ON THE plans accompanying model kits sometimes shown two CG locations ex perienced fliers half inch forward labeled Beginners can infer CG location Balance Point must some importance flying aft CG requires greater flying skilL true aft CG location exper ienced flier can perform spine tingling spins snap rolls will whereas beginner like myself will also perform spine tingling spins etc unin tentionally fact CG moved further further aft expert will encounter increasing difficulties until finally airplane becomes unstable extremely difficult dangerous fly will have become sensitive little no provocation will embark adventurous maneuvers its own usually disastrous results other hand moving CG progressively forward increases stability improves socalled penetration move forward airplane becomes less less sensitive elevator control until elevator power inade quate pull nose up sufficiently effect slow safe landing evident safe sensible CG range must lie between two extremes conjunction relative incidence tween wing horizontal tail fore andaft position CG powerful single factor achieving Static Longitudinal Stability therefore something should under stood anyone flying kind aircraft large small So lets look three aspects matter First What determines acceptable CG limits Second physically limits Third During design construction phase large -scale model some way figure place things CG will wind up its supposed article will discuss first item subsequent issues will discuss items two three Any discussion CG location must necessarily involve discussion Stability Stability definition tendency return original set conditions disturbed Fig 1 shown three cones Cone resting firmly its base something tried tip over will tend return its original position long dis turbance isnt great knock offthe table condition Stable B cone lying its side disturbed will continue remain its side will roll around new position making no effort return original condition maybe said Neutral condition corrective action must taken C cone shown unlikely attitude balancing its point Obviously slightest disturbance will cause fall down no hope restoration original position Unstable condition An airplane can turn roll pitch can perform maneuver singly combination others ie climbing turn rolling dive etc must have stability about three axes other words should direction ally stable laterally stable longitudinally stable latter longitudinal stability about lateral axis ability recover inad vertent changes pitch attitude will concern ourselves because longitudinal stability almost entirely dependent CG location Stability about lateral axis two kinds Static Dynamic airplane longitudi nally stable will tend automatically re establish its original Speed Attitude dis EUJI] ULIL 4 4 Determining well RC model flies controllable alL bear pilot CGs proper location important aerodynamic scheme things heart brain human body Reducing complex matter everyman 5 language author deals subject three installments What deter mines CG limits month Physical location limits Placement things CG winds up supposed turbed Thus encounters gust tending initiate stall causing nose up slow down instead stalling will drop its nose recover speeding up assuming its original flight attitude does quickly smoothly both Statically Dynamically Stable recovers has increasing difficulty re establishing its original attitude because overshooting hunting said Dynamically Unstable will concern ourselves Static Longitudinal Stability As said model Longitudinally Stable recovers impending stall dive itself Neutrally Stable disturbed will require prompt corrective control part prevent stall dive will indeed require attention during its flight stay groove speak other hand will very maneuverable undoubted ly liking skilled pattern flier model Unstable will very difficult keep under control sudden disturbance can cause stall before can much prevent Further its sensitivity will engender highG maneuvers vertical-banked tight turns etc might cause structural failure Flight maintained forward surface inclined greater angle relative wind rear surface wing produces lift rear surface trims holds its proper angie attack angular difference between two surfaces sometimes called decalage French word roughly trans lates mean wedgeshaped implying angular difference necessary also forward surface fly higher angle standpoint stability will stall first thus contributing recovery true types aircraft tailless conventional canard anything else conventional tailless configurations forward surface trimmed aft surface horizontal tail conventional elevon tailless case canard other way around rear surface provides lift trimmed smaller forward surface nevertheless being forward surface must fly higher angie attack main wing rear too will stall first However angular difference wing tail settings does itself insure stable flight must always proper relationship between forces play tendency effect recovery dive stall will always present Webruer von Flintstone research devel opment engineer lived aboutthirty thousand years ago operated out cave high foothills Cantabrian Cordillera discov ered stick free body thrown would rotate about its balance point CG After years principle still holds good can apply Fig 2 weight constant acts CG Lift ries angle attack arm behind CG tail force T also varies arm t CG stable airplane forces act together produce moments will cause airplane recover inadvertent maneuver might due gust wind such encountered passing through thermal recovery must accomplished automatically no change tail setting So lets see can figure out might done moment product force times distance sometimes area times distance Thus large tail will produce larger moment will small tail same arm con versely longer arm small tail can produce Brad PowersApril 1980 19 Fig 1 / / C N V NEUTRALUNSTABLE moment equal large tail shorter arm etc Lets call stalling nose-up moments posi tive anddiving nose-down moments negative take another look Fig 2 free flight glider lets say no means corrective control part see behaves move CG back forth Lift L times its arm produces moment L about CG tail also produces its momentTt about CG maintain level flight moments must balance other dont airplane will MACs 12 provide reader visual frame reference series begins have shown climactic drawing part three months copy course does deal specific detail various items called out drawing youll get drift things nose up stall nose down dive So sustain level flight constant speed Ll equals Tt Ll-Tt must equal zero say another way sum moments must equal zero Fig 2 have things arranged CG forward Lift Lets see works OK stalls dives Ifwe try stall first Fig 3 see Lift has greatly increased due large angle attack thus producing much greater negative moment did level flight tending push nose back down because attitude glider now pointing up horizontal tail forced upward relative wind also produce negative diving moment thus contributing further recovery So far good Now lets try dive nose drops about 3 degrees wing angle zero Lift thus producing no moment about CG tail still produces positive moment tending push nose back up belongs shown Fig 4 Thus see having CG forward Lift produces stable arrangement providing recovery both stalls dives point might well put information down graph Fig 5 Lets say glider maintains level flight wing 3 degrees angle attack Since said level flight means zero moment represented point intersec tion zero moment 3 degrees angle attack approaching stall condition lets say 9 degrees angle attack moments negative ones condition shown point B 20 Model Aviation STABLE dive condition zero angle attack accompanied positive moment due tail condition shown point C Thus see thatwith CG forward Lift have stable configuration plot moment versus angle attack produces curve having negative slope other words angle attack increases moments must become more negative acting sort like spring restore equilibrium Now just sure lets try other extreme place CG behind Lift shown Fig 6 again dive assumed zero lift angle attack zero will no moment other tail condition will duplicate first dive condition will also lie point C chart level flight condition however shown Fig 7 moments dont reduce zero become increasingly positive lift wing has increased tail setting still such produce its positive moment both conditions tending raise nose placing level flight condition now appears momentary best point chart Fig 8 relative wind tail will produce negative moment will insuffi cient overcome large amount positive moment produced wing high angle attack glider will stalL condition will lie point Boo chart see having CG behind Lift Unstable produces curve shown dashed line curve has positive slope denotes Instability Neutral Stability occurs CG Lift coincide acting pure opposition thus producing no moment thus very sensitive control lack will demand constant attention condition analogous dive conditions wing produced no moment about CG because zero Lift moment present due decalage producing positive moment So dive condition neutral stability will also plotted C since moment tail will remain unchanged Neutral Stability curve simply horizontal line originating C As can seen line demarcation between stability instability Thus clear stability simply means placing CG forward lift Before going further better pin down just what meant Lift hold model wing having symmetrical airfoil between fingertips plac ing finger 25% chord wing tip could run fast enough would feel wing develop some lift still maintain its attitude placed fingers forward aft 25% wingwould tend rotate shows place along chord symmetrical airfoil no moment moments along chord balance point called Aerodynamic Center AC 25% chord airfoils cambered sections 25% point also AC point constan4 rather zero moment wing alone Lift may considered act through point designing airplane having wing CG would want placed forward 25% point stability Disregarding elevon However elevon glider tail present something equivalent AC representative airplane whole must considered point forward CG must lie point called Neutral Point N its position determined r w L April 1980 21 6 Angle Attack0 total moment produced factors capable affecting trim airplane factors include destabilizing effects power thrust line below CG tending push nose up front end fuselage nacelles horizontal stabilizer f V ab ~neu ral B K 9 etc Factors tend produce positive moments raise nose destabilizing tend move N forward Factors such flap deflection fixed landing gear floats other items producing negative moments stabiliz ing tend move N aft 30 determination N laborious process involves detailed information about specific design can treat very general way proper procedure evaluate add together slopes ofthe moment curves various items mentioned above ascertain total effect Since cant attempt will treat matter simplistic