NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS TECHNICAL NOTE 2727 EXPERIMENTS IN EXTERNAL NOISE REDUCTION OF A SMALL PUSHER-TYPE AMPHIBIAN AIRPLANE

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1 ft '* NATINAL ADVISRY MMITTEE FR AERNAUTICS TECHNICAL NTE 2727 EXPERIMENTS IN EXTERNAL NISE REDUCTIN F A 1 SMALL PUSHER-TYPE AMPHIBIAN AIRPLANE By Jhn P. Rberts and Le L. Beranek Aernautical Research Fundatin DISTRIBUTIN STATEMENT A Apprved fr Public Release Distributin Unlimited Washingtn Reprduced Frm Best Available Cpy July » M-P~MM

2 NATINAL ADVISRY MMITTEE FR AERNAUTICS TECHNICAL NTE 2727 EXPERIMENTS IN EXTERNAL NISE REDUCTIN F A SMALL PUSHER-TYPE AMPHIBIAN AIRPLANE By Jhn P. Rberts and Le L. Beranek SUMMARY The wrk reprted is part f a prgram the bjective f which is t find practicable ways f reducing the external nise level f light airplanes. This reprt cvers nise measurements n a representative pushertype amphibian airplane. The wrk supplements earlier wrk by the Aernautical Research Fundatin n the external nise f tractr-type airplanes. Mdificatins f the pusher-type airplane and nisemeasurement prcedures used fr the present wrk were similar t thse used fr the earlier nise study f the tractr-type airplane. Tests were made (l) with a standard pusher-type airplane; (2) with the same airplane using a fur-bladed prpeller; (3) with a mdified versin f the airplane using a geared engine and fur-bladed prpeller but n exhaust muffler; and (h) with a mdified versin f the airplane using a geared engine, exhaust muffler, and prpellers varying in number f blades frm three t eight. Tests were als made n the eightbladed cnfiguratin f the geared and muffled airplane with the muffler relcated s that the exhaust did nt pass thrugh the prpeller. Fr all cnfiguratins these tests included sund-level recrdings f take-ffs and f verhead flights at 100- and 500-ft altitude. They als included analyses f sund-frequency cmpnents with the airplane n the grund frm a distance f 50 feet and at varius psitins arund the airplane. Fr sme f the cnfiguratins sund-level recrdings were made fr flights at 500-ft altitude passing 3000 feet away. These measurements frm 3000 feet away were als made fr a standard tractr-type airplane. Finally, frequency analyses were made fr the nise f ne f the mdified cnfiguratins f the pusher-type airplane during a flight passing 3000 feet away. Valuable infrmatin was btained during this study cncerning methds f external-nise measurement and interpretatin f such measurements. In general, it was demnstrated that significant reductin in the external nise f the pusher-type airplane can be achieved by the use

3 NACA TN 2727 f slwer-turning prpellers in cnjunctin with engine exhaust mufflers. This reductin can be achieved withut serius sacrifice in perfrmance if a variable-pitch prpeller is used. This result is in agreement with that f previus wrk n tractr-type airplanes. With a given tip speed and engine-pwer utput, it was fund that increasing the number f prpeller blades abve fur did nt decrease the nise level. This result differs frm that f previus wrk n tractr-type airplanes. Directing the discharge frm a muffler s that it passed thrugh the prpeller disk did nt increase the ver-all nise generatin under the cnditins f the present study. There is evidence t indicate that it actually reduced the nise generatin as cmpared with a lcatin utside the prpeller disk. The nise f the standard ungeared mdel f the pusher-type airplane was nt significantly reduced by the use f a fur-bladed prpeller in place f the standard tw-bladed prpeller perating at cmparable pwer, even when the fur-bladed prpeller had a lwer tip speed. The pusher-type airplane, in bth the standard and mdified cnfiguratins, radiated sund mre unifrmly in all directins in flight than the tractr-type airplanes. While the maximum nise level in a flight verhead r passing 3000 feet away is abut the same fr the tw types, the lack f sharp directivity in the pusher means that the nise level fr the pusher is higher at mst measuring psitins. The difference in nise generatin fr the tw types tested can be explained by the difference in engine and prpeller lcatin and the disturbance f the air flw thrugh the pusher prpeller which did nt exist fr the tractr. The lack f directivity in the nise f the pusher-type airplanes as cmpared with the tractr-type airplanes tested means that the pusher types make a disturbing nise fr a lnger time and at any given time in flight make a disturbing nise ver a larger area. In additin, the character f the nise generated by the pusher-type airplanes appears t be mre annying than that f the tractr-type airplanes tested. Bth f these cnditins mean that the standard pusher-type airplane tested is a nisier airplane than the standard tractr-type airplane tested and the mdified pushers are nisier than the mdified tractr-type airplanes under cmparable flight cnditins. INTRDUCTIN Experiments by the Natinal Advisry Cmmittee fr Aernautics (references 1 t 5) and the Aernautical Research Fundatin (reference 6) have shwn that it is bth pssible and practical t achieve

4 NACA TN 2727 significant reductin in the external nise level f light airplanes by the use f slw-turning multibladed prpellers and engine exhaust mufflers. These studies investigated principally the effectiveness f this technique in reducing the external nise f cnventinal tractr-type airplanes. The present study was undertaken t determine hw effective this technique wuld be in reducing the external nise f a representative pusher-type amphibian airplane. A secnd bjective was t investigate the effect n the nise level fr a geared and muffled pusher-type airplane when the number f blades was increased while maintaining bth engine pwer and prpeller tip speed cnstant. A third bjective was t investigate the effectiveness, n the pusher airplane, f simpler mdificatins fr reducing the nise such as mitting the muffler with a slw-turning prpeller r increasing the number f prpeller blades withut reducing the prpeller speed r using engine muffling. The experiments reprted herewith were cnducted during the years by the Aernautical Research Fundatin under the spnsrship and with the financial assistance f the Natinal Advisry Cmmittee fr Aernautics. The prject was under the general directin f Dr. Lynn L. Bllinger, Executive Directr f the Fundatin, and under the technical directin f Prfessrs tt C. Kppen and C. Fayette Taylr f the Massachusetts Institute f Technlgy and Mr. Arthur H. Tully, Jr., f the Harvard Business Schl. Mr. Fred S. Elwell, Administrative Supervisr f the Fundatin, crdinated and supervised the prject and, in additin, assisted with part f the sund measuring, pilting, and data pltting. Mr. Jseph Garside, Directr f peratins fr the Fundatin, directed the cntrl f airplane safety and maintenance and pilted the airplane n many ccasins. Mr. William W. Dean, Administrative Assistant f the Fundatin, prvided assistance in pilting, sund-level measurements, and data pltting during the summer f 19^9. The fllwing individuals and rganiatins generusly cntributed equipment and assistance n this prject: Aircled Mtrs, Inc., lent an experimental geared engine. The Gdyear Aircraft Crp. lent tw GA-2 amphibians. The first airplane was cnverted with the experimental geared Franklin engine, special prpellers, and silencers. The secnd airplane was a standard mdel GA-2 which was used fr sund-level cmparisn purpses. Additinal gratuitus services were extended by this Crpratin, namely, structural engineering mdificatins and engine

5 NACA TN 2727 changes; als cmpany pilts were prvided t ferry the aircraft several times between the Fundatin's facilities and their factry. The Maxim Silencer C. gave the silencers fr the experimental pusher airplane. Sensenich Brs, prvided all experimental prpellers at cst. Mr. Jseph Garside, President f Wiggins Airways, gave use f his cmpany's shps and facilities. DESCRIPTIN F APPARATUS Airplanes The fllwing airplanes were used in this study: (1) A standard Gdyear Aircraft GA-2 amphibian, equipped with a Franklin six-cylinder, direct-drive engine, rated at 145 hrsepwer at 2600 crankshaft rpm. The engine n this airplane was equipped with shrt exhaust stacks pinted straight up. This airplane, shwn in figure 1, will be referred t in this reprt as the standard pusher. (2) A mdified Gdyear Aircraft GA-2 amphibian, equipped with an experimental Franklin six-cylinder, geared engine, rated at l80 hrsepwer at 3050 crankshaft rpm. The adjustable-pitc* - sellers used n this airplane were set fr the wrk f the pr^ reprt s that the maximum pwer f the engine was apprximately ±45 hrsepwer, cmparable with the pwer f the standard pusher. This experimental engine and its planetary gearbx with a t-l rati is the same as the ne used in the experimental tractr reprted n in reference 6. It was equipped with a single Maxim exhaust silencer munted abve the engine and exhausting thrugh the prpeller. This airplane, shwn in figure 2, with the muffler exhausting thrugh the prpeller will be referred t in this reprt as the mdified pusher. This airplane was als flwn withut the muffler and with the muffler relcated s that the exhaust did nt pass thrugh the prpeller. These tw cnfiguratins f the mdified pusher will be referred t as the mdified pusher, unmuffled, and the mdified pusher, muffler relcated. They are shwn in figures 3 and 4. Back-pressure measurements were made n the three different exhaust arrangements fr the mdified pusher at a pint in the exhaust pipe near the engine. The back pressure fr the mdified pusher, unmuffled, was 3/8 inch f mercury at 2500 rpm, full thrttle. The back pressure fr the mdified pusher with the nrmal muffler arrangement was 3^ inches f mercury at 2525 rpm, full thrttle, and fr the mdified pusher with

6 NACA TN 2727 the muffler relcated it was 3^ inches f mercury at 2525 rpm, full thrttle. Figure 5 is a drawing f the muffler used. (3) A standard I9A8 Stinsn Vyager 165, equipped with a Franklin six-cylinder, direct-drive engine, rated at 165 hrsepwer at 2800 crankshaft rpm. This airplane is similar t the ne referred t as cnfiguratin 5 in reference 6. It was used during the present measurements t prvide data n the nise levels prduced by this airplane when flwn 3000 feet away since this type f measurement had nt been made in the previus wrk. Figure 6 is a phtgraph f this standard tractr airplane. (k) Measurements made fr the previus reprt f the nise prduced by bth a standard and a mdified tractr airplane are presented fr cmparisn with thse fr the pusher airplane in.the present reprt. Detailed infrmatin n these airplanes may be fund in reference 6. Prpellers The standard pusher was first equipped with a tw-bladed Aermatic cnstant-speed prpeller. This prpeller was adjusted s that the engine turned at a maximum f 2600 rpm, delivering apprximately 1^5 hrsepwer. The inset f figure 1 shws the standard pusher equipped with this prpeller. The standard pusher was als flwn with a fixed-pitch, slid, furbladed prpeller. This prpeller prvided maximum- and cruising-pwer flight peratin fairly cmparable with that prvided by the Aermatic prpeller. It prvided pr take-ff perfrmance, hwever, since it perated at a lwer maximum speed and pwer n the grund than an Aermatic prpeller. This prpeller is shwn n the standard pusher in figure 7. The mdified pusher with the muffler exhausting thrugh the prpeller was equipped with three-, fur-, six-, and eight-bladed prpellers using special wden blades assembled in ne f tw hub adapters. These prpellers were grund-adjustable and, as previusly mentined, were set t prvide a maximum pwer cmparable with the 1^5 hrsepwer f the standard pusher. Figure 8 shws each f these fur experimental prpellers munted n the mdified pusher. The three-bladed prpeller may als be seen n the mdified pusher in figure 2. The mdified pusher, unmuffled, was flwn nly with the experimental fur-bladed prpeller. This cnfiguratin is shwn in figure 3. The mdified pusher, muffler relcated, was flwn nly with the experimental eight-bladed prpeller and is shwn with this prpeller in figure it-.

7 NACA TN 2727 In rder t simplify the prblem f referring t the varius airplane prpeller cnfiguratins each has been assigned a cnfiguratin number in a numbering system -which is a cntinuatin f that used in the earlier reprt (reference 6). Table I shws these cnfiguratin numbers and gives engine, muffler, and prpeller infrmatin fr all the cnfiguratins (including thse f the earlier wrk) referred t in this reprt. It may be nticed in table I that reference is made t tw cnfiguratins f the standard tractr. In the present reprt the cmparisns between the pusher and tractr fr verhead flights and grund analysis use data fr the standard tractr (cnfiguratin l) frm the previus reprt (reference 6). The new measurements made fr the flights 3000 feet away were made using cnfiguratin 5 since cnfiguratin 1 was n lnger available. The difference between these tw cnfiguratins is in the prpellers used. Cnfiguratin 1 had a tw-psitin prpeller which was always used in the steeper, r cruising, pitch. Cnfiguratin 5 had a slid wden prpeller. The difference between these tw cnfiguratins is nt very great but was cnsidered imprtant enugh s that it culd nt be neglected. Blade-frm curves fr the varius prpellers used n the pusher airplanes are shwn in figures 9 "t 12. Blade-frm curves fr the prpellers used n the tractr airplanes may be fund in reference 6. Sund-Measuring Equipment The sund-measuring equipment used in this study cnsisted f: (1) Sund-level meter, General Radi C., with its micrphne munted inside a duble clth wind screen at the end f a 25-ft extensin cable (2) Sund analyer, General Radi C. (3) Graphic level recrder, Sund Apparatus C., a high-speed level recrder (k) Magnetic tape recrder, Magnecrd Inc., used with sund-level meter and sund-level-meter micrphne. The intercnnectin f instruments used fr the take-ff and flight measurements and fr the grund analysis is shwn in figure 13, and the frequency respnses f the sund-level meter and sund-level meter and sund-analyer cmbinatin are shwn in figure lk. The intercnnectin f instruments used fr the magnetic tape recrdings and fr the subsequent analysis f the tapes is shwn in figure 15-

