DTIC ADA529846: Flow Visualizations and Extended Thrust Time Histories of Rotor Vortex Wakes in Descent

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Flow Visualizations and Extended Thrust Time Histories
of Rotor Vortex Wakes in Descent
James Stack
!
University of California Berkeley
Berkeley, California 94720-1740
Francis X. Caradonna
†
Army/NASA Rotorcraft Division, U.S. Army AMRDEC
Ames Research Center
Mo!ett Field, California 94035-1000
O¨mer Sava¸s
‡
University of California Berkeley
Berkeley, California 94720-1740
Anexperimentalstudyisperformedonathree-bladedrotormodelintwowatertanks. Theblade
pitch, rotational velocity, descent angle, and descent speed are all varied in order to simulate a
wide range of rotorcraft operating states, focusing on descent cases where the rotor is operating
in or near vortex ring state — an area in which there is currently very little available data. Flow
visualization is done by injecting air bubbles and uorescent dye tangentially from the blade tips
to mark the vortex core, showing the development of both short-wave (“sinuous”) and long-wave
(“leapfrogging”) instabilities on the helical vortices in the wake. Strain gages are used to record
transientloads, allowingacorrelationbetweentherotorthrustperformanceandthedevelopment
of the vortex wake. Reynolds numbers are of order 105 and test runs are performed for extended
periods — up to 500 rotor revolutions — demonstrating the repeatability of the patterns of thrust
variation. The data indicate that as the instabilities develop, adjacent vortices merge and form
thick vortex rings, particularly during descent. Periodic shedding of these rings from the wake
associated with vortex ring state is observed, resulting in peak-to-peak thrust uctuations of up
to 95% of the mean and occurring at regular intervals of 20—50 rotor revolutions, depending on
ow parameters.
Notation
V free stream (towing) speed
V velocity induced at rotor in hover, (T/2"A)1/2
h
c blade chord V rotor tip speed, "R
tip
A rotor disk area, !R2 V rotor forward ight speed, V cos$
x
CT thrust coe!cient, T/"AVt2ip Vz rotor descent speed, V sin$
L blade length $ descent angle
N number of rotor revolutions % collective pitch angle at 0.75 R
R rotor radius & short-wave instability wavelength
Rec Reynolds number based on chord, cVtip/# # kinematic viscosity
T rotor thrust " water density
’ standard deviation of mean rotor thrust
GraduateStudentResearcher,[email protected]
!
† Sta!Scientist,[email protected] ’s rotor solidity ratio
‡ Professor,[email protected] ( vortex ring shedding period
Presented at the AHS 4th Decennial Specialists’ Conference " rotor rotation rate
on Aeromechanics, San Francisco, California, January 21-23,
2004. Copyright c 2004 by the American Helicopter Society
"
Internation, Inc. All rights reserved.
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Flow Visualizations and Extended Thrust Time Histories of Rotor Vortex
5b. GRANT NUMBER
Wakes in Descent
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Division,Ames Research Center,Moffett Field,CA,94035
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Introduction
harnessed by researchers using more sophisticated, de-
tailedmodelsoftheoweld. Thecurrentstateofthe
art in rotor wake modeling is “free wake analysis”, a
An accurate understanding of the physics of helical
methodinwhichthewakeoweldiscalculatedbased
vortexwakeshaslongbeenregardedasoneofthemost
on the induced velocities of all the tip vortices as well
di!cultproblemsinuiddynamics. Withimplications
as the inboard vortex sheets (see Refs. 21,22). And
on the performance of propellers, wind turbines, and
while such techniques have led to signicant progress
helicopter rotors, the problem is of practical interest
overthepasttenyearsintheunderstandingofsimpler
to many. Even before the days of modern production
cases of ight such as hover, they are not yet capable
helicopter ight, the issue of the nature and stability
of predicting the behavior of the wake in more compli-
of ring vortices and helical vortices had been analyzed
catedightstateswheretheaircraftismaneuveringor
extensively. Levy and Forsdyke (Ref. 1) performed a
descending. At this point, there is no reliable physical
stability analysis on a single helical vortex in 1928,
model for rotor wake ow, nor is one possible without
and more recently, Landgrebe (Ref. 2) and Widnall
better experimental knowledge of the ow than cur-
(Ref.3)haveaddedtoandcorrectedthisstudy. Gupta
rently exists.
