O.S. FL-70

Model Airplane News, Mar 2005 by Gierke, C David

A revolutionary 4-stroke design

Twenty-eight years after it marketed the first mass-produced 4-stroke, O.S. has released its latest masterpiece: the FL-70. According to the accompanying literature, this single-cylinder, air-cooled, overhead-valve 4-stroke engine "... is suitable for trainer, sport and scale models." In addition, "This engine reduces maintenance by incorporating the first O.S. ringless piston/liner assembly. Also, a sealed front bearing prevents oil leaks. This engine is designed so that more pilots, from hobby beginners to skilled Sunday fliers, may enjoy the performance advantages of 4-stroke engines: greater fuel economy, higher torque, lower noise and realistic sound."

DESIGN FEATURES

Any new engine that crosses my bench is subjected to basic measurements and a perusal of its specifications from the instruction manual. Over the years, I've digested statistical data from hundreds of engines; it's what "engine nuts" do! I did a double-take, however, when I read the FL-70's bore and stroke data. It has a bore of 27.7mm (1.090 inch) and a stroke of 19.0mm (0.748 inch), so a quick calculation revealed a bore/stroke ratio of 1.457:1-the highest I've ever seen. Although the ratio of bore to stroke has nothing to do with torque delivered to the crankshaft, big-bore, short-stroke engines can lay claim to certain performance and longevity benefits (see the "Bore/Stroke" sidebar).

The FL-70 incorporates a very short connecting rod. When a short connecting rod is used with a high bore/stroke ratio, the result is a compact (height-wise), low-weight design. Short connecting rods have one objectionable feature: they produce greater rod angularity at mid-stroke than designs with longer rods. Excessive rod angularity acting through the piston generates undesirable side-thrust loads on the cylinder wall, increasing the likelihood of power loss due to friction. The FL-70 has a connecting-rod length (center to center) -to-stroke ratio of 1.64:1-decidedly on the low side. It's interesting to observe the compromises that designers make to achieve an objective-in this case, producing a compact, lightweight, 4-stroke engine with a high power-to-weight ratio.

Although the FL-70 is O.S.'s first ringless design, model engines without rings have been produced since the 1930s. Until recently, large 4-stroke engines (those with a greater than 0.60ci displacement) have used Meehanite (fine-grain iron) rings to ensure a good gas seal between the piston and cylinder wall. With the advent of CNC machinery, engine manufacturers are able to maintain much closer tolerances (fit) when fabricating matching parts such as lapped (ringless) piston-and-cylinder assemblies. The ringless assembly does have disadvantages, such as piston wear and early loss of the compression seal, but this deficiency is somewhat offset by the engine's relatively low piston speed, as detailed in the "Bore/Stroke" sidebar. A special note accompanies the FL-70 to inform the purchaser: "... the piston will feel tight at the top of its stroke, or top dead center (TDC), when the engine is cold." Users of 2-stroke model engines are familiar with this interference-fit condition; known as "pinch," it typifies modern lapped-piston designs.

For the FL-70, O.S. uses a proprietary system known as ABN (aluminum piston with a brass cylinder sleeve that's nickel-plated). Nickel-plated brass does a better job of "wetting" its surface with lubricating oil than chrome-plated brass sleeves. Chrome rejects lubricant (imagine water running off a duck's back). Although chrome plating has long been recognized for its durability, O.S.'s ABN system achieves improved piston longevity while maintaining the critical compression and combustion-gas seal that is needed for reliable starting and long-lasting, wide-open-throttle (WOT) performance.

Another noteworthy design feature of the FL-70 is the composite-plastic rear cover that contains the crankcase breather nipple. The breather nipple is connected by a length of medium-size silicone tubing to a similar nipple on the muffler. Excess oil is forced from the engine's crankcase to the muffler, where it is discharged along with the exhaust-a clever solution to an age-old disposal problem for 4-stroke engines.

CONSTRUCTION NOTES

* Carburetor. The engine is fitted with the new, easy-to-use, one-piece O.S. 60W airbleed carburetor/intake manifold. Two adjustable controls are provided on this carburetor. The needle valve is used to establish the correct mixture strength required for full power when the throttle is full open, and the airbleed screw is used to find the correct mixture strength needed for steady idling and a smooth transition to medium speeds. The mixture strength between medium speeds and full-throttle operation is automatically adjusted within the carburetor, specifically for the FL-70. The 60W carburetor/manifold is an updraft unit that is "hung" behind the cylinder in typical O.S. fashion.

* Valve train. The engine's overhead poppet valves are activated by rocker arms, pushrods and lifters from the transverse(crosswise-) mounted camshaft that is actuated by the crankshaft and located directly behind the drive hub. The gear drive for the cam is spiral-cut into the aft portion of the crankshaft journal (directly behind the counterbalance). The one-piece crank is fully counterbalanced and case-hardened, along with the gear teeth, for long, trouble-free operation. The cam gear must be driven at ½ crankshaft speed to actuate the valves in accordance with the piston's motion. As you know, 4-stroke engines complete two revolutions of the crankshaft during a single cycle. During this time, the intake and exhaust valves each need to open only once. Therefore, the camshaft rotates at ½ crankshaft speed.