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NASA Technical Memorandum _._2_ _°2
An Experimental Investigation ofHelicopter Rotor Hub FairingDrag Characteristics
D. Y. Sung, M. B. Lance, L. A. Young,and R. H. Stroub
September 1989
NASA
https://ntrs.nasa.gov/search.jsp?R=19900002385 2020-07-14T21:52:39+00:00Z
NASA Technical Memorandum 102182
An Experimental Investigation ofHelicopter Rotor Hub FairingDrag CharacteristicsD. Y. Sung, Sterling Software Corporation, Palo Alto, CaliforniaM. B. Lance, Planning Research Corporation, Hampton, VirginiaL. A.Young and R. H. Stroub, Ames Research Center, Moffett Field, California
September 1989
IMASANational Aeronautics and
Space Administration
Ames Research CenterMoffett Field, California 94035
CONTENTS
Page
SYMBOLS .......................................................................................................................................... v
SUMMARY ........................................................................................................................................ 1
I. INTRODUCTION ...................................................................................................................1
. TEST APPARATUS AND DATA ACQUISITION, RELIABILITY,AND PRESENTATION .......................................................................................................... 2
2.1 Test Apparatus................................................................................................................22.2 Data Acquisition and Reduction ..................................................................................... 22.3 Tunnel Corrections ......................................................................................................... 3
2.4 Accuracy of the Internal Strain-Gage Balance ............................................................... 32.5 Data Repeatability .......................................................................................................... 32.6 Reynolds Number Effects .............................................................................................. 3
° TEST CONFIGURATIONS AND TEST SWEEPS ............................................................... 4
3.1 Test Configurations ........................................................................................................ 43.2 Test Sweeps ..................................................................................................................... 4
. RESULTS AND DISCUSSION ............................................................................................. 6
4.1 Data Correlation with the Ames 7- by 10-Foot Wind Tunnel Test Data ........................ 64.2 Interference Effects Between Hub Fairing and Pylon .................................................... 74.3 Hub Fairing Camber and Surface Curvature ................................................................... 94.4 Hub Fairing Thickness Ratio ........................................................................................ 10
4.5 Pylon Height ................................................................................................................. 104.6 Hub Fairing/Pylon Gap ................................................................................................ 11
4.7 Pylon Wake Shields ...................................................................................................... 11
5. CONCLUDING REMARKS ................................................................................................ 12
APPENDIX: Tabulated Data ............................................................................................................13
REFERENCES ................................................................................................................................... 39
TABLES ............................................................................................................................................. 41
FIGURES .......................................................................................................................................... 41
PRECEDING PAGE BLANK NOT FILMED
..°
111
SYMBOLS
Aref
CD
CDI
CD t
CDf
CDf/h
CDfh
CDfp
CDf/p
CDh
CDh/f
CDb/p
CDp
CDp/f
CDph
CDp/h
CL
CpM
CRM
CSF
CyM
D
DQ
g
h
L
Lref
LQ
PM
q
RM
SF
reference area = frontal area of the fuselage, 1.43 ft 2
drag coefficient, D/(q Are f )
total mutual interference drag between fuselage, pylon, and hub fairing
total drag of fuselage+pylon+hub fairing
isolated fuselage drag
fuselage-on-hub interference drag
mutual interference drag of the fuselage+hub fairing
mutual interference drag of the fuselage+pylon
fuselage-on-pylon interference drag
isolated hub fairing drag
hub-on-fuselage interference drag
hub-on-pylon interference drag
isolated pylon drag
pylon-on-fuselage interference drag
mutual interference drag of the pylon+hub fairing
pylon-on-hub interference drag
lift coefficient, L/(q Aref )
pitch moment coefficient, PM/(q Lref Aref )
roll moment coefficient, RM/(q Lre f Are f )
side force coefficient, SF/(q Are f )
yaw moment coefficient, RM/(q Lre f Are f )
drag, lb
equivalent flat plate drag area, D/q (ft 2)
hub/pylon fairing gap width, ft
hub fairing height, bottom of fairing to top of fuselage, ft
lift, lb
reference length, 1.26 ft
lift to dynamic pressure ratio, L/q (ft 2)
pitch moment, ft-lb
dynamic pressure = 1/2 r V.o 2 (psf)
roll moment, ft-lb
side force, lb
PRF.CLDw,_ PAGE
Vi
VOO
YM
a
d axial
d normal
d pitch
d roll
d side
d yaw
r
free-stream velocity, ft/sec
yaw moment, ft-lb
angle of attack, deg
uncertainty m an axial load measurement of the strain gage balance, lb
uncertainty m a normal load measurement of the strain gage balance, lb
uncertainty in a pitch moment measurement of the strain gage balance, in-lb
uncertainty m a roll moment measurement of the swain gage balance, in-lb
uncertainty m a side force measurement of the strain gage balance, lb
uncertainty m a yaw moment measurement of the strain gage balance, in-lb
density, slug/ft 3
vi
SUMMARY
A study was done in the NASA 14- by 22-Foot Wind Tunnel at Langley Research Center on the
parasite drag of different helicopter rotor hub fairings and pylons. Parametric studies of hub-fairing
camber and diameter were conducted. The effect of hub fairing/pylon clearance on hub fairing/pylon
mutual interference drag was examined in detail. Force and moment data are presented in tabular and
graphical forms. The results indicate that hub fairings with a circular-arc upper surface and a flat lower
surface yield maximum hub drag reduction; and clearance between the hub fairing and pylon induces
high mutual-interference drag and diminishes the drag-reduction benefit obtained using a hub fairing
with a flat lower surface. Test data show that symmetrical hub fairings with circular-arc surfaces gener-
ate 77% more interference drag than do cambered hub fairings with flat lower surfaces, at moderate
negative angle of attack.
1. INTRODUCTION
Rotor hub and pylon drag constitute 20-30% of the total parasite drag of single rotor helicopters
(refs. 1-3). Since parasite drag represents 40-50% of the total power requirement of a single rotor heli-
copter (ref. 3), the drag of the rotor hub represents roughly 10% of the total power required. The fuel
savings from the reduction of helicopter rotor hub drag has stimulated many research efforts. Further-
more, current civilian and military requirements call for helicopters with high speed and long range
capabilities, and therefore low drag is an important design criterion.
The idea of using a hub fairing to streamline the rotor hub dates from the late nineteen-fifties.
One of the earliest studies of isolated hub fairing drag with different fairing thickness and camber was
done by Sikorsky Aircraft (ref. 4). Bell Helicopter Company and the Boeing Company Vertol Division
also have conducted some studies of rotor hub fairings (refs. 5-7).
