Old-Timers
Bob Angel | [email protected]
Spark ignition fuels
FUEL QUESTIONS pop up frequently from people beginning Old-Timers modeling, and from those who might just want to bench-run an old spark-ignition engine. I mentioned the subject briefly in my October 2008 column. Joe Wagner also covered fuel for the large, modern sparkers in his "Engine Shop" column in that same issue.
It isn't as critical or mysterious as some believe, so here are the basics. In the early days, the near universal mix was three parts white gas plus one part 70-weight motor oil, commonly called "three to one" or written just as 3:1. Most modelers mixed their own.
Today, unless perhaps you live in certain Amish communities, you'll not find white gas and 70-weight (SAE 70) oil is also scarce, so modern substitutes must be used.
The white gas problem is easily solved by a trip to a sporting goods outlet. Coleman and other camp fuel is white gasoline and it is mostly additive free. It's low octane, which is as good if not better than high octane for our small displacement engines, and doesn't separate from the oil as sometimes happens with our government-specified gasoline.
SAE 70 oil can be found in some motorcycle shops, but a good substitute is SAE 60 aircraft oil, which can be picked up at some airport service facilities. Modern synthetic/castor oil blends work fine and are easier to find at motorcycle shops. They're slicker and probably superior at reducing long-term wear.
Make sure the label states that it contains castor and "mixes with gasoline or alcohol." Regular thick-viscosity castor oil (such as Sig AA) doesn't mix properly with gasoline and must be chemically altered (polymerized) to do so.
Many OT fliers prefer gas and oil, if only to smell that familiar old aroma. It's also less costly per gallon and provides much better mileage. Just be sure to stick with the camp fuel. The alcohol mixed into government pump gasoline can ruin old fuel tanks, as I'll mention later.
But a good alternative is methanol based (glow) fuel with no nitromethane. Most old sparkers were not built to handle nitro power. And nitro isn't allowed in Society of Antique Modelers (SAM) OT sparker competition. That rule keeps people from getting hit by flying engine parts.
I and many competition fliers prefer methanol-based fuel to gasoline. The engines run cooler while producing slightly more power and less carbon on the spark plug. Most model fuel suppliers can supply FAI fuel, which is a correct blend of methanol and castor oil. We usually use the same 3:1 ratio (25% oil).
You can buy the ingredients and mix your own. Except for SAE motor oils, the lubricants I've mentioned blend well with alcohol. Thick (Sig) castor also blends with methanol and is a good choice for some of the less finely machined older engines.
One cautionary note is that alcohol melts the old attached plastic fuel tanks, so you need to switch to a metal tank or one of newer plastic material. The alcohol tank also needs to be bigger due to the higher consumption rate. But have some sympathy for future generations and don't discard that original old collectible tank.
In addition to giving better performance, the higher flow rate of alcohol fuel makes the needle-valve adjustment less temperamental. In a few rare cases, I've had to drill out a spray bar outlet hole a couple of drill sizes bigger to get enough fuel flow. For a single-hole spray bar, a better way is to just drill straight through to add the second hole.
Oops, I almost forgot I was going to keep this short and simple ... The odds are quite low that you'll need to open up that spray bar.
"Miss Seldom Seen" is a title sometimes given to models shown in a special concours event for obscure aircraft at the SAM Champs. They are usually flown just once in a single qualification flight as required by the concours. Many aren't competitive in the regular events.
I'd like to mention another group of models that can be very competitive, yet are usually scarce at contests. As in the good old days, simple-to-build designs are usually chosen for competition.
Rectangular fuselage cross-sections are preferred along with constant chord wing planforms so that most ribs are identical. These models are easier to build close to the minimum weight requirements for competition.
Designs with oval or rounded fuselages, elliptical wings, many curves, a number of parts, etc., are usually overlooked as too hard to build. Carl Goldberg's Valkyrie is one such model, but oddly enough that particular design is the most often selected by those masochistic modelers who choose to tackle complex construction.
Although more difficult to build, curvy aircraft are also among the most artistic and beautiful. They always draw attention and praise when they appear at the field. I've chosen photos to represent some of those designs. I've seen both the Big Gull and Nomad designs fly. With their long wings and minimum drag they can compete with the best.
If you'd like to build something more challenging that looks great and attracts attention while being competitive, consider some of these designs: The Mystery Man, Mayfly, Record Hound, Lancer, Torpedo II, Blitzkrieg, KC Clipper, or Vee-Tailed Swallow. You can find three-views of these and many others on the SAM website found in the "Sources" listing.
At the website, click on "approved lists" then "model recognition page" and search by model name. When you select a model and click "view details," you'll get either a three-view or a picture of the model. It may say "picture not available" until you move the cursor away from the square.
Spark coil polarity was discussed, tested, and rediscussed at length among the active SAM fliers a few years back. The two primary poles of automobile coils were always marked for battery polarity. But spark coil polarity was discussed, tested, and rediscussed at length among the active SAM fliers a few years back. The two primary poles of automobile coils were always marked for battery polarity.
Transcribed from original scans by AI. Minor OCR errors may remain.
