levels for drag and download reduc-
60◦. Thebaseline,Figs.7(a)and7(c),doesshowsomeslight tion that are achieved by just the P1-configuration is quite
changesastheblade1movesfurtherawayfromthemeasure- impressive. The first quarter of the Cµ invested in the
ment plane, however the general features are the same. The load reductions achieved half of the overall reduction, a
widthofthewakeisroughlythewidthofthefuselage. There ∆CD of -12.6% for just the P1-configuration versus -25.2%
are two regions of low out-of-plane velocity on either side for the U-configuration and a ∆CL of -27.0% for just the
of the tailboom. In between them, there is a region of large P1-configuration versus -53.8% for the Ux45-configuration,
vertical velocity that is roughly as wide as the tailboom. As the configuration which produces the maximum download
was seen in Fig. 3(a), since the flow behind the ramp in the change. Adding the second set of slots, so that P1 and P2
baselineconfigurationisfollowingtheshapeoftherampeven areactive,bringstheperformanceveryclosetothemaximum
thoughtheflowisseparated,theflowwillexhibitalargever- valuesachieved. Anadditional5%dragreductioncanbehad
ticalvelocitycomponentwhenitencountersthePIVmeasure- if the last two sets of slots are activated, resulting in the U-
mentplane. IthasbeennotedbyMartinetal.(Ref.12),that configuration, but it comes at a cost of increasing theCµ re-
cases where a region of large vertical component of velocity quiredforthefirst20%reductionby53%. Additionally,very
existdirectlyunderthetailboomarealsocasesthatexhibita little change in download reduction is achieved by adding in
largeamountofdownloadactingonthefuselage. Hereitcan the third pair of slots, which is the Ux45-configuration, and
beseenthatitisthetrajectoryofthewakethatcausesboththe thereisthesamepenaltyforthedownloadreductionwiththe
regionoflargeverticalcomponentofvelocityandthesurface U-configurationthatwehaveseenbefore. Thecenterlinesur-
pressuredistributionthatgeneratesthedownload. facepressuredistributionontherampsectionforeachofthese
casesispresentedinFig.9.
FortheU-configuration,Fig.7(b),thelowout-of-planeve-
locityregionontheportsidehasbeeneliminatedandtheone In Fig. 10, the baseline, P1-configuration, P1+P2-
on the starboard side has been moved a significant distance configuration,andtheU-configurationvelocityfieldsarepre-
away from the tailboom. Also, the vertical velocity in the sented. The rotor azimuth angle for the acquisition of all of
area directly under the tailboom has been reduced. As was thesewas23◦. Forthebaselinecase,Fig.10(a),therearetwo
seen in Figs. 3(b) and 3(c), when the flow over the ramp is regionsoflowout-of-planevelocitybelowandoneitherside
being controlled in either the U-configuration, or the Ux45- ofthetailboomwithanareaofhighupwash,orhighvertical
configuration, the result is the wake is moving in more of a velocity,betweenthemdirectedtowardsthelowersurfaceof
streamwise direction and as it encounters the PIV measure- the tailboom. As noted earlier, this indicates that, while the
mentplane,theobservedverticalvelocitycomponentwillbe flowinthebaselinecaseisseparated,theflowisstillfollow-
small. In fact for the Ux45-configuration, Fig. 7(d), that re- ingthegeneralshapeoftherampsuchthattheflowhasahigh
gionoflargeverticalvelocityhasbeenreplacedbyalargere- verticalvelocitycomponentwhenitencountersthePIVmea-
gionoflowout-of-planevelocitydirectlyunderthetailboom. surementplane. Evidenceofthisflowpatterncanbeseenin
It is this region that generates the characteristic shape of the theCFDsimulationresultspresentedinFig.3forthebaseline
surfacepressuredistributionfortheUx45-configurationseen case. Thewidthofthewakeisapproximatelythewidthofthe
inFig.4andacorrespondingreductionindownload. Forthe fuselage,thefuselagewidthis±Y/RF = 0.164.
Ux45-configuration, the experimental aerodynamic load re- In Fig. 10(b), the effect on the velocity field in the wake
duction was a 53.8% download reduction with a 24.4% drag from steady blowing in the P1-configuration can be seen.
reduction. FortheU-configuration,theexperimentalaerody- There is still a region of large vertical velocity, but its mag-
namicloadreductionwasa39.2%downloadreductionwitha nitudeclosetothetailboomisgreatlyreduced. Itcanalsobe
25.2%dragreduction. seenthatthewakeiscontractedbyasmallamount. Thereisa
muchlargerlow-speedregiontotherighthandsideofthetail-
“BuildingTheU” boomcenterline.Whenthesecondslotpairisbroughton-line,
this low-speed area grows in size, as seen in Fig. 10(c), and
InFig.8,thechangeindragandliftcoefficientsasafunction the area of high vertical velocity component directly under
ofthemomentumcoefficient,C ,ispresented.Typically,data the tailboom has been removed. This indicates that the flow
µ
presentedlikethiswouldbeusedtoindicateanincreaseinthe over the ramp and into the wake is no longer following the
levelofexcitation,e.g.,increasingthejetvelocityoutofasin- generalshapeoftheramp, butisinsteadseparatingfromthe
gleslot. Here,however,thejetvelocityisremainingapprox- rampandmovinginamorestreamwisefashion,aswesawin
imately constant and it is the number of active slots that are theCFDiso-surfacespresentedinFig.3forthecontrolcases.
