IN THE LAST article propeller noise sources discussion centered noise caused torque loads propeller blades article will address noise due blade thickness noise due thrust loading Thrust noise generally held less efficient sound-generating mechanism both torque thickness noise re sults manner noise produced although complete discussion mechanism outside scope article Thickness noise re spects similar torque noise Separation thrust noise evidence presented previous arti cles has suggested thrust noise rela tively unimportant analysis based part measurements obtained microblades shown vary angle along side line propeller notes level sound generally highest about 900 side propel ler line its plane rotation addition data roughly sym metrical about 900 indicates thrust noise contribution may minimal Data obtained modified 7/2 x 8 propeller also included figure special case data depicted dashed line will discussed com pletely later article As mentioned symmetry direc tivity patterns shown Figure 1 suggests lack thrust-related noise However cannot guaranteed except working out method estimating relative magni tudes propeller noise sources will consider thrust torque thickness noise help theory de veloped researchers Gutin Demming can develop equations types noise noise can expressed product acoustic pressure amplitude directivity pattern Finally can separate symmetric noise sources torque thickness nonsymmet nc noise due thrust As noted earlier refer symmetry about 900 directiv ity pattern define symmetric acoustic pressure amplitude due torque blade thickness B thrust acoustic pres sure amplitude values B may calculated existing data Two meth ods used calculate values both methods essentially fit measured Ilodellier NOISE$ Propeller noise caused blade thickness thrust loading will discussed third report in-depth propeller noise study conducted North Carolina State University spon sored AMA ER Vess D Heathcoat R Nagel phone position 900 axis propeller location thrust noise theoretically insignificant since thrust-related noise simply does project direction variation directions noise sources can serve tool deter relative importance Torque thickness noise ideally symmetric about 900 reach maximum level point thrust noise drops minimum 90 0 point symmetrical examining patterns loudness generated propeller can draw conclusions about principle noise source Such measurements must obtained anechoic wind tunnel corrected effects wind tunnel shear flow etc data has measured magni tude propeller blade passing fre quency BPF determined Examining BPF eliminates other sounds may generated motors wind tun nel prop drive system turbu lence shear flow number such directivity patterns shown Figure 1 figure requires great deal discussion explanation data Figure 1 various propellers same operating condition Sound Pressure Levels SPL frequency corresponding propeller Pf B rickne 0227 0255 0317 0314 0604 0101 0189 0395 data theory two methods aver aged together produce results Ta ble summary shape magnitude curves similar shown Figure 1 fit classic theories thrust torque thickness noise relative lev els noise components could esti mated method took account torque thickness noise symmetri cal about 900 whereas noise due thrust Since theories use classic directiv ity patterns values B depend largely symmetry data similar shown Figure 1 According calculations propellers data base show little no influence thrust noise can seen noting generally least three times large B propeller result dicates dominance blade loading due torque rather thrust agreement low efficiencies stock propel lers discussed previous articles Thrust noise has greatest influence modified 7/2 x 8 propeller indicated higher value B prop modified attempt decrease pro peller noise result some noise re Continued page 34 February 1990 33 0227 * Without boundary layer turbulator 0 * boundary layer turbulator * * 0 Modified propeller 900 CD u 800 C C I 0 C 5 700 U 0 0 0 600 -j 0 n 500 40 Figure 1 0 Master 10 nger106 Zinger126 Zinger96 RevUp11 Modified 75 0 6080i60 Corrected Microphone Angle Degrees Sound Directivity Lab duction well higher efficiency Further evidence provided examin ing efficiencies propellers Recall discussions subject earlier arti cles stock propellers data base efficiencies between 25% 44% shown Table 1 efficient modified prop value about 55% Since efficient propeller re quires less power tb produce given amount thrust under test conditions equal rpm forward speed greater rel ative amount thrust-related noise ex pected modified propeller special case noted Figure 1 Note al though directivity pattern differs other propellers peak noise level establishes quieter props remains higher noise level angles greater 900 hence nearly symmetrical stock propellers fit theory suggests creased noise angles greater 900 due thrust loading shown higher value B Table noise reduction angles less 900 also due increased thrust noise Since thrust noise out phase thickness torque noise region cancels other noise sources results calculations listed Table 1 indicate inefficient stock propellers dominated symmetrical noise sources show little influence thrust-related noise However higher ef ficiency modified propeller results higher thrust-to-torque ratio greater degree thrust-related noise Data obtained during flight tests propellers also support observations Flight data first article ex plained significant variations noise aircraft nearly identical flight speeds rpm must due pro peller flight test data support further conclusion noise due thrust cannot dominant mechanism cause flight speed constant cases thrust also constant exten sion noise resulting thrust loads propeller blades may sumed nearly constant well Thus laboratory results under flight conditions corroborate findings pre sented preceding articles varia tions noise among different propellers due sources other thrust-related noise Separating torque thickness noise Torque noise blade thickness noise have similar directivity patterns symmetrical about peak 900 mechanisms such acoustic effi ciencies noise radiation differ very little Therefore theoretical scaling laws must used separate two mecha nisms simply means prop rpm creases noise due torque loading noise due blade thickness effects will increase different manner Figure 2 shows measured acoustic power PWL two propellersa standard Zinger prop modified propeller dis cussed earlier Although mentioned above modified prop probably exhibits degree thrust-related noise has selected use show noise power radiating still appears dominated torque noise figure illustrates sound power blade-passing frequency varies rpm Recall sound power PWL representative noise di rections whereas sound power blade-passing frequency BPF noise fundamental frequency pro peller useful tool estimating source noise use available theories deter sound varies rpm ac curately predict noise generated thick34 Model Aviation / 0 I7 40 1100 7 o Ziriger 10 Modified 75 00 m 0 0 ID 900 0 -J 0 800 700- 80009000100001100012000 