FS .81a 4-Stroke

A brand-new 4-stroke design from O.S.!

Written by Andrew Coholic; Photos by Andrew Coholic

O.S. introduced production model 4-strokes to the engine world many years ago (back in 1976!), and modelers have been using them with great success in all types of aircraft ever since. The Surpass line has been the mainstay of the line for many years. With the introduction of the totally new FS .56a and FS .81a engines, a new era of 4-stroke engine design from O.S. has begun. The same basic plan form (front mounted pushrods, cam housing with a camshaft 90 degrees to the crankshaft driven with helical cut gears) as the Surpass engines has been maintained - however, with many new features the new “a” series of engines is sure to be as popular in the future as the Surpass line has been for years. This article was initially published in the April 2008 issue of Fly RC. This expanded version includes test data on the O.S. FS .56a that was not included in the original article, as well as a glow plug test comparison using the FS .81a. I will look closely at these new engines, both of them, in this in-depth review!

INITIAL IMPRESSIONS
Both of these new style four strokes arrived in the typical O.S. engine blue box with excellent protective bubble padding wrapped around them and top-quality O.S. F glow plugs installed. Tools are not included, unfortunately. The engine was
well oiled and the carburetor was mounted.

The brand-new crankcase design has a more compact look with the crankcase fins extending forward to the cam housing. The fin shape has also been redesigned and has a more “faceted” look. Also differing quite a bit is the new muffler design. The new design is compact yet should provide better cooling with its fins as well as better sound reduction with its baffled design. And there was no crankcase vent; more on that soon!

RUNNING THE ENGINES
The ambient temperature during break-in was 64degrees F. The relative humidity was 73 percent, and the pressure was 30.24 inches of mercury and falling. Altitude is just above 1,000 feet above sea level.

FS .81a
I mounted the .81a engine on my test stand and broke it in with the supplied F type plug, an APC 14x6 propeller and some Wildcat 10 percent Premium Xtra fuel (10 percent nitromethane with 18 percent oil content in a 20/80 blend of castor/synthetic). My test stand tank holds 10 ounces of fuel.

With the high-speed needle set 3.5 turns out and the low-speed needle left as set at the factory, the .81a started quickly with a few bumps of my electric starter. Break-in consisted both of running wide open and at idle while using the high-speed needle to vary the mixture from very rich to just about leaning out into a clean run. I burned two tanks of fuel (approximately 35 minutes total run time) with breaks to let the engine cool.

FS .81a with 10% Wildcat Premium Xtra:

APC Prop Size	Idle rpm	Max rpm	Max CHT (°F)	Max EGT (°F)	Thrust (kg / lbs)
13x6	2,800	11,400	236	623	2.5 / 5.5
13x7	2,800	11,000	240	638	2.6 / 5.72
13x8	2,800	10,700	230	608	2.6 / 5.72
14x5N	2,800	11,800	238	658	2.9 / 6.38
14x6	2,400	10,100	238	661	3.1 / 6.82
14x8	2,400	8,900	218	636	2.8 / 6.16
15x4W	2,200	10,000	207	677	3.5 / 7.0

FS .81a with 15% Wildcat Premium Xtra:

APC Prop Size	Idle rpm	Max rpm	Max CHT (°F)	Max EGT (°F)	Thrust (kg / lbs)
13x6	2,800	11,700	225	583	2.6 / 5.72
13x7	2,400	11,000	228	627	2.7 / 5.94
13x8	2,400	10,800	226	604	2.7 / 5.94
14x5N	2,400	11,900	218	633	2.9 / 6.38
14x6	2,300	10,200	221	627	3.2 / 7.04
14x8	2,300	9,000	215	584	2.8 / 6.16
15x4W	2,200	10,100	197	603	3.6 / 7.81

During break-in, I recorded the following figures:
Minimum rpm: 2,500
Maximum rpm: 8,800
Cylinder Head Temperature (CHT): 107 to 118 deg. F
Exhaust Gas Temperature (EGT): 372 to 579 deg. F

All data was recorded on my laptop computer with the aid of the RCATS telemetry system. I also used a pair of optical tachometers and a handheld IR temperature gun to verify my readings.

After completing the break-in, I noticed that the engine was very easy to hand-start (it generally required only a flip or two), and I seldom resorted to using my electric starter again. The FS .81a never kicked back on me either—not even once! The .81a is definitely one of the nicer handling engines I have run in my experiences with 4-strokes.

