As most AMA members are aware, O.S. Engines is well known and respected for producing some of the finest model airplane engines ever made. The company’s detailing is exemplary and performance is first rate.
What you may not know is that O.S. was established in 1936 to produce miniature steam engines and also introduced its first gasoline model airplane engine that same year. Forty years later, O.S. introduced the first four-stroke engine to the RC market. It met with considerable success and remains enthusiastically sought after by engine collectors.
Along the way, O.S. has given us a staggering array of glow, gasoline, and diesel engines. Its current line includes 67 single- and multi-cylinder engines for aircraft, marine, and surface use. I can’t recall a single negative conversation about O.S. products during my 35 years in RC, and if I have had any, they haven’t been numerous or memorable.
Last year, I discovered O.S. released its first four-stroke gasoline engine: the GF40. At 40cc, this engine promises to be a welcome option for modelers with larger aircraft looking for the economy of a gasoline engine and the distinctive exhaust note only a four-stroke engine can deliver.
I quickly learned that the GF40’s power output is approximately equivalent to that of the earlier O.S. GT33 two-stroke engine, making it the perfect option for those 25-30cc models that deserve more power and a more realistic sound.
First Impressions
The O.S. GF40 arrived in a glossy, durable cardboard box. The supplied accessories include a silencer, electronic ignition, and CM-6 spark plug, along with the manual. The propeller nut features a collet-style locking nut to help avoid throwing the propeller if the engine backfires.
The workmanship is pure O.S., with a flawless appearance throughout. The GF40 features a cast-aluminum crankcase, cylinder, and head, all with an attractive, finely textured finish. My intention was to tear down the engine for a closer look, but the socket head cap screws securing the major castings were all tighter than I was comfortable forcing. I suspect they are using a thread-locking compound during final assembly. The fit and finish is so precise that I could barely detect the parting line between the backplate and crankcase castings.
The two-piece, cast-aluminum silencer has a baffle plate sandwiched between the main shells. These shells are easily separated, allowing you to reposition the angled exhaust port to best suit your installation. Soft aluminum gaskets minimize leaking at this intersection.
The silencer mounts to the cylinder on a threaded standoff tube, offering 15mm of linear adjustment. A pair of lock nuts secures the silencer in your desired position.
The rear carburetor features two readily accessible needle valves as well as traditional throttle and choke levers. And here’s a nice touch: one of the mounting lugs has a ring cast onto it to support and guide a pushrod for manual choke operation if desired.
Setting a Baseline
It just so happened that my 1/4-scale SR Batteries Eindecker had been gathering dust in my hangar and was a ripe candidate for an upgrade. The 17-pound World War I model was originally designed for a popular 26cc gas engine and it flies well on its large wing, but a bit more power never hurts, right?
Before swapping engines, I made one last trip to the field to gather some reference data with its original engine. I didn’t expect excess power to noticeably speed up this draggy WW I kite, but I did know it could significantly boost the rate of climb.
To watch that trend, I installed a ZLog recording altimeter from Hexpert Systems and headed out to the field. This nifty gadget tracks altitude across time, making it easy to calculate rate of climb. The original engine pulls the Eindecker off the ground in slightly less than 100 feet and gives a maximum climb of 450 feet per minute.
Time to head home and install the new O.S.
Installation
Mounting the GF40 is straightforward with mounting tabs integral to its backplate. The rear carburetor configuration requires 1.5-inch standoffs. Because none were provided, and I hadn’t thought to order them with the engine, I grabbed a length of 6061-T6 from my metal stash and then spent some quality time with my Sherline lathe. Hobbico offers standoffs in several sizes and I could have just ordered a set, but at that point making them was much faster.
T-nuts and 10-32 socket-head cap screws with lock washers did the honors, as I secured the GF40 inverted with a slight angle to use the existing throttle pushrod installation and tuck the muffler under the chin.
I drew and printed a mounting pattern with my computer to locate the new hole locations on the firewall. Watch clearances with your installation. I could not spin the full silencer into place without fuselage interference. Instead of removing the engine, I removed the rear silencer shell and baffle. I also rotated the rear shell on reinstallation by 90° to direct the exhaust farther from the Eindecker’s belly.
On the electronic ignition, I used self-adhesive Velcro and a length of Vel-Tye’s Velcro OneWrap to secure it in place. I powered it with a Thunder Power 2S 2,100 mAh pack mounted behind the firewall, again using self-adhesive Velcro. The ignition draws 400 mA at 6,000 rpm, so the 2,100 mAh pack should be good for plenty of flying.
