AD-780 720

ROTOR BLADE TREATMENTS TO MINIMIZL-ELECTROSTATICALLY GENERA TED NOISEON THE HLH HELICOPTER

R. DeRosa, et al

Bo-!ing Vert ol Company

Prepared for

Department of Defense

I May 1974

DISTRIBUTED BY:

Nztualscuic Ifutrii sfflkUi. S. DEPARTMENIT OF COMMERCE5M8 Pet Royal Road. *bi~feM Va. 22151

LS j

WRIOLt DfW=O ft6LAtLM PI~fYLVA

CODE IDEH-T. NW. 77272

NtW8ER D301-10279-1

TI-TLE ROTOR BLADE TRE:kriM-S TO Mini-MIZE

El :TROSTATlCALLY.W CGiEtPAiED, IZOXSE

ON. TME ELK HELICOPTER

ORSGKAL RELEASE DATE. FOR THE RELEASE DATE OFSSEQUENT REVI10CAS, SEE THE REVISON SHEET. FOR LWTAToNS

NPOSED OH THE DISMTRIUM AND, USE OF wFoSUTOm coxTAiIEDI THIS VOCUU-NT, SEE THlE LOUTATIWS, SHEET.

MODEL ELM aOTRACTDA 01- 3 -0! 7 ;Delvery Order 11

~SU NO_________ISPWE TO:_____________

PREPARED BY L::-- DATE _______

APPROVED BY . DATE______

APPROVED BY ~-DATE -

u FrieeAPPROVED BY ~ 2A.~DATE

NTOaL TEOW*CALDCR(IeATDOC SOEInm.t Ir

SHEET I

W4JU8ER D301-10279-1AZACo 45 ROAYREV LTR 1

I ______ - -ACTIVE SHEET RECORD

I ADDED SHEETS II ADDED SHEETS

w SHEETISHE f1ISEE- SHEET -i SHEET -1 SHEET -5 SKET A

NUBE > >1 HOER >11>NUS~ ' MBER 08 NUMBER NUMBEER J NIMI8ER ua

- 36 F2 373 314 39

5 406 -41

7 4

Appendix 44

2 4613 474 485 496 5so7 - 151

5631405

15 55

14851595

20

242I25

261I271I2829 i30131 I33

35 _ _ _1 _ _ _ _

%IEET 2

_ _ _ _ __ _ __7_ _ _ _

I*

NWISER D301-10279-1nc C7&~S7JYeW c~na..v 2EV LTR

" IWITATIONS IU

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I j- j III

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This dscwtsn ;~ cnajld ~ RUE DUde Design

All uerWsioes n *is deomi ,s.ui so .~pwn' by ~

*0'C fled mgnizelin prim ~* I

9r .n.I inn I

___ _ eJvWe~ -~-zr _- -nw-__-- -

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VC MSER D301-10279-I

TECH IfICAL RiEOKT

!his dociuent was prepar.-d by the Starford ;.;seLgrch Institute -for the _ Boein Vertol C'zny-a..4 Is si-mitted to satisfy Cc.t.rzact,.ber DAAJOI-73-A-0017.

Sni Laboratory testing has shown tk -a:fect of 11st anria ice_particles in generating static electricity on t. blade surface.The electrical acu=lations result in radio freqt.-y n-iseinterference.

Since the rotor blade paini coat is fabricatedof materials whichare electrically insu!ating, the first cons:.deration is torende.: the blade surface electrically conductive. Several typesof corductiye paints werc tested and are nou available for thispurpose. Test. ~pnels were fabricated, siniLar to blade structure,for evaluation of 13 different paint coating configurations-. Anoise reduction of 20 do from~ the noise level of the insulatingpaint system would ca.s'e interference to the LOMAN-D guidance'system, whereas a reduction of 40 db v>uld provide noise-freeoperation of LORAN-D, with fall range capability. 7vo of thepaint systems which reduced the noise level by over 40 db fronreference have been crosen for use on the RI blade. .Oz the inboard blade shmnk, nagna 3-8-6 Sz u.sed wi-th no !_.rcoatingrequired. This ccatizig aimr provides erosicn resistance to thefingerglass shank. On the blade fairizng area, aft of the no~ccap, BXS 10-21 Coductive coating s used. The titaniu= nose cap.being conductiv, does not require a conductive paint coating.The nose cap and fairing areas are then overlayed with epoxyprimer and the acrylic nitrocelulose laquer paint syste currentlyused on production rotor bla s.

This combination of paints provides tba erosion and moistureprotection required on composite structures yet mkes the bladeelectrically condu-ctiv , while providing in the field touch upcapability.

A surface resistivity Ieasurexent technique has been devised,Ahich Fill aller a good in-service test netkod to a-sess thepaint coat's ability to drain the surface charges nois2lessly.Tests have shown a dorrelafion oE surface conductivity to noisereduction capability.

TO prevent the helicopter potent ;&l, generated thru precipitationstatic, from reaching the corovw Aischarge level, staticdischargers, similar to those in use on production aircraft,are roq'ired. 'The discharger coyr:na threshold 1'zim at whichdischarge occurs) must obviously :e lower than that of any rotorblade sharp edges. An area whilch requires attention is the tipof the rotor blade where the pres*z-re reduction can cause the

ISHEEI

- 1E D301 10279-1

r-m aAW AYW c..REV UR

WIIICAL PXFORS .

Continzued

ou-toard edge of te nose cap to rearh it s L ona th . -hodprior to the diachargers. "he recomnded com'isguration is toinstall tv-o dischargers, on the trailing edge at the tip, and adaa rorded tip cover vith discharger installed. The designproposed for the HLH prototype will use two discharcers mamntedon the triling edge at t"e outboard erd of the blade. Thsedischargers will be ren7able for shipe.t or replacement, vithw special tools requirea. To reduce weight and cost, thepresent. flat ip co;er will be retained with provision foradition of a *ini discharger if required.

-* 6

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I II3

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-

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3£T

SUPPORTING LABORATORY STUDIESFOR THE DEVELOPMENT OF ROTOR-BLADE

TREATMENTS TO MINIMIZE ELECTROSTATICALLYGENERATED NOISE ON THE HLH HELICOPTER

- -

II

FOR TE EVE LOMN OF ROTR-BAD

TIW 30EMG CMWANWVERTOL 0WZ2GX 8ENG COMIERA ED. BX TMUMMAE~~A& F9MV1VA TU2

T. ICRTA. D "

RAY I- LEADAWAO Eaffwir Dvor&AEt* sa Apso som OMAN

aw ASL

A~ Uor2tory IltwstIgatla was r-"t4 to evzacate the sever~ty

of precipitauIoo-5tatIc o~~- to be expected co he Seavy Lift Feliccpter

CE) louw-fr e~evy systems, azd to deffIze the rKcrIremc-.ts of tecB=Iqe~s

for no~se eIlf.,.aatioc a the ME.

7Wis Investi~atioe, usftc a 22th scale model of tLe EMS and actmad

prcpIosed rotO7r.bla6e panel. samples zrwlaed the zecd for Mi Frotecta

athe HER. MC atlizs we, e top frstaI1 P-static dischzxezs

ct!e =I roor blades,, od tbe op'.u ==ber and locatica of these

disebares as discsse

Solse rteft-tftm afforded by raefums roto -bI3de-ecztI=C systems

*a 'I" aLsireagated and epfed. A rewoomftiom ~ made for a

7axeIculzar Iatue systam, 1ased CI woise-zreactLOM ama 5-rae-resistirity

P IS.ri pge!C ~ ii

II- P- - _ _ - - -

LLrO AIS---- - - - - -' -_ -- - - -

I . .... ........

1 I30FW-WIC3LM A M S .................. 5

IT IIA ~ D Wm ot -AU - - - - - - - 3

ApnuxXDMMlr AM~~rz XES -ZC r 0300=

VEZ CUM~s A. S ~.mii . - . .. . . . . . . . . 47

. .13 .- . .. . . 5 S

pp

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21

I

I -

I Ytwslble F z~d ZF Ltt=;a tfocs cosidemd

2 zest Srtz for Cocpftcf ..........-

3 A-teumz copl~...... 9

S 3YoIsc at A?*tt~ EacetL@ 2..~ aa...... .. .. ...

6 SO@se at A--tea ItcU 3 .. . .. .a----------------a.a. . 314

a Y*1se at ;=c - 1 xm 4 9 . . . a a . . . . . . . a a . . S

irS Coa Yas4 Z~sm!s at 7VA7 Ts11cz9 .~~ a .aa. .

