How to convert a rubber-powered free-flight to electric R/C

Model Airplane News, Nov 1999 by Hunt, Tom

SINCE I STARTED flying R/C models in the '70s, the size of receivers, servos and other hardware (except transmitters!) has been slowly decreasing. Now, with true "nano" stuff available, R/C model aircraft will soon be kept in shoeboxes in the closet and flown in living rooms! Well, the subject of this article is not quite that small, but it's close!

After spending a few years keeping my eyes and ears open for glow models that I could convert to electric power, I took a break and started a new quest: rubber to electric conversions. At the last Toledo show, I picked up the new, 30-inch-- span, rubber-powered Dumas* F8F Bearcat. It appeared as though it would be a relatively easy task to convert this model to electric R/C flight. I decided to give the Bearcat aileron/elevator control because I wanted to keep the dihedral more scale-like. (The model would have needed quite a bit more dihedral to turn properly with rudder, although in hindsight, the model could have supported the extra 1/2 to 3/4 ounce that rudder control would have added.) As long as the Bearcat weighed less than 12 ounces, I wanted to power it with the GD-280 (Titanic Airlines/Graupner Speed 280 motor/gearbox combo imported by Modelair-Tech*).

I don't intend to make this a blow-by-blow, stick-by-stick construction article. I will, however, describe my modifications to make this a fine flying, RIC electric aircraft. I intended to use as many parts as possible from the basic kit-even some of the scrap wood from the laser-cut sheets!

THE BASIC KIT

The kit is quite complete and comes with laser-cut parts, stripwood, Insignia Blue tissue, dry-transfer decals, vacuum-- formed canopy and cowl, wheels, rubber, prop, wire and some miscellaneous hardware. Only glue and dope are needed to build and fly the model as intended. The instructions and plans are clear and well thought out. The laser-cut parts can be removed from the sheet by slicing through two to three tabs per part. Use the plans to identify them first, as no printing is on the wood. Much of the stripwood (the stringers) is very soft. This might be a desirable attribute for an extremely light, rubber-power model, but for a heavier R/C model, it's quite a nuisance because it's easily broken during handling. I suggest that you weed out the softer sticks and replace them with some of a harder grade.

THE MODIFICATIONS

The wing. I hate building wings, so I built this first, just to get it out of the way. The wing is built in three sections: the outer panels and a flat center section. I scanned the outer wing section into my computer and drew in an aileron. Slightly larger than scale, the proportions still looked good. The outer ribs would have to be cut back and a sub spar installed just ahead of the aileron hinge line. The aileron itself still uses the remainder of the ribs; however, I duplicated an aft portion of RS for the inboard side of the aileron. I like top-hinged devices. They're easy to make and install and are completely sealed, so they're very effective. I pinned the leading-edge sheet of the aileron to the board at a slight angle to provide the gap for the down-aileron travel.

I built the remainder of the wing stock. After I had joined the three sections, I added 1/32, vertical grain, balsa shear webs to the main spar out to the inboard side of the aileron. I didn't make a dihedral brace because the covering material was going to add more strength to the wing than was probably required for the flight loads. I made aileron torque rods out of 1/32 piano wire and 1/16 aluminum tube and installed them in the top of the wing before covering it. At first, it did appear that this light gauge wire (used to keep the weight down) was a bit flimsy. I had some concerns about flutter, but it never materialized. If you can find a suitable tube, you could increase the wire size to .040 or .055 if you'd like. The extra weight should not be a problem.

I used an FMA* S-60 servo for aileron and mounted it very typically on a pair of rails supported across the center-section ribs. Using some of the tube that's used to run the torque rod out to the aileron, I flattened the end of about a 3/4-inch-long piece and drilled a 1/32-inch-diameter hole through the flattened end. I then glued this piece to the 1/32 wire torque rod with thick CA, then installed simple, solid, 1/32-- inch-diameter wire pushrods with Zbends at each end and a mid-span V-bend (for adjustment).

Empennage. I built the vertical tail stock. As mentioned earlier, no rudder was used. Both the free-flight and scale horizontal tails are shown on the plan; I compromised and built one somewhere in between using the available laser-cut parts and the hardest 1/8-inch-square balsa I could find in the kit. After I had designed and built scale-size elevator, I made a joiner between the two elevators out of some 0.055 wire that was in the kit. A piece of lMs-inch dowel with a 1/32 hole drilled through the end functions as the control horn.

Fuselage. What a monster! You could get three or four complete micro radio systems in this fuselage! After building the fuselage per the instructions, I added some 1/32 sheet wood to the inside on which to mount the equipment. I also added some sheet wood to the fuselage sidewall, inside the stringers just aft of the trailing edge (TE) of the wing, to provide a tougher grip area for hand launching. I also framed the section around the cowl as if the cowl were going to be removable (like the rubber versions). I mounted the Titanic Airlines GD280 motor/gearbox on a plate fastened to the frame around the cowl. I also added some braces as required for this assembly. I decided to retain the cowl/drive with a pair of small rubber bands looped around metal hooks fastened to the cowl, with the opposite end secured to a pair of wooden "hooks" glued to the inside wall of the fuselage, near the wing's leading edge (LE). This arrangement works great! I can access the motor/gearbox by just pulling the cowl forward or by removing the rubber bands. This "soft" mount will also reduce damage to the drive in the event of an accident.