NewComers
Bob Underwood
Box 40, St. Peters, MO 63376
Modeling is like every other activity that involves technically oriented products: the breed undergoes continuous evolution and refinement. Not only do we find new approaches, but older, established methods are tweaked to make them better. Sometimes "better" is just a tad, but ten tads over a few years adds up to a great change.
An excellent example is propulsion units—engines or motors—whatever is used to make our powered models fly by providing thrust. The most recent addition to the power mode is the true jet engine. At several thousand dollars a pop, and given its level of sophistication, it's not a NewComer issue, but it illustrates the point regarding development.
Within the more standard engine forms, the old two-strokes (dating to the early 1900s) underwent constant, subtle changes. Power levels and rpm increased steadily through improvements in materials and experimentation with timing, porting, etc. Four-strokes and the converted chain-saw gasoline engines made their appearance, and the modeler suddenly was armed with a multitude of weapons to wage the power battle.
The task became even more complicated with the advent of propeller research. Prompted by the desire to reduce sound (noise), the revolution (pun intended) in props has been very helpful. The use of highly refined, high-pitch props on RC pattern airplanes has produced a power/sound ratio that is fantastic. The ramifications of this progress through competition have rippled through the model community.
Unfortunately for the newcomer, the task of getting started has become more complicated. The choices are greater. So what do you use as a guideline? Manufacturers and dealers generally attempt to provide some help.
The tendency is to equate model size (weight, wingspan, etc.) to an engine size or size range. There are two schools of thought regarding the size range:
- One club "expert" will tell you to hug the low end of the displacement range (to keep you out of trouble from having too much power).
- The other club "expert" will tell you to bang the top end of the range (or sometimes go larger!): "You can always throttle back, and you'll have the extra power if you need it."
There can be pitfalls with each approach. When I started flying RC in 1967, I chose a Falcon 56 (56-inch wingspan) and a .19 two-stroke engine. This was a standard combination at the time, and the model flew nicely. Now a model with a 50–60-inch span is considered a .40-sized model. That's in spite of the fact that even a modern sport .40 is usually a whole lot more powerful than one from 23 years ago.
Getting started — my observations
Stay within the power/size range recommended by the manufacturer, with a tendency to lean toward the top end. The larger size is suggested for several reasons:
- If you use a smaller-size engine, you may have to wring out every rpm to make the model fly well. You always run the risk of leaning out the engine too far (too little fuel in the fuel/air mixture) and frying it in short order. It is always possible to underprop (use a lower-pitch prop) to manage excess power. You can also run the engine richer with a richer fuel/air mixture.
- Since most early models built tend to be tailheavy, a little extra "engine ballast" may not be all bad.
There are other considerations for selecting a size range for a starting model. A general rule suggests that smaller models are more difficult to fly. They are more sensitive to overweight conditions and are generally more control-sensitive. On the other hand, "larger" can equate to "expensive." This is true not only for the initial expense for the model, finishing material, and engine, but also for overall upkeep. Check the cost of fuel, for instance.
As a result of these factors, you will notice that most of the trainers are in the .40–.60 size range for RC and .40-and-slightly-below for control line. This produces a reasonably sized model that flies well, is easy to transport, and doesn't bend the pocketbook.
There is one other factor that may affect your selection: if you have thought ahead to stage two of your aeromodeling growth, there may be an engine size or other requirement dictated by your future model choices. It would be a shame to have a perfectly good engine that isn't usable in a future project.
In RC, the same is true for radio systems. In a previous column I suggested a minimum of five channels, even though your trainer may only utilize three (throttle, rudder, elevator). Yes, I know the basic functions only take three or four channels, but if you really want retracts or other options, more channels give you flexibility.
Two-stroke vs. four-stroke
Another difficult decision is whether to use a two-stroke or a four-stroke engine. Some considerations:
- Cost. Four-strokes are generally more expensive. However, they use less fuel. But they've got more parts to wear!
- Sound. Even unmuffled, four-strokes tend to be quieter. At least the engine "note" (harshness) is less objectionable. Does the club have a restriction?
- Weight and rpm. As a general rule, four-strokes are heavier and operate at a lower rpm than two-strokes. However, they will often swing a bigger prop, creating as much (or more) thrust. During the last few years, significant strides have been made in closing the gap between two- and four-strokes.
Propellers and matching
For a long time, one of the least efficient parts of our power system was the prop. This was true regarding power and sound reduction. As modelers, we tended to complicate the issue by having relatively closed minds about the engine/prop size correlation.
At one time, the charts all said "4.0: 10 x 6" or "6.0: 11 x 7." Well, props have gotten much better and modelers much wiser in selecting proper diameter and pitch for a specific application.
Every engine has a power curve that is dictated by various factors. Every model, due to its configuration or potential use, has specific prop/engine needs. The job is to link all this up.
Fortunately for most trainer applications, the adaptability covers a wide range. If you stick somewhere in the middle of the engine manufacturer's diameter/pitch range, you should have no problem. But it doesn't hurt to experiment a little; just don't lug the engine down too far. In this field, competition communities have been very helpful in spurring development.
Buying an engine
You've probably noticed that the cost of engines, even for a given size, covers a wide range. If you subscribe to the theory espoused by my mother-in-law, you'll select the most expensive engine! She based this concept on the premise you should buy the best there is. This served her well, and indeed the furniture she owned would wear forever (even if the style didn't!). But beware following this philosophy in buying an engine: the cost of an engine may be dictated more by its intended use and how it is set up and finished.
Purchasing a high-performance competition engine for a trainer or sport model is probably not a wise decision. I'm not certain I'd want an Indy-car engine in my Dodge van!
Again, it's wise to nose around, look, and ask questions. It doesn't take an Einstein to figure out what combinations fly well at the field.
Transcribed from original scans by AI. Minor OCR errors may remain.
