~j~AngIe Critical Models U Bill McCombs ITS HELPFUL Free Flight modeler know given model critical climb angle above model becomes spirally unstable Regardless trimmed flight path straight turning its critical climb angle reached model will always veer off roll off undesired less-steep spiraling flight results less altitude duration Unfortunately trouble cannot corrected trim changes side thrust rudder setting wing-warping etc since instability trouble although modelers frequently try way eliminate trouble make change increases critical climb angle sufficiently course critical climb angle 900 trouble will occur since steepest model can climb 900 straight up Suppose has high-powered gas model capable 900 climb has critical climb angle say 550 As long model trimmed downthrust climb less 550 will no trouble no veer-off trimmed climb 550 highly desirable veer-off will occur climb straight up nearly change must made get critical climb angle 900 See Fig 4b Ref 3 Similarly typical high-powered rubber model will capable very steep initial climb first few seconds power burst say 80 critical climb angle 550 veer-off would occur model would first go power-wasting spiral climb upbut much smaller climb angle because reduced power Much altitude duration would lost often seen flying sessions prevent change must made get critical climb angle about 900 FF Scale models highly powered climb steeply However because small dihedral usually relatively large vertical tail critical climb angle may quite smallsometimes 00 less Therefore roll-off frequently occurs resulting spiral often spiral dive Fig 4a Ref 3 unless change made prevent course bad trim can also cause undesired spiraling spiral dive easily corrected trim changes will correct trouble spiral instability known cause models have large dihedral spiral instability does result spiral dive very little effective dihedral Ref 1 2 Scale models spiral dive common result shown aeronautical literature critical climb angle 9 can calculated 9 Arctan rCD-AB 1 L DEAF] 1 variables through F depend upon characteristics model such vertical tail size location prop size location wing dihedral effective dihedral wing sweep angle fuselage side area etc defined Ref 2 An example calculation 9 shown Appendix 1 Although calculated value 9 precise since data used calculate precisely known Equation 1 can very helpful can also used find what changes characteristics will increase critical climb angle vice versa much change Controlling Critical Climb Angle 9 controlled changed changing characteristics model affect variables through F must know what effects using Equation 1 proceeding shown Ref 2 Appendix 1 data below obtained shows changes increase 9 Opposite changes will course decrease 9 Increasing 9 increases spiral stability vice versa effects found calculating 9 typical model changing selected characteristic see increases decreases 9 Changes increase 9 include ldecreasing vertical tail area 2decreasing aspect ratio vertical tail 3locating vertical tail forward aft horizontal tail 4decreasing vertical-tail-to-CU distance gliders models folded props decreases 9 5decreasing wing aspect ratio 6decreasing trimmed coefficient lift CL 7increasing prop diameter pitch blade area pusher props decreases 9 8increasing prop-to-CU distance 9increasing fuselage depth 1 0increasing airfoil thickness camber profile drag 1 lincreasing wing taper 12increasing wing effective dihedral 13increasing wing height location 14increasing drag items out along wing changes require structural modification except item 6 non-Scale models CL set trim requirements downthrust steepest climb Some changes less effective others practical best way control 9 per items 1 2 fact sufficiently small vertical tail size will allow model have critical climb angle 900 Ref 2 Therefore needs reduce cut away vertical tail size small amounts between successive flights until no veer-off trimmed flight path easiest tail initially sheet balsa mostly can replaced later built-up construction desired models having large dihedral tail needs changed FF Scale models however must first present reasonable minimum amount total effective dihedralabout 50 Outdoor Scale 40 Indoor Scaleand vertical tail changed needed Total effective dihedral wing dihedral plus other effective dihedral Ref 1 2 FF Scale models hinging rudder portion float freely about 148 November1997 350 either direction essentially same removing rudder particularly helpful maintaining scale appearance often occurs scale vertical tail much too large Ref1 However vertical tail must made too small undesirable Dutch roll wandering flight will occur mild Dutch roll will lessen duration violent will cause taiispin crash Fig 3 Ref 3 large dihedral violent Dutch roll usually causes barrel roll first followed tailspin tailspin spiral dive usually occurs right after being launched powered flight very helpful time- effort-saving procedure getting best vertical tail size also adequate minimum dihedral FE Scale models certain glide tests described