RPM Figure 2 ured Power Blade Passing Frequency ness torque effects example must know details about blades cluding torque loads vary along blade quite easy however predict noise will change rpm changes can use parameters such shaft power rpm simulated flight speed thrust pitch prop dimensions etc predict varia tion determine well pre diction fits measured data mea sured PWL does vary predicted may reasonably assume dominant noise-generating mechanism problem approach ever indicates trends absolute measurement dB Nevertheless can normalize results com parisons may made cases fol low dB levels normalized maximum predicted level 100 dB Again must stressed level exact wish examine compare trend prediction rpm changes Figure 3 shows predicted variation thickness noise applied two propel lers Figure 2 mentioned above Fig ure 2 PWLs blade-passing fre quency seen converge about same level rpm approximately 11325 The standard propeller produces PWL about 101 dB rpm modified propeller produces nearly 100 dB Figure 3 shows noise gener ated thickness effects no such conver gence would occur difference thickness noise two propellers predicted nearly constant over rpm range tested interesting compare type eval uation torque noise Most results presented articles have indicated noise due torque main source propeller noise model aircraft indeed case might expect convergence noise levels measured Figure 2 11325 rpm predicted torque noise theory Figure 4 provides comparison Examination Figure 4 clearly shows acoustic power levels due torque loads predicted converge higher rpm manner similar measured shown Figure 2 Again absolute levels have normalized max imum occurs 100 dB though no ferences have drawn absolute level trends prediction mean ingful revealing predicted trend Figure 4 reveals difference between propellers low rpm range almost no difference higher rpm trend agreement measured results propellers modified propeller Figures 2 through 4 likely contain higher compo nent thrust-related noise course cause higher efficiency remarkable trends predicted Figures 3 4 apply closely two propellers again suggests torque-related noise dominant acous tic source model propellers hence model aircraft noise conclusions must qualified uncertainty whether results presented series articles apply equally model aircraft propellers Although pro pellers tested data base shown Ta ble 1 representative typically used props no assurance conclu sions can extrapolated other propellers Also similar props same manu 1100 v 1000 -r 0 U N E L 0 z 900 10000 RPM ii 6oo 12000 800 aoio Figure 4 9000 Predicted PWL due Torque BPF facturer often show differences great can some importance fact several supposedly identical propellers exhibit subtle differences means course some variation re sults possible significance results re search project clear Three main points should emphasized Assuming engine equipped reasonably effi cient muffler major source model airContinued page 142 February 1990 35 1100 o Zinger 10 Modified 75 1000 -r 0 6 N 900 800-900010000iiooo12000 RPM Figure 3 icted PWL due Thickness BPF o Zinger 10 Modified 75 n Len Funds ESCAPE SPECIFICATION. Wing Span62Y2 inches Wing Area770 square inches Engine Size 10 cc 90 120 faur stroke Designed AMA FAI Turn-around pattern Foam wing stab 3-32 Balsa sheet covering Tricycle conven tional gear fixed retracts Rear side exhaust fiber glass canopy Veiy positive man'verable XLT SPECIFIC Wing Span65 inches Length65 inches Wing Area845 square inches Recommended Engine Size 10 cc 90 120 four stroke XLT designed tuned pipe re tract landing gears Capable AMA Turn-around pattern Rear side ex haust SPECIFICATIONS Wing Span63Y4 inches Wing Area700 square inches Engine Size50-60 Glow 90 four stroke Forfun sport pattern turn-around can done Utter Chaos completely built up Balsa construction Canopy engine mount included years proven eying reliablity BRIDI AIRCRAFT DESIGNS INC 23625 Pineforest Lane Harbor City California 90710 213 549-8264 213 326-5013 SEE YOUR FAVORITE HOBBY SHOP OR RETAIL OUTLETW Dealer Inquries Invited AOSORO Sf1000 00 ISF0A1 001 00000 DOFFOODfTIAL 15010 00000 0000 00000 00000 000000 00000000 00t40000000 0SflY 0J05TA0t0 00000000-0000 001000 0S0 ON000LLATIOO OAT 01 100010100 00 00 0110008 0000011 01IOT0 INSTAOUTIOO P0CO000 flhnO 0n0ded LII Ole Reliable/Pfeifer Continued page 111 Lii Ole Reliable hadnt flown six feet 137 seconds knew worth effort Jeffreys sixyear-old instincts right target model flies dad want fly Jeffrey like No buts Prop Noise Continued page 35 craft noise prop noise 2 main com ponent prop noise caused torque loading blades 3 propellers available generally very inefficient inefficiency leads high torque loads relative thrust produced path model aircraft noise reduc tion therefore clear although perhaps easy Making propellers efficient would reduce noise levels compro mising aircraft performance Next months conclusion will present several useful guide lines modelers follow choosing pro pellers own aircraft Addendum Corrections Part 2 table inadvertently omitted some revisions editorial staff error resulting meanings different intended authors ask read ers review following items con junction Model Propeller Noise article January 1990 issue order obtain proper meanings case italics added emphasis Pg 39 first col first para first sen tence Should read opeller noise sources hibit characteristic pattern noise radiates away propeller Pg39 second col second para first sentence Should read echoic re fers condition sound nor re flected use sound-absorbent material simulate free-field acoustic environ ment aircraft flight COMPACT ELECTRIC RC HERE 0edSI0 or50 Wolt tigh 1ff oeoey Motor Sorogsol Propoo4tells Powers 310 24oo Models Si lSSlwPostegeeeHendllng Pg 40 first col first full para omitted Table listing propellers used testing follows Table 1Propellers Included Data Base Manufacturer Size ManufacturerSize Zinger8 x 6Rev-Up11 x 6 EW Zinger9 x6Rev-Up lix 7 Rev-Up9 x 6 Zinger 12 x 6 Master10 x 6Tornado 9 x 6 Three-blade propeller Pg 40 first col first full para second sentence Propellers chosen ing typical use today abil iry 05 Max 046 SF engine turn Pg 40 third col second full para De lete Propeller noise always occurs blade passing frequency does reach crescendo Pg 40 third col last para third fourth sentences An efficient propeller will produce thrust same en gine output same flight conditions An inefficient propeller causes engine work turn prop producing little thrust Pg 41 first col first para ad vance ratio denoted symbol J pro portional ratio Pg 148 second col first para last sen tence ops efficient high speeds will provide reduced noise lev els compared less efficient props same performance levels Pg 148 second col first full para last sentence n reasonably suspect measured noise source fact torque noise regret omission errors Carl R Wheeley Editor Publisher responding advertisers mention read about Model Aviation 142 Model Aviation
Edition: Model Aviation - 1990/02
