Frequently Asked Questions - 2007/10
Bob Aberle | [email protected]
Q316 — Using Lithium batteries in your RC transmitter
Q: "I’ve read a lot recently on the various model nets and forums about replacing Ni-Cd and NiMH RC transmitter batteries with lithium batteries. Is this technically possible? Would I benefit by doing this?"
A: It is certainly possible from a technical standpoint. I suspect that many RC manufacturers will be switching to lithium-type batteries with their next-generation equipment.
At full charge, Ni-Cd and NiMH eight-cell battery packs provide approximately 10.5 volts. The built-in transmitter low-voltage alarms generally go off at roughly 8.8 volts. A three-cell Li-Poly battery will supply 11.1 volts (nominal) at full charge. The slight extra voltage shouldn’t be a problem for most RC transmitters, but if you go this route you might check with your particular RC-system manufacturer. You might also consider using Li-Ion cells such as the A123. With Li-Ion cells you can obtain close to 10 volts at full charge (about 3.3 volts per cell with three cells). The alarm’s going off at 8.8 volts (if it does!) will still prevent your lithium cells from being discharged to less than the minimum recommended discharge voltage of approximately 3.0 volts per cell.
Your biggest considerations are going to be how to fit the lithium battery pack into your transmitter and how to charge it. If you use a 2100 mAh-capacity Li-Poly battery, it would normally be charged at 1C, or 2.1 amps. Many RC-transmitter internal charging circuits have a fuse, which might blow at that amount of current. Therefore, I suggest that you charge the lithium battery externally to the transmitter.
In my example, I removed the rear battery-compartment cover on my Hitec Optic 6 transmitter and swapped in a three-cell ParkZone 2100 mAh Li-Poly battery pack, which is attached with Velcro tape. The main power leads are connected directly to the transmitter connector (you should be able to obtain a mating connector from the RC manufacturer). The balanced charger connector on the pack is easily accessible when it comes time to charge, allowing for direct charging of the battery (not through the transmitter circuit).
Because of the size of the particular Li-Poly battery I selected, it is impossible to use the battery-compartment cover. But since the pack remains almost flush with the case, I didn’t see that as a problem. I did find several three-cell Li-Poly packs that were configured to fit into a transmitter so the compartment cover would fit in place. If you go that route I still recommend that you access the battery and charge it directly.
From a cost standpoint, a replacement eight-cell, 1700 mAh Ni-Cd battery pack would cost me approximately $35 with a connector. I’ve found three-cell, 2100 mAh Li-Poly packs for as little as $40. You will have to purchase and attach the necessary mating transmitter connector, but the difference in price is almost negligible.
One of the advantages of using a Li-Poly battery is that it is charged at a 1C rate, which should take only an hour to bring a depleted pack to full charge. I use a new FMA Direct Cellpro 4S (4 amp) balanced charger that allows me to use a 2C (4 amp) charge current that requires only 30 minutes for a full charge. That means it would take little time to get the transmitter ready for a flight session.
In addition, the lithium batteries' extremely low self-discharge rate means a charged pack will remain that way for a few months, although for long-term storage the packs should be at roughly half charge (half capacity). I like using lithium batteries on RC transmitters that have an LCD screen; it continually displays the transmitter voltage with a digital readout. That way I always know where I am with respect to the alarm point.
As I mentioned, RC manufacturers will soon be adapting their transmitter battery compartments to accept Li-Poly and Li-Ion. Internal charging circuits will have to be adapted to the higher current levels. Improved battery safety, thanks to balanced charging, should make it possible and safe to charge the lithium packs when they are located inside an RC transmitter. I believe this is the way we will be going in the future.
Q317 — Plans showing only half a wing
Q: "I purchased a set of full size plans from a hobby publication and was disappointed to see only half of the wing shown. How am I supposed to build the other wing panel that is not shown?"
