O.S. GT-15 GASOLINE ENGINE
Frank Granelli [email protected]
An economical alternative to glow power
My RC flight experience is extensive, but the thrill of watching my airplane climb into the sky, cruise in that wild blue, and then softly return to Earth never seems to pale. Each flight is more rewarding than its predecessor.
Many of us love the smell of burning glow fuel in the morning. It smells like fun. But enjoying that “eau de castor” is getting expensive and the fuel is becoming difficult to obtain. Glow engines can quit at the worst moments, and in the worst positions. It’s after those “exciting” landings that I usually think about the power and reliability of the gasoline engines that propel my larger airplanes.
With their constantly live spark, gasoline engines idle slowly without quitting. Fortunately, O.S. Max has a gas engine that will work in most airplanes that require a two-stroke .61-size or four-stroke .91-size glow engine. The new GT-15 features a beefed-up .90-size (15cc class) two-stroke gasoline powerhouse in a .60-size crankcase that fits nearly anywhere.
Even the small IG-06 ignition module (2.5 x 1.06 x 0.87-inch) is easy to position and weighs only 3.35 ounces. Using a two-cell, 1,800 mAh LiPo ignition battery saves additional room. (Any 4.8 to 8.4-volt battery can be used.)
The engine, muffler, and ignition weigh 31.89 ounces. The O.S. .90 glow-powered engine that it replaced weighed 31 ounces (that was the older version with a heavy muffler). The return for this small penalty is an increase in low-throttle reliability and reduced fuel costs.
Both advantages are important because I have begun adding more 3-D maneuvers to my flying repertoire. Learning 3-D torque rolls requires plenty of practice and burns more fuel. At roughly $4 per gallon, however, gasoline is far more affordable than the $25-per-gallon cost for glow fuel.
Much of this unforgiving flight regimen is performed at low altitude and lower throttle settings. Ignition engines idle lower and transition with greater reliability than glow fuel powerplants. That’s important to all pilots. Scale pilots seem to spend decades building a cockpit; engine reliability is critical when that much talent and hard work are on the line during every flight. Sport fliers also benefit from lower costs and the certainty that their airplanes will return to the runway with “that ‘thing’ on the front” still rotating.
The Engine
Before flying the GT-15, let’s take a closer look. It is based on the O.S. Max .90 glow engine. Its 14.95 cc displacement is slightly smaller than the 91AX’s 15.55 cc, but matches the older engine. The mounting bolts and dimensions are identical for both glow engines and to the O.S. .65AX, but that is where the similarities end. This engine is internally tougher.
The connecting rod is thicker and wider than the .90 glow version and is attached to the crankshaft using a roller bearing, not just the glow .90’s bronze bushing. A large bolt holds this assembly in place. Because of this high-grade design, I didn’t attempt to disassemble the engine any further. Removing the head showed the typical flat-topped piston and bronze sleeve, and a port is a port, so why take the engine apart?
Fuel is fed to the GT-15 via the PD-08 pump-equipped carburetor. This regulated fuel pump not only increases engine reliability, but makes tank positioning less critical.
Although this carburetor resembles the glow version, setting it is different. Because of the pump, no muffler pressure is used. The muffler is an E-4040, which is common in many O.S. engines of this size range. Possibly because this is a gasoline engine, airborne noise is not as loud or annoying as with glow engines.
Mounting the Engine
To get the engine airborne, I chose my tried-and-true U-Can-Do 3D. The GT-15 easily replaced its O.S. .90 glow engine. The small ignition module fits in the cooling air outlet 4.2 inches from the engine's rear (specified minimum is 4 inches). The LiPo battery fits directly under the fuel tank. The only conversion work was modifying the fuel tank and installing an ignition switch/charge port.
The Sullivan Main Fuel Stopper Kit contains everything needed except Tygon fuel tubing, which is available at most hobby shops. Glow-fuel silicone stoppers and fuel lines can be ruined by gasoline. The tank can hold either fuel type, but you should always use an in-line fuel filter with a gasoline engine because the fuel ports have narrow passages that can easily clog.
The 30-minute conversion was straightforward. While the tank was out for conversion, I slipped the LiPo battery into place and secured the ignition module with hook-and-loop strapping.
I placed a standard receiver switch/charging jack between the battery and the ignition module. The switch allows engine shutdown independent of the radio system and is an added safety measure. I followed the AMA safety recommendation. The IG-06's connector is universal and was secured with 1/2-inch heat-shrink tubing.