Besides the work of developing low-drag hub fairing and pylon components, researchers are
faced with a major obstacle when solving the hub-drag problem: the additional drag engendered by the
aerodynamic interactions between the hub fairing and pylon, Because this interference drag could
amount to 35% of the total hub and pylon drag (ref. 3), it remains a major barricade to the successful
development of low-drag hub fairings. A research program was initiated at NASA Ames Research
Center to study the aerodynamic interactions and interference drag between hub fairing and pylon. The
goal of the Ames Hub Drag Reduction Research Program is to devise hub fairing designs that can
achieve 50-80% hub/pylon drag reduction.
As part of this program, two small-scale wind tunnel tests were conducted in the NASA Ames
7- by 10-Foot Wind Tunnel to investigate the drag characteristics of new hub fairing and pylon design
concepts (refs. 8-12). A substantial data base was established on the effects of various hub fairing and
pylon aerodynamic attributes on hub drag reduction. However, since additional information was needed
to further this hub drag reduction effort, a third wind tunnel test was conducted in the NASA Langley
14- by 22-Foot Wind Tunnel. The data from this test are presented in this paper.
The main objectives of the Langley 14- by 22-Foot Wind Tunnel test wore to (1) confirm the test
methodology used in the earlier Amos 7- by 10-Foot Wind Tunnel hub drag tests by con'olating data
from independent test setups; (2) identify aerodynamic characteristics of different hub and pylon fairing
designs; and (3) conduct a more extensive study of the effect of hub/pylon clem'anco on hub/pylon
interference drag.
The wind tunnel test used a 1/5-scale model of the XH-59A as the baseline fuselage upon which
various hub and pylon fairing assemblies were mounted. The hub assemblies were nonrotating. Data
acquired were drag, lift, side force, yawing moment, rolling moment, and pitching moment. This report
presents major fmdings of the test. Included are all the aerodynamic load data in tabulated form, and the
graphical presentation of the drag data. Test configurations and data reliability are discussed.
2. TEST APPARATUS AND DATA ACQUISITION, RELIABILITY, AND PRESENTATION
2.1 Test Apparatus
The 1/5-scale XH-59A model was used as the baseline fuselago. The XH-59A is the Advancing
Blade Concept Helicopter devoloped by the Sikorsky Aircraft Division of United Technologies
Corporation. Tho XH-59A model had been tested extensively in the Ames 12-Foot Pressuro Wind
Tunnel and the 7- by 10-Foot Wind Tunnel with both nonrotating (refs. 8,9) and rotating (ref. 13) hard-
ware. Instead of the dual-hub configuration used in the Advancing Blado Concept, a single-hub
configuration was used in this test.
The model installation is shown in figure 1. The hub and shaft fairings were mounted on a
nonrotatingshaft.The modol was sting-mounted,and used an internalstrain-gagobalanco as the load-
measurement unit. The mounting scheme as weLl as the dimonsions of the modol are given in figure 2.
2.2 Data Acquisition and Reduction
Quantitative data in the form of six component forces and moments were obtained by an internal
strain-gage balance. Each data point taken was an average of 40 sample points. All data reduction was
done using a MODCOMP Classic computer where weight tare corrections and the balance-axis-to-wind-
axis transformation were applied to the raw data.
Data were acquired fora range of dynamic pressuresand model pitchanglosatzero yaw anglo.
Dynamic pressuresweeps were conducted atO" anglo of attack,with dynamic pressurevarying from
40 psf to 120 psf in increments of 20 psf. Most of the runs were pitchangle sweeps testedata dynamic
pressureof 80 psf with the anglo of attackvarying from -10"to2" inincrements of 2". The refcrencc
coordinatesystem used in datareductionisshown in figure3.
The force and moment data are prosented in coefficient form. The frontal aroa of the fuselage,
Are f = 1.43 ft 2, was used as the referonce area to normalize the force data. The height of the fuselage
cross section, 1.26 ft, was used as the reference length, l-,ref.
2
2.3 Tunnel Corrections
Corrections of solid blockage and wake blockage effects were deemed unnecessary because the
model blockage was less than 1% of the test section area. The model was unpowered and the span of the
only lifting surface, the hub fairing, was less than 14% of the tunnel span; therefore, wall interference
effects and jet boundary corrections were not applied to the data. Other effects, such as tunnel buoy-
ancy, were also negligible. The internal mounting scheme obviated the correction for sting tares, and no
attempt was made to account for the sting-on-fuselage interference drag.
2.4 Accuracy of the Internal Strain-Gage Balance
The accuracies of the force and moment measurements from the internal balance were within
0.5% of the corresponding maximum load of each measurement component. The resolution of the
balance in engineering units is given below:
8_ = :i:0.375 lb
8no_ = +8.0001b
8 side = + 2.500 lb
8p_h = :t: 1.25 ft-lb
8 roll = + 0.625 ft-lb
_i yaw = + 0.625 ft-lb
The drag, lift, side force, pitch moment, roll moment, and yaw moment of the aerodynamic loads
were measured by the axial, normal, side, pitch, roll, and yaw gages of the balance, respectively. At a
dynamic pressure of 80 psf, the drag level of the low-drag test configuration (H50,$40) was about 14 lb;
therefore, at most a 3% uncertainty in measured drag could be ascribed to inaccuracy of the balance.
2.5 Data Repeatability
In order to study the repeatability of the test data, the H50,$40 configuration (see section 3.1)
was tested on two different days with many configuration changes. Figure 4 displays the drag data (in
engineering units) of the two repeated runs. The repeated data fell within an acceptable range of
uncertainty dictated by the resolution of the internal balance.
2.6 Reynolds Number Effects
Reynolds number effect or scale effect on the drag measurement of the H50,$40 test configura-
tion (see section 3 for definitions of test configurations) is shown in figure 5. Problems relating to the
Reynolds number effects on the drag measurements of helicopter models (high-drag bodies) have been
well noted in the past ( refs. 14,16). However, the test data reported here exhibit only a very slight
dependence on the Reynolds number based on the height of the fuselage; that is, within the range of
testedReynoldsnumbers,nonoticeabletransitional effect has been observed. Most of the data pre-
sented were taken at a Reynolds number of 1.5 x 106/ft.
3. TEST CONFIGURATIONS AND TEST SWEEPS
3.1 Test Configurations
All test configurations were given a two-digit designation for easy reference. The test matrix
given in table I shows how the test was structured, and it can be used as a quick reference to the test
configurations. Test configurations were defined based on a combination of geometric parameters,
shown in figure 6.
1. Hub falrings. Basically, a hub fairing is designed to reduce the aerodynamic drag of the rotor
hub in forward flight. All of the hub fairings tested have a circular planform. The profiles of the hub
fairing cross sections are of key interest. Hub fairing profiles with different camber, diameter, and
thickness ratio were studied, and were identified with a two- or three-digit designation with an H prefix.