and Loewy (Ref. 4) have performed a similar stability
analysis on multiple interdigitated helical vortices. The problem for aerodynamicists is that the ight
Despite the focus that modern helicopter ight has regimes that are of greatest interest to the rotorcraft
brought to the problem in the last hundred years, and community are precisely the ones in which CFD pre-
despite the power of modern computers and experi- dictions are least accurate. When a helicopter is de-
mental tools, a truegrasp of the physics of helical vor- scending rapidly, the very thing that makes the wake
tices has remained elusive. While somewhat reason- solution so di!cult — the intense interaction between
ableapproximationsoftheirbehaviorcanbemadeun- the rotor and its vortex wake — causes large, un-
der highly simplied scenarios, there is much progress steady dynamic loads on the blades. Under certain
stilltobemadeontheproblemofreal helicalvortices. circumstances (when the rotor descent velocity ap-
The current state of rotor wake modeling is that while proximately matches the wake velocity), the aircraft
accurate computations of relatively simple wake ows can encounter a condition known as vortex ring state
(such as hover) are beginning to produce results that (VRS), where the tip vortices merge together, forming
are quantitatively interesting, computations of rotor a thick vortex ring that remains near the rotor plane,
wakes that undergo massive interaction with the rotor disrupting the inow and causing a dramatic reduc-
(suchasforthevortexringstate)arecurrentlyofonly tion in lift. This unstable ring typically undergoes
qualitative interest at best. shedding/reformation cycles that result in large uc-
In the early days of helicopter ight, a number of tuationsinthrustthatmaketheaircraftquitedi!cult
simplied models appeared — such as the classic mo- to control. As retired test pilot Mott F. Stancheld
mentumtheoryandthebladeelementmomentumthe- says, “In my opinion, a mature VRS is the most haz-
ory(Refs.5,6)—whichwerecapableofpredictinggross ardous condition that exists in the realm of helicopter
performance characteristics for a rotor (such as thrust aeronautics” (Ref. 23).
andpower)butwereunabletocapturethedetaileddy- Although the aerodynamics of rotors in descending
namics of the wake ow eld. In view of the acute ef- ight — and in VRS in particular — has been the sub-
fectthewakehasontherotorperformance,researchers ject of research for many years (see Refs. 7—10,24),
began to undertake more detailed experimental stud- that work has largely focused on measurements of the
ies of the wake ow eld, beginning with wind tun- rotor thrust and power with very little ow eld vi-
nel testing and smoke ow visualization as early as sualization, and even less simultaneous measurement
the 1920s (Refs. 7,13,14). Later, experiments were of thrust/power and ow elds. This is likely due
done using hot wire anemometry (HWA) and other to the relative “disorder” of such ows and the di!-
techniques (Ref. 15) to measure the velocity eld it- culties associated with facility/model sizing, turbulent
self. Even more recent work has used laser Doppler di#usion, and injection of ow markers. The present
velocimetry (LDV) (Refs. 16—18) and particle image studyis aimed at addressingthesedi!culties andpro-
velocimetry(PIV)(Refs.19,20)asnon-invasivemeans viding a relatively complete description of the wake of
of achieving the same. the descending rotor and its e#ect on rotor loading.
The power of modern computers has recently been Ultimately, it is planned to develop a data set with
2
su!cient information to improve and validate compu-
tational methods for the prediction of rotor descent
behavior.