Page Numbers: 33, 34, 35, 142
IN THE LAST article propeller noise sources discussion centered noise caused torque loads propeller blades article will address noise due blade thickness noise due thrust loading Thrust noise generally held less efficient sound-generating mechanism both torque thickness noise re sults manner noise produced although complete discussion mechanism outside scope article Thickness noise re spects similar torque noise Separation thrust noise evidence presented previous arti cles has suggested thrust noise rela tively unimportant analysis based part measurements obtained microblades shown vary angle along side line propeller notes level sound generally highest about 900 side propel ler line its plane rotation addition data roughly sym metrical about 900 indicates thrust noise contribution may minimal Data obtained modified 7/2 x 8 propeller also included figure special case data depicted dashed line will discussed com pletely later article As mentioned symmetry direc tivity patterns shown Figure 1 suggests lack thrust-related noise However cannot guaranteed except working out method estimating relative magni tudes propeller noise sources will consider thrust torque thickness noise help theory de veloped researchers Gutin Demming can develop equations types noise noise can expressed product acoustic pressure amplitude directivity pattern Finally can separate symmetric noise sources torque thickness nonsymmet nc noise due thrust As noted earlier refer symmetry about 900 directiv ity pattern define symmetric acoustic pressure amplitude due torque blade thickness B thrust acoustic pres sure amplitude values B may calculated existing data Two meth ods used calculate values both methods essentially fit measured Ilodellier NOISE$ Propeller noise caused blade thickness thrust loading will discussed third report in-depth propeller noise study conducted North Carolina State University spon sored AMA ER Vess D Heathcoat R Nagel phone position 900 axis propeller location thrust noise theoretically insignificant since thrust-related noise simply does project direction variation directions noise sources can serve tool deter relative importance Torque thickness noise ideally symmetric about 900 reach maximum level point thrust noise drops minimum 90 0 point symmetrical examining patterns loudness generated propeller can draw conclusions about principle noise source Such measurements must obtained anechoic wind tunnel corrected effects wind tunnel shear flow etc data has measured magni tude propeller blade passing fre quency BPF determined Examining BPF eliminates other sounds may generated motors wind tun nel prop drive system turbu lence shear flow number such directivity patterns shown Figure 1 figure requires great deal discussion explanation data Figure 1 various propellers same operating condition Sound Pressure Levels SPL frequency corresponding propeller Pf B rickne 0227 0255 0317 0314 0604 0101 0189 0395 data theory two methods aver aged together produce results Ta ble summary shape magnitude curves similar shown Figure 1 fit classic theories thrust torque thickness noise relative lev els noise components could esti mated method took account torque thickness noise symmetri cal about 900 whereas noise due thrust Since theories use classic directiv ity patterns values B depend largely symmetry data similar shown Figure 1 According calculations propellers data base show little no influence thrust noise can seen noting generally least three times large B propeller result dicates dominance blade loading due torque rather thrust agreement low efficiencies stock propel lers discussed previous articles Thrust noise has greatest influence modified 7/2 x 8 propeller indicated higher value B prop modified attempt decrease pro peller noise result some noise re Continued page 34 February 1990 33 0227 * Without boundary layer turbulator 0 * boundary layer turbulator * * 0 Modified propeller 900 CD u 800 C C I 0 C 5 700 U 0 0 0 600 -j 0 n 500 40 Figure 1 0 Master 10 nger106 Zinger126 Zinger96 RevUp11 Modified 75 0 6080i60 Corrected Microphone Angle Degrees Sound Directivity Lab duction well higher efficiency Further evidence provided examin ing efficiencies propellers Recall discussions subject earlier arti cles stock propellers data base efficiencies between 25% 44% shown Table 1 efficient modified prop value about 55% Since efficient propeller re quires less power tb produce given amount thrust under test conditions equal rpm forward speed greater rel ative amount thrust-related noise ex pected modified propeller special case noted Figure 1 Note al though directivity pattern differs other propellers peak noise level establishes quieter props remains higher noise level angles greater 900 hence nearly symmetrical stock propellers fit theory suggests creased noise angles greater 900 due thrust loading shown higher value B Table noise reduction angles less 900 also due increased thrust noise Since thrust noise out phase thickness torque noise region cancels other noise sources results calculations listed Table 1 indicate inefficient stock propellers dominated symmetrical noise sources show little influence thrust-related noise However higher ef ficiency modified propeller results higher thrust-to-torque ratio greater degree thrust-related noise Data obtained during flight tests propellers also support observations Flight data first article ex plained significant variations noise aircraft nearly identical flight speeds rpm must due pro peller flight test data support further conclusion noise due thrust cannot dominant mechanism cause flight speed constant cases thrust also constant exten sion noise resulting thrust loads propeller blades may sumed nearly constant well Thus laboratory results under flight conditions corroborate findings pre sented preceding articles varia tions noise among different propellers due sources other thrust-related noise Separating torque thickness noise Torque noise blade thickness noise have similar directivity patterns symmetrical about peak 900 mechanisms such acoustic effi ciencies noise radiation differ very little Therefore theoretical scaling laws must used separate two mecha nisms simply means prop rpm creases noise due torque loading noise due blade thickness effects will increase different manner Figure 2 shows measured acoustic power PWL two propellersa standard Zinger prop modified propeller dis cussed earlier Although mentioned above modified prop probably exhibits degree thrust-related noise has selected use show noise power radiating still appears dominated torque noise figure illustrates sound power blade-passing frequency varies rpm Recall sound power PWL representative noise di rections whereas sound power blade-passing frequency BPF noise fundamental frequency pro peller useful tool estimating source noise use available theories deter sound varies rpm ac curately predict noise generated thick34 Model Aviation / 0 I7 40 1100 7 o Ziriger 10 Modified 75 00 m 0 0 ID 900 0 -J 0 800 700- 