detail Ref 1 along formulas data effective dihedral values highly recommended Pusher models jet-powered models air intake fuselages nose special problem good glide very steep straight-up climb can achieved previously described tractor models ie vertical tail size require reasonably sized front fin floats freely retracts steep climb ends undesirable gadgetry Ref 1 References 1 Making Scale Model Airplanes Fly 1997 revision also non-Scale $1595 William F McCombs 2106 Siesta Dr Dallas TX 75224 2 A Practical Approach Spiral Stability 1994 NFFS Symposium paper corrections computer program $295 two-stamp SASE above address 3 V-Tails Models Model Aviation July 1996 Appendix 1 Formulas calculating numerical values variables through F Equation 1 presented Ref2 fairly typical P-30-type rubber model capable very steep initial climb values A-F calculated B C D E F value e calculated per Equation 1 e an r 0125 0342 157_1 L 0342 540 207 0063] an[1622] very steep angle prevent probability veer-off during initial very steep dllmb e should 900 reducing vertical tail area sufficient amount using successive trial calculations per Ref 2 B becomes 116 D becomes 00239 resulting calculated value e 900 also illustrates calculations might used estimate initial value vertical tail area before doing flight glide tests Such repetitive calculations tedious time-consuming simple BASIC computer program now available Ref 2 Appendix 2 Per Ref 1 formula fairly reasonable initial value vertical tail area Av Aw X bILv Av ical tail area Aw ected wing area b wingspan Lv ance Center Gravity center Av N numerical factor Table dimensions inches example FF Rubber Scale model having Aw b v calculated area Av x 150 x 30/16 quare inches model gas powered value N would 027 resulting calculated value Av would 76 square inches Another tedious way illustrated Appendix 1 either case initial size would changed flight tests indicated necessary since both methods approximations FREE Info Pak Backed service training videos free technical hotline -OOO-35-O32 1 Ask o ope o MA7 2 owe @mitkv AnAb 2 P0 Box 5 7 UJUIIIIIIIJo M 48 06 5 7 November1997 149 mdd A4tk Features Wire Spoked Wheels Lewis Gun Deardmore Engine Full Cockpit Turnbuckles & Hardware Color Plans Photo Package Scale Documents Book CNC Machined Laser Cut Parts F OR FULL LINE OF KITS attLaa LEfl i us. uu talalug mussy sas 0 ARIZONA MODEL AaRcRAFrEk 90S0 EAST GRAY ROAD SCOTi aALE aou TO PLACE ORDER CALL 602 314-9937 FAX ORDERS 602 314-3409
Edition: Model Aviation - 1997/12
Page Numbers: 148, 149
~j~AngIe Critical Models U Bill McCombs ITS HELPFUL Free Flight modeler know given model critical climb angle above model becomes spirally unstable Regardless trimmed flight path straight turning its critical climb angle reached model will always veer off roll off undesired less-steep spiraling flight results less altitude duration Unfortunately trouble cannot corrected trim changes side thrust rudder setting wing-warping etc since instability trouble although modelers frequently try way eliminate trouble make change increases critical climb angle sufficiently course critical climb angle 900 trouble will occur since steepest model can climb 900 straight up Suppose has high-powered gas model capable 900 climb has critical climb angle say 550 As long model trimmed downthrust climb less 550 will no trouble no veer-off trimmed climb 550 highly desirable veer-off will occur climb straight up nearly change must made get critical climb angle 900 See Fig 4b Ref 3 Similarly typical high-powered rubber model will capable very steep initial climb first few seconds power burst say 80 critical climb angle 550 veer-off would occur model would first go power-wasting spiral climb upbut much smaller climb angle because reduced power Much altitude duration would lost often seen flying sessions prevent change must made get critical climb angle about 900 FF Scale models highly powered climb steeply However because small dihedral usually relatively large vertical tail critical climb angle may quite smallsometimes 00 less Therefore roll-off frequently occurs resulting spiral often spiral dive Fig 4a Ref 3 unless change made prevent course bad trim can also cause undesired spiraling spiral dive easily corrected trim changes will correct trouble spiral instability known cause models have large dihedral spiral instability does result spiral dive very little effective dihedral Ref 1 2 Scale models spiral dive common result shown aeronautical literature critical climb angle 9 can calculated 9 Arctan rCD-AB 1 L DEAF] 1 variables through F depend upon characteristics model such vertical tail size location prop size location wing dihedral effective dihedral wing sweep angle fuselage side area etc defined Ref 2 An example calculation 9 shown Appendix 1 Although calculated value 9 precise since data used calculate precisely known Equation 1 can very helpful can also used find what changes characteristics will increase critical climb angle vice versa much change Controlling