Main rotor wake - tail rotor interaction

Paper No. 25

MAIN ROTOR 1_{1/AKE/TAIL ROTOR INTERACTION}

•

John

VI.

Leverton,

J.S.

Pollard, C.R. :iills

Y/estland Helicopters Ltd., Yeovil, Somerset.

1. INTRODUCTION

During the early stages of the flight programme on the Lynx it was noticed

that the Lynx generated a distinctive noise which can be best described as a deep

thro~ted ~urbling'

noise. The burble noise could be heard over and above normal

helicopter noise from the main rotor, engines, tail rotor, etc. and was

consider-ed to be subjectively aPnoying.

Comparisons of the Lynx external noise levels

with those of other helicopters showed that the Lynx was no noisier than

heli-copters of comparable size and it >·ias decided, therefore, to conduct a detailed investigation into the ap~:arently unusual noise characteristics of this heli-copter.

Fro~

listening tests made during the early flying

it

was found that the

burble noise r:ras not present during the b.over condition, but was very noticeable

during the a"proach of a flyover, BSJ:BCially at relati,:ely large distances from

the observer. After passing by the observer this noise vras no longer disting-uishable. It 'das :wt certaitt if h::.e origin of the burble noise ;.1as actually on tr..e helicopter or if it 'll2S due to an interaction some distance away from the

helicopter of individual ~elicopter ncise sources. Also it was not clear if the noise 'Nas associated ·,.;it~, the rotors or ~vit:--, the gearbox and at one stage

credibility '.'las siven tc.; the 12 tter ;-ossi bili ty since the main noise source was :r_::.rcminent around 1 k·~z and the gearbox meshing frequencies occurred in this region. :=n s.dditi.::;n to ge2.rbox noise excitations, other possible generating mechanisms ',·,,ere tl:e interaction of the rotor noise sign:~ls with themselves or v1ith the gearbox noise givi!';g rise to a modulation effect. In order to clarify the

situation, therefore, it was decided to measure the external noise of the Lynx in flight both on the ground and at positions on the helicopter structure.

-T·his paper describes the theoretical and experimental studies that were

carried out to determine the nature of the burble noise ( 1).

t!uch of the

~<ork

1-.ra~ experimental but since some of the analysis techniques were not fully under-stood, it Has necessary 1:D assist the comparison by considering the theoretical approach and developing a computer program for the Fourier analysis of periodic

signals.

Based on the findings of this •,;ork a further theoretical study (2) was

undertaken to assess the effect of reversing the direction of rotation of the

tail rotor and recently the opportunity Has taken to repeat the noise

measure-ments during flight trials on a Lynx helicopter fitted «ith a reversed tail

rotor (3).

2, STANTik1D TAIL ROTOR - EXPERIMENTAL STUDY

The experimental flight programme consisted of hovers and level flight

flyovers over a ground array of microphones with three microphones mounted

externally on the helicopter structure; these were on the transmission decking

around the gearbox, on the undercarriage skid and on the tail cone corresponding

to approx.

1t

diameters from the centre of the tail rotor (see Figure

1).

The preliminary analysis of the noise recordings at the ground positions

showed that the Lynx external noise levels compared quite favourably with those

of other aircraft in that the Lynx does not exhibit unusually high noise levels,

The burble noise was most noticeable on the tail mounted microphone and the

ground microphone positions during the flyover conditions but not the hover conditions. The burble noise could be heard as a rise and fall in level versus

time and comparisons of the fre-C!.:_:ency analyses for the parts of the recording with and 1-rithout burble noise showed th::,t the former analysis was richer in

harmonics of the tail rotor blade passing frequency 4T (4

=

no. of tail rotor

blades and T

=

tail rotor speed in cycles/sec) and contained a large number of

unidentifiable discrete frequency peaks in the 500- 1500Hz frequency region.

