Vibration

U GeorgeF Abbott Vibntion model engines vibrate airframe affected What can about following discussion may help understand dynamics engine vibra tion compensate THE VAST MAJORITY power mod els equipped single-cylinder inter nal combustion engines Since no way simple single-cylinder reciprocat ing engine can balanced engines very nature vibration genera tors Hence vibration unpleasant fact life gas model airplane business vibration potentially dam aging model radio equip ment it also contributes substantially noise generated flight purpose article explain en gines produce vibration can balanced model may react vibration what methods may em ployed aircraft cope prob lem Engine balance section dynam ics reciprocating engine balance will discussed general way have tried avoid mathematics much possible Readers inclined can follow what little math verify conclu sions Also since intention provide understanding principles involved rather rigorous analysis have made some simplifying assumptions First should understood im possible balance single-cylinder en gine Further vibration forces produced engine increase square en gine rpm means engine speed doubled vibration forces crease fourfold may have heard read comment particular engine runs smooth cer tain speed range rough others concluded however engine balance given speed range would have mistaken vibration forces proportional square engine speed frequency vibration directly proportional engine speed engine whatever may LI I ROTATING MASS TDC 90 after TDC SCHEMATIC S NGLE Li N BDC FIGURE 2 REPRESENTATION OF CYLINDER ENGINE 270 after TDC PISTON I cONNECTINS ROD COUNTERWE GHT CENTER OF MASS OF COUNTER WE SHT R stroke r ance center crankshaft center ma. counterweight mounted inan airplane car test standtogether form elastic system may have natural frequency vi bration corresponds certain engine speeds called resonance Resonance may greatly magnify effects vibration certain engine speeds Figure i shows typical arrangement single-cylinder engine should quite familiar Note three moving parts crankshaft piston connecting rod four-cycle engine May 1990 53 Figure 1 tiwee moving parts typical single-cylinder two-stroke engine Above fhe crankshaft its counterbal ance below piston connect ing rod three parts primary cause engine vibration Because op posing forces involved see text its next t6 impossible completely eliminate vibration single-cylinder engine configuration RECIPROCATING MASS TDCDDCTDC top Xl 1d9018027O360 I crank angle bottom TDC FIG 3A PISTON POSITION DDC 0 x R cos 0 crankshaft angle R utrok TDC I Vp 00 10901902703&0 IIIII crank angle 0 FIG 3D PISTON VELOCITY course has moving parts valve mechanism isnt relevant discussion crankshaft rotating part piston reciprocating partthat travels back forth cylinder straight line bottom end connect ing rod travels circular path top end follows reciprocating motion piston Its general practice engine balancing weigh end connect ing rod opposite end supported pivot assign resulting weights crankshaft piston respec tively analysis can ignore weight connecting rod entirely out way affecting conclusions Figure 2 show schematic represen tation single-cylinder engine en gine represented two masses pis ton crankshaft counterweight crankpin connecting rod assumed weightless assumption justified because interested unbal anced masses engine general en gine practice mass crankpin would balanced part counter weight mass end con necting rod would add piston crankpin masses Figure 2 also shows engine four ref COUNTERWEIGHTs MASS Mc ye mM Vp cos N rey/min C 2rIR erence positions top dead center TDC piston top cyl inder 90 degrees after TDC pis ton descending about halfway down bottom dead center BDC piston bottom stroke 270 degrees piston its way up TDC mass counterweight taken centered CG symbol triangle represents counter weight Reciprocating forces Figure 3 shows curves describe position velocity acceleration resulting vibration forces piston mathematical expressions describe curves shown figures Figure 3A shows position pis ton crankshaft rotates engineering terms called simple harmonic mo tion because may described sine function Actually since connect ing rod infinitely long piston does follow exact sine curve gives rise secondary vibration forces discussed later section arti cle point concerned understanding basic princi ples Figure 3B shows piston velocity DISTANCE FROM CENTER OF ROTATION TO CENTER OF MASS N TIONAL SPEED Fc IT rN FIGURE 4 CENTRIFUGAL FORCE RESULTING FROM ROTATION OF COUNTERWEIGHT IS REPRESENTED DY ROTATING VECTOR Fc function crank shaft position Note TDC BDC piston ve locity zero 90 270 de grees has creased maxi mum Thats cause dead cen ter piston stops reverses direc non course its moving maximum velocity halfway point Figure 3C shows acceleration piston function position crankshaft very important since vibration forces result re ciprocating movement piston pro duced acceleration mass piston about later Its important note acceleration piston greatest dead center also point piston velocity zero piston moving maximum velocity 90- 270-degree points slows down decelerates stop dead center accelerates maxi mum velocity next midpoint curve Figure 3C illustrates must now consider mathematical expression piston acceleration Ap C cos 0 C umference crankpin circle N tional speed engine 0 crankshaft angle Its very important note N ro tational speed engine squared mathematical background because acceleration second derivative respect time posi tion piston Since have expression accelera tion piston can relate move ment vibration forces will result Newtons second law states force equal product mass ac celeration Fp mass piston Figure 3D shows force Note follows same sine curve piston p0sition shown Figure 3A vibration force reaction acceleration force curves Figures 3C 3D opposite sign course vibration forces result movement piston will direc tion cylinder Let us assume upright engine can refer forces up down As piston rises cylinder slowed down applies upward force engine As piston descends cyl inder accelerates pulled down crankshaft downward force applied engine opposite occurs during next 180 degrees crankshaft rotation Thus vibration forces caused re ciprocating motion piston straight line up down forces maximum top bottom dead center reversing twice revolution vibration forces due movement piston given following equation Fp 2 C cos Ot Whats important note force Fp varies square engine rotational speed Rotating forces crankshaft rotat 54 Model Aviation Pp / Ruuul tant FI6URE 5 RESOLUTION OF PISTON & OUNTERWEIOHT FORCES BY VECTOR ADDITIOI4 CASE 1 No counterweight provided Engine vibrates response ta piston forces Pp CRANaSNAPT POSITIONPp Resultant Pr TDCzero lFr zerozerozero SOC Ppzero }Pr DIGINE VIUIATES 570zerozero zeroUP S DOHN ing member engine Figure 2 shows counterweight located opposite crankpin counterweight can de signed compensate reciprocating forces piston since piston forces up down counter weight forces rotating result will sideways vibration forces Just acceleration piston resulting vibrational forces proportional square engine speed cen trifugal force produced rotating coun terweight also proportional square engine speed Figure 4 shows rotating crankshaft counterweight Its center mass dis tance r center rotation ex ression centrifucal force fol lows Fc c N2 Mc mass counterweight Kc tant relating physical di mensions crankshaft counter weight N tional speed Note expression centrifugal force relates radius curvature tangential velocity Kc above expres sion represents relationship case Kc r important point cen trifugal force Fc proportional square engine speed also true reciprocating force Fp course force rotating engine speed same direction engine Resolution forces Now have two forces operating enginethe recipro cating force acts linearly up down rotating force need able combine two forces easily done process vector addition vector representation force showing both its direction magnitude CASE 2 cRAMSNAFT POSITION TOC 90 laS 570 TOC Counterweight fully balances piston PpaPc j~1PC Resultant Fr zero JPp \- Pc Pp zeroPc Pr Jpp/ zI Pp Pr zero DDC}PPjc 570zero lOCJP CASE 3 cRM*SHAPT POSITION TOC Fr ENGINE VIDRATES SIDEIdAYS zero Counterweight balances half reciprocating ease Ppa Pc PpPc jP~rC Y0zeroPc 13Ac NOCIPc length line represents magnitude orientation ar rowhead represent direction Figure 4 centifugal force Fc shown vec tor pointing lower right curved arrow indi cates rotat ing counter clockwise direction Figure 5 illustrate combination