J9 AfmwmI fttCTitexJzr ftitfQ&ip 3e4 t

10 et~to De"ime merzmmsoiz Nn=t1.t

Fl I T&b.1dif NWt~als of~ M Blade . . a . . a a a 2

13 S7*ce-.C~z.4 u "t~td V.4scb.3u from USa of C 2lm~tz a aa

14 fF lsaz Jta11ztimm am =E ----- e .. . aIi25 Test Setup Esed ft 51 a-coatlzc Tess ..-.. a-.--.-..- 31Is Test-Sangize reftUS s .... a a .* 5 . . .. .. . .a. 33

27 Track= of Szmcr =1,c~xrjs Gep--&am Y~emctswr~ Omted# Test ?cla .. . .. a a . . a a a a . . 32

19 CoaxIal C4=do.*,wlty P.obe Vied to Dve.=vme theElcrtrIcal CcbatetrUly of Tszio'ms WX 3336e Pmls. . .. 40

19 Pb-qrj of TypIcal Blade Sinple i1th OMS 106-213"e 5Xat*i31 (101) . .a . 43

20Pbbo-w o fam fSw .=.......... 4

ppi v

22 fttc-a of T.1c±1 MM S3*w!c PWce1 szrfz

T"I

TABLES

~ Description of Blade-aterial Test-Panel Coatings ..... 36

2 Results of -lzC--Sample Charging tudles. ......... 37

3 Comparison of Noise Reduction and Surface Resistivity

of Panels with BUS 10-21 Base Coat. . ............ . 42

14

2-!

IJ

I,.

- I@

When c unication zId navigation equipment was first instulled on

aircraft, the aircraiL operators found that radio noise was observed in

these systems whenever the aircraft was operated In clouds -f dust or

precipLts.tion.1- 4 * These early investigators found that the noise steamed

fzam frictional electrification of the tircraft as a whole or of individual

parts as particles in the cloud struck the lrcraft and deposited incre-

ments of charge. The problem subsequcntly was studied in considerable

detail to identify the noise mechani3as responsible and to devise schemes

for reducing the noise to tolerable levels. -1 1 Many techniques have

bee'n developed xnd applied to conventional aircmft.

Static electrification occurs on helicopters also when they are

0operated in clouds of dust or snow. The most serious problem recognized

in connection with helicopter operation was the electrostatic discharge

to ground during ze-no operations, where it was found that personnel

%v.e 1oetimes kcnocked off their feet by shock and suffered miscellaneous

injuries such as sp .it tips and edge of fingers and nails. Various ac-

tive scbeses were tried to eliminate the static charge on helicopters.;3 -1 6

J In a joint SRI-Boeing Vertol investigation, it was concluded that the

chp-rge on the dust cloud stirred up by the hovering helicopter completely

dominsted the electrostatic euvirparent, and that it would cot be pos-

sible to sense helicopter-ti?-ground potentiel without touching the ground.

-Accordinglyj a passive aircraft grounding scheme was devised and demon-

strated.

References are listed at tie end of the,,rort.

03

In addition z .argo-handling problens caused by eOectrification.,

helicopters are als )ject to the noise problens encountered in conven-

tionali aircraft. IM this regard, the helicopter !s no different. Elec-

trification will o'>zr. and radio noise will result. In thze case of th--

ElM, the problem o-e radic noise became of particular significance with

the developmtent of modern low-frequ~ncy navigationial aids suchi as LORAN D,

which operates at roughly 100 k~z. These systez provide capobilities

that-cannot be achieved in any other way. Since they operate at IF,

however, they are particularly vuln-erable to precipitation-static Inter-

ference.

it was recognized that mozt precipitation-static fftes are simple

and inexpensive to implement if they are considered as =rt of the vehicle

design. As retrofits, these same fixes often become so expensive as to

preclude their consideration. Accordingly., this program was !=stit-zted

4 to Investigate the severity of precipitation-static noise to be expected

on the RIX and to define the requirements for technique's for noise elhIain-

tion on the HIA helicopter.

In conversations; with Boeing Vertol personnel,, rarious; approaches

were discussed for ehialutting radio-frequency noise steming fro fric-

tional electrification of the RIX helicopter. During these discussions

it was concluded that thae most promising approach to noise elimination was

through proper design and treatment of the helicopter blades.

Electrification of the vehicle as a whole leads to high' electric

fi elds and corona discharges at the rotor extremities.'- 1 01 Noise fro

dischargers capable of providing sufficient noise reduction to bring

the resulting corona-noisc levels do.n to acceptable values.!"' 1 In

planning such a discharger :installation, It is necessary-to specify the

noise reduction required of the dischargers. It Is also necessary to

2

define the number of dischargers required and to specify theIr locatkns

on the rotor blades.

ChZarging of the 'plastic blade surfaces as the result of impact by

snm or dust causes charge to acctuilate on the surface of the blade

until streamer disciarges occur Ow!r the blade surface t- some metallic

structure to relie7e some of the charge. These discharges also generate

RF noise that can disable radio commication and navigation systexs. 6 "

Straner -ose Us best eliminated by applying a coat of coudictive paint

to the surface of *4oe blade to remove the charge as rapidly as It arrives

opa the bladft.7 In this way, the high electzilc fields that ;--4 to stre2ucr

discharges are eliminit d.

aIn eneral, conductive paint is produced by adding conductive powder

(metal or graphite) to a paint vet-icle until the desired conductivity is

achieved. 'This usually reqidrer so much powder that the mechanical prop-

erties of the paint are degraded. For this reason, it is planned that,

0on the WAY the conductive paint will be overcoated with a thin layer of

noco.ductive paint having the desired =-hadcal a abrasion-resistant

properties. The argument is '-at this thin outer layer of paint will

pncture at -low voltages and perut t e charge accumlating on the outer

surface to flow to the conductive layer without generating large strem~er

discharges on tke outside surface. It was necessary to test the workability

of various materials that wnre lz ected for - In frnataon.

It was Czncluded that the needs of the ON could be met by accomplish-

Izg tbe lolloving three tasks on the present progra:.

(1) C-nduct the necessary model coi.plizg measmrements andtnalyses to deteraide the distharer-notAse reductionriquired to afford the same degree of no'se protectionfor the NIX as is currently available on airliners suchas the 707 using 1AXI C/D.

03

_ . .._. . . .. . . .._. .._._. .. ..__ _. .__ _ _ _-'- .j

(2) Conduct the necessary electrostatic nodel Keasurezentsand analyses to establish the wi~ber and placement of

passive dischargers on the lilH rotor blades.

(3) Conduct triboelectric charing tests on rs many as 10

saxples of candidate ronductive coating systems to

detersine their adequacy for use on the MA rotor blades.

Stace the nofte problem has been studied at great length on conven-

tional aircraf, extensive use will be made here of this earlier sork.

In particular, reference will be made to specific equations and tech-

niques developed Iti previous publications. Ceaerally, the required

equation will be abstracted and used without rederivation cz extevsive

justification. Attepting to Justify every step here old result in

an inzppropriately long and complex report.

4

II NOISE-M-PLIIU AMALMYSIS

As Indicted in "-he Introduction. stklic elect.-irfic-tioo of the

helicopter as a whole raises its potential until corona discharges accur

from the re~ions of highest elect-Ic field at the rotor tips. These dis-

chrLes generate radio noise that couples into ,ecevivng systess on the

vehicle. SiIlarly, charging of the dielectric outer Uurfaces of the

blades will result in streamer discharge- across the surfaces that also

couple wise into receiving systems. he severity of the resulting

Interference must be cstimatn In order to veigh the Importan-ce of devel-

oping fixos. Thus;, it Is necessary to determine the degree of couplingIbetween fIse sources on the blades znd regions on the NIA whe-v antennas

:ght be located.

Pozitioms where antennas might conceiyably be located on the NLN

and where coupling neasurements were made are shown in Figure 1. Posi-

tion 1 Is on tkz "Leer pert of the nose, where antennas could clearly

be located wlbout difficulty. The feasibility of antenna locations

such as tho'e in positions 2 ezd 3 -an the top of the fuselage depecds on

the degree to which performance of tte associated system will be degraded

by rotor-bbade modulation. It was felt that such Installations were

definitely possible, awd that measurenents of noise coupling to thiss

gvneral region -ioId b'e made. Anotter clear area where 2z=teunas ight

be installed was the back of the aft pylon. Accordingly, measurewo ts

were made of coupling to position 4. Since the HIX design incorporates

provisions for carrying large container cargoes along the flat part of

the belly, it will not be possible to Install antennas here. For this

reason, no measurements-of coupling to this part of the belly were uate.

. ,.. ..---.------ ---- -

PO TTIMO00N

FIGURE 1 VOWNBE LF AND HF M(TENKA I-CCATIOS6 CONSDERED IM T1EST

The neasurrai" techniqres used In the present: progr= were patterned

after those descrLW,~ In Re-.. S. The test setup Is shoirn O- zigore 2.= e a =1=Izt=.e RF osc41"ator is shown, in position v~ the edof -n of

the rotor blades. DC power Is fed to the oscillator YJ12 a high-resistance

f lead that Is of sufficiently high value to appear transparent at the IF

frtquencies of Interest. The t~wer-supply grouwm retar- f.o the belicopter

is made via a second high-resistarec, lead. Thus the setvp simulutes an

electrically Isolated helicopter with a small, Isolated BF geukZrator at

the tip of one of its blades.

