The Engine Shop - 2010/06

I LIKE SMALLER model engines. The
airplanes they power are quicker to construct,
less costly, and easier to transport than giantscale
types. Probably for those reasons, model
power plants with .25 through .46 cu. in.
displacements have long been the most
popular varieties for RC flying.
An excellent engine of that size is the
SuperTigre (ST) G-34. It’s a two-stroke, ringtype,
double-ball-bearing glow power plant
that is remarkably easy to start directly out of
the box. And since it’s a ringed-piston
design, it needs only a brief break-in.
There’s nothing startlingly radical about
the G-34. Its design is conventional, tried and
true. Yet it includes two special features that
I found noteworthy.
One is the way its carburetor is retained.
That’s via a “pinch bolt” that clamps across
the base of the carburetor and holds the part
firmly in place. Since that base is a true
cylinder, the carburetor can be rotated to
position the needle away from the propeller
disk.
I like that—and to take full advantage of
it, I filed away the front corner of the bottom
fin on the ST’s case. That allows more
clearance for the fuel inlet fitting and its
tubing, and it permits roughly 15° of
backsweep for the needle.
The G-34 muffler features a cylindrical
juncture too, between itself and its header.
This allows considerable freedom in
positioning the muffler, both inward/outward
and in rotating its longitudinal centerline
upward or downward.
June 2010 85
Joe Wagner The Engine Shop | [email protected]
Also included in this column:
• Fuel tank tubing alternatives
• Annealing and bending
aluminum tubing
• The four-stroke that
wouldn’t fly but does now
The SuperTigre ringed G-34
The Chinese SuperTigres are as wellmade
as the Italian original engines,
and they run as reliably. This is a
ring-type G-34 with a Master
Airscrew 10 x 4 G/F propeller.
The G-34 runs in easily. Notice the angled-back needle. ST’s
carburetor attachment method allows doing this; filing the lower
case fin helps maximize the angle.
Above: A film of lampblack deposited on
aluminum tubing over a candle flame
provides a convenient way to gauge the
proper annealing temperature for kinkless
bending.
Left: Small Parts, an excellent source for
parts and materials for modelers, sells
stainless-steel tubing that is suitable for
engine fuel tanks.
06sig3.QXD_00MSTRPG.QXD 4/23/10 9:19 AM Page 85
That also makes it easy to mount and
remove the engine, because loosening a single
clamp bolt on the muffler permits sliding it off
of its header. That gives good access to the
mounting bolts on the engine’s exhaust side.
The ST .34’s muffler is rather bulky for an
engine this size. However, it’s extremely
effective. I measured the sound output 10 feet
from the engine at a mere 82 dBA; that was
with a Graupner gray 10 x 4 propeller turning
11,800 rpm.
Two things about the ST muffler could be
improved. One is its exhaust outlet location,
on the muffler body centerline.
With the muffler axis aligned horizontally,
it allows exhaust oil to accumulate within—
which could be embarrassing when putting the
model back in one’s vehicle after a full day of
flying.
The second thing is that the muffler
pressure fitting is straight upright. It works
fine with an upright engine installation, but it’s
not good with an inverted engine. See the
troubleshooting paragraphs at the end of this
column for more information about this
potential problem.
On my ST .34 I plan to relocate the muffler
pressure fitting so that it points toward the
cylinder at close to 45°. While I’m altering
that, I’ll add a similar fitting at the bottom aft
end of the muffler, as a drain for exhaust oil.
My mention in a recent column about
corrosion in model fuel tanks, caused by brass
tubing, brought considerable input from
readers. Bob Whitney (Palm Bay, Florida)
suggested using stainless-steel tubing instead
of brass.
He wrote that suitably sized stainless
tubing can be obtained from Small Parts, and
although the material is rigid, it can be easily
annealed. Also, it’s solderable with Sta-Brite
solder and flux.
Other readers suggested Electroless nickel
plating the brass tubing. They pointed out that
this process plates both the inside and
outside of the tubing.
However, locating a convenient source
for Electroless nickel plating for a few
small model parts might be difficult in
many places. I couldn’t find any near
here, in the southeast corner of Alabama.
Doug Dahlke (Oshkosh, Wisconsin)
mentioned that copper tubing won’t cause
corrosion in model fuel. That’s true—but
1/8-inch-OD copper tubing has a thick
wall. Its ID is only .055. That’s less than
1/16 inch in diameter and will considerably
restrict fuel flow.
Mostly with Dave West’s
(Streamwood, Illinois) advice, we’ve
come up with a nearly ideal alternative
for brass fuel tank tubing. It’s aluminum.
