SPORT AEROBATICS Ron Van Putte LAST month promised answer question Doug Dahlke Oshkosh WI sent about what part propeller important producing thrust Part reason put off answering question last months article already too long other reason difficult question answer reader group widely varying backgrounds background aeronautical engineering could make explanation would satisfy engineer leave rest readers scratching heads other hand could make explanation simple Id insult 90% readers way dont mean confuse background experience intelligence Just because someone has lot college training doesnt mean hes intelligent someone hasnt often matter back ground experience intelligence deciding expert particular subject example doubt very people PhD Astrophysics know much about doing carburetor overhaul cars point aeronautical engineers dont know much about propeller theory because never ex posed training What Im going present physical description propeller works show part contributes producing thrust would like details let know can exchange letters can put column appears sufficient interest section propeller blade small airfoil just like might find wing object airfoil sections cause change momentum air direction perpendicular plane propeller disc amount thrust propeller produces equal rate change momentum air Newtons second third laws Note what Im saying about propellers can also said about wings substitute word lift wherever see thrust Fig 1 BLADE ROTATION Lets take look very simple airfoil section shown Figure 1 doesnt look like much airfoil does However know low-speed rubber-powered airplanes get along very well propellers having blade sections look like So dont see blade sections look like typical RC airplane propellers simplest reason blade sections like have too much drag creating thrust changing momentum air direction perpendicu lar propeller disc design also causes significant changes momentum air direction propeller moving causes drag motion propeller impeded keeps engine reaching rotational speed produces maximum power So flat-plate airfoil would work propel ler werent concerned power required turn efficient ways produce thrust reduce drag same time BLADE ROTATION Figure 2 shows another potential airfoil shape propeller doesnt look too sexy either does However does betterjob producing thrust previous shape Again youve seen airfoil shape lot lowspeed propellers has virtue being easy fabricate just like flat however also produces too much drag efficient enough strong enough used highspeed propellers Fig3 BLADE ROTATION typical airfoil used high-speed propel lers something like Figure 3 reason design works us other two dont answer question Doug Dahlke asked propeller produces thrust drag because changes momentum air around change momentum air results pressure distribution vicinity propel ler air pressure acting propeller produces thrust drag Drawing Fig5 picture what air pressure distribution looks like real job because difficult because hard reach conclu sions drawing Take look Figure 4 see what mean lengths arrows indicate magnitude pressure particular point trick can used aid making conclusions about pressure acting airfoil Suppose pressure distribution acting over airfoil exactly equal add up effect pressure case youd find thrust drag propeller exactly zero means can subtract some fixed constant pressure pressures acting propeller airfoil wouldnt change net effect pressures acting airfoil Usually local ambient pressure used constant pres sure take same pressures present Figure 4 subtract local ambient pressure will get picture like Figure 5 minus signs pressure arrows top airfoil indicate pressure less ambient pressure appears suction acting top airfoil no such thing negative pressure representation shown Figure 5 just accurate mathematically determining thrust drag using actual pressures shown Figure 4 looking figure easy tell what part propeller airfoil important producing thrust top surface wins landslide very little can done increase pressure bottom side airfoil angle attack shown contrary shape upper surface airfoil can dramatically affect pressure Consequently top airfoil has much greater effect producing thrust amount effect can significantly changed altering surface optimum shape top surface function airplane engine Fig 4 32 Model Aviation purpose aircraft system Its easy either problems propeller design would have solved years ago Ive received several letters taking task over statements about relationship propel ler pitch airplane speed Most letters claim authors airplanes traveling faster supposed example long letter received Donald Stephens Oklahoma City OK wrote Some us recently got quite surprise about local clubs ran cross-country RC air race town about 47 