8 NACA TN 2727 Flight-Cntrl Apparatus A Dewey and Almy Chemical C. "Kytn" captive balln was used t aline flight altitude. In additin, a sensitive altimeter was carried in the airplane t help the pilt maintain flight at a cnstant altitude when he was nt near the Kytn altitude marker. The airplane carried the usual instruments including, particularly, the engine tachmeter which was used t bserve engine speed during all sund measurements. TEST PRCEDURE Lcatin f Tests All sund measurements and perfrmance tests n the standard and mdified pusher airplanes were made at Nrwd Memrial Airprt Nrwd, Mass., between December 1948 and January Figure'16 is a map f this airprt and the surrunding cuntry, shwing the curse ver which the airplane was flwn fr the flight measurements. The nrmal test prcedure was divided int three parts, namely: Take-ff measurements, flight measurements, and grund analysis. In additin, sme measurements were made f flights at an altitude f 500 feet frm a distance f 3000 feet. These measurements will be discussed separately. Take-ff Measurements Fr the take-ff measurements, the pilt was instructed t make his take-ff s that he was just leaving the grund as he passed ver a marker 50 feet frm the sund-level-meter micrphne. In rder t simulate the perfrmance f a cnstant-speed prpeller, the pitch f the experimental grund-adjustable prpellers was set s that the engine at full thrttle wuld turn at 2500 rpm at the start f the take-ff. During the take-ff the pilt gradually reduced the thrttle setting s that the engine speed thrughut the entire take-ff did nt exceed 2600 rpm. Fr the take-ffs with the slid fur-bladed prpeller f cnfiguratin 7, take-ffs were made with a full thrttle setting thrughut since the maximum speed fr these take-ffs did nt exceed 2600 rpm. During each take-ff the peak reading f the sund-level meter was bserved and cntinuus recrds were made, n the graphic level recrder f the sund level frm the start f take-ff until the sund f the airplane died dwn int the backgrund nise. A reference mark was made n the sund-level recrds by mmentarily shrting the input t the graphic level recrder as the airplane passed ver the marker 50 feet frm the

9 NACA TW 2727 micrphne. Fur take-ffs were made fr each airplane-prpeller cnfiguratin tested. Tw f these were measured with the sund-level meter set fr flat weighting and tw, fr 40-decibel weighting. Flight Measurements Fr the flight measurements, the pilt was instructed t make level flights in a straight line, first at 100 feet and then at 500 feet, ver the curse shwn n figure l6. This curse extended abut 6000 feet in each directin frm a pint n the nrth-suth runway. The pilt alined his altitude n each flight by reference t the Kytn altitude marker. Since the flights were all in a line with ne side f the nrth-suth runway it was a simple matter t make all flights accurately ver the specified curse. At each altitude, eight flights were made at maximum pwer and eight, at cruising pwer. The pitch f the experimental grund-adjustable prpellers used n the geared engine was s adjusted that the full thrttle setting f the engine in level flight wuld prvide 1^5 hrsepwer at 2500 rpm crrespnding t the maximum rating f 1^5 hrsepwer at 2600 rpm f the standard pusher with the Aermatic prpeller. In the case f the standard pusher with the slid fur-bladed prpeller (cnfiguratin 7), n adjustment was pssible. With this cnfiguratin the full thrttle setting gave a maximum-pwer speed f apprximately 2750 rpm and cruising was chsen as 2*1-50 rpm. Prpeller tip speeds, engine pwers, and ther engine and prpeller infrmatin fr all cnfiguratins tested are shwn in table II. Measurements f the sund level f these verhead flights were made frm a pint n the nrth-suth runway directly beneath the flight path. Fr each flight the peak reading f the sund-level meter was bserved, and cntinuus recrds f the sund level were made. Fr each engine pwer, at each altitude, fur flights were measured with the sund-level meter set fr flat weighting and fur, with the ^0-decibel weighting. All sund-level recrds were marked with a reference mark by mmentarily shrting the input t the graphic level recrder as the airplane passed verhead. Grund Analysis Fr the grund analysis, the airplane was chcked tail dwn n the grund with the prpeller hub ver the center f a cmpass rse laid ut n ne f the taxying strips at the airprt. Fr these measurements the pitch f the experimental grund-adjustable prpellers was set as it was fr the take-ff measurements, that is, s that the engine turned at 2500 rpm at full thrttle. The pitch f the adjustable fur-bladed

10 NACA TN 2727 prpeller f cnfiguratin 9B was set n a different day frm the day it was tested. n the day the grund tests were made it wuld turn at a maximum f 2^75 rpm, but it was decided that this slight variatin in speed was nt serius enugh t warrant the lng jb f resetting the prpeller pitch. The maximum speed btainable with the slid furbladed prpeller f cnfiguratin 7, during the grund analysis, was apprximately 2125 rpm. Measurements were made f the ver-all level and the significant frequency cmpnents present in the nise f the airplane with the micrphne 50 feet frm the prpeller hub. After the measurement was cmpleted fr the psitin directly in frnt f the airplane, the micrphne was mved 30 alng the 50-ft circle and measurements Were again made. This prcedure was repeated fr each 30 n bth sides f the airplane with the exceptin f the l80 psitin which was mitted because f the prpeller slipstream. Measurements frm 3000 Feet Away In additin t measuring the nise prduced by flights directly ver the sund-level-meter micrphne, certain cnfiguratins were measured at a distance f 3000 feet frm the flight path. Fi? these measurements, the pilt was instructed t make flights at 500-ft altitude, eight at maximum pwer and eight at cruising pwer, ver the same curse used fr the verhead measurements. Prpeller pitch settings and engine pwers and speeds fr these flights were the same as thse used fr the verhead flights. The sund-measuring instruments were set up at the east end f the east-west runway at a distance f 3000 feet frm the nearest pint f the flight path. Fr each engine pwer fur flights were measured with the sund-level meter set fr flat weighting and fur were measured with the 40-decibel weighting. A reference, mark was made n the sund-level recrds as the airplane passed the airprt beacn twer. Analysis f Nise f Airplane in Flight In rder t get an analysis f the nise f ne f the cnfiguratins in flight, fr cmparisn with its grund analysis, a magnetic tape recrder was used. These flights were made at 500-ft maximum pwer ver the usual flight curse using the same prpeller pitch, engine pwer, and speed as fr the verhead flights. The instruments were set up at the lcatin used fr the measurements frm 3000 feet away, and the nise f the airplane was recrded n magnetic tape. Peak readings f the sund-level meter were bserved fr each flight. The tape recrding was prvided with a reference mark by shrting the input t the tape

11 10 NACA TN 2727 recrder as the airplane passed the airprt beacn twer. These tape recrdings were made using flat weighting f the sund-level meter. Analysis f the sund recrded n the tape was made with the analyer used fr the grund analysis. Several shrt sectins f the recrdings f tw different flights were cut ut f the tape and spliced tgether frming a cntinuus belt. This belt f tape with its recrding f abut 3^ secnds f flight nise was then run thrugh the tape recrder cntinuusly. The resultant repeating shrt sample f the nise f the airplane was analyed t determine the imprtant frequency cmpnents present in the nise. PRECISIN Successive measurements f the length f the Kytn nyln line and crrelatin f the Kytn altitude indicatins with that f a sensitive altimeter carried in the airplane indicated that airplane altitudes were held within +10 percent. This variatin shuld crrespnd t a variatin f +1 decibel in the sund-level measurements. It will be nted that sund-level measurements f similar flights under suppsedly similar cnditins smetimes shwed large differences. It is believed that the majr part f such differences was real and was due t variatins in atmspheric cnditins, terrain, and s frth. This prblem is mre fully discussed in the sectin "Discussin f Results." Labratry calibratin shwed that the sund-level-meter errr did nt exceed +1 decibel. A reasnable estimate f reading errrs under field cnditins appears t be abut +1 decibel. The tw errrs cmbined wuld give a maximum errr in the sund-level measurements f +2 decibels, aside frm thse errrs arising at the level recrder. Because the peak readings f the level recrder were assumed t be thse read simultaneusly n the sund-level meter, the errr in them wuld still be ±2 decibels. Fr data depending n recrder readings cnsiderably lwer than the peak readings it is nt pssible t determine the errr but, since the machine is generally accepted fr wrk f this kind and was kept in gd running rder, it is believed that this errr was prbably nt ver th decibels. Accurate calibratin f the tape recrder was nt pssible in the limited time available fr its use. In the play-back f the tape recrding the level was calibrated apprximately by cmparisn f the peak utput f the recrder with the peak reading f the sund-level

12 NACA TN 2727 n meter during the flight recrded. When the recrdings were made sme truble was experienced with variatin f the sensitivity f the tape recrder as a result f variatin in the utput vltage f the vibratr pwer supply. It is likely that the sensitivity f the instrument was quite cnstant during play-back, but n definite infrmatin is available n any sensitivity variatins which ccurred. Fr this reasn, the abslute levels indicated fr the analyses f these recrdings may have an errr f as much as ±5 decibels. The amplitudes f the varius frequency cmpnents relative t each ther were prbably nt affected by the sensitivity variatins. Frequency respnses fr the sund-level meter and sund analyer shwn in figure 8 were determined at the Massachusetts Institute f Technlgy Acustics Labratry. Interpretatin f the readings f the sund analyer was made by using these curves. Instrumental errrs in the sund analyer are prbably within ±2 decibels, but the actual levels presented in the grund analyses are subject t smewhat larger errrs in sme cases because readings frequently had t be made f levels that fluctuated as much as 10 decibels. This prblem was particularly severe with the higher harmnics f the prpeller, s the levels indicated fr these higher harmnics cannt be cnsidered mre accurate than ±6 decibels. METHD F PRESENTING RESULTS Sund measurements were made n the eight cnfiguratins f the pusher airplane described in table I. Measurements were als made n cnfiguratin 5 f the standard tractr. In additin, averages f measurements n cnfiguratins 1, 2A, 2B, 2F, 2C, and 2D f the tractr, made fr the previus reprt (reference 6), are presented here fr cmparisn with the pusher measurements. The results f these measurements n the pusher and tractr airplanes are presented in eight series s that the effect f different variables may be cnsidered separately. These eight series are as fllws: Series A: Series A is a cmparative study f the external nise f a standard pusher with an Aermatic prpeller (cnfiguratin 6) and a mdified pusher using experimental three-, fur-, six-, and eight-bladed geared prpellers (cnfiguratins 9A, 9B, <?C, and 9D) in cnjunctin with an exhaust muffler. These measurements were all made at cmparable maximum pwers, since the pitch f the experimental prpellers was set separately fr grund and flight measurements t give perfrmance cmparable with that f the Aermatic prpeller in bth situatins. (See figs. 17 t 30.)

13 12 NACA TN 2727 Series B: Series B is a study f the effect f muffler installatin n the external nise f the fur-bladed, geared-drive pusher airplane. This series cmpares the mdified pusher, unmuffled (cnfiguratin 8), with the mdified pusher (cnfiguratin 9B) as used in series A with the muffler exhausting thrugh the prpeller. The same fur-bladed experimental prpeller was used with each cnfiguratin at cmparable pwers and tip speeds. (See figs. 31 t 35.) Series C: Series C is a cmparisn f tw muffler lcatins n the mdified pusher using the eight-bladed prpeller. This series cmpares the mdified pusher (cnfiguratin 9D) as used in series A with the exhaust passing thrugh the prpeller and the mdified pusher with the muffler relcated s that the exhaust did nt pass thrugh the prpeller (cnfiguratin 10). (See figs. 36 t k.) Series D: Series D is a cmparisn f the grund analyses at 1900 and 2500 rpm f the fur-bladed, mdified pusher, unmuffled and muffled (cnfiguratins 8 and 9B). (See fig. 4l.) Series E: Series E is a cmparative study f the external nise f the standard pusher using an Aermatic prpeller (cnfiguratin 6) and a slid fur-bladed prpeller (cnfiguratin 7). (See figs, k-2 t 46.) Series F: Series F is a cmparisn f the external nise levels f the standard and mdified pusher cnfiguratins studied in this reprt with the nise f the standard and mdified tractrs studied in the earlier reprt (reference 6). (See figs. ^7 t k9.) Series G: Series G includes measurements frm 3000 feet away f the standard tractr (cnfiguratin 5); the standard pusher (cnfiguratin 6); the mdified pusher, unmuffled (cnfiguratin 8); and the mdified pusher, muffled, with the fur-bladed prpeller (cnfiguratin 9B). (See figs. 50 t 53«) Series H: Series H is a frequency analysis f the nise f the mdified pusher, muffler relcated (cnfiguratin 10), passing 3000 feet away. This analysis frm magnetic tape recrdings is cmpared with the grund analysis fr this cnfiguratin at the same speed and pwer. (See fig. 5^.) With the exceptin f the measurements frm 3000 feet away, the results f all flight measurements are presented in the frm f graphs in which the sund level is pltted against hrintal distance frm micrphne t airplane. Fr the measurements frm 3000 feet away, the results are pltted in terms f sund level against the hrintal distance f the airplane frm the pint n its flight path nearest t the sund-level-meter micrphne. The recrds fr all flights at bth 100- and 500-ft altitudes are pltted t 5000 feet either side f the er

14 NACA TN pint. Negative distances pltted t the left f er crrespnd, in all cases, t the airplane -appraching, while psitive distances t the right f er crrespnd t the airplane ging away. As in the previus reprt (reference 6) it was decided t use the data taken with the flat weighting f the sund-level meter fr the flights at 100-ft altitude and the data taken with 40-decibel weighting fr the flights at 500-ft altitude. This means that the pltted data fr the tw altitudes are nt directly cmparable. This chice prvides data taken with flat weighting fr sund levels reaching 90 t 100 decibels and data taken with 40-decibel weighting fr sund levels frm 1*5 t 70 decibels. It was felt that this prcedure wuld prvide the mst meaningful data. It is wrth nting that the graphs f these flights, shwing sund levels against hrintal distance, are nt crrected fr the finite velcity f sund. In ther wrds, the sund level shwn when, fr example, the airplane was 3000 feet frm the er pint f the graph is actually the sund level at the micrphne at that time. Since that sund tk sme time t reach the micrphne, it was actually generated when the airplane was smewhat farther away as it apprached, r nearer when it was ging away. This crrectin wuld make the largest difference in the measurements frm 3000 feet away. Fr instance, the nise generated in these flights by the appraching airplane when it was 3000 feet frm the nearest pint f its flight path wuld reach the micrphne when the airplane was abut 2300 feet frm the nearest pint. The recrds, as pltted, indicate what an bserver at the measuring psitin wuld hear and it is evident that the absence f the crrectin has n effect n cmparisns between different cnfiguratins mving at the same velcity and measured frm the same pint. Crrectin has been made fr the finite velcity f sund fr the graphs f figures 53 and 5k. Fr the take-ff measurements, it was nt pssible t plt the sund level against distance since the airplane was cnstantly changing its velcity; therefre, the sund level was pltted against time. The data taken with flat weighting f the sund-level meter have been used in pltting take-ff sund levels. Infrmatin frm the grund analyses is presented in the frm f plar plts f the ver-all level and the amplitude f each significant harmnic cmpnent present in the nise f the airplane. Grund analyses were all made with flat weighting f the sund-level meter. The analyses f the tape recrdings, and. the grund analyses cmpared with them, are pltted in separate graphs fr each analysis which shw sund level against frequency. These graphs indicate the level f each imprtant harmnic frequency cmpnent present in the nise analyed.