Flow visualization and thrust measurement results
are presented from experiments performed on a model
rotor in a wide range of operating states — from hover
to forward ight to rapid descent — with the focus be-
ing on the vortex ring state regime. Experiments were
performed in a 60 m long water tow tank, which al-
lowedforextended runtimesthatweremorethansuf-
cientforthestudyoflong-periodunsteadyowssuch
as those encountered in descending ight, and which
weremuchlongerthanhavepreviouslybeenperformed
insimilarstudies(whichhavegenerallybeenconducted
in wind tunnels — see Refs. 24,25). The rotor’s perfor-
manceisquantiedbymeasurementsofitsthrust,and
this information is correlated with simultaneous ow
visualization images. Figure 1: Model 3-bladed rotor. Diameter is 25.4
Thetime-historycharacteristicsoftherotor’sthrust cm. Dye and air enter the tubes at the root, ow
are examined for a broad range of descent speed, de- through the blades to the tips, and enter the ow
scentangle,andcollectiveanglecombinations. Bytest- tangentially.
ing the rotor’s performance over a wide variety of con-
gurations, the rotor’s performance can be fully char-
mm ID stainless steel tube embedded along its span,
acterized,andthedescentconditionsinwhichVRSbe-
allowing dye or air to be injected into the ow from
haviorisobservedcanbeclearlyidentied. Thethrust
the tip to mark the vortex cores. The airfoil modi-
time-historiesoftheseperiodicsheddingcasesarethen
cations included a chordwise-linear thickness increase
compared in order to determine how the descent con-
which thickened the trailing edge and provided more
gurationa#ectstheamplitude,frequency,andoverall
room for the dye/air tubes. In addition, the aft cam-
“orderliness” of the observed thrust uctuations. For
ber of the root airfoil was increased.
these particular cases, the ow visualization images of
The rotor was driven by a digitally-controlled mi-
the experiment provide clues as to the nature of the
crosteppermotor(25,000pulsesperrevolution),allow-
vortexwakeformationandsheddingphenomenonthat
ing for precise control of both the rotor’s position and
makes VRS such a dangerous ight regime.
velocity. The motor was mounted atop an 89 cm ver-
tical shaft and drove a 23 cm horizontal shaft onto
which the rotor was xed (Fig. 2). The vertical shaft
lay downstream of the rotor during descent testing, so
Experimental Setup astoavoidinterferingwiththerotor’sinow. Justbe-
neaththemotorwasa2.5cmthickrectangularmount-
ingplatewhichsupportedthemodelassemblyandalso
Rotor Model measured rotor thrust, using a pair of exures instru-
Experiments were performed with a three-bladed 25.4 mented with 120 ohm strain gage bridges. The thrust
cmdiameterrotormodelfeaturingmanuallyadjustable readings were fed to the computer controlling the ex-
blade pitch. The blades (Fig. 1), which were 9.5 cm periment. Data are corrected for drag tares and low-
long,weremoldedfromcarbonberreinforcedplastic. pass ltered during post-processing to eliminate high-
The blades were untapered, with a 1.9 cm chord, and frequency electrical and vibrational noise while retain-
had a twist of about 5 (compared with twists of 35— ing the main features of the signal.
!
40 for typical tilt-rotor aircraft). The low blade twist Tovisualizetherotor’swake,airbubblesandsodium
!
was chosen in order to minimizerotor separation. The uorescent dye were released from the blade tips in a
blade airfoils were modied ARAD-10 at the tip and direction tangential to the blade path. The dye and
modied ARAD-13at theroot. Each bladehad a0.36 air were supplied to the dye reservoir at the base of
3
the vertical shaft through thin, exible plastic tubing
(Fig. 2). The dye reservoir was directly connected to
therotor dye tubes through thehorizontal drive shaft.
The pressure decit at the blade tips drew some uid
from the supply tube into the wake, but in order to
achieve clear visualization of the ow it was necessary
to supply external pressure.
Stationary Tank
Initialtestingwasperformedina1.22 2.44 1.68m
× ×
deep stationary water tank at the University of Cali-
forniaBerkeley. Withthemodelxedinplaceatopthe
tank, thesetestssimulatedahoveringhelicopter’sow
eld. A 10-W Argon ion laser provided both planar
and volumetric illumination. For the two-dimensional
lightingtests,averticallightsheetwasalignedwiththe
axis of the rotor. In all cases, a digital video camera
recordedtheowfromthesideofthetank,perpendic-
ular to the wake direction and the light sheet. Due to
thesmallsizeofthetank,testrunswererelativelybrief
(and between-test intervals relatively long) in order to
minimize recirculation e#ects which would lead to an
unwanted climb condition.