The following hub fairings were tested: H10, I-D.0, H30, H40, H50, H60, H220, H230, H240, H250,
H260, H270, and H280. See figures 7-10 for the profdes of these hub fairings.
2. Pylons (shaft fairings). The use of a pylon, or shaft fairing, is similar to that of the hub fairing
except that the pylon is used to fair the rotor shaft. Pylons were identified with a two-digit designation
with an S prefix. Two configurations were tested: $40 and $80. The cross sections of these shaft
fairings are shown in figure 11.
3. Hub/pylon gap width. The gap (orclearance)between the hub fairingand thepylon has been
observed to hamper theeffectivenessof thehub fairingas a drag-reductiondevice (refs.8,9).Therefore,
the effectof hub/pylon gap on drag was studiedcloselyinthistest.Except as noted,thehub/pylon gap
iszero forallconfigurations.
4. Pylon height. Pylon height is one of the parameters that influences the overall hub/pylon drag,
and it is defined as the length of the rotor shaft between the top of the fuselage and the bottom of the hub
fairing (fig. 6). Pylon heights of 0.1667 ft, 0.3333 ft, 0.5 ft, and 0.5833 ft were tested with the H50,$40
configuration. All other configurations were tested with a 0.5833-ft pylon height.
5. Wake shield.A wake shieldisa streamlinedsurfaceplaced on the top of thepylon,and itis
designed mainly to minimize theinterferencedrag incurredfrom hub fairing/pylonclearance.Itisalso
referredto inthisreportas a pylon end plate.A more detaileddiscussionon the wake shieldispresented
inthe next section.Two differentwake shields,designatedWI and W2, were tested.
3.2 Test Sweeps
The test was organized into six test sweeps. This section outlines the purpose of each test sweep
and which parameters were varied.
4
1. Hub fairing camber with constant hub fairing diameter and thickness. This set of hub fairings
(H20, H30, H40, H50, and H60) was used to investigate the effect of hub fairing camber on the
hub/pylon mutual interference drag (fig. 8). Since the main objective of this test sweep was to study the
hub/pylon mutual interference drag, the set of hub fairings was designed to have comparable levels of
skin-friction drag and profile drag. This was accomplished by using hub falrings with the same amount
of wetted surface area, thickness ratio, and thickness distribution.
2. Hub fairingthicknessratiowith constanthub fairingdiameter. This setof hub fairings(HS0,
H220, and H230) was used toinvestigatethe profde drag of hub fairingswith a circular-arcupper sur-
faceand flatlower surfacecrosssection(fig.9). The purpose of thissweep was todetermine the effects
of hub fairingthicknessratioon drag by varying thefrontalareaof the fairingwhile keeping the diame-
terconstant. A hub fairingwith a higherthicknessratioentailslessweight penaltybecause of a more
efficientuse of fairingvolume. Because the weight of each hub fairingisan importantfactorinevalu-
atingthe am'activenessof a particulardesign,itisessentialtomeasure the overalleffectsof hub fairing
thicknesson drag.
3. Hub fairingthicknessratiowith constanthub fairingthickness.This testsweep was devised to
study the trade-offbetween profde drag and skin-frictiondrag with respecttothe change inthe hub
fairingthicknessratio.This change in thicknessratio(hub fairingI-IS0,H240, H250, H260, H270 and
H280; see fig.10) was done by increasingthe fairingdiameterwhile keeping thethicknessconstant.
The crosssectionsof thesefairingshave the same geometric attributes:circular-arcupper surfaceand
flatlower surface.The reason forkeeping the hub fairingthicknessconstantisthatallthe fairingswill
house thesame rotorhub. The circular-arcupper surfaceand flatlower surfacefairingcross sectionwas
chosen because ithad been shown tobe a low-drag configurationin previouswind tunneltests
(refs. 8,9).
4. Pylon height. The scope of this sweep was to measure the pylon+hub fairing drag as a function
of pylon height. The potential flow interaction between the hub and fuselage depends directly on the
height of the shaft fairing. It is thus desirable to determine the drag trend with respect to the pylon
height. In this sweep, the pylon height was varied from 0.1667 ft to 0.5833 ft. The H50,$40 configura-tion was used.
5. Hub/pylon gap width. In this sweep, attention was focused on the small gap between the hub and
pylon. Previous wind tunnel tests indicated that the hub/pylon gap induces a high shaft-on-pylon inter-
ference drag. The objective of this test sweep was to measure how this extra drag penalty impacts the
drag reduction when using a hub fairing. Both high-drag and low-drag hub fairing designs, H10 and
H50, respectively, were selected for this study (fig. 7).
6. Wake shield (or pylon end plate) concept. The purpose of this sweep was to study the potential
drag reduction from using wake shields. See figure 12 for drawings of the wake shields tested. The
wake shield is attached to the top of the pylon such that the sharp edge of the shield aligns to the free
stream.
The clearance between the hub fairing and pylon had been observed to cause high interference drag.
The wake shield is a design concept which attempts to minimize the impact of the gap on drag. If the
hub/pylon gap cannot be eliminated, the exposed part of the rotor shaft between the hub fairing and the
pylon produces a turbulent wake. At a negative fuselage pitch angle, we may postulate that this turbu-
lent wake, with wake entrainment, may convect downstream and intermix with the boundary layer of the
flow over the aft portion of the pylon. Such interaction may disturb the pressure distribution of the flow
over the pylon and cause flow separation. In order to avoid the flow separation induced by the wake,
and hence the resultant drag penalty, the wake shield, a wide edge plate, is installed atop of the pylon to
delay or prevent the turbulent wake from interacting with the flow over the pylon.
4. RESULTS AND DISCUSSION
The data are organized by run number. A cross reference between run number and configuration
definitions is provided in tables 11 and HI. The force and moment data are tabulated in the appendix.
Much of the hub-drag data presented here was obtained in order to confirm the methodology
used in previous hub drag tests, and therefore most of the configurations had been tested before
(refs. 8-12). However, this study went further than previous studies in the following areas:
1. The hub/pylon gap effect on the interference drag was examined more closely in the area in
which a large change in drag had been observed previously.
2. The effect of pylon height on drag was studied in detail.
3. More elaborate wake shield designs were tested.
For completeness, discussions of all of the test results are included in the following sections.
Some of the observations reiterate important findings published previously; the reader should refer to
references 8-12 for the original work on hub fairing development.
4.1 Data Correlation with the Ames 7- by 10-Foot Wind Tunnel Test Data
This section presents correlations between the Langley 14- by 22-Foot Wind Tunnel test data and
the Ames 7- by 10-Foot Wind Tunnel test data (ref. 11). Care was taken to ensure proper matching of
the model hardware, installation, and experimental configurations between these tests.