yet valid believe manner determining N simple problem Statics purposes explanation assume condition Neutral Point will lie resultant wing tail regarding weighted areas will have value proportional its area times its unit loading can take moments find position N Fig 9 shown plan view wing/tail configuration glider said decalage 3 degrees assume airplane attitude tail 1 degree wing 4 degrees lift varies directly angle attack does wing will have loading of4 unitsounces grams etc per sq tail will have loading unit Thus Fig 9 wing will have weighted area 600 X 4 2400 units tail will have 100 X 1 100 units So taking moments about AC wing Moment Wing 2400 X 0 equals 0 Moment Tail 100 X 30 equals 3000 - 2500 12 3000 arm Neutral Point 3000/2500 12 inches aft AC wing places 25 plus 12 inches 37 inches aft leading edge wing Since wing 1 2 has chord oflO inches amounts 37% Chord point presupposing up-load tail aft position N destabilizing factors present will act move point forward Lets look Fig 2 again see exist horizontal tail will now produce positive moment addition have generous nose section forward wing lies induced upflow ahead wing will tend produce stalling positive moment particularly high angles attack Also drag vertical fin will produce moment Fig 9 tending push tail downthe nose up also destabilizing Without attempting estab lish values said laborious lets assume effect will such move Neutral Point forward 4% up 33% chord give ourselves stability margin 5% forward CG will lie 28% chord glider tail larger tail arm longer Neutral Point will further aft will thereby permit further aft CG example some freeflight powered models seen CG locations way back near trailing edge wing come decalage reduced tail made very large tail arm very long maybe lifting airfoil tailwhich normally no value fuselage very slender nose very short maybe inverted fin possible push Neutral Point aft far enough have aft trailing edge thus obtain stability CG trailing edge wing such arrangement can get pretty touchy demand considerable fine tuning no control available Now lets fatten up fuselage glider put engine keepingthe same wing tail shown Fig 10 addition destabilizing effects glider have some new ones L 25 Continued page 101 22 Model Aviation Fig5 E 0 sta N 0 3 5 N -I10 AT LAST SOME Primer U ndercoeter Nowin Weve taken popular two-part white primer-undercoat formula made gray Just gray enough please have told us want see dents nicks ripples youre sanding Now make little visible just efficient ever course its still available white too Wood Plastic Metal Fiberglass Hobbypoxy primer-undercoater covers well cures fills fast shrinking easy sand provides smooth-as-glass surface. great adhesion wood plastic metal fiberglass Whether youre balsa builder molded plastic buff whether brush spray it. can now put little color life Hobbypoxy gray undercoat. good ever lot visible complete information how-to-do-it Hobbypoxy Finishing Guide write today ARNIIHAUIIvision Pettit Paint Co Inc H UDUIrUAT reooncis Street Rockaway NJ 07866 HOBBYPOXY ct group finishing products especially formulated hobbyists including Epoxy Glues Epoxy-enamel Paints Fillers Primer-undercoaters Resins R2 hole outside lug potentiometer opposite side want 1 seconds/stroke thing found testing servos column new servos arent necessarily fast advertised Some some arent what work theyre fast enough Log values find new look changes fact have changed information really want anyway Keep letters coming George M Myers 70 Froehlich Farm Rd Hicks yule NY 11801 About CG/Powers Continued page 22 I The engine thrust below CG will produce nose-up moment destabilizing tending move N forward 2 The propeller slip stream increases downwash over tail loading up pushing tail down nose up again destabilizing pushes N further ahead 3 The fatter fuselage presenting greater area upflow ahead wingmore instability N moves forward again 4 The fixed landing gear stabilizing tends pull nose down cancel effects other items some extent net result scale models similar basic configuration Neutral Point usually winds up very close 30% chord wing Thus good safe comfortable CG location about 5% forward 30% Neutral Point 25% chord So now understand CG forward beginners can further aft experts As matter fact have several models balanced approximately 25% tickles see little birds recover nicely stall generally forgiving mistakes controlling fliers dont seem pay much attention CG Location just balance airplane about third way back put seem fine because pretty good fliers would guess may struggling against some stability realizing way sure course test fly model varying CG locations Move CG forward little bit time until its hard get nose up can slow down landing move CG aft small increments until becomes sensitive skittish Ease back little both extremes let limits beginners CG location 25% chord good place start have expressing CG location percent chord because have used constant chord rectangular wing example airplanes made way course Some have tapered wings some have elliptical wings some have triangular wings some have two three wings what chord next installment will discuss Mean Aerodynamic Chord find configurations just mentioned Questions comments may addressed author care editor continued RC Aerobatics/Van Putte Continued page 23 thrust two propellers will deliver compar ing load factor props load factor determined multiplying prop pitch cube diameter example 1 1-8 prophas aloadfactorof8 X1 l 8 comparison 11 -7 prop has load factor 7 X1I53 6 According rule thrust developed 11-8 prop should about same 11 -7 prop know small engine will tum large prop So trick using load factor rule determine typical load factor range different engines vary prop characteristics get desired performance What follows table engine sizes versus load factor ranges flier should find displacement engine question read across load factor range engine table presented included article B J Rich Richburg Central Virginia Radio Control Association Virginia Newsletter Engine Displacement 05 10 15 20 25 30 35 40 45 50 60 80 Load Factor Range 400- 680 900- 1500 1500- 2500 2000- 3000 2600- 3650 3350- 4600 4000- 5500 4600- 6200 5200- 7000 6000- 8400 7600- 10800 9900-14000 flier should rate engine compared others same displacement new Schnuerle ported ringed piston engine will able swing higher load factor prop old boostported lapped piston engine ofthe same displace ment Pick out load factor corresponding April 1980 101 TTER FROM HOBBYPOXY
Edition: Model Aviation - 1980/04
Page Numbers: 19, 20, 21, 22, 101
ON THE plans accompanying model kits sometimes shown two CG locations ex perienced fliers half inch forward labeled Beginners can infer CG location Balance Point must some importance flying aft CG requires greater flying skilL true aft CG location exper ienced flier can perform spine tingling spins snap rolls will whereas beginner like myself will also perform spine tingling spins etc unin tentionally fact CG moved further further aft expert will encounter increasing difficulties until finally airplane becomes unstable extremely difficult dangerous fly will have become sensitive little no provocation will embark adventurous maneuvers its own usually disastrous results other hand moving CG progressively forward increases stability improves socalled penetration move forward airplane becomes less less sensitive elevator control until elevator power inade quate pull nose up sufficiently effect slow safe landing evident safe sensible CG range must lie between two extremes conjunction relative incidence tween wing horizontal tail fore andaft position CG powerful single factor achieving Static Longitudinal Stability therefore something should under stood anyone flying kind aircraft large small So lets look three aspects matter First What determines acceptable CG limits Second physically limits Third During design construction phase large -scale model some way figure place things CG will wind up its supposed article will discuss first item subsequent issues will discuss items two three Any discussion CG location must necessarily involve discussion Stability Stability definition tendency return original set conditions disturbed Fig 1 shown three cones Cone resting firmly its base something tried tip over will tend return its original position long dis turbance isnt great knock offthe table condition Stable B cone lying its side disturbed will continue remain its side will roll around new position making no effort return original condition maybe said Neutral condition corrective action must taken C cone shown unlikely attitude balancing its point Obviously slightest disturbance will cause fall down no hope restoration original position Unstable condition An airplane can turn roll pitch can perform maneuver singly combination others ie climbing turn rolling dive etc must have stability about three axes other words should direction ally stable laterally stable longitudinally stable latter longitudinal stability about lateral axis ability recover inad vertent changes pitch attitude will concern ourselves because longitudinal stability almost entirely dependent CG location Stability about lateral axis two kinds Static Dynamic airplane longitudi nally stable will tend automatically re establish its original Speed Attitude dis EUJI] ULIL 4 4 Determining well RC model flies controllable alL bear pilot CGs proper location important aerodynamic scheme things heart brain human body Reducing complex matter everyman 5 language author deals subject three installments What deter mines CG limits month Physical location limits Placement things CG winds up supposed turbed Thus encounters gust tending initiate stall causing nose up slow down instead stalling will drop its nose recover speeding up assuming its original flight attitude does quickly smoothly both Statically Dynamically Stable recovers has increasing difficulty re establishing its original attitude because overshooting hunting said Dynamically Unstable will concern ourselves Static Longitudinal Stability As said model Longitudinally Stable recovers impending stall dive itself Neutrally Stable disturbed will require prompt corrective control part prevent stall dive will indeed require attention during its flight stay groove speak other hand will very maneuverable undoubted ly liking skilled pattern flier model Unstable will very difficult keep under control sudden disturbance can cause stall before can much prevent Further its sensitivity will engender highG maneuvers vertical-banked tight turns etc might cause structural failure Flight maintained forward surface inclined greater angle relative wind rear surface wing produces lift rear surface trims holds its proper angie attack angular difference between two surfaces sometimes called decalage French word roughly trans lates mean wedgeshaped implying angular difference necessary also forward surface fly