15 llj. NACA TN 2727 In presenting the data fr series A, B, C, E, and G a standard prcedure is fllwed. In each series the basic data, cnsisting f separate graphs fr each cnfiguratin f that series in each measurement cnditin, are presented first. These data are fllwed by cmparative plts in which averaged curves fr each cnfiguratin are cmpared, fr each f the measurement cnditins. Fr the grund analyses the cmparative plts present separate cmparisns f the ver-all levels, engine fundamentals, and prpeller fundamentals fr each f the cnfiguratins f the series. In series D and H nly basic data are presented since fr these series it was felt that the mst useful cmparisn culd be made by studying the data in this frm rather than by trying t plt data fr different cnfiguratins n the same graph. Series F cmpares averaged data frm the present reprt with averaged data frm the earlier reprt (reference 6). DISCUSSIN F RESULTS Series A. Cmparisn f Standard and Mdified Pusher Airplanes (Cnfiguratins 6, 9A, 9B, 9C, and 9D) Take-ffs.- The results f the take-ff measurements fr the five cnfiguratins f series A are shwn in figure 17. Each f the five graphs shws sund level against time fr each f tw successive takeffs. Averages f the tw curves shwn in each f the five graphs are presented tgether fr cmparisn in figure 23. In figure 17, the differences between the recrds taken under suppsedly identical cnditins indicate the degree f reprducibility achieved between flights made n the same day. Since the flights with different cnfiguratins were made n different days with different atmspheric cnditins, the cmparisns shwn in figure 23 are subject t larger uncertainties. An additinal prblem was intrduced by the necessity f cmparing the fixed-pitch (grund-adjustable nly) prpellers f the mdified pusher with the Aermatic cnstant-speed prpeller f the standard pusher. With the Aermatic prpeller at full thrttle the take-ff started at 2500 rpm and increased t a maximum f 26 rpm. As previusly explained, the experimental prpellers were set fr 2500 rpm (1^5 hp) at full thrttle at the start f the take-ff and the pilt was instructed t limit the speed t 2600 rpm during the takeff run. This prcedure was fund t be difficult, hwever, and it is therefre likely that the 2600-rpm limit was exceeded during sme f the take-ffs. This wuld affrd a reasnable explanatin f why the maximum level f the three-bladed, geared prpeller n the mdified pusher (which was measured first when this technique was new) is nearly as high as that f the standard pusher (fig. 23).

16 NACA TN If the high peak fr the three-bladed take-ff is ignred, the average curves f figure 23 shw the standard pusher t be frm 5 t 10 decibels nisier than the average f the fur mdified cnfiguratins. Cmparing the varius mdified cnfiguratins, it appears that the three-bladed cnfiguratin (9A) was 2 t 3 decibels nisier than the average f the mdified cnfiguratins, at psitins away frm the take-ff pint, while the fur- and six-bladed cnfiguratins (9B and 9C) were slightly quieter than the average. The curve fr the eightbladed cnfiguratin (9D) falls between that fr the three and thse fr the fur- and six-bladed cnfiguratins. Cnsidering the difficulty f maintaining accurate cntrl f the take-ffs, hwever, it is prbably nt reasnable t ascribe much significance t these small differences. Flight measurements.- The results f the flight measurements, fr the five cnfiguratins f series A, are shwn in figures 18 t 21. Figure 18 presents the basic data fr each f the cnfiguratins fr the maximum-pwer flights at 100-ft altitude. Figure 19 presents the data fr 100-ft cruising pwer; figure 20, the data fr 500-ft maximum pwer; and figure 21, the data fr 500-ft cruising pwer. Each graph shws sund level against hrintal distance fr fur successive flights. Averages f the fur curves shwn in each f the five graphs f figure 18 are presented fr cmparisn in figure 2k. Similarly, averaged curves fr the ther flight cnditins are presented in figures 25 t 27. It can be seen frm figures 18 t 21 that successive flights with the same airplane, at the same pwer and altitude, smetimes prduced nearly identical successive sund-level recrds. At ther times the difference between successive flights was as much as 10 r 15 decibels at certain pints. This variatin in the sund level f successive flights and the ccasinal large fluctuatins in individual sund-level recrds were nticed in the earlier wrk (reference 6). During the present study, it appeared as the wrk prgressed that these fluctuatins culd nly be the result f atmspheric cnditins. It was very clear that the fluctuatin in a single recrd was nt a prduct f the terrain r f stray reflectins, because all flights were made ver the same curse in the same way, while the fluctuatins were very different n different recrds. Furthermre, evidence frm the earlier wrk (reference 6) shwed that mre vilent fluctuatins were bserved n windy days and that the magnitude f the fluctuatins tended t increase with an increase f the hrintal distance f the airplane. All these things suggested that turbulence and temperature and wind velcity gradients in the air might be majr causes f the bserved fluctuatins in the sund-level recrds. As a result f this evidence, it was decided t make sund-level measurements nly when the wind was 5 miles an hur r less. It was fund that infrmatin abut the wind cnditins near the altitude f \

17 l6 NACA TN 2727 the 500-ft flights culd be btained frm the Blue Hill weather bservatry lcated at an altitude abut 600 feet abve that f the Nrwd airprt and abut 3 miles away frm it. A cmbinatin f the reprt frm Blue Hill, the pilt's reprt f turbulence, and the visual bservatin f the behavir f the Kytn altitude marker gave a fairly clear picture f what the wind and turbulence cnditins were during each series f flight measurements. Crrelatin f this infrmatin with the amunt f fluctuatin in the sund-level recrds revealed that the best cnditins fr testing did nt ccur when there was dead calm. Apparently, when the air was very calm the airplane began t encunter its wn turbulence, in successive flights, as a result f flying repeatedly ver the same curse. Additinal truble with fluctuatin was als experienced n calm sunny days because f turbulence due t uneven heating f the grund and the resultant thermal air currents. The best test results were btained when the sky was vercast, s that there was little radiant heating f the grund, and when there was a 2- r 3-mile-an-hur wind acrss the flight path. Apparently, in this case the air was smth, and the turbulence in the wake f the airplane, due t ne flight, drifted away frm the flight curse quickly enugh s that the airplane was always flying thrugh undisturbed air. The variable which best crrelated with the fluctuatin bserved in the sund-level recrds was the pilt's reprt f air cnditins. The pilt generally reprted rugh air when there was a large amunt f fluctuatin in the sund recrds. Furthermre, when the sund recrds were exceptinally free f fluctuatin, the pilt nticed exceptinally smth flying. ccasinally, hwever, when the pilt nticed smth flying, sme slw fluctuatin was bserved in the sund recrds. This seemed t be particularly true n calm days when it seemed likely that the airplane was flying thrugh its wn turbulence. Anther prblem that must be cnsidered in making sund-level measurements f verhead flights is the prblem f the existence f relatively unifrm gradients f temperature r wind velcity. Such gradients cause a refractin f sund prpagating thrugh them (see reference 7). This refractin may cause sund levels measured several thusand feet away frm the surce t differ cnsiderably frm what they wuld be with unifrm atmspheric cnditins. Furthermre, if the abnrmal refractin is stable there will be little indicatin that it is ccurring. It appears that in rder t reduce the uncertainty in measurement caused by the prblems f turbulence and abnrmal gradients it wuld be necessary t make several series f measurements n different days with different atmspheric cnditins and average all the results. Since the numerus flight measurements f the present reprt culd nt be repeated n different days, these tw prblems lend an element f uncertainty t the results f the flight measurements, especially fr the sund levels measured when the airplane was 3000 r 4000 feet away.

18 3V NACA TN Even with these variables, hwever, the averaged cmparative plts (figs. 2k t 27) shw cnsiderable similarity in the shapes f the * sund-level recrds fr the different airplane-prpeller cnfiguratins. Figure 27, in particular, shws a great similarity in the shapes f the sund-level recrds fr the fur mdified cnfiguratins. An interesting feature f the flight recrds at 500 feet is the absence f a definite peak at the verhead psitin. This is in marked cntrast with the crrespnding recrds f the earlier wrk (reference 6). The difference is presumably due t the shielding effect f the fuselage in the case f the pusher-type airplane (with the verhead munting f the engine, figs. 1 and 2), which was absent in the case f the tractr (with a nse munting f the engine, fig. 6). A cmparisn f the averaged flight recrds fr the pusher and tractr airplanes is made in the discussin f series F and this difference in shape is discussed mre fully there. As was the case with the take-ff measurements, the flight measurements shw a distinct separatin between the curves f sund-level variatin fr the standard pusher and these curves fr the mdified pushers, «withut shwing differences in the mdified cnfiguratins which lie in the rder f the number f blades. Fr the flight recrds, the threebladed cnfiguratin (9A) appears t be ne f the quietest and the eight-bladed (9D), ne f the nisiest. There is certainly n evidence t indicate that it wuld be desirable t use a six- r eight-bladed prpeller.n the pusher in an attempt t quiet it. n the cntrary, it was the pinin f everyne that heard the varius mdified cnfiguratins that the eight-bladed prpeller made the mst annying nise. The high-pitched whine f the eight-bladed cnfiguratin was particularly nticeable at a distance when the sund levels invlved were such that the ear culd hear a high-pitched sund as luder than a lw-pitched ne f the same intensity. Anther reasnable bjectin t the sund f the eight-bladed cnfiguratin was that it did nt sund like a nrmal airplane. This feature might call attentin t its presence, when it might nt therwise be nticed. Grund analyses.- The results f the grund analyses fr the five cnfiguratins f series A are shwn in figure 22. Each f the five plar plts shws sund level against psitin arund the airplane, fr the ver-all level and significant harmnic cmpnents. Figure 28 cmpares the plar plts f the ver-all levels fr each f the five cnfiguratins. Figure 29 makes a similar cmparisn f the engine fundamentals, and figure 30 cmpares the prpeller fundamentals. Fr each cnfiguratin the frequency referred t as the engine fundamental is three times the engine crankshaft speed, while the prpeller fundamental * is simply the tip-passage frequency. Since the ver-all level fr the standard pusher exceeded 110 decibels fr certain psitins arund the airplane, the basic data fr this cnfiguratin and the cmparative plt

19 l8 NACA TN 2727 fr the ver-all levels have been pltted s that the uter circle f the plar crdinates crrespnds t 120 decibels. The remaining plar plts have been pltted s that the uter circle crrespnds t 110 decibels in rder that the detail at levels belw 90 decibels maybe clearly shwn. It may be seen in the plt fr the standard pusher (cnfiguratin 6) in figure 22 that the ver-all level fr the test cnditins used is largely determined by the level f the prpeller fundamental. At nly three psitins des the engine fundamental exceed the prpeller fundamental, and. at these psitins it is nly slightly luder. At the 90 psitin n the right, the engine fundamental was s much lwer than the prpeller nise that it culd nt be picked ut n the analyer. It wuld appear that the additin f a muffler t the standard pusher (cnfiguratin 6) withut ther mdificatins wuld nt result in an appreciable reductin f the nise generatin at 2500 rpm n the grund. The grund analysis fr the three-bladed, mdified pusher (cnfiguratin 9A) in figure 22 shws a great predminance f the prpeller cmpnents ver the engine fundamental. Furthermre, the ver-all-level curve crrespnds very clsely in shape t the curve fr the prpeller fundamental. The muffler n the mdified cnfiguratin (9A) evidently is effective in reducing the engine nise t the pint where it is nt significant fr this test cnditin. Fr the fur-bladed prpeller n the mdified pusher (cnfiguratin 9B) the engine nise was s lw that it culd nt be picked ut n the analyer except at tw f the measuring psitins. Fr this reasn it has been mitted frm the plar plt f figure 22. Fr this cnfiguratin, as fr the three-bladed cnfiguratin, the ver-all level is very similar in mst psitins t the prpeller fundamental. The nly significant exceptin is the 150 psitin n the right where the verall level is 8 decibels greater than the strngest harmnic cmpnent. At this measuring psitin a large amunt f nnharmnic nise was bserved, prviding a fairly unifrm backgrund nise indicated n the analyer frm abut 250 t 1000 cycles per secnd and nly a few decibels lwer than the amplitude f the third harmnic. This unifrm, nnharmnic nise, which seems t be an exaggerated vrtex nise, appears again in the analysis f the six-bladed cnfiguratin f the mdified pusher (cnfiguratin 9C, fig. 22). In this case it was f sufficient amplitude t mask the third harmnic f the prpeller. This vrtex nise was very prnunced fr the psitins in frnt f and behind the airplane, but at the side the harmnic tip-passage frequency was still nearly as lud as the ver-all level.