resolution two forces represented vectors Fp Fc may start either vector place tail head other line connect ing tail first head sec ond represents sum two forces called resultant Engine balance now able ana lyze balance single-cylinder engine Figure 6 illustrates three cases first represents no effort balance recipro cating forces Well call underbal anced second case reciprocating forces fully balanceda situation well refer overbalanced final case half reciprocating forces bal anced represents best compromise situation sketches show crankshaft posi tion vectors reciprocating forces Fp rotating forces Fc result ant two Case 1 absence counterweight means reciprocating forces present engine vibrates up down Case 2 counterweight exactly bal ances reciprocating forces Thus instead up-and-down vibration side ways force exactly same mag zeroPc lPc Resultant Pr Pri p - ENgINE VIDRATES IN CIRCLLAR NOTION Fri FIG 6 THREE DALANCE CONDITIONS FOR SINGLE CYCLINDER ENGINE nitude reciprocating force simply different direction Case 3 illustrates situation counterweight balances half recip rocating mass TDC BDC posi tions up-and-down vibrating forces reduced one-halfthe sum Fp Fc 90- 270-degree positions Fc ap pears sideways vibrating force 45 135 degrees resultant Fp Fc preduces vector same magnitude Fc opposite direction Conse quently now have vibration force equal magnitude one-half would present no balance rotating opposite direction engine rota tion best compromise can achieve single-cylinder engine Secondary vibration Earlier pointed out piston movement strictly speaking simple harmonic motion because connecting rod finitely long piston travel does fol low sine curve exactly Its well beyond scope article include fact above analysis However few com ments should made sake com pleteness actual movement piston short connecting rod say 1 /2 times stroke engine produces reciprocating May 1990 55 noises irritating listener 1 IIgnition TI O I compression R Istroke DOCTDCBDCTDCBDC U Ipower E Istroke ports Iopen II II 1one revolutlo1 FIG 7A TwaCYCLE IIgnItIon. -I-i TI0 coUprEEsionpowerExhaustintake R strokstrokestrolestinks Q BOCTOCBC TDCBOC U El I IIFIG 7flI IFOURCYCLE II II 1 revolutlo1 revolutlo1 FIG 7 TORQUE IIIPULSES OF TWOCYCLE AND FOIJRCYCLE ENGINES forces wbich include substantial compo nent twice engine speed result ing vibrations called secondary vibra tions case connecting rod length 1 /2 times stroke second ary vibrations will about one-third force primary vibrations Since its desirable both sake engine cornCENTER OF GRAVITY CCjj5 VIDRATIO\ FORCES FROMMINIMUM MOVEMENT ENGINEAT CS MPXIMUM MOVEMENT AT NOSE AND TAIL FIG 9 AIRPLANE ROTATES ABOUT ThE CS IN RESPONSE TO VIBRATION FORCES APPLIED DY THE ENGINE pactness reduce reciprocating mass have connecting rod short possi ble secondary vibrations significant factor engine balancing some multiple-cylinder engines sec ondaries can balanced others canpot example six-cylinder line engine 90-degree V-8 may pri mary second ary balance sim ple in-line fourcylinder can primary balance ly Secondary vibra tion model en gines significant because noise generates Since vibration forces contain sub stantial components twice engine speed noise generated airframe will like wise contain fre quencies twice engine speed higher flquency Multicylinder engines Its obvious twin-cylinder engine two-throw crankshaft pistons move op posite directions should vibration free practical problem such engines ever fact because twothrow crankshaft cylinders line other results twist ing force called couple will create some vibration Although such engines will somewhat less ideal balance vibration encountered low little practical concern such engine will primary secondary balance Also available op posed twin-cylinder engine singlethrow crankshaft pistons both move same directionan arrange ment course offers no vibration ad yantage Torsional vibration previous sec tion discussed vibration caused rotating reciprocating masses engine Another type vibration results way engine produces its power Engine power transmitted series impulsesthat force produced during power stroke power engine delivered propeller whatever load engine driving torque twisting force applied crankshaft addition producing torque en gine must also receive torque input order carry through compression stroke Thus engines must have flywheel store up energy produced power stroke some may returned compression stroke well smooth out power pulses aircraft en gines propeller serves flywheel Vibration due torque called torsional vibration twisting type vibration produced along axis crankshaft frequency related rpm engine Figure 7A illustrates twocycle engine top dead center TDC see large pulse torque power stroke followed trans fer period lasting about 120 degrees onethird rotation crankshaft dur ing no torque During transfer period ports open cylinder being scavenged burned gases charged fresh fuel/air mixture ports close compression strQke commences see pulse torque opposite direction power stroke cylinder fires initiates another power pulse process repeats Thus addition vibration created imbalance its moving parts en gine also producing torsional vibration modelers already well aware torsional vibration may very dam aging airframe As Figure 7B illustrates four-cycle en gines power stroke occurs other revolution two-cycle four-cycle engine produce same power 56 Model Aviation same rpm since four-cycle has half power impulses obviously impulses must twice large Thus would expect four-cycle engine produce torsional vibration half frequency two-cycle Ob servation bears out since four-cycle en gines seem lumpy run ning two-cycle counterparts Again multicylinder engines should somewhat smoother respect tor sional vibration provided course theyre configured firing order uniform flat opposed engines ex ample V twins common crankpin arrangement evenly spaced firing order impossible Coping vibration Since vibration pro duced engines unavoidable must plan best methods coping falls several categories can isolate vibration source engine can isolate parts airplane likely suffer effects vibration radio system can de sign airframe withstand effects vibration Vibration-isolating engine mounts wide ly used full-scale airplanes automo biles now coming use model air planes mounts reportedly quite successful particularly reducing model engine noise number considera tions arising differences between THE BEST FUSELAGE JIG EVER PERIOD Provides Solid Support Accurate Building Rugged constniction 7 Ply Aircraft Plywood Provides Lifetime Use Two Sizes Available Regular Fuselages up 6 Wide Jumbo Fuselages Up 9 Wide Use Own Building Board Work Bench Base Mount Jig Station Uni-Strut Guide Rails Most Versatile Set-Up contains complete Hardware Instructions Including Uni-Strut Information Bos 4 Standard Jig StationsS1795 Box 4 Jumbo Jig Stations1995 TwoS ft long Guide Rails including mounting nuts 2195 model full-scale applications must taken account Since inertial mass model engines small comparison magnitude vibration forces isolating mount ings model applications have quite limber will permit engine vi brate great deal reference air frame particularly low speeds causing potential problems airframe parts such cowlings addition fatigue failure engine parts such needle valves Other possible consequences fuel foaming fuel lines difficulty adjusting engine running power loss due energy absorption flexible mountings current applications seem cop ing problems successfully lim iting excursions engine re straining structures using mounting methods materials provide var iable stiffness rate Theres lot room further innovative work area Unless installing vibrationisolation system best practice mount engine firmly possible airframe idea tie structure together well-integrated mass upon vibration forces will applied Newtons Second Law F fec tive situation vibration forces F applied mass airframe Obviously mass M large ac Continued page 58 IVANS MAGIC GADGET protractor device used Ivan Kristensen accurately measure aileron elevator rudder throw Takes guess work out trimming out airplane quality tool complete instructions log book carrying case$795 PLUG-IN ADJUSTABLE STAB KIT last can optimize stab incidence best flight charac teristics pair glass filled nylon adjusters 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celeration will less Thus very light airframe will subject vibration ef fects corollary foregoing recom mendation entire airframe con structed rigidly possible should little no relative motion between vari