higher angle standpoint stability will stall first thus contributing recovery true types aircraft tailless conventional canard anything else conventional tailless configurations forward surface trimmed aft surface horizontal tail conventional elevon tailless case canard other way around rear surface provides lift trimmed smaller forward surface nevertheless being forward surface must fly higher angie attack main wing rear too will stall first However angular difference wing tail settings does itself insure stable flight must always proper relationship between forces play tendency effect recovery dive stall will always present Webruer von Flintstone research devel opment engineer lived aboutthirty thousand years ago operated out cave high foothills Cantabrian Cordillera discov ered stick free body thrown would rotate about its balance point CG After years principle still holds good can apply Fig 2 weight constant acts CG Lift ries angle attack arm behind CG tail force T also varies arm t CG stable airplane forces act together produce moments will cause airplane recover inadvertent maneuver might due gust wind such encountered passing through thermal recovery must accomplished automatically no change tail setting So lets see can figure out might done moment product force times distance sometimes area times distance Thus large tail will produce larger moment will small tail same arm con versely longer arm small tail can produce Brad PowersApril 1980 19 Fig 1 / / C N V NEUTRALUNSTABLE moment equal large tail shorter arm etc Lets call stalling nose-up moments posi tive anddiving nose-down moments negative take another look Fig 2 free flight glider lets say no means corrective control part see behaves move CG back forth Lift L times its arm produces moment L about CG tail also produces its momentTt about CG maintain level flight moments must balance other dont airplane will MACs 12 provide reader visual frame reference series begins have shown climactic drawing part three months copy course does deal specific detail various items called out drawing youll get drift things nose up stall nose down dive So sustain level flight constant speed Ll equals Tt Ll-Tt must equal zero say another way sum moments must equal zero Fig 2 have things arranged CG forward Lift Lets see works OK stalls dives Ifwe try stall first Fig 3 see Lift has greatly increased due large angle attack thus producing much greater negative moment did level flight tending push nose back down because attitude glider now pointing up horizontal tail forced upward relative wind also produce negative diving moment thus contributing further recovery So far good Now lets try dive nose drops about 3 degrees wing angle zero Lift thus producing no moment about CG tail still produces positive moment tending push nose back up belongs shown Fig 4 Thus see having CG forward Lift produces stable arrangement providing recovery both stalls dives point might well put information down graph Fig 5 Lets say glider maintains level flight wing 3 degrees angle attack Since said level flight means zero moment represented point intersec tion zero moment 3 degrees angle attack approaching stall condition lets say 9 degrees angle attack moments negative ones condition shown point B 20 Model Aviation STABLE dive condition zero angle attack accompanied positive moment due tail condition shown point C Thus see thatwith CG forward Lift have stable configuration plot moment versus angle attack produces curve having negative slope other words angle attack increases moments must become more negative acting sort like spring restore equilibrium Now just sure lets try other extreme place CG behind Lift shown Fig 6 again dive assumed zero lift angle attack zero will no moment other tail condition will duplicate first dive condition will also lie point C chart level flight condition however shown Fig 7 moments dont reduce zero become increasingly positive lift wing has increased tail setting still such produce its positive moment both conditions tending raise nose placing level flight condition now appears momentary best point chart Fig 8 relative wind tail will produce negative moment will insuffi cient overcome large amount positive moment produced wing high angle attack glider will stalL condition will lie point Boo chart see having CG behind Lift Unstable produces curve shown dashed line curve has positive slope denotes Instability Neutral Stability occurs CG Lift coincide acting pure opposition thus producing no moment thus very sensitive control lack will demand constant attention condition analogous dive conditions wing produced no moment about CG because zero Lift moment present due decalage producing positive moment So dive condition neutral stability will also plotted C since moment tail will remain unchanged Neutral Stability curve simply horizontal line originating C As can seen line demarcation between stability instability Thus clear stability simply means placing CG forward lift Before going further better pin down just what meant Lift hold model wing having symmetrical airfoil between fingertips plac ing finger 25% chord wing tip could run fast enough would feel wing develop some lift still maintain its attitude placed fingers forward aft 25% wingwould tend rotate shows place along chord symmetrical airfoil no moment moments along chord balance point called Aerodynamic Center AC 25% chord airfoils cambered sections 25% point also AC point constan4 rather zero moment wing alone Lift may considered act through point designing airplane having wing CG would want placed forward 25% point stability Disregarding elevon However elevon glider tail present something equivalent AC representative airplane whole must considered point forward CG must lie point called Neutral Point N its position determined r w L April 1980 21 6 Angle Attack0 total moment produced factors capable affecting trim airplane factors include destabilizing effects power thrust line below CG tending push nose up front end fuselage nacelles horizontal stabilizer f V ab ~neu ral B K 9 etc Factors tend produce positive moments raise nose destabilizing tend move N forward Factors such flap deflection fixed landing gear floats other items producing negative moments stabiliz ing tend move N aft 30 determination N laborious process involves detailed information about specific design can treat very general way proper procedure evaluate add together slopes ofthe moment curves various items mentioned above ascertain total effect Since cant attempt will treat matter simplistic yet valid believe manner determining N simple problem Statics purposes explanation assume condition Neutral Point will lie resultant wing tail regarding weighted areas will have value proportional its area times its unit loading can take moments find position N Fig 9 shown plan view wing/tail configuration glider said decalage 3 degrees assume airplane attitude tail 1 degree wing 4 degrees lift varies directly angle attack does wing will have loading of4 unitsounces grams etc per sq tail will have loading unit Thus Fig 9 wing will have weighted area 600 X 4 2400 units tail will have 100 X 1 100 units So taking moments about AC wing Moment Wing 2400 X 0 equals 0 Moment Tail 100 X 30 equals 3000 - 2500 12 3000 arm Neutral Point 3000/2500 12 inches aft AC wing places 25 plus 12 inches 37 inches aft leading edge wing Since wing 1 2 has chord oflO inches amounts 37% Chord point presupposing up-load tail aft position N destabilizing factors present will act move point forward Lets look Fig 2 again see exist horizontal tail will now produce positive moment addition have generous nose section forward wing lies induced upflow ahead wing will tend produce stalling positive moment particularly high angles attack Also drag vertical fin will produce moment Fig 9 tending push tail downthe nose up also destabilizing Without attempting estab lish values said laborious lets assume effect will such move Neutral Point forward 4% up 33% chord give ourselves stability margin 5% forward CG will lie 28% chord glider tail larger tail arm longer Neutral Point will further aft will thereby permit further aft CG example some freeflight powered models seen CG locations way back near trailing edge wing come decalage reduced tail made very large tail arm very long maybe lifting airfoil tailwhich normally no value fuselage very slender nose very short maybe inverted fin possible push Neutral Point aft far enough have aft trailing edge thus obtain stability CG trailing edge wing such arrangement can get pretty touchy demand considerable fine tuning no control available Now lets fatten up fuselage glider put engine keepingthe same wing tail shown Fig 10 addition destabilizing effects glider have some new ones L 25 Continued page 101 22 Model Aviation Fig5 E 0 sta N 0 3 5 N -I10 AT LAST SOME Primer U ndercoeter Nowin Weve taken popular two-part white primer-undercoat formula made gray Just gray enough please have told us want see dents nicks ripples youre sanding Now make little visible just efficient ever course its still available white too Wood Plastic Metal Fiberglass Hobbypoxy primer-undercoater covers well cures fills fast shrinking easy sand provides smooth-as-glass surface. great adhesion wood plastic metal fiberglass Whether youre balsa builder molded plastic buff whether brush spray it. can now put little color life Hobbypoxy gray undercoat. good ever lot visible complete information how-to-do-it Hobbypoxy Finishing Guide write today ARNIIHAUIIvision Pettit Paint Co Inc H UDUIrUAT reooncis Street Rockaway NJ 07866 HOBBYPOXY ct group finishing products especially formulated hobbyists including Epoxy Glues Epoxy-enamel Paints Fillers Primer-undercoaters Resins R2 hole outside lug potentiometer opposite side want 1 seconds/stroke thing found testing servos column new servos arent necessarily fast advertised Some some arent what work theyre fast enough Log values find new look changes fact have changed information really want anyway Keep letters coming George M Myers 70 Froehlich Farm Rd Hicks yule NY 11801 About CG/Powers Continued page 22 I The engine thrust below CG will produce nose-up moment destabilizing tending move N forward 2 The propeller slip stream increases downwash over tail loading up pushing tail down nose up again destabilizing pushes N further ahead 3 The fatter fuselage presenting greater area upflow ahead wingmore instability N moves forward again 4 The fixed landing gear stabilizing tends pull nose down cancel effects other items some extent net result scale models similar basic configuration Neutral Point usually winds up very close 30% chord wing Thus good safe comfortable CG location about 5% forward 30% Neutral Point 25% chord So now understand CG forward beginners can further aft experts As matter fact have several models balanced approximately 25% tickles see little birds recover nicely