File: 09OS.ht1.doc
[Headline: O.S. GT15 Gasoline Engine]
[Subhead: An economical alternative to glow power]
[Author: Frank Granelli]
[Email: GranelliF@aol.com]
[Photo credit: Photos by the author unless noted]
[Sidebars within text.]
[Callouts: none]
SPECIFICATIONS
Displacement: .912 cubic inches (14.95cc)
Bore: 1.091 inches (27.7mm)
Stroke: 0.976 inches (24.8mm)
Advertised practical rpm: 2,000-11,000
Rpm as tested: 1,400-10,000
Hp output: 2.37 at 15,000 rpm (2.4 psi)
Engine weight: 22.26 ounces
Muffler weight: 6.28 ounces
IG-06 module weight: 3.35 ounces
PLUSES
• Identical size replacement for .60-.90-size O.S. Max engines.
• Very reliable.
• Excellent transitioning.
• Low gasoline consumption.
• Nice sound qualities.
• Lightweight and small.
• Easy glow replacement.
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 to propel my larger airplanes.
With their constantly live spark, gasoline engines idle slowly without quitting. How do you stuff a reliable, powerful 50cc .60-size gasoline engine in a warbird without making the airplane nose heavy? Adding 3 pounds of lead to the tail slightly increases wing loading, so I haven’t been able to add a gasoline-powered engine to my glow-powered fleet. Fortunately, O.S. Max has an engine that will work. The new GT-15 features a beefed up .90-size (15cc class) two-stroke gasoline engine powerhouse in a .60-size crankcase that fits nearly anywhere.
Even the small IG-06, 2.5 x 1.06 x .87-inch ignition module is easy to position and weighs only 3.35 ounces. Using a two-cell, 1,800 mAh LiPo ignition battery saves additional room. (Any 4.8-8.4-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. (It was the older version with the 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 $4 per gallon, however, gasoline is more affordable than the $25 per gallon for glow fuel.
Much of this unforgiving flight regimen is performed at low altitude and lower throttle settings. Ignition engines idle slower 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 would also opt for 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 at this new engine. It is based on the O.S. Max .90 glow engine. Its 14.95cc displacement is slightly smaller than the 91AX’s 15.55cc, 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. This connecting rod is attached to the crankshaft using a roller bearing, not just the glow .90’s bronze bushing. A huge 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 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 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 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 2 hours.
Getting Started
Having a roller bearing means that the GT-15 will run well on a 50:1 gas/oil mixture, but a 30:1 mix was used to break it in. The engine required roughly 10 flights to break in. The first few flights were a 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. Open the high-speed needle three turns from closed.
Prime the engine using the starter for 10 seconds with the throttle open approximately 30% with the ignition off. Start the engine using an electric starter because it can’t be started by hand. (This is a good safety feature in case the ignition switch is left on and someone decides to flip the propeller.)
With the engine running at full throttle, close the needle valve until peak rpm is reached, then close the high-speed valve another 90° with the rpm constantly decreasing. Slowly open the valve until the engine again reaches peak rpm.
The final step is to open the valve 90° from this peak point. Setting the high-speed mixture will not affect the idle mixture. This is set in the factory and should never need to be adjusted.
If you adjust the idle—called a mixture control valve—this 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 was between 1,400 and 9,400. Although 10,000 rpm is possible using the smallest propeller, it didn’t provide enough thrust and flew as if it slightly stalled in the U-Can-Do.
The chart shows 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 ft. ASL. Surprisingly, the GT-15 gained 200 rpm at the higher elevation.
Flight performance was identical at both elevations. The higher rpm likely was caused by the decreased air resistance on the propeller. The pleasant surprise is that this GT-15 engine retained its full power at the higher altitude. Most glow-powered engines are down 10% at this field 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 propeller 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.
Propeller High-Speed rpm* Best Idle Speed
14 x 8 APC 9,200 1,500
15 x 6 APC 9,200 1,600
15 x 6 Xoar 9,400 1,700
16 x 4 Wide APC 8,900 1,800
16 x 6 APC 7,800 1,300
*Zero feet ASL
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 time periods, so I could not determine how long 14 ounces of fuel could 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.[dingbat]
—Frank Granelli
GranelliF@aol.com
MANUFACTURER/DISTRIBUTOR:
Hobbico
4105 Fieldstone Rd.
Champaign IL 61822
(800) 338-4639
www.osengines.com
SOURCES:
Landing Products
(530) 661-0399
www.apcprop.com
Xoar Propeller
www.xoarintl.com
Sullivan Products
(410) 732-3500
www.sullivanproducts.com