20 WACA TN The eight-bladed prpeller n the mdified pusher (cnfiguratin 9D, fig. 22) prduced a little mre f this vrtex nise. In mst f the psitins arund the airplane the nise did nt sund particularly like a cnventinal airplane whine but like an unpleasant tearing nise. Althugh this nise was nt s lud as that f the three-bladed cnfiguratin (9A), it seemed particularly annying. Fr the 150 psitin n the right fr the eight-bladed cnfiguratin, the analyer indicated a sund level frm 83 t 87 decibels thrughut the frequency range frm 250 t 1050 cycles per secnd. Abve the frequency f 1050 cycles per secnd the level f this nise decreased and it appeared that there was a rather sharp upper limit t its frequency spectrum. This upper frequency limit is a characteristic f the vrtex nise cmmnly nticed n tractr-type airplanes when the prpeller tip speed is reduced (references 3 and 8). The vrtex nise generated by the pusher type seems t be similar t that fr the tractr airplane, except that it ccurs at a much higher nise level fr a given tip speed and pwer. The cmparative plt f the ver-all sund levels (fig. 28) shws a well-defined separatin f the curve fr the standard pusher (cnfiguratin 6) and the curves fr the varius cnfiguratins f the mdified pusher (9A, 9B, 9C, and 9D). In additin, there seems t be a tendency fr the sund level t decrease with an increase in the number f blades. Anther interesting change with the increasing number f blades is the change frm the cnditin where the prpeller nise, predminant n the sides, is the ludest nise t the cnditin, with the eight-bladed prpeller, where the psitins near the nse and tail are nisiest because f the predminant vrtex nise. The cmparative plt f the engine fundamentals (fig. 29) shws hw effective the muffler was in reducing nise ccurring at three times the crankshaft frequency. Differences in the engine fundamentals fr the varius mdified cnfiguratins which had the same muffler and engine installatins (9A, 9B, 9C, and 9D) are nt clearly understd. They may be partly the result f sme srt f intermdulatin phenmenn in which energy at the engine fundamental frequency is cnverted t energy at a frequency which is the sum r difference f tw frequencies present in the nise f the airplane. This phenmenn has been discussed in the earlier reprt (reference 6). Sum and difference frequencies have been nticed at certain psitins in the grund analyses fr the present reprt, affrding evidence that this type f intermdulatin des ccur n this airplane as well as n the tractr airplane. Since in the mdified cnfiguratins f series A the exhaust frm the muffler is being chpped at the prpeller tip-passage frequency, the nnlinear cnditins necessary fr the prductin f sum and difference frequencies are almst certainly present. The cmparative plt f the prpeller fundamentals (fig. 30) shws nce again a separatin between the standard (cnfiguratin 6) and the

21 20 NACA TN 2727 mdified (cnfiguratins 9A, 9B, 9C, and 9D) pushers. There als appears t be a difference between the shape f the curve fr the pr,, peller fundamental f the standard pusher and the general shapes f the prpeller-fundamental curves fr the mdified cnfiguratins. In the prpeller fundamentals, there is a definite tendency fr the level t decrease with an increase in the number f prpeller blades. This effect wuld be expected frm Guti^s thery, althugh it did nt shw up clearly in the ver-all levels and in the flight measurements. The lack f agreement with the Gutin thery in the ver-all levels f the grund analysis is prbably a result f the increased vrtex nise with the six- and eight-bladed cnfiguratins. The lack f agreement with the Gutin thery in the flight measurements might als be explained by an increase in vrtex nise fr the six- and eight-bladed cnfiguratins, but there is evidence t indicate that this is nt the crrect explanatin. Further infrmatin n this prblem is presented in the discussin f the flight analysis f series H and in the general discussin f all the results. Series B. Effect f Installing a Muffler n Fur-Bladed, Geared-Drive Pusher (Cnfiguratins 8 and 9B) Take-ffs and flight measurements.- The basic data fr the tw cnfiguratins f series B fr take-ff and flight are shwn in figures 31, 32(a), and 32(b). The data fr cnfiguratin 9B are, f curse, the same as thse fr 9B in series A, but they are presented again in series B fr cnvenience. A similar prcedure is emplyed thrughut the rest f the data. Averaged curves fr the take-ff and flight measurements are cmpared fr the tw cnfiguratins f this series in figures 33 and 3k. N particular cmment seems necessary n the basic data. The recrds fr the mdified pusher, unmuffled (cnfiguratin 8), at 500-ft altitude shw cnsiderable fluctuatin at distances f several thusand feet, but this prblem has been discussed in the discussin f the flight recrds fr series A. The cmparative plts shw significant differences between the external nise levels f the unmuffled and muffled mdified pusher. The averaged curves fr the take-ffs shw a difference averaging 5 t 8 decibels between the tw cnfiguratins. The muffler evidently reduced the take-ff nise. This result is in agreement with the measurements f the ver-all levels fr the grund analyses. The cmparative plts fr the flight measurements d nt shw such a marked separatin between the curves fr the unmuffled and muffled pusher. A maximum difference f abut 5 decibels ccurs near the verhead psitin fr the recrds at 100-ft maximum pwer, 100-ft cruising pwer, and 500-ft maximum pwer. Fr the flights at 500-ft

22 NACA TN cruising pwer n.such difference was bserved. This discrepancy is prbably explained by the use f the 40-decibel weighting netwrk fr the flights at 500-ft altitude. The change in the relatin between the curves fr the tw cnfiguratins when the ^-decibel netwrk was used may be the result f changes in the relatinship between the harmnics and the fundamental f the engine nise with the reductin in speed. It might als be the result f changes in the relatinship between engine and prpeller nise with the reductin in speed. Reference t table II n page Ik f reference 9 shws that changes in the relatinships between the fundamental and the harmnics f the engine nise d ccur with changes in engine speed. Grund analyses.- The grund analyses fr the unmuffled and muffled mdified pusher (cnfiguratins 8 and 9B) are presented in figure 32(c). Cmparisn f the ver-all levels, engine fundamentals, and prpeller fundamentals frm these analyses are presented in figure 35. The grund analysis fr cnfiguratin 8 (fig. 32) shws that the prpeller fundamental and engine fundamental are abut equally imprtant in the nise f the unmuffled, geared pusher running at 2500 rpm n the grund. The engine fundamental predminates slightly in frnt f and t the rear f the airplane, while the prpeller fundamental predminates at bth sides. In the grund analysis fr the muffled cnfiguratin (9B, fig. 32) the engine fundamental was measurable at nly tw psitins s it has been mitted frm the basic data. The cmparative plt f the engine fundamentals shws t what extent the engine nise was reduced by the muffler. In the tw psitins where the engine fundamental f the muffled cnfiguratin was measurable it was 20 decibels lwer than that fr the unmuffled cnfiguratin. It was prbably at least this much lwer fr thse psitins where it culd nt be bserved in the grund analysis. The cmparative plt f the ver-all levels frm the- grund analyses (fig. 35) shws a significant difference in the shapes f the curves fr the unmuffled and muffled cnfiguratins. The unmuffled cnfiguratin was luder at all measuring psitins than the muffled cnfiguratin, and in additin the level fr the unmuffled cnfiguratin was nearly cnstant all the way arund the airplane. The ver-all-level curve fr the muffled cnfiguratin shws the shape characteristic f the prpeller fundamental curve, with the maximum levels ccurring at the 90 psitins n bth sides. The difference between the radiatin pattern fr these cnfiguratins des nt appear t be in agreement with the results f the flight measurements which shwed the unmuffled cnfiguratin t be nisier than the muffled ne nly near the verhead psitin f the flight path. It might be expected that the verhead psitin f the flight path wuld mst nearly crrespnd t the 90 psitin f the grund analysis, but it was at this psitin that the minimum difference existed between the tw cnfiguratins in the ver-all levels

23 22 NACA TN 2727 f the grund analyses. This discrepancy cannt be explained satisfactrily with the available data. It must be nted that, as previusly mentined, the take-ff measurements are cnsistent with the results f the grund analysis. This wuld suggest that there is a difference between the radiatin characteristics f the airplane perating at maximum pwer n the grund and perating at the same pwer and engine speed in flight. This cnclusin seems reasnable if the lcatin f engine and prpeller with relatin t the hull and wings f the pusher airplane is cnsidered in cnjunctin with the psitins frm which the indicated nise levels were measured. Fr the grund analyses and the take-ff measurements the sunds radiated t either side are measured. Fr the flight measurements near the verhead psitin, the sund underneath the airplane is measured. Figures 1 and 2 shw why these psitins present a different view f the principal surces f nise n the airplane. Since the engine and prpeller are shielded by the hull and the wings when the airplane is near the verhead psitin fr the flight measurements, it is nt surprising that the relatinships between the nise f different cnfiguratins bserved in the grund analysis des nt hld fr the peak levels measured verhead. It is nt s clear why there shuld be a difference between the results f the grund analyses in frnt f and behind the airplane and the results f the flight measurements when the airplane was several thusand feet away either appraching r ging away. These discrepancies between the results f the grund analyses and the flight measurements ccur thrughut the data f the present reprt and have previusly ccurred in ther measurements f this nature (reference 6) and in unpublished data taken by the staff f the Electr-Acustic Labratry, Harvard University, 19^0-19*15, under the general directin f Dr. L. L. Beranek. The cmparisn f the prpeller fundamentals frm the grund analyses f the unmuffled and muffled pusher (fig. 35) shws a difference between the curves fr the tw cnfiguratins. Since the same prpeller was used n bth cnfiguratins and perated at very nearly the same speed and pwer it is nt evident why there shuld be such a difference. Series B was intended t study the effect f adding a muffler t the unmuffled cnfiguratin while hlding ther variables cnstant; hwever, the prpeller is invlved in the muffler installatin because the exhaust cming ut f the muffler passes thrugh the prpellers (see figs. 2 and 8). This might be expected t increase the nise generatin at the prpeller tip-passage frequency since the blades f the rtating prpeller slice thrugh the rapidly mving stream f exhaust gases. Actually the grund analyses shwed the prpeller nise generatin t be less fr the muffled cnfiguratin than fr the unmuffled. It appears that the exhaust passing thrugh the prpeller reduces nise generatin at the tip-passage frequency f the prpeller. This

24 NACA TN can be explained if the prpeller blades slicing thrugh the clumn f exhaust gas prduce a surce f nise f an intensity cmparable with that f the nrmal prpeller nise. In this case the pressure wave cming frm the vicinity f the tip f each prpeller blade will be a maximum at the same time that the pressure wave prduced as the blade cuts thrugh the clumn f exhaust gas is a minimum. This effect prvides tw surces f nise at the tip-passage frequency which are ut f phase and may partially cancel each ther, at least fr certain directins f radiatin arund the airplane. This wuld cause a reductin in the nise at the tip-passage frequency which culd be quite significant at sme psitins if the tw surces are f nearly the same magnitude. Evidence t supprt this hypthesis is als fund in the data f series C where the mdified pusher with the eight-bladed prpeller and the exhaust passing thrugh the prpeller is cmpared with the same engine-prpeller cnfiguratin with the muffler relcated s that the exhaust did nt pass thrugh the prpeller. In this case, as in the similar cmparisn f series B, the cnfiguratin with the exhaust passing thrugh the prpeller had lwer levels fr the prpeller fundamental than the cnfiguratin in which the exhaust did nt pass thrugh the prpeller (see fig. k). In additin, the grund analysis shwed the secnd harmnic f the prpeller t be almst as lud as the fundamental fr the cnfiguratin where the exhaust passed thrugh the prpeller but shwed it t be cnsiderably quieter than the prpeller fundamental fr the cnfiguratin with the muffler relcated. This suggests a relative reductin in the prpeller nise generatin at the tip-passage frequency when the exhaust passes thrugh the prpeller. Unfrtunately, this clear-cut difference in the relatinship between the prpeller fundamental and secnd harmnic in the grund analysis did nt appear in the analysis fr series B, where a reductin in the relative level f the prpeller fundamental wuld be expected t ccur n the cnfiguratin with the exhaust passing thrugh the prpeller. Furthermre, it must be remembered that a reductin f as much as 8 r 10 decibels cannt be explained by the hypthesis f tw surces, if the surce prduced by the prpeller chpping the exhaust des nt make almst as much nise as the nrmal prpeller nise. Calculatins have been made f the magnitude f velcity variatin which wuld have t be intrduced in a 2-inch clumn f exhaust t prduce a secnd surce sufficiently intense t prduce a cancellatin reductin f 10 decibels. These calculatins shw that a peak-t-peak velcity variatin f abut 30,000 centimeters per secnd r apprximately 1000 feet per secnd wuld be necessary fr this large cancellatin under the assumed cnditins. It is evident that this much variatin culd nt be prduced in a clumn f exhaust gas mving at a velcity certainly ht exceeding 500 feet per secnd. It des seem reasnable, hwever, that a cancellatin f smewhat less than 10 decibels might ccur since this wuld nt require s large a variatin in velcity. Furthermre, the assumed cnditins may be sufficiently mdified, under the actual perating cnditins, s that the calculatins d nt present a fair picture f the nise generatin by the prpeller chpping the exhaust.

25 2U NACA TN 2727 The evidence n this subject is thus incnclusive, and further study wuld he necessary t determine whether the suggested slutin t the prblem presented is the crrect ne. Series C. Effect f Relcating the Muffler n Eight-Bladed, Geared-Drive Pusher s That Exhaust Did Nt Pass thrugh Prpeller (Cnfiguratins 9D and 10) Take-ffs and flight measurements.- The basic data fr the tw cnfiguratins f series C, fr the take-ff and flight measurements, are shwn in figures 36, 37(a), and 37(b). The data fr the mdified pusher, muffler relcated (cnfiguratin 10), at 500-ft cruising pwer (fig. 37(b)) are f particular interest since the measurements were made under the best test cnditins encuntered thrughut the entire series f measurements fr this reprt. Fr these recrds the maximum difference between successive recrds at any pint is k decibels, and the average maximum difference is abut 2 decibels. Cmparisn f these recrds f successive flights at 500 feet with the recrds fr ther cnfiguratins in this flight cnditin will reveal the unusual cnsistency f these recrds fr cnfiguratin 10. These measurements were made just after sunset n a day when there was a very slight wind acrss the flight path and the sky was partially vercast. These cnditins prvided what the pilt described as perfectly smth air and prvided sund-level recrds which were particularly smth and regular. These recrds prvide additinal cnfirmatin f the theries advanced in the discussin f the flight recrds f series A abut the surces f the fluctuatins usually encuntered in flight recrds. Averaged curves fr the take-ff and flight measurements fr the tw cnfiguratins f this series are cmpared in figures 38 ant l 39- With the exceptin f the take-ff recrds the mdified pusher with the muffler exhausting thrugh the prpeller (cnfiguratin SD) averages slightly quieter than the mdified pusher with the muffler relcated (cnfiguratin 10). This difference is small enugh s that its existence is nt clearly established. Since this difference des ccur fr all flight measurements it may be statistically a significant difference, but it is certainly nt a very imprtant ne. The results f the takeff measurements shw the mdified pusher with the muffler relcated t be slightly quieter, but these results are nt in agreement with the results f either the flight measurements r the grund analyses. The measured difference is small and prbably nt significant. The measurements fr series C were made n the eight-bladed cnfiguratin f the mdified pusher because it was expected that any difference in the nise generatin f the airplane caused by relcating the