Noise picked up by the test antenna an the aircraft Is fed to a= J

atten-uator followe L-, a battery-operated receiver tuned to t1a'e oscillator

friequency. The -attewuatj~ is adjazted to produce a standa-vl ieadIrg in

the meter cennected to the :e*ceivzr c=tput. The meter Is al"~ contai!--ed

In the model ard is read cptically. To cbcain, absolute ralues of coupling,

the IF source is placed against the ier conduc-tor of the standard co-

axial coupliq; str.Yatur*e, vhich Is attached.' to the recelving system In

place of the test autonna. The attenuator is adjvstd to produce the

standard reading on the -celver oiatput meter. In this vaT the ratio -.f

6

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'CP

p.-w1 0I-I..

C:a z -I

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the measurv&I co-rpllg tr the staddzrd taxplitV is glren by xba ratio of

the atteuucat'c'a settIK- rvquIred to produce tie standard receirer cutport.

It Is reciolzed, of corse, that In a rformairr the caplfzg -e

v~~m~t. s dA-srabc, the 4aat-~sis.f the recefwci ar~tM-e

used oc the model *rL a.!.towa:Icallyr .1corporated 1= receiver autpalt =d

ceo2scred Z~ping. Tab tke the =ca urcmoes more gemnerily useft.i t -vere-

fore, the test =nteuna prriperties have beurezoed from the results

us~ing =tenna data deveoe by Soll~ah.7 Adtloaa- disenssloca of

this procedure Is given. In We. S. '"Ib rezwisr of the MA co'plfrg o 1 -

sareens sbov= Ina FUI-re 3(2) tVro 3(d) =-,e presented In terms of

couplirv to an 2ateana of ruLit isdaLvt~oa area. 7bas the couplinag da2ta

c~r e th*=S2t if as expressIn the ewplII between a disr-1arge source

a a blz.-e.. andm a location on the belgccvter abere mw ulght wish to

locate a peceirlut atm=. The couaU. to an zc*.=1 =4rema comidereds

for use --a be detemined simply by mltiplying the normw7_izzd cc~1Ung

fac-tor of Ft~wre 3 by the antenna Inductios lib. -~dtrzm the

In~action zx".. ap of a partica2ar antema It Is cr.nve=Ie~t to use the

C PAL

It Is eTidet fnnFgr htteltrmnsvx ofndt

fr___________below_2.5_____Thisrestrictionwas__________________Me

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GO 2:PU 10ATUMIts &Q &SW 20 AW1t L z 2

LIIMS 3 /KOACOPL4

prizary coerm I= this pr*Cr= a2 systems ope.--ztirg In tbe LF portio

of Me. spect . To ~eaeC*ofidc im the dat*a, bowever, the area-

STU -its were exteoded I z the low part of the mv reZ!ze wbere 2irCaf t

reS'Ia=.~s s-*ad beic to m~ifes' thaeles I= the cocplI=C wzLIet.

oRM exzrPle, I= Ffgcx* 34 of IMef Sp5,a~zer sbovs a=alytically tlh-j the

first -=d-~m In mols- c'ocpIz,- r.'X tme !%x ez of zz zsymtrIcsl

dipole occurs woe= the lei~tb of Mhe locg half is 0.37W~.. Cow-sIdex1--

the ccplftg bt-tweem the positIco-l amtea *=d z disch'r~e cathe t~d

of the aft rotor-, the fIrit zaximm shGald ocicu when the dista~m betatee

the nose of the helicopoer 2=d thbe aft rotor-blise tipA, a dista~c of

roq1by m3 ft (for 6 S() is 0.37-54.. 7his cozzzes~ouds to a frequezcyr

of 2.9 30. Fkvu Figcre 3!W), we see th.zt the zeiscred cousminl fom~ the

aft SOWor to amtsm Poet-ISO I Is IzCzvXS!M wi IMCreaMSInv fzequenc7

up to tha hlhest frequency mscred Im thbe laboratory tests.

isbvmd Z2=o ur =-mm-.e --us c.dl ccw-- U=-Ci pec-.

for emc rotor. 35 swm ets were actually m-5e fvr fire bl.- fis~zoms,

~ 4 but kZ aras fow that the comPlizC data chaz*ed wezy Little after the

bWse aus 90" or nsreorrd f--v the fuselage. MA will be sem= in

the mext seetia, this Is the blade locatio. n which coroma discharges

w-im occur..) AcordtagJ3, to simpli the presentatiomad the cacua-

"Os shngle composite curve of coupling Is presexted .'or each rotor

In 5rwmral, the couplizg data of Figr.e 3 bare the same behavior

aste77lwfeum-r-opb aao e.S b copl- Is rela-

7bTe oocqlftg 0!Q,.o FJZ!Zxe 3 were used to&*tbhr %ith the coe-

noise spectral data of ZeL. 3 co generate the currs of coreou-wise,

euivalent field sb- su for the vxrt~ =er=a PwItIcs In FIjure 4

thzcg 7. CStree~r =rise tIsQ, sYbowi 1 tLese figzres; will be discmssed

2-ster in this SectIOa..) To Carying cat these ca~cclatfocss It was 3ss'=td

that thoe bel~mpter ebarrizC carent vs OCO VA, thd at It split zqpally

betweem the tv4 rotors. The noises gemerated froo the Zwo zrat rs wr

Zded OM 21= ba sis to deterunze the tcvzl =*Ise ccra--t izd.sced In

the atem. F4lloving tb'e prnced~mvs of Mef. SO the noise ccuzwt v,

the expressed In teras of the eufralezt zoise UI'eJld reqcfried at the

antens location to prodace the sae short-cIrcuit 2te" ccre

rsswemtiazy, the data of FIgures 4 tkoqf 7 sbos' the noise fier'Af

tht Old exrist at the autewm locations om the 3-S If It wer-ev

wIthoat dicagez van the rotor tVades; In a regIom of sevre cftuSb.

EsIng tbe comlIMg data of FIgure 3 agaft, bet this tine In com.-

juctlow with stremersos dat-o f Ilef. 7, c~t-czlztI~ms: maze waft, to

deteaulue the lavvi of noise that Wuld *MIS-, at the internis locelow

If the -- tor b1lJes were vatmrted md WG :,& of cbargimV to tbe bladie

surfaces retwaad to the wetalli t~r,,t of the blz~m via st z

presented for tko vauioms anterne-positlos:s n 51are 4 thzosg 7.

Also~ sibou In 7igures 4 tbroq* 7 are 7.4mmU of atuospheric noisefield obtained 1from 3e. 1 an& adjusted fo. a I-&Ez bdth. Tb

coros and streamer moisia flds are sees to be 40 to 60 dB greter ta

the ateo'sperJlc mol? taroot the entire freuescy --svge c, Ps Idered !a

thte case of antewm positiat L ad 4. The noise Is soseshat less sen re

ft the came of amitem pcwtitions 2 ad 3. The 1splcatioms of not"e an1the oeratin of I4AN-D systums are as follows-. 12be IMR3-D systemdesigmer can aimm Improred system perxrozaae util1 be bas; rded

- 11

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0

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10

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FIGURE 7 NOISE AT ANTENNA LOCATION 4

__ F

his system input noise figure to the atmospheric noise level. From this

observation we can infer that such systems are now, or ultimately will

be, operting at the atmspheric noise level limit. From Figares 4

$through 7, we see that corona or streaner noise from the blades will

severely degrade the performance of such a 3ystem. In particular, the

analysis indicates that from 20 to 40 dB of noise reduction is rrquired

to reduce the electrostatically generated noise levels to the daytime

atmospheric level.

Shown in Figure 8 fcr comparison are corona-noise fields at the 707

t ilcap antenna location for two conditions. In the first, corona dis-

charges are permitted to occur fro the rudder resulting in high coupling

to the antenna and hlh equivalent noise fields. Eliminating the dis-

j I charges from the rudder has the effect of reducin the noise levels to

the values given by the lower cur . The data of Figure 8 were verified

n the series of flight tests described n Refs. 8 and 9.

It should be roted at this time that in order to achieve satisfactory

operation of com-nicatlon and navigation equipment, aircraft such as the

707 are equipped with dischargers capable of proyiding 45 to 60 dB noise

reduction, and the dischargers are nmerous enough and in sufficiently

effectlie locations to permit all of the charging current to leave the

aircraft. To reduce e7,ectrostatically generated noise to similar levels

on the-Ib& , it will be necessary to provide comparablc degrees of noise

reduction to both stremwr and coronk-interference sources.

I 16-

V

ilo

I"I

MI

0

1S)

G.IS IAR~ FR0 I.O

FIGURE 8 C A NOISE LEVELS AT 707 TAIL_ ANTENNA

@ I

rz

. ' - - - ' z,

III EECrROSrATIC NOUELIrG

Laboratory electrostatic measurements were Kade to determine corona

thresholds of the HIM blades, and to estimate discharge currents from

passive dischargers located on the blades. The measurements were carried

out in two steps. First, the setup Illustrated in Figure 9 was used zo

determine the relationship between th. helicopter potentisz and a reference

on a Lelicopter blade as a function of angle e with respect to the fuse-

lage. (The reference point chosen ww; 12 inches inboard of the tip and

12 inches for.ard of the trailing edge on the full-scale vehicle.)

