A one-hour flight on one charge?

Model Airplane News, Sep 1999 by Aberle, Bob

You can do it with LiMn batteries and micro R/C

THIS ARTICLE IS iNTENDED as an introduction to the new lithium manganese (LiMn) rechargeable

batteries. We have certainly heard a lot about these cells in the past year or more, but their cost and the difficulty in obtaining a suitable charger have kept a lot of modelers from actually trying this new technology. What t have done is to take an existing ARF model intended for CO2 power and quickly convert it to small electric power; providing a test bed for the new lithium rechargeable batteries.

The model featured in this article is the popular AIKA ARF, built in the Czech Republic and sold in the U.S. by Hobby Club Inc.* The little AIKA comes all framed in balsa with a tissue-like covering. At 128 square inches, it is quite small. I originally flew this model with a GM-120 CO2 engine and a CETO single-channel (rudder-only) micro RIC system. Total weight at the time was 2.7 ounces. Like most CO2 engines, the GM-120 could not be throttled. That lack of a throttle or engine control along with rudder-only flight control made it quite difficult to fly in an indoor environment.

The basic idea of my conversion scheme was to produce a plane with a motor throttle, rudder and elevator control that did not weigh much more than when it had the CO2 engine installed. I was able to end up with those desired features in a model that weighed just 4.6 ounces. That produced a very respectable wing loading of only 5.2 ounces per square foot. Included in that 4.6-ounce total weight were two of the new Tadiran LiMn rechargeable battery cells (available from David Lewis*). The bonus of using this particular new battery is a flight time per charge of at least 1 hour. Hence, small model; long flights!

MOTOR AND BATTERY COMBINATION The first item to select in this conversion process was the electric motor. Thanks to a very complete motor data table prepared by Dick Miller (see his website at www.enter.net/rcm65/motdata.html), I was able to easily select a VL Products* HY-50B electric motor, which is supplied with a 5:1 gearbox. This motor is also available from John Worth at Cloud-9 R/C*. My selection was based on using a 6V power source, which is the characteristic voltage of two lithium rechargeable batteries connected in series. The HY-50B was claimed to swing a 7.25x4.50 VL Products black prop at 2,700 to 2,900rpm while drawing 0.8 amp of current. Dick's figures stated that this combination was capable of flying a 6.3-ounce (total weight) model with up to 229 square inches of wing area. As it turned out, the HY-50B proved that it had more than enough power to fly my 4.6-ounce model of 128 square inches. The motor current turned out to be exactly 0.8 amp, and the prop speed was 2,700rpm.

The next trick was to select a battery that would supply around 6 volts to the Vl Products motor. I could have used a 6-- cell, 150mAh Ni-Cd battery pack, but that would have weighed 2.1 ounces and would have provided only a 10-minute motor-run time. Instead, I made a bold move and invested in two of the new Tadiran lithium metal rechargeable battery cells. Each cell costs $12.50, has a rating of 850mAh, the physical size of an AA cell and weighs 0.7 ounce. As already stated, the characteristic voltage of each cell is 3 volts, so only two in series would give me my 6 volts. At 850mAh capacity, my flying time would be up to around 1 full hour, yet the 2-cell battery weight would be 0.7 ounce lighter than the 6-cell 150mAh Ni-Cd pack.

I might mention as a side note that Tadiran recently introduced a 1/2-size lithium cell that's 2/3 the size of an AA cell and has a capacity rating of 430mAh. It weighs just 0.4 ounce and has the same voltage characteristics but, unfortunately, this cell is more expensive than the larger 850mAh cells. Using these cells, the AIKA would weigh 4 ounces and fly for approximately 30 minutes.

There are, however, several drawbacks to using the new lithium rechargeables. First, they can only be charged to a peak voltage of 3.4 volts per cell or, in my case, 6.80 volts peak for 2 cells in series. Also, you can discharge these cells only to a minimum of 2 volts per cell (4 volts, in my case). Failure to keep the lithium cells below 6.80 volts and above 4 volts can damage them. I'll talk more about the necessary charging techniques later.

R/C SYSTEM

I obviously needed the smallest and most light-- weight R/C components available for this application. For a receiver, I tried the new micro receiver from Paul Garrett Receivers* (it's also available from David Lewis). Paul's single conversion circuit has 4, full-channel-control capability and yet weighs just 0.2 ounce, including a small connector block attached to the end of a 1 1/2-inch-long pigtail lead. The Garrett receiver measures 1 1/8 x 5/8 x 5/16 inches. It isn't very narrowband in performance, so several channels generally have to be blocked out on either side of the operating frequency. But in a controlled environment and operating with similar equipment, this hasn't proved to be a problem at all. The stated radio range is approximately 300 feet, which isn't a whole lot. In my ground tests, I obtained 360 feet of solid control before the receiver started glitching. This would probably be a little more when the model is airborne, and when you consider the size of the models being flown, 300 feet of control won't give you any real problems.