ous parts structure will reduce problems fatigue wearout will also prevent secondary vibrations oc cur parts airframe rattle against other solution isolate sensitive crit ical parts particularly radio against damaging vibration generally done wrapping receiver battery sponge rubber mounting servos through rubber grommets method seems work quite well long sponge rubber wrapping kept loose possible servos screwed down too tightly Its important understand modes vibration will found airframe consider entire airplane freely suspended body three degrees free domthat free move direc tion outside restraint As mat ter fact aerodynamic forces impose re straints movement airplane can ignore purposes analysis vibration forces ap plied airplane engine usually front craft plane will re spond input vibrating about center-of-mass center-of-gravity commonly referred CC Figure 8 shows amplitude vi bration will minimal CC crease distance point Thus vibration-sensitive equip mentthe receiver servos particular should located close possible CC its necessary place electronic components distance the CC example situating battery pack ward purposes balancethey should carefully packed sponge rubber Since its almost always positioned close en gine fuel tank subject severe vibra tion practice placing servos tail near ailerons Giant Scale models should carefully weighed subjects greater vibration locations near CC chain saw engines pro duce some awesome vibration forces Tor sional vibration places severe stress tail surfaces aft fuselage Giant Scale models using either chain-saw-type large four-cycle engines its good idea use brace wires between fin stabilizer fuselage Strengthening aft fuselage helps too properly balanced propeller also helps minimizing vibration Obviously im balance propeller 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U GeorgeF Abbott Vibntion model engines vibrate airframe affected What can about following discussion may help understand dynamics engine vibra tion compensate THE VAST MAJORITY power mod els equipped single-cylinder inter nal combustion engines Since no way simple single-cylinder reciprocat ing engine can balanced engines very nature vibration genera tors Hence vibration unpleasant fact life gas model airplane business vibration potentially dam aging model radio equip ment it also contributes substantially noise generated flight purpose article explain en gines produce vibration can balanced model may react vibration what methods may em ployed aircraft cope prob lem Engine balance section dynam ics reciprocating engine balance will discussed general way have tried avoid mathematics much possible Readers inclined can follow what little math verify conclu sions Also since intention provide understanding principles involved rather rigorous analysis have made some simplifying assumptions First should understood im possible balance single-cylinder en gine Further vibration forces produced engine increase square en gine rpm means engine speed doubled vibration forces crease fourfold may have heard read comment particular engine runs smooth cer tain speed range rough others concluded however engine balance given speed range would have mistaken vibration forces proportional square engine speed frequency vibration directly proportional engine speed engine whatever may LI I ROTATING MASS TDC 90 after TDC SCHEMATIC S NGLE Li N BDC FIGURE 2 REPRESENTATION OF CYLINDER ENGINE 270 after TDC PISTON I cONNECTINS ROD COUNTERWE GHT CENTER OF MASS OF COUNTER WE SHT R stroke r ance center crankshaft center ma. counterweight mounted inan airplane car test standtogether form elastic system may have natural frequency vi bration corresponds certain engine speeds called resonance Resonance may greatly magnify effects vibration certain engine speeds Figure i shows typical arrangement single-cylinder engine should quite familiar Note three moving parts crankshaft piston connecting rod four-cycle engine May 1990 53 Figure 1 tiwee moving parts typical single-cylinder two-stroke engine Above fhe crankshaft its counterbal ance below piston connect ing rod three parts primary cause engine vibration Because op posing forces involved see text its next t6 impossible completely eliminate vibration single-cylinder engine configuration RECIPROCATING MASS TDCDDCTDC top Xl 1d9018027O360 I crank angle bottom TDC FIG 3A PISTON POSITION DDC 0 x R cos 0 crankshaft angle R utrok TDC I Vp 00 10901902703&0 IIIII crank angle 0 FIG 3D PISTON VELOCITY course has moving parts valve mechanism isnt relevant discussion crankshaft rotating part piston reciprocating partthat travels back forth cylinder straight line bottom end connect ing rod travels circular path top end follows reciprocating motion piston Its general practice engine balancing weigh end connect ing rod opposite end supported pivot assign resulting weights crankshaft piston respec tively analysis can ignore weight connecting rod entirely out way affecting conclusions Figure 2 show schematic represen tation single-cylinder engine en gine represented two masses pis ton crankshaft counterweight crankpin connecting rod assumed weightless assumption justified because interested unbal anced masses engine general en gine practice mass crankpin would balanced part counter weight mass end con necting rod would add piston crankpin masses Figure 2 also shows engine four ref COUNTERWEIGHTs MASS Mc ye mM Vp cos N rey/min C 2rIR erence positions top dead center TDC piston top cyl inder 90 degrees after TDC pis ton descending about halfway down bottom dead center BDC piston bottom stroke 270 degrees piston its way up TDC mass counterweight taken centered CG symbol triangle represents counter weight Reciprocating forces Figure 3 shows curves describe position velocity acceleration resulting vibration forces piston mathematical expressions describe curves shown figures Figure 3A shows position pis ton crankshaft rotates engineering terms called simple harmonic mo tion because may described sine function Actually since connect ing rod infinitely long piston does follow exact sine curve gives rise secondary vibration forces discussed later section arti cle point concerned understanding basic princi ples Figure 3B shows piston velocity DISTANCE FROM CENTER OF ROTATION TO CENTER OF MASS N TIONAL SPEED Fc IT rN FIGURE 4 CENTRIFUGAL FORCE RESULTING FROM ROTATION OF COUNTERWEIGHT IS REPRESENTED DY ROTATING VECTOR Fc function crank shaft position Note TDC BDC piston ve locity zero 90 270 de grees has creased maxi mum Thats cause dead cen ter piston stops reverses direc non course its moving maximum velocity halfway point Figure 3C shows acceleration piston function position crankshaft very important since vibration forces result re ciprocating movement piston pro duced acceleration mass piston about later Its important note acceleration piston greatest dead center also point piston velocity zero piston moving maximum velocity 90- 270-degree points slows down decelerates stop dead center accelerates maxi mum velocity next midpoint curve Figure 3C illustrates must now consider mathematical expression piston acceleration Ap C cos 0 C umference crankpin circle N tional speed engine 0 crankshaft angle Its very important note N ro tational speed engine squared mathematical background because acceleration second derivative respect time posi tion piston Since have expression accelera tion piston can relate move ment vibration forces will result Newtons second law states force equal product mass ac celeration Fp mass piston Figure 3D shows force Note follows same sine curve piston p0sition shown Figure 3A vibration force reaction acceleration force curves Figures 3C 3D opposite sign course vibration forces result movement piston will direc tion cylinder Let us assume upright engine can refer forces up down As piston rises cylinder slowed down applies upward force engine As piston descends cyl inder accelerates pulled down crankshaft downward force applied engine opposite occurs during next 180 degrees crankshaft rotation Thus vibration forces caused re ciprocating motion piston straight line up down forces maximum top bottom dead center reversing twice revolution vibration forces due movement piston given following equation Fp 2 C cos Ot Whats important note force Fp varies square engine rotational speed Rotating forces crankshaft rotat 54 Model Aviation Pp / Ruuul tant FI6URE 5 RESOLUTION OF PISTON & OUNTERWEIOHT FORCES BY VECTOR ADDITIOI4 CASE 1 No counterweight provided Engine vibrates response ta piston forces Pp CRANaSNAPT POSITIONPp Resultant