Full low-throttle trim is used for radio shutdown. The cowling's needle-valve opening was slightly enlarged. Conversion was completed in about two hours.
Getting Started
Because the GT-15 uses a roller bearing, it will run well on a 50:1 gas/oil mixture, but a 30:1 mix was used for break-in. The engine required roughly 10 flights to break in. The first few flights were rough, but the engine always transitioned and idled well.
It was time to set the high-speed mixture. Although the carburetor looks similar to its glow counterpart, setting the high-speed mixture is the opposite of glow. Follow these steps:
- Open the high-speed needle three turns from closed.
- Prime the engine using the starter for 10 seconds with the throttle open approximately 30% and the ignition off.
- Start the engine using an electric starter (it cannot be started by hand — a good safety feature if the ignition switch is left on).
- With the engine running at full throttle, close the high-speed needle until peak rpm is reached.
- Close the high-speed valve another 90° while the rpm constantly decreases.
- Slowly open the valve until the engine again reaches peak rpm.
- Finally, open the valve 90° from this peak point.
Setting the high-speed mixture will not affect the idle mixture. The idle is set at the factory and should never need to be adjusted. If you do adjust the idle—called a mixture control valve—it affects the high-speed setting, so you will need to readjust the high-speed needle valve. The idle is adjusted to peak idle rpm using 3,000 rpm as the starting point after opening the control valve 60°.
Flying
O.S. recommends propeller sizes that range from 13 x 8 to 15 x 8. For sport and aerobatic flying, the engine/airplane combination seemed to work best with a 15 x 6 propeller. The useful rpm range was between 1,400 and 9,400. Although 10,000 rpm is possible using the smallest propeller, it didn't provide enough thrust and the U-Can-Do felt slightly stalled.
The test results show that the GT-15 is truly a powerhouse for this size of gasoline engine. The rpm tests were performed at both sea level and at 1,770 feet above sea level (ASL). Surprisingly, the GT-15 gained 200 rpm at the higher elevation. Flight performance was identical at both elevations; the higher rpm likely resulted from decreased air resistance on the propeller. The pleasant surprise is that this GT-15 retained its full power at the higher altitude. Most glow-powered engines are down about 10% at this level and have poorer aircraft performance.
I used two 15 x 6 propellers. Sport flight was great using the APC version, but there was little reserve power when flying torque rolls. My aircraft often falls out of this maneuver, so excess recovery power is appreciated. The Xoar 15 x 6 supplied that extra power.
The best overall 3-D performance was provided by the APC 16 x 4W propeller. Not only did it deliver the most excess climb power, but the larger diameter supplied more aircraft rotational torque, making flying this maneuver easier. Airspeed was slower using the 4-inch pitch propeller, but lower airspeed is good for the U-Can-Do because it reduces flutter risk. For scale airplanes—a natural home for this engine—I suggest using 15 x 6 propellers because most of these aircraft have moderately higher flight and approach airspeeds.
Similar to most gasoline engines, the GT-15 is fuel efficient. After 20 minutes on the 14-ounce tank, my neck got tired. For some reason, my neighbors and the other pilots at the field seemed to become annoyed by listening to an engine running at 6,000 rpm on the ground for long periods, so I could not determine exactly how long 14 ounces of fuel would last. Based upon fuel used in 20 minutes, my estimate would be 25 to 30 minutes. This is roughly half of what glow-fuel consumption would have been.
Lower fuel costs, better transitioning, more reliable and lower idling speeds, plus longer flight times in a small, light, and powerful package is a difficult combination to beat. This engine has opened my eyes to using gasoline power in smaller aircraft.
Its superior reliability, great sound, and lower operating costs would be great in the 86-inch P-38 Lightning that I am working on. At a street price of $350, it is affordable and a great value considering the cost savings.
— Frank Granelli [email protected]
Propeller — High-speed rpm* — Best idle speed
- 14 x 8 APC — 9,200 rpm — 1,500 rpm
- 15 x 6 APC — 9,200 rpm — 1,600 rpm
- 15 x 6 Xoar — 9,400 rpm — 1,700 rpm
- 16 x 4W APC — 8,900 rpm — 1,800 rpm
- 16 x 6 APC — 7,800 rpm — 1,300 rpm
*Zero feet ASL
Transcribed from original scans by AI. Minor OCR errors may remain.
