By Neal Capener
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Reprinted from November 2000, Vol. 5, No. 6 |
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By Neal Capener |
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I've always had a soft spot for J-3 Cubs. The very first model airplane I built was a Cub. My first float plane was a Cub. And, my first electric plane was a Cub. There are no surprises in building and flying Cubs. The large wing area and light wing loading makes it an easy flying sport trainer. No wonder the Cub is an often modeled subject. Global's Cub As soon as I saw Hobby People's advertisement for a new 81-inch-wingspan Cub, it looked like a good candidate for electric power. I instantly knew it was time to add another J-3 to my fleet. Moreover, I've had good experience with Global's Almost Ready to Fly (ARF) planes. Their planes are built light, the covering is tight, and the hardware is complete. The "kit" arrived in a large, colorfully decorated box. All the components were packed in plastic bags and separated from each other with card- board dividers. The manual covers the assembly in detail with the help of pictures and drawings; consequently, I will not. Instead, I will concentrate on the conversion to clean electric power: how to mount the motor and speed controller and how and where to place the batteries. In less than 10 hours, the plane was ready to receive the radio gear and the electric motor components.
Electric Conversion There is a lot of information available on several web sites on the subject of selecting the right electric motor for a plane. The manufacturers of motors do a good job presenting suggestions for applications and provide valuable data. Using a program like MotoCalc or Ecalc lets the builder computer-try different motors, props and cell counts before actually buying them. Selecting a motor for the Cub A slow flying model like a Cub needs a geared motor and a large prop One of the "Orme's Rules" suggests one NiCad cell per 50 square inches. The Cub has 800 in.2, consequently, 16 cells are a starting point. The empty weight of the Cub is 65 ounces. To that number, I add the weight of the radio gear (8 oz), battery weight (16 cells x 2 oz = 32 oz) and the weight of the motor (approximately 15 oz) to get its finished weight of 120 oz = 7.5 lb. For a better-than-Cub-like performance, it needs 50 to 60 watts per pound (another rule of thumb), which calculates to 375 to 450 watts. Now I know I need a motor that works on 16 cells and has an output of 450 watts. Since these numbers are guidelines and not etched in stone, the motors I tried work with 16 to 20 cells, depending on the prop used. I decided to try three different motors. One is an Aveox 1409/3 motor, an Aveox L260 controller and a Modelair Tech belt drive with a 3.6: 1 reduction, turning a 16/8 prop using 16 cells. The second propulsion system consists of an Astro 25 with super gear- box and an Astro 204 D controller. The gear ratio is 3.1:1, allowing it to turn a 16/10 prop on 16 to 18 cells. The third is a 18 V Dewalt industrial motor, mated to a BD 102 Modelair Tech belt drive with a 3.6:1 reduction and an Astro 204 D controller. This combination is good for 16 to 18 cells, with a 16/10 prop. Using an Astro 217 D controller instead of the Astro 240 D reduces cost by $60. The Aveox motor is also available with a planetary gear- box (3.7: 1 ratio). The first motor I installed was the Aveox. I bolted the motor to the belt drive unit with two screws supplied with the motor. Next, I installed the pinion on the motor shaft and tightened the belt to the recommended tension. Make sure you order a pinion gear with a 5-mm hole for the Aveox motor, and follow the instructions to achieve the right amount of tension. A belt that is too tight will rob power from the motor and wear quickly. The assembled unit mounts to the nylon motor-mount included with the kit. I pre-drilled four holes and used #4 x l-inch socket head sheet metal screws. The front of the belt drive unit is flush with the front of the motor mount. To allow cooling air to enter the fuselage, I cut a 2- by 1-1/2-inch opening in the firewall below the motor mount. Size or location is not critical. I left one of the molded side windows off for air to escape. Next, I soldered a battery pack from sixteen 2000 mAh Sanyo cells. Four rows of four cells were connected with braided wire. I used Astro Zero Loss connectors on the battery, speed controller and motor. Astro connectors are polarized and eliminate the danger of ruining components by accidentally plugging them in the wrong way. Battery tray I admit, usually I take the