Pr TDCzero lFr zerozerozero SOC Ppzero }Pr DIGINE VIUIATES 570zerozero zeroUP S DOHN ing member engine Figure 2 shows counterweight located opposite crankpin counterweight can de signed compensate reciprocating forces piston since piston forces up down counter weight forces rotating result will sideways vibration forces Just acceleration piston resulting vibrational forces proportional square engine speed cen trifugal force produced rotating coun terweight also proportional square engine speed Figure 4 shows rotating crankshaft counterweight Its center mass dis tance r center rotation ex ression centrifucal force fol lows Fc c N2 Mc mass counterweight Kc tant relating physical di mensions crankshaft counter weight N tional speed Note expression centrifugal force relates radius curvature tangential velocity Kc above expres sion represents relationship case Kc r important point cen trifugal force Fc proportional square engine speed also true reciprocating force Fp course force rotating engine speed same direction engine Resolution forces Now have two forces operating enginethe recipro cating force acts linearly up down rotating force need able combine two forces easily done process vector addition vector representation force showing both its direction magnitude CASE 2 cRAMSNAFT POSITION TOC 90 laS 570 TOC Counterweight fully balances piston PpaPc j~1PC Resultant Fr zero JPp \- Pc Pp zeroPc Pr Jpp/ zI Pp Pr zero DDC}PPjc 570zero lOCJP CASE 3 cRM*SHAPT POSITION TOC Fr ENGINE VIDRATES SIDEIdAYS zero Counterweight balances half reciprocating ease Ppa Pc PpPc jP~rC Y0zeroPc 13Ac NOCIPc length line represents magnitude orientation ar rowhead represent direction Figure 4 centifugal force Fc shown vec tor pointing lower right curved arrow indi cates rotat ing counter clockwise direction Figure 5 illustrate combination resolution two forces represented vectors Fp Fc may start either vector place tail head other line connect ing tail first head sec ond represents sum two forces called resultant Engine balance now able ana lyze balance single-cylinder engine Figure 6 illustrates three cases first represents no effort balance recipro cating forces Well call underbal anced second case reciprocating forces fully balanceda situation well refer overbalanced final case half reciprocating forces bal anced represents best compromise situation sketches show crankshaft posi tion vectors reciprocating forces Fp rotating forces Fc result ant two Case 1 absence counterweight means reciprocating forces present engine vibrates up down Case 2 counterweight exactly bal ances reciprocating forces Thus instead up-and-down vibration side ways force exactly same mag zeroPc lPc Resultant Pr Pri p - ENgINE VIDRATES IN CIRCLLAR NOTION Fri FIG 6 THREE DALANCE CONDITIONS FOR SINGLE CYCLINDER ENGINE nitude reciprocating force simply different direction Case 3 illustrates situation counterweight balances half recip rocating mass TDC BDC posi tions up-and-down vibrating forces reduced one-halfthe sum Fp Fc 90- 270-degree positions Fc ap pears sideways vibrating force 45 135 degrees resultant Fp Fc preduces vector same magnitude Fc opposite direction Conse quently now have vibration force equal magnitude one-half would present no balance rotating opposite direction engine rota tion best compromise can achieve single-cylinder engine Secondary vibration Earlier pointed out piston movement strictly speaking simple harmonic motion because connecting rod finitely long piston travel does fol low sine curve exactly Its well beyond scope article include fact above analysis However few com ments should made sake com pleteness actual movement piston short connecting rod say 1 /2 times stroke engine produces reciprocating May 1990 55 noises irritating listener 1 IIgnition TI O I compression R Istroke DOCTDCBDCTDCBDC U Ipower E Istroke ports Iopen II II 1one revolutlo1 FIG 7A TwaCYCLE IIgnItIon. -I-i TI0 coUprEEsionpowerExhaustintake R strokstrokestrolestinks Q BOCTOCBC TDCBOC U El I IIFIG 7flI IFOURCYCLE II II 1 revolutlo1 revolutlo1 FIG 7 TORQUE IIIPULSES OF TWOCYCLE AND FOIJRCYCLE ENGINES forces wbich include substantial compo nent twice engine speed result ing vibrations called secondary vibra tions case connecting rod length 1 /2 times stroke second ary vibrations will about one-third force primary vibrations Since its desirable both sake engine cornCENTER OF GRAVITY CCjj5 VIDRATIO\ FORCES FROMMINIMUM MOVEMENT ENGINEAT CS MPXIMUM MOVEMENT AT NOSE AND TAIL FIG 9 AIRPLANE ROTATES ABOUT ThE CS IN RESPONSE TO VIBRATION FORCES APPLIED DY THE ENGINE pactness reduce reciprocating mass have connecting rod short possi ble secondary vibrations significant factor engine balancing some multiple-cylinder engines sec ondaries can balanced others canpot example six-cylinder line engine 90-degree V-8 may pri mary second ary balance sim ple in-line fourcylinder can primary balance ly Secondary vibra tion model en gines significant because noise generates Since vibration forces contain sub stantial components twice engine speed noise generated airframe will like wise contain fre quencies twice engine speed higher flquency Multicylinder engines Its obvious twin-cylinder engine two-throw crankshaft pistons move op posite directions should vibration free practical problem such engines ever fact because twothrow crankshaft cylinders line other results twist ing force called couple will create some vibration Although such engines will somewhat less ideal balance vibration encountered low little practical concern such engine will primary secondary balance Also available op posed twin-cylinder engine singlethrow crankshaft pistons both move same directionan arrange ment course offers no vibration ad yantage Torsional vibration previous sec tion discussed vibration caused rotating reciprocating masses engine Another type vibration results way engine produces its power Engine power transmitted series impulsesthat force produced during power stroke power engine delivered propeller whatever load engine driving torque twisting force applied crankshaft addition producing torque en gine must also receive torque input order carry through compression stroke Thus engines must have flywheel store up energy produced power stroke some may returned compression stroke well smooth out power pulses aircraft en gines propeller serves flywheel Vibration due torque called torsional vibration twisting type vibration produced along axis crankshaft frequency related rpm engine Figure 7A illustrates twocycle engine top dead center TDC see large pulse torque power stroke followed trans fer period lasting about 120 degrees onethird rotation crankshaft dur ing no torque During transfer period ports open cylinder being scavenged burned gases charged fresh fuel/air mixture ports close compression strQke commences see pulse torque opposite direction power stroke cylinder fires initiates another power pulse process repeats Thus addition vibration created imbalance its moving parts en gine also producing torsional vibration modelers already well aware torsional vibration may very dam aging airframe As Figure 7B illustrates four-cycle en gines power stroke occurs other revolution two-cycle four-cycle engine produce same power 56 Model Aviation same rpm since four-cycle has half power impulses obviously impulses must twice large Thus would expect four-cycle engine produce torsional vibration half frequency two-cycle Ob servation bears out since four-cycle en gines seem lumpy run ning two-cycle counterparts Again multicylinder engines should somewhat smoother respect tor sional vibration provided course theyre configured firing order uniform flat opposed engines ex ample V twins common crankpin arrangement evenly spaced firing order impossible Coping vibration Since vibration pro duced engines unavoidable must plan best methods coping falls several categories can isolate vibration source engine can isolate parts airplane likely suffer effects vibration radio system can de sign airframe withstand effects vibration Vibration-isolating engine mounts wide ly used full-scale airplanes automo biles now coming use model air planes mounts reportedly quite successful particularly reducing model engine noise number considera tions arising differences between THE BEST FUSELAGE JIG EVER PERIOD Provides Solid Support Accurate Building Rugged constniction 7 Ply Aircraft Plywood Provides Lifetime Use Two Sizes Available Regular Fuselages up 6 Wide Jumbo Fuselages Up 9 Wide Use Own Building Board Work Bench Base Mount Jig Station Uni-Strut Guide Rails Most Versatile