Apart from being conducted at different wind tunnel facilities, the two tests differed in one major
respect: the 14- by 22-Foot Wind Tunnel test used an internal swain-gage balance, whereas the 7- by10-Foot Wind Tunnel test used the wind tunnel external scales. By correlating the test data, one can
determine whether the following factors introduced unacceptable distortion into the 7- by 10-Foot Wind
Tunnel test data:
1. Sting tare correction. In the 7- by 10-Foot Wind Tunnel test, the sting tare amounted to
approximately 68% of the total drag measured for most of the test configurations. In addition, the
model-on-sting interference effects were not accounted for in the sting tare correction.
6
2. Wind tunnel blockage effects. Model blockage correction was assumed to be negligible in the 7-
by 10-Foot Wind Tunnel test. The model blockage was 3.5% in the 7- by 10-Foot Wind Tunnel test. It
was reduced to 0.8% in the 14- by 22-Foot Wind Tunnel test.
Data correlation of four important test sweeps in the 14- by 22-Foot Wind Tunnel test and the
7- by 10-Foot Wind Tunnel test are presented in figures 13-16. From these comparisons, it can be con-
cluded that the model-on-sting interference and the model blockage had only minor impacts on the drag
data obtained in the previous 7- by lO-Foot Wind Tunnel tests.
4.2 Interference Effects Between Hub Fairing and Pylon
Data from different wind tunnel tests (ref. 3) clearly indicate that interference drag has encum-
bered the development of low-drag hub fairings. Mutual interference drag is engendered from the aero-
dynamic interactions between solid bodies when they are placed in proximity in the free stream. These
aerodynamic interactions are potential flow interaction, vortex interaction, and three-dimensional
boundary-layer interaction. Because of the interdependent nature of these interactions, a detailed and
accurate account of interference drag is impractical to obtain experimentally. It is important, however,
that the interference drag be conceptually understood. To aid in our analytical study of the drag data, the
following notations will be used in the discussion.
In general,
CD x =
CDx/y =
CDy/x =
CDxy =
X-body component or isolated drag
X-body-on-Y-body interference drag
Y-body-on-X-body interference drag
mutual interferencedrag between X body+Y body
CDx/y+ CDy/x
and specifically,
CDI = CDf-p + CDph+ CDfh
= CDf/p + CDp/f + CDp/h+ CDh/p + CDf/h + CDh/f
= CD t-CDf-cDp -CD h
where
CD I = totalmutual interferencedrag between fuselage,pylon,and hub fairing.
7
Three types of drag data were measured: CDf, CDf + CDp + CDfp, and CDt. I Without the dragdata of each isolated component, it is not possible to quantify the individual interference-drag compo-
nents. However, with careful reasoning, valuable insight can be obtained from these data.
Interference drag can be examined for a case in which the interference effects were readily ap-
parent. In figure 17, the drag as a function of the pitch angle is displayed for the following configura-
tions: fuselage alone, fuselage + $40 pylon, and fuselage + $40 pylon + H50 hub fairing. From this plot
we can observe the drag buildup from adding the pylon and then the hub fairing to the fuselage; that is,
we are looking at the magnitudes of CDf, CDf + CDp + CDfp and CDf + CDp + CDh+ CD_ + CDph +
CD m. It may be noticed immediately that CDph + CD h + CD m assumed a negative value at negative
pitch angles. In other words, the drag reduction caused by the aerodynamic interactions between the
H50 fairing and $80 pylon exceeded the additional component drag of the hub fairing. We should also
note that the negative magnitude of CDph + CD h + CDth diminished as the angle of attack increased, and
CDph + CD h became positive at ¢x= 0".
An explanation of the negative CDph is given as follows. In the case of the fuselage + $40 pylon
configuration, when the pylon is set at a negative pitch angle, the flow over the top surface of the pylon,
under a favorable (negative) pressure gradient, tends go around the comer to the side surfaces of the
pylon in an effort to align with the free stream. At the right-angle comer of the $40 pylon, a local flow
separation, a separation bubble, is formed because of the failure of the flow to negotiate the abrupt
change in pressure gradient at the comer. Figure 18 is a depiction of this scenario. However, with the
I-IS0 hub fairing placed atop the pylon, covering most of its upper surface, the flow is redirected over the
upper surface of the fairing, and the area of separation created at the corner of the pylon is thus preven-
ted. Based on the above reasoning, it can further be asserted that the drag reduction caused by the
elimination of local flow separation at the side surfaces of the pylon is higher than the skin friction and
profile drag of the 1-150 hub fairing.
At a positive angle of attack and in the absence of the hub fairing, a local flow separation is
likely to occur on the top surface of the pylon. However, the area of the top surface tangential to the free
stream is much less than that of the side surfaces at small incidence angles. Consequently the drag
penalty caused by the comer flow separation is small, at a = 0-2". On the other side, the induced drag of
the cambered hub fairing, a lifting surface, becomes a contributing entity at a positive incident angle.
Therefore, the negative CDr_ decreases and CD h increases as the pitch angle becomes more positive. Itfollows that the drag reduction caused by the favorable interaction between the hub fairing and pylon
diminishes at positive angles of attack.
The data thus demonstrate that the aerodynamic interactions between the hub fairing and pylon
have a major impact on the effectiveness of different hub fairing designs.
1 Because of the simultaneous interactions between all of the aerodynamic bodies placed in lxoximity, the interference drag
between any two of the components is influenced by the presence of the other components. Likewise, the mutual interference
drag between the pylon and fuselage, CDfp, is influenced by the presence of the hub fairing. In this report, it is assumed that
the change in CDfp due to the presence of the hub fairing is negligible. CDfp is treated as a constant in the discussion.
4.3 Hub Fairing Camber and Surface Curvature
Previous testsindicatedthata hub fairingwithpositivecamber can yieldsubstantialhub drag
reductioninforward flight(refs.8,12).The presenteffortwas a parametricstudy of the effectof hub
fairingcamber on drag. The hub fairingswere designed tohave the same diameter,thicknessratio,and
thicknessdistribution(fig.8). The wetted surfaceareasof thesehub fairingarc alsovery similar.
Therefore,the skin-frictiondrag and theprofiledrag of thisseriesof fairingsarccomparable.
