a________ 47oIk qbout Brad Powers does wing cambered airfoil alone have negative moment What kind airfoil needed flying wing design wasnt North rop flying wing good bombing platform IT WAS IN 1978 first submitted article Model Aviation entitled About Size Presented exercise dimensional analysis explained things such weight speed power etc vary size scale tried make material understandable bright 12-yearofd well-received readers letters received part modelers SOs 60s people like myself re kindled youthful interest modeling No 1 2-year-olds bright otherwise AC mmiii seemed reading stuff least didnt correspond think too bad because something spell binding about model airplanes seems stay life case letters readers indicated quite knowledge able presumes write arti cles better make blunders lest incur wrath experts article December 1984 issue stated mbered airfoil alone exhibits positive moment.. case sloppy proofreading own material before sending magazine What meant say cambered wing alone exhibits negative moment attached fuselage having no tail combination generally produces positive moment Needless say received lot mail about also received some mail saying decalage criteria stability realize decalage really form trim stability parameter nevertheless fundamental flight itself case tailless designs does fact become closely allied stability Lets see cambered sections fact have negative diving moments cambered section having curved median line shown Fig 1 Clark Y airfoil good example such section definition Aerodynamic Center point no moment Failing point moment constant respect Aerodynamic Center does change angle attack given speed Fig 1a section shown high angle attack AC 25% chord cp center pres sure resultant Lift actsin case just aft AC thus producing moment about AC product high Lift very small arm b moderate angle attack Fig3 72 Model Aviation Ij 4-CP La are among things author discusses b C L L H Fig constant moment cp moment product less Lift however acts longer arm thus producing moment equivalent c angle attack very small Lift also very small acts very long arm way back near trailing edgeand again produces moment equivalent b can say least infer angle attack given speed moment about AC will constant will lie curve con stant moment shown Fig 2 moment will Negative Diving moment because lift acts aft AC tending pitch nose down Such airfoil said unstable ie cant made fly itself must have trimming surface either behind conventional design front canard integral part tailless design Such trimming surface must set angle trim wing keep pitching down angular differ ence between two surfaces called decalage sometimes called lon gitudinal dihedral misnomer since has nothing dihedral earlier article discussed sta bility conventional configuration said stable airplane must have CG forward neutral point defined AC whole airplane Now lets see applies tailless design Obviously cant use unstable cambered airfoil dont have tail can So will have use stable airfoil such shown Fig 3 Its mean line turns up trailing edge cause nose pitch UP rather down dive did Clark Y example proper placement GO such wing alone can made fly stable fashion Turning up trailing edge has effect moving AC forward bit usually about 23% 24% chord Neutral Point stable airfoil point aft CG must go stability maintained Since decalage short coupled can regard upturned TE tail limited far forward CO can placed have good pitch control 20% chord MAC far forward practicable result tailless designs have very limited permissible CG travel range about 3% MAC limited CG range reason Northrops flying wing wasnt very suc cessful bomber too limited CO travel provide good bombing platform whereas conventional de signs long tails such B-24 B-17 comparatively steady rock other hand modern tailless de [ AR8 S sq V 322 27 60 KL MAV 78 9 Permissible Travel 24 inches Fig 4a 1 1861 56 inches AR sq signs utilize very low aspect ratios such delta configuration have such long chords 3% CG travel isnt bad Also extending long forebody adding lateral fins travel can extended somewhat shown Fig 4b AC moved forward Because long chord high speeds tend prevent oscil lations steady platform achieved Just see works lets make comparison Lets design wing similar Northrops compare delta wing equal area An empirical rule once developed determine proper Taper Ratio versus Aspect Ratio retained good stalling characteristics 20 AR TRavg wingsnot deltas 20 AR say aspect ratio 8 will fair approximation Northrops wing TR 2 decide area say 450 sq wing will have dimen sions shown Fig 4a MAC will just under 8 permissible CG travel will slightly less quarter inchpretty limited model 5-ft span other hand delta configu ration shown 4b will have over half inch permissible travelcomparable conventional model Because pressure distribution over both upper lower surfaces essentially same symmetrical air foil no camber has no moment about AC its AC 25% chord Such airfoils usually used tailless designs because simpler design build shown Fig 3 UP elevon deflection converts section stable Questions comments may ad dressed Brad Powers 5470 Castle Hills Dr San Diego CA 92109 Flying Near Al rports Careful Safe Flying No Accident October 1985 73 IiMAC 9 Travel Fig4b I
Edition: Model Aviation - 1985/10
