The Engine Shop
Joe Wagner 212 S. Pine Ave., Ozark, AL 36360
Rossis are back
I HAVE GOOD news for fans of high-powered Radio Control (RC) engines: Rossis are back! These superb Italian-made power plants were off the market for a while, but they've been available again since last September in a new Matrix model.
Several U.S. dealers will probably be carrying Rossis by the time you read this. One that I recently visited (Hobby House, Inc., 17721 Vanowen St., Reseda, CA 91335; E-mail: [email protected]) stocks all four Matrix sizes:
- .40 — rated at 1.9 horsepower
- .45 — 2.1 horsepower
- .53 — 2.3 horsepower
- .60 — 2.6 horsepower
However, as I've stated before at least twice in previous columns, horsepower ratings for model engines are far less important than propeller selection—much less important!
An example from history
For example, let's look at the Good brothers' famous pre-World War II RC airplane. This model—the Big Guff, now on display at the Smithsonian—won the RC event at the AMA Nationals in 1938, 1939, and 1940.
The Big Guff weighed 8 pounds—2 pounds of that was the RC system—and it had an 8-foot span with 1,350 square inches of wing. It was powered by a Brown Junior .60 spark-ignition engine rated at a mere 1/5 horsepower. That turned a 14 x 4 wooden propeller at approximately 6,500 rpm.
For comparison, one of today's most potent competition Free Flight engines is the Cyclon .061. It develops roughly 0.8 horsepower, with a 5-inch carbon-composite propeller spinning at more than 30,000 rpm. The Cyclon's horsepower rating is four times that of the ancient Brown sparker that powered the Big Guff, even though it has only one-tenth the Brown's displacement.
The Cyclon pulls one of today's Fédération Aéronautique Internationale Free Flight models in a 90+ mph vertical climb, but can you believe that the shrieking little Cyclon .061 could haul the Good brothers' 8-foot, 8-pound RC model off the ground and into the air? I can't. Horsepower rating alone doesn't provide performance. This holds equally true in full-scale aviation. Early-model P-47 Thunderbolts couldn't outclimb Spitfires or Focke-Wulfs, but changing the P-47's propeller to a different, "paddle-blade" type fixed that! The engine's horsepower remained the same, but with the new propeller the P-47's rate of climb improved tremendously. As P-47 ace Robert Johnson wrote (in Thunderbolt, Ballantine, New York, 1958): "Never again did a Focke-Wulf 190 or a Messerschmitt 109 out climb me in the Thunderbolt. The new prop was worth 1000 horsepower more—and then some. Later I had the opportunity to mix it up with a Spitfire 9B, the same model that had flashed past me in a climb. This time the tables were reversed: I was astonished as we both poured the coal to our fighters, and the Thunderbolt just ran away from the Spit."
Propeller selection matters
Correct propeller choice is vital for optimum performance in model airplanes too. Merely specifying the diameter and pitch is far from sufficient. Propellers of the same nominal size but of different makes can vary enormously in the thrust they produce. This shows up distinctly in Control Line (CL) models: comparative takeoff ability, flight speed, and ease of maneuvering can readily be judged. In RC models, differences in propeller effectiveness are harder to distinguish, but they are just as important to performance.
There is yet another aspect of propeller selection. My good friend Zach Allerton (Volant, PA) once built a big, lightweight Old-Timer model—a Scientific Red Zephyr—for relaxed RC flying. He powered it with a four-stroke O.S. engine, and it flew beautifully.
However, there was one problem: the O.S. couldn't idle slowly enough to let Zach’s Red Zephyr land under power. I suggested he try a flatter-pitch propeller on the model. That solved the problem!
A useful rule of thumb
A classic engineering rule of thumb explains why. The speed (in miles per hour) at which any propeller wants to move its vehicle (airplane or boat) equals the propeller revolutions per minute in thousands multiplied by its pitch in inches.
For instance, a 10 x 5 propeller spinning at 11,000 revolutions per minute tries to pull its model at 11 (thousand rpm) × 5 (inches pitch) = 55 miles per hour. Of course, no propeller is 100% efficient, and air drag holds model airplanes back in flight.
Still, the rule of thumb gives us a useful relationship. In the example given, changing the propeller to a 10 x 4 (of the same make) ought to reduce the model’s flight speed by roughly 20%.
However, as with almost everything else in model airplanes, things aren’t quite that simple. Reducing the propeller’s pitch will usually let the engine rpm increase—maybe even enough so that the model’s top speed stays approximately the same.
In fact, if the original pitch was so high that the propeller’s rotating blades were forced to work at an excessive angle of attack, reducing the propeller pitch will make the model fly faster—not slower.
But altering propeller pitch doesn’t affect idling rpm much. The throttle setting is the major determinant there. Thus, changing the propeller on Zach’s Red Zephyr from a 6-inch pitch to a 4-inch pitch reduced the model’s flying speed at idle by something like 33%. And that’s all it took to permit touch-and-gos and power-on landings.
Starting tightly fitted engines
Bob Rode (Sauk Village, IL) provided a useful tip for initial starts of a tightly fitted new aluminum-brass-chrome (or similar taper-bore design) model engine. Some of those have pistons that fit their sleeves so squeaky tight at the top of the stroke that they can hardly be turned over. Bob gets around that difficulty by lightly heating the head and cylinder top "just until the piston will go over top center without binding or a 'squeak.'"
Bob uses a propane torch to do the heating. I tried his method but used a MonoKote heat gun instead. That’s safer to use around model fuel, and it worked fine for me. I tried Bob’s head-heating technique to start an especially tight O.S. .10 that had stubbornly resisted every previous attempt of mine to hand-start it. (The little O.S.’s shaft is too short to accept a spinner for electric starting.)
My heat gun probably took longer than a propane torch would have to get the O.S. top end hot enough for easy hand-starting, but it did work—and after five one-minute fast-revving runs, the piston-cylinder fit of my O.S. .10 has freed up noticeably. Now the engine seems ready for me to fly an RC airplane with it.
By the way, have you noticed how few .10 model engines are available anymore in the USA? Looking through the latest mail-order catalogs recently, I found hardly anything listed under .15 displacement. The .10s are still popular in Europe and the Orient, and since 1946 I’ve greatly enjoyed flying with that size engine. I wonder why they’re becoming so rare in the USA.
A cure for ruined glow-plug igniters
In a previous column I discussed the problem of ruining Hot Shot–type glow-plug igniters by longtime overcharging. Hughes RC (1733 Campus Plaza Ct., Suite #17, Bowling Green, KY 42101; Web site: www.hughesrc.com) has come up with a cure for that.
Its CHARGE+ model IC4S provides foolproof automatic peak charging of glow igniters—the Hot Shot clip-on type powered by integral C-size Ni-Cd cells and larger "improvised" glow igniters such as the 2000 mAh one I’ve been using so successfully for more than a decade.
The complete IC4S system contains everything one could possibly want for recharging glow igniters. The basic charging system itself requires a 12-volt power source; that’s provided with a 12-volt heavy-duty plug-in-the-wall unit.
Hughes also includes:
- a long (and fused) cord for connecting the CHARGE+ to a car’s cigarette-lighter socket
- a pair of insulated alligator clips to allow powering the CHARGE+ unit from a field-box 12-volt gel cell
The complete CHARGE+ IC4S system costs approximately $60 and is available direct from Hughes RC.
Transcribed from original scans by AI. Minor OCR errors may remain.
