Author: Joe Wagner
Edition: Model Aviation - 2009/08
Page Numbers: 80,81,82
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The Engine Shop

Testing the O.S. .72 Alpha from Great Planes

O.S.'s latest "Alpha"-series four-stroke engine—the 72FS-a, available from Great Planes—has all the advanced features of the earlier 56FS-a. Those include a compact die-cast muffler, shortened pushrod tubes, an advanced carburetor assembly, and the absence of a crankcase breather tube.

Instead of a breather tube, the O.S. Alpha four-stroke engines contain a novel internal system for lubricating the shaft, ball bearings, gears, and camshaft/follower assembly. For lower-end lubrication, other four-strokes depend on the small amount of oil that combustion-chamber pressure forces through the tiny clearance gap between the piston and cylinder sleeve.

The Alpha design uses a better system. A small hole through the head alongside the intake valve allows crankcase vacuum on the intake stroke to draw a small amount of fuel-and-oil mixture downward and into the space surrounding the shaft and camshaft assembly. From there, a hole in the shaft and a crossport between the bearings delivers oil to those parts—but not a lot; Alphas don't spit oil from behind their propeller drivers. Enough oil is forced into the front bearing to provide the lubrication it needs for long service life.

The carburetor assembly has a 3/4-inch-long intake extension with a smoothly radiused "bellmouth" entry. And instead of the "reversed layout" of the earlier 56FS-a, the 72FS-a carburetor has its needle on the opposite side from the muffler. That design feature is a more convenient arrangement.

O.S.'s owner's manual for the 72FS-a advises using only an electric starter with this engine, because it lacks convenient provisions for manual choking. However, especially with a new engine, I much prefer hand-starting (with a reinforced leather glove in case of kickback). Hand-starting gives a better feel for what's happening during startups.

Starting and break-in

The 72FS-a wasn't easy to hand-start in my test stand, mostly because of its upright mounting. The highly intermittent airflow up the 2 3/4-inch vertical passage between the carburetor inlet and the head made it difficult to prime the engine "wet" enough. Side or inverted mounting would eliminate that difficulty.

For break-in I used a 13 x 8 Master Airscrew K-Series propeller. After following O.S.'s recommended "Running-In" procedure—I always read and follow the manufacturer's instructions—I obtained 9,800 rpm at full throttle, burning 15% nitro fuel. The engine idled reliably at 1,800 rpm after only four tanks of fuel. The O.S. .72 Alpha runs smoothly indeed. Its short stroke—only 76% of the cylinder bore—and lightweight ringed-aluminum piston contribute greatly to that. The engine's sturdy, compact design helps too.

Double Star .60 CL engines (Tom Dixon)

Tom Dixon of Marietta, Georgia, kindly sent two of his .60-size Double Star (D-S) CL engines for experimentation. Made with high precision in Moldova (an eastern European nation between Romania and Ukraine), these power plants were designed and manufactured purely for CL precision aerobatics (stunt) flying. The D-S engines feature aluminum pistons and cylinders and use bushing-type main bearings.

"Ball bearings add nothing to an 8,500 rpm Stunt engine's performance and can be a source of front bearing air leaks, plus corrosion problems," said Tom.

He also stated that the D-Ss require no test-bench running. They can be taken directly out of the box, put in a model, and flown. One reason this can be done safely is that these engines are intended to be run in an almost steady "four-cycle" setting in flight. Stunt fliers don't need or want high speed, and the only time a D-S is expected to break into two-cycle running is during a sudden change of flight direction—such as when starting into a wingover.

With the rich fuel-mixture setting that four-cycling requires, D-S engines run extremely cool. Tom wrote that excessive oil content in the fuel will overcool a D-S and cause erratic running or flameouts. D-S power plants run so cool that Tom found they need an RC-type glow plug. He supplies all D-S engines with Sig plugs (item SIGGP003) installed. This is an "idle-bar" plug that many four-stroke fliers prefer for its heat-holding ability at idling speeds.

The rear-exhaust D-S 61 RE's muffler extends roughly 5 inches behind the cylinder. That makes it resemble a tuned pipe, but it's not. Tuned pipes have been tried in CL stunt aircraft, but they don't do enough good to justify their added bulk and cost. The D-S 61 RE is designed for inverted installations, with its muffler exiting underneath the fuel tank and the exhaust leaving the bottom of the fuselage. D-S engines run best with Uniflow fuel and require no muffler pressure, so the D-S mufflers have no pressure fittings.

Fuel

A new supplier—Maxy's Fuel in Miami, Florida—makes a glow-fuel blend that's ideal for both the O.S. Alphas and the D-Ss. The instructions for those engines call for 20% oil content, preferably half castor and half synthetic, and up to 15% nitro. I test-ran the engines with Maxy's Fuel, meeting precisely those specifications.

As I expected, I had no problems whatever with the fuel. I was pleasantly surprised by its exhaust aroma. Back in the "Good Old Days," the popular glow-fuel blends that Francisco Laboratories made had a distinctive and pleasant exhaust odor. My nose says that Maxy's Fuel smells the same; it's nostalgic for me.

Glow-fuel manufacture is the most competitive area in the model and hobby industry. All of the major fuel blenders use the best-quality ingredients they can get, and most have fully computerized blending and bottling facilities.

I asked Stan Jackson, Maxy's representative for model airplane fuels, what special qualities to expect from this new glow fuel. He provided some background:

  • Maxy Velazco, president of Maxy's Fuel, is an avid RC car racer, and the flying fuels he developed for that sport hold many state and national RC car racing records. His fuels are widely known in the RC car world, both nationally and internationally.
  • Maxy's is the premier fuel at many RC car racing events; last year he decided to expand his product line into the model airplane and helicopter fields.

Maxy's will gladly custom-blend glow fuel to any desired "formula." The fuel I obtained for testing was a new blend I requested. The highest nitro-content model aircraft fuel Maxy's provided previously was 10%. The company has an online "Club Price" list for its stock fuel blends; "Special Recipe" prices are comparable.

Hand-starting glove tip

Earlier I mentioned the "reinforced leather glove" I use for hand-starting big model airplane engines. I'd used a plain leather glove for years, but the sharp kickbacks that .60 and larger engines sometimes deliver can be painful through that relatively thin protective layer.

I tried "chicken sticks" for a while. They work okay but don't deliver the same feel I get through traditional hand-starting. Then I thought, "I only use the right-hand glove of the pair." I cut off the first two fingers of the left-hand glove and forced those onto the index and middle fingers of the right glove. Now I have plenty of protective padding on my flipping fingers and no longer dread hand-starting engines for 1/4-scale RC models.

MA

Sources

Transcribed from original scans by AI. Minor OCR errors may remain.