Interior noise levels of two propeller-driven light aircraft

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INTERIOR NOISE LEVELS OF TWO PROPELLER-DRIVEN LIGHT AIRCRAFT By John J. Catherine , , and William. H. Mayes

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Q0 r V r LT e^ l11 0 t'I 1,l This informal documentation medium is used to provide accelerated or special release of technical information to selected users. The contents may not meet NASA formal editing and publication standards, may be re- vi;,,,d, or may be incorporated in another publication.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION LANGLEY RESEARCH CENTER, HAMPTON, VIRGINIA 23665

1 Neport No • ..meent t C&U" No 2 Govunment Accoamm Nu 3 R NASA TM X-72716 4 r,llt anti Suht,tlr r Report Dote July 1915 INTERIOR NOISE LEVELS OF TWO PROPELLER-DRIVEN %riormmp Orpmratioo Code LIGHT AIRCRAFT Author(%) 11 Kwform,ny O sn,reoon Resat No John J. Catherines and William H. Mayes TM X-72716 10 Wo ► k Unit No 9 Performing D r9ani ;u1,on Name and ldarew 504-09-21-01 NASA-Langley Research Center No contract or Grant 11.

Hampton, VA 23665 Tytr of Report ain't Pr-od covered 12 ;ponsnr.,w Ageni , Name and Addrrss Technical Memorandum 1 National Aeronautics and Space Administration jt%.nwrngAwn-,cone ^ll Washington, DC 20546 1 _ . -_ _ . _^ — __....

S,pplemr+ntery Notes --M- IbAnstra. t ^ I An experimental investigation was conducted on the interior noise of light ai-eras The purposes of the investigation were to determine the relationships I b"tween aircraft operating conditions and interior noise and to determine the degree to which ground testing can be used in lieu of fliqht testing for performing interior noise research. The results of this study show that the noise inside light aircraft is strongly influenced by tae rotational Speed of the In particular, the A-weighted JB levels increase with engine and propeller.

increased engine/propeller rpm for all ground and flight operations.

Both the overall noise and low frequency spectra levels were observed to decrease with increasing high speed rpm operations during flight. This phenomenon and its significance is not presently understood. Comparison of spectra obtained!

in flight with spectra obtained on the ground suggests that identification of I frequency components and relative amplitude of propeller and en<iine noise sources may be evaluated on stationary aircraft.

• 4p'ry underlmodl 17 Key Wor's ISu(g paed by 4- 1 h^a15); i',1r'•H cm IP Drstrrhutron StatemM r j Noise Unclassified - Unlimited General Aviation Acoustics 71 Pace' 21 No of Paget 22 (jenof. lot this r,• I^Ottr ^^-- ate'. U+rt,. V 'his Pepe) r 19 Security Unclassified Unclassified T nr r I.shonat i „in 1 "save a '^,e ing r ,.•Ir1, Vugm , :.'1'i?

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-ITXRIOR NOIf:i: LEVELS OF ^I4'1 PROFELLLR-DRIti'FN LTGN:' ATRCPAF^_' :)y ti.illiWr; H. lrayee John J. Catherirec and Langley Research :,enter Ifa pton, Virginie Paper Tirr.::entrd at 11CTSFXFO ;.tl.ar.ts, Georgic Apr 4 -.' 26-May 2, 1971,

11A:1A - Larg. ' ey INTERIOR NOISE LEVELC. OF :'WC I140PELLER-PRIVET! LIGPT AIRCRAFT by John .1. Cutherinns and William H. Mayes Langley Fe!- ILrch Center }lruap`cr,, `lirQinib ALSTRA(71r An experimental investigation was conducted cn the interior noise cf light aircraft. The purposes of the i.nve;;tiFatic,n were to detern.4 ne the relationships between aircraft operating conditions and interior noise and to determine the degree to which ground testing can be used iu lieu of fight testing for performing interior noise rer.earch. TLe resu.L-cs of this study show that th.- noise inside light aircraft. is ,:trongly influenced by the rotational sprad of the engine and pro peller. In particulr_r, the A-weigh + ed dB levels increase• with increased engine/propeller rprr. fcr nJa Ercund and fight cperatinn!.