Figure 19 displays the drag as a function of the hub fairing camber. The dam suggest that a hub
fairingwith a 9% camber yieldsminimum drag. There was aprecipitousdrop in drag when thecamber
was at9% with both the $40 and the $80 pylons. Subtractingthefuselagedrag,the9%-cambered HS0
fairingshows a drag reduction,when compared with the0%-cambered H20 fairing,of 85% with the $80
pylon and 74% with the $40 pylon. Ifone ascribesthe substantialdrag reductionto thecamber of the
hub fairing,then one isobligedto fredout how the camber causes such drag reduction,and why the drag
reaches a minimum attheparticularcamber of 9%. Aftera carefulreview of the data,an alternative
cause-effectrelationcan bc seen.
First,letus focus our attentionon the lower aftsurfaceofthe hub fairing.Because therotorshaft
iscylindricalin crosssection,itislikelythatflow separationoccurson thelower aftsurfaceof thehub
fairingjustbehind the shaft.Because thelocalflow separationhas a strongeradverse pressuregradient
(caused by positivesurfacegradient),itismore extensiveon the H20, H30, and H40 fairingsthan on the
1-150fairing.Moreover, forthose hub falringswith positivelower surfacecurvature,theflow over the
lower surfacewillbc restrictedeven more when thehub fairingisplaced tightlyon the top surfaceof the
pylon. Such a flow conditionescalatesflow separationand produces higherpressuredrag. Therefore,
hub fairingswith a positivelower surfacecurvature,with more areatangentialto thefreestream,have
higherpylon-on-hub-fairinginterferencedrag,CDp/h,thanfairingswith flatlower surfaceshave.
Secondly, hub fairings with a flat lower surface have favorable interactions with the pylon (see
section 4.1). In contrast, for the H20, H30, and H40 fairings, the corner flow separation is aggravated by
the acceleratingflow atthe leadingedge of the fairingbecause of thepositivelower surfacecurvature.
Concomitantly, the reductionsin both CDh/p and CDp/h reduce thedrag of the I-LS0fairingtoa much
lower levelthan thatof theH20-40 seriesfairings.
The above reasoningoffersa plausibleexplanationforthe substantialdrag reductionachieved by
using the I-LS0hub fairing.The drag reductioncan bc ascribedto theeffectof the lower surfacecurva-
ture.Itisreasonabletodismiss the9% camber as the primary cause of thedrag reduction.
We now turnour attentiontowhy thedrag-reductionbenefitwas more pronounced when the
$80 pylon was used. Ifwe considertheflow over the $80 profile,we can see thatthereisa strong
adverse pressuregradient(aconsequence of high surfacegradient)on the pressure-recoveryregion
between 0.2 chord lengthand 0.7 chord length.That is,the boundary layerformed over the $80 pylon is
lessstablethan thatover the$40 pylon,which has a more moderate surfacegradientdistribution.Thc
boundary layeron the $80 pylon ismore prone toflow separationand thusismore sensitiveto interfer-
ence effects.Itfollows thateven small interferenceeffectsbetween the hub fairingand pylon may trig-
gcr flow separationof the unstableboundary layerover the $80 pylon. However, when interactionswith
9
the pylon are favorable, as when fairings with a fiat lower surface are used, the boundary layer of the
$80 pylon remains attached, and substantial drag reduction is observed.
4.4 Hub Fairing Thickness Ratio
Hub fairings with circular-arc upper surfaces and flat lower surfaces were used to study the wade-
off between profile drag and skin-friction drag with respect to the change in the hub fairing thickness
ratio. Figure 20 shows the drag trend with respect to the hub fairing thickness ratio at two different pitch
angles. In this case the change in hub fairing thickness ratio was accomplished by increasing the fairing
diameter while keeping the fairing thickness constant. Note that the H280 fairing, with more wetted
surface, represents high skin-friction drag; while the H240 fairing, with a larger thickness ratio, repre-
sents high profile drag.
At moderate angles of attack, from 3" to -Y, the data indicate that profile drag and skin friction
drag were nearly even. The overall drag reached a minimum at a thickness ratio of 20%. At a more
negative angle of attack, about -6", the profile drag caused a higher penalty than did the skin-friction
drag. That is, the drag increase was substantial with a thickness ratio of 25% or more. It may also be
concluded that a hub fairing with a 20% thickness ratio is less sensitive to change in pitch angle, and
yieldsminimum drag.
An additional study examined the profile drags of the same type of hub fairings with the same
diameter but different thicknesses (see fig. 9). The data, plotted in figure 21, clearly indicate that the
drag increasedwith the thicknessratio within therange studied.
4.5 Pylon Height
The presence of the pylon and the fuselage can alter both the magnitude and direction of the local
velocity in the hub region. Consequently, part of the interference drag can be attributed to the potential
flow interaction between the fuselage and the hub fairing. One of the important parameters influencing
the potential flow interaction is pylon height. This test sweep was done to determine the pylon+hub
fairing drag as a function of pylon height.
Before the data are examined, two important principles of potential flow theory should be noted.
First, there is a local increase in dynamic pressure on the top surface of the fuselage. Second, the direc-
tion of the flow near a surface tends to align with that surface. It is also clear that if the potential flow
interactions between the fuselage and the pylon/hub fairing were negligible, then the drag increase with
respect to pylon height would be linear. Figure 22 shows the drag versus pylon height for the H50,$40
configuration at two different angles of attack. The data show a nonlinear trend in drag with respect to
the pylon height. This nonlinearity in drag gives credence to the assertion that hub fairing/pylon/
fuselage potential flow interactions had an appreciable influence on the interference drag.
Moreover, the data indicate that there was a favorable effect when the hub fairing was placed
close to the fuselage. There are two counteractive factors involved, namely the increase in local velocity
and the decrease in angle of attack. The dynamic pressure near the surface of the fuselage is higher than
that of the free stream. This causes a higher hub fairing component drag. However, because the flow
close to the fuselage tends to align with the surface, the hub fairing component drag is lower because of
10
thereductionin angle of attack.In thiscase,the datasuggestthattheeffectof reductionin angle of
attackhad a greaterimpact on the drag than did theincreaseinlocaldynamic pressure.
A more direct approach to the study of the hub fairing/pylon/fuselage potential flow interactions
is to use potential flow codes to calculate the flow field around the fuselage. With potential flow calcu-
lation, the changes in magnitude and direction of the local velocity near the fuselage can be accounted
for quantitatively. It should also be noted that the interference drag caused by potential flow interactions
depends strongly on the actual hub fairing/pylon/fuselage configuration.
4.6 Hub Fairing/Pylon Gap
The detrimentaleffectsof the hub fairing/pylongap on thedrag reductionachieved by using the
low-drag hub fairing(H50) were observed in previouswind tunneltests.In thisstudy,the gap effects
were examined more closely,with specialattentiontoa small fairing/pylongap in which a steepascent
indrag was observed (ref.1I).