8
Withinthelowspeedregion,theflowisreversed,asshownin on either side of the tailboom and between them, there is a
Fig.11,butthemagnitudeoftheout-of-planevelocityisvery regionoflargeverticalvelocitythatisroughlyaswideasthe
small,withthemeasurementsindicatingamaximumreversed tailboom. Atthishigheradvanceratio,thewidthofthewake
flow velocity of 0.7% U . If the last two pairs of slots are isslightlysmallerthanataµ =0.25andslightlysmallerthan
∞
brought on-line, resulting in the U-configuration, and shown thewidthofthefuselage. FortheU-configuration,Fig.13(b),
inFig.10(d),someofthelargeverticalvelocitydirectlyunder thelowout-of-planevelocityregionontheportsidehasbeen
the tailboom returns and the low-speed region is greatly re- eliminatedandtheoneonthestarboardsidehasbeenmoved
ducedinsizeandmovesfurtherawayfromthetailboomthan away from the tailboom. Again, the vertical velocity in the
inthepreviouscases. TheU-configurationpresentedherehas area directly under the tailboom has been reduced compared
the largestdrag reductionof thisset andthe smallestarea of tothebaseline. TheUx45-configuration, Fig.13(c), alsoex-
reversedflow,butproduceslessdownloadreductionthanthe hibitsalargeregionoflowout-of-planevelocitydirectlyun-
Ux45-configuration. This is due to the wake having less of der the tailboom. In Fig. 13(d), it can be seen that this is a
astreamwiseorientationfortheU-configuration,asindicated region of slightly reversed flow with the recirculation reach-
by the larger vertical velocities directly underneath the tail- ingamaximumspeedof18%ofthefreestream. Thewidthof
boom. thewakeintheUx45-configurationcloselymatchesthewidth
ofthewakeinthebaseline,withthewakeexpandingslightly
ontheretreatingsidefortheU-configuration.
ForwardFlightatanAdvanceRatioof0.35
ForwardFlightatanAdvanceRatioof0.225
For the current research effort, the ultimate goal was to
achieve drag and download reductions in a range of angles
The highest freestream velocity at which the synthetic jets
ofattackandadvanceratiosthatarerepresentativeofarotor-
were able to demonstrate sufficient authority for drag and
craftinforwardflight. Theseweredeterminedtobefuselage
downloadreductioncorrespondstoanadvanceratioof0.225.
anglesofattackintherangefrom-3◦ to-6◦ atanadvancera-
The performance of the synthetic jet actuation over a range
tioof0.35witharotorthrustcoefficient,C /σ ,of0.075. At
T of angles of attacks for primarily the Ux45-configuration is
the advance ratio of 0.350, only the steady blowing had the
presented in Fig. 14. For this data, the frequency was the
required control authority to reduce the aerodynamic loads
same for both configurations, 6/Rev, which corresponds to a
acting on the fuselage. Fig. 12 presents the effect of steady
F+=1.28,andtheexcitationonaper-slotbasiswasthesame
blowing on the fuselage drag and lift as a function of angle
resulting in a C = 1.03% for the Ux45-configuration and a
of attack at a constant level of excitation on a per-slot basis µ
C = 1.33% for the U-configuration. The synthetic jets were
throughout, whichresultsinaC =1.84%fortheUx45con- µ
µ operatedsothateachjetwas180◦outofphasewithitsneigh-
figurationandaC =2.28%fortheUconfiguration. Thedata
µ bor.
presentedindicatesthatsteadyblowingintheU-configuration
reduces the drag uniformly across the angle of attack range The application of control in the Ux45-configuration at
tested providing about a 30% reduction for all angles of at- zero degrees angle of attack results in an increase in drag.