80009000100001100012000 RPM Figure 2 ured Power Blade Passing Frequency ness torque effects example must know details about blades cluding torque loads vary along blade quite easy however predict noise will change rpm changes can use parameters such shaft power rpm simulated flight speed thrust pitch prop dimensions etc predict varia tion determine well pre diction fits measured data mea sured PWL does vary predicted may reasonably assume dominant noise-generating mechanism problem approach ever indicates trends absolute measurement dB Nevertheless can normalize results com parisons may made cases fol low dB levels normalized maximum predicted level 100 dB Again must stressed level exact wish examine compare trend prediction rpm changes Figure 3 shows predicted variation thickness noise applied two propel lers Figure 2 mentioned above Fig ure 2 PWLs blade-passing fre quency seen converge about same level rpm approximately 11325 The standard propeller produces PWL about 101 dB rpm modified propeller produces nearly 100 dB Figure 3 shows noise gener ated thickness effects no such conver gence would occur difference thickness noise two propellers predicted nearly constant over rpm range tested interesting compare type eval uation torque noise Most results presented articles have indicated noise due torque main source propeller noise model aircraft indeed case might expect convergence noise levels measured Figure 2 11325 rpm predicted torque noise theory Figure 4 provides comparison Examination Figure 4 clearly shows acoustic power levels due torque loads predicted converge higher rpm manner similar measured shown Figure 2 Again absolute levels have normalized max imum occurs 100 dB though no ferences have drawn absolute level trends prediction mean ingful revealing predicted trend Figure 4 reveals difference between propellers low rpm range almost no difference higher rpm trend agreement measured results propellers modified propeller Figures 2 through 4 likely contain higher compo nent thrust-related noise course cause higher efficiency remarkable trends predicted Figures 3 4 apply closely two propellers again suggests torque-related noise dominant acous tic source model propellers hence model aircraft noise conclusions must qualified uncertainty whether results presented series articles apply equally model aircraft propellers Although pro pellers tested data base shown Ta ble 1 representative typically used props no assurance conclu sions can extrapolated other propellers Also similar props same manu 1100 v 1000 -r 0 U N E L 0 z 900 10000 RPM ii 6oo 12000 800 aoio Figure 4 9000 Predicted PWL due Torque BPF facturer often show differences great can some importance fact several supposedly identical propellers exhibit subtle differences means course some variation re sults possible significance results re search project clear Three main points should emphasized Assuming engine equipped reasonably effi cient muffler major source model airContinued page 142 February 1990 35 1100 o Zinger 10 Modified 75 1000 -r 0 6 N 900 800-900010000iiooo12000 RPM Figure 3 icted PWL due Thickness BPF o Zinger 10 Modified 75 n Len Funds ESCAPE SPECIFICATION. Wing Span62Y2 inches Wing Area770 square inches Engine Size 10 cc 90 120 faur stroke Designed AMA FAI Turn-around pattern Foam wing stab 3-32 Balsa sheet covering Tricycle conven tional gear fixed retracts Rear side exhaust fiber glass canopy Veiy positive man'verable XLT SPECIFIC Wing Span65 inches Length65 inches Wing Area845 square inches Recommended Engine Size 10 cc 90 120 four stroke XLT designed tuned pipe re tract landing gears Capable AMA Turn-around pattern Rear side ex haust SPECIFICATIONS Wing Span63Y4 inches Wing Area700 square inches Engine Size50-60 Glow 90 four stroke Forfun sport pattern turn-around can done Utter Chaos completely built up Balsa construction Canopy engine mount included years proven eying reliablity BRIDI AIRCRAFT DESIGNS INC 23625 Pineforest Lane Harbor City California 90710 213 549-8264 213 326-5013 SEE YOUR FAVORITE HOBBY SHOP OR RETAIL OUTLETW Dealer Inquries Invited AOSORO Sf1000 00 ISF0A1 001 00000 DOFFOODfTIAL 15010 00000 0000 00000 00000 000000 00000000 00t40000000 0SflY 0J05TA0t0 00000000-0000 001000 0S0 ON000LLATIOO OAT 01 100010100 00 00 0110008 0000011 01IOT0 INSTAOUTIOO P0CO000 flhnO 0n0ded LII Ole Reliable/Pfeifer Continued page 111 Lii Ole Reliable hadnt flown six feet 137 seconds knew worth effort Jeffreys sixyear-old instincts right target model flies dad want fly Jeffrey like No buts Prop Noise Continued page 35 craft noise prop noise 2 main com ponent prop noise caused torque loading blades 3 propellers available generally very inefficient inefficiency leads high torque loads relative thrust produced path model aircraft noise reduc tion therefore clear although perhaps easy Making propellers efficient would reduce noise levels compro mising aircraft performance Next months conclusion will present several useful guide lines modelers follow choosing pro pellers own aircraft Addendum Corrections Part 2 table inadvertently omitted some revisions editorial staff error resulting meanings different intended authors ask read ers review following items con junction Model Propeller Noise article January 1990 issue order obtain proper meanings case italics added emphasis Pg 39 first col first para first sen tence Should read opeller noise sources hibit characteristic pattern noise radiates away propeller Pg39 second col second para first sentence Should read echoic re fers condition sound nor re flected use sound-absorbent material simulate free-field acoustic environ ment aircraft flight COMPACT ELECTRIC RC HERE 0edSI0 or50 Wolt tigh 1ff oeoey Motor Sorogsol Propoo4tells Powers 310 24oo Models Si lSSlwPostegeeeHendllng Pg 40 first col first full para omitted Table listing propellers used testing follows Table 1Propellers Included Data Base Manufacturer Size ManufacturerSize Zinger8 x 6Rev-Up11 x 6 EW Zinger9 x6Rev-Up lix 7 Rev-Up9 x 6 Zinger 12 x 6 Master10 x 6Tornado 9 x 6 Three-blade propeller Pg 40 first col first full para second sentence Propellers chosen ing typical use today abil iry 05 Max 046 SF engine turn Pg 40 third col second full para De lete Propeller noise always occurs blade passing frequency does reach crescendo Pg 40 third col last para third fourth sentences An efficient propeller will produce thrust same en gine output same flight conditions An inefficient propeller causes engine work turn prop producing little thrust Pg 41 first col first para ad vance ratio denoted symbol J pro portional ratio Pg 148 second col first para last sen tence ops efficient high speeds will provide reduced noise lev els compared less efficient props same performance levels Pg 148 second col first full para last sentence n reasonably suspect measured noise source fact torque noise regret omission errors Carl R Wheeley Editor Publisher responding advertisers mention read about Model Aviation 142 Model Aviation
Edition: Model Aviation - 1990/02