Critical Climb Angle 9 controlled changed changing characteristics model affect variables through F must know what effects using Equation 1 proceeding shown Ref 2 Appendix 1 data below obtained shows changes increase 9 Opposite changes will course decrease 9 Increasing 9 increases spiral stability vice versa effects found calculating 9 typical model changing selected characteristic see increases decreases 9 Changes increase 9 include ldecreasing vertical tail area 2decreasing aspect ratio vertical tail 3locating vertical tail forward aft horizontal tail 4decreasing vertical-tail-to-CU distance gliders models folded props decreases 9 5decreasing wing aspect ratio 6decreasing trimmed coefficient lift CL 7increasing prop diameter pitch blade area pusher props decreases 9 8increasing prop-to-CU distance 9increasing fuselage depth 1 0increasing airfoil thickness camber profile drag 1 lincreasing wing taper 12increasing wing effective dihedral 13increasing wing height location 14increasing drag items out along wing changes require structural modification except item 6 non-Scale models CL set trim requirements downthrust steepest climb Some changes less effective others practical best way control 9 per items 1 2 fact sufficiently small vertical tail size will allow model have critical climb angle 900 Ref 2 Therefore needs reduce cut away vertical tail size small amounts between successive flights until no veer-off trimmed flight path easiest tail initially sheet balsa mostly can replaced later built-up construction desired models having large dihedral tail needs changed FF Scale models however must first present reasonable minimum amount total effective dihedralabout 50 Outdoor Scale 40 Indoor Scaleand vertical tail changed needed Total effective dihedral wing dihedral plus other effective dihedral Ref 1 2 FF Scale models hinging rudder portion float freely about 148 November1997 350 either direction essentially same removing rudder particularly helpful maintaining scale appearance often occurs scale vertical tail much too large Ref1 However vertical tail must made too small undesirable Dutch roll wandering flight will occur mild Dutch roll will lessen duration violent will cause taiispin crash Fig 3 Ref 3 large dihedral violent Dutch roll usually causes barrel roll first followed tailspin tailspin spiral dive usually occurs right after being launched powered flight very helpful time- effort-saving procedure getting best vertical tail size also adequate minimum dihedral FE Scale models certain glide tests described detail Ref 1 along formulas data effective dihedral values highly recommended Pusher models jet-powered models air intake fuselages nose special problem good glide very steep straight-up climb can achieved previously described tractor models ie vertical tail size require reasonably sized front fin floats freely retracts steep climb ends undesirable gadgetry Ref 1 References 1 Making Scale Model Airplanes Fly 1997 revision also non-Scale $1595 William F McCombs 2106 Siesta Dr Dallas TX 75224 2 A Practical Approach Spiral Stability 1994 NFFS Symposium paper corrections computer program $295 two-stamp SASE above address 3 V-Tails Models Model Aviation July 1996 Appendix 1 Formulas calculating numerical values variables through F Equation 1 presented Ref2 fairly typical P-30-type rubber model capable very steep initial climb values A-F calculated B C D E F value e calculated per Equation 1 e an r 0125 0342 157_1 L 0342 540 207 0063] an[1622] very steep angle prevent probability veer-off during initial very steep dllmb e should 900 reducing vertical tail area sufficient amount using successive trial calculations per Ref 2 B becomes 116 D becomes 00239 resulting calculated value e 900 also illustrates calculations might used estimate initial value vertical tail area before doing flight glide tests Such repetitive calculations tedious time-consuming simple BASIC computer program now available Ref 2 Appendix 2 Per Ref 1 formula fairly reasonable initial value vertical tail area Av Aw X bILv Av ical tail area Aw ected wing area b wingspan Lv ance Center Gravity center Av N numerical factor Table dimensions inches example FF Rubber Scale model having Aw b v calculated area Av x 150 x 30/16 quare inches model gas powered value N would 027 resulting calculated value Av would 76 square inches Another tedious way illustrated Appendix 1 either case initial size would changed flight tests indicated necessary since both methods approximations FREE Info Pak Backed service training videos free technical hotline -OOO-35-O32 1 Ask o ope o MA7 2 owe @mitkv AnAb 2 P0 Box 5 7 UJUIIIIIIIJo M 48 06 5 7 November1997 149 mdd A4tk Features Wire Spoked Wheels Lewis Gun Deardmore Engine Full Cockpit Turnbuckles & Hardware Color Plans Photo Package Scale Documents Book CNC Machined Laser Cut Parts F OR FULL LINE OF KITS attLaa LEfl i us. uu talalug mussy sas 0 ARIZONA MODEL AaRcRAFrEk 90S0 EAST GRAY ROAD SCOTi aALE aou TO PLACE ORDER CALL 602 314-9937 FAX ORDERS 602 314-3409