stall generally forgiving mistakes controlling fliers dont seem pay much attention CG Location just balance airplane about third way back put seem fine because pretty good fliers would guess may struggling against some stability realizing way sure course test fly model varying CG locations Move CG forward little bit time until its hard get nose up can slow down landing move CG aft small increments until becomes sensitive skittish Ease back little both extremes let limits beginners CG location 25% chord good place start have expressing CG location percent chord because have used constant chord rectangular wing example airplanes made way course Some have tapered wings some have elliptical wings some have triangular wings some have two three wings what chord next installment will discuss Mean Aerodynamic Chord find configurations just mentioned Questions comments may addressed author care editor continued RC Aerobatics/Van Putte Continued page 23 thrust two propellers will deliver compar ing load factor props load factor determined multiplying prop pitch cube diameter example 1 1-8 prophas aloadfactorof8 X1 l 8 comparison 11 -7 prop has load factor 7 X1I53 6 According rule thrust developed 11-8 prop should about same 11 -7 prop know small engine will tum large prop So trick using load factor rule determine typical load factor range different engines vary prop characteristics get desired performance What follows table engine sizes versus load factor ranges flier should find displacement engine question read across load factor range engine table presented included article B J Rich Richburg Central Virginia Radio Control Association Virginia Newsletter Engine Displacement 05 10 15 20 25 30 35 40 45 50 60 80 Load Factor Range 400- 680 900- 1500 1500- 2500 2000- 3000 2600- 3650 3350- 4600 4000- 5500 4600- 6200 5200- 7000 6000- 8400 7600- 10800 9900-14000 flier should rate engine compared others same displacement new Schnuerle ported ringed piston engine will able swing higher load factor prop old boostported lapped piston engine ofthe same displace ment Pick out load factor corresponding April 1980 101 TTER FROM HOBBYPOXY
Edition: Model Aviation - 1980/04
Page Numbers: 19, 20, 21, 22, 101
ON THE plans accompanying model kits sometimes shown two CG locations ex perienced fliers half inch forward labeled Beginners can infer CG location Balance Point must some importance flying aft CG requires greater flying skilL true aft CG location exper ienced flier can perform spine tingling spins snap rolls will whereas beginner like myself will also perform spine tingling spins etc unin tentionally fact CG moved further further aft expert will encounter increasing difficulties until finally airplane becomes unstable extremely difficult dangerous fly will have become sensitive little no provocation will embark adventurous maneuvers its own usually disastrous results other hand moving CG progressively forward increases stability improves socalled penetration move forward airplane becomes less less sensitive elevator control until elevator power inade quate pull nose up sufficiently effect slow safe landing evident safe sensible CG range must lie between two extremes conjunction relative incidence tween wing horizontal tail fore andaft position CG powerful single factor achieving Static Longitudinal Stability therefore something should under stood anyone flying kind aircraft large small So lets look three aspects matter First What determines acceptable CG limits Second physically limits Third During design construction phase large -scale model some way figure place things CG will wind up its supposed article will discuss first item subsequent issues will discuss items two three Any discussion CG location must necessarily involve discussion Stability Stability definition tendency return original set conditions disturbed Fig 1 shown three cones Cone resting firmly its base something tried tip over will tend return its original position long dis turbance isnt great knock offthe table condition Stable B cone lying its side disturbed will continue remain its side will roll around new position making no effort return original condition maybe said Neutral condition corrective action must taken C cone shown unlikely attitude balancing its point Obviously slightest disturbance will cause fall down no hope restoration original position Unstable condition An airplane can turn roll pitch can perform maneuver singly combination others ie climbing turn rolling dive etc must have stability about three axes other words should direction ally stable laterally stable longitudinally stable latter longitudinal stability about lateral axis ability recover inad vertent changes pitch attitude will concern ourselves because longitudinal stability almost entirely dependent CG location Stability about lateral axis two kinds Static Dynamic airplane longitudi nally stable will tend automatically re establish its original Speed Attitude dis EUJI] ULIL 4 4 Determining well RC model flies controllable alL bear pilot CGs proper location important aerodynamic scheme things heart brain human body Reducing complex matter everyman 5 language author deals subject three installments What deter mines CG limits month Physical location limits Placement things CG winds up supposed turbed Thus encounters gust tending initiate stall causing nose up slow down instead stalling will drop its nose recover speeding up assuming its original flight attitude does quickly smoothly both Statically Dynamically Stable recovers has increasing difficulty re establishing its original attitude because overshooting hunting said Dynamically Unstable will concern ourselves Static Longitudinal Stability As said model Longitudinally Stable recovers impending stall dive itself Neutrally Stable disturbed will require prompt corrective control part prevent stall dive will indeed require attention during its flight stay groove speak other hand will very maneuverable undoubted ly liking skilled pattern flier model Unstable will very difficult keep under control sudden disturbance can cause stall before can much prevent Further its sensitivity will engender highG maneuvers vertical-banked tight turns etc might cause structural failure Flight maintained forward surface inclined greater angle relative wind rear surface wing produces lift rear surface trims holds its proper angie attack angular difference between two surfaces sometimes called decalage French word roughly trans lates mean wedgeshaped implying angular difference necessary also forward surface fly higher angle standpoint stability will stall first thus contributing recovery true types aircraft tailless conventional canard anything else conventional tailless configurations forward surface trimmed aft surface horizontal tail conventional elevon tailless case canard other way around rear surface provides lift trimmed smaller forward surface nevertheless being forward surface must fly higher angie attack main wing rear too will stall first However angular difference wing tail settings does itself insure stable flight must always proper relationship between forces play tendency effect recovery dive stall will always present Webruer von Flintstone research devel opment engineer lived aboutthirty thousand years ago operated out cave high foothills Cantabrian Cordillera discov ered stick free body thrown would rotate about its balance point CG After years principle still holds good can apply Fig 2 weight constant acts CG Lift ries angle attack arm behind CG tail force T also varies arm t CG stable airplane forces act together produce moments will cause airplane recover inadvertent maneuver might due gust wind such encountered passing through thermal recovery must accomplished automatically no change tail setting So lets see can figure out might done moment product force times distance sometimes area times distance Thus large tail will produce larger moment will small tail same arm con versely longer arm small tail can produce Brad PowersApril 1980 19 Fig 1 / / C N V NEUTRALUNSTABLE moment equal large tail shorter arm etc Lets call stalling nose-up moments posi tive anddiving nose-down moments negative take another look Fig 2 free flight glider lets say no means corrective control part see behaves move CG back forth Lift L times its arm produces moment L about CG tail also produces its momentTt about CG maintain level flight moments must balance other dont airplane will MACs 12 provide reader visual frame reference series begins have shown climactic drawing part three months copy course does deal specific detail various items called out drawing youll get drift things nose up stall nose down dive So sustain level flight constant speed Ll equals Tt Ll-Tt must equal zero say another way sum moments must equal zero Fig 2 have things arranged CG forward Lift Lets see works OK stalls dives Ifwe try stall first Fig 3 see Lift has greatly increased due large angle attack thus producing much greater negative moment did level flight tending push nose back down because attitude glider now pointing up horizontal tail forced upward relative wind also produce negative diving moment thus contributing further recovery So far good Now lets try dive nose drops about 3 degrees wing angle zero Lift thus producing no moment about CG tail still produces positive moment tending push nose back up belongs shown Fig 4 Thus see having CG forward Lift produces stable arrangement providing recovery both stalls dives point might well put information down graph Fig 5 Lets say glider maintains level flight wing 3 degrees angle attack Since said level flight means zero moment represented point intersec tion zero moment 3 degrees angle attack approaching stall condition lets say 9 degrees angle attack moments negative ones condition shown point B 20 Model Aviation STABLE dive condition zero angle attack accompanied positive moment due tail condition shown point C Thus see thatwith CG forward Lift have stable configuration plot moment versus angle attack produces curve having negative slope other words angle attack increases moments must become more negative acting sort like spring restore equilibrium Now just sure lets try other extreme place CG behind Lift shown Fig 6 again dive assumed zero lift angle attack zero will no moment other tail condition will duplicate first dive condition will also lie point C chart level flight condition however shown Fig 7 moments dont reduce zero become increasingly positive lift wing has increased tail setting still such produce its positive moment both conditions tending raise nose placing level flight condition now appears momentary best point chart Fig 8 relative wind tail will produce negative moment will insuffi cient overcome large amount positive moment produced wing high angle attack glider will stalL condition will lie point Boo chart see having CG behind Lift Unstable produces