After running the test props, I was pretty amazed that my test stand was spotless; it’s usually covered in oil! The FS .81a was easy to start, it throttled very well up and down and was very smooth at all rpm settings. I couldn’t find any fault with the engine. The muffler kept the noise level very reasonable as well.

GLOW PLUG TESTING THE FS .81a        
For testing a few various plugs out and seeing how they differ from the included O.S. F plug (which has historically been a benchmark for four strokes) I chose the following two plugs: a new Evolution 2/4 stroke plug and also a Merlin 2011 which is designed for four stroke engines using <20% oil content.

Here are the results, running the plugs in the FS .81a with 15% Wildcat Premium Xtra fuel:

Plug	Idle rpm	Max rpm	CHT (°F)	EGT (°F)	Thrust (kg / lbs)	Transition
O.S. F	2,200	10,100	180	656	3.5 / 7.7	Excellent
Evolution	2,500	10,050	170	657	3.5 / 7.7	Very good
Merlin 2011	2,300	9,800	188	660	3.5 / 7.3	Good

With the F plug, the transition was excellent & the idle was extremely low and reliable.  The Evolution plug provided similar top end power, but with a less reliable idle.  Transition was excellent but not quite as good as with the O.S. plug.  Switching to the Merlin, a noticeable decrease in top end rpm was noticed at the same settings while the temps were higher, and transition was still good but not nearly as crisp as with the F plug.  I suggest sticking with the supplied O.S. F plug for top performance (and although I cannot test longevity in the time I have to do a review, in my experience as a long time four stroke user, it is very hard to beat an O.S. F plug for long life.

FS .56a
I also mounted the .56a engine on my test stand and broke it in with the supplied F type plug, an APC 13x6 propeller and Wildcat 10 percent Premium Xtra fuel. With the high-speed needle set 31/2 turns out and the low-speed needle left as set at the factory, the .56a started as quickly as the .81a did.

Engine Specs:

Engine: O.S. FS .56a 4-stroke
Displacement: 9.32cc (0.569 ci)
Weight: 16.2 oz. with supplied header and muffler
Type: Single-cylinder, glow-ignition, poppet-valve 4-stroke, single conventionally ringed aluminum piston in a nickel-plated steel liner, ball-bearing-supported cantilevered crank, dual-needle metering carb.
Prop Range: 12x6 - 13x8 (for normal sport/aerobatic flying)
Manufacturer: O.S. Engines Mfg. Co. Ltd, Japan
Distributed by: Great Planes Model Distributors
Street Price: $249.99
The totally new O.S. FS .56a and FS .81a 4-stroke engines are, quite frankly, awesome running engines! The new design cases are stronger with improved sealing. The completely new recirculating lubrication system keeps the engines spotless while providing excellent lubrication to all moving parts of the engines, including the bearings. The new mufflers are very quiet and positionable to exhaust in any direction.

During break-in, I recorded the following figures:
Minimum rpm: 5,000
Maximum rpm: 9,900
Cylinder Head Temp.: 122 to 208 deg. F
Exhaust Gas Temp.: 334 to 580 deg. F

The FS .56a was as easy to run and tune as the .81a; both were smooth, responsive and very, very easy to hand-start hot or cold. Besides the differing sizes of propellers, running the .56a was in many ways identical to running the .81a without any negative tendencies.

Prop testing, again with a wide range of APC brand propellers and two blends of Wildcat fuel gave the following results:

FS .56a with 10% Wildcat Premium Xtra: .

APC Prop Size	Idle rpm	Max rpm	Max CHT (°F)	Max EGT (°F)	Thrust (kg / lbs)
11x7	2,300	11,400	290	542	1.7 / 3.74
12x6	2,300	10,300	281	526	1.9 / 4.18
12x7	2,300	10,000	285	532	1.9 / 4.18
12x8	2,200	9,400	267	497	1.9 / 4.18
12.25x3.75	2,200	12,300	287	589	2.2 / 4.84
13x6	2,200	10,100	289	536	2.1 / 4.62
13x7	2,100	9,250	275	504	1.9 / 4.18
13x8	2,100	8,900	269	498	1.9 / 4.18
14x4W	2,000	8,800	283	511	2.2 / 4.84

FS .56a with 15% Wildcat Premium Xtra:

APC Prop Size	Idle rpm	Max rpm	Max CHT (°F)	Max EGT (°F)	Thrust (kg / lbs)
11x7	2,100	11,500	265	550	1.8 / 3.96
12x6	2,100	10,500	272	515	2.1 / 4.62
12x7	2,100	10,050	269	522	2.1 / 4.62
12x8	2,100	9,500	272	498	2.0 / 4.4
12.25x3.75	2,000	12,500	278	584	2.4 / 5.28
13x6	2,000	10,150	248	530	2.3 / 5.06
13x7	2,000	9,360	246	501	2.1 / 4.62
13x8	1,900	9,040	241	484	2.1 / 4.62
14x4W	1,900	9,040	254	591	2.3 / 5.06

NEW LUBRICATION PATH. NO CRANKCASE VENTING
One really cool thing about the new “alpha” series of 4-strokes is the way the oil is routed through them. In a traditional 4-stroke glow engine, the oil that blew by the piston was exhausted from the crankcase through a breather nipple. During a few tankfuls of running, quite a mess could be made, especially if you did not route the oil outside the firewall area. Also of some concern was whether the many moving parts of the engine and the crankshaft bearings were adequately lubricated. All these concerns have now been addressed!

This is how it works: the oil that blows by the piston accumulates in the crankcase as it did before. Instead of being exhausted out through the case, the oil is transferred through the drilled-out crankshaft that has a port opening into the camshaft housing. When the piston is on the downstroke, the case is pressurized and the port opens and allows the oil to flow into the camshaft housing. There, it lubricates not only the camshaft, the camshaft bearings and the cam followers, but it also lubricates the front and rear crankshaft bearings. The oil is then forced up through the sealed pushrod covers and into the rocker box. The rockers are well lubricated (oil isn’t able to leak past the well-machined head-cover junction), and then the excess oil is “sucked” through a small orifice back into the intake where it’s recirculated through the engine once again.
The whole system keeps the engine’s exterior spotless!

TEST FLYING THE O.S. FS .81a
Because it is winter and I live in Northern Canada, I was able to fly the FS .81a only a few times in my SIG Four Star 60 ARF, and it was worth braving the cold and snow to do so! Mounted in the nose of the Four Star 60, the .81a balanced the plane neatly, and its RTF weight on skis was within a few ounces of 7 pounds.

I chose to test fly the engine and plane with an APC 14x8 propeller and 10 percent Wildcat Premium Xtra fuel. Even though it was close to zero degrees Fahrenheit, I did not “doctor” the engine in any way for cold-weather operation. During test flights, the relative humidity was around 85 percent and the pressure was denser than when bench-running—30.4 inches of mercury. It was cold, and my batteries were less than pleased! The O.S. FS .81a, however, started up without much trouble (I used my starter), and I let it warm up for a few minutes, during which time I took sound readings.
  
Standing in front, behind and off to each side of the model at 10 feet away (with the engine running wide open), I took readings of 89, 87, 88 and 88 decibels, respectively. The muffler supplied with both of these new O.S. engines is very good at reducing noise.

After a few minutes, I set the idle at 2,700rpm (a lower idle was achievable but prolonged running at idle would have eventually stopped the engine) and recorded a maximum rpm of just under 9,000. This was 1,000rpm less than I had recorded on the bench, but keep in mind that the high reading recorded during bench running is a peak rpm. During flight tests, I always reduce the rpm slightly for flying, and the cold weather and denser air are responsible for the remaining reduction in rpm.

On to the good stuff! When I throttle the .81a up while holding full up-elevator, the skis came up onto the surface of the powdery snow, and the plane was airborne in less than 100 feet! Once in the air, I forgot about my frozen fingers and started having fun. I and my flying partner, Brandon, took turns at the sticks so that I could snap a few pictures, and the .81a got a good workout with many aerobatic maneuvers (looping and rolling, inverted and upright flight, stall turns and so forth). We made many low passes with full-throttle climbouts, and the engine throttled extremely well and never sounded as if it was going to quit.

Trying some full power climbouts, I could get the Four Star 60 to climb a few hundred feet vertically before coming to a stop and then hanging on the prop. My thrust readings for this fuel/prop combination were just about the same as the plane’s RTF weight, so that verified the thrust readings. Needless to say, with the .81a pulling the test aircraft around, its performance was excellent—just about perfect for sport flying with no trouble pulling multiple loops from a horizontal flight path.

I was very impressed by the engine’s characteristics—as I had expected to be. I imagine that in warmer weather, the engine will perform even better, and you should be able to get a lower idle and higher top end for even better performance.

