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