Figure 23 shows thedrag as a functionof the hub fairing/pylongap forthe H50-$40 and HI0-
$40 configurations.Note thatthepylon heightwas kept constant.In the caseof the low-drag hub
fairing(H50), thedata show thatthe drag risewas precipitouswhen the first0.5-in.of hub fairing/pylon
gap was introduced.For the symmetrical hub fairing(HI0),the drag was virtuallyunchanged with a
gap of less than 0.08 ft.
For a given shaftlength,the shaftproduces substantiallyhigherdrag than thepylon. Therefore,
the drag increasecaused by theincreaseinthe exposed shaftin the gap was anticipated.Ifthe interfer-
ence effectsareonly a minor factor,the datashould have reflecteda more lineardrag trend.
The following reasoning is offered to account for the nonlinearity of the drag trend. The exposed
shaft in the gap creates a turbulent wake, which in turn induces extensive boundary-layer separation in
the flow over both the lower aft portion of the hub fairing and the upper aft portion of the pylon. The
resulting drag penalty contributes much to the sharp rise in CD_. Moreover, in the presence of a gap,
the fiat-lower-surface hub fairing was no longer able to eliminate the comer flow separation (refer to
section 4.1), and this further diminished the favorable interactions between the HS0 fairing and pylon.
The drag penalty of the hub/pylon gap on the H10 configuration was less significant because the
hub/pylon interference drag was already high. The drag actually dropped slightly when the first 0.25-in.
of gap was introduced. The reason for this is that the flow on the lower surface of the H10 fairing was
less restricted when a small gap was present. That is, the gap allowed the flow more room to turnaround the shaft and thus alleviated some of the flow separation at the low aft surface of the H10 fairing.
As the gap increased, the high-pressure drag produced by the rotor shaft became dominant.
4.7 Pylon Wake Shields
The reasoning behind the wake shield design concept is summarized in section 3.2. Two wake
shield designs (fig. 12) were studied with different pylon/hub fairing configurations. The impacts on
drag of the application of these wake shields can be seen in figures 24 and 25. The data indicate that this
design concept failed to meet its objectives. Instead of drag reduction, appreciable drag penalty was
11
observed. This means either that the wake produced by the shaft had only minor impact on the flow
over the pylon, or that the extra profile drag of the wake shield exceeded any drag savings.
5. CONCLUDING REMARKS
The principal test results are summarized below:
1. The Langley 14- by 22-Foot Wind Tunnel test results agree with those of the previous Ames 7-
by 10-Foot Wind Tunnel hub drag tests. Good correlation between the different test data was observed,
substantiating the test methodology in both test programs.
2. Aerodynamic interactions between the fairing and pylon are the fundamental factors that deter-
mine the drag level of seemingly similar hub fairing designs. The hub/pylon mutual interference drag
contributes significantly to the overall drag reduction. Therefore, hub fairing design should be coupled
with the pylon design in order to achieve optimal results.
3. Hub fairings with a circular-arc upper surface and a flat lower surface yield maximum hub dragreduction.
4. The symmetrical hub fairing with circular-arc surfaces (H20) generates 74% more interference
drag than the cambered hub fairing with fiat lower surface (H50) at moderate angles of attack, 2" to -4"
5. A gap between the hub fairing and pylon induces high mutual interference drag and diminishes
the drag-reduction benefit obtained by using a hub fairing with a fiat lower surface.
12
APPENDIX: TABULATED DATA a
Hub Drag Reduction Test Data Summary
Langley 14- by 22.Foot Wind Tunnel
June 8, 1988
a Note: Some runs were used for weight taring; the data from these runs are not included in this data set.
13
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38
REFERENCES
1. Sheehy, Thomas W.; and Clark, David R.: A General Review of Helicopter Rotor Hub Drag Data.
31 st Annual National Forum of the American, May 1975.
2. Sheehy, Thomas W.: A Method for Predicting Helicopter Hub Drag. Army Air Mobility Research
and Development Laboratory, AD-A021 201, Jan. 1976.
3. Keys, C. N. and Rosenstein, H. J.: Summary of Rotor Hub Drag Data. NASA CR-152080, 1978.
4. Wilson, John C.: Wind Tunnel Tests of Models of Rotor Head Fairing. UCA Research Dept.,
Report UAR-0495, Aug. 7, 1959.
5. Foster, R. D.: Results of the 1/2 Scale HU-1 and High Speed Helicopter Pylon and Hub Model
Wind Tunnel Investigation. Bell Helicopter Company, Report 8025-099-012, March 1961.
6. Smoke Tunnel Test of Faired Rotor Hubs on the 1/8-Scale HC-1B Model. The Boeing Company
Vertol Division, Report R-359, Sept. 1963.
7. Results of Rotor Hub Fairing Analytical Study. The Boeing Company Vertol Division, Relxrrt
R-356, Sept. 1964.
8. Young, Larry A.; and Graham, David R.: Experimental Investigation of Rotorcraft Hub and Shaft
Fairing Drag. AIAA 4th Applied Aerodynamics Conference, June 1986.
9. Young, Larry A.; Graham, David R.; Stroub, Robert H.; and Louie, Alexander W.: Reduction of
Hub- and Pylon-Fairing Drag. 43 rd Annual Forum and Technology Display of the American
Helicopter Society, May 1987.
10. Stroub, Robert H.; Young, Larry A.; Graham, David R.; and Louie, Alexander W.: Investigation of
Generic Hub Fairing and Pylon Shapes to Reduce Hub Drag. 13th European Rotoreraft Forum,
Sept. 1987.
11. Graham, D. R.; Sung, D. Y.; Young, L. A.; Louie, A. W.; and Stroub, R. H.: An Experimental
Study of Helicopter Hub Fairing and Pylon Interference Drag. NASA TM-101052, Jan. 1989.
12. Louie, A. W.: Evaluation of VSAERO in Prediction of Aerodynamic Characteristics of Helicopter
Hub Fairings. NASA TM-101048, Feb. 1989.
13. Felker, Fort F.: An Experimental Investigation of Hub Drag on the XH-59A. AIAA 3rd Applied
Aerodynamics Conference, Oct. 1985.
14. Montana, Peter S.: Experimental Evaluation of Analytically Shaped Helicopter Rotor Hub-Pylon
Configurations using the Hub Pylon Evaluation Rig. David W. Taylor Naval Ship Research And
Development Center, Report ASED-355, July 1976.
39
15. Logan,A. H.; Prouty, R. W.; and Clark, D. R.: Wind Tunnel Tests of Large- and Small-scale Rotor
Hubs and Pylons. USAAVRADCOM TR 80-D-21, April 1981.
16. Montana, Peter S.: Helicopter Drag Technology Program Fiscal 1973 Progress Report. Naval Ship
Research and Development Center, Sept. 1983.
