Kyosho Super Stearman ARF, The

Model Airplane News, Feb 2000 by Hunt, Tom

A perfect candidate for glow to e-power conversion

For some time, electric modelers have thought of ARFs as heavy, poor performers. However, things seem to be changing with the Kyosho* Super Stearman. This model has a well-designed, lightweight structure that makes it a perfect candidate for e-power.

In the November 1999 issue of Model Airplane News, Chris Chianelli reviewed this model in its glow configuration. The model consists of all-balsa construction, with only enough ply to add strenght where it's needed. it is completely covered, ready for radio- and power-system installation after the tail has been glued on, and the wings halves are assembled.

The fiberglass cowl and wheel pants are light and durable. The aluminum landing gear easily supports the slightly higher weight of the electric-powered version. The high-temperature covering material used on the model is similar to MonoKote, and although it is not the lightest available, a model this size does not warrant using the lighter, softer, more easily dented low-temperature coverings. Painted, 1 1/16-inch birch ply could be substituted for the aluminum N-struts and cabane struts, but if that's all you were going to do to lighten the model, it would translate into only only an ounce or two of savings.

Just to let the cat out early, the model weighed 7 pounds, 2 ounces ready-to-fly with 16, 20mAh cells on board. This is only about 1 1/4 pounds more than Chris's glow version. This brought the wing loading up to 22.9 ounces per square foot from 18.5. As you will read later, this is not a problem. If I had designed and built this model from scratch, I doubt it would have weighed less than 6 1/2 pounds with the same equipment installed. There is a fair amount of extra balsa wood in the fuselage and wing, but the tail is as light as I would care to make it.

CHOOSING A PROPULSION PACKAGE

When deciding what I'll need to power a potential electric model, I start by choosing an acceptable "do not exceed" wing loading. For this semi-scale biplane, I chose 25 ounces per square foot. The model's wing area is 717 square inches. Computing the maximum weight, we get approximately 74 pounds. Using a minimum 60 watts per pound (for scale-like aerobatics), we need to input 465 watts to power the Stearman. Let's set the maximum current at 30 amps for takeoff and full-power aerobatics. This now tells us to use 16 cells (cells = watts / amps).

Before we select a motor, we should first decide what size of prop we would like to swing. I always start with one that has a diameter as near to scale as possible, if not slightly larger-especially on scale models. Scale size will give the best compromise of climbing power, speed and duration. A diameter of 14 to 15 inches would be "scale" for this model. I chose to go with a 14-inch; it allows room for growth if I need it. I don't like to swing a prop with a pitch that's less than half the diameter; I generally prefer the pitch to be between 0.6 to 0.75 the diameter. This, again, is a compromise between climbing, speed and duration. I settled on a l4x8 prop.

Now, 16 cells. Which motor works well on 16 cells and can swing a 14x8 prop at 30 amps? A few choices come to mind. An Aveox* brushless motor on a Robbe* planetary gearbox would be nice; an AstroFlight* 15 on an Astro Superbox might do it, too. I decided on the Modelair-Tech* H-1000 belt drive combined with a DeWalt 14.4V powertool motor. This system can be run on as few as 14 and as many as 21 cells. It is available in three ratios: 3.07:1, 3.33:1 and 3.63:1. On 16 cells and the 3.07:1 ratio, this setup will swing the intended 14x8 prop with 29 amps. This system is a bit heavier, but it turns out that the weight was an asset. To get the CG in its proper place, I needed to shove the 16-cell pack as far forward as I possibly could. If I had used a lighter motor system, I would have ,needed to add lead to move the CG forward, and this would have negated any weight savings I'd gained.

DRIVE SYSTEM INSTALLATION

The firewall on the model is quite robust. It's made out of what looks like 6mm birch ply, so it's more than sufficiently strong to take the 460 or so watts from the electric motor drive system. The H-1000 belt drive is usually mounted on beams like a glow engine. I used the glass-filled T-beam mounts that were supplied in the kit and shortened them a little with a band saw. I screwed the H-1000 to the T-beams and then screwed the beams to the firewall. The firewall had to be cut to clear the back of the motor and to provide cooling air to the battery pack. This was easily done using my Dremel* tool with a router bit and, later, a sanding drum. To keep the entire drive system inside the cowl, I elevated the prop shaft only about 3/16 inch from the center of the cowl (barely noticeable on the ground or in the air). I then mounted the cowl as per the instructions, with the exception of removing the lower centerline mounting block because it would interfere with the motor.

As I mentioned before, the 16-cell, 2000mAh battery pack (two, 8-cell "flat" packs, one on top of the other) had to be placed right up against the firewall to achieve the proper CG. I removed the factory-in stalled bulkhead in the lower wing bay to facilitate the installation of this pack. I used the convenient internal "deck" in the roof of the fuselage to mount the pack. I placed hook-and-loop fastener inside the fuel-tank bay on this deck to support the front of the pack. The rear of the battery sticks into the lower wing bay-perfect for getting at the connectors and for mounting the batteries. At this end, I used 8-inch-long hook-and-- loop strap fasteners. I glued one strap into the roof of the fuselage with epoxy, placed the battery pack in the fuselage and wrapped the strap around the pack. I then added a second strap over the first.