A brief introduction
Eric Henderson | [email protected]
First, my best wishes go out to Joe Wagner and his family. I hope that Joe has a speedy recovery and is soon back to his former self.
Second, a quick personal introduction. Engines and I go back to my preteens, when I was flipping diesel engines to get my CL airplanes to fly. After a brief foray into 500cc British four-stroke motorcycle racing engines with aluminum castings and cylinder heads, I returned to model engines with my first Merco .61 glow engine. That was soon followed by the early YS 60 short-stroke, pressure-regulated, two-stroke, tuned-pipe screamer.
A long procession of O.S., Rossi, Moki, YS, Magnum, ASP, Enya two- and four-stroke power plants followed, to name a few. They found their way into my airplanes, cars, and boats, and they ranged in sophistication from no-throttle CL engines to electronic-fuel-injected, supercharged four-stroke varieties. Add a string of Desert Aircraft, 3W, and RCS gas/ignition singles and twins, and you have my model-engine résumé fairly wrapped up.
Next, the phone rang from Muncie, and here I am in print again with MA, reviewing and writing about my favorite area of the hobby!
O.S. FS-95V overview
In this column I review the O.S. FS-95V glow engine. The company indicates that it is the first in a new line of four-strokes, which prompted a much closer look at all the "bits" to see if O.S. had changed things. When an engine is the first of a line, you often find new engineering that will be carried forward in future releases.
There is an obvious major external change. Gone are the single rocker-box cover and parallel pushrods. In their place are two pushrods that spread out toward individual black covers. This configuration clearly indicates that both valves are now angled in the head. The pushrod protectors are positioned in a "V" to line up with the new cylinder head design. It's reasonable to assume that the "V" designation was derived from this feature and could be a signature of future designs and releases.
The locations of the ports and glow plug leave only a few places where the head bolts could go. O.S. is clearly confident that this layout works. My only comment is that one has to be judicious when retightening the cylinder head. For cylinder-head tightening on a model engine, using an inch-ounce torque driver is mandatory in my book. At first each bolt should be tightened lightly and equally — a slight turn at a time on the four bolts until you experience some resistance. Then, and only then, hit them with the full force of the torque wrench.
The angled valves can be opened with pressure from a finger. It was reassuring that they could not touch each other, even at well above maximum opening settings.
One head-design item puzzled me: the angle of the valve does not match the angle of the pushrod to the rocker arm — the valves are angled more. Therefore, neither pushrod, when viewed from the front, is going to be directly in line with its lifting action. The valve-lifter end is square with the top of the valve itself. The purist in me was uncomfortable with the alignment, but I could not see why it would not work. Call it an aesthetics fetish if you like; time will tell.
While in the "puzzled" zone, I noticed the muffler and the crankcase have pressure and vent nipples pre-cast in the mold. No longer evident are the brass screw-in nipples that we have grown to know and love. This is a brilliant casting achievement on O.S.'s part. I hastily add that the camshaft casing is also extremely thin and of excellent caliber.
What concerned me was that the fuel tubing might not want to stay on a nipple that has no increase or change in width near the end. I understand it could not be cast that way, but silicone fuel tubing tends to come off when it gets oily or old. The crankcase vent line is noncritical other than the inconvenient mess if it fails. The muffler exhaust/pressure feed to the tank is much more important for successful operation. During testing the pressure and crankcase vent lines stayed firmly in place. It helps if you clean the metal with alcohol first.
The engine uses a 16002 main ball bearing and the same front thrust bearings as many current O.S. engines. There are also two small bearings that support the camshaft. Experience dictates that it is good practice to use the crankcase vent as an after-run oiling port to help prevent these bearings from rusting in more humid climes.
Telemetry and test setup
Telemetry
I used a Spektrum TM1100 telemetry module. It took only a few minutes to bolt the rpm pickup onto/into the backplate and add a temperature sensor wire, which I rigged as a noose to hold it against the cylinder head. I wrapped the AR600 and TM1100 telemetry bits in a plastic bag to protect them from fuel and exhaust contamination.
Telemetry allows you to read the temperature and engine rpm from a safe distance. The rpm reading on the transmitter display updates rapidly and accurately. I carefully monitored head temperature changes because this was a new power plant being run hard on a relatively cool day. I watched the head temperature track the power requested by the throttle stick and saw the rpm rise as the engine was broken in.
Test conditions and initial break-in
It was a 38° day with 15 mph winds cooling everything. Testing this way was safer and more comfortable — I sat mostly in my vehicle with the heater on. The FS-95V fired up with the first spin of the electric starter. The main needle was initially set at three turns out. There was never a need to touch the low-end needle factory setting. I used a 12 x 6 propeller first to avoid overloading moving parts during the initial break-in process.
During break-in I recorded rpm relative to approximate throttle-stick positions. Most model engines have neither a linear nor directly proportional response to the throttle stick. The FS-95V had a reasonably good rpm-to-stick position ratio:
- Idle start point: 2,210 rpm
- One-quarter throttle: 5,700 rpm
- Half throttle: 7,100 rpm
- Three-quarters throttle: 9,200 rpm
- Full throttle: 10,500 rpm
The response was consistently smooth and rapid. There was a perceived increase in exhaust note and a power surge upon the application of full throttle. O.S. has redesigned its Jet Stream muffler with a new shape to obtain a better four-stroke sound. There was a distinct “bark” at full bore, but overall it was a pleasant sound that I could talk over.
Performance results
After using a full 24-ounce tank of fuel, I took the following rpm and head-temperature measurements:
- 12 x 6 propeller: idle rpm 2,600; full-throttle rpm 11,700; head temperature 97°
- 13 x 6 propeller: idle rpm 2,510; full-throttle rpm 11,300; head temperature 102°
- 14 x 6 propeller: idle rpm 2,210; full-throttle rpm 10,700; head temperature 109°
Please note that O.S. recommends only 13 x 7–9 to 15 x 6–8 propellers for normal operation. Smaller propellers should be used for break-in only. The test rpm figures were above O.S.'s specified practical rpm of 11,000. I recommend reducing to 15% nitro or using a bigger propeller if the engine revs too high for the valve train.
This engine design is 10 grams lighter than the O.S. FS-91, which helps the power-to-weight ratio. Getting 10,700 rpm with a 14 x 6 prop on a nonpumped, nonsupercharged power plant is excellent. This strong-performing four-stroke runs smoothly and sounds great. Check the website for pricing.
Specifications
- Bore: 1.14 inches (29 mm)
- Stroke: .93 inch (23.6 mm)
- Volume: .95 cu. in. (15.59 cc)
- Weight: 20.88 ounces (592 grams)
Sources
O.S. Engines (217) 398-8970 www.osengines.com
Transcribed from original scans by AI. Minor OCR errors may remain.