4O
Table I. - TEST MATRIX SHOWING PYLON AND HUB FAIRING COMBINATIONS
c INDICATESCONFIGURATIONNUMBER
PYLON
$40
$40 (0.25-in. gap)
$40 (0.50-In. gap)
S40 (0.75-1n. gap)
S40 (1-in. gap)
$40 (3-in. gap)
SS0
S40 (6 in. high)
S40 (4 in. high)
I HUB FAIRING
H20 H30 H40 H50 : H60 H220 IH230
i C21; oc
C22; .c
C23; .c
$40 (2 in. high)
S40 + Wl (1.In. gap) C40; .f
S40 + W2 (l-in. gap) C41; ,f
S40 + Wl C4.4; .f
S40 + W2 C45; .li
Notes:
•W- wake shield
C12; .b C13; ob iC14; ,b
, i
C06; .a,b,c,d C15; .b C34; od C35; .diC28; .c
C29; .c
C24; .c C30;
C25; .c C31;
C26; .c C32;
C16;-biC17;.b C18;.b C19;
C36;
C37;
C38;
C42;
.c
.c
,c
• b C20; .b
.e
°e
.e
C43; .f
C46; .f
I C47; .f
•a- hub fairing diameter sweep (constant thickness)•b- hub fairing camber sweep (constant diameter and thickness)•c- hub/pylon gap width (constant height between hub fairing and fuselage)
•d- hub fairing thickness sweep (constant diameter)
•e- pylon height sweep•f- wake shield sweep
]H240 [ H250 ! H260 H270 I H280
C07;.a !C08; .a i C09; .a I ClO; .a i C11;.a
i
i I
i I
! I
J
I i
i
i
Figure 1. - Model installation in the NASA Langley 14- by 22-Foot Wind Tunnel.
41ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
FORCE AND MOMENTREFERENCE POINT
SECTION A-A
J
13.75 in.
-F15 in.
REFERENCE AREA DIMENSIONS
STING
INTERNAL STRAIN GAGE BALANCE
Figure 2. - Model dimensions and mounting scheme.
YAWMOMENT
LIFT, PITCH MOMENT _ ROLL
_-- __ S,OE/ .OMENT/y /FORCE / DRAG / / /
FORCE AND "__ -_ "r'_ /- /MOMENT ,/"'-- -t
I
Figure 3. - Reference coordinate system.
42
20 ¸
dfr"Q
15
10
II
D RUN 109• RUN 309
0 | I I I | I I J-12 -8 -4 0 4
MODEL PITCH ANGLE, dig
Figure 4. - Data repeatability, H50-$40 configuration.
.14
OC_.10
[3- -- --'EI---D-.-B-.C]
.06 I I1E+06 2E+06 " 3E+06
REYNOLDS NUMBER
Figure 5.- Reynolds number effects.
43
HUB/PYLONGAP WIDTH
FUSELAGE
HUB FAIRING
1
Figure 6. - Test configuration.
DIAMETER, ft
THICKNESS RATIO (FRACTION OF DIAMETER)
CAMBER (FRACTION OF DIAMETER)
LOWER SURFACE SECOND DERIVATIVE AT CENTER
UPPER SURFACE SECOND DERIVATIVE AT CENTER
H10 H50
0.92
0.24
0.00
- 0.45
0.45
I 1.25I
0.18
! 0.o9_ 0.00
- 0.62
Figure 7. - Hub fairing cross sections: H10 and HS0 hub fairing.
44
H20
DIAMETER, ft
THICKNESS RATIO (FRACTION OF DIAMETER)
CAMBER (FRACTION OF DIAMETER)
UPPER SURFACE SECOND DERIVATIVE AT CENTER
LOWER SURFACE SECOND DERIVATIVE AT CENTER
J H20 I 1"130 1"140
1.25 1.25 1.25
0.18 0.18 0.18
0.00 0.03 0.06
- 0.34 - 0.44 - 0.54
0.34 0.23 0.12
HS0 H60
1.25 1.25
0.18 0.18
0.0g 0.11
- 0.62 - 0.60
0.00 - 1.00
Figure 8. - Hub fairing cross sections: variation of camber.
DIAMETER, ff
THICKNESS RATIO (FRACTION OF DIAMETER)
CAMBER (FRACTION OF DIAMETER)
UPPER SURFACE SECOND DERIVATIVE AT CENTER
H50
1.250.18
0.09
-0.62
H220 i H230
1.25 1.25
0.24 0.29
0.12 0.14
- 0.78 - 0.87i
Figure 9. - Hub fairing cross sections: variation of thickness ratio (constant diameter).
45
_'_ H280
DIAMETER, It
THICKNESS RATIO
(FRACTION OF DIAMETER)
CAMBER
(FRACTION OF DIAMETER)
UPPER SURFACE SECOND
DERIVATIVE AT CENTER
1126O
1.67
0.13
0.07
.0.49
H270
1.46
0.15
O.O8
- O.SS
1t50
1.25
0.18
0.00
-0.62
1"1260
O.92
O.24
0.12
- 0.78
H250
0.74
O.29
0.15
-0.87
H240
0.66
O.33
0.17
-0.92
Figure 10. - Hub fairing cross sections: variations of thickness ratio (constant thickness).
CHORD LENGTH, ft
LOCATION OF MAXIMUM THICKNESS
(FRACTION OF CHORD LENGTH)
TRAILING EDGE SLOPE
THICKNESS RATIO
$40
1.58
0.30
- 1.17
0.34i
$80
1.58
0.20
- 1.17
0.34
Figure 11. - Shaft fairing cross sections: S40 and $80 pylons.
46
SECTION A-A T• _ 2.375-in. DIAM
a) Wl WAKE SHIELD
_L.1i..T.<_'/////_[_I .Tsin.
SECTION A-A T
A2.375-in. DIAM
19.0 in.
b) W2 WAKE SHIELD
Figure 12. - Top views of the wake shields testexi.
4?
fJ
.25
,20
.15
.10
-.02
---'',/r'l 14- BY 22-FOOT WIND TUNNEL TEST DATAO 7- BY 10-FOOT WIND TUNNEL TEST DATA
I I L I I I.02 .06 .10
CAMBER
Figure 13. - 14- by 22-Foot and 7- by 10-Foot Wind Tunnel test data comparison, effect of hub
fairing camber on drag with $80 pylon.
¢2
.25
.20
.15
.10
-.02
0
0
El 14- BY 22-FOOT WIND TUNNEL TEST DATA0 7- BY 10-FOOT WIND TUNNEL TEST DATA
I I I I I I I.02 .06 .10
CAMBER
Figure 14. - 14- by 22-Foot and 7- by 10-Foot Wind Tunnel test data comparison, effect of hub
fairing camber on drag with $40 pylon.