Generally, air was used as the injectionuid for ini-
tial experiments because of its non-contaminating na-
ture. Also, in contrast to the neutrally-buoyant dye,
which marked the vortex cores but di#used to areas
surrounding the core as well, the majority of the air
bubbles initially migrated directly to the low pressure
vortex cores. Thus in the near-wake of the rotor, be-
fore the vorticity had di#used signicantly and atten-
uated the pressure decit at the cores, the air bubbles
nicely captured the details of the lament structure.
However, once the transition to the far-wake region
occurred, the buoyancy of the bubbles caused them to
rise to the surface quickly, rendering the details of the
wake indecipherable for distances greater than about
onediameterdownstreamoftherotor. Latertestsused
neutrally-buoyantuorescentdyeastheinjectionuid,
Figure 2: Entire model assembly, showing locations which clearlyshowedthebreak-upand di#usionofthe
ofstraingagesanddyetubing. Flowwouldbefrom wake at greater downstream distances.
left to right in a descent test.
In general, thrust measurements were not recorded
for the stationary tank tests. Rather, these tests were
performed primarily for visualization purposes, as the
quality of the images was signicantly better in the
stationarytankthaninthetowingtankandthestruc-
ture and evolution of the wake could be more clearly
discerned.
4
0.035
0.030
0.025
0.020
nt
cie 0.015
effi 3 rev/s
o 0.010 4 rev/s
C
st 5 rev/s
u
hr 0.005 6 rev/s
T
7 rev/s
0.000
-0.005
Figure3: Side viewofcarriage platform with model -0.010
assemblymountedonturntable. Directionoftravel
-0.015
is from left to right.
-10 -5 0 5 10 15 20 25 30 35
Collective Angle (deg)
Towing Tank Figure 4: Variation in mean thrust coe!cient with
collectivepitchangle. Valuesforarangeofrotation
Thecharacteristicsofadescendinghelicopterweresim-
rates collapse — approximately — to a single curve.
ulatedbypullingthemodelthroughwaterinthe60m
long tow tank at the University of California’s Rich-
mond Field Station. The 2.4 m wide, 1.5 m deep tank
featuresalarge,low-speedcarriagerunningalongaset representative case in order to limit the number of ex-
of rails on top of the tank. The carriage speed, which perimental variables and also because it produced the
for these tests ranged from 0—50 cm/s, could be con- best ow visualization results. In addition, since ro-
trolled manually or by computer. tor thrust is proportional to the square of the rotation
Themodelassembly(Fig. 2)wasmountedona1.22 rate, the non-dimensional thrust, C , should be inde-
T
1.52mplywoodplatform,whichwassuspendedjust pendent of rotation rate (in the absence of viscous or
×
above the water surface by a steel frame connected to aeroelastice#ects),beingafunctionoftherotor/airfoil
the carriage (Fig. 3). The model could be rotated geometry and the blade pitch angle only.
using a turntable on the plywood platform, enabling
This result was tested experimentallyforthe caseof
the descent angle of the rotor to be varied in 0.5 in-
!
ahoveringrotor. Themeanthrustcoe!cientwasmea-
crements, from 0 (forward ight) to 90 (vertical de-
! !
suredforawiderangeofcollectiveanglesandrotation
scent). A set of halogen track lights with blue dichroic
rates in the water tow tank. The expected result is
lterswasmountedontothefrontofthecarriagetoil-
that the individual curves — marking the variation in
luminate the ow and highlight the yellow uorescent
thrustcoe!cientwithcollectiveangle—foreachofthe
dye. Unltered white light was used when air was the
rotation rates tested ("=3—7 rev/s) should collapse to
injectionuid. Forvisualization,thevideocamerawas
a single curve. However, Fig. 4 shows a displacement
mounted on the platform vertically, looking downward
of the ve curves that is a regular function of rotation
at the ow and xed in position with respect to the
rate (except for the highest speed). For the rotation
rotor.