Thin-wall aluminum tubing is easy to
find (e.g., K&S), inexpensive, and won’t
corrode in any kind of model engine fuel.
Using it instead of brass requires doing
things differently.
Although aluminum can be soldered,
it’s not easy. But we don’t need solder to
assemble aluminum tubing into metal fuel
tanks when J.B. Weld is available.
That material is as strong and almost
as adherent to metal as solder is. It’s easy
to achieve a reliable, airtight juncture
between the tubing and the tank with J.B.
Weld too.
The biggest problem I encountered in
working with thin-wall aluminum tubing
was bending it without its kinking. That’s
because the process used in manufacturing
it “work-hardens” the metal, making it stiff.
Trying to achieve anything more than
gentle bends in thin-wall aluminum tubing
usually results in a kink.
That’s where Dave West came to the
rescue. He told me about a quick and easy
way to anneal (soften) hard aluminum for
easy bending. Dave’s procedure is as
follows.
1. Pass the area of the tubing that you
want to make “bendable” through a candle
flame just long enough to form a solid, jetblack
film over its surface.
2. With a small pencil torch, evenly reheat
the blackened area only enough to burn the
black film off.
To that I add, immediately after the last of
the black film vanishes, quench the hot
tubing by plunging it into ice water. That’s
because nonferrous metals behave differently
in hardening and annealing than steel. To
harden steel, you heat it and then quench it
rapidly. To anneal it, you heat it and let it
cool as slowly as possible.
Nonferrous metals work oppositely. They
soften after being heated and fast-quenched.
Then they regain their hardness with time
and/or stress—such as by bending or
stretching.
That’s why soft brass, copper, and
aluminum tubing get harder and more
difficult to bend as bending progresses.
However, they can be annealed again and the
bending continued.
Dave West’s candle-and-minitorch
method of annealing hard aluminum works
equally well with spring-temper sheetaluminum
landing gear struts. The original
springiness will return after a week or so at
room temperature.
Troubleshooting Exploits: The first
problem was with one of my new engines: an
especially tightly fitted ABC (aluminumbrass-
chrome) that needed considerable
breaking in. I had put in only a half-hour of
run time when I was suddenly called out of
town. I put everything on my engine test
stand on hold before I left.
Upon my return a few weeks later, I
wanted to resume the ABC’s break-in. But to
my surprise, I couldn’t get it running again.
The engine started only with difficulty.
When I did get the power plant going, it
died as soon as I disconnected the plug
battery. I couldn’t see anything wrong or
different—until I checked the fuel tank. It was
still partly full from the previous run, more
than a month earlier. I discovered that enough
evaporation had occurred through the tank
vent to raise the oil percentage far above
normal.
Sure, I prefer oilier fuel than most fliers
do, but this was ridiculous. There was
probably more than 50% oil content! The
moral is to empty your model engine’s tank
before leaving home for weeks—especially in
hot weather.
A reader e-mailed me about a mysterious
problem he had with a four-stroke engine. It
was mounted inverted in a big-scale RC
airplane. It always started immediately and
ran nicely in the air—but only for a few
minutes. After that, it would slow and die.
The reader checked everything he could
imagine. He was far from being a novice.
He removed the engine from his aircraft
and put it on the bench, and it functioned
normally—no problems. Yet when he
reinstalled it in his model, the same thing
happened every time: power slowly declined
and then it stopped after a few minutes.
This modeler even filled the tank, started
the engine, and then held the airplane upsidedown.
The power plant perked merrily
along—full speed, idling, or anywhere in
between—until the tank ran dry. After that,
the man asked me by e-mail, “Is inverted
running a problem with four-strokers?”
I checked with my old friend, Zach
Allerton (New Castle, Pennsylvania), who has
been flying with four-stroke engines almost
exclusively for more than 35 years. Zach told
me that he had never encountered a problem
such as the one the reader reported. He was as
mystified as I was.
Then I remembered a two-stroke Rossi I’d
owned that behaved similar to the
troublesome four-stroker. Yet its installation
had been as a “sidewinder.”
Solving the Rossi’s difficulties turned out
to be easy, because its plumbing was visible.
The trouble stemmed from the location of its
muffler pressure tap.
In my installation, the tap was on the
underside of the muffler. After a minute or so
of run time, enough oil accumulated inside the
muffler to flow into the pressure line to the
tank. That affected the tank pressure enough
to kill the engine.
The solution was to relocate the
muffler’s pressure fitting. That solved the
side-mounted Rossi’s problem—and the
same solution worked for the reader’s
inverted four-stroke. MA
Sources:
SuperTigre
(800) 637-6050
www.supertigre.com
Small Parts, Inc.
(800) 220-4242
www.smallparts.com