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by John Pavlick
O.S. .46LA Conversion
Expanded power for 3D applications
I’m about to introduce you to a different type of model airplane engine: the diesel.
Some think that a conventional glow engine is a diesel, since it has a glow plug like an automotive diesel engine. Well that isn’t entirely true. A glow engine uses “glow ignition.” The main difference between these engines and a diesel is what causes the fuel to ignite. Since they run on different types of fuel—glow fuel or “diesel” fuel—the process is different. Whether it’s glow or diesel, when the air-fuel mixture is compressed inside the cylinder, heat is generated. With glow ignition, this heat alone will not ignite the fuel because it requires more heat than can be achieved with a reasonable compression ratio. The heat from the glow plug ignites the fuel.
TIMING, COMPRESSION & HEAT
Does all that make sense? Good; now here’s something else that you need to understand: fuel does not ignite immediately. Whether it’s nitromethane and methanol (glow fuel) or kerosene and ether (diesel fuel), it simply takes time for fuel to ignite. To make optimum power (torque), ignition must happen when the piston is in the proper position. If it happens too late, power will be compromised. Worse yet, if it happens too early, not only will power output suffer, but engine parts may also be stressed. The challenge is even greater when there isn’t an ignition system.
When using a modern automotive fuel, for example, in an injected internal-combustion engine, a micro-computer determines the correct time to fire the spark plugs based on sensors that monitor engine conditions. We don’t have anything like that with our carbureted model engines. We only have a few tuning elements at our disposal: Our primary tool is the carburetor needle valve that we use to set the fuel mixture. A leaner setting means more heat inside the combustion chamber; more heat means that the fuel is ignited “earlier”; it’s what can be referred to as “advanced” timing.
Now, here’s something interesting: the compression ratio affects the heat generated during the compression cycle and, therefore, the ignition point of the fuel. This is key to understanding how to tune a diesel engine.
Diesel fuel costs a little more than glow fuel, but you’ll use a lot less. Some people report using half as much diesel as glow fuel but I’ve found it to be closer to two thirds as much. It depends on your
GLOW & GAS COMPARISON
To see whether this conversion was worth the effort, I wanted to compare the glow-fueled version of an O.S. 46LA with its diesel counterpart. I installed the engine in a Lanier R/C Radical 330 profile 3D airplane. I started with a brand-new out-of-the-box O.S. 46LA. This is a reasonably priced sport engine with bronze bushings and a relatively small, air-bleed carburetor. Nothing fancy here.
The real performance gains when using a diesel conversion show up when you increase the prop size to the outer limits and beyond what a glow engine can spin.
The plane had plenty of top speed; I estimated about 70mph, which is faster than you should fly a profile 3D airplane. Grass-field takeoffs were short and the landing speed was reasonably slow. I easily flew aerobatic maneuvers from level flight, but vertical performance was somewhat limited. Basically, the airplane flew very well, but long uplines with rolls presented a bit of a challenge. Hovering was not easy and required almost full throttle. This is where you need thrust, and an 11x6 clearly was not enough for this 4.5-pound airplane. As for noise, I measured 87dB at 9 feet from the muffler. No surprises here, and then it was time for the Davis Diesel conversion.
While the engine was cooling, I removed the fuel tank and converted the fuel system to diesel with a new stopper and new fuel lines. Don’t forget to replace the fuel line on the remote needle valve! Having done that, I removed the original head and head gasket and installed the Davis Diesel head with a new head gasket. I began with the prop that Davis Diesel recommended for this engine: an APC 13x4. I must admit that I had my doubts at this point, since I’ve never seen a .46 engine that could turn a 13-inch prop without a tuned exhaust!
After reading the instructions (always a good idea), I filled the tank with diesel fuel and cranked the engine. After some cranking with the starter, the engine came to life (you have to flush all of the glow fuel out of the crankcase), and I dialed in the compression setting with my tachometer. The first flight proved that I was a little on the lean side because I heard the engine surge on uplines. I landed and opened the needle slightly. Much better. Now I was able to hover at about 1/2 to 3/4 throttle. The punch-out wasn’t great, but it was much better than with the glow-engine setup. In fact, it was almost comparable to an electric setup.
This little engine turns a 13-inch prop at 10,000rpm, and the idle is extremely low. How does an idle of 1,800rpm sound? Remember, this is a little .46! The diesel is quieter than the glow setup, too (3 to 5dB). This is partly the result of turning a larger prop at a slower speed, but it also has to do with the way the diesel operates. Throttle response is excellent, and you don’t have to worry about flameouts cause by the glow plug’s cooling off because there isn’t a glow plug. I’m impressed.
DIESEL BEATS GLOW IN 3D
In fun-fly and 3D, the diesel was clearly superior to its glow-fueled counterpart. It swings a larger-diameter prop, increases throttle response and has a super-low idle speed, so it makes the plane easy to handle. I must admit that when I started this project, I had my doubts, but the Davis Diesel conversion setup performed much better than I expected once I had the engine running right.
There’s a bit of a learning curve, but once you get past that, you shouldn’t have any problems with setup and tuning.
So why don’t we see more diesels at the flying field? I don’t know. When you look at its advantages—being able to swing a larger prop, better fuel economy, lower idle speed and quieter operation—you have to wonder. There is, of course, a downside: diesel fuel doesn’t exactly smell pretty. But some people find the odor of glow fuel offensive, so I guess it’s just a matter of preference. The slight discharge of burnt oil and fuel that leaves a properly tuned diesel system’s muffler and leaves a dark residue on your plane is not something you want. The solution is a hose extension: add a piece of latex tubing, automotive fuel line, or something similar to the muffler; unlike with a glow engine, this will not noticeably reduce performance.
With the current nitro shortage affecting the sales and price of glow fuel, many people are saying that the only choices are gas or electric, but they should think again. There is another option: diesel! I recommend the diesel conversion experience to anyone who is looking for an interesting project that’s sure to attract attention at the flying field. Diesels have a unique sound that you have to hear for yourself. If your club has noise restrictions, you’ll make new friends among the club’s officials and any nearby neighbors. And wait until someone sees your .46 engine with a 13-inch prop. Be prepared to answer many questions. You may even become known as the club’s “diesel guru”!