An Easy Bit Charger Conversion

by Greg Covey

The stock Icara Hangar Rat ARF

Introduction:

R/Cers have recently utilized a new technique for providing a low-cost entry into the world of micro flight. By borrowing from the more popular R/C car industry, the new Hobbico palm-sized MicroSizer or Tomy "Bit Char-G" micro cars can be used to control a small free-flight plane. These new micro-sized cars can be purchased on-line or at many discount department stores for $10-$20. When used as intended, all you need to do is to snap on the gears, motor, tires, and body. After charging your car for 45 seconds...you're ready to go!

The same transmitter, receiver, motor, and, magnetic actuator steering can be retrofitted into a small free-flight airplane. The amount of skill and work required to accomplish creating a workable R/C airplane can vary upon your selection of micro car and plane. My goal for this conversion was to use a minimum of both skill and effort to help entice more people into the exciting world of micro-flight!

Conversion:

For my project, I choose the Icara "Hangar Rat" rubber-powered free flight ARF sold at FAI Model Supply. It is a 15 gram (0.53oz) almost ready-to-fly tissue covered balsa design and is listed as having an 18.5" wingspan. I measured a 19.5" wingspan with a chord of 4" from center to tip so the resultant wing area is about 78 sq. in. The model should provide relatively light wing loading which will help make it fly slow even with the increase in overall weight.

The Hangar Rat kit parts were well made and assemble easy

The Hangar Rat ARF design provided a quick build model for my first bit-charger (or "Bit Char-G") conversion project. The few parts assembled easy using medium thickness CA glue.

The Hobbico MicroSizer car kit disassembles easily

For my micro R/C car model, I choose the popular Hobbico MicroSizer Skyline White 49MHz version from Tower Hobbies.

The car body weighs only 0.4oz as is, and, there are several easy ways to reduce the weight, if needed later. I typically like to start a conversion with minimal effort and see how it flies. I even left on the plastic receiver cover plate. The stock MicroSizer receiver was flipped up to remove the single battery cell only. I simply soldered my motor wires and battery connector wires to the existing metal tabs on the car body.

The car body is easily wedged into the wing struts after cutting notches in the plastic ends to surround the balsa struts. The angled strut in the stock Hangar Rat wing frame slides down on top of the car body and is held with a small drop of CA.

The motor power seemed ok but it is difficult to tell if i'll need to modify it until it is test flown. I would like it to be just enough power for cruising around the gym so I don't need to keep turning it on and off. 

My initial range check also appeared ok for a gym sized flying area. I have read about 2 options for increasing the transmitter power (and therefore range). You can use 4 of the AA cells instead of 2 cells in your transmitter or use a 9v battery with a wired-in regulator modification. I would reserve these options until needed.

The power system was tested before installing it into the plane

I choose not to use the stock motor since it would require making a gearbox. My purchased KP-00 motor from Dynamic Web Enterprises weighs only 0.2oz with gearbox, wires, and, U-80 prop. My Kokam 145mAh Lipoly cell with wire, connector, and wrap weighs another 0.2oz. I tested all the electronics before putting it in the plane.

The KP-00 motor is CA'ed to the balsa fuselage and the Lipoly cell is attached using Velcro

The plane is now powered by a KP-00 motor and a single 145mAh LiPoly cell. The Hangar Rat design allows the entire wing assembly to easily slide forward or backward to obtain the perfect CG. There are two tiny rubber rings that hold the wing structure to the fuselage. The rings are simply rolled into place. I left off the stock landing gear to reduce weight until I determine how it flies at 1.0oz all up.

A tiny carbon rod turns the stock rudder

The stock rudder is connected to the MicroSizer axle via a tiny carbon rod and solid wire using "Z" bends. The wire is wrapped around the carbon rod tightly but can be moved to fine tune the position. I sliced a "V" wedge (or channel) into the stock rudder so that it easily turned in either direction about 30 degrees. I also cut off the plastic wheels and glued a tiny control horn, made from thin plastic, directly onto the metal axle. Some Zap CA "kicker" spray helps with this gluing step.

One key area for good rudder control is in cutting a "V" channel hinge into the stock rudder so that it swings with minimum force needed. If you cut through too far and go through the paper on the other side, it can probably be easily fixed by either using rubber hinges or thread hinges.

Another key area is in the control horn distance from the rudder and the "servo" arm length. You can play with these a bit to get the turning radius you want. The Hanger Rat turns well with little rudder throw but it doesn't turn on a dime.

An alternate technique for controlling the rudder is to use a pull-pull system using thread tied from each side of the metal axles to each side of the rudder.

The Hangar Rat is RTF at 1.0oz

Almost no modification was performed on the stock MicroSizer chassis other than cutting off the plastic wheels and plastic ends to fit the wing frame. The Hangar Rat ARF also built up in a few minutes. The entire project took about hours to make ready.

Test Flying:

The plane flew but slowly descended to the gym floor

My first test flights were very promising. Although the Hangar Rat didn't have enough power to sustain flight, it flew very long and stabile paths across the gym floor to the opposite side before touching down. I was happy that my plane was balanced properly and still flew very slow. I needed a bit more power from my motor. Since I measured only 1.5v across the motor with the stock receiver circuit, I knew that I was only using about 1/3 of the KP-00 capability.

Modifications:

The lightened Hangar Rat now weighs 0.9oz

After researching the motor circuitry a bit, it appeared there was no easy way to boost the power level to the motor without adding an additional MosFET driver. The motor was drawing about 300mA at 1.6v. I did, however, re-wire the motor + line to the battery + line. This eliminated the reverse direction control which is not needed for flight.

The Hangar Rat flew outdoors at 0.9oz

There didn't appear to be any easy jumper fix to add more power, other than adding an extra MosFET transistor to switch the motor on harder. The controlling signal goes right into the IC itself. I decided that, for ease of reproduction, I will try to drop some weight in unused plastic and metal from the car body.

I was able to reduce the weight by 10%, or 0.1oz, by removing half of the car body. The plane now weighs 0.9oz RTF.

The drop in weight proved to be a winner! When the next sunny day arrived, my Hangar Rat flew outside like a champ without modifying the motor power. The warm air currents flowing upward may have helped too. In either case, I went home very happy with the results.

Summary:

Although my Hangar Rat flew ok outdoors in no wind conditions, it lacked precise control to fly indoors in smaller basketball court size gyms. I decided to replace my bang-bang bit charger setup with a fully proportional RFFS100 module from DWE. The model still weighed only 0.9oz but I had proportional throttle control and rudder control. No elevator was needed as I could change the motor speed to go higher or lower.

The 0.9oz Hangar Rat uses an RFFS100 module, KP-00 motor, and a single Kokam 145mAh cell. Rudder and throttle controls only.

Hangar Rat Video (2.8meg)