Both the overall noise and lcw fregiiercy spectra levels were oLserved to deerr-ase with increasing high speed r; r^ operaticr: durirf fl ght. This phenomenon and its significance is not presently understood. Ccmpor, son of spectra obtained in suggests that :dcr.ti_ication of flight with spectra obtained or the f: • eund frequency components and relative anFlitude of propeller and engine noise sources may be cva:uated on staticnary aircraft.

1IITERIOh NOISE LEVLL£ OF 1140 PROPELLIF-DRIVEN LIGHT AIRCRAFT BY John J. Catherir.en and William 11. Mayes Langley Rereurch k'enter Hampton, Vi rgi W a :'LTRODUCTi 05 ;c techrcicgy has been conducted at NASA An assesse:ent of aircraft interior noi , Langley Research Center tc dc-termine whether improvements are r.cedei to control the noise for the comfort. of crew anal. pas::t-nEers. A:. part c1' this assesoment,

interior noise levels of light aircrt ft ariel other current types o1' aircraft

were examined. ;. sample of tY:r_E;e interior noise levels is pre:;erted in figure 1.

Interior noise levels of busses, railcars, and P%toriotiles are included fcr -6.

comparison. The data shx.wr, in figure 1 ware cbta: r,ec from refs. 1 FigI: re 1 shows that the noise leve'_e measured it.side light aircraft were between. 84 and 104 dBA. Comparison of these levels with levels found in other vehiclev, and consideration of the resr.onse of prop]~ tc these 1evF1s, leads to the ccr:clusicn that substantial benefits can be obtair.ea ky reduction o1' this noise.

Control of light aircraft interior noise require; r.ncwledge of such factor: as the noise sources and paths of ertr,; irnt.c the irt f-rior. Sore of thesr: factors are described in figure 2. Sources of r.oi:;e inclt:de propellers and recil,rcccatirg engines, as well as auxiliary cquiprsent and flew ^,f air over the a.ircra"t. Noise car. enter through the lig: tweight fi.ise.inge structurc, through window: (that comprise a large percentage of the fuse:.rtee area;, cr through acous'.ic ".eaks"

in the nonpreenurizea structure. Noise can alto be transmitted directly by structural vibrations induced by the engird-. It can be seen that the many factors contributing to the interior noise end the complex interactions among these factors makes it a difficult task to control the noise. The task is made more li::cult by the need to mairtain the jw rformarce and economy of the aircraft while reducing the interior noise. In reviewing the tecrnclogy, it wau found that insufficient information is avallahic on the cha rite teristics of the interior noise, or on nethods for its control. The lrincipal previous work is the st.uie:, of the effects of altitude.

by Tobias (refs. 2,3)

As indicated in figure 3, the work describes: in this paper har two ob,4ectives, namely, (a) to determine the relationships between aircraft operating conditions

and interior noise and (b) to deterr.ine tl.e degree tc which ground tectitg can

be used in lieu of flight testing for ierfoznming interior noise research. The interest in ground testing ariu-eo fro g, the thougt.t that interior noise studies may be performed more easily and more systcr..atically cn a s` .•.ry aircraft

because of the accessibility of instrumentct _or., hddod reasurr., _:.t c-9pability, and

a more controlled acoustic envirormsent. Euwever, before such studies, can be performed, the important characteristics of the inter: r noise court be understood

paler presents results of

fcr both flight and ground test cor,ditloriz. 'Ahiz

interior noise measurements obtained fcr a two Fae,cenger, single engine, propeller

driven aircraft for both ground c ,^rat i-r:: and for a range of in—flight. -.yvrat ing

conditions. Measurements cbta`_neu in n tsar, engine, propeller driven tyFe aircraft during normal flight operations• are also l:resented.

TEVT Di?: CRIPT10I4 Test Aircraft the passer;ger seet l.ocaticn for two Interior noise measuren.er.tc were ubtrsir.ec: itt types of light aircraft. The aircraft were unmodified ar.d have specifications as presented in figure 4. The _ ingle ^:.ginF, two :.errt aircraft model) is

(1973

believed to to represer,tnt i •: a of one of the most popular currently rranufacturcd models. The '-din engine ai roraft was a four scat (19`,7) model and had a known door seal leak.