In carrying out the measurents Illustrated in Figur.' 9y the /48-

scaLe model was sprayed wita conductin silver-loded psin'. to render it

co.-dhatug. 72k- model was suspended in the -laboratory a,A courected to 2

high-voltage power supply. A small metal probe - uted on a thin,, elec-

I trically iulating handle was touched to the blade at the reference

point. lbe charge acquired by the probe was transferred to a Faraday

"Ice pail" attached to the input of an infilite-impedance electrometer.

?be relationship between probe 4*arge and surface fie d was deter-

mimed by repeating the charge-transfer measurement in the known surfack -

field between a pair of metal plates where the field In given by

E = V/d

wvere

V = Applied voltage

d = Spacing between plates.

0i9

T EFLO

AmL

U44WLEMOIE

LADCKATORY T-

ELICTWC ra

PARAilLIL-ftATE

ARDYELECTWOMMTER E m VAO

POTEKTIAL AND REERENCEPOINT FIELD

20

( In the second step of the electrostxtic: studies, a full-scale see-

tiort of 5tH helicopter rotor blade a2 suspended In the laboratory as

j ~Illustrated In Figure 10. The pomr-suapply voltage was increased urntil

corona ocset occurred (at corona cur-rent of ZG VA was used to indicate

onset). The reference-point field at threshold was then measured In the

setup of Figure 10. Since the earlier experiments using tMe setup of

Figure 9 developed a relationship between reference-poimt: field and

helicopter potential1 It Is now possiLble I-* state the potential at which

corona threshold will, occur as a function of blade position.

RTORADJE

o f

~1FIGURE 10 SETUP TO DETEFi1MM REFERDSCE-OIT FIELD CORRESWOMDNGTO COMMN THRESHOLD

Figure 11 shows the result of the corona-threshold measurements

made :in the laboratory- It Is evident that the threshold !S sensitive

to k-lade position. In particular.. minim threshold occurs when e 1800,

and the threshold Icresass markedly When the blade Is shielded by the

fuselage (for 0 < 450). The shielding effect on the foward rotor blades

Af21

son -- -

L£

noI

weI

A um

L>~iW

IKOPX LEQVN CONZ

A" SAE

0EDN CVNI

'---MM SLADE

AGE 0 WEAVE TO nFRAGE-do

FWPLME It THRESH"L POTHMlAL OF HIH SLAME STPUCTM

is more promouced tbam o the aft rotor bccause the forxa.d rotor blades

pass closer to the faselage (see Figure 9).

The tralling edge of the a~t blade has the lowest inIgrL. coro

ttresbold (roogly 150 k). The nlnizm th.lesbol of the fanrd rotor

Is somesint hblger, but still substantially below the threshold of the

leading cormer of either bla.e (250 to 275 k). As is eridemt fro

FIgre l1, soe expertmemtatiom was dome to dete-zine the effect of

Installing a roamded cap am the tip of the blade. The cap has tle effect

of greatiy tacreasing the coro threshold of the leadln corer of the

rotor (425 to 4S5 kU). The iuplicz-tons of these measurnts will be

dealt wih later.

The fact that coroma thresold Increases so rapidly for C < 45e

means that most d rgin will occmr for e > 45V-

Ye Implications of Figvre 11 should be comsidered further. For

~o - mo !C. with no dischargers Installed., dc- ulth no airflow to generate

locauzed pressure redctions., the model stuies Indicate that coroma

thresbold al the blade-tip tr ling-edge structure will cc imr the

vehicle potestial reaches 150 k. Assming that the blades are equipped

with trail7g-type passive disckargers of the sort used on cxLrciAl

transport aJreraft, coram threshold of the discharrs will occur at

rouhly 1/10 the tkreskold of the trailing efte Itself. lbus, long before

trailing-edge threshold is apprm'cbedv the dischargers famictica and tem-

erate colms of space charge thsit have the effect of shielding the blade

trailing edge proper, thereby imtreasim ts corom thresbold by as much

as a factor tpf 2 to a potential c&1 say, 20 to 300 kr (see Figure 43

of 3ef. 8). Ths the moftl mstwoments indicate that, with t-ailig

dischargers installed, corosa thi*sbold of the blafe trailizg edge and

blade leading corner will occur st 2S0 to 300 kT, assimlng that there

are v0 vortices present to produce localized priosn reductIcAs (which

23

redc- coroma thresbold In proportioa to pressure redrction). ibTis momms)

that we =crt umder amy circumstance ailoir the helicopter voltage to

epproa~cbh 250 to 300 WX because If I.t does, ws-producing coroca dis-

crges wIll occur f.~ the blade structure.

',et us mw assume that we find a factor-of-2.5-to-3 .~rgft of safety

In vehicle voltage acceptable (this zeas that we feel -ofideut that

the COmbi~atl*M Of UMce--tSztieS I= locMlized PreSS--re red&Ctfom *ad

errom.o ;= or laboratory studies is less ta 2.5 to 3). Le-t as estfsate

the current that would be discharged by a al- la st llatiorv of two

trailing dischagers per blade wben the helicopter potential Is 100 MY

MTe discharr,--r are assmed to be 6 to 7 11he Z*W In the ammer of am

ortho-decoopledt disckazger as describeZ Iu Refs. S, 9j, ad 12..) Ffr--t,,

SSiMg the resW1tS Of the ref eceSoutfild- -tSX We det*72Iue

the reference-polst field as a fauctIom of blade position for an afreraft

pot-.tial of 100 MI' The results of such a deteruination are sboaw in

Figu-re 12.)

1A Eq. (31) of Def. S.. = ez~resscin Is gives for the space-cftrge--

ltgsoted current discharged from the end of a rod capped with a zero-

Mum shold 1G. sc md placed In a uistrem (see F4gure 13). (Thbe

vafk'_ of this expresslow was verified In flight for the case of qxtba-

decoupled dischargers, whIch ka"e a coum tkreshold rogly 1/10 that

of the shamp tmalI edge on which they are muted.) The eqcatios Is:

I= AE 2M' (l)a 0

where

I1a Discharge current

.a W adus of dlschargic cylinder

24

- AFT WSA.AI

I 41a

^ME OF SUO!E TO RAGE-0

I FUVMUM 12 RDIERWiE#GCf P.ELW FOR Kill FOTBIIAL OF M0 IkV

- 25

i7

E a= Field Imemsity at scrface of rod,, TM

S= L/36-- x 10 fa--adf ii.

-OAUK MMPML TO SWACf F CrLV4O~t AT COnO CF CtHUE

I ~ FLW

CKFPIG PM U1CE

OF NOM~ AT VA OF CWW

RQAW 13 SPACE-OIHflGE-LONTE 09POWARG FROM 00 OF CaXWOER

First = --Ge that MR:, 01 Is "Imesr wIth v . £m It is pemsssible to

aa

fled the aird E -wt tZe tol samt by fzg* E - Letraus Sref2e iic-t )WIt a

quzagats 3, C, am,-- D in 7ire 12 using linear Inttarpolutt~s betwe

oe poi.nts of the quadrats. This av rsgiwg yialifs

Agts aloag the lIte of thoe on p. 91. of Jkf. 9 Indicate that for

theEKEblae~,E'Z -2S.S. Tkusg for 200 ki a the KIN,

E 1.2-.l xi0,/na

Let as ass that the tip of the rotor has a welocity of 250 m~s (this

corrmepo to a pedof X 0.75, which Is comervatively below the

26I

j _________ __

Xac I i~uit for lbelicopter blzdes) - Let ta also ctoose 2a - 116 Imj~mC)=6.35 10 a.. tbs fting all ttese vaimes iz~to rq, (I yelds

-362!0(6.3:S X 10 )(L.27 x 10 2=

S#me both distbargers omthe blade sbaed 4f1sickazre roq;Uy tie sm

carrmt, dsfm oa1y dischargers I= vomdats B, --a. -e cfe

thetatl, ur~ntdischazrW byta camplxaect x h elXc'aptae-r

of 100 WT Is

Ira =12(56)

(I)'M Uhs . sllk* t eu " off am 40--,A pedt" CbrgIm "Sfef tlzated

for tae =a hencer4.

let us m c~side tho desiM of divicberzi imtmaltls smitaboe

fr HE. The a" it Calunt" vialat of total disCharge wMAMse

that two tralling dischazgers per blade cm rvasasbly be exrectad to

beadle tbe mimum vaicpatod hAwrgag CUzwa? W the ma Wkne holag

the wvklcle pottials below 100O hr. Mabe zmults obtalmed abnr also

ltdlcate that in the abeence of lcal~med pceewr zwmtosome of

the blade strucizze scoeld reach cozma tkrtsaold zUtl %he vablele

ptential Is in the rae 25M to 3W MY. TUvxit-test expezieae am jet

aircraft,, howvrr, Im-icate that amtatial pzussaro redectices do

ocu at the *aft of aWrfoisr ps~itt.Ug coy discharges at potem-Mals

subtantially below those piwdictad an the basis of still air misure-

meats. * r r example,, It was foun that a-*Is *meet occurred whem the

*lrzft votetLw v~ 1.3 txes t~e tr!i-te, thrsLRd1 (uaee P. 1r7

of rbef. 8). ThIS r*3xt a2S CttSmtd wItk: o=3d tips amthe airroilsm

Satisftctozy qpet ioc of PzsSIre dlzdrro s~st am co==zt!o~x1 air-

craft W OW'a~ ackler. Mti1 airfoIl-tIp 41sclrers wew 6evelopedl to

tozerte 2 calinu of spe-v dOrre aleag t&r tip of the air.-oi to praridc

ShIel1wi 3:00 a cv wrsple~L cea-zsh 1 icrre I= the regfocs

aircraft evz ewiCes It 2a~e=s pc=Se~t to a=tIcipzte . t4-rF cfitoo,

0the WA blades a=a tv take st"S Ls tbe waial bm aerfz= to clz-

The preferred 91sca.-er iawalatIo is sbom Im yigure tC.