Further analyses on a Spectral Dynamics

.)D

301 C '\eal Ti!!le Analyser showed that

the frequency spectrum of the tail microphone recording :;as dominated by the tail

rotor harmonics 1 2T, 16T, 20T, 24T, etc. together :;i th a large number of

prominent peaks occurring at intervals of 4R (main rotor bladP. passing frequency)

after each harmonic. The latter appeared to be given only by the frequencies

n4T + m4R Hhere m = 1 to 5, but there may :Jave been more peaks at n4T - m4R

frequencies buried beneath the ::;eneral ncise level. Similar res-:11 ts ,,,ere

obtaiYled for the -round microphone 1,iher.. a!1_alysing those parts of the recordir:.g

for which burble noise could be heard. l'he 4R discretes 'dere not preser:.t ,..,::,en

the burble noise Nas no lo::-!.ger audible c:n tape.

'i}1e zrou...YJ.d :;:icrophone recordings 'dere subject to a Doppler ~3:-:ift i;,

frequency dependent on the fonrard flig~~t speed of the helicopter a:1.d t~;e

relative angle bet':!een the flight pat~ a~:.d tb..e liYJ.e j.oining the observer to ::::e

source. This had to be taken int'J account :·rhen comparing the results ,~.f t~~e

ground r.;icrophone recordings 'dith t:r:ose of -tLe tail !:licrophone.

It appeared from the results that the :J.oice si,;r.al around 1 OCOEz (.::~d to

a lesser extent 2000Hz) ·.-ras bei!lg modulated by other sig;:als aYJ.d it ·.-122. dec:ded

to identify these r:od;.;,lati:::::: frequencies filterir.g ')Ut the sif;:.al beloH .SOO:·::z

«n~1 a"C'fe jl~t.j,.,. (\i o rr::.MOVl·r.~ th"-'t p"r+ ('f" -t-';,8, "1."'>"'·'"'1 '·/hich 1'"' ~,·+c::l",-1,., ~.-::. Qrr _

_ ,,v. "" .!\...: • .:.. -•"='• ~... .1<::> ~-;,<,.<. _,ct_"' ~·-.. "·· ~· b·-0. ... ~ ·---~l<-~ u.~ li~-

'-''~'-1 OOO~Iz frequer.cy ran,z:e), full :·rave rectifying- and t~:sn re-a:,alysing. ..'t.lt:·, . ..,.

the process of rectifics.tio:: introduces ::deli

t

:.anal frequency peaks, it dc·eS,

:-:.o·.-:ever, 8!1.ha!1ce the amplitudes of the ::!Od-..:.l-'3. t ior. signals ar.d, t~-:.er>Sfors, :-:12kes

: e ~odulaticn frequencies easier to detect.

FL:;:.1re 2 s!"':ows a!1 instantan.eous spectr..tr::. ( 0-500Ez freq'lency .ra~g·e .: for +:;}:e :ail :::cmnted ~icrophone signal filtered ·'.'it the 1 k.'-:z octave bared f'~!:d full

':rave

rectified. .l.};_e levels of the discretes at tl:e :ncduJa tio;: freqwo::.cies ::2·~,re ·:~eer:. incre.e.sec). by the rectific2 tion process 2!'.d t!">.e freque:,cies 1re 9.ccur".:. te ly :;i·:c:: by !'AT ::::m4?. -:dhere m :::::: 0, 1, 2, 3, 4 or 5 and n deter::;ir:es tr.e :·:e.rr.:onic 8f 4T. ?}"";e positions of the discretes ar.d tl'.e sl:ape of tl:e spectra are consister.t \·:ith the Fourier analysis of impulsi'Je sig!lals and not r.1.odulated si!"lusoidal sig!"lals.

':.'ime history analyses r.-rere obtained or.. a standard frequenc.J" a::~al.J"ser al:'!.d

level recorder and :?igure

3

sho~·rs the d.BA and filtered 11C·:z octave band time

histories for the flyover condition recorded O!". the tail microphone. It can be

seen fror:1 the results (especially the filtered 1kHz octave band trace) fu ~~t tb.e

~ignal. is beinii: mo~ulased b~"r anoth~r sign~ of period LR and to.~ les:er extent oy a s1gnal of per1od LR" .1. .. e per1od of

4,l correspondS to the t1me ·;J.r,en one

blade of the tail rotor·'" coincides Hi th any-'blade of the :nain rotor.

