PHOTOS: CAPT. PAT DAILY (USN RET)
Capt. Pat Daily (USN ret) takes many of the photos for Stew. Here (at left) he captures his tarted up GWS Tiger Moth. Capt. Pat builds beautiful scratchbuilt models as well. Again (above) a self photo of his Velie Monocoupe.
sight and/or the battery is discharged below its self-destruct point. Small brushed mo- tors drawing less than 2 amps can be run di- rectly by a timer like the Pico timer or the Atomic Work Shop
http://www.atomicwork
shop.co.uk Zombie which has a built in ESC (Electronic Speed Control). The excellent Zombie has a throttle function, the Pico does not—more about how to cope with that later.
I might mention here it is highly desirable to use a small fuse with a Zombie. I have been given several to diagnose that were burned out by a stalled motor. Brushless motors and brushed motors drawing more power require the use of an external ESC and a timer with a PPM (R/C servo) pulse to drive them. K&P Aero
http://www.kpaero. com has suitable timers as does Micro Flier Radio
http://www.microflierradio.com.
Of
course my Stone Age E-36 timer will work as well. BMJR,
http://www.bmjrmodels.com has these.
For trimming, I prefer using a small re- ceiver in lieu of the speed adjustable timer, since you can cut the power immediately in the case of an impending flight disaster. This not only keeps from burning out a stalled motor wrapped with grass, breaking a prop or burning out a timer, but usually reduces the force of the ensuing crash to the point of little or no damage to the model. There are several small receivers on the market that weigh less than the timer they replace in this scenario. You don’t need a full range receiver since trimming is done close enough to you for you to see what is going on.
For the smallest models a Plantraco re- ceiver will run up to a J-3 motor, you will
need to change the battery connection to suit your setup. If you have a DSM2 compatible transmitter, I like
http://www.deltang.co.uk DT receivers. These weigh less than half a gram with connectors, and have both a built- in brushed 2-amp ESC and a PPM pulse out- put to drive an external ESC. I would even go so far as to recommend taking the weight hit of three grams to use a PKZ brick receiver strapped on the model for initial trim flights. You are only going to use the built-in ESC, not the servos. An AR6400 will provide the option of driving an external ESC for larger models.
Let’s look at an actual example of this pro- cedure, really a cautionary tale, with a PKZ J-3 motor. I have built several Comet Fokker D-7 Dime Scalers over the years. These have always flown well even if of du- bious scale appearance. I decided to see what one would do with electric power. I pulled a beat-up mediocre example down from its ceiling perch. The fuselage was a bit narrow for a PKZ brick so I decided to make it freeflight. I installed a J-3 motor with a 70 mAh battery and used a Plantraco receiver to control the motor. It did not weigh much more than the rubber version.
Being Winter, I went to the gym to try it out. It took off nicely and I immediately throttled back for a gentle climb. I shortly cut power completely to keep it out of the rafters. It had a nice glide to landing and was trimmed to circle. Outside on a calm day, half throttle resulted in leisurely circu- lar climb and glide.
Thus encouraged, I removed the radio and installed a Pico timer. The resulting flight was a disastrous power stall. I re- placed the Cub prop with a Citabria prop
put on backwards to reduce thrust. The stall was still there. I put in a ridiculous amount of down thrust and shifted the c.g. forward. I then had to increase the decalage to keep the glide from being a crash dive and the climb was still too fast as it spiraled up unrealistically.
Let’s face it, that little 4-inch prop looked anomalous compared with the 7-inch rubber prop I had been flying it with. That was re- ally the clue to my problem; small fast spin- ning props require more thrust offset than larger slower turning props even with rub- ber power. That and way too much power were the problem. The 16-inch span D-7 was quite a bit smaller than the 24-inch J-3 Cub the motor came from.
The solution was to take all of the ill con- ceived adjustments out and re-trim it for a decent glide. Then put the receiver back and note the throttle position for a happy climb. I used a DT receiver and my DX6i transmit- ter so I could better judge throttle position. I then used a tach to measure the resulting rpm for that setting. If I were going to use a Zombie or other timer with a speed adjust- ment, I would simply set it for that rpm. You can do this with a ducted fan as well. Since I wanted to use one of my Pico timers and save the Zombie for a nicer mod- el, I measured the voltage that resulted in the same rpm/throttle setting with a fully charged battery. I then removed the receiver and put a couple of 1N4000 diodes in series with the battery to drop the voltage. You get about a 0.75 volt drop per diode. It’s a bit crude but it works. The diodes get a bit hot but not dangerously so, since we are draw- ing less than an amp. Don’t try this with smaller diodes or bigger motors.
PHOTOS: JOHN KROUSE
John Krouse’s Micro-E Solar Challenger (above left), a TCFF (Throttle Controlled Free Flight). A model of Paul MacCready’s trans-channel electric plane that flew from Paris to London in 1981. Bottom view of John Krouse’s Solar Challenger (above center). Span is 16 inches, with 32 square inches area. Tail boom is 4 mm thin wall tube. Some specs on John Krouse’s
FLYING MODELS
Challenger (above right): fuselage 1⁄8-inch balsa. Motor 6x12mm 4.5 ohm pager direct drive. Prop BP 56mm trimmed to 50 mm. Receiver 72 MHz Micro Invent Minor. Battery 60 mAh Li-Po. Weight 10.9 grams with battery. The motor is mounted with a “Krouse rod” of soft aluminum that makes for easy thrust adjustment.
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