miles northeast back friends flew Supertigre 23-powered MEN Trainer another flew old Sig Cadet eye being able land anywhere chase car MEN Trainer old WW II surplus Jeep top speed about 60 mph 15 mph south breeze Onthe crosswind legs Jeep wide open keep up MEN less half throttle downwind legs MEN flown minimum power sinuous course Jeep keep up Cadet pilot same problems except less Y3-power crosswind comfortable flying either coming back upwind response claims airplanes can fly faster would think based propeller pitch engine rotational speed other factors must wrong perhaps both Propeller pitch described ways manufacturers wrong Secondly engines unload substantially airplane speeds up rotational speed engine ends up much higher published values Ill say againyou dont get something nothing forward speed aircraft approaches value zerolift angle propeller based published values propeller pitch engine speed either both values wrongor black magic present Ron Van Putte 111 Sleepy Oaks Rd Ft Walton Beach FL 32548 -.---- -4 V V j 6 vx- 3 SOMEBODY PLEASE SPOT MY PLANETHOSE BIRDS GOT ME CONFUSED -SUPER -12 STARTS REVOLUTIONS Modelers saw specs side-by-side recently have beaten path favorite dealers take look SUPER-12 themselves Imagine 12-volt starter 250/0 power can used 24-volts start just about anything Ask dealer show fact sheet FAMOUS SULLIVAN HI-TORK STARTERSUPER HI-TORK SULLIVAN STARTER USED WITH 12-VOLT BATTERYUSED WITH 12-VOLT BATTERY Torque230inoz Horsepower2/5 HP CapabilityStarts almost engine Free-Turning RPM4400 ModelsHT HTD HTD has aluminum cone pulley stead nylon 2/5 Horsepower Torque........... 288 oz Horsepower..... 1/2 HP Capability. Starts almost engine including 1/4-scale engines Free-Turning Model ........... HTD-24 1/2 Horsepower SUPER HI-TORK SULLIVAN STARTER WITH 24 VOLTS ITwo 12-Volt Batteries Other Combination 1HP Torque........... 570 oz Horsepower .......... 1 HP Capability... Starts engines used including 1/4-scale boats helicopters etc Free-Turning Model............ HTD-24 1 Full Horsepower Free-turning RPM RPM startermeetsnoresistance Whenstartersareappliedtoa modeisengine differences RPM5 areless pronounced starters use engines Byron reduction gear WARNING Modelers Fly Near Overhead Power Lines fUu%fr P R 0 D U C T S N C 535 DAVISVILLE ROAD ow GROVE PA 19090 215 6593900 L March 1981 33 1/2 HP
Edition: Model Aviation - 1981/03
Page Numbers: 32, 33
SPORT AEROBATICS Ron Van Putte LAST month promised answer question Doug Dahlke Oshkosh WI sent about what part propeller important producing thrust Part reason put off answering question last months article already too long other reason difficult question answer reader group widely varying backgrounds background aeronautical engineering could make explanation would satisfy engineer leave rest readers scratching heads other hand could make explanation simple Id insult 90% readers way dont mean confuse background experience intelligence Just because someone has lot college training doesnt mean hes intelligent someone hasnt often matter back ground experience intelligence deciding expert particular subject example doubt very people PhD Astrophysics know much about doing carburetor overhaul cars point aeronautical engineers dont know much about propeller theory because never ex posed training What Im going present physical description propeller works show part contributes producing thrust would like details let know can exchange letters can put column appears sufficient interest section propeller blade small airfoil just like might find wing object airfoil sections cause change momentum air direction perpendicular plane propeller disc amount thrust propeller produces equal rate change momentum air Newtons second third laws Note what Im saying about propellers can also said about wings substitute word lift wherever see thrust Fig 1 BLADE ROTATION Lets take look very simple airfoil section shown Figure 1 doesnt look like much airfoil does However know low-speed rubber-powered airplanes get along very well propellers having blade sections look like So dont see blade sections look like typical RC airplane propellers simplest reason blade sections like have too much drag creating thrust changing momentum air direction perpendicu lar propeller disc design also causes significant changes momentum air direction propeller moving causes drag motion propeller impeded keeps engine reaching rotational speed produces maximum power So flat-plate airfoil would work propel ler werent concerned power required turn efficient ways produce thrust reduce drag same time BLADE ROTATION Figure 2 shows another potential airfoil shape propeller doesnt look too sexy either does However does betterjob producing thrust previous shape Again