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Fill 'er up With? When four-stroke model
engines came onto the scene, much attention was paid to fuel selection. Many
manufacturers offered special fuels with reduced oil content designed
exclusively for four-strokes. Since oil is the poorest-burning ingredient in
model fuel, less oil content made the early four-strokes run more consistently.
Today, low oil content is not only unnecessary, but is probably a negative. Most
engine manufacturers recommend at least 16-18% oil; high-performance engines
demand 20% or more.
The myth that four-strokes require low-oil-content fuels started because early
modelers used regular two-stroke glow plugs. Now that four-stroke glow plugs are
available, the oil's heat-removing ability is a benefit—not a problem.
Although four-stroke sport engines run cooler than equivalent two-strokes,
cylinder pressures are much higher. The extra oil helps protect parts such as
the ring, cylinder lining, and wrist pin that are exposed to this higher
pressure. As I have discussed, model fuel cools the engine by lubricating it and
carrying away excess "top end" heat as unburnt oil exits the exhaust.
Most model fuels use a mixture of synthetic oil and castor oil. Except for
high-performance, supercharged four-strokes that require synthetic oil only,
approximately 5% castor oil is a good amount for two- and four-strokes. The
total recommended oil content is the same as for two-strokes: 18-20% minimum.
This provides a small error margin during extreme operation.
Unlike in a two-stroke, there is no refrigeration cooling of the four-stroke's
lower crankcase since the fuel never gets there in quantity. Many Pattern
competition pilots have learned that providing extra cooling air to a four-stroke's
lower crankcase area is beneficial. It provides extra cooling, but then the
cooler air flows past the crankcase and into the "upside-down"-mounted
carburetor, making the entire fuel/air mix denser for extra power.
Make sure the lower crankcase receives cooling air when you install any
four-stroke. Regardless of the power advantages, having a cool lower end
prolongs bearing life.
What about nitromethane content? Since four-strokes have just one power stroke
per two crankshaft revolutions, nitromethane content less than 10% makes it
harder to keep the glow plug operating at peak efficiency. In most sport
four-strokes, nitromethane contents higher than 25% can result in extra
detonation and thrust-washer and spinner-backplate wear unless everything is set
perfectly. Even high-performance, supercharged four-strokes experience problems
when nitromethane content exceeds 35%.
For sport use, consider 15% nitromethane content when flying at lower than
5,000-foot density altitudes and in temperatures lower than 95¡. Consider 20%
nitromethane content if conditions exceed these figures.
Sport four-strokes actually burn less fuel than equivalent-size two-strokes.
This is partly because of their better combustion efficiency and higher internal
pressures, but mostly because fuel is burned only on every other piston stroke.
However, four-strokes do no get twice the "mileage" of two-strokes. At best,
sport four-strokes enjoy 20-40% better fuel economy. Since they use less fuel,
it is easier to feed them higher nitromethane- and oil-content fuels that might
cost slightly more.
Click on photo to view large image with caption
Propellers: Propeller choices for four-strokes
may be slightly different than for two-strokes. Both produce roughly the same
torque (twisting force) for a given displacement engine size. Two-strokes still
develop more horsepower, but it is usually at high rpm (exceeding 13,000) that
most sport fliers at club fields cannot readily use. The noise is excessive, the
propellers must be small, and high-nitromethane-content fuels must be used.
Besides, turning so fast prematurely wears out most sport engines.
Four-strokes have horsepower peaks in the 9,000-11,000 rpm range. Sport fliers
find it easier to choose a propeller that allows the engine to operate in this
range. Only sport fuels are required, and everything is quieter and easier to
set up at these low rpm.
The four-stroke's power curve makes it possible for Sport Scale fliers to use
larger-diameter propellers and still reach their engine's peak ratings.
Bigger-diameter propellers are more efficient if big obstructions such as scale
cowls or wide fuselages are located just to the propeller's rear. The more the
propeller's swept area that is located outside the obstruction, the less
interference the propeller receives from deflected airflow.
Through the years, four-strokes earned a reputation for having more torque and
therefore being able to turn larger-diameter propellers with higher pitches.
After extensive research by modeling's Engine Gurus, we know that this is untrue
and that four-strokes have nearly the same peak torque as two-strokes. Yet
four-strokes seem to have more torque because all that they do have is fully
available.
If two-strokes' peak torque could be reached at 8,000 rpm, they could use the
same larger-diameter propellers. But the torque peak is higher in the rpm range,
and they can't.
However, the rules for choosing a propeller are the same for four-strokes as
they were from last month for two-strokes. Pick the largest-diameter propeller,
with sufficient pitch to fly at the speed you want, which allows the engine to
turn approximately 1,000 rpm higher than the engine's peak torque rpm. Make fuel
and glow-plug choices first—they affect an engine's top rpm ability—and then
choose the propeller.
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