ratestestedhere,bladeReynoldsnumbersrangedfrom
Re =45,000—106,000, a region which is certainly sub-
c
Results ject to viscous e#ects. However, the regularity of the
displacement, regardless of collective angle, is surpris-
Allresultspresentedinthispaper,unlessstatedoth- ing. TheliftvariationseeninFig. 4couldbeaccounted
erwise, refer to experiments conducted at a rotor rota- for by a twist of about 5 , which is certainly possible
!
tion rate of "=4 rev/s (V =319 cm/s) and at a col- given the present blade construction. It is therefore
tip
lective pitch angle of %=11.6 . This was taken as a considered plausible that aeroelastic twist may also be
!
5
passes through it. In this three-dimensional image,
the short-wave “smooth sinuous wave type” instabil-
ity — which was discussed by Leishman (Ref. ?) and
Fukumoto and Miyazaki (Ref. 11) and analyzed the-
oretically by Widnall (Ref. 3) and Gupta and Loewy
(Ref. 4) — can clearly be seen along thelament in the
near-wake of the rotor. Ortega et al. (Ref. 26) have
suggested that this is an elliptic instability of the vor-
texcoresthatdevelopscooperativelyonadjacenthelix
turns, though this observation is unable to be veried
here with air being released from only one blade. In
this case the wavelength of the instability is approxi-
mately 3.75 cm, or 2c.
Figure 6 shows a series of two-dimensional images
of the upper half of the rotor plane with dye being
injected from all three blade tips. This view of the
wake shows cross-sections of the cores of the three tip
vortices and illustrates the inuence that each vortex
lament has on its neighbors. The induced velocities
of adjacent turns cause the helices to expand and con-
Figure 5: Three-dimensional ow visualization im- tract, thus altering their propagation speeds and re-
ageofrotorusingairinjectionfromtipofoneblade.
sultingintheclassic“leapfrogging”or“vortexpairing”
Rotor speed is !=3 rev/s. The short-wave sinuous
phenomenonoftenseenwithparallelvortexrings. This
instability of the vortex is just visible.
e#ect—whichwasstudiedcomputationallybyJainand
Conlisk (Ref. 12) and experimentally by Ortega et al.
occurring. (Ref. 26) — can be seen in the pairing of the second
The stalling of the lift trend at the highest rotor and third vortex cores downstream of the rotor in (b)
speed is also notable. Whereas the lower speed curves and (c), and quickly leads to the complete merger of
show continued increases in C at collective angles as all three vortices in (e). The merger of adjacent tip
T
highas30 ,the"=7rev/scurveplateausatabout20 . vortices is a general phenomenon of rotors regardless
! !
Normally, stall would be expected to be more likely to of the number of blades — for example, Ortega et al.
occur at lower speeds. A possible explanation is that, (Ref.26)observeditinthewakeofatwo-bladedrotor.
at this high speed and collective, the lift is enough to However, with fewer blades the adjacent helix turns
twist the blades and lead to a major separation. are farther apart and thus the leapfrogging and merg-
Nevertheless,thehighthrustsobtainedforallhigher ing processes take longer to transpire. The location
collectives cases (C is over 0.03, with a blade load- wherethethreevorticesmerge—abouthalfadiameter
T
ing coe!cient, C /’ , of over 0.2) are surprising and downstream of the rotor — can be taken as the point
T s
not currently understood. Future studies will include where the wake loses its helical structure.
surface ow visualizations, torque measurements, and
higher sti#ness blades to determine the nature of this Thrust Measurements
behavior.