youve seen airfoil shape lot lowspeed propellers has virtue being easy fabricate just like flat however also produces too much drag efficient enough strong enough used highspeed propellers Fig3 BLADE ROTATION typical airfoil used high-speed propel lers something like Figure 3 reason design works us other two dont answer question Doug Dahlke asked propeller produces thrust drag because changes momentum air around change momentum air results pressure distribution vicinity propel ler air pressure acting propeller produces thrust drag Drawing Fig5 picture what air pressure distribution looks like real job because difficult because hard reach conclu sions drawing Take look Figure 4 see what mean lengths arrows indicate magnitude pressure particular point trick can used aid making conclusions about pressure acting airfoil Suppose pressure distribution acting over airfoil exactly equal add up effect pressure case youd find thrust drag propeller exactly zero means can subtract some fixed constant pressure pressures acting propeller airfoil wouldnt change net effect pressures acting airfoil Usually local ambient pressure used constant pres sure take same pressures present Figure 4 subtract local ambient pressure will get picture like Figure 5 minus signs pressure arrows top airfoil indicate pressure less ambient pressure appears suction acting top airfoil no such thing negative pressure representation shown Figure 5 just accurate mathematically determining thrust drag using actual pressures shown Figure 4 looking figure easy tell what part propeller airfoil important producing thrust top surface wins landslide very little can done increase pressure bottom side airfoil angle attack shown contrary shape upper surface airfoil can dramatically affect pressure Consequently top airfoil has much greater effect producing thrust amount effect can significantly changed altering surface optimum shape top surface function airplane engine Fig 4 32 Model Aviation purpose aircraft system Its easy either problems propeller design would have solved years ago Ive received several letters taking task over statements about relationship propel ler pitch airplane speed Most letters claim authors airplanes traveling faster supposed example long letter received Donald Stephens Oklahoma City OK wrote Some us recently got quite surprise about local clubs ran cross-country RC air race town about 47 miles northeast back friends flew Supertigre 23-powered MEN Trainer another flew old Sig Cadet eye being able land anywhere chase car MEN Trainer old WW II surplus Jeep top speed about 60 mph 15 mph south breeze Onthe crosswind legs Jeep wide open keep up MEN less half throttle downwind legs MEN flown minimum power sinuous course Jeep keep up Cadet pilot same problems except less Y3-power crosswind comfortable flying either coming back upwind response claims airplanes can fly faster would think based propeller pitch engine rotational speed other factors must wrong perhaps both Propeller pitch described ways manufacturers wrong Secondly engines unload substantially airplane speeds up rotational speed engine ends up much higher published values Ill say againyou dont get something nothing forward speed aircraft approaches value zerolift angle propeller based published values propeller pitch engine speed either both values wrongor black magic present Ron Van Putte 111 Sleepy Oaks Rd Ft Walton Beach FL 32548 -.---- -4 V V j 6 vx- 3 SOMEBODY PLEASE SPOT MY PLANETHOSE BIRDS GOT ME CONFUSED -SUPER -12 STARTS REVOLUTIONS Modelers saw specs side-by-side recently have beaten path favorite dealers take look SUPER-12 themselves Imagine 12-volt starter 250/0 power can used 24-volts start just about anything Ask dealer show fact sheet FAMOUS SULLIVAN HI-TORK STARTERSUPER HI-TORK SULLIVAN STARTER USED WITH 12-VOLT BATTERYUSED WITH 12-VOLT BATTERY Torque230inoz Horsepower2/5 HP CapabilityStarts almost engine Free-Turning RPM4400 ModelsHT HTD HTD has aluminum cone pulley stead nylon 2/5 Horsepower Torque........... 288 oz Horsepower..... 1/2 HP Capability. Starts almost engine including 1/4-scale engines Free-Turning Model ........... HTD-24 1/2 Horsepower SUPER HI-TORK SULLIVAN STARTER WITH 24 VOLTS ITwo 12-Volt Batteries Other Combination 1HP Torque........... 570 oz Horsepower .......... 1 HP Capability... Starts engines used including 1/4-scale boats helicopters etc Free-Turning Model............ HTD-24 1 Full Horsepower Free-turning RPM RPM startermeetsnoresistance Whenstartersareappliedtoa modeisengine differences RPM5 areless pronounced starters use engines Byron reduction gear WARNING Modelers Fly Near Overhead Power Lines fUu%fr P R 0 D U C T S N C 535 DAVISVILLE ROAD ow GROVE PA 19090 215 6593900 L March 1981 33 1/2 HP