curve shown dashed line curve has positive slope denotes Instability Neutral Stability occurs CG Lift coincide acting pure opposition thus producing no moment thus very sensitive control lack will demand constant attention condition analogous dive conditions wing produced no moment about CG because zero Lift moment present due decalage producing positive moment So dive condition neutral stability will also plotted C since moment tail will remain unchanged Neutral Stability curve simply horizontal line originating C As can seen line demarcation between stability instability Thus clear stability simply means placing CG forward lift Before going further better pin down just what meant Lift hold model wing having symmetrical airfoil between fingertips plac ing finger 25% chord wing tip could run fast enough would feel wing develop some lift still maintain its attitude placed fingers forward aft 25% wingwould tend rotate shows place along chord symmetrical airfoil no moment moments along chord balance point called Aerodynamic Center AC 25% chord airfoils cambered sections 25% point also AC point constan4 rather zero moment wing alone Lift may considered act through point designing airplane having wing CG would want placed forward 25% point stability Disregarding elevon However elevon glider tail present something equivalent AC representative airplane whole must considered point forward CG must lie point called Neutral Point N its position determined r w L April 1980 21 6 Angle Attack0 total moment produced factors capable affecting trim airplane factors include destabilizing effects power thrust line below CG tending push nose up front end fuselage nacelles horizontal stabilizer f V ab ~neu ral B K 9 etc Factors tend produce positive moments raise nose destabilizing tend move N forward Factors such flap deflection fixed landing gear floats other items producing negative moments stabiliz ing tend move N aft 30 determination N laborious process involves detailed information about specific design can treat very general way proper procedure evaluate add together slopes ofthe moment curves various items mentioned above ascertain total effect Since cant attempt will treat matter simplistic yet valid believe manner determining N simple problem Statics purposes explanation assume condition Neutral Point will lie resultant wing tail regarding weighted areas will have value proportional its area times its unit loading can take moments find position N Fig 9 shown plan view wing/tail configuration glider said decalage 3 degrees assume airplane attitude tail 1 degree wing 4 degrees lift varies directly angle attack does wing will have loading of4 unitsounces grams etc per sq tail will have loading unit Thus Fig 9 wing will have weighted area 600 X 4 2400 units tail will have 100 X 1 100 units So taking moments about AC wing Moment Wing 2400 X 0 equals 0 Moment Tail 100 X 30 equals 3000 - 2500 12 3000 arm Neutral Point 3000/2500 12 inches aft AC wing places 25 plus 12 inches 37 inches aft leading edge wing Since wing 1 2 has chord oflO inches amounts 37% Chord point presupposing up-load tail aft position N destabilizing factors present will act move point forward Lets look Fig 2 again see exist horizontal tail will now produce positive moment addition have generous nose section forward wing lies induced upflow ahead wing will tend produce stalling positive moment particularly high angles attack Also drag vertical fin will produce moment Fig 9 tending push tail downthe nose up also destabilizing Without attempting estab lish values said laborious lets assume effect will such move Neutral Point forward 4% up 33% chord give ourselves stability margin 5% forward CG will lie 28% chord glider tail larger tail arm longer Neutral Point will further aft will thereby permit further aft CG example some freeflight powered models seen CG locations way back near trailing edge wing come decalage reduced tail made very large tail arm very long maybe lifting airfoil tailwhich normally no value fuselage very slender nose very short maybe inverted fin possible push Neutral Point aft far enough have aft trailing edge thus obtain stability CG trailing edge wing such arrangement can get pretty touchy demand considerable fine tuning no control available Now lets fatten up fuselage glider put engine keepingthe same wing tail shown Fig 10 addition destabilizing effects glider have some new ones L 25 Continued page 101 22 Model Aviation Fig5 E 0 sta N 0 3 5 N -I10 AT LAST SOME Primer U ndercoeter Nowin Weve taken popular two-part white primer-undercoat formula made gray Just gray enough please have told us want see dents nicks ripples youre sanding Now make little visible just efficient ever course its still available white too Wood Plastic Metal Fiberglass Hobbypoxy primer-undercoater covers well cures fills fast shrinking easy sand provides smooth-as-glass surface. great adhesion wood plastic metal fiberglass Whether youre balsa builder molded plastic buff whether brush spray it. can now put little color life Hobbypoxy gray undercoat. good ever lot visible complete information how-to-do-it Hobbypoxy Finishing Guide write today ARNIIHAUIIvision Pettit Paint Co Inc H UDUIrUAT reooncis Street Rockaway NJ 07866 HOBBYPOXY ct group finishing products especially formulated hobbyists including Epoxy Glues Epoxy-enamel Paints Fillers Primer-undercoaters Resins R2 hole outside lug potentiometer opposite side want 1 seconds/stroke thing found testing servos column new servos arent necessarily fast advertised Some some arent what work theyre fast enough Log values find new look changes fact have changed information really want anyway Keep letters coming George M Myers 70 Froehlich Farm Rd Hicks yule NY 11801 About CG/Powers Continued page 22 I The engine thrust below CG will produce nose-up moment destabilizing tending move N forward 2 The propeller slip stream increases downwash over tail loading up pushing tail down nose up again destabilizing pushes N further ahead 3 The fatter fuselage presenting greater area upflow ahead wingmore instability N moves forward again 4 The fixed landing gear stabilizing tends pull nose down cancel effects other items some extent net result scale models similar basic configuration Neutral Point usually winds up very close 30% chord wing Thus good safe comfortable CG location about 5% forward 30% Neutral Point 25% chord So now understand CG forward beginners can further aft experts As matter fact have several models balanced approximately 25% tickles see little birds recover nicely stall generally forgiving mistakes controlling fliers dont seem pay much attention CG Location just balance airplane about third way back put seem fine because pretty good fliers would guess may struggling against some stability realizing way sure course test fly model varying CG locations Move CG forward little bit time until its hard get nose up can slow down landing move CG aft small increments until becomes sensitive skittish Ease back little both extremes let limits beginners CG location 25% chord good place start have expressing CG location percent chord because have used constant chord rectangular wing example airplanes made way course Some have tapered wings some have elliptical wings some have triangular wings some have two three wings what chord next installment will discuss Mean Aerodynamic Chord find configurations just mentioned Questions comments may addressed author care editor continued RC Aerobatics/Van Putte Continued page 23 thrust two propellers will deliver compar ing load factor props load factor determined multiplying prop pitch cube diameter example 1 1-8 prophas aloadfactorof8 X1 l 8 comparison 11 -7 prop has load factor 7 X1I53 6 According rule thrust developed 11-8 prop should about same 11 -7 prop know small engine will tum large prop So trick using load factor rule determine typical load factor range different engines vary prop characteristics get desired performance What follows table engine sizes versus load factor ranges flier should find displacement engine question read across load factor range engine table presented included article B J Rich Richburg Central Virginia Radio Control Association Virginia Newsletter Engine Displacement 05 10 15 20 25 30 35 40 45 50 60 80 Load Factor Range 400- 680 900- 1500 1500- 2500 2000- 3000 2600- 3650 3350- 4600 4000- 5500 4600- 6200 5200- 7000 6000- 8400 7600- 10800 9900-14000 flier should rate engine compared others same displacement new Schnuerle ported ringed piston engine will able swing higher load factor prop old boostported lapped piston engine ofthe same displace ment Pick out load factor corresponding April 1980 101 TTER FROM HOBBYPOXY
Edition: Model Aviation - 1980/04
Page Numbers: 19, 20, 21, 22, 101
ON THE plans accompanying model kits sometimes shown two CG locations ex perienced fliers half inch forward labeled Beginners can infer CG location Balance Point must some importance flying aft CG requires greater flying skilL true aft CG location exper ienced flier can perform spine tingling spins snap rolls will whereas beginner like myself will also perform spine tingling spins etc unin tentionally fact CG moved further further aft expert will encounter increasing difficulties until finally airplane becomes unstable extremely difficult dangerous fly will have become sensitive little no provocation will embark adventurous maneuvers its own usually disastrous results other hand moving CG progressively forward increases stability improves socalled penetration move forward airplane becomes less less sensitive elevator control until elevator power inade quate pull nose up sufficiently effect slow safe landing evident safe sensible CG range must lie between two extremes conjunction relative incidence tween wing horizontal tail fore andaft position CG powerful single factor achieving Static Longitudinal Stability therefore something should under stood anyone flying kind aircraft large small So lets look three aspects matter First What determines acceptable CG limits Second physically limits Third During design construction phase large -scale model some way figure place things CG will wind up its supposed article will discuss first item subsequent issues will discuss items two three Any discussion CG location must necessarily involve discussion Stability Stability definition tendency return original set conditions disturbed Fig 1 shown three cones Cone resting firmly its base something tried tip over will tend return its original position long dis turbance isnt great knock offthe table condition Stable B cone lying its side disturbed will continue remain its side will roll around new position making no effort return original condition maybe said Neutral condition corrective action must taken C cone shown unlikely attitude balancing its point Obviously slightest disturbance will cause fall down no hope restoration