In order tc examin.e the burble n::::·ise signal in more detail, pressure

c~r:1pli tude 'peak-to-peak' time histories ':rere obtained Hi th a U-V recorder and

Fii.,--v..re 4 shaHs a dBA time history for tr.e ;round :nicrophone. This is not a true

dBA Heighted analysis since i t was r.ecessary to play back the recorded sigr.al at

f

of its recorded speed in order to provide a suitable time scale. It is clear

t::.at the sigr1.al co!lsists of a r..1.unber of inpulses 11hich form fairly ':tell defined

and repeatable groups of

3,

4 or 5 impulses. Also although the nu...rnber of

positive and !1egative peaks in tte individ'J.al impulses varies from 2 .to 5 there

is a predomir~nce of

3

peak L~pulses. The i~pulses follow closely a sir..usoidal

accounting for the fact that burble noise is subjectively most noticeable around

this frequency.

Narrowband filtered TJ-V tir:J.e histories, ho;·!ever, \·lere subject to

limit-ations sirtee the pulse si~nal '.'ias ah~ays affected to sol!le extent by the

filter-ing process befilter-ing used. ..~.he fi 1 te:r 1 _{rise time' must be short cor:J.pared to the}

typical impulse duration and since the impulse frequency 1_t!US about 1k:Ez tl',is

meant ideally a filter bandHidth greater than 4k.B:z. Such a filter, ho1;.rever,

\·rould not be able to detect the impulses buried in the normal broadbaEd

helicoc-ter noise. Ttus analyses obtained.;·:ith a 100Hz hetrodyne crystal filter '{iere ·

affected by the filter rise time as shoi'm in ?igure 5 and closely rer;.emble the

filter :.:!'.2..po cho.n:wteri:::;tics. ?he+ octave R-C type filter results, on t}:e

other hand, (FigtJ.re 5) Here not -'lffected so !:luch by the filter shane but the:r do

have a 'spiky' appearance \·lhich is due to a 'ringing' (oscillating) effect caused by the impulses.

It is :::;till pc;3:::i'olc to ')bt.o.ln information on ~lur::1.tions from such traces, :-:c:'::ever. S:.i r:lcas,J.rin.~; tl':c ti:ne intervals 'oet·::cen eac;. impulse and bet'.·!ee:, ".'C~c:-: .;r':,up Df :.:::pulses, it -... ,~:; fG'J.nd ':>.·--.t the repetition frequency -:>f a;; i::1pulsc :·r::o3

-;~-: c~voro.co 14-0~~z ar:d t):e ~~epetitie;:" frequency of a group cf i:::1pulses :·E"L3 2.:: t!-.'2

.-_·.rder ·:.:"' 21:-Iz. ,',_lt~_oug!: t::.e:3e frequencies are only approxir:J.ate the~l appear tc

c0rr:;:pond t--~ 4~ + 4-R :1:,d !~R resrcctively.

-;:I;c; ::-:,:.1ir: freq:;.or.cies c.:-:iscci<Ited ;;rith tt:e bu.rble :::.oise are the l!:2p1J.lse

froJ_';o:::c:/, t:~.o i!:"."8ulsc rcpeti tier:. frequenc:r a:-:.d t}:e Ere::- up repetition freq1JJ::-.c:l.

·.<ri~-:.:-: f::::·r:r.~:rd flL:~~::t ··:·: e.b-.:::~tt · ~5 kn.ot3 ·ttit:: the- tail rotor operc~tir.g i::-1 tl:e :·.::;:r-:::al :~ir·:cti·-::-:, E:.e.::e frer;_:_te:~cies :tr'3 a;:proximatelJ :000 to 1200:-~z, 140Ez a~d ~~J::z rs:;;pectivsly. -~!~e :naxi::rv .. :n repetit-:·Jn rate at Hhich individual impul3c-s ca::-.

