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John Krouse’s PowerUP Twin Boom Pusher (above left) weighs 11 grams with 31 square inches of area. (1.75 ounces/square foot). Thrusters Microflyer Cougar (above center) from Radio Shack 16 grams. Stew finally got this to


pacitor to keep the planes out of the rafters. Of course the power profile of a capacitor discharge motor is an exponential curve. The motor will continue to run at ever lower rpms long after it has stopped producing enough power to climb or even maintain lev- el flight. It takes a minute or two to bleed off the voltage to the point where it won’t turn the prop at all.


Let’s look at the capacitor discharge motor system in detail. These have been around for many years, but the development of the lightweight supercapacitor a few years ago made them more practical. A capacitor con- sists of two electrical conductors separated by a dielectric (insulator). Early capacitors, known then as a condensers, consisted of metal foils separated by a thin layer of con- denser paper. (Yes, the same stuff we use to cover indoor models.)


When there is a potential difference across the conductors, an electric field devel- ops across the dielectric, causing positive charge to collect on one plate and negative charge on the other plate. Energy is stored in the electrostatic field. Capacitance is the ratio of the electric charge on the conductors to the potential difference between them. It is expressed in farads or coulombs per volt. An electrolytic capacitor is a capacitor that uses an electrolyte (an ionic conducting liquid) as one of its plates to achieve a larger capacitance per unit volume than other types. Electrolytic capacitors are polarized; hence, they can only be operated with a low- er voltage on the terminal marked “–” with- out damaging the capacitor. Supercapaci- tors is the generic term for a family of electrical double-layer capacitors (EDLC) that have energy densities that are approxi-


Thruster Current Supercap Charge-Discharge 3.5 3


2.5 2


1.5 1


0.5 0


0 20 40 60 80 100 120 140 160 Time


Volts mAh


400 800 1200 1600 2000


-800 -400 0


3.5 3


2.5 2


1.5 1


0.5 0


Volts Power


make two complete circuits of the gym. Flash Fighter(above right) from Radio Shack. Model weighs in at 8 grams, but Stew found it a bit awkward to get on and off charger.


mately 10% of conventional batteries. Their power density is generally 10 to 100 times greater. These are low voltage devices and there is a limit to the rate of discharge (cur- rent) they can tolerate.


Every year at Christmas time some capac- itor powered model plane toys are in the off- ing. After Christmas, last year I picked up a Thrusters Microflyer from Radio Shack for under $10, and this year a Flash Fighter for $5. The Thrusters Microflyer has a geared motor and 84mm tractor prop. The Flash Fighter is direct drive with a 40mm pusher prop and uses three AAA cells for charging. Both actually fly pretty well, but are not well suited to indoor flying. That initial pow- er burst tends to put them in the rafters with little cruise time. The Thruster Mi- croflyer and PowerUP are charged by three AA cells for a max of 20 seconds. I took one of these chargers apart and discovered the push button switch applies the voltage from the cells directly to the capacitor and also lights the LED to indicate charging. I dug the power unit out of the Thrusters Microflyer. It weighs 10 grams all-up and looks to have a supercap of between 1–10 Farads. This supercap weighs 2 grams. I de- cided to test the unit by detaching the super- cap and using my Arduino-INA219 setup as a recording VOM to measure its characteris- tics. I had written a Visual Basic program to stick the data in an Excel spreadsheet. The results are shown in the plot labeled Thruster Current.


The charge was started at 8 seconds and completed at 34 seconds indicated by the black vertical reference line. Note the volt- age has dropped a bit after the charging has been stopped. The motor unit was removed


from the charge stand at 44 seconds indicat- ed by the blue vertical reference line and started to run. (The charge plug had kept the motor turned off while it is engaged.) By about 70 seconds on the plot, the power (volts times current) has fallen to 0.3 watts where it’s not very useful. The supercap con- tinues to discharge for another 90 seconds. Integrating the area under the charge current curve we find the capacitor has tak- en on a charge of 21.3 coulombs. Dividing by the voltage of 3.1 we arrive at a value of 6.8 farads for the supercap—about what I had guessed. Maximum power is 2.0 watts, but it sure decays fast. A 2.6-gram 70 mAh Li-Po would do a much better job of producing sus- tained power; admittedly with some compli- cation of a more sophisticated charging sys- tem and the requirement for a timer. The capacitor system power profile is more like a high start catapult launch.


When I went to the Easy Built Models web site I came across an article on Modifying kit JX09 EVG Flying Wing for PowerUP Ener- gy Boost Flight by Rich Pignata, e-mail: rpignata@nc.rr.com. The Flying Wing weight (ballasted) without the PowerUp was 27.5 grams. With the PowerUp it weighs about 30.9 grams with no ballast required. It flies for 45 seconds climbing to 40 feet in 50-foot circles. Impressive for a 28-inch span, 165 square inch model. A few years ago I built one of these Flying Wings and powered it with a geared M20 LV motor driving a K&P Aero 96–60 folding propeller powered by a 60 mAh Li-Po with a Pico timer. Not much heavier at 33 grams, but a lot more power. It climbed to 90 feet or so in 20 seconds. I needed a viscous timer to keep it from going OSS.


Thruster Power Charge-Discharge


1.5 2


0 1


-0.5 0.5


-1.5 -1


-0.50 10 20 30 40 50 60 70 80 90 100110120130140150160 Time Seconds


These two tables better illustrate Stew’s findings. Plot of charge/discharge of the Thusters Microflyer capacitor (above left). Current out is negative in this set


FLYING MODELS -2


up. Plot of power (above right) for the Thusters Microflyer unit. This time power out is positive in Watts.


47


Watts


mAh


Volts


Volts


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