Set-Up contains complete Hardware Instructions Including Uni-Strut Information Bos 4 Standard Jig StationsS1795 Box 4 Jumbo Jig Stations1995 TwoS ft long Guide Rails including mounting nuts 2195 model full-scale applications must taken account Since inertial mass model engines small comparison magnitude vibration forces isolating mount ings model applications have quite limber will permit engine vi brate great deal reference air frame particularly low speeds causing potential problems airframe parts such cowlings addition fatigue failure engine parts such needle valves Other possible consequences fuel foaming fuel lines difficulty adjusting engine running power loss due energy absorption flexible mountings current applications seem cop ing problems successfully lim iting excursions engine re straining structures using mounting methods materials provide var iable stiffness rate Theres lot room further innovative work area Unless installing vibrationisolation system best practice mount engine firmly possible airframe idea tie structure together well-integrated mass upon vibration forces will applied Newtons Second Law F fec tive situation vibration forces F applied mass airframe Obviously mass M large ac Continued page 58 IVANS MAGIC GADGET protractor device used Ivan Kristensen accurately measure aileron elevator rudder throw Takes guess work out trimming out airplane quality tool complete instructions log book carrying case$795 PLUG-IN ADJUSTABLE STAB KIT last can optimize stab incidence best flight charac teristics pair glass filled nylon adjusters included provide micro screw adjusonent smaller front tube Main tube aluminum matching phenolic socket $695 Include 52 SOiltem UPS esidents Add 6% Sales Tax GATOR RIC PRODUCTS 3713 Pompano Dr acola FL 32514 476-8639 U DJU 31 1 [Cl 12 PLUG-IN WING ADJUSTER KIT Four glass filled nylon brackets included key wing panels front rear allow accurately adjust panel straight flight complete hardware & instructions$995 WING TUBE & SOOETS Aluminum alloy tubing plug-in wings Included pair special slip-fit phenolic sockets boild wing structure Now size models 718 diax2$ 895 1diax2S 995 IWdiax3$1495 May 1990 57 Which Thermal Competition Glider Would Rather Flying Surprised nearly 26 [ID Yes according results Selig/Donovan/Fraser Princeton teals mighty LOVESONG might er everl adding trip 1/8 wide 020 thick 20% chord Eppler 214 has become killer thermal airfoil 90s sizable performance boost puts PIXY CAMANO & LOVESONG farther ahead tips up competition Beat Designs Mrfoil Performance can buy 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celeration will less Thus very light airframe will subject vibration ef fects corollary foregoing recom mendation entire airframe con structed rigidly possible should little no relative motion between vari ous parts structure will reduce problems fatigue wearout will also prevent secondary vibrations oc cur parts airframe rattle against other solution isolate sensitive crit ical parts particularly radio against damaging vibration generally done wrapping receiver battery sponge rubber mounting servos through rubber grommets method seems work quite well long sponge rubber wrapping kept loose possible servos screwed down too tightly Its important understand modes vibration will found airframe consider entire airplane freely suspended body three degrees free domthat free move direc tion outside restraint As mat ter fact aerodynamic forces impose re straints movement airplane can ignore purposes analysis vibration forces ap plied airplane engine usually front craft plane will re spond input vibrating about center-of-mass center-of-gravity commonly referred CC Figure 8 shows amplitude vi bration will minimal CC crease distance point Thus vibration-sensitive equip mentthe receiver servos particular should located close possible CC its necessary place electronic components distance the CC example situating battery pack ward purposes balancethey should carefully packed sponge rubber Since its almost always positioned close en gine fuel tank subject severe vibra tion practice placing servos tail near ailerons Giant Scale models should carefully weighed subjects greater vibration locations near CC chain saw engines pro duce some awesome vibration forces Tor sional vibration places severe stress tail surfaces aft fuselage Giant Scale models using either chain-saw-type large four-cycle engines its good idea use brace wires between fin stabilizer fuselage Strengthening aft fuselage helps too properly balanced propeller also helps minimizing vibration Obviously im balance propeller 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vastly improved MicaFilm its characteristics well suited smaller Old-Timers Electric models certainly RCSD negative side Litespan bit tricky use materials adhesive built also doesnt add much torsional strength films samples some color-blotch troubles Litespan definitely has place area hobby recommend might also want check out rest Charlies product line specialize Electric flight Free Flight RC products True confessions Normally use short captions photos dont write about subjects main part column ever month just have say few extra words about Howard DGA Bob Maurer have photo file sev eral years awaiting just right time use often since columns tend matic Bob Maurer built model Paul Lind berg kit 1938 Lindberg prolific de signer several magazines particularly Flying Aces popularity designs led Paul kit business shortand apparently unsuccessfultime Maurer powered Sky Chief cheap Dennymite never flew bad wasnt RCSD event back certainly would have reassuring know model wasnt going crash fly away Bob went medical school served US Army settling San Antonio after war 1946 two anesthesiologists long successful career followed Dr Maurer remained active modeling well loved San Antonio area generosity support local clubs sad part story Bob passed away before got around publishing pic ture memory shelter has erected club field Bob Maurer represents what makes hobby attractive involved supportive no interest national recognition desire fame just sim ply loved modeling modelers will missed us Vibration/Abbott Continued page 58 knew certain particular engine underbalanced overbalanced might possible reduce overall vibration installing unbalanced propeller ever thats probably practical approach take As long single-cylinder reciprocating engine remains predominant power plant model airplanes vibration will remain constant concern can best cope problem building solid airframe struc tures providing vibration isolation sensitive components such radios As technology improves vibration isolating en gine mounting seems very promising CL Avanti/Wuif Continued page 59 pIe does control configuration like Avanti Reading few articles airplane re assured could use same basic control system full-scale ship Avanti has unique three-surface design forward canard helps lift long nose fuselage placement wing far rear lifts main weight aircraft An other reason placing wing far back allow maximum length passenger cargo area interruption wing spars braces high stabilizer well Out path fuselage turbulence makes good bite clean air small canard flapped slow-speed flight landing doesnt act elevator Altogether singularly neat efficient package twin-engined business craft Corn 174 May 1990 173 NEW FAI TAN RUBBER 1/4 3116 1/8 3/32 1116- 042 thick 1 lb box- *1475 llncludes4oz Slick Lubel $1375 no lube k costa $17S1 10 lb box $11000 UPS Deliveredl PostpaId US v18 UPS Cal rae add 6% aslas tax Made USA Slow cure conalatant qualIty Ask FAI-TAN 89/90 catalog lists 100FF kital CROCKET HOOKS 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U GeorgeF Abbott Vibntion model engines vibrate airframe affected What can about following discussion may help understand dynamics engine vibra tion compensate THE VAST MAJORITY power mod els equipped single-cylinder inter nal combustion engines Since no way simple single-cylinder reciprocat ing engine can balanced engines very nature vibration genera tors Hence vibration unpleasant fact life gas model airplane business vibration potentially dam aging model radio equip ment it also contributes substantially noise generated flight purpose article explain en gines produce vibration can balanced model may react vibration what methods may em ployed aircraft cope prob lem Engine balance section dynam ics reciprocating engine balance will discussed general way have tried avoid mathematics much possible Readers inclined can follow what little math verify conclu sions Also since