:>: .l.:;i:>c-cted .:-;-.:.'ojectivelJ is iY?. t::o :)rder

o:

25 per .second ar~d t:~_us tte i~divid';al

~~!:~l~0s (:f rspetiti:!: frca~e~cy 14Qiz) q~s not ~~ard by the observer. ~he 21~z -::r .::.~. re;-etiti::·:; rate is t 'oeloH t!-:.e audible de-tectability limit E~r:.d he!lce ·:rill be ::s·.;;_rd. ,~::u.:: t::-:: ccr.;bined ,::;ffect from 8. subjective point cf vio1v is a -<..-.r.Y". +-~,yo,~r1 ... 0d hJ.-rbl-i.,..._:· c:.·~-"Pd '·f}'-;"'r' con""i<cts of n c~"''ri""r '"'igral "'rowd 1'k,:"z

(

~.;.:~"v-~:v~;,:o ,.,~.,...--r::-~~~-~)~~-~-:'"'','.--~'--- .-~·..., '-' ~"'

'C.;

'r'-'-~-C' :;~,-,,~ ~--:.; ~ i -

.~--~---::--._.~.1..;;~ ! ... vqlA---C,,.; -~e ... ,::- ... vau.l-ted :::.v a 1r-.::qu~::.~cJ ,).l a,___._.._,~_t c:::L"z. _,_t l.::> ·t:-:is ::·.-:-. .i·_~l:::.ti·:-~-:. ·::f:>::ct ·.-thic~~ is t::c'J.:;~:t to acco:_mt for t~e b:.ll''ole r:ci::oe 'eeL;:.- zo eli.::. ti::.cti "'/8 :_:_;_d 3"'---'b j ccti ve ly anr:.oying •

.

~.

It ;ro.:J clear from the experimental results tb.2t t~:e burble ::cise :·;-23 ;,ot

(li!'ectly associated ·.-ii th normal helicopter noise sources a:.d i~ order to

deter-~:i:--,e if tr~e burble noise consisted of impulsive noise signals :::.risi:,E fr:)n an interaction effect beb·:een the main rotor ':take and the tail rotor, it ·::as

decided to exa'Tiine the ~ .. .rake patterns produced by the Lynx :::ain rotor. Jn the

Lj--:--,x helicopter the tail rotor rotates in an anticlocklvise direction ard in

fori·rard fligl:t the tail rotor blades cut thr·-:''J.gh the Hake produced by t~:.e :::ain

r:::tcr as shovm in Figure 6. During hover, hmvever, the main rotor :;·:ake is sted

;~r.,my fron the tail rotor. In fonmrd flight each vortex from a main rotor blade enters tl:e tail rotor disc and, depending on the relative positions of the vortex

2~d the tail rotor blades, it trill intersect Hith several blades before leaving

the di3c. Since there are 4 tail rotor blades, it is usual for there to be 4

intersections per vortex, although

3

or 5 intersections are also possible. After

the first vor~ex has passed through the disc there will be a certain time delay,

dependent on

Jn

before the second vortex reaches the disc. Thus every time a

vortex passes ·through the tail rotor there will be a group of 4 or 5

inter-sections follo11ed by a time delay before the next group. Figure 7 shows on a

time scale the calculated intervals between each intersection (represented by a straight line) and between each group of intersections assuming that at time

rotor disc. After applying a time delay correction to account for the relative

positions of the microphones e.nd the tail rotor, it w."?..s found the.t the

inter-section frequency or impulse repetition frequencJ 'das about 140~~z and the gro·.1p

frequency was about 21 Ez. i'hese figtLres agree :·rell ':lith the meas1J.Ted data. As

a vortex passes tl'.:.rough the tail rotor disc it t·lill intersect '.-rith each blade at

different parts of the blade dependir;.g 0:: the blade azi:nuth position. An

inter-section ::~t a blade tip t·lill produce a ;Tes.ter impulse tha..>: one further down the

blade (ot-.ring to relative velocities of blade and vortex) and tl:.us it is nossible

to assess each impulse on an e.mpli tude scale. r: Figu.re 7 sho\·rs !"low the amPlitudes

vary ~ .. dth time with an appr0Aimatc: period of t-.~~ These results can only be

taken as a general guide, hm·rever. ··

As a blade cuts a tip vortex it is subjected to ar, impulsive blade

loading having positive and n_~gative cl:aracteristics and t::.is sey;_erates a bang

or impulsive pressv.re i·rave. .;.'he '..ridt~: of t:-:e ir.:pulse a!1d its cr.aracteristic

frequency are depe!1dent r:'.ai:'.ly )~ ~.:!-:.e velocit,y pr0file c;f tb? tip vortex, as