26 NACA TN muffler s that the exhaust did nt pass thrugh the prpeller wuld be largest with the greatest number f prpeller blades. The measurements shw that the cnfiguratin with the exhaust passing thrugh the prpeller is, if anything, slightly quieter than the cnfiguratin with the muffler relcated. Since this is true, there seems t be n reasn why a muffler used n this airplane, under the cnditins fr which it was tested, shuld nt be munted abve the engine in the psitin f cnfiguratins 9A, 9B, %, and 9D. n the pusher airplane this munting psitin appears t affrd the simplest installatin. Grund analyses.- The grund analyses fr the mdified pusher (cnfiguratin 9D) and the mdified pusher, muffler relcated (cnfiguratin 10), are shwn in figure 37(c). Cmparisn f the ver-all levels, engine fundamentals, and prpeller fundamentals frm these analyses are presented in figure k. An interesting difference is immediately evident between the grund analyses fr cnfiguratin 9D, with the exhaust passing thrugh the prpeller, and cnfiguratin 10, with the muffler relcated. The grund analysis fr cnfiguratin 10 des nt shw the wide separatin between the ver-all-level curve and the curves fr the harmnic cmpnents f the engine and prpeller nise that was bserved fr cnfiguratin 9D. It was pinted ut in series A, where the analysis fr cnfiguratin 9D was discussed, that the ver-all level fr this cnfiguratin was much greater than the strngest f any f the harmnic cmpnents f the engine r prpeller nise. This separatin between the ver-all-level curve and the curves fr the harmnic cmpnents was accmpanied by a high level f, nnharmnic nise between the frequencies f 250 and 1050 cycles' per secnd. In the grund analysis fr the mdified pusher with the muffler relcated s that the exhaust did n't pass thrugh the prpeller (cnfiguratin 10) sme nnharmnic nise was als nticed particularly at the 150 psitins; hwever, this nnharmnic nise ' ccurred at a level several decibels belw the level at which it ccurred fr cnfiguratin 9D. It is bviusly the lwer level f the nnharmnic vrtex nise fr cnfiguratin 10 that explains the difference between the grund analyses f cnfiguratins 9D and 10. The cmparisn f the ver-all levels frm the grund analyses fr the tw cnfiguratins f this series shws very little difference between the curves. There is a slight predminance f the ver-all level fr the cnfiguratin with the exhaust passing thrugh the prpeller (9D) at the frnt and t the rear f the airplane. At the 90 psitins the cnfiguratin with the muffler relcated is slightly nisier. The cmparisn f the engine fundamentals fr the tw cnfiguratins shws a significant difference between the tw curves. The muffler as used in cnfiguratin 10, allwed much mre engine nise t be radiated

27 2g NACA TN 2727 than it did fr cnfiguratin 9D. Tw pssible explanatins f this phenmenn are as fllws: First, it is quite pssible that the cnfiguratin f muffler and exhaust pipes used in the relcatin f the muffler prduced a system in which sme resnance at the exhaust frequency ccurred at 2500-rpm engine speed. The difference in length f exhaust pipes frm the tw sides f the engine may have been a cntributing factr t the reductin in efficiency f muffling (see fig. h). Secnd, it is pssible that intermdulatin between engine and exhaust frequencies, fr cnfiguratin 9D where the exhaust was chpped at the prpeller tip-passage frequency, cnverted sme f the energy at the engine fundamental frequency t energy at a frequency which was the sum r difference f engine and prpeller frequencies. As previusly mentined, these sum and difference frequencies have been bserved in sme f the analyses. The cmparisn f the prpeller fundamentals fr the cnfiguratins f series C has been mentined in the discussin f the cmparable plt in series B. It was suggested there that the reductin f nise at the prpeller tip-passage frequency, when the exhaust passed thrugh the prpeller, might be explained by cnsidering the nrmal prpeller nise and the nise generated by the prpeller chpping the clumn f exhaust as tw surces which are l80 ut f phase. It is interesting t nte, in this cnnectin, the high level f vrtex nise fr cnfiguratin 9D with the exhaust passing thrugh the eight-bladed prpeller. As is shwn by the cmparisn f the ver-all levels fr cnfiguratins 9D and 10, the reductin in the level f the prpeller fundamental where the exhaust passed thrugh the prpeller is balanced by an increase in the generatin f the nnharmnic vrtex nise. Since the ver-all levels in either case are abut the same, the change in the character f the nise is the nly significant difference. There des nt seem t be any particular reasn fr preferring ne type f nise t the ther. A frequency analysis f the nise generated by cnfiguratin 10 in flight is presented in the discussin f series H. The results f this flight analysis are quite different frm the results f the grund analysis fr this cnfiguratin. Fr this reasn the cnclusins abve must be restricted t peratin n the grund. An analysis f the nise f cnfiguratin 9D in flight wuld be needed t shw whether it prduces as much nnharmnic nise in flight as it des n the grund. Series D. Cmparisn f Grund Analyses at 1900 and 2500 rpm fr Fur-Bladed, Geared-Drive Pusher, Unmuffled and Muffled (Cnfiguratins 8 and 9B) When the grund analyses were first made fr cnfiguratins 8 and 9B they were made with the prpeller pitch set as it was fr the flight

28 NACA TW measurements. This measurement prvided a grund analysis at 1900 rpm, full thrttle, fr each f these cnfiguratins. It was subsequentlydecided t reset the grund-adjustable prpellers and make the grund analyses fr all the cnfiguratins at 2500 rpm, full thrttle, s that they wuld be mre nearly cmparable with the grund analysis fr the standard pusher with the Aermatic prpeller. Examinatin f these analyses at tw different speeds seemed interesting enugh t make it wrth presenting the fur analyses fr cmparisn. These fur plar plts are shwn in figure kl. N cmparative plts are presented since the differences are easily seen in this presentatin f the basic data. A cmparisn f the grund analyses fr the unmuffled pusher (cnfiguratin 8) at the tw speeds shws that the nise at the engine exhaust frequency is almst exactly the same fr the tw engine perating cnditins. Since fr bth cnditins the engine was perating at full thrttle and, therefre, maximum manifld pressure, this result is nt surprising. Similar results were measured fr a similar engine by Davis and Carnecki (reference 9, table II). The prpeller nise generatin is very different, hwever, fr the tw cnditins f peratin. This result was expected since bth the engine pwer and the prpeller tip speed are reduced at the lwer speed (see table II herein). This reductin in prpeller nise, withut a crrespnding reductin in the engine nise, means that the engine nise which was nly abut equally as lud as the prpeller nise at 2500 rpm is predminant at 1900 rpm. It is prbably generally true, even fr engine perating cnditins which are nt s extreme as the full-thrttle peratin at 190Ö rpm, that peratin at lwer engine speeds will be accmpanied by an increasing imprtance f engine nise. In the case f cnfiguratin 8, where the engine nise is as lud as the prpeller nise at 2500 rpm, this wuld mean that peratin at lwer pwers and speeds wuld prduce a cnditin where the engine nise wuld predminate. These relatinships must be cnsidered if the decisin is t be made whether the additin f a muffler t the mdified pusher wuld be desirable. It will be nted in the grund analysis fr cnfiguratin 8, at 1900 rpm, that a harmnic cmpnent at kl times the crankshaft frequency attributable t the engine was bserved at all measuring psitins arund the airplane. Since this cmpnent was fairly imprtant at sme psitins, it has been pltted. A similar cmpnent at k^ times the crankshaft fre- quency was nticed in the grund analysis fr cnfiguratin 8 at 2500 rpm, but it was nly measurable at abut half f the measurement psitins. This cmpnent was as lud during peratin at 1900 rpm as it was at 2500 rpm and, in sme psitins, luder. The engine-nise analyses by Davis and Carnecki (reference 9) have als shwed this cmpnent t : be an imprtant ne in the nise f an unmuffled engine.

29 28 NACA TN 2727 Cmparisn f the grund analyses fr the muffled pusher at 1900 and 2500 rpm shws that, with the engine nise reduced by the muffler, the ver-all level is reduced by the reductin in prpeller nise at the lwer pwer and speed. The muffler is sufficiently effective at this lwer speed and engine exhaust frequency s that the prpeller nise is still the predminant cmpnent f the nise at 1900 rpm. Cmparisn f the grund analyses fr cnfiguratins 8 and 9B, at 1900 rpm, des nt shw the large difference in prpeller nise generatin that ccurred with these tw cnfiguratins at 2500 rpm (see series B). Furthermre, at 1900 rpm, cnfiguratin 9B makes mre prpeller nise than cnfiguratin 8, where there is a difference. This is in cntrast with the greater prpeller nise f cnfiguratin 8 at 2500 rpm. This difference in prpeller nise generatin suggests that the exhaust passing thrugh the prpeller in cnfiguratin 9B is nt prducing a secnd surce f nise f sufficient magnitude t cause the cancellatin at the prpeller frequency hypthesied in the discussin f peratin at 2500 rpm in series B. Since the exhaust mves at a smewhat lwer velcity at 1900-rpm engine speed, hwever, this difference may still be cnsistent with the hyptheses f series B. Series E. Effect f Replacing Aermatic Prpeller with a Slid Fur-Bladed Prpeller n Standard Pusher (Cnfiguratins 6 and 7) Take-ffs and flight measurements.- The basic data fr the tw cnfiguratins f series E, fr take-ff and flight measurements, are shwn in figures h2, 43(a), and 43(b). Averaged curves fr the take-ff and flight measurements are cmpared fr the tw cnfiguratins f this series in figures kk and k$. In cnsidering the data fr this series it must be remembered that cnfiguratin 7 used a slid wden prpeller which had a diameter 10 inches less than that f the Aermatic prpeller f cnfiguratin 6. This meant that cnfiguratin 7 culd nt be perated under cnditins cmparable with the cnditins fr cnfiguratin 6. Table II shws that maximum-pwer peratin prvided the mst cmparable cnditins fr the tw cnfiguratins. In this perating cnditin the pwers were apprximately the same, the speed fr the fur-bladed cnfiguratin was slightly higher than that attained by the Aermatic prpeller, and the tip speed was slightly lwer. The perating cnditins fr the grund analysis and take-ff measurements were quite different fr the tw cnfiguratins, speed, pwer, and tip speed fr the furbladed cnfiguratin being cnsiderably lwer than fr the tw-bladed Aermatic.

30 NACA TN These differences in perating cnditins explain sme f the differences that appear in the cmparative plts. Fr the take-ffs the higher pwer and prpeller tip speed f cnfiguratin 6 explains the higher nise level thrughut the take-ff fr this cnfiguratin. The cmparisn fr the flights at 100-ft maximum pwer (fig. kk) shws very little difference between the tw cnfiguratins. This result is nt in agreement with Gutin 1 s thery f prpeller nise generatin since the pwers were cmparable, while the tip speed f the fur-bladed prpeller was lwer than that f the tw-bladed Aermatic. Theretically, a fur-bladed prpeller shuld be quieter, at the same tip speed and pwer, than a tw-bladed prpeller, and the reduced tip speed f the fur-bladed prpeller shuld make it even quieter. Evidently this thery which is based n the assumptin f prpellers turning in undisturbed air des nt apply t prpellers perating under the cnditins fr the pusher airplane where the prpeller blades turn thrugh air which has been disturbed by passing arund the engine nacelle and ver the hull and wing surfaces (see figs. 1 and 2). The cmparisn fr the flights at 100-ft cruising pwer shws that the lwer pwer and tip speed f the fur-bladed cnfiguratin prvided smewhat quieter peratin. This des nt appear fr flights under the same perating cnditins at 500-ft altitude; hwever, the use f the it-0-decibel weighting fr these measurements wuld make the higherfrequency nise f the fur-bladed prpeller relatively mre imprtant. Use f the ^0-decibel weighting gives results, fr levels belw 70 decibels, which cmpare mre nearly with the ludness f the sunds, as heard by the ear, than measurements with flat weighting wuld prvide. It is interesting t nte, therefre, that the measurements indicate that the fur-bladed cnfiguratin perating at a lwer pwer and prpeller tip speed than the Aermatic wuld still sund as lud t the human ear as the Aermatic cnfiguratin at a distance f several thusand feet. The cmparisn fr the flights at 500-ft maximum pwer is nt cnsistent with the cmparisns fr the ther flight cnditins. Reference t the basic data fr this perating cnditin shws that the recrds fr the standard pusher with the Aermatic prpeller varied ver a wide range when the airplane was appraching and was several thusand feet away. ne f the recrds shws a nearly cnstant sund level at the micrphne thrughut the entire apprach. It seems likely that abnrmal refractin (see discussin f series A) was ccurring fr these measurements which made the average level during the apprach cnsiderably higher than it wuld nrmally have been. Cmparisn f the average curves fr the fur-bladed cnfiguratin, at 500-ft maximum pwer and 500-ft cruising pwer, shws that the curve fr 500-ft maximum pwer fell belw that fr 500-ft cruising pwer during the apprach. This again suggests abnrmal refractin ccurring, in this case in such a manner as t reduce the level during the apprach fr cnfiguratin 7

31 30 NACA TN 2727 at 5-ft maximum pwer. It is prbable that abnrmal prpagatin cnditins f this srt explain the incnsistently wide separatin between the average curves fr cnfiguratins 6 and 7 at 500-ft maximum pwer. Grund analyses.- The grund analyses fr the tw- and fur-bladed cnfiguratins f the standard pusher are shwn in figure ^3(c). Cmparisns f the ver-all level, engine fundamentals, and prpeller fundamentals frm these analyses are presented in figure k6. The analysis fr cnfiguratin 6 shws the prpeller nise generatin t be mst imprtant at all but tw f the measuring psitins. Fr cnfiguratin 7, n the ther hand, the engine fundamental is by far the largest cmpnent except at the tw 150 psitins. This difference between the tw analyses -is easily explained by the much lwer pwer and speed fr cnfiguratin 7 in the grund analysis. The cmparisn f the ver-all levels frm the grund analyses (fig. 46) shws that the standard pusher with the Aermatic prpeller (cnfiguratin 6) was nisier than cnfiguratin 7 with the fur-bladed prpeller in the psitins t the side and the rear where the high nise level f the prpeller, fr cnfiguratin 6, predminated. In the cmparisn f the engine fundamentals the nearly unifrm radiatin pattern fr the unmuffled engine is clearly shwn, and the engine fundamental fr the engine perating at the lwer speed fr cnfiguratin 7 averages abut 2 decibels higher than that fr cnfiguratin 6. The cmparisn f the prpeller fundamentals shws the great increase in prpeller nise generatin with the increased tip speed and pwer f the Aermatic cnfiguratin. It must be kept in mind, when cmparing the results f the grund analyses in this series, that the analysis fr cnfiguratin 6 has been pltted s that the uter circle f the plar plt crrespnds t 120 decibels. The same scale has been used fr the cmparisn f the ver-all levels, but the ther plar plts have been pltted s that the uter circle crrespnds t 110 decibels. Series F. Cmparisn f Standard and Mdified Pushers with Standard and Mdified Tractrs Flight measurements.- ne f the imprtant purpses f the present study was t cmpare the external nise generated by a representative pusher-type airplane with that f a cnventinal tractr-type airplane. The nise generatin by tractr airplanes has previusly been thrughly investigated and theries have been develped which prvide fairly accurate predictin f the nise level t be expected (see references 1 t 6, 8, and 10). In cntrast with the wrk n tractr airplanes very