original position Unstable condition An airplane can turn roll pitch can perform maneuver singly combination others ie climbing turn rolling dive etc must have stability about three axes other words should direction ally stable laterally stable longitudinally stable latter longitudinal stability about lateral axis ability recover inad vertent changes pitch attitude will concern ourselves because longitudinal stability almost entirely dependent CG location Stability about lateral axis two kinds Static Dynamic airplane longitudi nally stable will tend automatically re establish its original Speed Attitude dis EUJI] ULIL 4 4 Determining well RC model flies controllable alL bear pilot CGs proper location important aerodynamic scheme things heart brain human body Reducing complex matter everyman 5 language author deals subject three installments What deter mines CG limits month Physical location limits Placement things CG winds up supposed turbed Thus encounters gust tending initiate stall causing nose up slow down instead stalling will drop its nose recover speeding up assuming its original flight attitude does quickly smoothly both Statically Dynamically Stable recovers has increasing difficulty re establishing its original attitude because overshooting hunting said Dynamically Unstable will concern ourselves Static Longitudinal Stability As said model Longitudinally Stable recovers impending stall dive itself Neutrally Stable disturbed will require prompt corrective control part prevent stall dive will indeed require attention during its flight stay groove speak other hand will very maneuverable undoubted ly liking skilled pattern flier model Unstable will very difficult keep under control sudden disturbance can cause stall before can much prevent Further its sensitivity will engender highG maneuvers vertical-banked tight turns etc might cause structural failure Flight maintained forward surface inclined greater angle relative wind rear surface wing produces lift rear surface trims holds its proper angie attack angular difference between two surfaces sometimes called decalage French word roughly trans lates mean wedgeshaped implying angular difference necessary also forward surface fly higher angle standpoint stability will stall first thus contributing recovery true types aircraft tailless conventional canard anything else conventional tailless configurations forward surface trimmed aft surface horizontal tail conventional elevon tailless case canard other way around rear surface provides lift trimmed smaller forward surface nevertheless being forward surface must fly higher angie attack main wing rear too will stall first However angular difference wing tail settings does itself insure stable flight must always proper relationship between forces play tendency effect recovery dive stall will always present Webruer von Flintstone research devel opment engineer lived aboutthirty thousand years ago operated out cave high foothills Cantabrian Cordillera discov ered stick free body thrown would rotate about its balance point CG After years principle still holds good can apply Fig 2 weight constant acts CG Lift ries angle attack arm behind CG tail force T also varies arm t CG stable airplane forces act together produce moments will cause airplane recover inadvertent maneuver might due gust wind such encountered passing through thermal recovery must accomplished automatically no change tail setting So lets see can figure out might done moment product force times distance sometimes area times distance Thus large tail will produce larger moment will small tail same arm con versely longer arm small tail can produce Brad PowersApril 1980 19 Fig 1 / / C N V NEUTRALUNSTABLE moment equal large tail shorter arm etc Lets call stalling nose-up moments posi tive anddiving nose-down moments negative take another look Fig 2 free flight glider lets say no means corrective control part see behaves move CG back forth Lift L times its arm produces moment L about CG tail also produces its momentTt about CG maintain level flight moments must balance other dont airplane will MACs 12 provide reader visual frame reference series begins have shown climactic drawing part three months copy course does deal specific detail various items called out drawing youll get drift things nose up stall nose down dive So sustain level flight constant speed Ll equals Tt Ll-Tt must equal zero say another way sum moments must equal zero Fig 2 have things arranged CG forward Lift Lets see works OK stalls dives Ifwe try stall first Fig 3 see Lift has greatly increased due large angle attack thus producing much greater negative moment did level flight tending push nose back down because attitude glider now pointing up horizontal tail forced upward relative wind also produce negative diving moment thus contributing further recovery So far good Now lets try dive nose drops about 3 degrees wing angle zero Lift thus producing no moment about CG tail still produces positive moment tending push nose back up belongs shown Fig 4 Thus see having CG forward Lift produces stable arrangement providing recovery both stalls dives point might well put information down graph Fig 5 Lets say glider maintains level flight wing 3 degrees angle attack Since said level flight means zero moment represented point intersec tion zero moment 3 degrees angle attack approaching stall condition lets say 9 degrees angle attack moments negative ones condition shown point B 20 Model Aviation STABLE dive condition zero angle attack accompanied positive moment due tail condition shown point C Thus see thatwith CG forward Lift have stable configuration plot moment versus angle attack produces curve having negative slope other words angle attack increases moments must become more negative acting sort like spring restore equilibrium Now just sure lets try other extreme place CG behind Lift shown Fig 6 again dive assumed zero lift angle attack zero will no moment other tail condition will duplicate first dive condition will also lie point C chart level flight condition however shown Fig 7 moments dont reduce zero become increasingly positive lift wing has increased tail setting still such produce its positive moment both conditions tending raise nose placing level flight condition now appears momentary best point chart Fig 8 relative wind tail will produce negative moment will insuffi cient overcome large amount positive moment produced wing high angle attack glider will stalL condition will lie point Boo chart see having CG behind Lift Unstable produces curve shown dashed line curve has positive slope denotes Instability Neutral Stability occurs CG Lift coincide acting pure opposition thus producing no moment thus very sensitive control lack will demand constant attention condition analogous dive conditions wing produced no moment about CG because zero Lift moment present due decalage producing positive moment So dive condition neutral stability will also plotted C since moment tail will remain unchanged Neutral Stability curve simply horizontal line originating C As can seen line demarcation between stability instability Thus clear stability simply means placing CG forward lift Before going further better pin down just what meant Lift hold model wing having symmetrical airfoil between fingertips plac ing finger 25% chord wing tip could run fast enough would feel wing develop some lift still maintain its attitude placed fingers forward aft 25% wingwould tend rotate shows place along chord symmetrical airfoil no moment moments along chord balance point called Aerodynamic Center AC 25% chord airfoils cambered sections 25% point also AC point constan4 rather zero moment wing alone Lift may considered act through point designing airplane having wing CG would want placed forward 25% point stability Disregarding elevon However elevon glider tail present something equivalent AC representative airplane whole must considered point forward CG must lie point called Neutral Point N its position determined r w L April 1980 21 6 Angle Attack0 total moment produced factors capable affecting trim airplane factors include destabilizing effects power thrust line below CG tending push nose up front end fuselage nacelles horizontal stabilizer f V ab ~neu ral B K 9 etc Factors tend produce positive moments raise nose destabilizing tend move N forward Factors such flap deflection fixed landing gear floats other items producing negative moments stabiliz ing tend move N aft 30 determination N laborious process involves detailed information about specific design can treat very general way proper procedure evaluate add together slopes ofthe moment curves various items mentioned above ascertain total effect Since cant attempt will treat matter simplistic yet valid believe manner determining N simple problem Statics purposes explanation assume condition Neutral Point will lie resultant wing tail regarding weighted areas will have value proportional its area times its unit loading can take moments find position N Fig 9 shown plan view wing/tail configuration glider said decalage 3 degrees assume airplane attitude tail 1 degree wing 4 degrees lift varies directly angle attack does wing will have loading of4 unitsounces grams etc per sq tail will have loading unit Thus Fig 9 wing will have weighted area 600 X 4 2400 units tail will have 100 X 1 100 units So taking moments about AC wing Moment Wing 2400 X 0 equals 0 Moment Tail 100 X 30 equals 3000 - 2500 12 3000 arm Neutral Point 3000/2500 12 inches aft AC wing places 25 plus 12 inches 37 inches aft leading edge wing Since wing 1 2 has chord oflO inches amounts 37% Chord point presupposing up-load tail aft position N destabilizing factors present will act move point forward Lets look Fig 2 again see exist horizontal tail will now produce positive moment addition have generous nose section forward wing lies induced upflow ahead wing will tend produce stalling positive moment particularly high angles attack Also drag vertical fin will produce moment Fig 9 tending push tail downthe nose up also destabilizing Without attempting estab lish values said laborious lets assume effect will such move Neutral Point forward 4% up 33% chord give ourselves stability margin 5% forward CG will lie 28% chord glider tail larger tail arm longer Neutral Point will further aft will thereby permit further aft CG example some freeflight powered models seen CG locations way back near trailing edge wing come decalage reduced tail made very large tail arm very long maybe lifting airfoil tailwhich normally no value fuselage very slender nose very short maybe inverted fin possible push Neutral Point aft far enough have aft trailing edge thus obtain stability CG trailing edge wing such arrangement can get pretty touchy demand considerable fine tuning no control available Now lets fatten up fuselage glider put engine keepingthe same wing tail shown