Crazy Power with a 60-90-Size Footprint
In 2011, O.S. introduced two new high-performance engines, the O.S. 91 Speed and O.S. 105HZ. The O.S. 91 Speed is available in 3D or F3C versions and they are very high-performance competition engines. The 105HZ is a bigger displacement engine; however, the price is slightly less than the 91 Speed. The 105 is designed for all types of flying and as the saying goes, “There is no substitute for displacement.” The 105HZ is a short stroke engine with its bore (29mm) bigger than its stroke (26mm). It is designed to be happy at high rpm. We have found the sweet spot is from 15,000 to 16,000 rpm. This is perfect for all existing 90-size helicopters on the market. Using the 105HZ will not require changing the helicopter gear ratio. In the past, O.S. and others have introduced long stroke engines for model helicopter use, for example the O.S. 61SF introduced in 1987, with the intention of taking advantage of the fact that long stroke designs produce more torque. But the problem was long stroke engines tend to produce more vibration. After the not so popular O.S. 61SF, O.S. went back to the short stroke O.S. 61SFN design, and afterward, most O.S. heli engines were all short stroke design.
There is no substitute for displacement, the 105HZ is the biggest 2-stroke glow fuel powered engine designed by O.S. for smackdown 3D flying. This is the powerhouse that we have been waiting for. With the help of an extra large venturi intake for the carburetor, the O.S. 105HZ develops 3.75 horsepower and fits in the same mounting dimension of a classic 61 to 91 size helicopter engine mount. Combined with the O.S. PowerBoost Pipe, the 105HZ engine is a brute powerhouse that looks like a jewel with it’s beautifully anodized blue heat sink.
NEED TO KNOW
MANUFACTURER: O.S. Engines
DISTRIBUTOR: Great Planes Model Distributors
DISPLACEMENT: 1.048 cu. In. (17.17cc)
BORE: 1.442 in. (29mm)
STROKE: 1.024 in. (26mm)
PRACTICAL RPM: 2,000-16,500
OUTPUT: 3.75 hp @ 15,000 rpm
105HZ WEIGHT: 21.03 oz. (596g)
105HZ-R WEIGHT: 21.46 oz. (608g)
PART NUMBERS: OSMG1960 (105HZ), OSMG1961 (105HZ-R)
PRICE: $479.97 (105HZ), $529.99 (105HZ-R)
The O.S. 105HZ was tested in an ElyQ Vision 90 Competition helicopter. In total, four different engines were tried in the Vision 90, an O.S. 105HZ, an O.S. 91HZ with black heat sink head, a YS 91, and a Thunder Tiger 90H. The aluminum fan hub on the Vision 90 does not have any female thread, instead it has two splits and that allows the fan hub to clamp on the round portion at the base of the engine crankshaft. This allows the same fan hub to fit O.S. and YS, which use different crankshaft threads. O.S. and Thunder Tiger uses 5/16-inch thread and YS uses 8mm metric thread. With the same Wildcat Curtis Youngblood Performance 30 heli fuel. No. 8 glow plug used on all four engines, the O.S. 105HZ gave the most stellar performance to the Vision 90.
The Vision 90 only offers one gear ratio; it has a 115-tooth nylon main gear and a 14-tooth steel pinion gear, which provides 8.214:1 ratio, which is a good all-around ratio for most 90 to 100 size engines. The O.S. 105HZ spec says it has a practical rpm range of 2,000 to 16,500 and develops 3.75 horsepower at 15,000 rpm. With a gear ratio of 8.214:1, that implies the main rotor rpm is 1,826 rpm when the engine is at 15,000 rpm. My Vision 90 was tached at 1,950 rpm during 3D which puts the O.S. 105HZ cranking away happily at 16,000 rpm. Because the O.S. 105HZ runs so smoothly and consistently, I did not have to use an engine governor. By carefully setting the U-shaped Idle-Up throttle curve, the 105HZ could easily maintain 1,880 to 1,930 rpm throughout 3D.
Two needles on the carburetor are used to control the top-end and mid-range, and a screw for adjusting the idle. The needles are not super critical in setting up. However, it still requires experience and practice to tune a 2-stroke model helicopter engine. The trick is to achieve a rich setting for hover with the muffler spilling out a thick and steady trail of blue smoke. If there is not enough smoke, then the engine is too lean (not enough fuel flowing into the carburetor). Turn the main needle counterclockwise a few clicks to open up the needle to allow more fuel flow. Too lean can be very dangerous because the engine can over heat and the piston and liner will get scratched and damaged permanently. Another symptom of too lean is the engine makes high pitch, scratchy sound, and the rotor rpm becomes too fast. The engine is too rich when there is too much smoke, and often wet fuel will sputter out of the exhaust. Symptom of too rich is the engine has no power and rotor rpm is slow. Either too rich or too lean can cause the helicopter to shake and stutter.
Test the top end setting by flying in high-speed level flight, as well as doing vertical climb at full-throttle. If the engine wants to die when full-throttle is given, or the engine can not sustain steady full-throttle, then the main needle is too lean. If the engine provides weak and sluggish power when full-throttle is given then the main needle is too rich.
It takes practices to tune a model engine, and even for experienced pilots it can take a few flights to make a new engine run perfect. Using the right glow plug and fuel is important. For testing, I used O.S. No. 8 glow plug and Wildcat 30-percent nitro helicopter fuel. The fuel and plug are important toward giving a cool running, reliable idle, smooth transition and a powerful top end. The deep blue fins on the heat sink not only looks good, but is functional in keeping the engine running cool.
O.S. heli engines are very well made, and I no longer break in new engines on the bench. The O.S. 105HZ was put in the Vision 90 and ran at a very rich setting during the first ten flights. Then I progressively leaned the needle one or two clicks during the next few flights to extract slightly more power. Most helicopter engines reach their peak performance after 10 to 15 flights. From the 15th to the 50th flight, the engine is at their peak. After 100 to 200 flights, the engine performance may start going down gradually. After 200 flights, the crankshaft bearing and/or piston ring may require changing, if you want the engine to run at peak performance. How long an engine can remain peak depends on how well you take care of the engine. The secret is to run the engine in a clean environment and keep the motor clean and well lubricated.
There is an O.S. 105HZ and an O.S. 105 HZ-R. The R version has a regulator attached to the carburetor system to meter the incoming fuel. O.S. calls it DRS (Demand Regulator System). The benefit is more consistent engine running during 3D flying because the fuel is pressure fed and not relying on the carburetor venturi effect to draw the fuel from the tank. The DRS version costs about $50 more and is ideal for 3D flying. For non-competition sport flying, the O.S. 105HZ without the regulator is fine.
In conclusion, the O.S. 105HZ is another example of fine workmanship from O.S. My first O.S. engine was an O.S. 40FSR airplane engine from 30 years ago when O.S. just entered into US market. That engine was used in my Kaos. 40 airplane, and it has also done a tour in my Schluter Helibaby helicopter many, many years ago. If treated carefully, a high-quality engine like the O.S. 105HZ can last many years. Proper treatment techniques include running the engine at the end of the day with the fuel line removed from the carb and let it run to a stop to burn off all the methanol alcohol inside. When not consistently flying the model always use after-run oil (available from hobby shops) or use high-quality Teflon oil, such as Triflow, into the carburetor in order to prevent the bearings from rusting and the inside surface from becoming pitted. You will not be disappointed with the workmanship and performance of the new O.S. 105HZ.