32 NACA TN little study has been made f the prblem f nise generatin by a pusher-type airplane n which the prpeller rtates thrugh air which has been disturbed by passage ver the engine nacelle, hull, and wing surfaces. Series F presents a cmparisn f the nise generatin in flights verhead at 500-ft altitude fr the standard and mdified tractr airplanes f reference 6 and the standard and mdified pusher airplanes f the present reprt. Cmparisns f the averaged curves fr the standard tractr and the standard pusher at 500-ft maximum and cruising pwer are presented in figure k"j. Cmparisn f the average f the curves fr several f the mdified cnfiguratins f the tractr (2A, 2B, 2F, 2C, and 2D) with a similar average f the curves fr several f the mdified pushers (9A, 9B, 9C and 9D) is presented in figure k3. These cmparisns shw a large difference in the way the sund level varied fr flights verhead with the tractr and pusher airplanes. In the cmparisns fr the standard airplanes (fig. k"j) the peak nise fr the standard tractr is 8 t 10 decibels higher than the peak fr the pusher, but the high nise level lasts fr a much shrter time. Evidently the area in figure kj under the curve fr the tractr is much less than the area under the curve fr the pusher. It is reasnable t suppse that the time integral f the nise level prduced by an airplane wuld be related t a persn's subjective estimate f the amunt f nise it made. Fr this reasn there is sme basis fr cnsidering the standard pusher nisier than the standard tractr under the cnditins fr which it was measured, even thugh the peak level fr the tractr is higher. This indicates the danger f cmparing the nise generatin by different airplanes simply in terms f the peak nise prduced in a flight verhead. The cmparisn f the averages f the mdified tractrs and pushers (fig. H8) shws clearly that the mdified tractrs were n the average much quieter than the mdified pushers. The peak levels are very similar, but the high nise level f the tractr lasted fr a much shrter time. The difference in the shapes f the curves fr the tractr and pusher, which is similar fr all the measurements presented, can be explained by the differences between these tw airplane types. First, äs previusly mentined, the majr nise surces n the pusher are shielded by the hull and wings f the airplane near the verhead psitin. It is this shielding that explains the dip in the pusher recrds ccurring where the peak ccurs in the tractr recrds. Secnd, there is evidently a difference in the directivity characteristics f the nise surces fr the tw airplane types. Shielding can explain the differences near the verhead psitin but cannt explain the increasing difference between the tw airplane types at distances f several thusand feet. Further evidence f a difference in directivity may be

33 32 NACA TN 2727 fund in the measurements frm 3000 feet away in series G which shw a difference in the shape f the sund-level variatin when measured frm the side. Shielding cannt be a majr surce f the difference under these cnditins. It may be, f curse, that the parts f the airplane which cause the shielding fr the verhead flights are respnsible fr the evident differences in radiatin characteristics fr the nises f the tw airplanes. A further reasn fr the difference in radiatin characteristics is t be fund in the difference in the character f the air flw int the prpellers fr the tw airplanes. In the tractr the prpeller turns thrugh undisturbed air, while in the pusher the air flw int the prpeller is disturbed by passage ver the engine nacelle, hull, and wing surfaces. This disturbed air flw, in the case f the pusher, wuld be expected t interfere with the nrmal prpeller nise generatin since it intrduces a nnunifrmity in the air flw thrugh the prpeller disk. ver-all levels frm grund analyses.- Cnsideratin f the radiatin characteristics in flight fr the pusher and tractr airplanes immediately raises the questin f hw the radiatin characteristics shwn by the ver-all levels f the grund analyses cmpare. Figure k9 shws the ver-all levels n the grund fr bth standard airplanes, and fr the fur-bladed geared and muffled cnfiguratins f bth mdified airplanes. The standard pusher appears t be as much as 10 decibels nisier withut as large a difference in shape as might be expected frm the results f the flight-measurement cmparisns. The mdified pusher (cnfiguratin 9B) is nisier than the mdified tractr (cnfiguratin 2F) in many psitins, and the shapes f the tw curves are quite different. The mdified pusher seems t have a mre sharply directinal radiatin characteristic than the mdified tractr, a result which is nt in agreement with the results f the flight measurements. The air flw int the prpeller when the engine is turning at 2500-rpm maximum pwer n the grund is different frm the air flw int the prpeller fr the same pwer and speed in flight. Presumably, this difference in air flw fr grund and flight measurements explains the differences in nise generatin fr these tw measuring cnditins. It must be nted in studying the cmparisns f this series that perating cnditins fr the tw airplane types were smewhat different. Maximum pwer in flight crrespnded t a higher pwer fr the tractr than fr the pusher. Furthermre, while the pusher measurements bth n the grund and in flight were dne at perating cnditins which were always cmparable with thse fr the standard pusher, this prcedure was nt emplyed in the measurements fr the tractr. Reference t table II is necessary t determine the pwers, tip speeds, and prpeller diameters which must be taken int accunt in making cmparisns.

34 NACA TN Series G. Measurements frm 3000 Feet Away fr Cnfiguratins 5, 6, 8, and 9B In rder t avid the shielding effect f the hull and wings f the pusher, which evidently had a majr effect n the sund levels measured fr flights verhead, sme measurements were made frm a psitin 3000 feet frm the nearest pint f the flight path. It was als felt that these measurements might give a truer picture f the nise heard in the vicinity f an airprt. The measurement technique presented varius prblems which made it seem less useful than had been expected, but sme f the results were bth unexpected and interesting. Basic data.- The basic data shwing the recrds f successive flights fr the fur cnfiguratins f this series at 500-ft maximum and cruising pwer, 3000 feet away, are presented in figures 50 and 51. As might be expected, these recrds shw a large amunt f fluctuatin. The difficulties f this nature encuntered with the measurements f verhead flights at distances greater than 3000 feet naturally ccurred thrughut the entire flight fr the present measurements. This fluctuatin makes the interpretatin f the data mre difficult but des nt bscure the large difference between the sund levels measured fr flights in ppsite directins. This difference between the sund levels fr flights nrth and flights suth is apparent in varying degrees in all the measurements n the pusher cnfiguratins. N significant difference f this srt may be seen in the recrds fr cnfiguratin 5 f the standard tractr. The maximum difference fr the pusher cnfiguratins ccurred when the airplane was abut 2000 feet past the nearest pint f the flight path. At this psitin a difference f abut 10 decibels ccurred fr mst f the recrds, with the nise radiated in the right rear quadrant f the airplane being luder than the nise radiated in the left rear quadrant. In attempting t determine a reasn fr this difference, cnsideratin was given t the fact that the prpeller rtates in a cunterclckwise directin as seen frm the rear f the airplane, which means that the maximum nise radiated hrintally cmes frm the side f the airplane n which the prpeller blades are mving up. Cnsideratin f the mechanism f nise generatin wuld make it appear that the maximum nise shuld be radiated n the ppsite side f this airplane where cmpressin wuld be expected t ccur each time a prpeller tip apprached the hull. This reasning is evidently incrrect since the measurements cnsistently shw that the maximum nise ccurs n the side where the blades are mving away frm the hull. The recrds fr the tractr airplane d nt shw a difference f this srt between the nrth and suth flights but d shw what appears t be a recurring reductin in the nise received frm the airplane at the nrth end f its flight path. This difference might be a terrain difference, since there were sme trees beneath a line frm the airplane t

35 3^ NACA 'TW 2727 the micrphne at this end f the flight path. It seems surprising, hwever, that such a terrain difference was nt nticed in the ther measurements frm 3000 feet away. Perhaps the large fluctuatins in the recrds and the difference between the nise n the tw sides f the pusher bscured this difference in the pusher recrds. Cmparative plts.- The basic data fr each cnfiguratin in figures 50 and 51 have been averaged and presented in figure 52. The cmparisn fr the flights at 500-ft maximum pwer clearly shws the large difference between the way the sund level varied fr the flights f the standard tractr and the way it varied fr the flights with the three different cnfiguratins f the pusher. The particularly high nise level f the tractr fr these flights at maximum pwer was caused by peratin at full thrttle which prvided 2800 rpm, 165 hrsepwer, and a prpeller tip speed ver 900 feet per secnd. This is a higher pwer than was used fr the measurements f the" standard tractr, cnfiguratin 1, presented in series F. peratin at this pwer prvided a nise level sufficiently high s that the nise f the tractr culd be measured thrughut the 10,000-ft flight curse. The prnunced directivity f the tractr nise may be seen when the variatin in distance frm airplane t micrphne thrughut the flight path is cnsidered. At the nearest pint the airplane is 3000 feet away, while at either end f the curse the airplane is less than 6000 feet away. This means that a surce f sund which radiated unifrmly in all directins shuld prduce a sund level varying a maximum f 6 decibels as it mved frm ne end t the center f the flight curse. The variatin with the tractr ver this distance was abut 25 decibels shwing that radiatin near the plane f the prpeller is abut 20 decibels greater than that at a 60 angle in frnt f r behind the prpeller plane. The tractr is evidently much mre directive than the pusher, a result previusly mentined in the discussin f series F. The standard tractr at cruising pwer made a peak nise abut equal t that fr the standard pusher, but the nise lasted fr a much shrter time fr the tractr. This result, similar t the result fr the flights verhead, means that the standard tractr at cruising pwer is effectively a much quieter airplane than the standard pusher. Measurements were nt made frm 3000 feet away n any f the mdified tractrs, because the airplanes were nt available. If these airplanes are as much quieter than the standard tractr 3000 feet away as they were in the verhead measurements, then the mdified tractrs wuld be much quieter than the mdified pushers. An apprximate sketch has been made f part f the directivity patterns in flight fr the standard tractr and standard pusher airplanes. This plar plt (fig. 53) was derived frm the recrds f figure 50 and crrectin has been made fr variatins in the distance f the airplane

36 NACA TN frm the micrphne which take int accunt the finite velcity f sund and the velcity f the airplane. The cnventinal inverse-square law f attenuatin with distance has been used. It must be emphasied, hwever, that this graph shuld nt be cnsidered as representing the result f a thrugh and careful measurement f the radiatin pattern f the airplanes in flight but rather as a best estimate f this pattern available frm the data f figure 50. Fr the sake f clarity a smthed curve has been drawn thrugh selected pints. In additin t any errrs intrduced by drawing a smthed curve the data are subject t the errrs that were pssible in the flight measurements. The sund-level scale used is a purely relative ne using 0 decibels as the level ccurring at the 90 psitin n the right fr bth airplanes. The sund levels fr the tw airplanes were fund t be abut alike at the 90 right psitin when the pwers and tip speeds were nearly the same. The difference between the directivities fr the pusher and tractr is made strikingly clear by the plar plt f figure 53. The reasns fr this difference are nt entirely clear, but the measurements f series H ffer ne pssible explanatin. In the case f the pusher airplane, the harmnic cmpnents are believed t be equal t r greater in intensity than the fundamental and the directivity patterns fr thse cmpnents have their maximums at ther than near -90. The nly analysis made f the nise spectrum fr flight is discussed in the fllwing sectin and it tends t cnfirm the abve explanatin. Further explanatin will be fund in the discussin f series H. It is interesting t cmpare the measurements f the three cnfiguratins f the pusher frm 3000 feet away with measurements fr these cnfiguratins fr the verhead flights. The data fr 500-ft maximum pwer in series A (fig. 26) shwed that the mdified pusher (cnfiguratin 9B) averaged 8 t 10 decibels quieter than the standard pusher (cnfiguratin 6). The measurements frm 3000 feet away shw a difference f 6 t 8 decibels fr these cnfiguratins which is nt seriusly different frm the results f the verhead measurements. Cmparisn f cnfiguratins 6 and 8 in series B (fig. 3k) als gave results similar t the cmparisn f these cnfiguratins frm 3000 feet away. Series H. Cmparisn f Analyses f Nise f Cnfiguratin 10 Flying n a Curse Which Passed 3000 Feet Away with Grund Analyses Made 50 Feet frm Prpeller Hub Sme recrdings were made n magnetic tape f the nise f the mdified pusher, muffler relcated, with the eight-bladed prpeller (cnfiguratin 10) frm 3000 feet away. Time did nt permit perfectin f this new prcedure r recrdings f ther cnfiguratins. Shrt samples

37 36 NACA TN 2727 f the recrdings fr tw f the flights at 500-ft maximum pwer using flat weighting were analyed. Calculatins have been made shwing the psitin f the airplane alng its flight path when it generated the nise recrded n each f the samples analyed.. The psitin f the airplane has been expressed in terms f the distance frm airplane t micrphne and the angle measured between a line frm the tail t the nse f the airplane and a line frm the airplane t the micrphne. In these calculatins crrectins have been made which include cnsideratin f the velcity f the airplane and the finite velcity f sund s that the indicated directin is the directin in which the sund analyed was radiated and the indicated distance is the actual distance that the sund traveled frm airplane t micrphne. The flight analyses and grund analyses fr cmparable psitins are presented in figure 54. The relatinships between the varius imprtant frequency cmpnents are quite different fr the tw cnditins. In general, fr the grund analyses f this series, nly the first and secnd harmnics f the prpeller tip-passage frequency were measurable abve the backgrund f nnharmnic nise, and the secnd harmnic was mre than 10 decibels lwer in amplitude than the fundamental. In additin, fr the grund analyses, the engine fundamental was measurable and usually at a higher level than the prpeller secnd harmnic. Fr the flight analyses, the engine fundamental was measurable at nly ne psitin, and the higher harmnics f the prpeller, particularly the secnd and third, were smetimes nearly as imprtant as the prpeller fundamental. Harmnics as high as the sixth f the prpeller frequency were measurable abve the nnharmnic backgrund. It is evident that nise at the prpeller tip-passage frequency is the majr nise fr cnfiguratin 10 in flight. The imprtance f the higher harmnics f the prpeller nise will be realied when it is cnsidered that, because f the characteristics f the human ear, the nise f the higher harmnics wuld in many cases sund luder at the distances fr which the measurements f this series were made than the nise at the fundamental frequency. In additin t the difference between the character f the nise prduced by the airplane in flight and n the grund, there is further evidence in the measurements f this series f a difference in the plar distributin f the ver-all nise level arund the airplane in flight and n the grund. The differences in ver-all level between flight and grund measurements in similar directins are nt what wuld be expected frm a simple cnsideratin f the theretical inverse-square law f attenuatin with distance. These results are als indicated by cmparisn f the data f series G (see particularly fig. 53) with the verall levels frm the grund analysis fr the same cnfiguratin (see fig. ^9).

