The Engine Shop 2003/09

76 MODEL AVIATION
UP TO NOW the material in these columns has been mainly aimed
at modelers who have had some experience with model engines.
But Model Aviation Aeromodeling Editor Bob Hunt recently
reminded me that many of today’s AMA members start in Radio
Control (RC) without any previous acclimatization period in Free
Flight or Control Line flying.
The engines used in those activities provide plenty of scope for
modelers to inexpensively accustom themselves to the quirks and
sometimes paradoxical behavior of model engines—and even have
fun doing so. However, someone who starts in modeling with an
Almost Ready-to-Fly RC trainer can be thoroughly baffled by its
engine. I exchanged E-mails with one newcomer who couldn’t keep
his RC engine running.
“It starts all right,” he wrote. “But as soon as I adjust the needle
valve, it slows right down and quits.”
In my replies I listed all of the possible problem areas I could
think of: gasket leakage, blown plug seal, a cut in the fuel line
somewhere that allowed air bubbles to enter the fuel stream, and
more. I was racking my brain trying to think of other possible
causes when a new E-mail arrived from the man.
“Sorry I bothered you,” he wrote. “I just found out that I’d been
adjusting my needle in the wrong direction ... ”
Needle valves are apparently so simple! Yet I could write an
article twice as long as this column about the various types of
needle valves, their advantages and disadvantages, and the
maddening problems that result from a bent point or an air leak or a
misdirected fuel orifice.
But most sport RC fliers I know don’t care much about the
finicky details of what makes their engines run—as long as they do
run. And when they don’t run, these fliers want a quick fix much
more than they do a lesson in internal-combustion fundamentals.
So many things can go wrong, though. In two-stroke engines,
several interacting processes go on at once while the engine runs.
For one, when the piston rises in its cylinder and compresses the
fuel-air mixture above it, it’s pulling in a fresh charge below it at
the same time. If both processes aren’t close to optimal, the engine
won’t run right—at least not for long.
(The term “fuel-air mixture” that is used so readily is an
oversimplification. What enters a model engine is akin to what
leaves the nozzle of a paint spray gun: a mixture of air,
Joe Wagner
T h e E n g i n e S h o p
212 S. Pine Ave., Ozark AL 36360
RC engine test mount. Scrap lumber, 1⁄2-hour’s work provide
perfect setup for breaking in engine, learning its quirks.
Irvine .40—typical swap-meet buy—looks and feels good, but has
no instruction sheet telling how to set carburetor.
Early model-engine speed controls. Bramco was ancestor of
generations of RC throttles—each with peculiarities.
No two of these carburetors are quite alike. Other types exist,
making it hard to write general rules for setting them.
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hydrocarbon vapor [the fuel], and an atomized viscous liquid [oil].)
Newcomers to model engines can best learn about their subtle
and simultaneous interactions on a test stand rather than in an
airplane. Even if the engine comes mounted in the airplane, it’s best
to remove it and install it on an open and accessible test stand—for
break-in (saving the airplane and its RC system from unnecessary
vibration and handling stresses) and for finding the proper
carburetor adjustments. (More about that later!)
Engine test stands can be a problem. I’ve owned almost every type
of “commercial product” in this line, and they’ve all been adequate
for non-RC, unmuffled engines, but mounting today’s RC model
engines presents problems. The muffler gets in the way of one of the
clamps, the throttle arm’s motion is blocked, or the fuel-tank
location needs changing.
I’ve learned that making a custom test mount for each RC engine
saves time and trouble. Common lumber (well glued and screwed
together) works well. All that is needed is a firm backing for the
molded plastic “radial mount” used in RC airplanes and a suitable
cutout for a fuel tank. (The one from the model is fine; you might as
well find out about any quirks in that!) A manually operated
pushrod for the throttle completes the setup.
Now for the running in! With a new engine—out of the box or
already installed in the model—it’s easy. Today’s model-engine
makers provide excellent instruction sheets. Never neglect reading
through these and doing what they recommend! The manufacturers
want you to be happy with their products—so you’ll brag to your
friends about them—and they do their best to supply plenty of
helpful information in their owner’s manuals.
The engine makers’ information is especially vital for carburetor
adjustments. There are so many types of those! Ever since the
original Bramco barrel throttle for RC-airplane use came out (circa
1960), model-engine makers have been industriously “improving
the breed.”
There’s always been a fundamental problem with barrel-type
throttles. Although the edges of the movable barrel restrict the flow
through the carburetor as the throttle is closed, the cross-sectional
area of the barrel where the fuel-oil mixture is drawn in remains
constant. So does the “pumping displacement” of each piston
upstroke. That’s why with “plain vanilla” barrel carburetors such as
the Bramco, the mixture gets richer and richer as the throttle is
closed.
Many ways of compensating for that have been devised: air
bleeds, double needles, sideways-sliding barrels, noncircular barrel
passageways, cam-operated needles, “idle disks,” etc. I’ve seen
dozens of different arrangements and have come up with a couple
more.
There’s no way except direct experience to fully learn the
peculiarities of a given type of carburetor for an RC model engine.
That’s why the manufacturers’ instructions are so vital; they save
considerable trial and error in learning. But some hands-on
learning is inescapable—and the open, accessible-from-all-sides
test mount is by far the best place for that.
These are propeller-shaft extensions. The Veco is a 1950 design
of Clarence Lee’s, and the Fox is a modern type.