Page Numbers: 33, 34, 35, 142
IN THE LAST article propeller noise sources discussion centered noise caused torque loads propeller blades article will address noise due blade thickness noise due thrust loading Thrust noise generally held less efficient sound-generating mechanism both torque thickness noise re sults manner noise produced although complete discussion mechanism outside scope article Thickness noise re spects similar torque noise Separation thrust noise evidence presented previous arti cles has suggested thrust noise rela tively unimportant analysis based part measurements obtained microblades shown vary angle along side line propeller notes level sound generally highest about 900 side propel ler line its plane rotation addition data roughly sym metrical about 900 indicates thrust noise contribution may minimal Data obtained modified 7/2 x 8 propeller also included figure special case data depicted dashed line will discussed com pletely later article As mentioned symmetry direc tivity patterns shown Figure 1 suggests lack thrust-related noise However cannot guaranteed except working out method estimating relative magni tudes propeller noise sources will consider thrust torque thickness noise help theory de veloped researchers Gutin Demming can develop equations types noise noise can expressed product acoustic pressure amplitude directivity pattern Finally can separate symmetric noise sources torque thickness nonsymmet nc noise due thrust As noted earlier refer symmetry about 900 directiv ity pattern define symmetric acoustic pressure amplitude due torque blade thickness B thrust acoustic pres sure amplitude values B may calculated existing data Two meth ods used calculate values both methods essentially fit measured Ilodellier NOISE$ Propeller noise caused blade thickness thrust loading will discussed third report in-depth propeller noise study conducted North Carolina State University spon sored AMA ER Vess D Heathcoat R Nagel phone position 900 axis propeller location thrust noise theoretically insignificant since thrust-related noise simply does project direction variation directions noise sources can serve tool deter relative importance Torque thickness noise ideally symmetric about 900 reach maximum level point thrust noise drops minimum 90 0 point symmetrical examining patterns loudness generated propeller can draw conclusions about principle noise source Such measurements must obtained anechoic wind tunnel corrected effects wind tunnel shear flow etc data has measured magni tude propeller blade passing fre quency BPF determined Examining BPF eliminates other sounds may generated motors wind tun nel prop drive system turbu lence shear flow number such directivity patterns shown Figure 1 figure requires great deal discussion explanation data Figure 1 various propellers same operating condition Sound Pressure Levels SPL frequency corresponding propeller Pf B rickne 0227 0255 0317 0314 0604 0101 0189 0395 data theory two methods aver aged together produce results Ta ble summary shape magnitude curves similar shown Figure 1 fit classic theories thrust torque thickness noise relative lev els noise components could esti mated method took account torque thickness noise symmetri cal about 900 whereas noise due thrust Since theories use classic directiv ity patterns values B depend largely symmetry data similar shown Figure 1 According calculations propellers data base show little no influence thrust noise can seen noting generally least three times large B propeller result dicates dominance blade loading due torque rather thrust agreement low efficiencies stock propel lers discussed previous articles Thrust noise has greatest influence modified 7/2 x 8 propeller indicated higher value B prop modified attempt decrease pro peller noise result some noise re Continued page 34 February 1990 33 0227 * Without boundary layer turbulator 0 * boundary layer turbulator * * 0 Modified propeller 900 CD u 800 C C I 0 C 5 700 U 0 0 0 600 -j 0 n 500 40 Figure 1 0 Master 10 nger106 Zinger126 Zinger96 RevUp11 Modified 75 0 6080i60 Corrected Microphone Angle Degrees Sound Directivity Lab duction well higher efficiency Further evidence provided examin ing efficiencies propellers Recall discussions subject earlier arti cles stock propellers data base efficiencies between 25% 44% shown Table 1 efficient modified prop value about 55% Since efficient propeller re quires less power tb produce given amount thrust under test conditions equal rpm forward speed greater rel ative amount thrust-related noise ex pected modified propeller special case noted Figure 1 Note al though directivity pattern differs other propellers peak noise level establishes quieter props remains higher noise level angles greater 900 hence nearly symmetrical stock propellers fit theory suggests creased noise angles greater 900 due thrust loading shown higher value B Table noise reduction angles less 900 also due increased thrust noise Since thrust noise out phase thickness torque noise region cancels other noise sources results calculations listed Table 1 indicate inefficient stock propellers dominated symmetrical noise sources show little influence thrust-related noise However higher ef ficiency modified propeller results higher thrust-to-torque ratio greater degree thrust-related noise Data obtained during flight tests propellers also support observations Flight data first article ex plained significant variations noise aircraft nearly identical flight speeds rpm must due pro peller flight test data support further conclusion noise due thrust cannot dominant mechanism cause flight speed constant cases thrust also constant exten sion noise resulting thrust loads propeller blades may sumed nearly constant well Thus laboratory results under flight conditions corroborate findings pre sented preceding articles varia tions noise among different propellers due sources other thrust-related noise Separating torque thickness noise Torque noise blade thickness noise have similar directivity patterns symmetrical about peak 900 mechanisms such acoustic effi ciencies noise radiation differ very little Therefore theoretical scaling laws must used separate two mecha nisms simply means prop rpm creases noise due torque loading noise due blade thickness effects will increase different manner Figure 2 shows measured acoustic power PWL two propellersa standard Zinger prop modified propeller dis cussed earlier Although mentioned above modified prop probably exhibits degree thrust-related noise has selected use show noise power radiating still appears dominated torque noise figure illustrates sound power blade-passing frequency varies rpm Recall sound power PWL representative noise di rections whereas sound power blade-passing frequency BPF noise fundamental frequency pro peller useful tool estimating source noise use available theories deter sound varies rpm ac curately predict noise generated thick34 Model Aviation / 0 I7 40 1100 7 o Ziriger 10 Modified 75 00 m 0 0 ID 900 