seen b,y the blade, u~1d :::e rslc-ltive cli:::2::--:sicr.s of tt:e blade c::ord and tip v:Jrtex. In order to detert":'ciY'£ t!-:<:.' s::.ape Jf tl:.e r ·81_{.\ltin..g acou3tic ir:-.p'...tlse it is :v:;cessary}

to know the loading o:. t!:e :ail rotc;r blc.de ;·J~.ich ir.:. turn is deper.dent on t~e

velocity profile of the ti~;. vortex. -='l:e blr::.de loading 'd2S calculated to a

fair approximation by o_G.apti:r__g a :net!:.od developed for !:lain rot:::.r bl';.de/!'::ain rotor tip vortex interactiGn ·:Jr bl<"~de sla:; (4). '.:.'~e blade \·ras cc:::.3idered to ir-;tersect t~e vortex \·Ii th the .:;pan of :~-,e bl.ctde ·:. t rL?,~:t .::-,r.::;les to the tip vortex ~:r'.d not parallel to t!;.e vortex: :l.S fc.r t::c: blc;.de slap ca:::;e. c~':-,e vortex velocitJ profile ~-ras taken fror.1 :::.easu.re:::.e~1ts c::

e.

·.-.·e.:·::::::ez rstor as ~.:rrr.x d2ta ;;J.s n-:;t ·:.vn.ilable a::,d t~ic i:r .. f'orns.tion '.-J.e.s fed into :,_ cor.,~~ut·:r ~·ro:;ro.:-:: de''l"elsped 2t .. '~~cut?:£:.;:!ptor.

~~iversity togetter ~itt tl:e fcr~urd speed of t~e telic~pter, the ta~l rat~r tip

:::;peed and the relati7e Jista~ces

and 2!:cles

bet~ee~

the

tail ~icr~pta~e ~~d ~he

::.r:.

teraction path. ·.::':---.e ::-:.co-~L~ tic :_;r'C...-:::;:;·_J:'•:: :n;·li L~de t .:.r.,r:: plc·4

• :;,l: tai::.ed frc:::', t!: e

'cl2.de loadir.g is :::::u.-r::: ir. ~-;--L:,:'J..ro S. ~·ras predicted aEd ·.-r::.e;;. t::.i.::; :-r::~s ?·.:'~J.l'ie::'

2.l'ound 1200Hz.

cl::.~r.J.ct:::ristic 3 pr:e..k imp-J.L:e respcnse

··~:~_:lly:-~.ed it ;;s.vs G. 2pBctr'J.::1. pr:a!:i~:i?'

1he arr.plit'J.de cf the t:·.ircl ~·e::tk (?i~;~u:r:: a) is, l:0· .. 'cver, :·ig~ificantly lo;·:er t~--:ar.. i11dicated by the '_Y-V ti::o.e ::.isL:-rie:::: .-:;,; .. -;, 2.lt!: t~-. -,·re is ::.o real explar:ation fer t:-:is, it i::::; tho'J.,;ht t-:. te ::-<:::::c-ci::;.tc<d ·.-:it!:. the blade loe~ding, since incre2si~g tte loadinc decay ro.te e::'.1:.E:""lces t!"".·:: pe2k:,·· r:c..-1::-:.J.rc of t~~e acoustic ir::pulse (see dotted line on ?igure 8).

In. order to link the experimental ar:.d theoretical rGo3-.<l ts .s.r:d to

under-st:al:d. the processes of rectific:::-,tio:J. and r.:.odulation, a computer procram ~.;as

developed for the ?::::.urier anal;y~sis ::;f sL:rr .. 2ls. UnfortTr1a tely there Here several

limitations it: the program...•rte CHing to the fact th2,t it Has difficult to simulate

the burble noise signal. It ~·ras possible, however, to co:.n,firm tte narrowband

frequency &.nalysis results it~ th_:~t rectificati·Jn displaces the rnodula ting

frequency from the rest of tr.e spectrum and r;akes it easier to detect. Also it

confirmed thst amplitude modul1tion of an impulse signal by a sine wave gives a

lobe sf1ape spectrum with sidebands about the individual discretes in. each lobe.

Since the burble noise characteristics are dependent on the direction of rotation of the tail rotor, reversi:J.G the tail rotor should siP'nificantly reduce the nagnitudes of the impulses and the subjective effect of the interaction noise.