Fig 10 addition destabilizing effects glider have some new ones L 25 Continued page 101 22 Model Aviation Fig5 E 0 sta N 0 3 5 N -I10 AT LAST SOME Primer U ndercoeter Nowin Weve taken popular two-part white primer-undercoat formula made gray Just gray enough please have told us want see dents nicks ripples youre sanding Now make little visible just efficient ever course its still available white too Wood Plastic Metal Fiberglass Hobbypoxy primer-undercoater covers well cures fills fast shrinking easy sand provides smooth-as-glass surface. great adhesion wood plastic metal fiberglass Whether youre balsa builder molded plastic buff whether brush spray it. can now put little color life Hobbypoxy gray undercoat. good ever lot visible complete information how-to-do-it Hobbypoxy Finishing Guide write today ARNIIHAUIIvision Pettit Paint Co Inc H UDUIrUAT reooncis Street Rockaway NJ 07866 HOBBYPOXY ct group finishing products especially formulated hobbyists including Epoxy Glues Epoxy-enamel Paints Fillers Primer-undercoaters Resins R2 hole outside lug potentiometer opposite side want 1 seconds/stroke thing found testing servos column new servos arent necessarily fast advertised Some some arent what work theyre fast enough Log values find new look changes fact have changed information really want anyway Keep letters coming George M Myers 70 Froehlich Farm Rd Hicks yule NY 11801 About CG/Powers Continued page 22 I The engine thrust below CG will produce nose-up moment destabilizing tending move N forward 2 The propeller slip stream increases downwash over tail loading up pushing tail down nose up again destabilizing pushes N further ahead 3 The fatter fuselage presenting greater area upflow ahead wingmore instability N moves forward again 4 The fixed landing gear stabilizing tends pull nose down cancel effects other items some extent net result scale models similar basic configuration Neutral Point usually winds up very close 30% chord wing Thus good safe comfortable CG location about 5% forward 30% Neutral Point 25% chord So now understand CG forward beginners can further aft experts As matter fact have several models balanced approximately 25% tickles see little birds recover nicely stall generally forgiving mistakes controlling fliers dont seem pay much attention CG Location just balance airplane about third way back put seem fine because pretty good fliers would guess may struggling against some stability realizing way sure course test fly model varying CG locations Move CG forward little bit time until its hard get nose up can slow down landing move CG aft small increments until becomes sensitive skittish Ease back little both extremes let limits beginners CG location 25% chord good place start have expressing CG location percent chord because have used constant chord rectangular wing example airplanes made way course Some have tapered wings some have elliptical wings some have triangular wings some have two three wings what chord next installment will discuss Mean Aerodynamic Chord find configurations just mentioned Questions comments may addressed author care editor continued RC Aerobatics/Van Putte Continued page 23 thrust two propellers will deliver compar ing load factor props load factor determined multiplying prop pitch cube diameter example 1 1-8 prophas aloadfactorof8 X1 l 8 comparison 11 -7 prop has load factor 7 X1I53 6 According rule thrust developed 11-8 prop should about same 11 -7 prop know small engine will tum large prop So trick using load factor rule determine typical load factor range different engines vary prop characteristics get desired performance What follows table engine sizes versus load factor ranges flier should find displacement engine question read across load factor range engine table presented included article B J Rich Richburg Central Virginia Radio Control Association Virginia Newsletter Engine Displacement 05 10 15 20 25 30 35 40 45 50 60 80 Load Factor Range 400- 680 900- 1500 1500- 2500 2000- 3000 2600- 3650 3350- 4600 4000- 5500 4600- 6200 5200- 7000 6000- 8400 7600- 10800 9900-14000 flier should rate engine compared others same displacement new Schnuerle ported ringed piston engine will able swing higher load factor prop old boostported lapped piston engine ofthe same displace ment Pick out load factor corresponding April 1980 101 TTER FROM HOBBYPOXY
Edition: Model Aviation - 1980/04
Page Numbers: 19, 20, 21, 22, 101
ON THE plans accompanying model kits sometimes shown two CG locations ex perienced fliers half inch forward labeled Beginners can infer CG location Balance Point must some importance flying aft CG requires greater flying skilL true aft CG location exper ienced flier can perform spine tingling spins snap rolls will whereas beginner like myself will also perform spine tingling spins etc unin tentionally fact CG moved further further aft expert will encounter increasing difficulties until finally airplane becomes unstable extremely difficult dangerous fly will have become sensitive little no provocation will embark adventurous maneuvers its own usually disastrous results other hand moving CG progressively forward increases stability improves socalled penetration move forward airplane becomes less less sensitive elevator control until elevator power inade quate pull nose up sufficiently effect slow safe landing evident safe sensible CG range must lie between two extremes conjunction relative incidence tween wing horizontal tail fore andaft position CG powerful single factor achieving Static Longitudinal Stability therefore something should under stood anyone flying kind aircraft large small So lets look three aspects matter First What determines acceptable CG limits Second physically limits Third During design construction phase large -scale model some way figure place things CG will wind up its supposed article will discuss first item subsequent issues will discuss items two three Any discussion CG location must necessarily involve discussion Stability Stability definition tendency return original set conditions disturbed Fig 1 shown three cones Cone resting firmly its base something tried tip over will tend return its original position long dis turbance isnt great knock offthe table condition Stable B cone lying its side disturbed will continue remain its side will roll around new position making no effort return original condition maybe said Neutral condition corrective action must taken C cone shown unlikely attitude balancing its point Obviously slightest disturbance will cause fall down no hope restoration original position Unstable condition An airplane can turn roll pitch can perform maneuver singly combination others ie climbing turn rolling dive etc must have stability about three axes other words should direction ally stable laterally stable longitudinally stable latter longitudinal stability about lateral axis ability recover inad vertent changes pitch attitude will concern ourselves because longitudinal stability almost entirely dependent CG location Stability about lateral axis two kinds Static Dynamic airplane longitudi nally stable will tend automatically re establish its original Speed Attitude dis EUJI] ULIL 4 4 Determining well RC model flies controllable alL bear pilot CGs proper location important aerodynamic scheme things heart brain human body Reducing complex matter everyman 5 language author deals subject three installments What deter mines CG limits month Physical location limits Placement things CG winds up supposed turbed Thus encounters gust tending initiate stall causing nose up slow down instead stalling will drop its nose recover speeding up assuming its original flight attitude does quickly smoothly both Statically Dynamically Stable recovers has increasing difficulty re establishing its original attitude because overshooting hunting said Dynamically Unstable will concern ourselves Static Longitudinal Stability As said model Longitudinally Stable recovers impending stall dive itself Neutrally Stable disturbed will require prompt corrective control part prevent stall dive will indeed require attention during its flight stay groove speak other hand will very maneuverable undoubted ly liking skilled pattern flier model Unstable will very difficult keep under control sudden disturbance can cause stall before can much prevent Further its sensitivity will engender highG maneuvers vertical-banked tight turns etc might cause structural failure Flight maintained forward surface inclined greater angle relative wind rear surface wing produces lift rear surface trims holds its proper angie attack angular difference between two surfaces sometimes called decalage French word roughly trans lates mean wedgeshaped implying angular difference necessary also forward surface fly higher angle standpoint stability will stall first thus contributing recovery true types aircraft tailless conventional canard anything else conventional tailless configurations forward surface trimmed aft surface horizontal tail conventional elevon tailless case canard other way around rear surface provides lift trimmed smaller forward surface nevertheless being forward surface must fly higher angie attack main wing rear too will stall first However angular difference wing tail settings does itself insure stable flight must always proper relationship between forces play tendency effect recovery dive stall will always present Webruer von Flintstone research devel opment engineer lived aboutthirty thousand years ago operated out cave high foothills Cantabrian Cordillera discov ered stick free body thrown would rotate about its balance point CG After years principle still holds good can apply Fig 2 weight constant acts CG Lift ries angle attack arm behind CG tail force T also varies arm t CG stable airplane forces act together produce moments will cause airplane recover inadvertent maneuver might due gust wind such encountered passing through thermal recovery must accomplished automatically no change tail setting So lets see can figure out might done moment product force times distance sometimes area times distance Thus large tail will produce larger moment will small tail same arm con versely longer arm small tail can produce Brad PowersApril 1980 19 Fig 1 / / C N V NEUTRALUNSTABLE moment equal large tail shorter arm etc Lets call stalling nose-up moments posi tive anddiving nose-down moments negative take another look Fig 2 free flight glider lets say no means corrective control part see behaves move CG back forth Lift L times its arm produces moment L about CG tail also produces its momentTt about CG maintain level flight moments must balance other dont airplane will MACs 12 provide reader visual frame reference series begins have shown climactic drawing part three months copy course does deal specific detail various items called out drawing youll get drift things nose up stall nose down dive So sustain level