0 -J 0 800 700- 80009000100001100012000 RPM Figure 2 ured Power Blade Passing Frequency ness torque effects example must know details about blades cluding torque loads vary along blade quite easy however predict noise will change rpm changes can use parameters such shaft power rpm simulated flight speed thrust pitch prop dimensions etc predict varia tion determine well pre diction fits measured data mea sured PWL does vary predicted may reasonably assume dominant noise-generating mechanism problem approach ever indicates trends absolute measurement dB Nevertheless can normalize results com parisons may made cases fol low dB levels normalized maximum predicted level 100 dB Again must stressed level exact wish examine compare trend prediction rpm changes Figure 3 shows predicted variation thickness noise applied two propel lers Figure 2 mentioned above Fig ure 2 PWLs blade-passing fre quency seen converge about same level rpm approximately 11325 The standard propeller produces PWL about 101 dB rpm modified propeller produces nearly 100 dB Figure 3 shows noise gener ated thickness effects no such conver gence would occur difference thickness noise two propellers predicted nearly constant over rpm range tested interesting compare type eval uation torque noise Most results presented articles have indicated noise due torque main source propeller noise model aircraft indeed case might expect convergence noise levels measured Figure 2 11325 rpm predicted torque noise theory Figure 4 provides comparison Examination Figure 4 clearly shows acoustic power levels due torque loads predicted converge higher rpm manner similar measured shown Figure 2 Again absolute levels have normalized max imum occurs 100 dB though no ferences have drawn absolute level trends prediction mean ingful revealing predicted trend Figure 4 reveals difference between propellers low rpm range almost no difference higher rpm trend agreement measured results propellers modified propeller Figures 2 through 4 likely contain higher compo nent thrust-related noise course cause higher efficiency remarkable trends predicted Figures 3 4 apply closely two propellers again suggests torque-related noise dominant acous tic source model propellers hence model aircraft noise conclusions must qualified uncertainty whether results presented series articles apply equally model aircraft propellers Although pro pellers tested data base shown Ta ble 1 representative typically used props no assurance conclu sions can extrapolated other propellers Also similar props same manu 1100 v 1000 -r 0 U N E L 0 z 900 10000 RPM ii 6oo 12000 800 aoio Figure 4 9000 Predicted PWL due Torque BPF facturer often show differences great can some importance fact several supposedly identical propellers exhibit subtle differences means course some variation re sults possible significance results re search project clear Three main points should emphasized Assuming engine equipped reasonably effi cient muffler major source model airContinued page 142 February 1990 35 1100 o Zinger 10 Modified 75 1000 -r 0 6 N 900 800-900010000iiooo12000 RPM Figure 3 icted PWL due Thickness BPF o Zinger 10 Modified 75 n Len Funds ESCAPE SPECIFICATION. Wing Span62Y2 inches Wing Area770 square inches Engine Size 10 cc 90 120 faur stroke Designed AMA FAI Turn-around pattern Foam wing stab 3-32 Balsa sheet covering Tricycle conven tional gear fixed retracts Rear side exhaust fiber glass canopy Veiy positive man'verable XLT SPECIFIC Wing Span65 inches Length65 inches Wing Area845 square inches Recommended Engine Size 10 cc 90 120 four stroke XLT designed tuned pipe re tract landing gears Capable AMA Turn-around pattern Rear side ex haust SPECIFICATIONS Wing Span63Y4 inches Wing Area700 square inches Engine Size50-60 Glow 90 four stroke Forfun sport pattern turn-around can done Utter Chaos completely built up Balsa construction Canopy engine mount included years proven eying reliablity BRIDI AIRCRAFT DESIGNS INC 23625 Pineforest Lane Harbor City California 90710 213 549-8264 213 326-5013 SEE YOUR FAVORITE HOBBY SHOP OR RETAIL OUTLETW Dealer Inquries Invited AOSORO Sf1000 00 ISF0A1 001 00000 DOFFOODfTIAL 15010 00000 0000 00000 00000 000000 00000000 00t40000000 0SflY 0J05TA0t0 00000000-0000 001000 0S0 ON000LLATIOO OAT 01 100010100 00 00 0110008 0000011 01IOT0 INSTAOUTIOO P0CO000 flhnO 0n0ded LII Ole Reliable/Pfeifer Continued page 111 Lii Ole Reliable hadnt flown six feet 137 seconds knew worth effort Jeffreys sixyear-old instincts right target model flies dad want fly Jeffrey like No buts Prop Noise Continued page 35 craft noise prop noise 2 main com ponent prop noise caused torque loading blades 3 propellers available generally very inefficient inefficiency leads high torque loads relative thrust produced path model aircraft noise reduc tion therefore clear although perhaps easy Making propellers efficient would reduce noise levels compro mising aircraft performance Next months conclusion will present several useful guide lines modelers follow choosing pro pellers own aircraft Addendum Corrections Part 2 table inadvertently omitted some revisions editorial staff error resulting meanings different intended authors ask read ers review following items con junction Model Propeller Noise article January 1990 issue order obtain proper meanings case italics added emphasis Pg 39 first col first para first sen tence Should read opeller noise sources hibit characteristic pattern noise radiates away propeller Pg39 second col second para first sentence Should read echoic re fers condition sound nor re flected use sound-absorbent material simulate free-field acoustic environ ment aircraft flight COMPACT ELECTRIC RC HERE 0edSI0 or50 Wolt tigh 1ff oeoey Motor Sorogsol Propoo4tells Powers 310 24oo Models Si lSSlwPostegeeeHendllng Pg 40 first col first full para omitted Table listing propellers used testing follows Table 1Propellers Included Data Base Manufacturer Size ManufacturerSize Zinger8 x 6Rev-Up11 x 6 EW Zinger9 x6Rev-Up lix 7 Rev-Up9 x 6 Zinger 12 x 6 Master10 x 6Tornado 9 x 6 Three-blade propeller Pg 40 first col first full para second sentence Propellers chosen ing typical use today abil iry 05 Max 046 SF engine turn Pg 40 third col second full para De lete Propeller noise always occurs blade passing frequency does reach crescendo Pg 40 third col last para third fourth sentences An efficient propeller will produce thrust same en gine output same flight conditions An inefficient propeller causes engine work turn prop producing little thrust Pg 41 first col first para ad vance ratio denoted symbol J pro portional ratio Pg 148 second col first para last sen tence ops efficient high speeds will provide reduced noise lev els compared less efficient props same performance levels Pg 148 second col first full para last sentence n reasonably suspect measured noise source fact torque noise regret omission errors Carl R Wheeley Editor Publisher responding advertisers mention read about Model Aviation 142 Model Aviation