easy route and use hook and loop fastener material to hold the batteries to the fuselage floor. I have often wondered about the reliability of this system, usually while my model is at the top of a loop or flying inverted. Since I wanted to fly the Cub with three different motors and different weights up front, I needed to shift the battery to balance at the center of gravity (CG) and still securely hold the battery in place. As a result, I first built a square box from 1/8-in. plywood to hold the battery. Then I glued 1/2- by 1/4- by 6- inch basswood rails to the sides of the fuselage. These rails extend from the servo tray forward to the front fuselage former. The battery box has cross-members attached that secure to the basswood rails with small screws. To shift the battery, I loosen the screws, slide the tray into the new position and tighten the screws. I attached the windshield with four small screws to allow easy access to the battery compartment. Radio Installation The Hobby People Cub uses separate aileron servos. I installed Hitec HS 81 micro-servos. Full-size Hitec HS 300 servos drive rudder and elevator. The pushrod housings are already inside the fuselage. Threaded wires were inserted on the plastic pushrods at both ends, a clevis at the control surface and a 90-degree bend and keeper at the servo end. The elevator pushrod uses a unique dual pushrod system. The wires for the aileron pushrods were undersized in my kit, allowing the clevises to pull off easily. I replaced them with rods from stock at hand. I fastened the receiver and airborne battery to the servo tray with hook and loop fastener. Flying Before heading to the field, I placed the Cub on the scale. Ouch! 8 Ibs, 2 oz! Where did I gain 10 oz over the projected weight? The battery tray, the larger (and heavier) wheels (to fly off the rough grass field), the prop, the pilot, some glue and paint; I guess it all added up. To check the ground handling I just wanted to taxi down the runway. I applied power to get rolling, steered a bit left, then right and suddenly the Cub was airborne. Whoa! Surprised! I gave it full power and climbed out steeply. Like the full-size Cub, it takes very little speed to get flying. At altitude, I made a few minor trim adjustments and the Cub was flying "hands off" at half stick. I like to check-out the stall behavior of a new model right away, so I reduced power and fed in up-elevator. As anticipated, the Cub slows to a crawl, loses altitude, but stays under control - good to know there are no surprises ahead when it is time to land. With so much power, there is no need to dive before pulling a loop. Rolls are slow and need elevator correction while inverted. The Cub flies upside down, but needs lots of up-elevator. With the Aveox motor, the Cub "leaps" off the ground in less than 15 feet. The 16/8 prop turns at 5900 rpm with a current draw of 30A. Clearly, the Cub is more in its element flying like the full-size plane. Since the wing loading is light, the Cub "floats" on landings. It is a great plane to practice side slipping on approach, one wheel touch-and-goes and hovering into the head wind. The next motor I installed was the Dewalt motor with the Modelair Tech belt drive. This motor is "borrowed" from power tools and is modified and mated to the BD-102 belt drive. The drive is available in three ratios, and changing the reduction is as easy as changing the drive pulley. The BD-102 has molded mounting lugs and that makes bolting the unit to the Nylon motor-mount a snap. The Cub with the Dewalt motor, 18 cells and a 16/10 prop weighed a full pound more and brought the wing loading up to 26.5 oz/ft2. This increase had surprisingly little effect on the performance. Turning the prop at almost 5000 rpm drawing 21 amps, there was plenty of power to fly the Cub in a non-scale-like fashion. The 14 V Dewalt motor would be a better choice than the 18 V version since a smaller battery pack can be used. I borrowed the Astro motor from a similar sized plane. The motor is bolted to the motor mount with angle brackets I made from sheet aluminum. I was using a 16-cell battery to turn the 16/10 prop at 6400 rpm drawing 23 A. It is difficult to compare the motors directly with each other. The gear ratios are different and I used different props from different manufacturers. Duration was 8 minutes and up with any of the motors, depending on flying style. Final Thoughts To sum it up, let me say any of these motors is a good choice and does an excellent job of flying the Cub. The Cub is a sweetheart to fly. I think everybody should have one in his or her hangar. |
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