While I was connecting the throttle pushrod, I noticed that the control arm was loose on its shaft. The throttle arm is plastic and screws to the end of its brass rod without indexing. Securing the arm at the right angle took some trial and error because the throttle shaft can spin under the arm as you tighten it.
Interestingly, the weight of the larger and more complex GF40, when fully installed with standoffs and ignition, is essentially the same as the smaller 26cc original engine.
Turning Over
Confirm the appropriate carburetor settings before you fire it up for the first time. The manual calls out 12/3 turns open from fully closed for both needle valves. These settings worked beautifully for my initial runs and, after some tuning trials during the propeller testing, I have returned to these recommended positions.
The rear-mounted carburetor helps keep your fingers away from the propeller, but that isn’t permission to adjust the needles while the engine is running. It is always best to shut down between needle changes.
I used regular unleaded fuel mixed 32:1 with high-quality oil. The manual recommends using an electric starter for safety and I concur. Although the engine does start readily by hand, why stick your precious fingers in harm’s way? The GF40 easily starts with a starter, usually with a bump after there is some gasoline in the carburetor.
The recommended first propeller is an 18 x 8, with up to a 20 x 10 to best suit your model and flying style after break-in. Following my first few flights, I ran the GF40 with four typical propellers within the recommended range, monitoring rpm with the telemetry capability of Hitec’s Aurora 9 transmitter, backed up with a handheld tachometer.
The engine started easily and ran smoothly with all of them, giving a dependable low idle and plenty of power on the top end. I thought it might be slightly rich with the recommended initial needle positions, but that wasn’t the case. The GF40 consistently transitions smoothly without sagging after it is warmed up.
In the Air
The question that really matters is what happens in the air. I had expected a performance jump with the O.S., but even I was surprised. Previously, the Eindecker would accelerate and rumble along the runway at for nearly 100 feet before rising.
Although it flew well on its wing and would loop and roll in a scale manner with appropriate coaxing, there wasn’t much excess energy. I found that pulling up for a stall turn after a full throttle pass had it running out of steam after climbing only 50-75 feet. It is fun to fly, but you do have to mind your energy while maneuvering. Again, more than 17 pounds is a lot of airplane behind a 26cc two-stroke engine.
All of that changed with the O.S. GF40. Although the full throttle rpm remained similar, I was now flying with a 20 x 8 propeller instead of the previous 18 x 6. In addition to the extra pitch, the propeller disk is now 23% larger. Combine the two and you have a completely different feel as you roll into the throttle.
The takeoff run can now be as short as I want. With full power, I have been off the ground in no more than 20 feet and climbing hard. The maximum climb rate now exceeds 1,200 feet per minute and the Eindecker can hover, although that does look wrong!
The envelope for appropriate maneuvers has also been significantly expanded. My loops, barrel rolls, chandelles, and Immelmanns are all now large, open, and graceful. In stall turns, the GF40 will pull as long as I ask, letting me stretch them out to my heart’s content before I kick in the rudder to bring it around.
Another joy is that distinctive four-stroke exhaust note, especially while dragging out the takeoff or floating by at reduced throttle. Although you may not always need the extra power, it is nice to know there is plenty in reserve, whether I need to get out of a tight jam or simply want to have some fun.
I think a 22 x 6 propeller will be the sweet stick for lazy WW I-style flying and I am already eyeing one of those nicely finished WW I-style Xoar propellers. Given a modern, lightweight, aerobatic airframe, the 20 x 8 propeller would be my first choice.
In Conclusion
I have no doubt that the O.S. GF40 will provide plenty of pull for aerobatic models weighing in the mid to low teens, and could easily fly sport and scale models up to 20 pounds or more, depending on wing loading and expectations.
Its reliable starting, easy handling, and abundant power will make this a ready favorite for anyone who appreciates a well-designed and precisely machined engine.
SPECIFICATIONS
Displacement: 39.96cc/2.439 cu. in.
Bore: 40.0mm/1.575 inches
Stroke: 31.8mm/1.252 inches
Practical rpm: 1,800-9,000
Output: 3.75 hp at 8,600 rpm
Weight: 3 pounds, 2.2 ounces
Engine: 42.65 ounces
Silencer: 3.98 ounces
Ignition: 3.53 ounces
Fuel consumption: approximately .75 to .8 ounces per minute at full throttle
Street price: $799.99
PLUSES
• Easy starting.
• Reliable idle.
• Smooth midrange and acceleration.
• Strong power.
• Great sound.
MINUSES
• None noted.
Propeller Testing
Propeller Idle Maximum rpm
Zinger 18 x 8 1920 7170
Xoar 18 x 10 1890 6730
Zinger 20 x 8 1740 6580
Zinger 20 x 10 1770 6220