Test Instrunent.aticn e.r.d erocedure y were Cbtaine f in flight using a (type 1, Sound pressure level ineasurt • n:ent he electrical outrut of the sound. level meter precision) scund level meter.

port,tle rr:agrietic take recorder fcr cutsequent detailed

was also recorded on zi.

frequency analysis. An ;amplitude modulation me''hod war 4:;-d the data

on tape, together with a 7.5 inches per r.ecc,n0 rt-corder tape speed allowed fcr

a frequency analysis of up to 10,000 Nz.

the c,ingle engine, pi-cfeller driven aircraft The procedure for the flight ttats of involved cruise engine rpm settirgs of 2,000, 2,200, and ,,CC. Noi£e measurer..ente were obtained fcr each of the rpn. setting:- a' flig:.t altitudes cf 1000, 2000, and 3000 :eet and fcr une cordi'.ir^c cf dlo- dencent. The flight conditions were identified on tape by the use of a tl..ee-digit, dial-set, battery operated encoder triggered by the observer. The coding device irrlarted a binary digital code to the tape which was used to identify the varicus flight events. A more complete description c.f coding device is give n t r. ref*. 7.

i A second part of the study irvolvtd interior noise measurer7 p r.ts on a stationary aircraft Lased on the obj(:cti •:es ner.tione.a curlier. in order to investigate ground reflective effects, the aircraft was oyeratec on both concrete and grasay surfaces for engine speeds of 1,200, 2,000, and x,400 rpm. Measurements were also obtained with windows open Find windows cooed for the engine operating at 1200 rpr to obtain information on noise path identificati)n.

For the twin engine eircrar't, whict. hau a known door seal l e ak, data were obtained cn a noninterference basic during a normal passenger carrying flight.

Dat.. Ana s it The data recorded on magnetic tope wn:. playa: tack to obtain an cscillogrcrh record cf the noise data, the three-digit hinar;i onde, and a 7'ASA-36 bit time code. .",e time code was added to the tale ir: the initial cnta reduction preeea.

to identify the digitizing times. The data ca.own in this rarer were , reduced from 5-second segments that were selected fcr each aircraft condition. The data

were digitized at a rate of 20,000 per second. 11 ased on the Nyqulot

requirement of two Samples per cycle, _hj L;arple r,.t.e yielded valid resultr up to 10,000 Hz. Before digitizirg, the atta wore filtered with an ana'og low pass filter having a cutoff fre,t.ercy of 1C,000 Hz, in order to eliminate the 8).

problems of folding or aliasing (see rv, Lastly, the data were reduced cr. n CDC F(OC series computer using a Fast Fourier Transform (FFT) program. (ref. 9) to obtain the desired outputs. The outpt.ts of this program were OACFL, daA, arl sound pressure spectra in the fora of constant bard (20 Hz bands), 'i3 octave and octave land analysis.

pf-ML,rs ANN, DIf;CL£SI01: El*feet: ci' Pi'V in ;:,.&,t Cound pressure spectra obtained It; the sing.` a t^ng l ne vArcraft :xre shown in figure 5 for three different engine-prci,t.j:er ryr. settings measured At a flight altitude of 1000 feet. Ir. aduiticn, tte indicated airsLee:l, overall dB, and dTA • rpr condition. it can be seen that values are given for each engine-1 ropeller l itionc, occurs below 1GC Hz.

the predominant pe%k of the c-i:ect.rrs., for all rpm con- Hz The fundamental prupeller-r:ng`.ne firing frequencies varied from 67 tc during the increase of engine rprr. from PCCC rpm to 2`,CG r}m, respect.vely; these frequencies correspond to the dominant hea' s nccwn L• elow 1CC }:z. For this= frequency range, wH ch eontro'_s the overall CPL, the spectral levels decreas.

