ElectricFlight Timer Decay 5.5mm Prop & 150 mAh LiPo 4
3.5 3
2.5 2
1.5 1
0.5 0 0 3 69 ILLUSTRATIONS: STEW MEYERS
Tail off for the Micro-Pico Timer Green line (above left) shows the start of power cut off at 17 seconds. Magenta line at 20 seconds indicates loss of power as FET shuts off. Since the motor voltage is rising as the current dies, the power curve (volts x amps) rises
T
here is a new electric free flight event—E-20 being promoted for small field flying. The event rules are a reaction to the spectacular performance of E-36 models and are de- signed to keep things simple and limit out-of- sight flights. The E-20 event rules have been submitted to NFFS and are under review by the Competition Committee. The proposed rules are as detailed in the sidebar.
E-20 power system Let’s look at the motor system and its im- plications. Horizon Hobby’s ParkZone #3616 motor is a 8.5×20mm coreless motor used on their P-51 and T-28. The motor shaft is 1mm. This comes with a pinion gear that en- gages the output spur gear on the model’s gear box. This pinion must be removed to use in the direct drive configuration. You need to use a GWS or similar gear/wheel puller to remove the pinion. Once it’s off, it’s off. You are never going to be able to replace it unless you heat the gear and chill the shaft. Trying to press it on will only pop out the brush assembly at the back. These motors come in two windings; a 1.0-
ohm version with a white back used on the Champ, early Sukhoi, and Mosquito and a
E-20 Resources MOTORS AND PROPS
Bob Selman Balsa Products
www.bsdmicrorc.com www.bphobbies.com
Horizon Hobby
www.horizonhobby.com Plantraco
www.microflight.com TIMERS
Peterborough Model Flying Club,
www.peterboroughmfc.co.uk
Willamette Modelers Club of Oregon,
www.willamettemodelersclub.weebly.com/
42
GWS gear puller removing a pinon (at left) from a ParkZone Champ motor. Micro-Pico timer (above) features an all-up weight of 2 grams and is almost identical to the Napier timer except that it uses a small pot rather than fixed resistors to set time.
MAY 2014
above the current curve. This shows more power is being produced than the current curve alone would indicate. Power Plot (above right) generated by Bergmeyer equations for the #3616 P-51 motor. Peak power @ 3.7 volts is 3.5 watts @ 19,000 RPM.
black back 0.8-ohm motor used on the rest of the ParkZone models. These differ only in the length of the motor leads. There is also a longer 8.5mm motor that is more powerful and the rules will have to cope with this. These motors can be easily burnt out if they are stalled, say by grass wrapped around the prop after a bad launch. It is a very good idea to use a small fuse that will blow when the current exceeds three amps. Using the motor in the direct drive mode without a gear box means any propeller larger than 2.6 inches in diameter will over- load the motor, causing it to overheat and produce less power. The high-revving, small diameter prop imposes some design con- straints on the model. Thrust offsets are less effective than with a larger prop. The small prop is not well matched to the airframe at low airspeeds. You need a clean design and a good heave at launch to get it up to speed where the prop will be more effective. What prop can you use? Bob Selman has GWS 2508 and 2510 props, Balsa Products, a 57×20 prop, as well as Plantraco’s Tri-Turbo 65mm prop. You might also look at helicopter tail rotors and mini quad- copter props. Just remember you need to let the motor rev up so it is operating below its peak power point.
The 3616 motor has a Kv (rpm/volt) of ap- proximately 10,000. So with 3.7 volts from a single LiPo the maximum rpm will be 37,000 rpm. The peak power point is at half
the maximum rpm or 18,500 rpm. Don’t load the motor below this rpm. You do want to use a good 160mAh LiPo as the voltage drop at three amps is considerable. We are using the cell at a 20C rate. A 40C rated cell would be a good bet. Get a few and check the inter- nal resistance to see which is best.
Keeping time In keeping with the simple approach for
E-20, the Willamette Modelers Club (WMC) of Oregon suggested the Napier timer, a simple capacitor-discharge (C-D) timer. This circuit has been around for ages, it is nothing more than a parallel resistor-capacitor circuit with the gate of a FET connected to the junction of the resistor-capacitor. A momentary switch in the circuit allows the battery to charge up the capacitor and start the timer. When the switch is depressed, battery power flows through the fuse, through the switch and charges up the ca- pacitor, this happens very fast. While the ca- pacitor is charged, the FET is turned on and battery power flows through the fuse, through the motor, through the FET back to the bat- tery. This is what makes the motor run. The motor will run until the capacitor is mostly discharged through the resistor, at this point the motor starts slowing down as the FET begins to turn off. This tail off process takes a few seconds, when the ca- pacitor discharges enough the FET will turn off completely and the motor stops. Miniature trim pots (potentiometers) can
12 15 18 21 Seconds
24 27
Volts mA.
Watts
1600 1400
1200 1000 800 600 400 200 0
50
40 30
10 20
0 0 0.05 0.1 0.15 0.2 0.25 0.3 Torque in-oz 0.35 0.4
RPM Amps Watts
by stew meyers You can reach Stew Meyers at 8304 Whitman Drive, Bethesda, Maryland 20817, or via e-mail at
stew.meyers@
verizon.net
P-51 Motor, Kv = 10.815 KRPM/Volt R = 0.895 Ohms, Bergmeyer Calculation @ 3.7 V
5
3 4
2 1 0 0.45 0.5
Volts - Watts
Milliamps KRPM
Amps - Watts
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68