Cen trffuga Force U George Abbott What reasonable numbers model airplane purposes Typical radio control sport airplanes fly 35 55 mph pattern airplanes 50 85 mph pylon racers 125 mph turn radius order 10 feet sharp pullout 100 feet big loop 50 100 feet pylon turn estimating size maneuvers just remember paved RC runways about 300 feet long 30 feet wide residential utility poles 18 30 feet high F THE SEVERAL FORCES act aircraft flight centrifugal force W important will help us both design flight understand force object traveling along curved path will experience force toward outside curve childhood example tying rock string whirling string around head force rock experiences called centrifugalforce very important factor behavior airplanes Whenever airplane changes direction centrifugal force causes apparent weight airplane change example during pullout dive apparent weight airplane will increase airplane pushes over level flight apparent weight will decrease term commonly used describe forces gforce force g equivalent force resulting acceleration earths gravity generally accepted 32 ft/sec/sec An object acted upon force gravity will exert force equal its mass multiplied g force called weight expressed pounds English system units accordance Newtons first law FMxa Weight mass related acceleration gravity follows WMxg M pounds slugs W ht pounds leration ft/sec/sec g acceleration gravity 32 ft/sec/sec its clear mass body its weight divided gthe acceleration gravity Thus object having weight 64 pounds lb would have mass two pounds lbm express acceleration terms g always get forces terms weight avoids lot confusion hope Centrifugal force results accel eration caused moving objects change direction acting upon mass object formula acceleration V2 r leration ft/sec/sec V velocity ft/sec r us curve feet can combine formula Newtons first law little arithmetic get formula will accept speed miles per hour provide result force g mph2 x 0672 r g e multiples weight mph d miles per hour r us curve feet Notice speed squared formulae centrifugal force will always directed outward center curving path total force acting object will vector sum resultant forces paper will use following terminology W ht pounds g centrifugal force multiples W G l force multiples W Thus object W 10 pounds subjected 3g would experience total force G 30 pounds further simplicity will normalize weight W pound everything comes out terms g Control line airplanes subject centrifugal force circular flight primary factor maintaining line tension models may also employ such aerodynamic means outside rudder ensure line tension low speeds overhead flying wind Speed models dont need help Control line models fly speeds 35 55 mph sport stunt models up around 100 mph combat carrier models well over 100 mph Speed models AMA Competition Regulations specify perfect control line stunt maneuvers will have turn radius 49 feet dont always make Figure 1 rule book illustrates several desired maneuvers Figure 1 AMA Competition Regula tions criteria square corners control line stunt maneuvers Plugging some numbers can relate g forces speed radius curvature some typical model maneuvers Table 1 shows results TABLE 1 RC MODELS Pullout Loop RadiusSpeed feetmph g 20303 205084 206012 508597 Pylon turniao125105 CONTROL LINE STUNT Maneuver5 Line tension525 60 70 CONTROL LINE SPEED Line tension70 50336 5032 5028 5024 150216 44 Model Aviation radius corners \ lev 2 loops 4-nd77Th should interpret numbers shown Table 1 Sig Cadet weighing 45 lb pulling Out dive 50 mph 20-ft radius will experience maximum force G 94g 84 due pullout plus lg normal its apparent weight will 423 lb An increase speed 60mph would increase G 585 lb An eight-lb pattern ship doing loop SOft radius 85 mph would have apparent weight 856 lb bottom maneuver Figure 2 shows way vertical forces act airplane control line stunt ship executing 49-foot-radius corners shown rule book would pull 336g Thus three-pound model would have apparent weight about 100 lb control line models also matter line tension consequence model flying horizontal circular path restrained control lines control line model flying 50 mph 525-ft lines would experience 32g horizontally model weighing three pounds would experience 96 lb line tension Speed model weighing three pounds flying 150 mph 70-ft lines would exert about 65 lb line tension figures lead several important considerations involving both structural ove Figure 2A bottom pullout plane experiences g force caused pullout plus weight plane Above right Figure 2B top push-over plane experiences g force less planes weight g force ane appears weightless g force greater 1 plane experiences -Gs Right Figure 2C During loop plane experiences G force sum W g Direction W always down g always outward actual flight planes speed changes throughout loop would constant perfect maneuver strength aerodynamic behavior Obviously wing structure particularly center sectionas well other parts airframemust tolerate loads wish avoid in-flight breakup control mechanism control line speed ships must very sturdy capable functioning smoothly under load addition stresses imposed upon airframe aerodynamic behavior airplane may also drastically affected centrifugal forces because increase g has effect increasing weight airplane often very significant amount affects behavior airplane turns well its stall speed BEHAVIOR IN TURNS important aspect airplane hehavior related centrifugal force angle hank turn coordinated turn bank angle will represent resultant vertical force gravity horizontal centrifugal force resulting turn coordinated turn will no skidding slipping real airplane ball turn bank indicator would remain centered full-scale airplanes particularly some designs Golden Age 20s 30s skidding unintentional slippingespecially close ground carefully avoided Both maneuvers greatly increase drag airplane may also encourage pilot cross controlsthat apply opposite aileron rudderwhich could lead spin Wq4fg La low speeds Modem airplanes tolerant its still mark good airmanship keep ball centered model airplanes dont have ball keep centered often usually make turns control ailerons airplanes equipped rudder airplanes ailerons aileron-less airplanes dihedral wings induces bank response rudder-induced skid resulting turn probably pretty well coordinated dynamics aileron turn bit different ailerons establish bank angle both cases bank angle induces horizontal component lift causes airplane turn poorly coordinated turn bank angle may induce slip rudder may cause skid Either can cause increased drag loss altitude may compensated additional lift either added angle attack up elevator added power both An important consequence poorly coordinated turns model airplanes loss altitude Most model pilots dont bother use rudder turns giant scale exception relying inherent roll stability models resist skids important aspect turns g forces consequent loading airplane particularly tight turns turn executed speed turn radius such centrifugal force g bank angle will 450 airplane will experience 14g turn results centrifugal force 1 73g bank angle will 600 airplane will experience 2g steeper bank angle greater g experienced airplane Figure 3 illustrates interested mathematical relationship follows Angle bank degrees atan g mph2 x 0672 r #-, V hOdVV,gThVVC Figure 3 turn centrifugal force g directed outward weight W directed downward Resultant bank angle July1993 45 ImRo pwc$16000 ALL KITS FEATURE BAND LAID FIBERGLASS FUSELAGEPANEL LINES SCALE DETAILSFOAM CORE WINGS MACHINE CUE FORMERS & BULKHEAD& SEE YOUR LOCAL DFALER OR CALL HOBBY 1L4NGAR 600-283-5746 FOR IMAiEDUTE DELIVERY DEALER INQUIRIFS