Instantaneousthrustmeasurementswererecordeddur-
ingthedescentexperimentsinthetowingtank. These
Flow Visualization tests were conducted over a range of towing speeds
Flowvisualizationtestinginthestationarywatertank from 0—50 cm/s, descent angles from $=0—90 , and
!
yielded numerous images clearly showing the develop- collective angles from %=6—18 . Descent runs were
!
ment of the rotor wake and the instabilities that cause typicallyperformedfor100rotorrevolutions,although
it to break down. In Fig. 5, air bubbles are injected somewereconductedforlongerperiodsinordertover-
from the tip of only one blade for the sake of clarity. ify the trends observed during shorter runs. The data
The large starting ring vortex can be seen on the left, sampling rate was 200 samples per revolution, and the
expanding and slowing down as the rest of the wake rst and last ve revolutions of the run were ignored
6
Figure 6: Two-dimensional ow visualization images of upper half of rotor in hover using uorescent dye
injection from all three blade tips. The “leapfrogging” of one vortex lament over another can be seen in
(b) and (c) as the three vortices orbit about each other and nally merge in (e).
(due to the starting transients from the carriage mo-
0.021
tion, and also to the width of the ltering window).
For many of the runs, thrust levels remained rel- 0.019
atively steady over the duration of the experiment.
0.018
Thiswasgenerallythecaseforhover,slowdescent,and
very steep or very shallow descent angle runs. Figure 0.016
7 shows a thrust coe!cient time-history for a hover- nt
ing rotor. The mean thrust coe!cient was 0.0078 and cie 0.015
thepeak-to-peakuctuationamplitudewas12%ofthe effi
o 0.013
mean. Figure 8 shows a thrust coe!cient time-history C
st
ofsimilarform,butforarapiddescentatafairlyshal- hru 0.012
lowdescentangle($=30 ). Notethatthemeanthrust T
!
coe!cient, 0.018, is more than two times greater than 0.010
in the hover case, but the total uctuation amplitude
0.009
is still only 12% of the mean.
However, for experiments featuring a combination 0.008
of moderate descent speed and steep descent angle,
0.006
the rotor thrust characteristics were markedly di#er- 0 10 20 30 40 50 60 70 80 90 100
ent. Insomesuchcases,thethrustexhibitedverylarge, Revolutions
regular uctuations. Figures 9—10, for instance, show
Figure 7: Thrust time-history for a hover test
thrust time-histories typical of this type of behavior.
(V=0). Mean thrust coe!cient is 0.0078 and uc-
TheseplotsexhibitclassicVRScharacteristics,with
tuation is 12% of the mean.
very large, regular thrust oscillations. The sustained
7
0.021 0.021
0.019 0.019
0.018 0.018
0.016 0.016
Coefficient 00..001135 Coefficient 00..001135
Thrust 0.012 Thrust 0.012
0.010 0.010
0.009 0.009
0.008 0.008
0.006 0.006
0 25 50 75 100 125 150 175 200 225 250 0 20 40 60 80 100 120 140 160 180 200
Revolutions Revolutions
Figure8: Thrusttime-history fora fast, shallow de- Figure 10: Thrust time-history for another VRS
scent run (Vx/Vh=1.62, Vz/Vh=-0.93). Mean thrust case with !=50" and V/Vh=1.5 (Vx/Vh=0.96,
coe!cient is 0.0181 and uctuation is 12% of the Vz/Vh=-1.14). Meanthrustcoe!cientis0.0173and
mean. uctuation is 50% of the mean. Fluctuation period
in this case is only about 20 revolutions.
0.021
0.019
regularityofthethrustoscillationswasveriedbyper-
0.018 forming long runs of over 500 blade revolutions (see
Fig. 9). (For most runs, a run of 100 revolutions will
0.016 su!ce.)
nt Otherdescentcongurationsdemonstratedverysim-
e 0.015
effici ilar oscillatory characteristics, but with signicantly
o 0.013 di#erent amplitudes and periods of uctuation. Fig-
C
st ure 10 shows a thrust coe!cient time-history that ap-
hru 0.012 pears very similar to Fig. 9. However, in this case —
T
where the descent angle was 50 and the towing speed
0.010 !
was V/V =1.5 — the thrust oscillations were smaller
h
0.009 in magnitude (peak-to-peak variation was 50% of the
mean) and of shorter period (approximately 20 revo-
0.008
lutions). The mean thrust coe!cient in this case was
0.006 slightly higher though, at 0.0173.
