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SPECIFICATIONS |
Wingspan: 20 in.
Length: 18.75 in.
Wing area: 80 sq. in. (0.56 sq. ft.)
Weight: 80g (2.82 oz.)
Wing loading: 5 oz./sq. ft.
Power: DC5-2.4 motor with 4.2:1 gears and 5.25-in. plastic prop
Battery power: 7, 50mAh Ni-Cd cells
Controls: 3-channel micro
Comments: easy to fly off water and in confined areas; handles moderate breeze; top speed—approximately 25mph; minimum speed—approximately 12mph.
Please
note that the engine displayed in the photographs is not a glow
engine, it is a balsa replica mounted over the "real"
engine. |
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I am completely inexperienced in the design of seaplanes. I lived near the ocean for many years, but salt water is very unfriendly to electronic gear, so I avoided operating models in it and was very satisfied as a landlubber. We now live on a farm with a pond. Considering how much fun I have had with land-based micro electric RC models, I decided to give a seaplane a try.
My inexperience probably worked in my favor because I had to check back issues of model magazines and study successful designs. In particular, I give credit to the late Don McGovern, who wrote much about seaplanes in Flying Models. Woody Blanchard also gave me many useful concepts.
Armed with this information, I pulled out a fresh sheet of paper and began to lay out the features I wanted in Pond Baby. I faced issues I had never dealt with: how deeply would it float? What about the very high thrust line? Would the hull behave normally at this small size? Out came the handbooks to calculate how much water Pond Baby would displace; this led to the hull width I selected. Thrust line? Mount the motor with some upthrust and give it a try. Hull effects? Table this one, and assume the model will work well! The rest of the design process followed Blanchard/McGovern thinking with some of my micro RC experience mixed in.
I avoided having to build a new wing by pulling the wing off my Pixie
prototype. This also helped to determine the size of the rest of the
airframe. The construction of the hull/fuselage (called a "hull" from
now on) went quickly. I cut the tail out of 1/16-inch-thick sheet
balsa and mounted it. I made the motor mount out of a sandwich of
balsa and plywood and glued it to the wing, then I carved a set of
tip floats out of soft balsa.
Because I’d be tossing this little bird into the water, I decided to paint the wooden parts and then wax them. I mounted the DC5-2.4 motor with a 4.2:1 gear set driving a 5.25-inch prop cut from a 6-inch Peck Polymers plastic prop. For good measure—and to cause a little confusion—I mounted a balsa replica of a glow-plug engine on top of the motor. The battery is a 7-cell 50mAh Ni-Cd positioned in the hull bottom to provide the correct CG. I used one WES-Technik servo and one cut-down Hitec HS-50 servo, and my speed control is an FMA Mini 5. A trusty Sky Hooks & Rigging receiver passes on the control wiggles, and the servos drive the controls with 0.015-inch-diameter wire pushrods run through a few bushings.
After strapping the tip floats into the predicted locations and clamping the wing assembly onto the cabin top, I fastened the long receiver antenna to several corners of the airframe—ending on the fin, to avoid its dragging in the water.
A braver person would probably have headed straight for the pond, but I still had a few concerns about flotation. When the family was elsewhere, I headed for the bathtub. Hallelujah! It floats—and at about the right level as well.
Bearing in mind all the unknowns, I headed for the pasture and a hand-launched flight. Right from the start, Pond Baby behaved like a dreamboat (sorry!). Power affects pitch trim slightly: full power holds the nose fairly level and fast, while power-off, trim is a little nose-up. Ready for sea trials!
With the wing removed, you can see there's ample room for the RC gear.
AT THE POND
With a fresh battery charge, I switched the radio on and slipped Pond Baby into the water. Would the rudder have any effect while taxiing on water? Sure; I taxied out from shore and turned the bird into the slight breeze. Full throttle brought Pond Baby up on step very quickly, and things looked great; then, while I held full throttle, it suddenly cut back and started to surge up and down. The solution was a "buggy whip" antenna for water operation.
Having solved the signal problem, I went back to the pond. Now, when Pond Baby started to pick up speed on the step, it would suddenly whip around in a "water loop." I had overdone the tip floats’ streamlining by bringing their rears to a point, and when one touched the water at speed, the drag was excessive. The solution? I cut out two patches of very thin soda-can aluminum and fastened one to each float to create a "planing" surface.
Recharge, and back to the pond. At last, Pond Baby whizzed across the surface and lifted off with just a touch of up-elevator. Neato! After a couple of laps, I realized that I had never made a water landing; perhaps I should try it. Power back and trim for a nose-high touchdown; looking good until it touched the water! I was reminded of a Disney film about poor birds making awful landings. Pond Baby was not broken, but I hadn’t sufficiently sealed the cabin around the wing, so water sloshed around inside.
Dried out and with a better cabin seal, things were coming together: smooth liftoff and cruise and then set up a landing with more speed so that the touchdown would be in the planing attitude similar to a takeoff (just like a tail-high landing with a tail-dragger). This was the fix! I explored the flight envelope: Pond Baby looped, rolled and did a nifty stall turn, but its real charm lies in the water operations.
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