intention provide understanding principles involved rather rigorous analysis have made some simplifying assumptions First should understood im possible balance single-cylinder en gine Further vibration forces produced engine increase square en gine rpm means engine speed doubled vibration forces crease fourfold may have heard read comment particular engine runs smooth cer tain speed range rough others concluded however engine balance given speed range would have mistaken vibration forces proportional square engine speed frequency vibration directly proportional engine speed engine whatever may LI I ROTATING MASS TDC 90 after TDC SCHEMATIC S NGLE Li N BDC FIGURE 2 REPRESENTATION OF CYLINDER ENGINE 270 after TDC PISTON I cONNECTINS ROD COUNTERWE GHT CENTER OF MASS OF COUNTER WE SHT R stroke r ance center crankshaft center ma. counterweight mounted inan airplane car test standtogether form elastic system may have natural frequency vi bration corresponds certain engine speeds called resonance Resonance may greatly magnify effects vibration certain engine speeds Figure i shows typical arrangement single-cylinder engine should quite familiar Note three moving parts crankshaft piston connecting rod four-cycle engine May 1990 53 Figure 1 tiwee moving parts typical single-cylinder two-stroke engine Above fhe crankshaft its counterbal ance below piston connect ing rod three parts primary cause engine vibration Because op posing forces involved see text its next t6 impossible completely eliminate vibration single-cylinder engine configuration RECIPROCATING MASS TDCDDCTDC top Xl 1d9018027O360 I crank angle bottom TDC FIG 3A PISTON POSITION DDC 0 x R cos 0 crankshaft angle R utrok TDC I Vp 00 10901902703&0 IIIII crank angle 0 FIG 3D PISTON VELOCITY course has moving parts valve mechanism isnt relevant discussion crankshaft rotating part piston reciprocating partthat travels back forth cylinder straight line bottom end connect ing rod travels circular path top end follows reciprocating motion piston Its general practice engine balancing weigh end connect ing rod opposite end supported pivot assign resulting weights crankshaft piston respec tively analysis can ignore weight connecting rod entirely out way affecting conclusions Figure 2 show schematic represen tation single-cylinder engine en gine represented two masses pis ton crankshaft counterweight crankpin connecting rod assumed weightless assumption justified because interested unbal anced masses engine general en gine practice mass crankpin would balanced part counter weight mass end con necting rod would add piston crankpin masses Figure 2 also shows engine four ref COUNTERWEIGHTs MASS Mc ye mM Vp cos N rey/min C 2rIR erence positions top dead center TDC piston top cyl inder 90 degrees after TDC pis ton descending about halfway down bottom dead center BDC piston bottom stroke 270 degrees piston its way up TDC mass counterweight taken centered CG symbol triangle represents counter weight Reciprocating forces Figure 3 shows curves describe position velocity acceleration resulting vibration forces piston mathematical expressions describe curves shown figures Figure 3A shows position pis ton crankshaft rotates engineering terms called simple harmonic mo tion because may described sine function Actually since connect ing rod infinitely long piston does follow exact sine curve gives rise secondary vibration forces discussed later section arti cle point concerned understanding basic princi ples Figure 3B shows piston velocity DISTANCE FROM CENTER OF ROTATION TO CENTER OF MASS N TIONAL SPEED Fc IT rN FIGURE 4 CENTRIFUGAL FORCE RESULTING FROM ROTATION OF COUNTERWEIGHT IS REPRESENTED DY ROTATING VECTOR Fc function crank shaft position Note TDC BDC piston ve locity zero 90 270 de grees has creased maxi mum Thats cause dead cen ter piston stops reverses direc non course its moving maximum velocity halfway point Figure 3C shows acceleration piston function position crankshaft very important since vibration forces result re ciprocating movement piston pro duced acceleration mass piston about later Its important note acceleration piston greatest dead center also point piston velocity zero piston moving maximum velocity 90- 270-degree points slows down decelerates stop dead center accelerates maxi mum velocity next midpoint curve Figure 3C illustrates must now consider mathematical expression piston acceleration Ap C cos 0 C umference crankpin circle N tional speed engine 0 crankshaft angle Its very important note N ro tational speed engine squared mathematical background because acceleration second derivative respect time posi tion piston Since have expression accelera tion piston can relate move ment vibration forces will result Newtons second law states force equal product mass ac celeration Fp mass piston Figure 3D shows force Note follows same sine curve piston p0sition shown Figure 3A vibration force reaction acceleration force curves Figures 3C 3D opposite sign course vibration forces result movement piston will direc tion cylinder Let us assume upright engine can refer forces up down As piston rises cylinder slowed down applies upward force engine As piston descends cyl inder accelerates pulled down crankshaft downward force applied engine opposite occurs during next 180 degrees crankshaft rotation Thus vibration forces caused re ciprocating motion piston straight line up down forces maximum top bottom dead center reversing twice revolution vibration forces due movement piston given following equation Fp 2 C cos Ot Whats important note force Fp varies square engine rotational speed Rotating forces crankshaft rotat 54 Model Aviation Pp / Ruuul tant FI6URE 5 RESOLUTION OF PISTON & OUNTERWEIOHT FORCES BY VECTOR ADDITIOI4 CASE 1 No counterweight provided Engine vibrates response ta piston forces Pp CRANaSNAPT POSITIONPp Resultant Pr TDCzero lFr zerozerozero SOC Ppzero }Pr DIGINE VIUIATES 570zerozero zeroUP S DOHN ing member engine Figure 2 shows counterweight located opposite crankpin counterweight can de signed compensate reciprocating forces piston since piston forces up down counter weight forces rotating result will sideways vibration forces Just acceleration piston resulting vibrational forces proportional square engine speed cen trifugal force produced rotating coun terweight also proportional square engine speed Figure 4 shows rotating crankshaft counterweight Its center mass dis tance r center rotation ex ression centrifucal force fol lows Fc c N2 Mc mass counterweight Kc tant relating physical di mensions crankshaft counter weight N tional speed Note expression centrifugal force relates radius curvature tangential velocity Kc above expres sion represents relationship case Kc r important point cen trifugal force Fc proportional square engine speed also true reciprocating force Fp course force rotating engine speed same direction engine Resolution forces Now have two forces operating enginethe recipro cating force acts linearly up down rotating force need able combine two forces easily done process vector addition vector representation force showing both its direction magnitude CASE 2 cRAMSNAFT POSITION TOC 90 laS 570 TOC Counterweight fully balances piston PpaPc j~1PC Resultant Fr zero JPp \- Pc Pp zeroPc Pr Jpp/ zI Pp Pr zero DDC}PPjc 570zero lOCJP CASE 3 cRM*SHAPT POSITION TOC Fr ENGINE VIDRATES SIDEIdAYS zero Counterweight balances half reciprocating ease Ppa Pc PpPc jP~rC Y0zeroPc 13Ac NOCIPc length line represents magnitude orientation ar rowhead represent direction Figure 4 centifugal force Fc shown vec tor pointing lower right curved arrow indi cates rotat ing counter clockwise direction Figure 5 illustrate combination resolution two forces represented vectors Fp Fc may start either vector place tail head other line connect ing tail first head sec ond represents sum two forces called resultant Engine balance now able ana lyze balance single-cylinder engine Figure 6 illustrates three cases first represents no effort balance recipro cating forces Well call underbal anced second case reciprocating forces fully balanceda situation well refer overbalanced final case half reciprocating forces bal anced represents best compromise situation sketches show crankshaft posi tion vectors reciprocating forces Fp rotating forces Fc result ant two Case 1 absence counterweight means reciprocating forces present engine vibrates up down Case 2 counterweight exactly bal ances reciprocating forces Thus instead up-and-down vibration side ways force exactly same mag zeroPc lPc Resultant Pr Pri p - ENgINE VIDRATES IN