WELCOME WEACCEPT 14 MASTERCARD & DISCOVER CARDS WEGLADLYSHY COD SHIPPING & H4NDLJNG $500 PER KIT COm1NFNTAL US4 OIlIER AREAS CALL KEN7VCKYRESIDENIS ADD 6% S4LES TAX HOBBY HANGAR ISA FULL SER VI CE HOBBY SHOP WITH OVER 11000 ITEMS IN STOCI4 CATALOGS AVAILABLE $400 EACH WIREBATE ON FIRST ORDER SPECIFY PLANEHELICOPTER OR CAR HANGAR EXaLcsIVEaTRwToRCOMING R SC4LE GIANT SCALE ME-i 09 606-28$-5889 FAX Consider what would happen airplane flying straight level flight weight suddenly increase factor two Obviously wing must work harder support additional weightthat coefficient lift must increase will happen consequence increase angle attack Induced drag also increases square coefficient lift means apparent weight airplane doubles coefficient lift must also double induced drag will increased four Thus its evident airplane will either slow down lose altitude both unless increase power think us have seen happen flying experience sometimes unfortunate results STALL SPEED other aerodynamic concern involves relation stall speed wing loading Stalling generally thought consequence airplanes flying too slowly will certainly result stall also possible stall airplane high speed often exciting results general stall speed proportional VszzkIWl/Clrn stall speed mph Wl loading ozIsq ft Clm maximum coefficient lift typically 12-15 k a constant relating air density Reynolds number typicaily about 5 Values Clm k typical RC models Since wing loading proportional apparent weight airplane turn proportional g force airplane sees follows stall speed will proportion square root g Thus airplane 600-square-inch wing weighing five pounds will have stall speed level flight 1 g around 20 mph airplane experiences 9g maneuver wing loading will increase factor nine stall speed will increased 60 mpha factor three square root nine important factor specifying maneuvering speeds full-scale airplanes structural limit airplane 9g maximum speed airplane could maneuvered no chance exceeding limit would three times stall speed Now lets think about moment trainer may have stall speed 15-20 mph pattern airplane 25 mph pylon racer 30 mph Consider Sig Cadet stall speed 15 mph diving 50 mph pulling out 20-foot radius bottom pullout airplane experiences 94g square root 94 307 stall speed 307 x 20 mphfaster airplane going Thus stalls may fall off spin called departurea particularly appropriate term happens close ground Another example tight turn can pylon racer stall speed 30 mph make 125 mph stalling departing Answer 605 feet Assuming stall speed 30 mph g 173g will give stall speed 125 mph 125 mph 173g will result turn radius 605 feet have watched pylon races have probably noticed squirrelly airplanes sometimes act turns consider fairly sharp leading edges wings racers surprising course drastic increase drag results high wing loadings means wider turns may result better lap speeds PRACTICAL RESULTS Structural integrity generally taken care airplane designed built Substantial construction wing center section very important Dihedral braces should made good hard aircraft plywood fiberglass reinforcement good insurance Shear webs spar structure good idea may seem silly mention adequate wing attachment helps means firm anchors wing bolts sturdy dowels rubber bands used course fresh rubber bands High g maneuvers low speeds can come close home really biteparticularly beginners flying sport airplane slightly windy day maybe 15 mph its right down runway too worried preparing land airplane has stall speed 15 mph usually fly downwind leg 25 mph slow down 20 mph over fence final Continued page 51 46 Model Aviation E WULF 190 Made USA SPECIFICATIONS WINGSPAN 65 ENGINE SIZE60-SW 2-SIROKE 5W-I20 4-SIROKE WEIGHT9-10 LBS I- SPECIFICATIONS WING SPAN 60 ENOlNE 560 2-SIROKE WEIGHT8-85 LBS CESSNA 150 SPECIFICATIONS WING SP4N 65 ENGINE SIZE WEIGIfl Made USA 45-60 2S7ROKE 7-75 LBS square root wing loading following formula shows relationship SPECIFICATlONS WING SPAN 60 ENGINE SIZE60 2-SIROKE WEIGHT7-8 LBS Centrifugal ForcelAbbott Continued page 46 Today fly downwind try slow down some because airplane seems have rather high ground speed 25 40 mph Still airplane seems pretty fast turn down wind final turn pretty tight induces additional g load Remember discussion above additional g load increases drag may slow airplane Now must deal combination two higher stall speed caused higher wing loading reduction airspeed caused increased drag also resulting higher wing loading result may very high sink rate ground spectacular snap ground Ever seen happen way lot mystique surrounds infamous downwind turn can traced steep turns speed reductions pilot executes attempting compensate wind drift maneuvering land 55 igh-speed stall ways set up Nil airplane make less susceptible control loss resulting abrupt maneuvers first obvious very often ignored way proper balance airplane tailheavy will have poor longitudinal stability making very pitch sensitive will make prone abrupt maneuvers may generate high g forces consequent highspeed stalls airplane already somewhat unstable may difficult recover safely Another adjustment can optimized elevator travel should possible reasonable speed apply full elevator entering high-speed stall emergency pullout close ground dont need airplane going crazy because pulled stick way back Control line tension control line models essential line tension maintained times Control line tension result two factors centrifugal force aerodynamics airplane such rudder offset leadout position asymmetrical wing span Line tension usually problem except flying sport stunt models strong winds long lines sometimes concern about flying maneuvers directly overhead Table 1 see 60-ft lines have 28g because centrifugal force directly overhead line tension would actually 1 8g because downward force gravity should still enough Regardless maneuver being performed centrifugal force dependent tangential velocity brings us question effect wind line tension Two factors work reduced speed airplane flies upwind re duces centrifugal force other aerodynamic force crosswind component counteracts centrifugal force upwind side circle Fortunately speed reduction greatest heading wind aerodynamic force greatest flying crosswind two dont occur same time can trouble flying overhead directly wind now speed reduced line tension reduced g As plane goes over top begins descend winds aerodynamic forces tend further reduce line tension Lets look few numbers 60-ft lines airspeed 50 mph see table line tension caused centrifugal force 28g 15-mph wind ground speed upwind will 35 mph formula gives result 137g Thus overhead line tension due centrifugal force will 037g some line tension caused aerody namics airplane would very comforting indeed CONCLUSION Model airplanes generally fly scale speeds often maneuvered violently Consequently often subjected flight loads far excess encountered full-scale counterparts Fortunately also usually much stronger size successful models combine great strength needed like wing center sections light weight tolerant aerodynamics proper balance conservative trim adjustment Pilot skill little good luck help too also safety considerations An airplane tends go out control may end up pits midst spectators An airplane has suffered in-flight structural failure can also go out control may pose serious safety hazard LASSES 195 i$2S&H 100% UV Protection Shatterproof Excellent Optical Quality Tints Fit Over Prescriptions S KPO BOX 1000 ID UMFRIES VA 22026 800-221-9318 July 1993 51 O Volt12AH$1200 12 Volt65AN$1500 VISA MASTERCARD DISCOVER $400 SHIPPING & HANDLING 5AI OR Fl RESIDENTS
Edition: Model Aviation - 1993/07
Page Numbers: 44, 45, 46, 51
