Cabal Report: r/c'ing the Trumpeter 1/144 KILO, part-2

The heart of a successful r/c conversion is, of course, the WTC; the correct amount and placement of fixed ballast weight and buoyant foam; and correct articulation of the control surfaces and spinning of the propulsion shaft.

As the kit control surfaces were already outfitted with pins, and stabilizer bores to accept them, it was an easy matter to integrate those features into the cast resin pieces I made from the horizontal stabilizers and control surface kit parts.

Placement of fixed ballast weight and buoyant foam would come later. First, however, I had to come up with a cylinder that would fit the tiny confines of the 1/144 KILO kit, yet still have enough internal volume to afford housing of the critical control, propulsion, and ballast elements within a dry environment as well as providing the required capacity to the main ballast tank to get the model submarine to or near the designed waterline.

As the flimsy kit hull halves needed re-enforcement anyway, and that I found the two-inch outside diameter of the Lexan cylinder selected to be a perfect fit, that drove the WTC cylinder diameter as well as length - I found that this kit represented a hull with a constant cylinder diameter than ran 6 7/8" inches in length. That number, of course, would be the maximum permissible length of the Lexan cylinder portion of the WTC. Any remaining WTC length (an extension needed to house the motor and control surface servos) would be made up in the form of a cast resin motor cup and motor bulkhead sub-assembly - elements of the WTC that would project aft into the tapered portion of the hull.

(And yes, I know that the Trumpeter KILO kit hull form is at variance with 'reliable' plans of the actual craft; that the kit does not represent the constant taper aft of the sail seen on the prototypes. Deal with it! The kit is what it is, don't get so wound up in such detail that you decline to use this kit in the way I suggest here).

This discussion deals with the design, manufacture and use of the WTC-2/KILO:

The completed WTC-2/KILO next to the hull halves. Forward, to the extreme right, is the ballast tank with half of the gas type ballast system onboard bottle projecting past the forward bulkhead -- this done to leave more floodable volume to the ballast tank proper. In the center of the cylinder is the ballast bulkhead -- it has attached to it, on the dry side, the ballast system servo and on the wet side it has the linkage which either vents the tank (the square resin item atop the ballast tank is the vent valve housing) or admits gas from the onboard bottle to empty the tank. Not yet installed will be an 'equalization tube' that will run from atop the ballast tank to an auxiliary ballast tank built into the stern of the lower hull.

The dry space aboard the WTC-2/KILO is aft of the ballast bulkhead -- this dry space continuing into the motor cup.

Five major castings are needed to make up a WTC-2/KILO. From left to right: The motor bulkhead in which are fitted, on the wet side, the two pushrod watertight seals, on the dry side is a foundation for one of those cheap little Radio-Shake 3v motors; foreground is the ballast system bulkhead equipped with servo and linkage; in background is the motor cup which has the volume to house three mini-servos, the APC-4 and the propulsion motor; to the right of the motor bulkhead is the motor cup servo mounting bulkhead; and to the extreme right is the ballast tank forward bulkhead. All these items were cast from Alumilite casting resin, an excellent product of quick cure, and a very good hardness which permits easy and sure post casting machining operations. As is my practice I endeavor to make as many subassembly connections through radial compressed O-ring as possible --it's the ease of disassembly they provide that makes adjustments and repairs to my WTC's such a relatively easy matter in the field.

I first worked out the physical dimensions of the motor cup (the length and taper angle driven by the geometry at the stern of the 1/144 Trumpeter KILO kit) on paper and then check its accuracy with paper templates placed within the hull. Once

assured that things would fit, I committed to master fabrication. In this case I'm laying out blanks that will be cut from Renshape 40, a very dense polyurethane pattern making medium which is very responsive to machining operations, yet will produce a smooth finish when properly worked with abrasives.

Cutting the blanks on the band saw. Two had to be cut and laminated together to get the required length.

Cutting inside and out with the Tag lathe. Not the working drawing to the right.

Almost ready for the final sanding. Since both the rubber and the casting resin shrink a bit, you have to render the master slightly oversize to account for the eventual loss of size through the process of tool and part fabrication. How much? You can do the math (working from the manufacturers specifications) or develop a 'feel' for how much. I'm a touchy-feely sort of guy, as most of you know.

Using the excellent Alumilite casting resin to produce the WTC-2/KILO plastic parts. This stuff cures quick so you have to work out an exacting methodology to follow as you mix, pour, and get the tools into the pressure pot. Note that I have set three of the tools on a 'caddy' to make transfer of them into the pressure pot a quick action.

For small work like this a common spray-paint pressure pot is best employed to crush any bubbles captured within the tool cavities. Only 30psi is required to get a void free casting.