Using the theoretical model discussed previously, calculations >~ere made for different forwc,rd speeds ranging from 60 knots to 200 knots olith the tail rotor rotatin,; in both clockwise and anticlockwise directions, For the standard tail

rotor direction the impulse repetition frequency increased with increasing

forward speed but 'i·lhen the tail rotor 'das reversed the freauency decreased \'lith

increasing forivard speed. Simil3Ily, as shown in Figure 9,~ the impulse

t·Ihile a fairly rapid decr2ase in noise level occurred N!:e:: the tail rc·tor :·ras reversed. _-':..t 130 knots fonrr1rd ~;peed t!'£ esti:r:1s.ted red1.tcti-::n i::: i:npulse peak

c-·'\''"'·a' n~eoc, lc · l ' c b "t l7rft:: ("'e"' '-"1:·:'"''"8 9) ~nri -·,~nne+>.·. l.r'l"'Ulce "'·::.airs

. _ _.vu.. !_~ ..,.;;;.ure <:.,V8 d9.;:,;, a 0;..;. .-~...__~ \"' e --·_.ve. ---'"' ._,.J.. •• -...., v.-- 41.1-' ,) ₁~<:: ••

c~re about 15/20d.B above the €eneral ~:elicoptcr ::.·Ji:::>e i::: the rwr!T:al ccnfigur-::-~tion,

r'::versing the t2il rotor :·rould sean that tl:e burble noise s!wuld be ":ffectively

::J.e.::>ked by tl-:e !:oise fror:1 the r.1air: rotor, enci~'le:--;, etc. particula..rlJ at hL;h for:·:c:-trd speocir:>. ?hu3 t!--.c bUrbL? n·:·L::e s::.c~ld no lor~er be distirlg'J.isf':.able fror:: ~ subjective point of vic~.

~-.eliccnt0~ fi:ted ~ith t2k0~ ~~t·- ~icr~p;--.~~s: ~~tsi :~- P~- ~o~nd n~d 0~ t~0 ~ail b~~~ ~t t~e 2ane

ti :r.:::~ ~:::.: f·)r -:;:':: ::>t::-::::.1:-:.:td ·:::..il r-::tor. t~lthougi~ t!:ere ·.-IGrs a f9'if ur-.~.explai::~ed ·'l ii'fore::c<_;;:·~ ·.i;, ·_.:,::··~::. ~; :1c .:l'O'J.r:U ~::icr·.J I-'!:::~:.e ar;.d ta,il :::.c·1n ted ::-.icrop!:.':J::~e l''C>Sul ts,

::istsris:~

. :·_r~r'- ·::e:.::- :::'):::e i:.Gic-::ti':'·-- )f puL:e

::ry_;_pi:-".;:_;-·-"~,-. :-_i:::;tori'~-::: o.t ~!-:e J.u:l·<:·r f'Jr:-rc~rcl 2~eed3 -.. i:::.~:.

"·· ·.:..;:r::-·.-.l'0'.l .. ; -+-- •·>.- .. :~:il :c·-::t::r ::.-::-i:~·-:: :-.•.::-:.3 :::c.~·t :·.::,tico8..Gle i:: t~_c:;; 2k~z bE_:_r.d. _·-..;.::_··c-

·c

::>.::·.-:s ,:::c ;y...::::: -:t~:'l 2l-::-;_z :ct:J..'/C >o.~:l +.:~-:: List;Jris-:3 fer the ~ 15 !::::.ot

L _;___, e:: ·:-;;r ~-l . :!'' - .. t-'-· · . ' . c ·c ::.··:.::.c ::·_;.;:: .... ,._, · ::.~-:rcr' bl~,ds field. !-.lt·::;1..:.gl:

::; i:-:liL:..r for botl:

rsvsrsed :ail ror:r

tnil r-:.-:~)r

--_;J:'0 11 t~·-'..: :--:vcr:;ei tL:.il ro-tor :::t 130 ~-::Ect::; i2 q·_:.ietor ··:t 2000f-t C:~· 'iCd.3:~ ::.::_d ~::..:·f-:-r·<::::c,? -:::--T:-2::-~r:::: t- i~:cr-:::J.:::e C\'C:r~ f"J.rt::-::r ~-:it~: di.:::tD.::cc. .]'.J-:.:: pri:Jr to