38 NACA TN N analyses are available at present f the nise f the tractr airplane in flight. bservers have reprted that the mdified tractrs made much less f a whining nise in flight than the mdified pushers. It is well-knwn that a whining nise is assciated with intense higherharmnic cntent. The measurements f reference 6 shwed that fr the mdified cnfiguratins f the tractr using a six r eight-bladed geared prpeller (2C and 2D) the flight nise at mst measurement psitins cntained a large amunt f nnharmnic vrtex nise. It is likely that the nise f the pusher airplane because its prpeller is turning thrugh disturbed air cntains bth a larger relative amunt f prpeller nise and a higher level f the higher harmnics f the prpeller nise relative t the fundamental than the nise f the tractr. If this difference in the character f the nise generated by the tw airplane types des exist, it affrds a pssible explanatin f the difference in the directivity f the nise as shwn in series G (fig. 53). The directivity f the tractr nise shuld be determined primarily by the directivity f the vrtex nise and the directivity f the predminant fundamental f the prpeller nise. Fr the standard tractr f figure 53 the prpeller fundamental is imprtant and prduces a directivity pattern resembling that calculated fr the prpeller fundamental frm Gutin's thery (see fig. 1, reference k). Fr the standard pusher, with the imprtant secnd and third harmnics f the prpeller nise, it is pssible that the different directivity patterns fr the different harmnics add up in such a way that their sum has the rather unifrm ver-all directivity shwn in figure 53. The ^-decibel weighting used fr figure 50, frm which figure 53 was calculated, wuld increase the relative imprtance f the higher harmnics f the prpeller nise. Admittedly, the evidence fr the abve hyptheses which wuld explain the difference in directivity fr the tractr- and pusher-type airplanes is sketchy. Evidence is drawn frm scattered data n several different cnfiguratins. An explanatin f this nature, hwever, seems likely t be crrect in view f the available data. It was mentined in the discussin f precisin that sme truble was experienced in the measurements fr this series with variatin f the sensitivity f the magnetic tape recrder. Pssible errrs arising frm this difficulty must be taken int accunt in cnsidering the ver-all levels fr the flight measurements. The relatinships between the amplitudes f the varius harmnic cmpnents in the flight analyses were prbably nt affected, hwever, s cmparisns f the spectra fr flight and grund measurements are subject nly t the usual instrumental errrs.

39 38 NACA TN 2727 PERFRMANCE TESTS In rder t get sme idea f the effect n perfrmance f the varius mdificatins made, take-ff runs were made with the varius cnfiguratins, and the distance required fr the wheels t leave the grund was carefully measured. In rder t eliminate, as much as pssible, differences in pilting technique all take-ffs were made in the same way by ne pilt using a standardied prcedure. Take-ffs with the standard pusher were made with a full thrttle setting thrughut the take-ff. Take-ff prcedure with the six experimental cnfiguratins using grund-adjustable prpellers (8, 9A, 9B, 9C, 9D, and 10) was similar t that used fr the sund-measurement take-ffs. This invlved reducing the thrttle setting during the take-ff s that the engine speed did nt exceed 2600 rpm. The prpeller pitch was set fr these perfrmance tests as it was fr the grund analysis and take-ff measurements, s that the maximum speed at the start f the take-ff wuld be 2500 rpm. This prcedure prvided the experimental cnfiguratins with a slightly higher pwer fr the start f the take-ff but with a reductin in pwer during take-ff. It was felt that this prcedure gave a reasnable apprximatin t the perfrmance f a cnstantspeed prpeller. This prcedure was, f curse, impssible fr the slid fur-bladed prpeller f cnfiguratin 7. In this case take-ffs were made with a full thrttle setting, but the lwer speed and pwer during the take-ff prvided relatively pr perfrmance. Table III presents the results f these perfrmance tests. The take-ff distances indicated in the table are the averages f the fur best runs in a series f apprximately eight take-ffs fr each cnfiguratin. The greater weight f the experimental cnfiguratins is simply that due t the gearing, muffler, and prpellers. With these weights the take-ff distance averages abut 10 percent greater fr the experimental cnfiguratins that were perated under fairly cmparable cnditins. Maximum airspeeds in level flight are presented in table III as an additinal indicatin f perfrmance. The prpellers f the experimental cnfiguratins were set fr these tests s that they all absrbed apprximately the same level-flight pwer as that f the standard prpeller. It must be nted in cnsidering these speeds that they were read frm tw different uncalibrated meters, ne used fr cnfiguratins 6 and 7 and the secnd, fr the ther six cnfiguratins. This prbably means that the small differences bserved are insignificant.

40 NACA TN GENERAL DISCUSSIN A study f the data presented in series A t H shws that several imprtant cnclusins can be drawn. In additin, it is evident that sme questins have been raised which remain unanswered. The mst general cnclusin frm the data f the present reprt is that nise generatin by the pusher-type airplane des nt fllw the theries which have been fund t apply t nise generatin by tractr-type airplanes. As suggested in the discussins f series E and F this result is nt-surprising since the air flw int the prpeller f a pusher-type airplane is disturbed by passage ver the engine nacelle, hull, and wing surfaces. The thery f prpeller nise generatin, which has been useful fr tractr-type airplanes, assumes a unifrm flw f undisturbed air int the prpeller. ne result f this interference with the nrmal flw f air in the pusher cnfiguratin is that the pusher airplane, perating at a lwer engine pwer and prpeller tip speed than the tractr-type airplane, radiates a larger amunt f nise energy. The cmparisn f series G shwed that at cruising pwer the standard pusher and tractr made cmparable peak nise levels in a flight passing 3000 feet away. The nise f the pusher, hwever, remained at a high level thrughut 10,000 feet f flight, whereas the nise prduced by the tractr drpped ff rapidly as the airplane flew away. Fr the mdified cnfiguratins f the pusher the nise was reduced by the lwer tip speed f the prpellers and, fr many cnditins, was further reduced by the use f engine exhaust muffling. This reductin thrugh lwer tip speed and mufflers which amunted t frm 7 t 10 decibels is f abut the same magnitude as that btained by similar mdificatins f the tractr-type airplane. A difference in the nise generatin by the tractr and pusher airplanes, which is prbably als explained by the disturbed air flw int the pusher prpeller, may be seen in the data fr series A which shw the effect f increasing the number f prpeller blades. Fr the geared and muffled pusher airplane increasing the number f prpeller blades beynd fur tended t increase the nise generatin ver that with a fur-bladed prpeller. With F. tractr-type airplane such as the ne tested bth thery and experience indicate a significant reductin in nise level with an increase f the number f prpeller blades frm fur t six r eight (see reference 6). It is likely that, with the geared engine used in this study, a simpler muffler and a three-bladed prpeller n the pusher airplane wuld prvide as quiet peratin as pssible at the prpeller tip speeds used. Since the reductin in prpeller tip speed frm the standard t the mdified cnfiguratins is the majr surce f the reductin in nise level, it is likely that a further reductin in the tip speed with a greater

41 t NACA TN 2727 gear-reductin rati wuld prduce still mre reductin in the nise generatin. If this change were made, it might be desirable t use a muffler as effective as the ne used in the present study, in rder t take full advantage f the reductin in prpeller nise. When cmparing the nise generated by the pusher airplane with the nise generated by cnventinal tractr-type airplanes, the nnharmnic vrtex nise generatin must be cnsidered as well as the harmnic nise at multiples f the prpeller and engine frequencies. In measurements fr the tractr airplane tested, and ther tractr-type airplanes, vrtex nise generatin has meant that the reductin in nise level with an increase f the number f blades was nt s great as wuld be expected if the tip-passage nise was cnsidered alne. In general, vrtex nise has prvided a limit t the nise reductin btainable by using multibladed prpellers turning at reduced speeds. This raises the questin f where vrtex nise generatin begins t limit the reductin in nise btainable with the pusher-type airplane. The grund analyses fr cnfiguratins 9A, 9B, 9C, and 9D shw that a nnharmnic nise which appears t be an exaggerated vrtex nise did increase in imprtance with an increase in the number f prpeller blades. Frm the grund analysis it appeared that this vrtex nise was at least partly respnsible fr the lack f prnunced change in the ver-all level with increasing numbers f blades. The analysis n the grund fr cnfiguratin 10, with the exhaust nt passing thrugh the prpeller, shwed a reductin in vrtex nise, accmpanied by sme increase in prpeller nise, ver that fr the cmparable cnfiguratin (9D) with the exhaust passing thrugh the prpeller. This result suggested that the exhaust passing thrugh the prpeller was invlved in bth the lwer tip-passage nise and the higher vrtex nise fr cnfiguratin 9D. An analysis was made f the nise in flight f cnfiguratin 10 (series H) which shwed a great predminance f harmnic nise ccurring at multiples f the prpeller tip-passage frequency. N analyses were made f the nise f cnfiguratin 9^ in flight, but bservers n the grund reprted that its nise in flight was similar t the nise fr cnfiguratin 10. This prbably means that the nise generated by the geared pusher in flight did nt cntain the high level f vrtex nise nticed in the grund analysis. This again implies that a further reductin in prpeller tip speed wuld be-useful in reducing the verall prpeller nise generatin. Such a cnclusin seems t be in cnflict with the results f the grund analyses, but the questin is immediately raised whether the nise measured in the grund analyses is fairly cmparable with the nise generated in flight. The data f series H shw that, at least in sme cases, the nise f the airplane running n the grund is quite different frm the nise generated in flight. ther evidence was fund thrughut the measurements fr this reprt that the results f grund analyses

42 NACA TN 2727 kl were nt cnsistent with flight measurements. It is nt cmpletelyclear frm the evidence available why this shuld be true. The preceding discussin has shwn evidence that the nise f the mdified cnfiguratins f the pusher airplane in flight is largely made up f harmnic cmpnents f the prpeller tip-passage frequency. A flight analysis shwed that this was true fr cnfiguratin 10, and it seems likely that a predminance f prpeller tip-passage nise als ccurred fr cnfiguratins 9A t 9D in flight. This annying harmnic whine f the geared and muffled pushers is in marked cntrast with the nise f the geared and muffled tractr airplane, especially fr the higher number f prpeller blades. Althugh n analyses have been made f the nise in flight f the mdified tractrs, there is bth theretical and practical evidence that nnharmnic vrtex nise is an imprtant cmpnent f the nise f these airplanes. bservers wh have heard bth the mdified tractrs and the mdified pushers have cmmented n the marked difference in the character f the nise f the tw airplane types as well as n the difference in the amunt f nise made. Evidence f this srt supprts the hypthesis f series H abut the difference in directivity f the pusher- and tractr-type airplanes. Practical experience with cmplaints frm peple living near the airprt frm which bth f these airplanes were flwn has indicated that, while very few cmplaints were received abut the nise f the tractr cnfiguratins, vilent bjectins were raised abut the nise f repeated flights ver the test curse fr the pusher cnfiguratins. This difference is prbably attributable bth t the difference in the duratin f the nise fr the tw types f airplanes and t the difference in the character f their nise. It is likely that even at the same nise level the swish f the mdified tractrs wuld be less annying than the whine f the mdified pushers. 'There are nt at present any measures f annyance available t check this hypthesis. Anther imprtant prblem which must be cnsidered in studying the effectiveness f mdificatins designed t reduce nise generatin is the effect these mdificatins have n perfrmance. 'The take-ff distances and airspeeds at maximum pwer f table III shw that perfrmance is nt seriusly affected by the mdificatins. It must be remembered that these figures were measured with the prpeller pitch fr the experimental cnfiguratins set differently fr take-ffs and fr flight. These prpellers, in effect, were treated as variable-pitch prpellers fr the perfrmance cmparisns. A fixed-pitch prpeller cannt prvide bth the shrt take-ff run and high tp speed f a variable-pitch prpeller. If a prpeller using mre than tw blades must be used with a geared engine, the cst f prviding variable pitch fr this multibladed prpeller must be cnsidered. The prblem f quieting the pusher-type airplane is thus a cmplex ne. Pusher-type prpellers turning thrugh disturbed air evidently are

43 k2 NACA TN 2727 surces f mre nise than tractr-type prpellers fr the same perating cnditins. Reductin f prpeller tip speeds in cnjunctin with sme exhaust muffling seems t he the nly slutin t the nise prblem fr pushers. The same general cnclusin als applies, f curse, t tractrs, but a greater reductin in tip speed is necessary with pushers t achieve the same effective nise utput. It is thus easier t quiet tractrs than pushers. NCLUSINS The fllwing cnclusins apply t the external nise level and the perfrmance f a three-place pusher-type amphibian airplane: 1. It is pssible t achieve a 7- t 10-decibel reductin f external nise by the use f slw-turning prpellers in cnjunctin with engine exhaust mufflers. 2. Increase in the number f prpeller blades abve fur n the geared and muffled airplane des nt prduce significant changes in the external nise level. 3. A muffler exhausting thrugh the prpeller under the cnditins f the present study des nt increase the ver-all nise generatin. k. The nise level is nt reduced under mst cnditins by the use f a smaller-diameter, fur-bladed prpeller at the same pwer withut gear reductin and engine muffling. 5. The sund radiatin frm an airplane perating n the grund appears t be quite different frm the radiatin in flight, bth as t directivity and harmnic cmpsitin. 6. The pusher-type airplane in bth the standard and mdified cnfiguratins radiates sund mre unifrmly in all directins in flight than d the tractr-type airplanes. While the maximum nise level in a flight verhead r passing 3000 feet away is abut the same fr the tw types, the lack f sharp directivity in the pusher means that the nise level fr the pusher remains high fr a lnger time. 7. In additin t the difference in duratin f the nise prduced by the tractrs and pushers, there is als a difference in the character f the nise which may make the pusher nise still mre bjectinable.

