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Tech Talk - ELECTRO - FLYING

by Gerry "ELEKTROPHART" Doubleday

           "OK,  Chapter 1 - What Do all of Those Numbers Mean ?? ........... IN SHORT - Very Little !!

           ELECTRIC MOTORS

           There are two basic types of electric motors used for model aviation.

            Brush type motors have been available for over 20 years . Typically these motors carry designations such as Speed 300 / 400 / 480 / 600 etc . The only dependable convention of these designations is that a Speed 600 is larger than a Speed 400 . With-in a " size " there may be many motors of varying power outputs and efficiencies . Some motors , such as cobalt motors , have external ( replaceable ) brushes and quite strong " rare - earth " magnets and may be relatively powerful and efficient .

           Most brushed motors are " can " motors with non - replaceable internal brushes . These motors are very inexpensive , but , ( unfortunately ) you get what you pay for ! These motors are very inefficient ( typically 50 - 60 % ) and have a short service life . Many , if not most , park flyer ARFs come with a can motor . I used to think that most were not worth the trouble to install ! Now with the advent of high power Lithium Polymer ( LiPo ) batteries enough weight has been taken out of park flyer models that even a can motor can give reasonable performance ! But should you install that can motor ??

           The darlings of the electric motor field are now the brushless motors

- particularly the outrunner motors. The outrunners are so named because the windings are in the center of the motor with the magnets and shaft are attached to the rotating case . Outrunners yield high torque since most of the mass of the motor is turning.

           Brushless motors are " controlled " by an electronic speed controller (ESC). The brushless ESC does does much than just control the motor speed it also performs the functions of brushes ( current switching ) much more efficiently with much less heating . With less heating and tighter tolerance manufacturing , brushless motors are much more efficient and can generally carry higher input voltage .

            Premium brushless motors have operating efficiencies over 90 % . Even " cheap ' brushless motors have 70+ % efficiency . With higher efficiencies and higher operating voltage , the power levels of brushless motors are MUCH greater than a "can" brushed motor of similar size & weight .

           BUT what ( or maybe I should say watt ) do the numbers mean . Motor numbers such as 4120/16 , 30/3 , 28/12/3 are common - it is all so confusing .

           Different manufacturers use differing numbering systems usually referring in some way to the motor diameter ( in millimeters ) , stator length , number of wire winding turns on the motor poles . With each manufacturer having their own system - how can you compare ?

           In truth , you can't very well . The most dependable number is the weight of the motor . You can pretty well believe that a 10 ounce motor will be more powerful than a 5 ounce motor - DUH , no kidding !!

           Honestly - you can depend that a 1 ounce motor can fly the hell out of a 10 ounce airplane and will fly a 15 ounce airplane pretty well . Using this rule of thumb , a 2 ounce motor can fly the hell out of a 20 ounce airplane and a 15 ounce motor can fly the hell out of a 150 ounce airplane .

           An old convention would compare input watts ( basically nominal volts times motor amps ) with airplane weight . As an example , a 3 ounce motor with a 20 amp rating operating on a 3 cell LiPo ( 11.1 nominal volts ) would have max input of 222 watts .

           There is an older " rule of thumb " that said that it took 100+ watts per pound for a high performance airplane , 75 watts per pound for a sport flyer and ( maybe ) 50 watts per pound for an easy flyer . Therefore our 222 watt motor should fly a 2.22 pound ( 35.5 ounce ) model strongly . This agrees with the new " rule of thumb which says the 3 ounce motor will fly a high performance 30 ounce model and a 45 ounce model strongly . The trouble with the old convention was that the can brushed motors - usually with a gearbox are less efficient than today's outrunner brushless motors .

           The old convention can understate the performance of current motor systems .

           Recently , the price of brushless motors and ESCs have dropped like a rock . The VERY popular 400 size motor and ESC - so named because they about the same physical size of a Speed 400 can brushed motor - can be bought for $ 50 !! These motors weight about 1.5 ounces and can make a 15 ounce Alpha warbird really scoot . It can also fly a 25 - 28 ounce Miss 2 easy flyer with authority .

           Incidentally , the can motor / gear box / brushed ESC previously recommended for the Miss 2 would also cost about $ 50 and would fly with less power for about half as long on the same battery pack .

           Larger , very powerful brushless motor systems are showing up at all levels of model aviation including scale and championship 3 D . Performance can be equal or some may say better than glow / gas - but at a hefty premium cost due to battery pack costs . The real impact of brushless electrics is in park flyers and indoor flying . Small systems have really opened up indoor flying with 3 D models - with some models weighing 1 ounce ( or less ) .

           NEXT - If you think motor numbers are confusing , just wait to hear about batteries !!

           Gerry Doubleday

           Macon Aero Modelers, Inc.

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