3kres two traIllaw &Isdiazguzs are covrfild to bagle the belk or a*e

disdcurg* Cwtot.. A roMM&d 21ip car Us lmtz2d as tbe ad of the Z:2~

to iacrwse cowow Mmsbohlft at the tip of the t-r.d. Ia urmcz

zrmbf! I-lervems the ttreibold oC the 1.adlz come by a ftctz. orf 2.

a" CULkasus the Shup aloe z1 the top aad bottom Of the blad ip

tbhe*3 tumresslX the ovemll coro tfresb*ls im the eatire tip rg.

of the Wlaie. A tip-type dischws-ge !wsllad cc Ce tdcap resulTS

in tL& geamtio of a cloim cf vsgpeL-e cberge aloft the rotor blafe tip

to rx~se c thzesbolds eww fartbez.. 7hm ve Iwtallatig of F4*We

14(a) Is &Itugad to tae lato scomat all of zbe comm Sac-.C that

exez enc am Jet tzT~ots; Ileaes are likely to oc m d uvo-

Perates PewLN UMS for rwdcisw the ,1hload of their rfP rmee.

It tbQ tip =ffI4=t1Qf requIxe$~ to acconpUlft the Imtwllat~ca*f

F~gure 14(a) Impose sePvtoble vweIit,, draw, or cot penulti"s, It. is

possible to simpifF the Instaflatlow somewat to the owet justated

Ina Fig=*e 14(b), with am* atdat less of pefoow"e. Mer* the momal

sq.az*d-off Wlade tip Is zetklmdj, sad *larve dt*Chkgers axe izatall"

as IilAutrated. Memwiag 'hbe roded tip cap x - meLes the* t3L-sP~*2d of-

the front Mla"e coe to zo~y 250 WT fta &tM aix. Alto elixisatu

_ _ --------------------- - - - - ~ -- - - -- ~-------- - -_ _ _ _

ROTATION O#OED BLADE* - TIP CAP

j I TIP-TYPEj HillROTORDISCHARGER

STO 2S" (8 PIWIFERMED

TRMILING-YPE V NIMMIMI

rfDiSTANcEir--m

ROTATIO

NiH ROTOR $LADE

DISCHARGERPoWB.LE

DISTANCE A~S1

FIGURE14 REC MDE ITCR IMALTIN NF4BAE

29

±he rounded tip egj*es the sharp edges along the top and bottom of the -

blade tip If it is possible for vortexes with their associated localized

pressure reductions ".o occur at any of these sharp corners, the threshold

may be reduced sufficiently that corona will occur at potentials below

100 kV. Before the d~sign of Figure 14(b) can be considered seriously,

it should be ascertained that vortices are not likely to occur in any of

the regions of high electric field at the blade tip. If, for example,

a two-to-one pressure reduction is conceivable as the result of ortex

generation, the design of Figure 14(b) will be highly marginal.

The minimum possible discharger installation is shown in Figure

14(c). Here two trailing dischargers are installed on each blade with

no provision for tip-type dischargers to inhibit corona from the sharp

corners on the tip Itstlf. This design has the merit of requiring no

rework of the blade tips, and Incorporates only trailing-type dischargers.

The results of the xodel studies conducted on the present program coupled

with experience gained on conventional Jet aircraft flight tests indicates

that the design of Figure 14(c) is not likely to be satisfactory. The

sharp corners on the blade tips will be highly prone to -. rona at voltages

comparable to those required to di!j-_.e liie expected currents fIr'- the

RIM. Attempting to increase the current-handling capability of the de-

sign of Figure 14(c) by adding more trailn dischargers will be only

marginally productive because additional dischargers after the first one

or two add smaller increments of discharging capability (see Figure 42

of Ref. 8). It should be observed at this time that the reason for

using tip-type dischargers is not that they add greatly to the current

discharged. Zqther, they sre 3sed to generate a column of space charge

behind them to increase the corona threshold of the region in which they

are installed, thereby preventing unwanted, noise-producing corona from

occurrlng on the aircraft structure in these regions.

30

L3__ _ -

IV BLADE-C0ATMl JESTS

Evaluation of the adequacy of the candidate rotor-blade coating

syst, %us was curried ou. using the SRI triboelectric charging facility-

shown :in Figure 15. In this :facility, a flow of particulate material

(lycopodium powder) Is directed or-to the surfita of the sample under

BLAST OF AELYCOPOOILWA I~^o

PO RTB E 15 TE TN., 'ID BLADECOATIN- T E CO E

test. Frictional charging resulting from the Impact of the particles

an the test surface charges the surface In the sam way as dust or Ice

crystals would charge the rotor-blade surface. Charging current density

is mneas.ured by monitoring the current arriving on~ the conducting ring

surrounding the target. In~ the tests, the currents ranged fro 2 to

6 IA/f .At 6 xnA/ft , only 100 ft of blade area would be required to

generate the maxium current of 600 pA postulatea for the RIX. Thus

It is felt that the tests were adequately severe. By darkening the

room, it was possible to observe the occurrence of streamer discharges

on the surface of the test sample. The occurrence of streamer discharges

0 31

was also detected by the existence of radio noise pulses. These were

detected using a radio receiver connected to the metal rim ar-,xmd the

periphery of the test sample. The attenuator shown in the receiver

system permitted a rough estimate to be made of the degree of noise

reduction achieved through the application of a particulsr conductive

coating system.

The strength of the triboelectric charging test is that It simulates;

all aspects of the plastic charging and dischargingC processes at the

same time. In addition, there are no wires leading: to power supplies

I and no electrodes attached to the test sample to cast douabt on the

validity of the tests.

Dar Lag the test program, 26 samles (soame were duplicates) of prowis-

Ing coating systems and bare plastic samples were tested in the tribo-

electric charging facility. The samples wore preper&.4 by Boeing Vertol

using materials and techniques acceptable for ultimate use In production.

The details of preparation of the test samples are shown In Figure

16. The simples were prepared by making each successive layer of the

coating system smaller- than the preceding layer to provide definite

access to each of the elements of the test panel. In particular, the

test-peal design allowd for the use of a simple scheme for insta~lling

a current-collecting 'lectrode around tMe panel. This was accomlished

by plscing a strip of copr tape around the On~ so that the tape pro-

traded 1/2-inch onto the anti-abrasion layer. In this way the copper

tape collected current flowing to groumd via the conducting layer as

well. as current discharged via streamers over the top surfcce. The

upper grounded metal rim in Figure 16 was an aluminam frame Installed

over the outside of tne entire structurie to provide a definite form to

the electromagnetic coupling fields. This is Important to assure rea-

sonale repeatability of the streamer nols-e pulse measurements. The

32

GFOUPIOAWPAWAM AWTA

L^YM ."T;MrM

LAYM NTAME

SA2-17

copper tape that t',e rin, did not collect any of the dc charging current.

Tess he hit-igaiity was istecu ran x tom equiaed with t~eaude

huidite an eer TheF to mle weesordIhiFom.o w

daspioto the starof zthe coatis to peit tre stisfryct oroughlye

to friinatlhagg throyg al adcevisible fls.estdincage s 4

eniomn 33uae prto ne b lwhmdt odtoseit

___ In the_ deetadI h rci.Dr1 h cua lcrfcto

tet

high levels of radio noise. The requirements for a SatiSfaCtor7 COatiM a

system are discussed In the Appendix. tl on completion of a dust-blowing

test., the lyeopodium Dowder used as the charging mat-erial was often

attracted to the surface of the nonconductive panel, and formed patterns

snowing the paths taken by streamer discharges originating on the grounded

conducting ris around the test panel and propagating oat to the center

of the panel as shown In Figure 17.

