Rabe 10 stunt engine
Snaggletooth was also used for several hundred test flights in the development of a significantly more powerful stunt engine of .60 displacement which would fit the narrow ST .46 mounts. It seemed like I could always use more power and had, more or less, constant problems with heat. The new engine started out with a much modified Como 40 case. It was modified in many ways. First, the bottom end of the case was bored out for rod clearance when running a crankshaft with a full .60 crank throw. This resulted in nearly cutting the case in half. The case thickness at the bottom of the bore was only a paper thin .0020th of an inch. What strength there was in the bottom of the case depended on the mounting lugs to bridge the outside of the rod clearance groove. Second, many engines of the time threw fuel from the front bearing. I grooved the case and installed an “O” ring behind the front bearing. This dried up the engine too well when it stopped any lubrication from reaching the front bearing. A solution for this was found by nicking the “O” ring to pass a small amount of fuel. The “O” ring did drag a bit, but this went away after the first few flights. Cooling was improved by welding additional metal onto to the top of the case and machining larger cooling fins. Finally, the case was sand blasted for a nicer appearance. The rod was custom machined to specified dimensions and fitted with bronze bushings with oil holes on both ends. There were a number of combustion chamber head buttons tried of various shapes. These buttons were held in place by an oversize head clamp featuring large cooling fins.
The crankshaft was custom machined and heat treated to my design drawings by Glen Dye. The design process for the crankshaft was begun by weighing the pistons, rods and crankshaft counterbalances of a number of engines. I related this data to an article on the theoretical counterbalancing of single cylinder engines. The design of the crankshaft featured a .090 thicker counterweight which I calculated to be still a bit less than necessary. The counterweight had three holes drilled, parallel to the crankshaft, to allow the installation of tungsten weights. Initial test flights began with no extra weight installed. Tungsten plugs of various lengths were added over a series of flights to progressively reduce vibration until a point was reached where vibration began to increase again. This resulted in a very smooth running engine.
Crankshaft timing was selected to match the timing of the ST .60.
The piston was a modified ST .60 part. It had the ring pinned, and the wrist pin was secured by circlips to protect the sleeve. The chromed sleeve was mostly a stock part as the ST .60 already had good port timing for stunt runs. The result was an engine which fit the Snaggletooth mounts and ran pretty much like a very good ST .60. It turned an inch longer prop than the ST .46. Once again, I was motivated by the idea that if I couldn’t buy what I need for top competitive performance, I would make it.
Al
Topic: Balance of Engines & Props (Read 1171 times)