Edition: Model Aviation - 1993/07
Page Numbers: 44, 45, 46, 51
Cen trffuga Force U George Abbott What reasonable numbers model airplane purposes Typical radio control sport airplanes fly 35 55 mph pattern airplanes 50 85 mph pylon racers 125 mph turn radius order 10 feet sharp pullout 100 feet big loop 50 100 feet pylon turn estimating size maneuvers just remember paved RC runways about 300 feet long 30 feet wide residential utility poles 18 30 feet high F THE SEVERAL FORCES act aircraft flight centrifugal force W important will help us both design flight understand force object traveling along curved path will experience force toward outside curve childhood example tying rock string whirling string around head force rock experiences called centrifugalforce very important factor behavior airplanes Whenever airplane changes direction centrifugal force causes apparent weight airplane change example during pullout dive apparent weight airplane will increase airplane pushes over level flight apparent weight will decrease term commonly used describe forces gforce force g equivalent force resulting acceleration earths gravity generally accepted 32 ft/sec/sec An object acted upon force gravity will exert force equal its mass multiplied g force called weight expressed pounds English system units accordance Newtons first law FMxa Weight mass related acceleration gravity follows WMxg M pounds slugs W ht pounds leration ft/sec/sec g acceleration gravity 32 ft/sec/sec its clear mass body its weight divided gthe acceleration gravity Thus object having weight 64 pounds lb would have mass two pounds lbm express acceleration terms g always get forces terms weight avoids lot confusion hope Centrifugal force results accel eration caused moving objects change direction acting upon mass object formula acceleration V2 r leration ft/sec/sec V velocity ft/sec r us curve feet can combine formula Newtons first law little arithmetic get formula will accept speed miles per hour provide result force g mph2 x 0672 r g e multiples weight mph d miles per hour r us curve feet Notice speed squared formulae centrifugal force will always directed outward center curving path total force acting object will vector sum resultant forces paper will use following terminology W ht pounds g centrifugal force multiples W G l force multiples W Thus object W 10 pounds subjected 3g would experience total force G 30 pounds further simplicity will normalize weight W pound everything comes out terms g Control line airplanes subject centrifugal force circular flight primary factor maintaining line tension models may also employ such aerodynamic means outside rudder ensure line tension low speeds overhead flying wind Speed models dont need help Control line models fly speeds 35 55 mph sport stunt models up around 100 mph combat carrier models well over 100 mph Speed models AMA Competition Regulations specify perfect control line stunt maneuvers will have turn radius 49 feet dont always make Figure 1 rule book illustrates several desired maneuvers Figure 1 AMA Competition Regula tions criteria square corners control line stunt maneuvers Plugging some numbers can relate g forces speed radius curvature some typical model maneuvers Table 1 shows results TABLE 1 RC MODELS Pullout Loop RadiusSpeed feetmph g 20303 205084 206012 508597 Pylon turniao125105 CONTROL LINE STUNT Maneuver5 Line tension525 60 70 CONTROL LINE SPEED Line tension70 50336 5032 5028 5024 150216 44 Model Aviation radius corners \ lev 2 loops 4-nd77Th should interpret numbers shown Table 1 Sig Cadet weighing 45 lb pulling Out dive 50 mph 20-ft radius will experience maximum force G 94g 84 due pullout plus lg normal its apparent weight will 423 lb An increase speed 60mph would increase G 585 lb An eight-lb pattern ship doing loop SOft radius 85 mph would have apparent weight 856 lb bottom maneuver Figure 2 shows way vertical forces act airplane control line stunt ship executing 49-foot-radius corners shown rule book would pull 336g Thus three-pound model would have apparent weight about 100 lb control line models also matter line tension consequence model flying horizontal circular path restrained control lines control line model flying 50 mph 525-ft lines would experience 32g horizontally model weighing three pounds would experience 96 lb line tension Speed model weighing three pounds flying 150 mph 70-ft lines would exert about 65 lb line tension figures lead several important considerations involving both structural ove Figure 2A bottom pullout plane experiences g force caused pullout plus weight plane Above right Figure 2B top push-over plane experiences g force less planes weight g force ane appears weightless g force greater 1 plane experiences -Gs Right Figure 2C During loop plane experiences G force sum W g Direction W always down g always outward actual flight planes speed changes throughout loop would constant perfect maneuver strength aerodynamic behavior Obviously wing structure particularly center sectionas well other parts airframemust tolerate loads wish avoid in-flight breakup control mechanism control line speed ships must very sturdy capable functioning smoothly under load addition stresses imposed upon airframe aerodynamic behavior airplane may also drastically affected centrifugal forces because increase g has effect increasing weight airplane often very significant amount affects behavior airplane turns well its stall speed BEHAVIOR IN TURNS important aspect airplane hehavior related centrifugal force angle hank turn coordinated turn bank angle will represent resultant vertical force gravity horizontal centrifugal force resulting turn coordinated turn will no skidding slipping real airplane ball turn bank indicator would remain centered full-scale airplanes particularly some designs Golden Age 20s 30s skidding unintentional slippingespecially close ground carefully avoided Both maneuvers greatly increase drag airplane may also encourage pilot cross controlsthat apply opposite aileron rudderwhich could lead spin Wq4fg La low speeds Modem airplanes tolerant its still mark good airmanship keep ball centered model airplanes dont have ball keep centered often usually make turns control ailerons airplanes equipped rudder airplanes ailerons aileron-less airplanes dihedral wings induces bank response rudder-induced skid resulting turn probably pretty well coordinated dynamics aileron turn bit different ailerons establish bank angle both cases bank angle induces horizontal component lift causes airplane turn poorly coordinated turn bank angle may induce slip rudder may cause skid Either can cause increased drag loss altitude may compensated additional lift either added angle attack up elevator added power both An important consequence poorly coordinated turns model airplanes loss altitude Most model pilots dont bother use rudder turns giant scale exception relying inherent roll stability models resist skids important aspect turns g forces consequent loading airplane particularly tight turns turn executed speed turn radius such centrifugal force g bank angle will 450 airplane will experience 14g turn results centrifugal force 1 73g bank angle will 600 airplane will experience 2g steeper bank angle greater g experienced airplane Figure 3 illustrates interested mathematical relationship follows Angle bank degrees atan g mph2 x 0672 r #-, V hOdVV,gThVVC Figure 3 turn centrifugal force g directed outward weight W directed downward Resultant bank angle July1993 45 ImRo pwc$16000 ALL KITS FEATURE BAND LAID FIBERGLASS FUSELAGEPANEL LINES SCALE DETAILSFOAM CORE WINGS MACHINE CUE FORMERS & BULKHEAD& SEE YOUR LOCAL DFALER OR CALL HOBBY 