flight constant speed Ll equals Tt Ll-Tt must equal zero say another way sum moments must equal zero Fig 2 have things arranged CG forward Lift Lets see works OK stalls dives Ifwe try stall first Fig 3 see Lift has greatly increased due large angle attack thus producing much greater negative moment did level flight tending push nose back down because attitude glider now pointing up horizontal tail forced upward relative wind also produce negative diving moment thus contributing further recovery So far good Now lets try dive nose drops about 3 degrees wing angle zero Lift thus producing no moment about CG tail still produces positive moment tending push nose back up belongs shown Fig 4 Thus see having CG forward Lift produces stable arrangement providing recovery both stalls dives point might well put information down graph Fig 5 Lets say glider maintains level flight wing 3 degrees angle attack Since said level flight means zero moment represented point intersec tion zero moment 3 degrees angle attack approaching stall condition lets say 9 degrees angle attack moments negative ones condition shown point B 20 Model Aviation STABLE dive condition zero angle attack accompanied positive moment due tail condition shown point C Thus see thatwith CG forward Lift have stable configuration plot moment versus angle attack produces curve having negative slope other words angle attack increases moments must become more negative acting sort like spring restore equilibrium Now just sure lets try other extreme place CG behind Lift shown Fig 6 again dive assumed zero lift angle attack zero will no moment other tail condition will duplicate first dive condition will also lie point C chart level flight condition however shown Fig 7 moments dont reduce zero become increasingly positive lift wing has increased tail setting still such produce its positive moment both conditions tending raise nose placing level flight condition now appears momentary best point chart Fig 8 relative wind tail will produce negative moment will insuffi cient overcome large amount positive moment produced wing high angle attack glider will stalL condition will lie point Boo chart see having CG behind Lift Unstable produces curve shown dashed line curve has positive slope denotes Instability Neutral Stability occurs CG Lift coincide acting pure opposition thus producing no moment thus very sensitive control lack will demand constant attention condition analogous dive conditions wing produced no moment about CG because zero Lift moment present due decalage producing positive moment So dive condition neutral stability will also plotted C since moment tail will remain unchanged Neutral Stability curve simply horizontal line originating C As can seen line demarcation between stability instability Thus clear stability simply means placing CG forward lift Before going further better pin down just what meant Lift hold model wing having symmetrical airfoil between fingertips plac ing finger 25% chord wing tip could run fast enough would feel wing develop some lift still maintain its attitude placed fingers forward aft 25% wingwould tend rotate shows place along chord symmetrical airfoil no moment moments along chord balance point called Aerodynamic Center AC 25% chord airfoils cambered sections 25% point also AC point constan4 rather zero moment wing alone Lift may considered act through point designing airplane having wing CG would want placed forward 25% point stability Disregarding elevon However elevon glider tail present something equivalent AC representative airplane whole must considered point forward CG must lie point called Neutral Point N its position determined r w L April 1980 21 6 Angle Attack0 total moment produced factors capable affecting trim airplane factors include destabilizing effects power thrust line below CG tending push nose up front end fuselage nacelles horizontal stabilizer f V ab ~neu ral B K 9 etc Factors tend produce positive moments raise nose destabilizing tend move N forward Factors such flap deflection fixed landing gear floats other items producing negative moments stabiliz ing tend move N aft 30 determination N laborious process involves detailed information about specific design can treat very general way proper procedure evaluate add together slopes ofthe moment curves various items mentioned above ascertain total effect Since cant attempt will treat matter simplistic yet valid believe manner determining N simple problem Statics purposes explanation assume condition Neutral Point will lie resultant wing tail regarding weighted areas will have value proportional its area times its unit loading can take moments find position N Fig 9 shown plan view wing/tail configuration glider said decalage 3 degrees assume airplane attitude tail 1 degree wing 4 degrees lift varies directly angle attack does wing will have loading of4 unitsounces grams etc per sq tail will have loading unit Thus Fig 9 wing will have weighted area 600 X 4 2400 units tail will have 100 X 1 100 units So taking moments about AC wing Moment Wing 2400 X 0 equals 0 Moment Tail 100 X 30 equals 3000 - 2500 12 3000 arm Neutral Point 3000/2500 12 inches aft AC wing places 25 plus 12 inches 37 inches aft leading edge wing Since wing 1 2 has chord oflO inches amounts 37% Chord point presupposing up-load tail aft position N destabilizing factors present will act move point forward Lets look Fig 2 again see exist horizontal tail will now produce positive moment addition have generous nose section forward wing lies induced upflow ahead wing will tend produce stalling positive moment particularly high angles attack Also drag vertical fin will produce moment Fig 9 tending push tail downthe nose up also destabilizing Without attempting estab lish values said laborious lets assume effect will such move Neutral Point forward 4% up 33% chord give ourselves stability margin 5% forward CG will lie 28% chord glider tail larger tail arm longer Neutral Point will further aft will thereby permit further aft CG example some freeflight powered models seen CG locations way back near trailing edge wing come decalage reduced tail made very large tail arm very long maybe lifting airfoil tailwhich normally no value fuselage very slender nose very short maybe inverted fin possible push Neutral Point aft far enough have aft trailing edge thus obtain stability CG trailing edge wing such arrangement can get pretty touchy demand considerable fine tuning no control available Now lets fatten up fuselage glider put engine keepingthe same wing tail shown Fig 10 addition destabilizing effects glider have some new ones L 25 Continued page 101 22 Model Aviation Fig5 E 0 sta N 0 3 5 N -I10 AT LAST SOME Primer U ndercoeter Nowin Weve taken popular two-part white primer-undercoat formula made gray Just gray enough please have told us want see dents nicks ripples youre sanding Now make little visible just efficient ever course its still available white too Wood Plastic Metal Fiberglass Hobbypoxy primer-undercoater covers well cures fills fast shrinking easy sand provides smooth-as-glass surface. great adhesion wood plastic metal fiberglass Whether youre balsa builder molded plastic buff whether brush spray it. can now put little color life Hobbypoxy gray undercoat. good ever lot visible complete information how-to-do-it Hobbypoxy Finishing Guide write today ARNIIHAUIIvision Pettit Paint Co Inc H UDUIrUAT reooncis Street Rockaway NJ 07866 HOBBYPOXY ct group finishing products especially formulated hobbyists including Epoxy Glues Epoxy-enamel Paints Fillers Primer-undercoaters Resins R2 hole outside lug potentiometer opposite side want 1 seconds/stroke thing found testing servos column new servos arent necessarily fast advertised Some some arent what work theyre fast enough Log values find new look changes fact have changed information really want anyway Keep letters coming George M Myers 70 Froehlich Farm Rd Hicks yule NY 11801 About CG/Powers Continued page 22 I The engine thrust below CG will produce nose-up moment destabilizing tending move N forward 2 The propeller slip stream increases downwash over tail loading up pushing tail down nose up again destabilizing pushes N further ahead 3 The fatter fuselage presenting greater area upflow ahead wingmore instability N moves forward again 4 The fixed landing gear stabilizing tends pull nose down cancel effects other items some extent net result scale models similar basic configuration Neutral Point usually winds up very close 30% chord wing Thus good safe comfortable CG location about 5% forward 30% Neutral Point 25% chord So now understand CG forward beginners can further aft experts As matter fact have several models balanced approximately 25% tickles see little birds recover nicely stall generally forgiving mistakes controlling fliers dont seem pay much attention CG Location just balance airplane about third way back put seem fine because pretty good fliers would guess may struggling against some stability realizing way sure course test fly model varying CG locations Move CG forward little bit time until its hard get nose up can slow down landing move CG aft small increments until becomes sensitive skittish Ease back little both extremes let limits beginners CG location 25% chord good place start have expressing CG location percent chord because have used constant chord rectangular wing example airplanes made way course Some have tapered wings some have elliptical wings some have triangular wings some have two three wings what chord next installment will discuss Mean Aerodynamic Chord find configurations just mentioned Questions comments may addressed author care editor continued RC Aerobatics/Van Putte Continued page 23 thrust two propellers will deliver compar ing load factor props load factor determined multiplying prop pitch cube diameter example 1 1-8 prophas aloadfactorof8 X1 l 8 comparison 11 -7 prop has load factor 7 X1I53 6 According rule thrust developed 11-8 prop should about same 11 -7 prop know small engine will tum large prop So trick using load factor rule determine typical load factor range different engines vary prop characteristics get desired performance What follows table engine sizes versus load factor ranges flier should find displacement engine question read across load factor range engine table presented included article B J Rich Richburg Central Virginia Radio Control Association Virginia Newsletter Engine Displacement 05 10 15 20 25 30 35 40 45 50 60 80 Load Factor Range 400- 680 900- 1500 1500- 2500 2000- 3000 2600- 3650 3350- 4600 4000- 5500 4600- 6200 5200- 7000 6000- 8400 7600- 10800 9900-14000 flier should rate engine compared others same displacement new Schnuerle ported ringed piston engine will able swing higher load factor prop old boostported lapped piston engine ofthe same displace ment Pick out load factor corresponding April 1980 101 TTER FROM HOBBYPOXY