The Engine Shop
Joe Wagner | [email protected]
2007/04
A review of Dave Gierke’s new 2-Stroke Glow Engines for R/C Model Aircraft book
Two-stroke model engines are essentially simple devices. With their few moving parts, it doesn't seem as though there could be much complication in operating them or many things to go wrong. Yet that's far from true. In spite of their simplicity, model-engine operation involves several subtle interactions that can greatly affect their power output and dependability.
One way to educate yourself about these power plants is personal experience. Learning that way can take a long time, though.
A better way is through other people's experiences. There's certainly no lack of information these days about that—especially on the Web. Internet forums such as RCUniverse contain an incredible amount of data about model engines: opinions, techniques, ideas, complaints, suggestions, problems, recommendations, etc.
Yet much of that model-engine information is dubious and even contradictory. Some of it is plain wrong. A far better source of dependable data on model airplane engines is Dave Gierke's book 2-Stroke Glow Engines for R/C Model Aircraft.
Published by Air Age, Inc. (which publishes Model Airplane News)—and available from AMA's online store—this book covers in exquisite detail the design and operation of two-strokers in their many versions. It explains everything.
The latter half of the book is particularly useful to RC fliers. Its well-illustrated chapters about fuels, starting, break-in, adjustment, idling technique, cleaning and maintenance, and more give you reliable information about all the "operational aspects" of model engines. These chapters not only tell you how to do things; they explain why.
A topic that Dave's book doesn't cover is propeller selection. I don't know of any book that does. Maybe that's because the subject is so much more of an art than a science.
The propeller pulls our models (ducted fans excepted) through the sky. No matter how well your engine runs, if it's spinning an improper prop, your model won't perform its best. And propeller selection depends just as much on the airplane as it does on the engine. A slow sport flyer needs a bigger, flatter-pitch propeller than a speedy airplane. That seems obvious, yet blade shape, material, and even stiffness can have more of an effect than you might think.
It's not nearly good enough to specify, say, a 10 x 5 propeller for flying a model. Propellers of different makes and materials with the same nominal diameter-pitch numbers can vary enormously in how well they fly an airplane.
A while back I got hooked on a certain kind of model engine. I owned approximately 10 and flew them in FF, CL, and RC airplanes. I did plenty of carefully observed test flying to find the optimum propellers. I was surprised to discover that each model needed a different propeller to perform its best. I used the same fuel and glow plugs in all the engines too. (Maybe I could have improved flight performance even more had I tried altering the plugs and fuel.)
It is usually a subjective task to determine the best propeller for a certain airplane-engine combination. However, with today's readily available handheld radar speed guns, a more accurate method can be used.
If a propeller were 100% efficient it would pull its vehicle at a speed (in mph) equal to its pitch (in inches) multiplied by its rpm (in thousands). A 10 x 5 propeller spinning at 12,000 rpm would produce an airplane velocity of 60 mph (5 x 12) at 100% efficiency. However, we all know that 100% efficiency is unattainable. To find out what efficiency a propeller actually delivers on a given model and engine, divide the measured speed in flight by the theoretical speed. Say the model flies at 42 mph. That shows that the propeller is working at 70% efficiency. (That's pretty good! Some of the propellers I've flight tested came out at less than 40%—and even the best never exceeded 75%.)
In general, propellers work more efficiently at low rpm. Have you ever watched one of the new "wind farm" electrical power generators working and been surprised by how slowly their blades rotate? There's a good reason for that, and it applies to model engines too.
High rpm wastes energy in producing useless turbulence and noise. Slower spinning results in more of the available energy being converted into useful work. A few kinds of models, such as racing airplanes and competition FF aircraft, require high rpm, but most don't.
Another way I've found to increase my engines' in-flight power efficiency is to boost my fuel's oil content. Many modelers think minimizing model-engine fuel's lubricant content will add power. After all, the part of the fuel that burns is what produces power, right?
Yes, that's right. But oil does much more inside a model engine than make the moving parts slippery. For one thing, it improves compression. Another thing it does is absorb excess heat and in a way act somewhat like the water injection used by World War II fighter-airplane engines for "War Emergency Power." Reducing the oil percentage in model-engine fuel can reduce power output to zero.
One day I was having such fun flying my CL Chickadee that I used up all the fuel I had brought with me. To save me the trouble of driving home for more, a friend offered to let me use some of his. Without thinking about it, I accepted.
I filled up the Chickadee's tank with my buddy's glow fuel and started its Cox Medallion .15 engine. It roared to life immediately. But the flight turned out to be quite short; the engine seized after just a few laps. The fuel's low oil content did it; Cox engines require lots of castor in their fuel.
On the topic of CL flying, Brodak Manufacturing (one of the supreme sources for CL flying supplies of all kinds) has come out with another CL engine. It's an .049 and an addition to, rather than a replacement for, the Chinese-made 1/2A Brodak engines I described in an earlier column.
The new Brodak .049 Mark II is a Schnuerle-ported design and has a chrome-plated aluminum piston running in an aluminum cylinder. That should make it a lightweight engine, but it's slightly heavier than the old Cox "Killer Bee" .049.
However, the Brodak .049 Mk II is ruggedly built with a hefty crankshaft and case casting. It uses a standard glow plug and its cylinder is retained with three easily accessible socket-head screws. That feature can come in handy. The engine is supplied with extra cylinder-head gaskets to make compression adjustment easy. The .049 runs well on fuels with 10%–20% nitromethane, but it needs 23%–26% oil content.
No muffler or RC throttle is available for the new Brodak .049. It's meant for use in 1/2A CL Stunt (Precision Aerobatics) and Combat models. I think it would also make an excellent power plant for some of yesteryear's sport FF models such as the Midwest Sniffer (now available again from BMJR Model Products).
Handy hint
When you're reassembling an engine with gaskets, after thoroughly cleaning the sealing surfaces, dip the gasket (a new one if possible) in castor oil and let it soak for a few minutes before installing. Put a little castor on the screws too.
Then when you reassemble the engine, the castor oil will help in tightening the screws firmly. In a few days the oil film's thickening from time and oxidation will ensure an airtight seal on the gasketed surfaces and protection against screws loosening from vibration.
It's good practice to retighten the screws after a couple days. Gaskets can and do compress.
Transcribed from original scans by AI. Minor OCR errors may remain.