CLOSER INSPECTION
Working on an engine like the O.S. FS .81a is very pleasant. The hardware is top quality and fits are as good as you could hope for. If you decide to disassemble your engine, remember that nothing needs to be overtightened. All the parts come apart easily. Note that the camshaft timing mark is to be at 6 o’clock when the crankshaft/piston is at TDC; this isn’t mentioned in the manual.

Since both engines have identical construction, so I will just describe the parts once. The one-piece crankcase is an extremely high-quality casting. O.S. has amazing quality control, and the exterior and interior surfaces are free of imperfections. The outside cast surface appears to have a fine bead-blasted finish, and the machined areas are shiny and without even the smallest scratch. The new case style with the extended fins over the cam housing area will be stronger and will give better case cooling. The crankcase holds the two interference-fit crankshaft bearings (rear is unshielded; front has a single shield).

The crankshaft is machined out of one piece of steel, and it seems to have matted nickel plating to prevent corrosion before the crankpin and bearing journals are ground to size. The crank is bored out and a port that’s aligned with the cam housing area has been cut. The crankshaft has a fully cut, dual-cutaway counterbalance. Helical gear teeth cut into the crank drive the camshaft. The crankshaft nose is threaded with an SAE 5/16-24 thread and there’s a Woodruff key way to hold the key, which locks the prop-drive washer to the shaft.

The engine’s heads are similar to those in the existing Surpass line; five machine screws hold the head to the case, and a single-thickness brass shim provides a gas-tight seal. The one-piece head castings feature a bathtub-style combustion chamber with a glow plug centrally located between the two valves. The intake valve is larger than the exhaust valve by 13 percent. The valve guides are aluminum and cast in place. The rocker mount is also cast as part of the head. The two high-temperature-steel valves/springs and split-collet-type keepers; machined-steel rockers and rocker shaft round out the head components. The rocker box/head cover is cast aluminum and machined to seal tightly on the head casting without needing any gasket. The cover is anodized blue and attached firmly with two machine screws. One other notable thing you see in the head is a small hole that connects the intake to the rocker box area.

Both engines feature new-style machined-aluminum pushrod covers that have an excellent O ring seal at either end and keep the hardened-steel pushrods clean and protected. The cam followers are hardened steel, and the one-piece camshaft is also hardened steel with integral cut teeth that mate with the crankshaft teeth.

The engines feature a true 2-needle-style metering carburetor that also comes with machined-aluminum intake stacks (which should prevent more fuel from blowing back at low rpm). There is a cast-aluminum elbow that connects the carb to the head intake. Again, the flange and head are machined so well that no gasket is needed to ensure a leak-free seal. The carb side of the elbow uses an O-ring to seal against air leaks. The carburetors are mounted on the back cover with two Philips-head machine screws, and they can be positioned to have the high-speed needle on either side of the engine. The machined aluminum-alloy drive washer has a steel shim washer between it and the front bearing. The prop washer, and dual nuts are steel with a corrosion-resistant black finish.

Finally, both engines feature a new-style cast-aluminum muffler. These mufflers are three-piece with an internal baffle plate, and they have two O-rings for sealing the separate parts, and a long draw bolt and nut hold the pieces together. Fully threaded steel tube/nuts connect the muffler to the engine’s exhaust port. The muffler can be positioned to have the exhaust exit in any direction.

WISH LIST
For the O.S. FS .56a and .81a engines, I hoped to see the inclusion of basic tools like those O.S. included with the older Surpass engines—namely Allen keys and feeler gauges to set the rocker clearances. A flex throttle extension similar to the one that comes with the O.S. FS 200 would also be nice.

CONCLUSION
Both of these fine engines were a real pleasure to look at and handle and a dream to run and fly. They are extremely well made, they’re easy to start by hand or with a starter, and they’re two of the best-behaved model engines I have run. They are both very smooth and have excellent transition up and down the entire throttle range. The engines can turn props on par with or better than the competition, and—more important to me—they do it with grace and do it quietly! And with the new lubrication design, maintenance should be a moot point (and no more routing of a messy crankcase vent line). How can a model engine be so “great”? Well, I suggest you buy one, fly the heck out of it and see for yourself!

Links

Landing Products, www.apcprop.com, (530) 661-0399

O.S. Engines, distributed exclusively by Great Planes Model Distributors,
www.osengines.com, (800) 682-8948

SIG Manufacturing, www.sigmfg.com, (800) 247-5008

Wildcat Fuel, www.wildcatfuel.com, (859) 885-5619