48
o
O 14- BY 22-FOOT WIND TUNNEL TEST DATAA 7- BY 10-FOOT WIND TUNNEL TEST DATA
0 2 4 6
GAP, in.
Figure 15.- 14- by 22-Foot and 7- by 10-Foot Wind Tunnel test datacomparison, hub/pylon fairing
gap effecton drag with I-LS0hub fairing,$40 pylon.
•25 ,.,.,.,"_
.20 sss
°t.05
0
O 14- BY 22-FOOT WIND TUNNEL TEST DATA
A 7- BY 10-FOOT WIND TUNNEL TEST DATA
I I I I I I2 4 6
GAP, in.
Figure 16. - 14- by 22-Foot and 7- by 10-Foot Wind Tunnel test data comparison, hub/pylon fairing
gap effect on drag with H10 hub fairing, $40 pylon.
49
Q(J
.2O
.15
.10
.05-
O,-12
O'" "-¢"---¢---.<>-...¢.._.¢._._ o
FUSELAGE+S40PYLON+HSOHUB FAIRING
O FUSELAGE+S40PYLON
O FUSELAGE ALONE
I I I I I I I I-8 -4 0 4
e_,deg
Figure 17. - lmcz'fcrcnce effects between hub fairing and pylon.
SEPARATION BUBBLE
LINEOFBOUNDARYLAYER REATTACHMENT
Figure 18. - Flow over $40 pylon without hub fairing at a negative angle of attack.
5O
.25
.20
.10
.05 "
i
0 _.02
,,
• \ J
H20 H30 _ _"__
He0W/SS0 PYLON _' $80_ H50A
13 W/S40 PYLON LS40__Imzmzmm--
I I I I I I0 .02 .04 .06 .08 .10
CAMBER
J.12
Figure 19. - Effect of hub fairing camber on drag.
.20
.15
o_ .10
.05
_......___ _._.._...-_.._.._oU,,... _ .-_.._R em_m_ m"c.'----
A (z - -6 °0 a-O °
0 I I I I I.10 .15 .20 .25 .30 .35
THICKNESS RATIO
Figure 20. - Drag as a function of hub fairing thickness ratio (constant thickness).
51
.20 _-
.15
OC_.10
.05
E)..._..="_
O" I I | I
• 15 .20 .25 .30 .35THICKNESS RATIO
Figure 21. - Drag as a function of hub fairing thickncss ratio (constant diameter).
Ou
.15
.10
.05
O-- .... ---O--- ..... ---O""
I i | I I I I0 2 4 6
PYLON HEIGHT, in.
Figure 22. - Effect of pylon height on drag.
52
Oc3
.25 j- .......ZZ
| _ .........
.15_.10 ZI H10 HUB FAIRING + S40 PYLON
/ O H50 HUB FAIRING + $40 PYLONO5 t
I I I _ -21 dog I I | I
0 2 4 6
GAP. in.
Figure 23. - Effect of hub/pylon gap width on drag (cambered I4.50 and symmetrical H10 hub
fairings with $40 pylon).
Oo
r_tJ
.25 -
.20 -
.15 _
10
.05
0
.25
20
15 I.10
.05 -
-15
O H10 + 840, Wl, 0-in. GAP
ZI H10 + 840, 0-in. GAP
I I I I
O H10 + $40, Wl, 1-in GAP•_, HI0 + $40, 1-in GAP
O H50 + S40, Wl, 0-in GAP
Z_ H50 + S40, 0-in GAP
I I I I
O H50 + $40, Wl, 1-in. GAP
A H50 + $40, 1-in. GAP
| I I I I I I I-10 -5 0 5 -15 -10 -5 0 5
o_,deg a, deg
Figure 24. - Effect of pylon wake shield (W1) on drag.
53
rJ
r,j
.25
.20
.15
.10
.05
0
.25
.20
.15
.10
.05
0-15
O H10 + $40, W2, 0-in. GAP
/" H10 + $40, 0-in. GAP
I I I I
O H10 +$40, W2, 1-in. GAP
H10+ 840, l-in. GAP
O HS0 + 840, W2, 0-in. GAP
ZI HS0 + $40, 0-in. GAP
I I I I
O HS0 + $40, W2, 1-in. GAP
'_- HS0 + $40, 1-in. GAP
I I I I I I I I
-10 -5 0 5 -15 -10 -5 0 5
,',, dq ,',,,mR
Figure 25. - Effect of pylon wake shield (W2) on drag.
54
N/LSA_mu
_wReport Documentation Page
1. Report No.
NASA TM-102182
2. Government Accassion No.
4. T_e and Subtitle
An Experimental Investigation of Helicopter Rotor Hub Fairing
Drag Characteristics
7. Author(s)
D. Y. Sung (Sterling Software Corp., Palo Alto, CA), M. B. Lance
(Planning Research Corp., Hampton, VA), L.A. Young, and
R. H. Stroub
9. Performing Organizalzon Name and Address
Ames Research Center
Moffett Field, CA 94035
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Wastdngton, DC 20546-0001
1'5. Supplementary Notes
3. Recipient# Catalog No.
5. Report DaZe
September 1989
6. Performing Organization Code
8. Performing Organization Report No.
A-89096
10. Work Unit No.
505-61-51
11. Conl_act or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
Point of Contact: L.A. Young, Ames Research Center, MS T-42, Moffett Field, CA 94035
(415) 694-4022 or FTS 464-4022
16. Abstract
A study was done in the NASA 14- by 22-Foot Wind Tunnel at Langley Research Center on the
parasite drag of different helicopter rotor hub fairings and pylons. Parametric studies of hub-fairing
camber and diameter were conducted. The effect of hub fairing/pylon clearance on hub fairing/pylon
mutual interference drag was examined in detail. Force and moment data are presented in tabular and
graphical forms. The results indicate that hub falrings with a circular-arc upper surface and a fiat
lower surface yield maximum hub drag reduction; and clearance between the hub fairing and pylon
induces high mutual-interference drag and diminishes the drag-reduction benefit obtained using a hub
fairing with a fiat lower surface. Test data show that symmetrical hub fairings with circular-arc sur-
faces generate 74% more interference drag than do cambered hub fairings with fiat lower surfaces, at
moderate negative angle of attack.
17. Key Words (Suggested by Author(s))
Interference drag
Rotor hub fairings
Aerodynamic interactions
18. Disthbution Statement
Unclassified-Unlimited
Subject Category - 02
19. Secudty Classif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified
21. No. of Pages
6O
22. Price
A04
NASA FORM 1626 OCTSSFor sale by the National Technical Information Service, Springfield, Virginia 22161