Touch momentarily to turn on
Napier Timer
Logic level MOSFET
1/8 W 1MEG
10
MFD 10 V
M
Fuse
Motor battery
Two common free flight timers. Luke Napier's timers (at left), note plugin fixed resistor about 1.5 grams with connectors. The Napier timer schematic (above). Pico timer with inline fuse (below). With its rugged pot and wiring it's a little heavier at 3.5 grams than a Napier or Micro-Pico timer. Pico timer schematic (below, at left).
9V Battery
120 K 1/8 W
2 MEG POT
1N914
Motor Battery
Touch 9 volt battery here to charge capacitor, turn on the MOSFET and start motor.
have very short cycle lives, so Luke Napier uses fixed plug-in resistors for setting motor run time. The values are 1megohm, 2megohm, 3megohm, 3.9megohm and 5.1megohm chosen to provide 10, 20, 30, 40 and 50 seconds run time, but in practice they will run somewhat longer or shorter de- pending on motor/prop combo. Napier sets the timer for 17 seconds of full power with a tail off to essentially no power at 20 seconds to match the rules. A handy feature of this design is the built-
in fuse. Of course if it blows you have a sur- face mount soldering job on your hands. The Pico timer is a slightly heaver C-D al- ternate with a rugged pot and in-line replace- able fuse. The power cut off characteristics
are nearly identical to the Napier timer. It re- quires an external battery to charge the ca- pacitor since the cutoff voltage of the larger MOSFET is higher than that of a single LiPo. The external battery is also somewhat of a safety factor. Another flier once picked up my model for me while I was flying with a C-D timer that used the onboard battery to start the motor. That’s right. He idly pressed the start button and was quite star- tled when it came to life. Fortunately no fin- gers were in the way of the prop and he did not drop it.
Another C-D timer is the Peterborough
MFC timer where the pot is placed in an external Zapper to vary the starting volt- age applied to the resistor-capacitor circuit.
Proposed Rules for E-20 Event
Model Eligibility: Projected wingspan not to exceed 20 inches (monoplane only) No auto-surfaces except dethermalizer
Minimum Weight: One ounce, ready to fly
Motor Eligibility: Models must use an 8.5mm diameter coreless electric motor. No gear drives allowed. (a Parkzone PKZ 3616 motor is commonly used)
Propeller: Battery:
Motor Timer: Flights:
The propeller is not to exceed 2.7 inches in diameter. The battery not to exceed 160mAh, 3.7 volt single-cell LiPo. Use of an electronic or mechanical timer is acceptable.
Three flights of 90 seconds out of unlimited attempts of 20 seconds or more.
Motor Run Time: Limited to 20 seconds max. Flyoff Flights:
For those who score three 90-second maximums, flyoffs will be with a 10-second motor run and a 2-minute max until a winner is declared. At the discretion of the CD, flyoffs may be in a mass launch style before the end of the contest.
Attempt:
A motor timer malfunction (fails to shut off or restarts once stopped) is considered to be an attempt. A flight of 20 seconds or less is con- sidered an attempt. A flight of 20 seconds or less can be used as an official flight if the contestant declares it at that time.
FLYING MODELS
The airborne unit can thus be very light. The time to cut off voltage and tail off are like any other C-D timer. For directions on how to build one, go to the 2009 technical- articles on the Peterborough web site at
www.peterboroughmfc.co.uk.
Simple is good
Allowing use of a mechanical or non-capac- itor discharge timer is opening a can of worms for the event. While all C-D timers have a similar tail off, other timers have a sharp cut off. Granted this makes transition to glide more difficult, but provides more legal power. C-D timers are cheap and in the spirit of the event. I’m not sure a $50 Atomic Workshop Zombie is. Specifying that everyone use some sort of C-D timer would even the playing field. Take a look at the timer decay plot. The green reference line is at 17 seconds, where the cut off starts and the magenta dashed line at 20 seconds. Since the motor voltage is rising as the current dies the power (volts × amps) curve is pulled up above the cur- rent curve showing more power is being pro- duced that would be indicated by the cur- rent curve alone.
The original rules proposal from the WMC specified a Napier timer. You can get one on the WMC web site as shown in the sidebar. WMC also specified a Parkzone PKZ #3616 motor. To exclude the longer more powerful 8.5mm motor, it is only necessary to specify a maximum length of 20mm.
Upcoming events
Capital Area Soaring Association Spring Sizzle, Memorial Day Weekend, Saturday May 24 and Sunday May 25, 2014, Gude Drive, Rockville, Maryland. Open flying both days from 9 a.m. No contests, just fun.
www.springsizzle.com.
Loudoun County Aeromodler’s Electric Aircraft Fly In, Saturday July 26, 2014. Fun Fly at Banshee Reeks Park, (near Leesburg, VA)
www.lcaa.org/ for details.
43
Pico Timer with Fuse MOSFET
47
MFD 10 V
M 2 Amp fuse
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