Rotary cylinder valve

Model Airplane News, Nov 1997 by Chianellli, Chris

You're probably thinking exactly what I did when my eyes first gazed on the RCV 60 (.60ci) engine. "Rotary cylinder?! They can't mean that literally ... can they?" "They" can, and they do use the term as a literal description. Yes, it's true; the cylinder sleeve turns; in fact, the cylinder sleeve, rotary-drum valve and prop shaft make up a single, inline, rotating component. Sounds strange, but it's really quite simple. The crankshaft, whose axis in this case is perpendicular to the thrust line of the airplane it's mounted in, turns a beveled gear that meshes with a beveled ring gear that's machined into the bottom edge of the cylinder sleeve, thereby turning the cylinder/drum-valve/propshaft unit. This multi-task rotary component is supported at the front, somewhat conventionally, by a caged ball-race bearing unit at the base of the prop-shaft and, at the rear, quite unconventionally, by a large bronze bushing that captures the cylinder sleeve just in front of the beveled ring gear. You with me so far?

Induction, exhaust and glow ignition are all carried out by means of the drum valve that's between the cylinder top and the prop shaft. Machined in this drum is a 90-degree duct. One end of the duct alternately passes over the intake port, ignition window (in which the glow plug sits) and exhaust port. The other end of the duct leads to the center of the combustion chamber (see? I told you it was simple). I almost forgot! The cylinder, hence the prop shaft, rotates once for every two crankshaft rotations. Yes! It's a 4-stroke/cycle engine.

Having the prop bolted to the top of the rotary cylinder instead of to the crankshaft does give the design an obvious scale-modeling advantage: it's streamlined and will fit into otherwise difficult cowled-in situations. Also, the designer has put the crankshaft end to use by passing it through the crankcase side as a convenient and safe starting drive. Inventor Keith Lawes claims the RCV 60 prototype is easy to start and will turn a 16x12 prop at 4,500rpm producing 5 pounds of thrust on 5-percent-nitro fuel. While a torque benefit is obviously the result of the 2:1 reduction ratio, because of the linear porting dynamics a rotary valve offers, attaining high horsepower outputs is a dubious affair, in my humble opinion. I have run several Webra 4-strokes that use an overhead rotating tube with flute holes machined in them for induction and exhaust in lieu of poppet valves. This system has similar breathing dynamics to that of the RCV 60, and while it's a very simple and reliable mechanism that produces respectable torque, it does not supply the high-rpm breathing necessary to produce power output comparable to that of a poppetvalve 4-stroke engine of similar displacement. Simply put, the opening and closing of a rotary valve is linear, while that of a poppet valve is exponential. A poppet opens faster and stays open longer. No contest.

When I first heard of the engine, I was sure the piston ring would have been fitted on some type of rollerbearing system so as to rotate with the cylinder sleeve. This is not the case, and it concerns me. In this engine, we have the piston moving in a conventional linear path combined with the sleeve's lateral rotation around it. This sets up a multi-directional wear pattern that brings into question the piston/sleeve fit over the long term.

One other feature I would like to see addressed is the straight beveled gears. Helical-cut gears are quieter and far more efficient. To be fair, the engine is in the prototype stage (production plans are in the works for the very near future), and both of these concerns could be addressed in production versions.

On the side of the RCV is its compact simplicity and tremendous novelty appeal for the mechanically curiousthe latter being an extremely appropriate quality in my book; after all, the realm of R/C airplanes is a world fraught with creative ingenuity-one of the things that keeps me coming back to this hobby.

For more information, contact RCV Engines Ltd., 6 Haviland Rd., Ferndown Industrial Estate, Wimborne, Dorset, England, GB BH21 7RF; phone 44 (0) 1202 877044; fax: 44 (0) 1202 861210.

-Chris Chianellli