Edition: Model Aviation - 1990/02
Page Numbers: 33, 34, 35, 142
IN THE LAST article propeller noise sources discussion centered noise caused torque loads propeller blades article will address noise due blade thickness noise due thrust loading Thrust noise generally held less efficient sound-generating mechanism both torque thickness noise re sults manner noise produced although complete discussion mechanism outside scope article Thickness noise re spects similar torque noise Separation thrust noise evidence presented previous arti cles has suggested thrust noise rela tively unimportant analysis based part measurements obtained microblades shown vary angle along side line propeller notes level sound generally highest about 900 side propel ler line its plane rotation addition data roughly sym metrical about 900 indicates thrust noise contribution may minimal Data obtained modified 7/2 x 8 propeller also included figure special case data depicted dashed line will discussed com pletely later article As mentioned symmetry direc tivity patterns shown Figure 1 suggests lack thrust-related noise However cannot guaranteed except working out method estimating relative magni tudes propeller noise sources will consider thrust torque thickness noise help theory de veloped researchers Gutin Demming can develop equations types noise noise can expressed product acoustic pressure amplitude directivity pattern Finally can separate symmetric noise sources torque thickness nonsymmet nc noise due thrust As noted earlier refer symmetry about 900 directiv ity pattern define symmetric acoustic pressure amplitude due torque blade thickness B thrust acoustic pres sure amplitude values B may calculated existing data Two meth ods used calculate values both methods essentially fit measured Ilodellier NOISE$ Propeller noise caused blade thickness thrust loading will discussed third report in-depth propeller noise study conducted North Carolina State University spon sored AMA ER Vess D Heathcoat R Nagel phone position 900 axis propeller location thrust noise theoretically insignificant since thrust-related noise simply does project direction variation directions noise sources can serve tool deter relative importance Torque thickness noise ideally symmetric about 900 reach maximum level point thrust noise drops minimum 90 0 point symmetrical examining patterns loudness generated propeller can draw conclusions about principle noise source Such measurements must obtained anechoic wind tunnel corrected effects wind tunnel shear flow etc data has measured magni tude propeller blade passing fre quency BPF determined Examining BPF eliminates other sounds may generated motors wind tun nel prop drive system turbu lence shear flow number such directivity patterns shown Figure 1 figure requires great deal discussion explanation data Figure 1 various propellers same operating condition Sound Pressure Levels SPL frequency corresponding propeller Pf B rickne 0227 0255 0317 0314 0604 0101 0189 0395 data theory two methods aver aged together produce results Ta ble summary shape magnitude curves similar shown Figure 1 fit classic theories thrust torque thickness noise relative lev els noise components could esti mated method took account torque thickness noise symmetri cal about 900 whereas noise due thrust Since theories use classic directiv ity patterns values B depend largely symmetry data similar shown Figure 1 According calculations propellers data base show little no influence thrust noise can seen noting generally least three times large B propeller result dicates dominance blade loading due torque rather thrust agreement low efficiencies stock propel lers discussed previous articles Thrust noise has greatest influence modified 7/2 x 8 propeller indicated higher value B prop modified attempt decrease pro peller noise result some noise re Continued page 34 February 1990 33 0227 * Without boundary layer turbulator 0 * boundary layer turbulator * * 0 Modified propeller 900 CD u 800 C C I 0 C 5 700 U 0 0 0 600 -j 0 n 500 40 Figure 1 0 Master 10 nger106 Zinger126 Zinger96 RevUp11 Modified 75 0 6080i60 Corrected Microphone Angle Degrees Sound Directivity Lab duction well higher efficiency Further evidence provided examin ing efficiencies propellers Recall discussions subject earlier arti cles stock propellers data base efficiencies between 25% 44% shown Table 1 efficient modified prop value about 55% Since efficient propeller re quires less power tb produce given amount thrust under test conditions equal rpm forward speed greater rel ative amount thrust-related noise ex pected modified propeller special case noted Figure 1 Note al though directivity pattern differs other propellers peak noise level establishes quieter props remains higher noise level angles greater 900 hence nearly symmetrical stock propellers fit theory suggests creased noise angles greater 900 due thrust loading shown higher value B Table noise reduction angles less 900 also due increased thrust noise Since thrust noise out phase thickness torque noise region cancels other noise sources results calculations listed Table 1 indicate inefficient stock propellers dominated symmetrical noise sources show little influence thrust-related noise However higher ef ficiency modified propeller results higher thrust-to-torque ratio greater degree thrust-related noise Data obtained during flight tests propellers also support observations Flight data first article ex plained significant variations noise aircraft nearly identical flight speeds rpm must due pro peller flight test data support further conclusion noise due thrust cannot dominant mechanism cause flight speed constant cases thrust also constant exten sion noise resulting thrust loads propeller blades may sumed nearly constant well Thus laboratory results under flight conditions corroborate findings pre sented preceding articles varia tions noise among different propellers due sources other thrust-related noise Separating torque thickness noise Torque noise blade thickness noise have similar directivity patterns symmetrical about peak 900 mechanisms such acoustic effi ciencies noise radiation differ very little Therefore theoretical scaling laws must used separate two mecha nisms simply means prop rpm creases noise due torque loading noise due blade thickness effects will increase different manner Figure 2 shows measured acoustic power PWL two propellersa standard Zinger prop modified propeller dis cussed earlier Although mentioned above modified prop probably exhibits degree thrust-related noise has selected use show noise power radiating still appears dominated torque noise figure illustrates sound power blade-passing frequency varies rpm Recall sound power PWL representative noise di rections whereas sound power blade-passing frequency BPF noise fundamental frequency pro peller useful tool estimating source noise use available theories deter sound varies rpm ac curately predict noise generated thick34 Model Aviation / 0 I7 40 1100 7 o Ziriger 10 Modified 