tack. For steady blowing in the Ux45-configuration, there is Themaximumdragreductionwasobservedatafuselagean-
an increasing level of drag reduction as the angle of attack gle of attack of -6◦, with the data obtained -3◦ also showing
becomes more negative. At 0◦ angle of attack, the Ux45- a decrease in drag. If the trend of the data holds, the Ux45-
configurationactuallyresultsinaslightincreaseindrag, but configurationreducesthedragwithintheangleofattackrange
at -6◦ angle of attack, there is a 25% reduction in drag. The of -1.2◦ to -6◦. There is a download reduction produced by
Ux45-configuration does prove more capable of producing a theUx45-configurationacrosstheentireangleofattackrange
downloadreduction. AsshowninFig.12(b),steadyblowing withthemaximumreductionalsoatafuselageangleofattack
in the Ux45-configuration delivered download reductions on of -6.◦ The maximum drag and download reduction for the
the order of 50-60% across the entire range of angles of at- Ux45-configurationwas22%and43%, respectively. Forthe
tacktestedwithamaximumreductionof59%occurringatan U-configuration,maximumdraganddownloadreductionalso
angleofattackof-2◦. SteadyblowingintheU-configuration occur at a fuselage angle of attack of -6◦ and were 16% and
produced a 30% increase in the download at 0◦ angle of at- 14%,respectively. Forthesyntheticjetexcitation,theUx45-
tack. However,oncethefuselagereaches-2◦ angleofattack, configurationperformsfarbetterthantheU-configuration.
the U-configuration is producing download reductions lead- ThepointforthezerodegreeangleofattackinFig14(b)
ing to a reduction of 25% at -6◦ angle of attack. Since the isincludedforcompleteness. Theuncertaintyassociatedwith
rotorprovidesboththeliftingforcefortherotorcraftandthe thatpointisveryhighduetotheaccuracyofthebalanceand
propulsiveforceforforwardflight,determiningwhichreduc- thefactthatthemagnitudeoftheliftforceforthebaselineis
tionismoredesirablewoulddependonotherrequirements. veryclosetozeroandtheeffectofthecontrolistoreducethe
magnitudeoftheliftforceevenclosertozero(Refs.11,13).
Using the same conventions established in the previous
section,thePIVresultsforanadvanceratioof0.350arecon- For the PIV acquisition, the synthetic jets introduce the
sidered and presented in Fig. 13. Overall, the features are need to look at several different rotor azimuth angles. The
similartothefeaturesatanadvanceratioof0.25. Thebase- drivesignalsforthesyntheticjetactuatorsweresynchronized
line velocity field has two low out-of-plane velocity regions tothe1/RevpulsegeneratedbyGRMSwhentheleadingedge
9
ofblade1passesoverthetail. Thesyntheticjetswereoper- control is directly related to the maximum peak jet velocity
ated at a frequency of 6/Rev, which means that as blade 1 thatcanbedevelopedbythesyntheticjets. Forthesynthetic
sweepsfrom0◦to60◦,theactuatorgoesthroughonecomplete jetexcitationtobeappliedathigheradvanceratios,thedevel-
cycle. PIVimagepairswereacquiredatthreedifferentrotor opmentofactuatorswithhigherpeakexitvelocitieswouldbe
azimuthangles,23◦,38◦,and53◦andarepresentedinFig.15. required.
The effect of the rotor blade passage on the baseline can be
seen in Figs 15(a)-15(c). The unsteadiness of the wake can Acknowledgments
be seen in the strength and direction of the in-plane velocity
ThisworkwassupportedbytheNASARotaryWingProject,
directly under the tailboom. The U-configuration reveals it-
now the Revolutionary Vertical Lift Technology Project, and
selflesscapableherewithunsteadyexcitationthanitdidwith
the support of Susan A. Gorton, Project Manager, is grate-
steadyexcitation. Thereisonlyaslightreductioninthemag-
fully acknowledged. The synthetic jet actuators used in this
nitudeoftheverticalvelocitydirectlyunderthetailboomand
research effort were designed, fabricated, and installed by
the regions of low out-of-plane velocity are only slightly al-
Dave Domzalski of Domzalski Machine. This work would
tered. TheUx45-configuration,Figs15(g)-15(i),issuccessful
not have been possible without the hard work and dedica-
inmaintainingthelargeregionofreversedflowthatwasseen
tion of the Army/NASA rotorcraft team, past and present,
in the steady cases. Given the performance of the synthetic
particularlyAustinOvermeyer,BryanMann,AndyHarrison,
jetexcitationatthisadvanceratioandthefindingspresented
Stephen “Fred” Mason, Brendon Malovrh, Preston Martin,
previouslyabout“buildingtheU,”thesyntheticjetconceptis
Jim Hallisy, Kevin Noonan, and Wayne Mantay. This work
stillattractiveforthisapplication.Itwillrequire,however,the
wouldnothavebeenpossiblewithoutthehardworkandded-
developmentofsyntheticjetactuatorswithhigherjetveloci-
icationoftheentire14x22staff,particularlytheleadtesten-
tiesthanthoseutilizedhere.
gineerJoshBallard,JimByrd,BenTrower,AndyBoneyand
JoeBurtonJr.
ClosingThoughts
References
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Paper161,TicinoPark,Italy,2011.
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