1L4NGAR 600-283-5746 FOR IMAiEDUTE DELIVERY DEALER INQUIRIFS WELCOME WEACCEPT 14 MASTERCARD & DISCOVER CARDS WEGLADLYSHY COD SHIPPING & H4NDLJNG $500 PER KIT COm1NFNTAL US4 OIlIER AREAS CALL KEN7VCKYRESIDENIS ADD 6% S4LES TAX HOBBY HANGAR ISA FULL SER VI CE HOBBY SHOP WITH OVER 11000 ITEMS IN STOCI4 CATALOGS AVAILABLE $400 EACH WIREBATE ON FIRST ORDER SPECIFY PLANEHELICOPTER OR CAR HANGAR EXaLcsIVEaTRwToRCOMING R SC4LE GIANT SCALE ME-i 09 606-28$-5889 FAX Consider what would happen airplane flying straight level flight weight suddenly increase factor two Obviously wing must work harder support additional weightthat coefficient lift must increase will happen consequence increase angle attack Induced drag also increases square coefficient lift means apparent weight airplane doubles coefficient lift must also double induced drag will increased four Thus its evident airplane will either slow down lose altitude both unless increase power think us have seen happen flying experience sometimes unfortunate results STALL SPEED other aerodynamic concern involves relation stall speed wing loading Stalling generally thought consequence airplanes flying too slowly will certainly result stall also possible stall airplane high speed often exciting results general stall speed proportional VszzkIWl/Clrn stall speed mph Wl loading ozIsq ft Clm maximum coefficient lift typically 12-15 k a constant relating air density Reynolds number typicaily about 5 Values Clm k typical RC models Since wing loading proportional apparent weight airplane turn proportional g force airplane sees follows stall speed will proportion square root g Thus airplane 600-square-inch wing weighing five pounds will have stall speed level flight 1 g around 20 mph airplane experiences 9g maneuver wing loading will increase factor nine stall speed will increased 60 mpha factor three square root nine important factor specifying maneuvering speeds full-scale airplanes structural limit airplane 9g maximum speed airplane could maneuvered no chance exceeding limit would three times stall speed Now lets think about moment trainer may have stall speed 15-20 mph pattern airplane 25 mph pylon racer 30 mph Consider Sig Cadet stall speed 15 mph diving 50 mph pulling out 20-foot radius bottom pullout airplane experiences 94g square root 94 307 stall speed 307 x 20 mphfaster airplane going Thus stalls may fall off spin called departurea particularly appropriate term happens close ground Another example tight turn can pylon racer stall speed 30 mph make 125 mph stalling departing Answer 605 feet Assuming stall speed 30 mph g 173g will give stall speed 125 mph 125 mph 173g will result turn radius 605 feet have watched pylon races have probably noticed squirrelly airplanes sometimes act turns consider fairly sharp leading edges wings racers surprising course drastic increase drag results high wing loadings means wider turns may result better lap speeds PRACTICAL RESULTS Structural integrity generally taken care airplane designed built Substantial construction wing center section very important Dihedral braces should made good hard aircraft plywood fiberglass reinforcement good insurance Shear webs spar structure good idea may seem silly mention adequate wing attachment helps means firm anchors wing bolts sturdy dowels rubber bands used course fresh rubber bands High g maneuvers low speeds can come close home really biteparticularly beginners flying sport airplane slightly windy day maybe 15 mph its right down runway too worried preparing land airplane has stall speed 15 mph usually fly downwind leg 25 mph slow down 20 mph over fence final Continued page 51 46 Model Aviation E WULF 190 Made USA SPECIFICATIONS WINGSPAN 65 ENGINE SIZE60-SW 2-SIROKE 5W-I20 4-SIROKE WEIGHT9-10 LBS I- SPECIFICATIONS WING SPAN 60 ENOlNE 560 2-SIROKE WEIGHT8-85 LBS CESSNA 150 SPECIFICATIONS WING SP4N 65 ENGINE SIZE WEIGIfl Made USA 45-60 2S7ROKE 7-75 LBS square root wing loading following formula shows relationship SPECIFICATlONS WING SPAN 60 ENGINE SIZE60 2-SIROKE WEIGHT7-8 LBS Centrifugal ForcelAbbott Continued page 46 Today fly downwind try slow down some because airplane seems have rather high ground speed 25 40 mph Still airplane seems pretty fast turn down wind final turn pretty tight induces additional g load Remember discussion above additional g load increases drag may slow airplane Now must deal combination two higher stall speed caused higher wing loading reduction airspeed caused increased drag also resulting higher wing loading result may very high sink rate ground spectacular snap ground Ever seen happen way lot mystique surrounds infamous downwind turn can traced steep turns speed reductions pilot executes attempting compensate wind drift maneuvering land 55 igh-speed stall ways set up Nil airplane make less susceptible control loss resulting abrupt maneuvers first obvious very often ignored way proper balance airplane tailheavy will have poor longitudinal stability making very pitch sensitive will make prone abrupt maneuvers may generate high g forces consequent highspeed stalls airplane already somewhat unstable may difficult recover safely Another adjustment can optimized elevator travel should possible reasonable speed apply full elevator entering high-speed stall emergency pullout close ground dont need airplane going crazy because pulled stick way back Control line tension control line models essential line tension maintained times Control line tension result two factors centrifugal force aerodynamics airplane such rudder offset leadout position asymmetrical wing span Line tension usually problem except flying sport stunt models strong winds long lines sometimes concern about flying maneuvers directly overhead Table 1 see 60-ft lines have 28g because centrifugal force directly overhead line tension would actually 1 8g because downward force gravity should still enough Regardless maneuver being performed centrifugal force dependent tangential velocity brings us question effect wind line tension Two factors work reduced speed airplane flies upwind re duces centrifugal force other aerodynamic force crosswind component counteracts centrifugal force upwind side circle Fortunately speed reduction greatest heading wind aerodynamic force greatest flying crosswind two dont occur same time can trouble flying overhead directly wind now speed reduced line tension reduced g As plane goes over top begins descend winds aerodynamic forces tend further reduce line tension Lets look few numbers 60-ft lines airspeed 50 mph see table line tension caused centrifugal force 28g 15-mph wind ground speed upwind will 35 mph formula gives result 137g Thus overhead line tension due centrifugal force will 037g some line tension caused aerody namics airplane would very comforting indeed CONCLUSION Model airplanes generally fly scale speeds often maneuvered violently Consequently often subjected flight loads far excess encountered full-scale counterparts Fortunately also usually much stronger size successful models combine great strength needed like wing center sections light weight tolerant aerodynamics proper balance conservative trim adjustment Pilot skill little good luck help too also safety considerations An airplane tends go out control may end up pits midst spectators An airplane has suffered in-flight structural failure can also go out control may pose serious safety hazard LASSES 195 i$2S&H 100% UV Protection Shatterproof Excellent Optical Quality Tints Fit Over Prescriptions S KPO BOX 1000 ID UMFRIES VA 22026 800-221-9318 July 1993 51 O Volt12AH$1200 12 Volt65AN$1500 VISA MASTERCARD DISCOVER $400 SHIPPING & HANDLING 5AI OR Fl RESIDENTS