No suc'bdust patterns were observe on any of the satisfactory

conductively coated panels. It should also be noted that there was

never any noise produced by electrification of any of the best conduc-

tively coated panels. The noise-monitoring and recording Instrumntation

-as t armed oc: at the start of each test so that no~se occuurrfrg at the

first electrification could certainly have bendetected.

at description of the blade-,coating samples tsleo Is given in

Table 1. (The film resistivity listed in Table I mas dejermined by

applying two strips of copper tape on opposite edges of the conducting

layer of Figure 16 and measuring the resistance between the two strips

using an c~meter.) The or.,Pran results of th&- tests are summrized In

Table 2. The data are presented in Thble 2 so as to permit comparison

of the effectiveness of various base materials and their effectiveness

In combination with ogercostings of different abrasion-resistant nterials.

The moisiest 1!ples tested were Numbers 19, 20, 25, and 26 (neither of

which Incorporated conductive material In Its formAulation). Accordingly,

the .soise levels generated by these samples were taken as he 0-dB ref-

erence typical of noise generated an an untreated blade, and the noise

level observe on all the other 3auples Is expressed as dB below this

reference level. Several of the multilayer conductive coating systems

were found to be very satisfactory In completely eliminating discharges

on the surfaces of the blades. PartIcularly good were:

34

I__ ___ _ _ ___ __ ___ ___ ___ ___ __ ____-was-__

-~-

L

I

I' Z

-4;00I-

I

4'

00

I07

a020z20a0I-'C

0200000

'C

4o0

'C0

1-0U-00

0'CI-

0

0

I 35 -~

0

-=~~

C _~t.=O -04C3= =-b. - _ _ -=

IM M12 wwdk~vvcowtig-pomy b~c* X* D

nz-camsft.rmemme wmwacmm=Wm jtlw R-2 omt D14p

4 NoD362 coem~e c~ta-opmy, tbe 0-40"-m W?zZ--W3. 7a= )tm mv~ U rM

$,g ~ ~ ~ nu 2aS aMt22u m-wu.1-1o cmZ"m tc

ep~wemmc a~~w. ouw &m pzw "z "at" ACC-MAGS Z9 R

on D-= emoellv cmtncovwz, Uew %403-4u 4- DI_ _ _ __ _ _ __ _ _ _ _gtz "" mluoFc-CWI- Io s

3UI-C-na2f scft- Cit" g (2C)S2 tuawwt me=I.13 aft" Cn s

11 13-2 Gasu t u &W-.~m7 urza "adellAI1I 9.

Coldbr11TV 9 =Xd

7, S - 32-31 cu w mag~ dkw on 4.2 a

U .- Mom w 54"...~ 4..xv ~reo rz~.&N w 39

W2-C-=" ".Pma ~.w3 CCH oue ub3SM"

oom7 "*MI W32st ct" 9v i

m-*"aw " f -

m"-l- ftZ~ al"Geal.ea u..s.r cc"= a31w ftsot4.~

n, CDP on MumTD

=1C-"oo 39Im. CCUmg ou" if a

=1-C-S14 waft PC. ft -

36

£ Ir o m y c f hc!mm

X~om P*-e t- ac a

- I I

ac I: - c

@ac te '3 . -

acm

9- cm*= * IcIc

IDI

-fiw - I- -

3itz~cmcmI~z~aim -e-6 .

1 6 1 1 6 c 1

AcI If

~itzut~z~o *c

16 -

de a

* WVS 10-21 + polytumetba~ + RA-2 (Samples I =Y-4 2).

a MCS 10-21 -f Epoxy Primier + Acrylic Nitrocellulose Lac er

(Samples 9 zad 10).

a Kagva S-R-6 + Epoxy Prlier + Acrylic Nitrocellulose Lzcqzer(SosAies 17 2=d IS).

0 Kazmm 9-8-6 + Epoxy Primer + Folyurethsae (Samples 13 2=d 14).

It Is Luteresting to cote that Smples 1 aid $ are mot as reliably quiet

2s Samples 9 aid 10, although the main differen-ce betveen the two sets

of samples :is that 7 and a harve LA-, ade to the lzeiquer top coat. Oz

the other hand, Samples 1 aid 2j, 2=0oPoratiUW Z%-2, are quieter tU==

Samples 3 and 4., which ha2e the saeformulation without KA-2.

In 2"IYIng the results show In Table li, It should always be borme,

In mind that ami standing coting systems prndsced no detectable noise im

the measuring system usedr and showed no indicaon of charge aec~uatic

or stre~.ermg. It Is possible, therefore, that the moise reductions

acble--vd were sus a-t I I lly Im excess of 40 di.. The effort to skiIn )azd otherwise Improve the messuzrIng system to lzcrease Its dynamic rarge

wins beyon the sop of the presemt conta. Elatll mare carefully com-

trolled measuremts catz be made,, however, It Is safer not t* try split-

ting hairs: Concerning the data of Tiable 2 by arguig that one system Is

only a few dB better than another. Instwead,, It should be conclcoded that

four of the systems (listed aborre) prodcced no detectable streamer noise

umder ffictional charging, while the rest produced noise of greater or

lesser degree.

Tbe results of the streziner-oise calculations, shown In Figures 4

tJ.aagh 7., Indicated that streamr noise am the blades must be reduced

by over 40 dB to bring It down to the daytine-atmospbere leiel. Tbs

there Is no merit In considering paint systeas ttat cannot prorlde this

degree of moise redaction.

To a effort to deterzfrie wvhy] 9*i of the coating systens were raire

satisfactory t'6-x* otbers: It w2s argened th:at o possible mecba--Is leaft-

Ing to tbh- obserreS zoise-redme"ti,~ dIfferemces - Iad be uoleco1ar !fzer-

actioc betweem the cct-Ire sa trate amd the itsslaitftg, abr-siom-

resstan. top coat that affected the i~szlatlzg zrd ToltaTge-breakd'm

cftaracteristics. It was felt that evidezce of thoese imteractiocs a~d

their effects cold be gafred by amasurimC the electrical coadactlylty

of the varioms surfaces. Accordingly, an eaperisiet to weasure the cc-

dotivity of the blade-p~bel comtiag-systew samples was desigxxd.

fle esperiammt used a coaxial resIstamee ceascrIzW probe illmstrated

iIn Figure 19. Acoaxial desigm was chosem because,, with this strzictcre,all currst flow Is cofimed betweem the Irmr and uter codators.

'Ths, accurzte mesee. of resistamce Is posslble ew ner the edgies[ of paels. The dine=oms of the coaxial probe were chosen so that the

condactiwity (In obasI) of my surface could be det-zaizmed ty mltiply-

'. Ing the aneasured resistamee by 4. Im order to obtalm umifom surfacecoctact over the *=tIre probe, soft, coumatimV wimyl pads were applied

to the b~rass costact peds of the probe. These peds had side-to-sile[2 ressees of less tba 1 Cbm.

Iaitial measurmts revealed that several ef the Iae saples

showed a s=rprIs!ly hi.* cowdectIvity at low (1.5 volt) voltages.

Jrertestimg, per."wrmed =wer tbe sa- teperatmre 2=d hbmdity COMn-

ditoms as described earlhi, shams that all of the Iae asles with

thbe US W0-21 base naterial gave hIgh coadctlvzty at low voltages.

Siame the abrastow-resistaxt top coatizigs cm these pazzels are gem-

erally nmoeanctiag,, the materliaLs were saspect and a microscopic exami-Ii matiom of the panels was made. FIX=*e 1.9 *bars a photographb of the

surface of pameI Sample No. 7i wapified 10 ilues. it %,== e seez from

the pbotograpa that the surface Is irrepujlar. A xicroprobe attacbed to

39

~AAD~I

RMtE 18 COAXIAL CO MUCflVI1Y MKOS I~ TO DETEUME THE E6IR1ALCOPIDUMflI1Y OF VARIOUS HMW SLAWE PAXELS

40

FORI-I 11 -SAHC Y-.XL LZ APEW~ M &21ESWATEFAL 1=

a= cmweer ere~ed hatthe sz=Z&7=11es: -%= i= e pwzorz£

weremacc~fetiv %ble ~be mex-tamocs Icps m th s~faw

cocdtiwjl re=w-*= cocbe -ath he ~ft5tes prssce- bcse -

sc~dbv te oexald probe th e 20fl ~esboshe ~ pb th~boe of th

were tmcmciw bemo while e 2 tiz0o mafis the Tsfc wcx':ae

are cc~et the UCmes with1 te 102 bs coat. Vad 2 eresstv~t wahe o

El cna, es to the mI b are of fci wty zt (on esacb paea-

I ~ I shaz be e taa- e~re Is h a brac re of fir uasrelaetlocm beavez te

41

red~ti s~faice ccctB~meiwy=5 th *l s4c rte=cc affb~ded 3y emcf

MLCrIWInf OF PA3ZLS RThm am 21-21 &SM CCIT1

__________I (as) I !!Q;p 2 >7 40 -QG

3,4 34..>4040

so,6 21,201

l11,12 29p 1.912

42

K' I

(k The measured resistivity data also are consistent with a general

description of the surface. To the observer it appears that the panels

with a high conductivity (and good noise reduction) were the roughest

-(and consequently had the greatest exposure of conductive base material).