A: Since I’ve designed and had published more than 50 models in the past 35 years, I can give you some insight into this situation and provide a suggestion. When I show only half a wing on my plans, it isn’t because I’m lazy! The larger the full-size plans, the more that set of plans costs. That being the case, I often draw only half the wing to keep the size of the plans to the minimum.
But how do you build the half of the wing that is not shown? Surely you can’t build two right wing panels! I turn the plans over and rub mineral oil onto them with paper towels. This type of oil is available at almost every pharmacy. Almost immediately the plans paper turns translucent, showing the reverse image you need to build the other panel.
After oiling the plans I position parchment paper (available by the roll in supermarkets) over them. This prevents the wood from sticking to the plans and prevents what is left of the mineral oil from getting on the model. The nice thing about mineral oil is that it dries quickly, leaving the plans almost like new for storage purposes and later use. I found that vegetable oils and olive oil did not work as well and that the residue practically ruins the plans.
Q318 — Small brushless motors for free flight
Q: "I've seen several of the new, very small and lightweight brushless outrunner motors. The prices seem to be continually coming down making them quite attractive. But I'm a free flight modeler and was wondering if these new motors can be adapted to aircraft that don't employ RC control?"
A: In one of last year’s columns I wrote about motors and CL use. For that application electric-power expert Sergio Zigras, who operates the company Z-Tron, developed a timer control. I have learned that Sergio has developed and is now manufacturing a motor timer for FF applications. The device costs $30 and weighs only 2 grams. It will work with brushed and brushless motors and their respective speed controllers.
Motor run time can be adjusted to between one and 63 seconds in one-second increments. The motor speed control can be varied from full off to full on in 63 steps. And if that weren’t enough, you can hook up a microservo to operate a DT (like a pop-up stabilizer), which can be set from 0.25 to 15 minutes.
I made a power package using either a BSD Micro RC Mighty Midget 13/4/15T brushless outrunner motor or the new Horizon E-flite Park 180 motor and the Phoenix-10 brushless ESC, the Z-Tron timer, a Blue Arrow 2.5-gram servo (to operate the DT), and an FMA Direct Cellpro two-cell, 300 mAh Li-Poly battery. All those components resulted in a total airborne weight of 1.75 ounces. An .049-powered (1/2A) FF model typically weighs 5 ounces.
Here is your chance to build a Nostalgia FF design, such as the Fubar 36, the Mini-Hogan 34, or the Baby Phoenix, and power it with electric. Being able to limit the motor-run duration and power level, along with a DT, allows you to fly it easily in a small field or school yard. With quiet electric power you can even fly at dawn and no one will know you are doing it.
You can purchase the Z-Tron FF timer from BSD Micro RC for $30. Bob Selman has the complete Z-Tron instructions posted at: www.bsdmicro.com/pdfs/Products/FF_Timer.pdf
Q319 — RTF indoor RC models trend
Q: "I saw several references recently to RTF models that are now being made available expressly for indoor RC flying. Is this becoming a trend? Should I expect to see more and more micro RTF RC indoor models coming on the hobby market?"
A: Yes! Plantraco in Canada was one of the first to offer a complete RTF indoor RC model package, which included the RC and power system. It has been reported about favorably in the modeling press.
In a recent FAQ column I mentioned the 18-gram ParkZone Cessna 210 Centurion that Horizon Hobby is selling. Another new entrant, which was shown at the 2007 WRAM Show in New York and the Toledo Show, is Cox Hobby Distributors' Lil Playboy. The replica Old-Timer design has an 18-inch wingspan and weighs 45 grams (1.6 ounces). A 135 mAh Li-Poly battery can provide about eight minutes of flight on a charge. The street price is expected to be $39, and the model will initially be available at www.coxmodels.com.
If you are already a sport RC flier and want more details about indoor RC flying, read "Intro to Indoor RC Guide" at: www.cloud9rc.com/indoorflying.asp
Transcribed from original scans by AI. Minor OCR errors may remain.