Edition: Model Aviation - 1993/07
Page Numbers: 44, 45, 46, 51
Cen trffuga Force U George Abbott What reasonable numbers model airplane purposes Typical radio control sport airplanes fly 35 55 mph pattern airplanes 50 85 mph pylon racers 125 mph turn radius order 10 feet sharp pullout 100 feet big loop 50 100 feet pylon turn estimating size maneuvers just remember paved RC runways about 300 feet long 30 feet wide residential utility poles 18 30 feet high F THE SEVERAL FORCES act aircraft flight centrifugal force W important will help us both design flight understand force object traveling along curved path will experience force toward outside curve childhood example tying rock string whirling string around head force rock experiences called centrifugalforce very important factor behavior airplanes Whenever airplane changes direction centrifugal force causes apparent weight airplane change example during pullout dive apparent weight airplane will increase airplane pushes over level flight apparent weight will decrease term commonly used describe forces gforce force g equivalent force resulting acceleration earths gravity generally accepted 32 ft/sec/sec An object acted upon force gravity will exert force equal its mass multiplied g force called weight expressed pounds English system units accordance Newtons first law FMxa Weight mass related acceleration gravity follows WMxg M pounds slugs W ht pounds leration ft/sec/sec g acceleration gravity 32 ft/sec/sec its clear mass body its weight divided gthe acceleration gravity Thus object having weight 64 pounds lb would have mass two pounds lbm express acceleration terms g always get forces terms weight avoids lot confusion hope Centrifugal force results accel eration caused moving objects change direction acting upon mass object formula acceleration V2 r leration ft/sec/sec V velocity ft/sec r us curve feet can combine formula Newtons first law little arithmetic get formula will accept speed miles per hour provide result force g mph2 x 0672 r g e multiples weight mph d miles per hour r us curve feet Notice speed squared formulae centrifugal force will always directed outward center curving path total force acting object will vector sum resultant forces paper will use following terminology W ht pounds g centrifugal force multiples W G l force multiples W Thus object W 10 pounds subjected 3g would experience total force G 30 pounds further simplicity will normalize weight W pound everything comes out terms g Control line airplanes subject centrifugal force circular flight primary factor maintaining line tension models may also employ such aerodynamic means outside rudder ensure line tension low speeds overhead flying wind Speed models dont need help Control line models fly speeds 35 55 mph sport stunt models up around 100 mph combat carrier models well over 100 mph Speed models AMA Competition Regulations specify perfect control line stunt maneuvers will have turn radius 49 feet dont always make Figure 1 rule book illustrates several desired maneuvers Figure 1 AMA Competition Regula tions criteria square corners control line stunt maneuvers Plugging some numbers can relate g forces speed radius curvature some typical model maneuvers Table 1 shows results TABLE 1 RC MODELS Pullout Loop RadiusSpeed feetmph g 20303 205084 206012 508597 Pylon turniao125105 CONTROL LINE STUNT Maneuver5 Line tension525 60 70 CONTROL LINE SPEED Line tension70 50336 5032 5028 5024 150216 44 Model Aviation radius corners \ lev 2 loops 4-nd77Th should interpret numbers shown Table 1 Sig Cadet weighing 45 lb pulling Out dive 50 mph 20-ft radius will experience maximum force G 94g 84 due pullout plus lg normal its apparent weight will 423 lb An increase speed 60mph would increase G 585 lb An eight-lb pattern ship doing loop SOft radius 85 mph would have apparent weight 856 lb bottom maneuver Figure 2 shows way vertical forces act airplane control line stunt ship executing 49-foot-radius corners shown rule book would pull 336g Thus three-pound model would have apparent weight about 100 lb control line models also matter line tension consequence model flying horizontal circular path restrained control lines control line model flying 50 mph 525-ft lines would experience 32g horizontally model weighing three pounds would experience 96 lb line tension Speed model weighing three pounds flying 150 mph 70-ft lines would exert about 65 lb line tension figures lead several important considerations involving both structural ove Figure 2A bottom pullout plane experiences g force caused pullout plus weight plane Above right Figure 2B top push-over plane experiences g force less planes weight g force ane appears weightless g force greater 1 plane experiences -Gs Right Figure 2C During loop plane experiences G force sum W g Direction W always down g always outward actual flight planes speed changes throughout loop would constant perfect maneuver strength aerodynamic behavior Obviously wing structure particularly center sectionas well other parts airframemust tolerate loads wish avoid in-flight breakup control mechanism control line speed ships must very sturdy capable functioning smoothly under load addition stresses imposed upon airframe aerodynamic behavior airplane may also drastically affected centrifugal forces because increase g has effect increasing weight airplane often very significant amount affects behavior airplane turns well its stall speed BEHAVIOR IN TURNS important aspect airplane hehavior related centrifugal force angle hank turn coordinated turn bank angle will represent resultant vertical force gravity horizontal centrifugal force resulting turn coordinated turn will no skidding slipping real airplane ball turn bank indicator would remain centered full-scale airplanes particularly some designs Golden Age 20s 30s skidding unintentional slippingespecially close ground carefully avoided Both maneuvers greatly increase drag airplane may also encourage pilot cross controlsthat apply opposite aileron rudderwhich could lead spin Wq4fg La low speeds Modem airplanes tolerant its still mark good airmanship keep ball centered model airplanes dont have ball keep centered often usually make turns control ailerons airplanes equipped rudder airplanes ailerons aileron-less airplanes dihedral wings induces bank response rudder-induced skid resulting turn probably pretty well coordinated dynamics aileron turn bit different ailerons establish bank angle both cases bank angle induces horizontal component lift causes airplane turn poorly coordinated turn bank angle may induce slip rudder may cause skid Either can cause increased drag loss altitude may compensated additional lift either added angle attack up elevator added power both An important consequence poorly coordinated turns model airplanes loss altitude Most model pilots dont bother use rudder turns giant scale exception relying inherent roll stability models resist skids important aspect turns g forces consequent loading airplane particularly tight turns turn executed speed turn radius such centrifugal force g bank angle will 450 airplane will experience 14g turn results centrifugal force 1 73g bank angle will 600 airplane will experience 2g steeper bank angle greater g experienced airplane Figure 3 illustrates interested mathematical relationship follows Angle bank degrees atan g mph2 x 0672 r #-, V hOdVV,gThVVC Figure 3 turn centrifugal force g directed outward weight W directed downward Resultant bank angle July1993 45 ImRo pwc$16000 ALL KITS FEATURE BAND LAID FIBERGLASS FUSELAGEPANEL LINES SCALE DETAILSFOAM CORE WINGS MACHINE CUE FORMERS & BULKHEAD& SEE YOUR LOCAL DFALER OR CALL HOBBY 