Another qualitative observation made while the panels were being examined

was that the top-coat materials over the FAS 10-21 base coat were easily

4 scratched, and seemed softer than the other top-coat materials tested.

i IThe remaining panels-those with the Magna 8-8-6 base material, and

the panels with no conductive basa coat-were similarly examined under

a microscope and subjected to a low- and high-voltage conductivity mea-

surements. These panels exhibited no measurable conductivity at low

voltages, so high-voltage measurements were made. The results of these

measurements are shown in Figure 21 together with noise-reduction data

from Table 2. It can be seen from this figure that the data for" three

distinct conductivity groups. It is also evident from the figure that

O there is a high degree of correlation between conductivity and noiseV

* .duction.

Figuie 22 shows a photograph of the surface of Sample 18, magnified

10 times. This panel has an epoxy top coat and the Magna 8-B-6 base.

This surface was typical of all panels with the Magna base coat. It can

be seen that the surface is very smooth, with no irregularities. Tho

surface was moderately hardj resisting easy scratching.

It was calculated from these tests that the Magna 8-B-6 base coat

covered with either the il-P-23377 plus Mil-L-19538, or the Mil-P-23377

plus Mil-C-83286 abrasion-resistant coatings were superior to thc other

coating systems from the standpoint of noise reduction and mechanical

properties of the surface.

0* 43

_ ___ __

o l I I I I I I II

19-2C 8 NOISC R. UMON

o

0 2 0

/4114

40

0 2 4 6 a 10APPUED VOLTAGE - kA-28-23

FIGURE 21 H.GH-VOLTAGE CONIOUC0XTITY OF HLH BLADE PANELS WITH MAGNA8-6-6 BASE MATERIAL

It was further concluded from these tests that the low- or high-

voltage conductivity measurements could be a viable technique for check-

ing noise suppression on the RIl blades In a field enviroment.

Some of the other consideratons Impo-tnt in designing a streamer- "

free surface are discussed In the Appendix.

4 )44

- I _ I_ -I I - -- - _-

FIGURE 22 PHOTOGRAPH OF TYPICAL HLH BLADE PANEL SURFACE WITH MAGNA8-",- COUCiTIVE BASE COAT

45

______- - -- ~ -- ---- ---- ---

V CO0CUSIONS AND RENOMTIONS

An analysis was made to determine the noise to be expected at

typical LF and HF antenna locations on an unmodified HIE helicopter.

This analysis, based on previous work substantiated by flight testing on

conventional aircraft, indicates that noist steming from corona dis-

charges from the blade titm ox from streamer discharges across the plastic

blade surfaces will produce interference levels 40 dB or more abeve day-

time atmospheric noise. The LOAX-D System designer can achieve Improved

system performance until he has reduced his system Input noisc figure to

the atmospheric noise level. From this observation it caa be inferred

that such systems are now, or ultimately will ba, operating at the

atmospheric-noise-level limit. This means that the predicted corona or

streamer noise from the blades will severely degrade the performance of

such a system. In particular, the analysis Indicates that 40 dB of

noise reduction is required to reduce the electrostatica3ly generated

noise levels to the daytime atmospheric level.

Electrostatic-model studies of the KIM helicopter indicate that it

is possible to devise a passive-,ischarger installation for the helicopter

blades that will handle the maximm anticipated discharge current of

600 pA predicted for the MEB helicopter while providing 45 to 60 dB of

corona-nolse reduction. Various d-scharger arrangements on the blades

were considered in light of prebable aerodynamic conditions at the blade

tips. The one recommeaded for use on the IE is based on presently beut

available infomation regarding uaximum expected pressure reductions in

the vortices generated at the corners of the blade tips.

The results of charging tests conducted on candidate conductive-

coating systems for the blades indicated that four of the coating systems

* ift ,go MW

tested were highly successful in eliminating corona discharges on the &

blade surface. The dynamic range of the setup used was such that noise

reductions greater than 40 dB could not be specified quantitatively.

When the four successful coating systems were tested, however, no noise

whatsoever was observed in the test setup. It is recommended that one

of the four successful coating candidates be selected and applied to the

blades of the HIM helicopter. The application of this treatnent will

provide the required 40-drB stre-mer-noise reduction needed to approach

the atmospheric noise lizit.

In connection with the conductive-costing-system tests, it was

found that two systems that were essentially Identical, except for .he

addition of an additive in one of the layers, produced markedly different

noise reduction. It was speculated that the differences might stem frc

differences in the ease with which the top layer of the coatinw system

punctures electrically. In order to pursue this question, and to identify

the "better" of the good panels, a measurement was made of the surface )conductivity of each candidate panel using a special, coaxial, surface-

conductivity probe.

These mersurements revealed a definite correlation between the con-

ductivity and noise suppression afforded by the different coating systems.

The panels with high conductivity showed good noise suppression, wn-ile

the panels wit low conductivity demonstrated poorer noise-suppressing

capabillt is.

A microscopic examination of the panel surfaces shoed that the

panels with the BUS 10-21 base coat had irrelular surfaces with what

appeared to be eruptions of the base-coat material through the top-coat

layer. tince the panels sub-coated with XMana 8-B-6 did not have these

eruptions, but some of them exhibited good noise reduction, It is sur-

gested that the Xagna 8-B-6 base coat is superior to the BUS 10-21

)

because the Magna 8-B-6 appears to resu2t in a tougher coat. So measure-

ments of abrasioa resistance were actually nade during the present program.

however.

These neasurenents also showed that a conductivity neasurement could

easily be a2dpted Cor an "on-site" quality-control technique to determine

the "Ise-suppressing capabilities of an HLH blade while it was wounted

on the helicopter.

I:I

ii

049

I

Appendix

PKUlREWNI S FOR ELECrOICALLT CO.MCIYE BLAE COATINM

1. General

The surfaces of helicopter blades are m2de of immulating plisir-

m.terials. In operation, these surfaces *=quirw electrostatic charge,

which ultimately results In electrical bwkdowns to metallic structures

of the blade. These breakdoms generate radio coise that can disable

counlation and navigation systems on the helicopter. Mae breakdowns

can be avoided by drawing my the charge as rapidly as it arrives.

This can be accomplished by making the plastic sufficiently conEuctive

or by aplyIng a conductive film ove- the plastic blace surface. Often

( 7 ) It is necessary to use a double-layer coating system because the con-

tOucting materizl is too fragile to withstand the abrasion of normal

belicopter operation.

2. Single-Layer Conductive CoatIM

It is of interest to calculate the axim value of the surface

resistance that can be used before the elcctric field at the surface of

the plastic becomes high enouZh to result in air breakdown. Let us

consider a surf.ace an which charging is occurring as s5o0n in Figure A-1.

Chsrging current density J = 50 x 10 - 6 A/ft 2 ft 500 V 10 - 6 Alm 2I arriv-

ing on the surface in question. This causes current to flow along the

surface to tha grounded conducting rim. At a distance r fzo. the center,

we w i1 have a total current cross!ng the boundary of

I(r) J r; r2 (A-l)

53

____ ____ ___ ____ ___ __ _ ____ ____ ___--il

FIGUIRE A-1 CU.RBET FLOW IN SURFACE CWNQUCnNG FILM

The ccrrent per unit distance Z arroand the cIrcunference of this b~andry

is

(r) jC..r2

L 2r)

=j r/2 .(A-2)

C

The voltage drop this current generates In the :Lnffnitesiaal distance

dr is

dy.=V r -Rdr (A-3)L

wbere R Is the surface resistivity :in obs per square. By definitioc,

the electTrfc-field Intensity caused by this current flow is, fa~ Eq.

ECO = R(A-4)dr £

54

ASubstituting Eq. (A-2) into Eq. (A-4) we obtain

E(r) = J r R/2 . (A-5)c

Solvig Eq. (A-5) for 2

2E(-S- . A-6)~Jr

If we specify the mnxt.m, field Imtensity we can tolerate before air

breakdown occurs over the surface of the blade and define the charging-

Scurreat density, E q. (A-6) defines the mxim perzissible -al?- of smr-

face resistivity.

Air breakiown occurs at sea-level pressure 7e E = 3 x 10'. Let

us asse that r = 0.5 a (a rather large surface). Recalling that we

said J = 500 V 10 - 6 A/z 2 , s s s l l m s m o . A - )

~we obtain

3 let

500W 10'(0.5)

'II

To allow for -ome safety margin, let us say that we require K = 1000

uegobas per square.