with increasing engine rpm and r'rspeed. The reason for the observed dLcrease is not fully understoca. Possitle renscmr are different l.rcreller inflow conditions, effects cf forward speed, an .u:;r1fi a .-tructurhl resronvec. However, at frequencies greater than 1C0 }iz, wi ict. CGr.t:rr1r tt,(- dBA level, the spectral levels in rease with eng,rtx ri.m eta wcu!.d be exrected with the correnpending incr y -. #rd airspeed ar.d er.rfte po.;4r. .hc Freaks ir, the : hectra rt•ove 100 1:z are a:;::ociated with prepcl:c:r-ene.r.c ! 1 •• jr. ► - frrg1 ,er.(,y hb.rmi-nics.

Car.,parisor. (A W.ght wit} "rouric Testr 11cise spe%.tra for both ground and flight condition: are shown in figure 6. These data were ottained for an engine-r. •c:pc1:-:• r-ettir:g cf 2000 :•p^, with the flight condition Pt on altitude of 1000 ft r..': un 1AS v:' 72 knots. It In Been that the

dominant low frequencies (below V I Hz) and most of the high frequency peaks measured in flight are in good agreement with those measured in the stationary aircraft. In the midfrequency range (100-2000 Hz), the noise levels are higher during ground tests by up to 6 dB and are believed to be due primarily to ground reflection. llowever, for frequencies above about 2000 Hz, the flight spectrum levels are higher than those measured on the ground. This result is believed to be due to aerodynamic noise associated with flight. the above results would suggest that identification of frequency components and relative amplitude effects of noise sources related to the propeller and engine may be studied on stationary aircraft.

A summary of the noise levels measured for all flight and ground test conditions are shown in figure 7. Overall and A-weighted levels are plotted as a function of propeller-engine rpm. Included in the data are results from ground tests on both concrete and grass surfaces and also flight data for each of three altitudes, including engine idle descent. The engine idle descent data are given by the two open circle data points at 1200 rpm. Curves are drawn through the data points for comparative purpases.

Several observations can be made from this figure. For all conditions tested, the dBA levels increase with Increasing engine-propeller rpm. The values of dBA associated with ground runup are seen to he about 6 dBA higher thap1 these measured during flight. It should be noted that the slopes of the curves associated with the dBA levels for both ground and flight conditions are approxi- mately the same. This result would imply that dBA levels are indenendent of of the study. Caution, how- forward speed for this aircraft over the operating range used for this interpretation, since ground reflection and airspeed werel ever, should be

uncontrolled variables and may h-we sel'-con per.cating effects. he effects of altitude on both the overall and dL'k levels nre r , •, n to be rnail (witt.in 1 dt3) an would be exp ected from the: te nt results rei.,..rted :n reference 2. For higher ri.m settings, the overall c.,FL ure . erj to dc-_rruue with increasing rir.. i-articularly for all the in-flight test c( r.^'.t onn.

Effects o: Vitt.dows Ac L,cn ticne •i earlier, winw v are convidtred to be at, important trannmiss_on path t.

for noio p to enter the interior c: light. Nircraf Zir.ce to t. ~P authorn, Y.r,owledge no quantitative information is available on window effects, sane preliminary infc rration was obtained or the e-ffe-rt of windows or. the meacurud Interior noise. The single crgine uircrh.t war, operand at a constant 1200 rpr on the ground ar.d noire tnensur-n:er.t.- were: obtait:ed with the windo.:s open, and with the windows closed. The results of the: . a med :nuremet.t r are presented in figure 8.

It can be ceen that !n get.eral the -r-.cc• tra c.ttained for there twc conditions have the sarne shape. ;lightly higher anp 1jitua,.z ^u.,oc • :atcd with the window eFen ► CL I n dition were Measured er the frequency range of nl.out -',CC to 6000 liz. it or should be notes that the• cveru'.i :.F'L and level ito •rcase d and 6 di?, respectively with the windows open. :'hio result iridic:atc tout windows do provide s4-r..e noise ve: reduction for the aircraft cabi r. and tt at the rt:lat i contr'_buticn of windowu a:- a noise tranzr..is ion path for light aircraft shculd he further investigated.