1L4NGAR 600-283-5746 FOR IMAiEDUTE DELIVERY DEALER INQUIRIFS WELCOME WEACCEPT 14 MASTERCARD & DISCOVER CARDS WEGLADLYSHY COD SHIPPING & H4NDLJNG $500 PER KIT COm1NFNTAL US4 OIlIER AREAS CALL KEN7VCKYRESIDENIS ADD 6% S4LES TAX HOBBY HANGAR ISA FULL SER VI CE HOBBY SHOP WITH OVER 11000 ITEMS IN STOCI4 CATALOGS AVAILABLE $400 EACH WIREBATE ON FIRST ORDER SPECIFY PLANEHELICOPTER OR CAR HANGAR EXaLcsIVEaTRwToRCOMING R SC4LE GIANT SCALE ME-i 09 606-28$-5889 FAX Consider what would happen airplane flying straight level flight weight suddenly increase factor two Obviously wing must work harder support additional weightthat coefficient lift must increase will happen consequence increase angle attack Induced drag also increases square coefficient lift means apparent weight airplane doubles coefficient lift must also double induced drag will increased four Thus its evident airplane will either slow down lose altitude both unless increase power think us have seen happen flying experience sometimes unfortunate results STALL SPEED other aerodynamic concern involves relation stall speed wing loading Stalling generally thought consequence airplanes flying too slowly will certainly result stall also possible stall airplane high speed often exciting results general stall speed proportional VszzkIWl/Clrn stall speed mph Wl loading ozIsq ft Clm maximum coefficient lift typically 12-15 k a constant relating air density Reynolds number typicaily about 5 Values Clm k typical RC models Since wing loading proportional apparent weight airplane turn proportional g force airplane sees follows stall speed will proportion square root g Thus airplane 600-square-inch wing weighing five pounds will have stall speed level flight 1 g around 20 mph airplane experiences 9g maneuver wing loading will increase factor nine stall speed will increased 60 mpha factor three square root nine important factor specifying maneuvering speeds full-scale airplanes structural limit airplane 9g maximum speed airplane could maneuvered no chance exceeding limit would three times stall speed Now lets think about moment trainer may have stall speed 15-20 mph pattern airplane 25 mph pylon racer 30 mph Consider Sig Cadet stall speed 15 mph diving 50 mph pulling out 20-foot radius bottom pullout airplane experiences 94g square root 94 307 stall speed 307 x 20 mphfaster airplane going Thus stalls may fall off spin called departurea particularly appropriate term happens close ground Another example tight turn can pylon racer stall speed 30 mph make 125 mph stalling departing Answer 605 feet Assuming stall speed 30 mph g 173g will give stall speed 125 mph 125 mph 173g will result turn radius 605 feet have watched pylon races have probably noticed squirrelly airplanes sometimes act turns consider fairly sharp leading edges wings racers surprising course drastic increase drag results high wing loadings means wider turns may result better lap speeds PRACTICAL RESULTS Structural integrity generally taken care airplane designed built Substantial construction wing center section very important Dihedral braces should made good hard aircraft plywood fiberglass reinforcement good insurance Shear webs spar structure good idea may seem silly mention adequate wing attachment helps means firm anchors wing bolts sturdy dowels rubber bands used course fresh rubber bands High g maneuvers low speeds can come close home really biteparticularly beginners flying sport airplane slightly windy day maybe 15 mph its right down runway too worried preparing land airplane has stall speed 15 mph usually fly downwind leg 25 mph slow down 20 mph over fence final Continued page 51 46 Model Aviation E WULF 190 Made USA SPECIFICATIONS WINGSPAN 65 ENGINE SIZE60-SW 2-SIROKE 5W-I20 4-SIROKE WEIGHT9-10 LBS I- SPECIFICATIONS WING SPAN 60 ENOlNE 560 2-SIROKE WEIGHT8-85 LBS CESSNA 150 SPECIFICATIONS WING SP4N 65 ENGINE SIZE WEIGIfl Made USA 45-60 2S7ROKE 7-75 LBS square root wing loading following formula shows relationship SPECIFICATlONS WING SPAN 60 ENGINE SIZE60 2-SIROKE WEIGHT7-8 LBS Centrifugal ForcelAbbott Continued page 46 Today fly downwind try slow down some because airplane seems have rather high ground speed 25 40 mph Still airplane seems pretty fast turn down wind final turn pretty tight induces additional g load Remember discussion above additional g load increases drag may slow airplane Now must deal combination two higher stall speed caused higher wing loading reduction airspeed caused increased drag also resulting higher wing loading result may very high sink rate ground spectacular snap ground Ever seen happen way lot mystique surrounds infamous downwind turn can traced steep turns speed reductions pilot executes attempting compensate wind drift maneuvering land 55 igh-speed stall ways set up Nil airplane make less susceptible control loss resulting abrupt maneuvers first obvious very often ignored way proper balance airplane tailheavy will have poor longitudinal stability making very pitch sensitive will make prone abrupt maneuvers may generate high g forces consequent highspeed stalls airplane already somewhat unstable may difficult recover safely Another adjustment can optimized elevator travel should possible reasonable speed apply full elevator entering high-speed stall emergency pullout close ground dont need airplane going crazy because pulled stick way back Control line tension control line models essential line tension maintained times Control line tension result two factors centrifugal force aerodynamics airplane such rudder offset leadout position asymmetrical wing span Line tension usually problem except flying sport stunt models strong winds long lines sometimes concern about flying maneuvers directly overhead Table 1 see 60-ft lines have 28g because centrifugal force directly overhead line tension would actually 1 8g because downward force gravity should still enough Regardless maneuver being performed centrifugal force dependent tangential velocity brings us question effect wind line tension Two factors work reduced speed airplane flies upwind re duces centrifugal force other aerodynamic force crosswind component counteracts centrifugal force upwind side circle Fortunately speed reduction greatest heading wind aerodynamic force greatest flying crosswind two dont occur same time can trouble flying overhead directly wind now speed reduced line tension reduced g As plane goes over top begins descend winds aerodynamic forces tend further reduce line tension Lets look few numbers 60-ft lines airspeed 50 mph see table line tension caused centrifugal force 28g 15-mph wind ground speed upwind will 35 mph formula gives result 137g Thus overhead line tension due centrifugal force will 037g some line tension caused aerody namics airplane would very comforting indeed CONCLUSION Model airplanes generally fly scale speeds often maneuvered violently Consequently often subjected flight loads far excess encountered full-scale counterparts Fortunately also usually much stronger size successful models combine great strength needed like wing center sections light weight tolerant aerodynamics proper balance conservative trim adjustment Pilot skill little good luck help too also safety considerations An airplane tends go out control may end up pits midst spectators An airplane has suffered in-flight structural failure can also go out control may pose serious safety hazard LASSES 195 i$2S&H 100% UV Protection Shatterproof Excellent Optical Quality Tints Fit Over Prescriptions S KPO BOX 1000 ID UMFRIES VA 22026 800-221-9318 July 1993 51 O Volt12AH$1200 12 Volt65AN$1500 VISA MASTERCARD DISCOVER $400 SHIPPING & HANDLING 5AI OR Fl RESIDENTS
