O.S. MAX .46AX

Model Airplane News, Oct 2004 by Gierke, C David

A CLASSIC POWERPLANT-NOW EVEN BETTER

Longtime Model Airplane News contributor Andy Lennon says, "The O.S. Max .46 is the best engine I've ever owned." He should know; Andy has designed and built many successful models around the venerable .46, including the Crow, Sea Hawk, Seagull III, Robin, Swift and Wild Goose.

Over the years, there have been many iterations of the Max .46, but its roots can be traced to the then revolutionary O.S. 40F-SR of 1976. This new design featured Schnuerle porting, front rotary-valve induction and a side exhaust. During this era of small pattern ships, the F-SR was the first .40-size pattern engine to exceed the 1hp barrier. Back then, as today, a high-quality engine was expected to have certain features, such as ball bearings for crankshaft support and bronze bushings for the connecting rod, but quality wasn't always enough: pilots constantly demanded better performance. Hardpressed, manufacturers met the horsepower challenge by increasing the .40's displacement by 15 percent without significantly increasing its physical size or weight. Enter the first O.S. .46. Sales showed that the new engine was an instant hit, and even competitors agreed. RC pattern and scale pilots were joined by hordes of sport modelers who valued the power and user-friendliness of the new .46. Momentum from its early triumphs has carried the .46 more than 25 years to the present, where it continues to thrive within the extensive O.S. engine line.

A CLOSER LOOK

The latest model, the .46AX, replaces the popular .46FX. Many features remain, however: the mounting-bolt pattern, dual ball bearings and the advanced bimetallic liner (ABL) cylinder sleeve. Although the cylinder sleeve (liner) has retained its "drop-in" configuration, engineers at O.S. advertise that "The cylinder liner is still ABL, but a new design helps align and level the head to ensure a perfect seal. And the head still snugs down with [machine] screws ... but there are only four, instead of six."

The new front-intake, sideexhaust, twin-ball-bearing O.S. is conventional in design and follows the successes of previous engines of this displacement. Its die-cast, one-piece aluminum-alloy crankcase has a fuel-metering carburetor as well as a remote primary needle valve. The drive washer is "keyed" onto the crankshaft, eliminating the previous removal problem associated with collet-lock assemblies.

DESIGN FEATURES

After I had disassembled the .46AX, my customary measurement of component parts (sizes and clearances) and design particulars revealed several interesting features:

* Connecting rod. The engine's relatively low connecting-rod-to-stroke ratio (1.76:1) indicated that the design was intended for low nitromethane-content fuel blends (see "The connecting rod and nitro content" click trip). Also, its effective compression ratio turned out to be quite high-9.6:1 (see the "Geometric and effective compression ratio" click trip)-further reinforcing the low nitro requirement. Why? Methyl alcohol (methanol) tolerates high compression ratios much better than nitromethane, which tends to detonate. Detonation is characterized by a frying-egg sound and sharply elevated cylinder-head temperatures; if allowed to continue, detonation can destroy an engine by melting a hole through the piston crown. Increasing the fuel's nitro content will often boost the engine's torque and power, but the possibility of detonation also increases. As stated in the owner's manual, 10- to 15-percent nitro (by volume) is about the limit for the .46AX.

* Porting system. The O.S. cylinderporting system uses the tried-and-true combination of two Schnuerle transfer ports and a single boost port. A subtle but significant departure from established practice is seen where the descending piston opens the Schnuerle transfers slightly before the boost port. This strategy requires a portion of the boost port's energy flow to redirect the primary air/fuel mixture streams toward the squish-band portion of the cylinder head, away from the glowplug element. In the early years of Schnuerle boost-port systems (late 1960s), this arrangement was featured with notable success. As time went on, designers began using the boost port more as a transfer port, rather than a control port, by opening it simultaneously with the primary Schnuerle ports; unfortunately, plug quenching during idle and throttle-up became more of a problem. I, for one, welcome O.S.'s return to an earlier paradigm!

* Crankshaft. A prominent feature of the .46AX is its massive crankshaft counterbalance. When viewed from the rear, the crank web is seen to be asymmetrical and overbalanced to the trailing side. Although a singlecylinder engine can't be perfectly balanced (there will always be speeds at which it shakes worse than others), O.S.'s engineers have compensated for the radial inductionhole cutout (below the carburetor) that was largely ignored in past designs. Testing has demonstrated very smooth operation throughout the engine's practical speed range. The crank's relatively small axial induction hole (0.395 inch) suggests that increased mixture velocities lead to superior crankcase filling and scavenging (clearing) into the engine's combustion zone.