:~,:::; fl~.'C:'l·?::~ ~-:::J..k, l1o·::cvsr, t!:o re\.r:::r~ed tsil r'Jtor ex):i'oits :--:ii{:.:?r ::::-ize 1::-vels. ,, ::~:.il :::icropl:-:::-:~:.e rs::;·_:.l-1:.:: :3U--~~·,;est tl::J.t the tail rotor noise ls'Iels i:--: t::·? far f:,.::ld "::--:.-_:.Ll l;c :::i::~iL:r .s.~-:d t::u:--:; it is cc~sidered th::-~t t~-:e l:~r;-e differe::.ces

:-:-J.:>2..3ured i:.~· tl:.:: __ :rCJ;J..:r;.j ::.icrcp!'lor.e c.re due to c!-:a:::.ges in t:---:e directivity and source :~·':re::,i~~tl; sf t::e tcdl rotor ::oi:_,e. ~[arror.·rbar.d freq-~e::.cy s.::,e.lysis t)f both ::icro-~~:--:cr:e posit::.ons c:)r',fir::'.eci. t!':e abse!lce of burble noise freq'Je::cies o::. tr~e reversed

tail rotor tests.

7he subjectively distir!.Ctive r.oise c~:aracteristics of tl'.e 'Synx ':",elicopter

during the approach fli;:;-ht condition nre _ge::·1erated by th.c interscctio:--. of tl:e

tail rotor ~-Iith tip vortices shed by the ~ain rotor. Duri~-:.~~ fcrv-rard flight t!::e

relative positions of tl:e vortices a!',d the tail rotor blades are such that tb..e

:ns.in rotor tip vortex is i21tersected by 4 or 5 tail rotor blades giving rise to

gro'J.pS of impulses as 0J.c!:. vortex passes t~rough the tail rotor disc. Oi<~ing to

impulses is effectively modulated and the interaction noise is heard as a deep

throated burbling sound.

Reversing the tail rotor has eliminated the burble noise, as predicted

and significantly reduced the tail rotor blade passing noise in the far field.

Prediction methods based on blade thickness effects have calculated the tail

rotor noise at tre tail microphone to within 2 or 3dB but owing to problems of

directivity etc. accurate predictions for the ground microphones have not been

possible.

On a dBA basis the far field noise has decreased by 15-20dBA at

distances of about 3000 ft. and thus the noise detectability of the helicopter

has effectively been doubled by reversing the tail rotor,

8, ACKtiO'!ILED0_{S:'T ?S}

The authors

wish

to

thar~

their colleagues

in

the Applied Acoustics Dept.,

the Research Experimental Dept., and the "-esearch Aerodynamics Jept. of ·.·estland

HeliCopters Ltd. for their help in preparation of this paper. 'l'he views

expressed in this paper are t!':ose of the authors and do not necessarily represent

those of ':lestland HeLcopters :td.

9. RE?SRSNC~S

1. J.S. Pollard and J.':T. Leverton, LyP..x External Noise- '3urble' :Toise

Investigation. ':r!IL Applied Acoustics Group ~~rote 1044. October 1973.

2. J.:3. Pcllard, Lyn.x 'Burble' Noise -Effect of ForHard 3peed ar..d

Direction of Rotation of Tail Rotor. ·:.~

.:r.:,.

Applied :\cou::tics ro'-.lp

:rote 1053.

Janu2.ry 1 974.

3. C .R. ·:/ills, ?.eversed Jirectio:1 Tail Rotor Ly:r.x Burble ~--rc-:.se

Investigation 1

I'est Results. ':l.H.L. Applied Acoustics ..:rroup ::ate 1115.

June 1975.

4. J.Jl. Leverton, Helicopter Blade Slap. f.1.Sc. Thesis 1966 I3'IE

Soutr~mpton

University,

5. I'.L. HaHkings, I··LV. Lovmon, 2~oise of High 3peed Rotors. :'-"'"~·.-.:..._paper

9 6'

TAIL ROTOR GEARBOX MOUNTED

MICROPHONE {ON T!-IE

STARBOARD SlOE l SKID MOUNTED MICROPHONE 7 25' DIAMETER iii 2

I

... L l

:

FIG. 1. MICROPHONE POSITIONS ON AIRCRAFT

100 200 300 400 500

FREQUENCY ~Hz.