CIRCLLAR NOTION Fri FIG 6 THREE DALANCE CONDITIONS FOR SINGLE CYCLINDER ENGINE nitude reciprocating force simply different direction Case 3 illustrates situation counterweight balances half recip rocating mass TDC BDC posi tions up-and-down vibrating forces reduced one-halfthe sum Fp Fc 90- 270-degree positions Fc ap pears sideways vibrating force 45 135 degrees resultant Fp Fc preduces vector same magnitude Fc opposite direction Conse quently now have vibration force equal magnitude one-half would present no balance rotating opposite direction engine rota tion best compromise can achieve single-cylinder engine Secondary vibration Earlier pointed out piston movement strictly speaking simple harmonic motion because connecting rod finitely long piston travel does fol low sine curve exactly Its well beyond scope article include fact above analysis However few com ments should made sake com pleteness actual movement piston short connecting rod say 1 /2 times stroke engine produces reciprocating May 1990 55 noises irritating listener 1 IIgnition TI O I compression R Istroke DOCTDCBDCTDCBDC U Ipower E Istroke ports Iopen II II 1one revolutlo1 FIG 7A TwaCYCLE IIgnItIon. -I-i TI0 coUprEEsionpowerExhaustintake R strokstrokestrolestinks Q BOCTOCBC TDCBOC U El I IIFIG 7flI IFOURCYCLE II II 1 revolutlo1 revolutlo1 FIG 7 TORQUE IIIPULSES OF TWOCYCLE AND FOIJRCYCLE ENGINES forces wbich include substantial compo nent twice engine speed result ing vibrations called secondary vibra tions case connecting rod length 1 /2 times stroke second ary vibrations will about one-third force primary vibrations Since its desirable both sake engine cornCENTER OF GRAVITY CCjj5 VIDRATIO\ FORCES FROMMINIMUM MOVEMENT ENGINEAT CS MPXIMUM MOVEMENT AT NOSE AND TAIL FIG 9 AIRPLANE ROTATES ABOUT ThE CS IN RESPONSE TO VIBRATION FORCES APPLIED DY THE ENGINE pactness reduce reciprocating mass have connecting rod short possi ble secondary vibrations significant factor engine balancing some multiple-cylinder engines sec ondaries can balanced others canpot example six-cylinder line engine 90-degree V-8 may pri mary second ary balance sim ple in-line fourcylinder can primary balance ly Secondary vibra tion model en gines significant because noise generates Since vibration forces contain sub stantial components twice engine speed noise generated airframe will like wise contain fre quencies twice engine speed higher flquency Multicylinder engines Its obvious twin-cylinder engine two-throw crankshaft pistons move op posite directions should vibration free practical problem such engines ever fact because twothrow crankshaft cylinders line other results twist ing force called couple will create some vibration Although such engines will somewhat less ideal balance vibration encountered low little practical concern such engine will primary secondary balance Also available op posed twin-cylinder engine singlethrow crankshaft pistons both move same directionan arrange ment course offers no vibration ad yantage Torsional vibration previous sec tion discussed vibration caused rotating reciprocating masses engine Another type vibration results way engine produces its power Engine power transmitted series impulsesthat force produced during power stroke power engine delivered propeller whatever load engine driving torque twisting force applied crankshaft addition producing torque en gine must also receive torque input order carry through compression stroke Thus engines must have flywheel store up energy produced power stroke some may returned compression stroke well smooth out power pulses aircraft en gines propeller serves flywheel Vibration due torque called torsional vibration twisting type vibration produced along axis crankshaft frequency related rpm engine Figure 7A illustrates twocycle engine top dead center TDC see large pulse torque power stroke followed trans fer period lasting about 120 degrees onethird rotation crankshaft dur ing no torque During transfer period ports open cylinder being scavenged burned gases charged fresh fuel/air mixture ports close compression strQke commences see pulse torque opposite direction power stroke cylinder fires initiates another power pulse process repeats Thus addition vibration created imbalance its moving parts en gine also producing torsional vibration modelers already well aware torsional vibration may very dam aging airframe As Figure 7B illustrates four-cycle en gines power stroke occurs other revolution two-cycle four-cycle engine produce same power 56 Model Aviation same rpm since four-cycle has half power impulses obviously impulses must twice large Thus would expect four-cycle engine produce torsional vibration half frequency two-cycle Ob servation bears out since four-cycle en gines seem lumpy run ning two-cycle counterparts Again multicylinder engines should somewhat smoother respect tor sional vibration provided course theyre configured firing order uniform flat opposed engines ex ample V twins common crankpin arrangement evenly spaced firing order impossible Coping vibration Since vibration pro duced engines unavoidable must plan best methods coping falls several categories can isolate vibration source engine can isolate parts airplane likely suffer effects vibration radio system can de sign airframe withstand effects vibration Vibration-isolating engine mounts wide ly used full-scale airplanes automo biles now coming use model air planes mounts reportedly quite successful particularly reducing model engine noise number considera tions arising differences between THE BEST FUSELAGE JIG EVER PERIOD Provides Solid Support Accurate Building Rugged constniction 7 Ply Aircraft Plywood Provides Lifetime Use Two Sizes Available Regular Fuselages up 6 Wide Jumbo Fuselages Up 9 Wide Use Own Building Board Work Bench Base Mount Jig Station Uni-Strut Guide Rails Most Versatile Set-Up contains complete Hardware Instructions Including Uni-Strut Information Bos 4 Standard Jig StationsS1795 Box 4 Jumbo Jig Stations1995 TwoS ft long Guide Rails including mounting nuts 2195 model full-scale applications must taken account Since inertial mass model engines small comparison magnitude vibration forces isolating mount ings model applications have quite limber will permit engine vi brate great deal reference air frame particularly low speeds causing potential problems airframe parts such cowlings addition fatigue failure engine parts such needle valves Other possible consequences fuel foaming fuel lines difficulty adjusting engine running power loss due energy absorption flexible mountings current applications seem cop ing problems successfully lim iting excursions engine re straining structures using mounting methods materials provide var iable stiffness rate Theres lot room further innovative work area Unless installing vibrationisolation system best practice mount engine firmly possible airframe idea tie structure together well-integrated mass upon vibration forces will applied Newtons Second Law F fec tive situation vibration forces F applied mass airframe Obviously mass M large ac Continued page 58 IVANS MAGIC GADGET protractor device used Ivan Kristensen accurately measure aileron elevator rudder throw Takes guess work out trimming out airplane quality tool complete instructions log book carrying case$795 PLUG-IN ADJUSTABLE STAB KIT last can optimize stab incidence best flight charac teristics pair glass filled nylon adjusters included provide micro screw adjusonent smaller front tube Main tube aluminum matching phenolic socket $695 Include 52 SOiltem UPS esidents Add 6% Sales Tax GATOR RIC PRODUCTS 3713 Pompano Dr acola FL 32514 476-8639 U DJU 31 1 [Cl 12 PLUG-IN WING ADJUSTER KIT Four glass filled nylon brackets included key wing panels front rear allow accurately adjust panel straight flight complete hardware & instructions$995 WING TUBE & SOOETS Aluminum alloy tubing plug-in wings Included pair special slip-fit phenolic sockets boild wing structure Now size models 718 diax2$ 895 1diax2S 995 IWdiax3$1495 May 1990 57 Which Thermal Competition Glider Would Rather Flying Surprised nearly 26 [ID Yes according results Selig/Donovan/Fraser Princeton teals mighty LOVESONG might er everl adding trip 1/8 wide 020 thick 20% chord Eppler 214 has become killer thermal airfoil 90s sizable performance boost puts PIXY CAMANO & LOVESONG farther ahead tips up competition Beat Designs Mrfoil Performance can buy 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FREE CATALOG Ti4esuala fWabg eidwiiuaa 392$-C Ss sa Cart Scala Plusda 32571 9a4 5g4-3531 5 Oelr gao m uoir ing No 