44 NACA TN 2727 ^3 8. Perfrmance f the pusher airplane evidently wuld nt be seriusly reduced by the use f a geared engine and an exhaust muffler prvided a variable-pitch prpeller were used. Aernautical Research Fundatin Bstn, Mass., March 20, 1950

45 hk NACA TN 2727 REFERENCES 1. Deming, Arthur F.: Prpeller Rtatin Nise Due t Trque and Thrust. NACA TN 7^7, 19^0. 2. Thedrsen, Thedre, and Regier, Arthur A.: The Prhlem f Nise Reductin with Reference t Light Airplanes. NACA TN 11^5, 19^6. 3. Hicks, Chester W., and Hubbard, Harvey H.: Cmparisn f Sund Emissin frm Tw-Blade, Fur-Blade, and Seven-Blade Prpellers. NACA TN 135^, 19^7. k. Hubbard, Harvey H., and Regier, Arthur A.: Prpeller-Ludness Charts fr Light Airplanes. NACA TN 1358, 19^7-5. Regier, Arthur A., and Hubbard, Harvey H.: Factrs Affecting the Design f uiet Prpellers. NACA RM L7H05, 19^7-6. Beranek, Le L., Elwell, Fred S., Rberts, Jhn P., and Taylr, C. Fayette: Experiments in External Nise Reductin f Light Airplanes. NACA TN 2079, Regier, Arthur A.: Effect f Distance n Airplane Nise. NACA TN 1353, 19^7. 8. Stwell, E. Z., and Deming, A. F.: Vrtex Nise frm Rtating Cylindrical Rds. NACA TN 519, Davis, Dn D., and Carnecki, K. R.: Dynammeter-Stand Investigatin f a Grup f Mufflers. NACA TN 1838, 19^9-10. Hubbard, Harvey H., and Regier, Arthur A.: Free-Space scillating Pressures near the Tips f Rtating Prpellers. NACA Rep. 996, 1950.

46 NACA TN ^ a) ä a H P ctf P ca J CM CM CM CM J & -* t s m ^^ d m CM * -p ^-^ J X C h rl a 8 1H bd H -p d^ <H d p IA LA LA ^ m Li.p 00 H en J H -=t H LA CD H CM CM J CM,H H H H H C H IH m LTN LA t- D t -=t -=f.d a ü ir\ m IA c' LA c' P H H CM Ö J CM CM CM rh H C4-8 m J H d -d d ^d -d d d 0 0 T) aj d cu J d 0 t) 0 0 rh d CU Tj 0 d TJ -d 0 d d H ^H cö d -d ct) d ca d ca ca 0 J 0) rh CD H cu rh 0 J CU H es H ca H H Pi,a -p Ö P H r c-t f a Ö a r H rh p P 3 Pi P 1 f t 1 P 1 P i U W cß & P P t P a Ä la -p i P t p p id X ft a ^ ft a a X ft ft H C H w H H H H H U J -P - - m 0) «V VD V V \D ^ i ö t -=t t- t C - t- t- 8 t- t t [- t- t ^-* P (1) *H <th d a p -d cu CU C Ö -d H d H 1 H 8 g» S= CU cu EH H CU cu 0 EH» E 53 CU s E X EH < a a a cu cd P d d Ä e d -d d P e Ö <H ri 0 0 TH 9! 3 P S3 CM CM m -* VD J CM -4" -Ü- n -* V fn 0 cu cu H CU CU d Ö «l l a a a H rh H H H CM J CM CM CM <H ca C8 ca ca P S3 fei a B a 3 E <D I -P 0) -d d -d d d I -P J 1. p P d d d d d d 3 H Ü i CU -H a M cu u 0 rl cu H u CD -H CJ cu -H CJ i -H cu u 0 H 0 u 0 IH 0 IH 0 IH Ml fi h CD cc3 CC3 c«<s h ^ IH H H IH ct) ca a C3 ca c3 a rl rd CU cu CU 0 tj ^H d cu m n C5 a C3 C5 ß Ö a C3 U rj d H H H r-\ rh H H a) M3 CM p CU CU CU 0 CU 0 H t- en J- u CJ CJ u u CJ t c fi, a d a a B a ü bd CU c cu P CU a 0 cu -rh H IH H U -d IH U H H m CS!H B JH!-i u 15 U *H H J CU cu CU f CU J 8) hd 60 i ca 00 a) b bd cu H In «1«CH <H rtf H r-1 CU 4; cu CU CU CU 0J Ö FH h i h PM ÜH PH PH H K ««K K K W cd 9 TU d. d. d. d, d d, d, -d -d cu U CU u cu ri ) M CU H tu tu ^ CU H 0 In 0 JH 0 FH 0 H 0 H cd D) rl H H C d cu CU CU H H n p <H P CH P CH P "H p CH P Td +J d p d fs CH A 4H J3 H fl =H Jl <P J3 <H S3 u U U TH H Ü U a CD a c C C C ca & a a d CD d a) d ca -d ccj ^2 as es a cs a d 3 d 3 d 3 d 3 d 3 P h U U?H H -p h p ft P Pi p, 0 p. p. Pi Pi Pi < P X P X P X P 52 P a p -P E X s S E X S8 -rt Ö 03 ^ H ä 9 & ^ a LA «5 t cl PH- N Ü a H cf/ 1

47 1+6 NACA TN 2727 fn Td J J ^-s H U Cl H ft a p^ c -*.* CM V <D t 0-4- H CM ft ""-^ t N LA LA LA LA h U -H?H i PH -P 1^ Pi ai bd a h*-^ a H W ft ir\ 0 LA 0 0 LA LA LA 0 LA 0 LA ffgi H H H H rh CM r-t CM Ch H 0 0 t H H H H H H r-i H H r-l H rh H H rl H ft B 0 0) Ö "d 0 LA 0 LA (U 0 m P bd 0 ft IA t~- 0 CM LA LA LA LA LA LA LA IA IA t N N V ^3 v -* -3- -=r CM CM CM CM CM CM c a ft u CM CM CM CM CM CM CM CM CM CU CM CM CM CM CM 0J 0 ^ 4J H T) 1 tu CU ^ rh CU H rh ft CU CVJ IA LA t LA LA t- r- t r- t- t (U ft ~~v % n -* -* CM CM H c c P t- N V V VD \ LA LA IA IA LA IA h 0 ft p h led»j J PL, +<-' ft tu a h^- 0J ft IA LA 0 IA LA IA LA LA 0 IA IA a IA V ffsi -* -*' -3- -=t J- J- -3- Jf H H r-i H r-t r-i H rh r-i H H rh H H H a W ft (U 0 a id tu ä LA LA SD (U ft V r-i H H M) t LA LA LA LA LA R a ft P CM c m m m m J CM CM CM CM CM CM CM CM &q c^-» 0< H EH c -P c tu P H TH" tu cu 0 H (U 0) H ft c ^0 m CM CM 8 -* t D CM t t- t- tu c ^. =t 0 t LA v LA 8 8 ft -p \D N LA LA LA LA v -3- LA LA -=i- LA LA IA LA ft <H M rl *-* PH P CU d CU ft IA 1A LA LA LA LA LA 0 IA IA IA N IA N LA \ ^ -4- H \ -d- -* c^ J- -=f J- J- H H H H r-l r-t H H H H r-i H 0 m ft H 0 0 LA 0 LA j a -* -=f -=1- CM CM 0 t b CU ft N ND t X t t LA H \ LA LA 0\ -=t- LA LA a ft fi H CM CM CM CM CM CM CM r-t CM CM H CM CM CM J H 0^-- R T) -d CU tu tu (U ft ft tin m c CM CM CM CM CM CM CM CM CM CM C\J ^ tu c c 0 r 0 tu d N N V V0 M3 0 V VX V MD \D ^ H -H i-5 -p rh b r-i H H a) tu a PS ft <U h u ftp rh H 0) SH ft (U LA -P ^ p< 1 Ö t- t t t t t t r- t t t- r- ' u N -* N v V V v 8 9P S? 9P P n CH 0 u tu a H 3,0 S3 CM CM r -* V CM CM JJ- -=)- -3- v a 0 -H 82 CH P H ä 8 & s a LA V t & m N R & d -d rd «V d,--d TU cu u u CU M 'S ^ u tu tu B u tu U r-t 0) u tu <U H cu CU ^1 cu 5 C <Ü h J -P nj H 0 H rt 0 cd 0 a cu a v CU CM r-i CU H J H r-i il r-i H ) H H (D H Cd H <d P Ch -p CH P «H P "+H -P <H P -d + d fi d ^4 CH J3 CH CH ij CH CH Ä CH CH ja ^ (H ^J <VM *H,CJ *-( a 0 Ü CJ U U r( U a 0 C c Ü c 3 CH CH CH <ih H CD <H 0 cd a! -d ct! -d ca ctj 3 d 3 3 d. 3 i "d3 3 -d 3 3 P ri gfi U H gfi 0 u + h -p ft -p ft 0 ft a Ö 3 3 TJ 33H ft a 0 ft a 0 ft a 0 pt a ft S flj. < P a + S -t 3 a -p a p X * P S 3 S X S»H

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49 1*8 NACA TN 2727 MD Ö s u 0) H H CD ft in PH u P B u cu < Si -p u 0) Si PH 13!H cd T3 Ö p b

50 7V NACA TN 2727 k9 ( MI Figure 2.- Mdified pusher with geared engine, muffler, and three-bladed prpeller, cnfiguratin 9A.

51 50 NACA TN 2727 Figure 3.-.Mdified pusher, unmuffled, with fur-bladed prpeller, cnfiguratin 8.

52 NACA TN V Figure k.- Mdified pusher, muffler relcated, with eight-bladed prpeller, cnfiguratin 10.

53 ! 52 NACA TN 2727 en i.z-1 = 1 cn "h <1 _ 1 ^ i.hi "11 X i-t a 1 f in C m rl 1 r- \ - T-I CM\ \ 1 V Ö ^ i 1 0) X! in c t 1 r-h rf _,» I-HN *; :H a i ', i L! d *c c 1 1 -J TH cn -is ^rt irr i i 1 I *HN r c 1**«c T-H :1 d HS 1 1 E r cl** i-h cn< Id k- i r* i

54 NACA TN Ö -P cd W H u u p V cd u -p cd Ö cd -P w VD

55 5^ NACA TN 2727 <v H H <D ft U ft <U T3 cd H r I H 03 Ö -P cd -H t ft M cd T3 Ö cd P I t^- b I

56 NACA TW P N Ö P a u CD H. r-f CD ft ft -Ö <D H r I N Ö P u Ü 0 H H CD ft U ft 13 CD -Ö H, I s M W rl Id U (Ü H H 0 ft U ft T3 CD Td H, C *H 01 H H CD ft ft C U cö P!* CD C ft CD <H T3 s PH

57 56 NACA TN ,10 ß'-ßj. DEG 15^ \ \ \ 1 \ 1 \ b N^ D \ \ < \ \ \ -ß'-ßj' \ \ \ D 25 \.04 0 J 20 -S-, PERCENT D \ \ \ t^ \ \ s N ^,.02 \ 10 \ V r/r NACA^' "^ ^ Figure 9.- Blade-frm curves fr Aermatic prpeller (see table I) D, diameter f prpeller; R, tip radius; x, radius f element; b, width (chrd) f element; h, maximum thickness f element; ß', pitch angle f element; ßij', pitch angle f tip element.

58 8V NACA TN "I 40 ß'-ßj, DEG I \ \/ -ß'-ßr b D D N N. 0 J 20 \ X \.02 -g-, PERCENT \ 1 15 \ % V 10 b \NACAp-^ r/r 1 Figure 10.- Blade-frm curves fr fixed-pitch, slid, fur-bladed prpeller (see table I). D, diameter f prpeller; R, tip radius; r, radius f element; b, width (chrd) f element; h, maximum thickness f element; ß«, pitch angle f element; ß T ', pitch angle f tip element.

59 58 20 T 40 ß-ßj, DEG J 20 Ü-, PERCENT Figure 11.- Blade-frm curves fr medium-bladed" prpeller (see table I). D, diameter f prpeller) R, tip radius; r, radius f element; ~b, width (chrd) f elementj h, maximum thickness f element) ß', pitch angle f element; ßij 1, pitch angle f tip element.

60 NACA TN T DEG b. D 02 ", PERCENT Figure 12.- Blade-frm curves fr thin-bladed prpeller (see table I), D, diameter f prpeller; R, tip radius; r, radius f element; b, width (chrd) f element; h, maximum thickness f element; ß', pitch angle f element; ß T ', pitch angle f tip element.

61 6 NACA TN 2727 MICRPHNE IN WIND SCREEN BATTERY EXTENSIN CABLE VIBRATR SUPPLY SUND- LEVEL METER ««^MMENTARY SHRTING SWITCH HAND SWITCH GRAPHIC LEVEL RERDER (a) Fr flight and take-ff measurements, MICRPHNE IN WIND SCREEN SUND- LEVEL METER SUND ANALYZER HEADPHNES (b) Fr grund analysis. Figure 13-- Equipment intercnnectins.

62 NACA TN l I r^ M- f \ X \ 1- * l _j LU Uj ** J. J u_<fr \ X \ 2, Kg <Z s D 8 U. ^ K H tü UNS Z-i-J 0<JJ.Z <0< IS X <a s I- < _l u. \ X X ^ X ^v \ U <a 1 N H s <D H t <1J fi H 1 nd Ö d Ö 3 3 C 03 A ts P fi -H cö t*?h C <D J -P -H 0) U S a) H <U Ö in l -H 0. <U ü H T3 c 1 CD Ü n3 -H 2 Ö H Pi ft 3 ft c c6 (E U. CH <U fa c6 <D -P c H Ö ft -p <D Ö h cd P tj c 0 ö Ö a 0) 0 ti 3 CTJ4^ 0) P ft!h -H P^!* in 1 -Ö -H a H 3 (H 0) <D CÖ P ass M PR ea 's9niav3a a313n-u3za1vnw 0 (0 aa 's9niv3a a3i3w--i3a3i-anns in

63 62 NACA TN 2727 ( MICRPHNE IN WIND SCREEN BATTERY v**-*.^ VIBRATR SUPPLY 1 22,000-HM RESISTR in re» ire» T/N BDnuinc f PWER SUPPLY IMPEDANCE MATCH AND PRPER LEVEL RERDING MECHANISM SUND- LEVEL METER ; / PRTABLE AMPLIFIER MMENTARY SHRTING SWITCH HAND SWITCH (a) Fr magnetic tape recrding f flight nise. NNECTED IN SERIES WITH SHIELD F CABLE BETWEEN MICRPHNE AND LEVEL METER MICRPHNE c PWER SUPPLY RERDING MECHANISM 0 SUND- LEVEL METER 0 SUND ANALYZER p PRTABLE AMPLIFIER HEADPHNES (b) Fr analysis f magnetic tape recrdings, Figure 15«- Equipment intercnnectins.

64 NACA TN » I2'30 Figure 16.- Map f sund test site. Sund equipment at er pint f flight curse fr verhead flights; sund equipment at psitin indicated by small circle at end f east-west runway fr flights passing 3000 feet away.