Edition: Model Aviation - 1993/07
Page Numbers: 44, 45, 46, 51
Cen trffuga Force U George Abbott What reasonable numbers model airplane purposes Typical radio control sport airplanes fly 35 55 mph pattern airplanes 50 85 mph pylon racers 125 mph turn radius order 10 feet sharp pullout 100 feet big loop 50 100 feet pylon turn estimating size maneuvers just remember paved RC runways about 300 feet long 30 feet wide residential utility poles 18 30 feet high F THE SEVERAL FORCES act aircraft flight centrifugal force W important will help us both design flight understand force object traveling along curved path will experience force toward outside curve childhood example tying rock string whirling string around head force rock experiences called centrifugalforce very important factor behavior airplanes Whenever airplane changes direction centrifugal force causes apparent weight airplane change example during pullout dive apparent weight airplane will increase airplane pushes over level flight apparent weight will decrease term commonly used describe forces gforce force g equivalent force resulting acceleration earths gravity generally accepted 32 ft/sec/sec An object acted upon force gravity will exert force equal its mass multiplied g force called weight expressed pounds English system units accordance Newtons first law FMxa Weight mass related acceleration gravity follows WMxg M pounds slugs W ht pounds leration ft/sec/sec g acceleration gravity 32 ft/sec/sec its clear mass body its weight divided gthe acceleration gravity Thus object having weight 64 pounds lb would have mass two pounds lbm express acceleration terms g always get forces terms weight avoids lot confusion hope Centrifugal force results accel eration caused moving objects change direction acting upon mass object formula acceleration V2 r leration ft/sec/sec V velocity ft/sec r us curve feet can combine formula Newtons first law little arithmetic get formula will accept speed miles per hour provide result force g mph2 x 0672 r g e multiples weight mph d miles per hour r us curve feet Notice speed squared formulae centrifugal force will always directed outward center curving path total force acting object will vector sum resultant forces paper will use following terminology W ht pounds g centrifugal force multiples W G l force multiples W Thus object W 10 pounds subjected 3g would experience total force G 30 pounds further simplicity will normalize weight W pound everything comes out terms g Control line airplanes subject centrifugal force circular flight primary factor maintaining line tension models may also employ such aerodynamic means outside rudder ensure line tension low speeds overhead flying wind Speed models dont need help Control line models fly speeds 35 55 mph sport stunt models up around 100 mph combat carrier models well over 100 mph Speed models AMA Competition Regulations specify perfect control line stunt maneuvers will have turn radius 49 feet dont always make Figure 1 rule book illustrates several desired maneuvers Figure 1 AMA Competition Regula tions criteria square corners control line stunt maneuvers Plugging some numbers can relate g forces speed radius curvature some typical model maneuvers Table 1 shows results TABLE 1 RC MODELS Pullout Loop RadiusSpeed feetmph g 20303 205084 206012 508597 Pylon turniao125105 CONTROL LINE STUNT Maneuver5 Line tension525 60 70 CONTROL LINE SPEED Line tension70 50336 5032 5028 5024 150216 44 Model Aviation radius corners \ lev 2 loops 4-nd77Th should interpret numbers shown Table 1 Sig Cadet weighing 45 lb pulling Out dive 50 mph 20-ft radius will experience maximum force G 94g 84 due pullout plus lg normal its apparent weight will 423 lb An increase speed 60mph would increase G 585 lb An eight-lb pattern ship doing loop SOft radius 85 mph would have apparent weight 856 lb bottom maneuver Figure 2 shows way vertical forces act airplane control line stunt ship executing 49-foot-radius corners shown rule book would pull 336g Thus three-pound model would have apparent weight about 100 lb control line models also matter line tension consequence model flying horizontal circular path restrained control lines control line model flying 50 mph 525-ft lines would experience 32g horizontally model weighing three pounds would experience 96 lb line tension Speed model weighing three pounds flying 150 mph 70-ft lines would exert about 65 lb line tension figures lead several important considerations involving both structural ove Figure 2A bottom pullout plane experiences g force caused pullout plus weight plane Above right Figure 2B top push-over plane experiences g force less planes weight g force ane appears weightless g force greater 1 plane experiences -Gs Right Figure 2C During loop plane experiences G force sum W g Direction W always down g always outward actual flight planes speed changes throughout loop would constant perfect maneuver strength aerodynamic behavior Obviously wing structure particularly center sectionas well other parts airframemust tolerate loads wish avoid in-flight breakup control mechanism control line speed ships must very sturdy capable functioning smoothly under load addition stresses imposed upon airframe aerodynamic behavior airplane may also drastically affected centrifugal forces because increase g has effect increasing weight airplane often very significant amount affects behavior airplane turns well its stall speed BEHAVIOR IN TURNS important aspect airplane hehavior related centrifugal force angle hank turn coordinated turn bank angle will represent resultant vertical force gravity horizontal centrifugal force resulting turn coordinated turn will no skidding slipping real airplane ball turn bank indicator would remain centered full-scale airplanes particularly some designs Golden Age 20s 30s skidding unintentional slippingespecially close ground carefully avoided Both maneuvers greatly increase drag airplane may also encourage pilot cross controlsthat apply opposite aileron rudderwhich could lead spin Wq4fg La low speeds Modem airplanes tolerant its still mark good airmanship keep ball centered model airplanes dont have ball keep centered often usually make turns control ailerons airplanes equipped rudder airplanes ailerons aileron-less airplanes dihedral wings induces bank response rudder-induced skid resulting turn probably pretty well coordinated dynamics aileron turn bit different ailerons establish bank angle both cases bank angle induces horizontal component lift causes airplane turn poorly coordinated turn bank angle may induce slip rudder may cause skid Either can cause increased drag loss altitude may compensated additional lift either added angle attack up elevator added power both An important consequence poorly coordinated turns model airplanes loss altitude Most model pilots dont bother use rudder turns giant scale exception relying inherent roll stability models resist skids important aspect turns g forces consequent loading airplane particularly tight turns turn executed speed turn radius such centrifugal force g bank angle will 450 airplane will experience 14g turn results centrifugal force 1 73g bank angle will 600 airplane will experience 2g steeper bank angle greater g experienced airplane Figure 3 illustrates interested mathematical relationship follows Angle bank degrees atan g mph2 x 0672 r #-, V hOdVV,gThVVC Figure 3 turn centrifugal force g directed outward weight W directed downward Resultant bank angle July1993 45 ImRo pwc$16000 ALL KITS FEATURE BAND LAID FIBERGLASS FUSELAGEPANEL LINES SCALE DETAILSFOAM CORE WINGS MACHINE CUE FORMERS & BULKHEAD& SEE YOUR LOCAL DFALER OR CALL HOBBY 