Turning, Turning, Turning - 2004/05

LAST MONTH I outlined the types of model
engines, highlighting performance and design
differences. But of all the available types, sizes,
and variations of model engines, the most
common kind used in trainers today is the twostroke,
.40-cubic-inch-displacement “Ol’
Reliable,” or “forty.”
In this installment I’ll cover this type of
engine’s initial care and feeding, including
mounting, break-in, and needle settings. Following this segment I’ll
cover propellers, glow plugs, fuel, maintenance, and repair. Except
for history and propeller sizes, everything I will discuss in these
articles will apply to most two-stroke engines from .10 to 2.10
displacement.
The .40 two-stroke has been the most popular RC engine for
several decades. A logical outgrowth of CL’s most popular engine
of the 1950s—the Fox .35—the .40 RC offered increased
displacement to compensate for the power that was lost when
incorporating a throttle.
The first .40 was familiar to CL pilots who were transferring to
RC; remained easy to hand start; was approximately the same
physical size, weight, and power as the .35; and offered good fuel
economy. These features made the .40 popular then and remain its
key advantages to this day.
Today, the old .40 format comes in many displacements (the
volume of the cylinder the piston travels). The same-size crankcase
(the aluminum engine “block” containing all the moving parts
except for carburetion) now varies from the original .40-cubic-inch
displacement all the way up to .51 cubic inch.
Naturally, the various .45s, .46s, and .50s produce more power
than the .40s, but they use more fuel and require a larger volume of
cooling air to operate. These slightly larger-displacement engines
usually swing larger-diameter propellers that may cause groundclearance
problems on normal “40-size” aircraft. You may have to
adjust the landing-gear length to accommodate them.
The .40s are offered in ringed or aluminum-brass-chrome (ABC)
configurations. The original ringed, sometimes baffled, engines
feature low fuel consumption and reliable, cool running. The ABC
engines are powerful without being temperamental, unless they are
solely racing engines—and those are definitely outside this article’s
scope.
Most, but not all, .40 engines sold today are Schnuerle ported
(have extra fuel-intake ports inside the engine) for more power.
Whether Schnuerle ported or not, the engine’s break-in procedure is
determined by its ringed or ABC (also AAC, or aluminumaluminum-
chrome) design.
Before the engine can be properly broken in, it has to be
mounted on the airplane or test stand. Mounting on a test stand is
easy; just follow the stand manufacturer’s directions. Be sure to
attach the muffler and tank pressure lines as well.
Almost all of today’s .40 two-stroke engines require muffler
pressure to the fuel tank to get sufficient fuel into the carburetor.
Why? Without muffler pressure the engine must create a vacuum in
the fuel feed line to draw fuel from the tank into the carburetor. It
does this by drawing air into the carburetor through the venturi
opening and then past a small hole (the spray bar) that mixes fuel
into the incoming air.
The venturi is that big hole in the carburetor that opens as the
throttle is advanced, and the spray bar is the small brass tube inside
the venturi. To get enough fuel suction, the incoming air must be
moving quickly through the venturi. For proper fuel suction, the
volume of moving air is not as critical as its speed.
Before mufflers became common, manufacturers had to make
the venturi bore small to increase the incoming air’s speed.
However, a smaller venturi restricts the total amount of incoming air
and therefore reduces power output. Venturi bore size had to be a
compromise between power and reliable fuel feed.
The advent of mufflers allowed manufacturers to divert some of
the exhaust gases into the fuel tank itself. This rerouting put pressure
inside the tank that forced fuel to flow into the carburetor.
While not actually acting as a fuel pump, the addition of muffler
pressure meant that venturi suction was no longer the sole source of
the engine’s fuel feed. As a result, the venturi bore diameter could
be made larger without reducing the carburetor’s fuel intake.
Making the venturi bore larger increases an engine’s power
output. Today’s engine’s larger venturi requires that the muffler be
attached every time the engine is run, to ensure that the fuel mixture
is “rich” enough (has a high enough fuel-to-air ratio) to lubricate and
cool the engine. This is especially important during break-in,
whether the engine is mounted on a test stand or in an airplane.
Mounting an engine in a model may seem daunting, but it is easy,
and model pilots eventually need to know how to do it. Although
many of today’s RTF trainers’ engines are already mounted, hard
62 MODEL AVIATION
Turning,
Turning,
Turning
by Frank Granelli
The O.S. .46 LA (L) is exactly the same as the .40 LA (R) except
for its larger displacement. The .46 is more powerful but has
higher fuel consumption.
landings may damage the original mounts. An ARF trainer requires
the assembler to mount the engine.
Depending on the airframe, you may need to adjust the engine’s
“thrust angle,” which is the angle between the airframe’s horizontal
centerline through the fuselage and the direction—right, left, up, or
down—in which the engine is pointing in relation to that centerline.
Remounting in a slightly larger mount is usually the best way to make
thrust adjustments, especially if the engine is cowled.
There are four types of engine mounts most commonly in use
today: aluminum “clamp-on” mounts; adjustable fiberglass or solid
fiberglass mounts; and independent, twin I-Beam, fiberglass mounts.
Of those, the aluminum clamp-on mount is the easiest and the
hardest to use correctly. It’s easy because two clamps hold the engine
in place; there is no need to drill mounting holes into the mount. It’s
difficult to ensure that the engine is centered and aligned inside the
mount.