It is of interest to see what this surface resistivity Implies re-

garding the bulk resistivity of the material compromIsing the conducting

surface film. Th surface resistivity is related to the bulk resistivlty

by

a =(A-7)t

10

7=a

There t Is the film thickmess. Ass~wing a fiUr thicklaess of t = 0.001

Irsch = 2.5 vr 10 -3a, 2zd 3 = 10 10obzs per square, we find from Eq. (A-7)

that the bulk resistivity required to the -surface naterial I,

1 10 (2.5 x10 -34

=2..5 w 0 oba --,v

3- Double-Layer Conuctive Coating

In considering the possible optiou for belicopter-blade treatmet

It Is evidect that oLc. has the cho!.ce of wail vn abrasIom-resIstant

cooductive coatiog, or of applying a more fragile coating and corering

:it uith a thin layer of transparent abrse-resIstt material that

either codcts or breaks doe . at 1,.,w voltage levels, to the conducting

layer underneath. Let us xor cousidier the two-1ayer system In wkicb ae

# ~relatIvely fragile condcting film Is applied over t1?e blade surface

and this Is them cowe'-ed with 2 more abrasics-resistazt layer. The outer

layer wast be carefully chosen. If It Is not, the systes will not func-

tCO Properly.

Ideafly, If the outer ant-abrasiom coating material Is a good ins=la-

tor, It should be sufficiently Imperfect that reas.-cabVy closely spaced

holes exist bet.ee the outside of the coating and the conducting layer

underneath. A thin film that is ]Likely to bare voids appears to be a

good first choice. Less energy I-- required to pancture a this film, and a

thin film when punctured probably wmld suffer less damage and gemerate

less radio noise than would a thick layer of aaterial.

Instead of assuming that the outer anti-abrasiom layer Is a perfect

I-sulator,%.it is of Interest also to consider the case of an amti-abrasioni

layer with some bulk elecltrical cooductivity. Surface charge will still

5'

(7 acemurlat-7 c the oatsidc suzrface, amd a b1Jgh electric field uxlll exist

Isde the cater layer, but cu yte %~III have smme cmar et Zfow 'hragb

t2* top layer to tba ~ fla Let as cansIder a reioo of anti-

*abrasion mat,:ei* "vawig a s=zrf==e =re-- A 2--e a tbiek=-ess L - The evarret~f

I arri-rime am this area Is

I1= JA (A-6)

wbere J Is the surf se-ckxMzia-c-rret, de~sI ty. The rw 3-!sta--ce R fro

the top -cr-fzce Co the condoctin film is gixem by

I wh~tere 9 is the bulk resIstivity of the ai-bzonaezmaterial -

The vnltge drop V acxoss the c tigamti-abrasiom layer Is girec-flO _by z's lzu.,

W~e want to make certain that thin v'oltzge appearimg across the azti-

Abrasion layer Ila" not exced te dielectrIc stre t E of tbe(il

Substituting ras. (A-8) and (A-9) Into Eq. (A-0). and them substituting

I Erq. (A-10) Into Eq. (A-11) ve find*

PLE=JA- . (A-12)

A L

* 57

Solving Eq. (A-12) for the bulk re3istirity we obtain

E

which tells us the bilk resistivity needed to perult the aztic~pated

chargim-ccrreit density J to flow throgb the a=tl-abraslaa cvating

withoct exceeding the dielectric stremgth of the coating. (The foregoing

argume-ts presume that tie anti-abrasiou layer is very th1- so that all

Of the fields prodced by the statIc clharging are norzal to the -,71aze of

the codctIng film caderx-eath) -

It will be Interesting to use Eq. (A-13) to calculate a typical

vzae of bulk resistIvity rejuired to met our criterioc cf alleviag

the car aex to flow throwgh tie azti-abrasioa sheet without pwnct~ring

It. In the fqlght tests descr~ted Im Itefs. 6 thr 9. It was fod

that surface charging curets cau reach valuies as higb as 50 pA~f t 2

-8 2so that J =5.4 X 10 Alczi . Let vs asme that the azti-abrastom

layer Isui of so e terial baving ,w dielectic strezgth stliar to

that of a piece of glass I = thick for vftc~a E =2.5 x 10 5to

45.5 x eO V/ca -I Substitaticc these nntberm into Eq. (A-13) we Sind

0 = 4.6 x 101 to 10 13ohm ca. These x-esistIvIties are several orders

e-f magnitude lewer than t~oe publisbed for =my of the moern plastic

materials used In fo-.n=tIng paints., accordingly It will be necessary

to either choose the palnce vterfal carefully or to Iced the palmt with

coaductire materlaa.

55

1. B_. G. B~cke, Olrecipitztiom Static fttezwitrece - roc. lEE, rot.27,ySo. 5 MKay 1_939).

2. 3t. C. Ayers 2ae_ 3. 0. Jarrard-, "Atrcraft Priec1pltztiOQ StaticlIrestizatiom, Coctract v 33 -10 SC-70, Tr=s -*orid Airlin~es,

3. 3. G== et a1.-* "Amay-XzXz7 Precipitattoo Static Pro~ect,'Proc. inE Vol. 34, Nos. 4 and 5 (L946).

Report 37, Comtract AT 19(604)-26G, SKI Project 591, tnfr

6. It. L. Tazr and J. X. Xauiler, "kwffo Noise Generated orm Air-

iicraft a y nts," Fena. Report, Coetract A T1(0)-6 IProject 126,Stnor7,sac Istitute, Ktolrark, aif.c

~~ii X~~~(eptembear C!"if. (e~br5)

8. J. X. lawnrad .E Snicz, A td f Precp~tatipSatcNos Ch rginoatioinoAroo-Grate Iz~yAtennas," ee. Rerat , ?Comtac 33,TeF

Report SR P3 Curo T3(0)35,~1P.ject 21499, StastfordRearhnsic,Reseo ar , Cattue (ecembeark Ma7i. (p1 93 D109

9. It. L. lraneand J.F e. L.wrcz Tawr, t-ando G.agi andiesbereo*-merted Itet fec n iues fo PreelpFta336, ttcb

Report)-723, CotatA 9(-USI Prject 24-0, StafordRearhIstteRsacItttMenlo Park, Calif. (April 11962), AD261 029.

90J .lzrcZ .Tne R .TneadG .Kles*a~lpa an S.Ko ~biv frPeiiainSai

Inerernc RdutinM SD714239 Fna RpotCotrctA__________fl _____ Project_2__,______________________ Jt

Xe~~~~~~~~ao~ri~ Pak Wlf JzayL6) D-7 W

1 . L. T r w-a J. E. ImaevIcz, -A- A=Iys"s of Cdr=GcrMtdtoterfere~e In- A-rc~ft,w Poe. 1=. Vol. 12. No. 1, Fp. 44-52(Jazzary MO4)

IE J. E. Sz-ricz *= 3. L-. Taer. wSoue Teci-gqes for the Ef~w;tioe of Eoxroea Disc-Bzre 36lse I= Airer ft A~tei=s~ -

Pr*C. 1=-E, Vol. 52, X-6. 1, pp. 53-454 (Ja-- L956;).

M2 lftzssve X=Ul F! Id Disc=res for CE-U4 Afferft, w* ESZ E 1 te=So. 26S, V.S. Army Viet-., LE4= Blrhb, teux-*1c of Tret~r. (T).

13. X. C. Beeber, "pCM-54 St~tic Mscrge Test Progr=~ a1,3=tio ofD_%ascie~ces ActIve EW 2=d Gr~ger Passive P-Stat Disd'=rewrs, -DCZ-299, Dyv-ascIe~cas C~cp~rTatLO (ScIeztIfIc Syste tss iso=)3lue Bell, P6.--- 0.1 AW-2 L969).-

14. X- C. Decler, '"Itvst!s~atIo of CZ-5&A E1ectrostatic CbhTrig =dof Active Eltctx',statIc D-csc-2arer C3zi11,0ics. V. S. A =7Awtat= NatC1ria Libzaortes. Sort Ecszls,. rfrgiinia (JZ=:=y

15. 3. X. rslexa C al.,* "bw~v. = CM-54 (FnyIn Cra-e) Electra-statlc DIs.bzr Evaltca, M Teccl Zea1- M=E-31W, .. S.Army E Je LcaIcs Ccaa, Ft. Zmmb --. J1. "1=211 L'99).

16. J. X. Y.znewz~ awd . Ga. LvAX2s, "kectr1c D!$C-22 System,FlmaL '~prCO@?Fact SRI~77~ P=~Ject M57, Stazfozd

ree~r~ mtitz, sZI~O Part, Calif. (Yz'rbr 17

17. D. G. Dougl=aas j.. X. Xmvic=, "".Ub~t Test Erslmtiou ofBellcogeter Ca-Em-Nm1zlg Systins--PassIve Staic-:lectrIcityDraiaae,- Cestzact DAO-1C$3,O.-6tr No. CLF-72xg SalPro~ect 254-0, Stamford Research lastittte, Neulo Paxk, Catif.

/ (September 19-M,

15S. J. T. BoUjicf, '"A*teinas For Airboro.a ADF Systems" Fi=1 Mep.%.t,Tast 11, Cctract AS-33(C-IS)413, Stafwrd Reseac 1rntItzte,3sezio Part, Cal-of. (,xv!y 1350).

19. 3*feresee Data For Kwglzbeer~s 4M~ W., V. 762 (Icter-atlocalTe-I.epbome =bd Telegrapb Corpo-atioc, New Tork, ;--T., MG5).

so