Effect ,. (A' Door Fea. Leak Another factor that mey contri but..r :c the it.tc;• ic : not ne of light a ircrt;ft are seal leeks that may occur around open'_n(r^ of wine.ows and dcorb of -)lder airora:t.

C' w noise data pertaining to a seal leak • tre obtained and are shown in figure 9.

Noise spectra were Treasured at a pensenger neat location in a twin + ^gine, four passenger aircraft for two flight conditions, namely, lift-off and , cuise. The speed of the aircraft during lift-off war anproxirately 87 kncts ct .pitro-d tc 161 kn,.)tz measured for cruise. The spectra' dat" har, a peak at at, ut 0 l 05 Hz which corrrt;pcnd: to the propeller "cl engine tiring frequencie:u Furt*ermore, nt the higher frequenclen atove 1000 Nz, the ncic • e level- rare h gher during crulue• anu, since the engine rpr. is less but the rpecu K o increas ed , the source o;' Furth(r evidence of this thir, noise is believed to be aerodynarric It nuturc.

^Aservation is shown t.y the third apectru:i which vat-, r..eurarc:d rear the door seed leak. The spectrum lever) at tee; higher Vr^qucT,cie:; Ir about 15 dB }:ighe°r v'ien measured near the door leak ec compared tc the seat lccaticr. and Is appreximate-ly 25 dB higher when compared tc, the lift.-c-ft corrittion.

CONCLUMNG FI S,ARKC The results of this study show that the noise levele inside light aircraft are strongly influenced by the rotational speed of the e=ngine and propeller. In parti- cular, the A-weighto-a di? levels inc-eare w'th increae;ed engine-propeller rpr.: for all ground and flight operations.

Poth the overfill r:oioe and low frequency .;pect.ra levels were obi erved to decrease with increasing high speed rlr.. oF:e•raticr.o aurirg flight. This phenomerc,n and its significance is not presently unt'.erstood.

th • ik:ound comparison of spectra obtained _n flight with spectra obtained on of frequency components and relative amplitude of suggests that identification propeller and engine noise sources may be evaluated on stationary aircraft.

Closing of the aircraft windows provided approximately b dBA of interior noise reduction under static conditions. Door seal leaks provided a source/path for aerodynamically generated in-flight noise.

Nf'FLTFNC *''I I. Stone, Richard B.: Cockpit. Noi::e Flivi l rcnrient of Airlfrr Aircraft.

1969, pl.. }89-993.

Aerospace Medicine, Cept.

-jias, Jerry V.: Cockpit Noire Intenrity: Fifteen, : , ingle Engine Light 2. Tr J.969, pp. 963-166• Aircraft. Aem-Space Medicine, ';ept.

in Ligi.t '"Min Engine Aircraft. Sound and V-.bratlon, 3. lobia:,, Jerry V.: Noise Sept. j- 16:-19.

1 9 6 9, p Sound and 1 +. Bray, Don E.: Noise Fnvirorxnet,t s :n Pudic Trarsportaticn.

Vibration, April 197 1+, fop. 16-2C.

Bei anek, Lec L. , ed.: Ncise anal V:br,,ticn Cor.`rol. !•'cCraw Hill Pook 5.

Cc., 1971.

6. Rudmoce, H. Wayne; and Berane; , Lec L.: 11oi:sN Reduction in Aircraft.

J. Aeron. Sci. , vol. 14, no. ?, Feb. 19 1: 7, pp • 79-96.

A.; and Catl crincs, ljohn Meesurement. acid Analysis 7. Clevenson, Cherman :'cience and Technology of Vibration in Transportation; Systems. AAS Seriev, 'Technology- Utilizaticn Ideas for the 70's and Beycnd, vol. 16, 1971, pp. 197-202.

Wasurement and Analysis of Fandon Bendat, J. S.; and liersol, A. C.: d, New 7c:rk, 196/_.

i:ac,a, Jour. Wiley eud Sons, 9. Brown, Thomas J. ; L'rcwr, Christine C. ; and iiardin, Jay C.: Program 2'or IJt.:A T ?r X- 988, ;'ept. 197L.

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^' 1