0 50 100 150 200 250 300 350 400 450 500 Obviously the descent conguration greatly inu-
Revolutions
ences the oscillatory behavior of the rotor thrust and
thus can spell the di#erence between a routine bumpy
Figure 9: Thrust time-history for a classic VRS
rideandacatastrophicloss of controlforadescending
case: !=60 , V/V =1.25 (V /V =0.62, V /V =-
" h x h z h
1.08). Lengthofrunis500rotorrevolutionsinstead aircraft. This di#erence can be seen by comparing the
of100,butperformanceparametersareunchanged. thrust envelopes — the region between maximum and
Meanthrustcoe!cientis0.0135,uctuationampli- minimum thrust levels — for two descent angles over
tude is 94% of the mean, and uctuation period is a range of towing speeds. Figure 11 shows the maxi-
about 43 revolutions. mum,minimum,andmeanthrustvaluesfor(a)$=90
!
8
(a) !""="90o descent (b) !""="#0o descent
VRS VRS
Pre-VRS Post-VRS Pre-VRS
Regime Regime
(aperiodic) (aperiodic) (aperiodic)
(aperiodic) (periodic)
Post-VRS
(aperiodic)
Figure 11: Thrust coe!cient envelopes — maximum and minimum thrust levels — for (a) 90 descent and
"
(b) 60 descent congurations.
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and(b)$=60 descentcongurations. Forthevertical tion. In this case it should be noted that the variation
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descent case ($=90 ), the thrust levels dropped pre- inthrust(theexpansionofthethrustenvelope)occurs
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cipitously as the descent speed increased to about 20 at speeds as low as V/V =0.5 — well before the fully-
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cm/s (V/V =1.0). However, there was no noticeable developed VRS region. The thrust variations in this
h
thrust periodicity accompanying the loss of lift as one region are still aperiodic, though quite large. If con-
wouldseewithaclassicvortexringstatecase,andthus trolled ight is sustainable in this region, this could
the envelope is relatively narrow. As the descent rate constitute an operationally useful precursor to the full
increased further, thrust was recovered and eventually vortex ring state. Note that no such precursor is seen
reached two to three times hover thrust levels as the for vertical descent.
rotor encountered turbulent wake state and windmill The mean thrust levels in Fig. 11 are also very re-
brake state. vealing. In the $=60 VRS region, the mean thrust is
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The rotor’s behavior in $=60 descent is similar, half-way between the maximum and minimum values,
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withlossofliftatapproximately20cm/s(V/V =1.0), as one would expect for a highly-organized, periodic
h
followed by recovery and increased lift as the descent oscillation pattern. But in the higher and lower speed
speed reaches windmill brake state. The major di#er- regions the thrust pattern is entirely aperiodic, and
ence between the two cases is in the size of the enve- thisisreectedinthefactthatthemeanthrustvalues
lope. In vertical descent, the thrust levels drop dras- arenolongermidwaybetweenthemaximumandmin-
tically in the 20—40 cm/s descent speed range, yet the imumbutclosertotheminimumvalues. Inthe$=90!
di#erencebetweenmaximumandminimumthrustlev- descent conguration the mean thrusts appear to lie
elsisrelativelysmall(comparedtothemean)andalso half-way between the maximum and minimum curves,
somewhat random. However, in the case of the$=60 but in this case the thrust patterns are aperiodic for
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descent, the rotor experiences only a slight reduction all descent speeds and the mean values happen to lie
in lift once VRS is reached, but also large, regular os- in the middle of a fairly random distribution of thrust
cillations in thrust as well. Thus the thrust envelope values.
for this case is noticeably larger — covering a signi- Figure 12 provides a detailed look at the dynamics
cant proportion of the total thrust. Controllability of ofthevortexringformationandsheddingprocessthat
the helicopter under these conditions is a major ques- precipitates the thrust oscillations discussed above. In
9