1L4NGAR 600-283-5746 FOR IMAiEDUTE DELIVERY DEALER INQUIRIFS WELCOME WEACCEPT 14 MASTERCARD & DISCOVER CARDS WEGLADLYSHY COD SHIPPING & H4NDLJNG $500 PER KIT COm1NFNTAL US4 OIlIER AREAS CALL KEN7VCKYRESIDENIS ADD 6% S4LES TAX HOBBY HANGAR ISA FULL SER VI CE HOBBY SHOP WITH OVER 11000 ITEMS IN STOCI4 CATALOGS AVAILABLE $400 EACH WIREBATE ON FIRST ORDER SPECIFY PLANEHELICOPTER OR CAR HANGAR EXaLcsIVEaTRwToRCOMING R SC4LE GIANT SCALE ME-i 09 606-28$-5889 FAX Consider what would happen airplane flying straight level flight weight suddenly increase factor two Obviously wing must work harder support additional weightthat coefficient lift must increase will happen consequence increase angle attack Induced drag also increases square coefficient lift means apparent weight airplane doubles coefficient lift must also double induced drag will increased four Thus its evident airplane will either slow down lose altitude both unless increase power think us have seen happen flying experience sometimes unfortunate results STALL SPEED other aerodynamic concern involves relation stall speed wing loading Stalling generally thought consequence airplanes flying too slowly will certainly result stall also possible stall airplane high speed often exciting results general stall speed proportional VszzkIWl/Clrn stall speed mph Wl loading ozIsq ft Clm maximum coefficient lift typically 12-15 k a constant relating air density Reynolds number typicaily about 5 Values Clm k typical RC models Since wing loading proportional apparent weight airplane turn proportional g force airplane sees follows stall speed will proportion square root g Thus airplane 600-square-inch wing weighing five pounds will have stall speed level flight 1 g around 20 mph airplane experiences 9g maneuver wing loading will increase factor nine stall speed will increased 60 mpha factor three square root nine important factor specifying maneuvering speeds full-scale airplanes structural limit airplane 9g maximum speed airplane could maneuvered no chance exceeding limit would three times stall speed Now lets think about moment trainer may have stall speed 15-20 mph pattern airplane 25 mph pylon racer 30 mph Consider Sig Cadet stall speed 15 mph diving 50 mph pulling out 20-foot radius bottom pullout airplane experiences 94g square root 94 307 stall speed 307 x 20 mphfaster airplane going Thus stalls may fall off spin called departurea particularly appropriate term happens close ground Another example tight turn can pylon racer stall speed 30 mph make 125 mph stalling departing Answer 605 feet Assuming stall speed 30 mph g 173g will give stall speed 125 mph 125 mph 173g will result turn radius 605 feet have watched pylon races have probably noticed squirrelly airplanes sometimes act turns consider fairly sharp leading edges wings racers surprising course drastic increase drag results high wing loadings means wider turns may result better lap speeds PRACTICAL RESULTS Structural integrity generally taken care airplane designed built Substantial construction wing center section very important Dihedral braces should made good hard aircraft plywood fiberglass reinforcement good insurance Shear webs spar structure good idea may seem silly mention adequate wing attachment helps means firm anchors wing bolts sturdy dowels rubber bands used course fresh rubber bands High g maneuvers low speeds can come close home really biteparticularly beginners flying sport airplane slightly windy day maybe 15 mph its right down runway too worried preparing land airplane has stall speed 15 mph usually fly downwind leg 25 mph slow down 20 mph over fence final Continued page 51 46 Model Aviation E WULF 190 Made USA SPECIFICATIONS WINGSPAN 65 ENGINE SIZE60-SW 2-SIROKE 5W-I20 4-SIROKE WEIGHT9-10 LBS I- SPECIFICATIONS WING SPAN 60 ENOlNE 560 2-SIROKE WEIGHT8-85 LBS CESSNA 150 SPECIFICATIONS WING SP4N 65 ENGINE SIZE WEIGIfl Made USA 45-60 2S7ROKE 7-75 LBS square root wing loading following formula shows relationship SPECIFICATlONS WING SPAN 60 ENGINE SIZE60 2-SIROKE WEIGHT7-8 LBS Centrifugal ForcelAbbott Continued page 46 Today fly downwind try slow down some because airplane seems have rather high ground speed 25 40 mph Still airplane seems pretty fast turn down wind final turn pretty tight induces additional g load Remember discussion above additional g load increases drag may slow airplane Now must deal combination two higher stall speed caused higher wing loading reduction airspeed caused increased drag also resulting higher wing loading result may very high sink rate ground spectacular snap ground Ever seen happen way lot mystique surrounds infamous downwind turn can traced steep turns speed reductions pilot executes attempting compensate wind drift maneuvering land 55 igh-speed stall ways set up Nil airplane make less susceptible control loss resulting abrupt maneuvers first obvious very often ignored way proper balance airplane tailheavy will have poor longitudinal stability making very pitch sensitive will make prone abrupt maneuvers may generate high g forces consequent highspeed stalls airplane already somewhat unstable may difficult recover safely Another adjustment can optimized elevator travel should possible reasonable speed apply full elevator entering high-speed stall emergency pullout close ground dont need airplane going crazy because pulled stick way back Control line tension control line models essential line tension maintained times Control line tension result two factors centrifugal force aerodynamics airplane such rudder offset leadout position asymmetrical wing span Line tension usually problem except flying sport stunt models strong winds long lines sometimes concern about flying maneuvers directly overhead Table 1 see 60-ft lines have 28g because centrifugal force directly overhead line tension would actually 1 8g because downward force gravity should still enough Regardless maneuver being performed centrifugal force dependent tangential velocity brings us question effect wind line tension Two factors work reduced speed airplane flies upwind re duces centrifugal force other aerodynamic force crosswind component counteracts centrifugal force upwind side circle Fortunately speed reduction greatest heading wind aerodynamic force greatest flying crosswind two dont occur same time can trouble flying overhead directly wind now speed reduced line tension reduced g As plane goes over top begins descend winds aerodynamic forces tend further reduce line tension Lets look few numbers 60-ft lines airspeed 50 mph see table line tension caused centrifugal force 28g 15-mph wind ground speed upwind will 35 mph formula gives result 137g Thus overhead line tension due centrifugal force will 037g some line tension caused aerody namics airplane would very comforting indeed CONCLUSION Model airplanes generally fly scale speeds often maneuvered violently Consequently often subjected flight loads far excess encountered full-scale counterparts Fortunately also usually much stronger size successful models combine great strength needed like wing center sections light weight tolerant aerodynamics proper balance conservative trim adjustment Pilot skill little good luck help too also safety considerations An airplane tends go out control may end up pits midst spectators An airplane has suffered in-flight structural failure can also go out control may pose serious safety hazard LASSES 195 i$2S&H 100% UV Protection Shatterproof Excellent Optical Quality Tints Fit Over Prescriptions S KPO BOX 1000 ID UMFRIES VA 22026 800-221-9318 July 1993 51 O Volt12AH$1200 12 Volt65AN$1500 VISA MASTERCARD DISCOVER $400 SHIPPING & HANDLING 5AI OR Fl RESIDENTS