75 00 m 0 0 ID 900 0 -J 0 800 700- 80009000100001100012000 RPM Figure 2 ured Power Blade Passing Frequency ness torque effects example must know details about blades cluding torque loads vary along blade quite easy however predict noise will change rpm changes can use parameters such shaft power rpm simulated flight speed thrust pitch prop dimensions etc predict varia tion determine well pre diction fits measured data mea sured PWL does vary predicted may reasonably assume dominant noise-generating mechanism problem approach ever indicates trends absolute measurement dB Nevertheless can normalize results com parisons may made cases fol low dB levels normalized maximum predicted level 100 dB Again must stressed level exact wish examine compare trend prediction rpm changes Figure 3 shows predicted variation thickness noise applied two propel lers Figure 2 mentioned above Fig ure 2 PWLs blade-passing fre quency seen converge about same level rpm approximately 11325 The standard propeller produces PWL about 101 dB rpm modified propeller produces nearly 100 dB Figure 3 shows noise gener ated thickness effects no such conver gence would occur difference thickness noise two propellers predicted nearly constant over rpm range tested interesting compare type eval uation torque noise Most results presented articles have indicated noise due torque main source propeller noise model aircraft indeed case might expect convergence noise levels measured Figure 2 11325 rpm predicted torque noise theory Figure 4 provides comparison Examination Figure 4 clearly shows acoustic power levels due torque loads predicted converge higher rpm manner similar measured shown Figure 2 Again absolute levels have normalized max imum occurs 100 dB though no ferences have drawn absolute level trends prediction mean ingful revealing predicted trend Figure 4 reveals difference between propellers low rpm range almost no difference higher rpm trend agreement measured results propellers modified propeller Figures 2 through 4 likely contain higher compo nent thrust-related noise course cause higher efficiency remarkable trends predicted Figures 3 4 apply closely two propellers again suggests torque-related noise dominant acous tic source model propellers hence model aircraft noise conclusions must qualified uncertainty whether results presented series articles apply equally model aircraft propellers Although pro pellers tested data base shown Ta ble 1 representative typically used props no assurance conclu sions can extrapolated other propellers Also similar props same manu 1100 v 1000 -r 0 U N E L 0 z 900 10000 RPM ii 6oo 12000 800 aoio Figure 4 9000 Predicted PWL due Torque BPF facturer often show differences great can some importance fact several supposedly identical propellers exhibit subtle differences means course some variation re sults possible significance results re search project clear Three main points should emphasized Assuming engine equipped reasonably effi cient muffler major source model airContinued page 142 February 1990 35 1100 o Zinger 10 Modified 75 1000 -r 0 6 N 900 800-900010000iiooo12000 RPM Figure 3 icted PWL due Thickness BPF o Zinger 10 Modified 75 n Len Funds ESCAPE SPECIFICATION. Wing Span62Y2 inches Wing Area770 square inches Engine Size 10 cc 90 120 faur stroke Designed AMA FAI Turn-around pattern Foam wing stab 3-32 Balsa sheet covering Tricycle conven tional gear fixed retracts Rear side exhaust fiber glass canopy Veiy positive man'verable XLT SPECIFIC Wing Span65 inches Length65 inches Wing Area845 square inches Recommended Engine Size 10 cc 90 120 four stroke XLT designed tuned pipe re tract landing gears Capable AMA Turn-around pattern Rear side ex haust SPECIFICATIONS Wing Span63Y4 inches Wing Area700 square inches Engine Size50-60 Glow 90 four stroke Forfun sport pattern turn-around can done Utter Chaos completely built up Balsa construction Canopy engine mount included years proven eying reliablity BRIDI AIRCRAFT DESIGNS INC 23625 Pineforest Lane Harbor City California 90710 213 549-8264 213 326-5013 SEE YOUR FAVORITE HOBBY SHOP OR RETAIL OUTLETW Dealer Inquries Invited AOSORO Sf1000 00 ISF0A1 001 00000 DOFFOODfTIAL 15010 00000 0000 00000 00000 000000 00000000 00t40000000 0SflY 0J05TA0t0 00000000-0000 001000 0S0 ON000LLATIOO OAT 01 100010100 00 00 0110008 0000011 01IOT0 INSTAOUTIOO P0CO000 flhnO 0n0ded LII Ole Reliable/Pfeifer Continued page 111 Lii Ole Reliable hadnt flown six feet 137 seconds knew worth effort Jeffreys sixyear-old instincts right target model flies dad want fly Jeffrey like No buts Prop Noise Continued page 35 craft noise prop noise 2 main com ponent prop noise caused torque loading blades 3 propellers available generally very inefficient inefficiency leads high torque loads relative thrust produced path model aircraft noise reduc tion therefore clear although perhaps easy Making propellers efficient would reduce noise levels compro mising aircraft performance Next months conclusion will present several useful guide lines modelers follow choosing pro pellers own aircraft Addendum Corrections Part 2 table inadvertently omitted some revisions editorial staff error resulting meanings different intended authors ask read ers review following items con junction Model Propeller Noise article January 1990 issue order obtain proper meanings case italics added emphasis Pg 39 first col first para first sen tence Should read opeller noise sources hibit characteristic pattern noise radiates away propeller Pg39 second col second para first sentence Should read echoic re fers condition sound nor re flected use sound-absorbent material simulate free-field acoustic environ ment aircraft flight COMPACT ELECTRIC RC HERE 0edSI0 or50 Wolt tigh 1ff oeoey Motor Sorogsol Propoo4tells Powers 310 24oo Models Si lSSlwPostegeeeHendllng Pg 40 first col first full para omitted Table listing propellers used testing follows Table 1Propellers Included Data Base Manufacturer Size ManufacturerSize Zinger8 x 6Rev-Up11 x 6 EW Zinger9 x6Rev-Up lix 7 Rev-Up9 x 6 Zinger 12 x 6 Master10 x 6Tornado 9 x 6 Three-blade propeller Pg 40 first col first full para second sentence Propellers chosen ing typical use today abil iry 05 Max 046 SF engine turn Pg 40 third col second full para De lete Propeller noise always occurs blade passing frequency does reach crescendo Pg 40 third col last para third fourth sentences An efficient propeller will produce thrust same en gine output same flight conditions An inefficient propeller causes engine work turn prop producing little thrust Pg 41 first col first para ad vance ratio denoted symbol J pro portional ratio Pg 148 second col first para last sen tence ops efficient high speeds will provide reduced noise lev els compared less efficient props same performance levels Pg 148 second col first full para last sentence n reasonably suspect measured noise source fact torque noise regret omission errors Carl R Wheeley Editor Publisher responding advertisers mention read about Model Aviation 142 Model Aviation