RCV: 120-SP

Model Airplane News, Sep 2001 by Yarrish, Gerry

Whenever I show the new beehive-shaped RCV 120-SP engine to my modeling buddies, the first thing they always ask is, "Oh! Is it a Wankel?" I tell them, "No, it's a rotating-cylindervalve engine." "A what?" is usually their secand question. Designed by Keith Lawes, this unconventional engine is a big departure from the standard model airplane engine-design layout philosophy. Manufactured by RCV Engines Ltd., the 120-SP is the second in its RCV line; the first was a .60-size engine. The 120-SP operates much like a 4-stroke engine but with two big differences: first, the piston travel is in line with the prop shaft/piston sleeve component, and second, the prop revolves at half the crankshaft speed. Let's take a closer look.

THE BIG DIFFERENCE

The most unusual thing about this engine is that the piston's cylinder sleeve is supported by two ball bearings that permit it to revolve and drive the propeller. At the front of the engine, the prop shaft is an integral part of the sleeve. At the cylinder sleeve's base is a large bevel gear that engages a smaller bevel gear driven by the

crankshaft. The piston and connecting rod that drive the crankshaft are inside the sleeve and travel fore and aft in line with the prop shaft.

Another interesting feature of the RCV engines is they are started with a hexshaped starter wand that engages the crankshaft well behind and at a 90-degree angle to the propeller. Since the crankshaft gear is half the diameter of the cylinder gear, the engine's prop rpm are gearreduced at a ratio of 2:1, which greatly increases the engine's torque output. (More on this later.)

The cylinder is made of steel, and a port opening is machined where the 5/16-inch prop shaft and the cylinder body meet; this port leads to the combustion chamber. This arrangement forms a rotary valve directly behind the front shaft bearing. The single opening acts as both the intake and the exhaust port as it revolves under the carburetor and exhaust portions of the engine case. The intake, ignition and exhaust portions of the ergine operating cycle are not quite 120 degrees apart from one another as can be seen by the front view shown in Figure 1.

PISTON AND CRANK

The piston is machined of aluminum and has a conventional cast-iron piston ring for proper sealing. The wristpin is press-fit within the piston body. Since the piston traverses within a revolving cylinder wall, it isn't surprising that the ring isn't pinned into a stationary position. Also, according to the manufacturer, the cylinder wall is not tapered toward the combustion chamber as it would be in a typical ABC engine. The connecting rod is machined of solid aluminum and is bushed at both ends. The lubrication hole drilled into the conrod's lower end faces toward the piston instead of away from it as is found in conventional setups. I suspect that this is because very little lubrication passes over the back of the crankshaft pin, as most of the fuel charge enters and exits from above the piston. During engine operation, however, I noted more than adequate lubrication of all internals.

The crankshaft is housed in a separate engine-case assembly that also forms the engine-mount portion of the case. The crank is counterbalanced and is supported by two needle bearings that are seated in the aluminum housing bolted to the main engine case and sealed with an O-ring. The crank end that's opposite the crank pin terminates with a large replaceable capscrew. This capscrew engages the hexshaped starting wand used to start the engine.

The engine case (cylinder jacket) is

RCV 120-SP

made of a solid piece of aluminum and is deeply finned to dissipate engine heat. The cylinder housing and the crank housing are held together with four cap-head bolts. This arrangement has an unanticipated benefit: the engine case halves can be assembled in any of four positions without changing the engine's timing or operation. This means you can adjust the engine's layout and the position of the starting socket relative to the exhaust pipe. The engine comes with the socket placed 180 degrees from the exhaust. Depending on your engine installation, you can have the exhaust pipe facing down and the starting socket on the left, top, right or bottom of the engine. This is a nice touch that any scale modeler can appreciate.

Opposite the starting socket is the crankcase breather fitting. This fitting shouldn't be connected to the fuel tank; it should be left to outside air pressure as is done with most conventional 4-stroke engines. As an experiment, I connected the muffler's pressure tap to the crankcase breather with a short length of fuel tubing. I found no advantage in doing this, as there was no increase or decrease in engine performance.

INTAKE, IGNITION AND EXHAUST The carburetor is a standard 2-needle arrangement with a 1/2-inch-diameter carb barrel and a 0.336-inch (8.Smm) intake diameter. The carb is attached to a 90degree intake manifold and is sealed with an O-ring. The manifold bolts onto the engine case with two cap-head screws. The venturi faces aft, and the high-end needle faces up. Throttle linkage attached to the throttle arm fits easily in a straight-line arrangement that makes the pushrod placement uncomplicated.