Page Numbers: 72, 73
a________ 47oIk qbout Brad Powers does wing cambered airfoil alone have negative moment What kind airfoil needed flying wing design wasnt North rop flying wing good bombing platform IT WAS IN 1978 first submitted article Model Aviation entitled About Size Presented exercise dimensional analysis explained things such weight speed power etc vary size scale tried make material understandable bright 12-yearofd well-received readers letters received part modelers SOs 60s people like myself re kindled youthful interest modeling No 1 2-year-olds bright otherwise AC mmiii seemed reading stuff least didnt correspond think too bad because something spell binding about model airplanes seems stay life case letters readers indicated quite knowledge able presumes write arti cles better make blunders lest incur wrath experts article December 1984 issue stated mbered airfoil alone exhibits positive moment.. case sloppy proofreading own material before sending magazine What meant say cambered wing alone exhibits negative moment attached fuselage having no tail combination generally produces positive moment Needless say received lot mail about also received some mail saying decalage criteria stability realize decalage really form trim stability parameter nevertheless fundamental flight itself case tailless designs does fact become closely allied stability Lets see cambered sections fact have negative diving moments cambered section having curved median line shown Fig 1 Clark Y airfoil good example such section definition Aerodynamic Center point no moment Failing point moment constant respect Aerodynamic Center does change angle attack given speed Fig 1a section shown high angle attack AC 25% chord cp center pres sure resultant Lift actsin case just aft AC thus producing moment about AC product high Lift very small arm b moderate angle attack Fig3 72 Model Aviation Ij 4-CP La are among things author discusses b C L L H Fig constant moment cp moment product less Lift however acts longer arm thus producing moment equivalent c angle attack very small Lift also very small acts very long arm way back near trailing edgeand again produces moment equivalent b can say least infer angle attack given speed moment about AC will constant will lie curve con stant moment shown Fig 2 moment will Negative Diving moment because lift acts aft AC tending pitch nose down Such airfoil said unstable ie cant made fly itself must have trimming surface either behind conventional design front canard integral part tailless design Such trimming surface must set angle trim wing keep pitching down angular differ ence between two surfaces called decalage sometimes called lon gitudinal dihedral misnomer since has nothing dihedral earlier article discussed sta bility conventional configuration said stable airplane must have CG forward neutral point defined AC whole airplane Now lets see applies tailless design Obviously cant use unstable cambered airfoil dont have tail can So will have use stable airfoil such shown Fig 3 Its mean line turns up trailing edge cause nose pitch UP rather down dive did Clark Y example proper placement GO such wing alone can made fly stable fashion Turning up trailing edge has effect moving AC forward bit usually about 23% 24% chord Neutral Point stable airfoil point aft CG must go stability maintained Since decalage short coupled can regard upturned TE tail limited far forward CO can placed have good pitch control 20% chord MAC far forward practicable result tailless designs have very limited permissible CG travel range about 3% MAC limited CG range reason Northrops flying wing wasnt very suc cessful bomber too limited CO travel provide good bombing platform whereas conventional de signs long tails such B-24 B-17 comparatively steady rock other hand modern tailless de [ AR8 S sq V 322 27 60 KL MAV 78 9 Permissible Travel 24 inches Fig 4a 1 1861 56 inches AR sq signs utilize very low aspect ratios such delta configuration have such long chords 3% CG travel isnt bad Also extending long forebody adding lateral fins travel can extended somewhat shown Fig 4b AC moved forward Because long chord high speeds tend prevent oscil lations steady platform achieved Just see works lets make comparison Lets design wing similar Northrops compare delta wing equal area An empirical rule once developed determine proper Taper Ratio versus Aspect Ratio retained good stalling characteristics 20 AR TRavg wingsnot deltas 20 AR say aspect ratio 8 will fair approximation Northrops wing TR 2 decide area say 450 sq wing will have dimen sions shown Fig 4a MAC will just under 8 permissible CG travel will slightly less quarter inchpretty limited model 5-ft span other hand delta configu ration shown 4b will have over half inch permissible travelcomparable conventional model Because pressure distribution over both upper lower surfaces essentially same symmetrical air foil no camber has no moment about AC its AC 25% chord Such airfoils usually used tailless designs because simpler design build shown Fig 3 UP elevon deflection converts section stable Questions comments may ad dressed Brad Powers 5470 Castle Hills Dr San Diego CA 92109 Flying Near Al rports Careful Safe Flying No Accident October 1985 73 IiMAC 9 Travel Fig4b I