308-i/S Scale s505 No307 .1/7 Scale s8 ULTIMATE 10 DASH 300 BIPLANE QUARTER-SCALE 13 LBS 62 SPAN SUPER TIGRE 3000 POWER FIBERGLASS FUSELAGE FOAM WINGS SOLO ONLY BY RESERVATION $75 refundable deposit includes data package $15 separately Camprehensive lsitS554 finished unpainted aircraft 11550 pomerplant india gear nat included SATISFACTION GUARANTEED ar yaur manny back Phone ar FAX 801-753-1971 bntueen 9 4 MT Visa MC check no COD PRODUCTION LIMITED eight hond-crafted individually numbered aircraft per month DAEDALUS RESEARCH INC tory creatingin An awardminning des odvance c civilian nmanned An DUCTED FAN PLANS SPANLENGTH SR-71 Blackbird5541077 F-86 0 Sabre430470 U-2BSpyplane1030525 F-8J Crusader380530 -4J Phantom370515 F9F Panther355495 F11-1 Grummanriger 500 630 Plans $3000 PP SR-71 Blackbird 4 sheets $4600 2C19964414 ENEGE-l17S TE1E AEEINC P0 Box 187 Pittatown NJ 08867 celeration will less Thus very light airframe will subject vibration ef fects corollary foregoing recom mendation entire airframe con structed rigidly possible should little no relative motion between vari ous parts structure will reduce problems fatigue wearout will also prevent secondary vibrations oc cur parts airframe rattle against other solution isolate sensitive crit ical parts particularly radio against damaging vibration generally done wrapping receiver battery sponge rubber mounting servos through rubber grommets method seems work quite well long sponge rubber wrapping kept loose possible servos screwed down too tightly Its important understand modes vibration will found airframe consider entire airplane freely suspended body three degrees free domthat free move direc tion outside restraint As mat ter fact aerodynamic forces impose re straints movement airplane can ignore purposes analysis vibration forces ap plied airplane engine usually front craft plane will re spond input vibrating about center-of-mass center-of-gravity commonly referred CC Figure 8 shows amplitude vi bration will minimal CC crease distance point Thus vibration-sensitive equip mentthe receiver servos particular should located close possible CC its necessary place electronic components distance the CC example situating battery pack ward purposes balancethey should carefully packed sponge rubber Since its almost always positioned close en gine fuel tank subject severe vibra tion practice placing servos tail near ailerons Giant Scale models should carefully weighed subjects greater vibration locations near CC chain saw engines pro duce some awesome vibration forces Tor sional vibration places severe stress tail surfaces aft fuselage Giant Scale models using either chain-saw-type large four-cycle engines its good idea use brace wires between fin stabilizer fuselage Strengthening aft fuselage helps too properly balanced propeller also helps minimizing vibration Obviously im balance propeller will cause substan tial vibration should avoided Continued page 173 AIR WOLF EPOXY FIBERGLASS FUSELAGES TAKES MOST 50-60 MECHANICS OTHER MODELS AVAILABLE LARRY JOLLY MODEL PRODUCTS $21000 5501 WCOMO SANTA ANA CALIFORNIA 92703 714 826-6861 58 Model Aviation THE PIPER BRAVE scale model Piper s latest Ag plane t wing span 30-33 lbs 5 cc power lb payload ational flaps hopper door onal Spreader Kit formed aluminum landing gear rglass cowls um-formed canopy tucted extruded foam ply hardwood l actuaiiy auslingBRAVE $269~ OPTIONAL SEND $100 FOR CATALOG OFRT 1 NEODESHA KS 66757 SAC KITS AND ACCESSORIES 316 325-2821 Free Shipping Kit OrdersScott Airplane CompanyVISA/MASTERCARD U ~i/8INCH3848 3/8-INCH3648 1/8 1/808113/8 o 3/82737 BALSA WOOD1/6 o 3/181115 3/6 o 1/23041 STICKS1/60 1/41317 3/8 o 3/44460 1/603/81621 1/16-INCH3648 1/60 1/217241/2-INCH3848 1/ie o 1/16061/8034425331/2 1/23746 1/1603/32073/16-INCH3648 1/2 3/45270 1/18 01/60811TRIANGULAR 1/18 03/1609133/1603/161317 1/160 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PLYWOOD 1/640120121821/40601274 1/640120243641/4012012146 1/640120497281/4o12o24293 1/6404604929121/4012048585 1/3206012603/6o12o12180 1/320120121193/80 12024360 1/320 120242373/60 12046720 1/32o12a48473__________ 1/2 0 120 12202 1/1606012591/2512024405 1/160120 121171/20 12048606 1/16a12o24234 1/160 12049466 3/320601291LITE PLY WOOD 3/320 120 12161 3/320120243621/801201261 3/32o12s497231/6012024122 ______________1/6012046244 1/6o6o 12100 1/6012012199 1/60 12024397 TAPERED TRAILING 1/60 12046794 EDGE3668 3/160601274 1/801/22230 3/160 120121463/160 3442940 3/160 120242931/4 14255 3/16012049586 5/16o1/o4461 3/8a16272 * OPEN TO THE PUBLIC * Superior Aircraft Materials 12020-G Centralia iian Gardens CalIfornia 90716 865-3220 Handling Charge $500 $300 COD mum Order S2000 plus UPS accept Personai checks COD fornia Residents add 6% Sales Tax proaches well-doped Japanese tissue tightness materials puncture resistance excellent theres no tendency rip tear Frankly material essentially like vastly improved MicaFilm its characteristics well suited smaller Old-Timers Electric models certainly RCSD negative side Litespan bit tricky use materials adhesive built also doesnt add much torsional strength films samples some color-blotch troubles Litespan definitely has place area hobby recommend might also want check out rest Charlies product line specialize Electric flight Free Flight RC products True confessions Normally use short captions photos dont write about subjects main part column ever month just have say few extra words about Howard DGA Bob Maurer have photo file sev eral years awaiting just right time use often since columns tend matic Bob Maurer built model Paul Lind berg kit 1938 Lindberg prolific de signer several magazines particularly Flying Aces popularity designs led Paul kit business shortand apparently unsuccessfultime Maurer powered Sky Chief cheap Dennymite never flew bad wasnt RCSD event back certainly would have reassuring know model wasnt going crash fly away Bob went medical school served US Army settling San Antonio after war 1946 two anesthesiologists long successful career followed Dr Maurer remained active modeling well loved San Antonio area generosity support local clubs sad part story Bob passed away before got around publishing pic ture memory shelter has erected club field Bob Maurer represents what makes hobby attractive involved supportive no interest national recognition desire fame just sim ply loved modeling modelers will missed us Vibration/Abbott Continued page 58 knew certain particular engine underbalanced overbalanced might possible reduce overall vibration installing unbalanced propeller ever thats probably practical approach take As long single-cylinder reciprocating engine remains predominant power plant model airplanes vibration will remain constant concern can best cope problem building solid airframe struc tures providing vibration isolation sensitive components such radios As technology improves vibration isolating en gine mounting seems very promising CL Avanti/Wuif Continued page 59 pIe does control configuration like Avanti Reading few articles airplane re assured could use same basic control system full-scale ship Avanti has unique three-surface design forward canard helps lift long nose fuselage placement wing far rear lifts main weight aircraft An other reason placing wing far back allow maximum length passenger cargo area interruption wing spars braces high stabilizer well Out path fuselage turbulence makes good bite clean air small canard flapped slow-speed flight landing doesnt act elevator Altogether singularly neat efficient package twin-engined business craft Corn 174 May 1990 173 NEW FAI TAN RUBBER 1/4 3116 1/8 3/32 1116- 042 thick 1 lb box- *1475 llncludes4oz Slick Lubel $1375 no lube k costa $17S1 10 lb box $11000 UPS Deliveredl PostpaId US v18 UPS Cal rae add 6% aslas tax Made USA Slow cure conalatant qualIty Ask FAI-TAN 89/90 catalog lists 100FF kital CROCKET HOOKS ADESYFAI SEND $150FAI Model Supply FOR CATALOG P0 Box 3557 Torrance CASOSlO ___ ELECTRONICS 900 Lucy Road ll Ml 48843 5462644 Wo sorvice makos RIO equipment utilizing state art electronics Spectrum Analyzer Precision Frequency Measuring equipment True 1991 Standards endorsed AMA & RCMA tho Transmitter Slicker Program FCC LicensedSpeed Controi RepairsQuality Service Transmitter Chocks Only includes GOLD/SILVER sticker Transmitter & Receiver Calibration$2400 Frequency Upgrades & Changes crystals extra$2900 Bench Time Repairs per hour$2500 Transmittors servico wil receive AMA/RCMA sticker ADD $500 FOR SHIPPING & INSURANCE TURN AROUND TIME TWO WEEKS OR LESSII UPSIVISA/MASTERCARD ACCEPTEDCQD