FIG. 2. NARROWBANO ANALYSIS ON REAL 1'1ME ANALYSER- SIGNAL FILTERED AT 1000HZ ',OCTAVE BAND AND F1_{JLL WAVE RECTIFIED.}

FIG.4.

-

=9-::;=J--FIG. 3. RMS TIME HISTORIES FOR TAIL MOUNTED MICROPHONE

~

TYPICAL 3 PEAK PULSE

?0"-.j [I I .

I

6o ,

I

;~~~~-~~,~~~~~~~~~~~~~~HW~~~~~~~~

10/ I

dBA TIME HISTORY FOR GROUND MOUNTED MICROPHONE (TAPE SPEED REDUCED BY A FACTOR OF 4).

~I

\

\

HOVE~·.

100Hz HETERODYNE FILTER (CENTRE FREQUENCY 600Hz I

001 SECOND

B&K

t

OCTAVE BAND FILTER 1800Hz I 001SECONO

FIG. 5. COMPARISON OF U-V TIME HISTORIES

60KNOTS 90 KNOTS JOKNOTS 11SKN0TS 130KNOTS 1SQKN0TS :HOVER 90KNOTS 11SKNOTS 130KNOTS 1SOKN0TS 30 KNOTS

FIG.6. MAIN ROTOR TIP VORTEX TRAJECTORIES DURING HOVER AND FORWARD FLIGHT

3rd 2nd VORTEX VORTEX

---

....

---·---

,--§

~:

h :.,

~~-~ ~

r

11~-~~~~-

- . -/

I~

1-

r:

-I

~

I

II I

1:1

II

w .. 1 . ~ 0

I

I

IO·S

hI

11

J

I~JJ

Ll

u

~·

--

.

....,,

I I

~A~MPLITUOE

VARIATION f'PROXIMATE PERIOD Fl· 2_ 4R

,~

r-~~--~~~~--~~=i~~~--~~~~~~--:

0 01 02 03 01. OS 06 0;7 08 f>9 c }() TIME.ISECOt-OSI.

6 ~~ 6 6 ~ 6 .) ~ ,"\ c.\ 6 6 :2:. ~'- ,.\ ...:-:, ..2- u <..\ MAIN ROTOR BLADE PASSAGES OVER TAL BOOM.

FIG.7. TAIL ROTOR BLADE/MAIN ROTOR VORTEX INTERSECTION POINTS

2·0 ~ -2·0 -3·0 -t.·O

~

~ 90 ~ 1i ~ "'80

"

'

_,

w > ~ 70 w

~

"'

~ 60 0.. 0 z => a "' 50 135·3d8 14(}6dB

FIG. B. ACOUSTIC PRESSURE·AMPLIT'JOE TIME PLOT

130~

128

₁₁

"8

12

130~

1KHz OCTAVE BAND.

~

"51

~

1401

1135i

u

!

i_ 130i

"'

!

~ 125~ \i: I 120j 130KNOTS I ' ! STANDARD TAIL ROTOR DIRECTION TAIL ROTOR REVERSED

~6'o

a'o ,Qo do

1Lo

H1o

11!0 260 220 rKuOTSI.

····6o 11lor•To ' 18o ' 2lo ' 26o ' 300 ' 3io ' 3&; '

!FEET/SECOND!. FORWARD SPEED.

FIG. 9. VARIATION OF IMPULSE PEAK SOUND PRESSURE LEVEL WITH FORWARD SPEED

FIG. 10. tJ.V TIME HISTORIES FOR REVERSED TAIL ROTOR- TAIL MICROPHONE (115 KNOTS)

STANDARD TAIL ROTO

,-800

'

'

- ... -<;:""

_,. '-..REVERSED TAIL ROTOR.

600 400 200

DISTANCE. lMETRESI.

0

SECONDS.

0 200 '00

FIG.1l. COMPARISON OF dBA TIME HISTORIES FOR STANDARD AND REVERSED TAIL ROTORS 50 M. ALTITUDE 130 KNOT FLYOVER CONDITION.