Clamp-on mounts are larger than the engine’s crankcase, allowing
the engine to be mounted too far to one side or twisted between the
mounting beams. Both situations affect the engine’s thrustline and
consequently the airplane’s handling characteristics—never for the
better. Compounding the alignment problem is that most trainers and
sport ARFs have right and/or downthrust built into the firewall (the
wood faceplate to which the mount is bolted).
The firewall’s offset means that it is impossible to align the engine
inside the mount by measuring from any point on the airframe, unless
you are a surveyor or mathematician. If you are not, all measurements
must be done in relation to the mount itself.
The initial step is to determine how far forward in the mount the
engine needs to be. If your model has a cowling and spinner, make
sure there is at least 1⁄16 inch clearance between the front of the
cowling and the rear of the spinner. A photo shows what happens
without this clearance. If the engine is not cowled, make sure the
propeller will clear the fuselage side plates.
Once you have established the engine’s fore and aft placement,
make a mark at the rear and front of the engine’s mounting plate.
Measure the mount’s outside width at the front and the rear of the
marks.
Measure the width of the engine’s mounting plates. Subtract this
number from the mount’s width, and the result is the total extra side
space at the front and rear of the engine’s position. Divide this extra
space—front and rear each—by two, measure in from the outside of
the mount by this amount at the proper locations, and mark. Draw a
line between the two marks on each side. Aligning the outside of the
engine’s mounting plates to these two lines centers the engine in all
directions inside the mount.
Lightly clamp only one side of the engine. Ensure that the engine
hasn’t moved by checking the reference line on the unclamped side,
and—just to make sure that everything is straight—mount the
propeller.
Make a mark in the top middle of the mount’s faceplate (the rear
mount part that holds the aluminum mounting beams), and measure
from this center mark to each propeller tip as a check. The distances
should be the same. If not, they will not be too different and can easily
be adjusted without moving the engine sideways.
Do not use this check measurement without centering the engine in
the mount first. If you do, it is possible to have the engine too far to
one side. Equal propeller-tip distances will then ensure that the engine
is twisted inside the mount.
Once everything checks out, install and tighten the second clamp,
and then secure the first clamp. It takes longer to read this than to do
it.
You can use the same method to position the engine in a solid
May 2004 63
“The .40 two-stroke has been the
most popular RC engine for several
decades.”
These reliable, well-used ringed engines are Schnuerle ported
and have piston rings. SuperTigre .40 (L) and Enya .45 (R) have
been sport-engine favorites for many years.
SuperTigre .45 (L) has smaller, square exhaust port typical of
ABC engines, compared to ringed SuperTigre .40 (R).
64 MODEL AVIATION
It’s easy to see larger fuel spray bar (R) in “down the throat”
venturi photo. On left is idle mixture adjuster, or needle valve,
that controls fuel/air mixture below half throttle.
The most common engine mounts. Metal “clamp” mount (second
from left) requires no mounting holes to be drilled but is most
difficult to align properly.
Make sure there is at least 1⁄16 inch between spinner backplate
and cowling. Flexible (soft) engine mounts require at least 1⁄8
inch spacing.
Marking front and rear of engine’s mounting plates is first step in
aligning engine in mount wider than its crankcase.
Inexpensive ($10-$15) dial micrometer is best way to measure
mount’s beam width, but small engineer’s ruler also works well.
Measure front and rear marks; there is a difference.
Same dial micrometer makes it easy to measure engine’s width
(which is 2.42 inches here). This measurement is hard to make
without micrometer but is usually printed in engine’s instructions.
Photos courtesy the author
fiberglass mount that may be too large for it. However, if you have
good karma and eat healthy, this type of mount usually fits the engine
securely and may even have the beams spread slightly apart to
accommodate it. In this case, only the engine’s fore/aft position needs
to be determined and the mounting holes drilled.
Drilling perfect mounting holes used to be tough and once served to
“build character” in a modeler. But now, several companies sell tools
that make this job so simple, fast, and troublefree that some of us have
to find other ways to become “characters.” A photo shows the Great
Planes Dead Center engine-mount-hole locator in use, and several
other manufacturers make almost identical tools.
To properly use this tool, you need to make a mount fixture or
possess a drill-press vise. The fixture is easy to make. Join two pieces
of 1⁄2 plywood (approximately 6 inches square) with epoxy and screws
so that they are perpendicular. You will use this fixture throughout
your entire modeling career, so make sure it is correct and well braced.
Screw the engine mount to this fixture, making sure that it is level and
square.
Position the engine, hold it in place, and use the tool to drill one
small, shallow mark in a mounting beam. Mark only one hole for now.
Remove the engine and drill the hole. What size hole? You should use
the largest hardened socket-head machine bolt that will fit inside the
May 2004 65
With the proper measurements established, it is simple to draw a
straight line on the mount with a ruler.
Loosely clamp engine between lines using clamp on side
opposite line. Once adjusted, tighten one clamp enough to
prevent engine movement.
Install second clamp, check final alignment. Engine must be
centered before using this measurement to double-check
alignment.
There is no easier way to mark engine mounting holes when
drilling is required. Mark one hole, drill and tap, remount engine,
mark remaining holes.
First, mount separate beam mounts to engine on fixture. Once
mounted, complete engine/mount assembly can be positioned on
“firewall” and mounting holes drilled.
engine’s mounting holes. The screws that came with your engine
mount are okay, but hardened steel bolts are stronger and easier to
install. Most .40 engines use 4-40 or 6-32 bolt sizes.
After you have drilled the hole, tap matching threads into it.
Fiberglass is softer than metal, so use a drill that is one size smaller
than what is printed on the tap. Use a No. 37 drill for 6-32 bolts and a
No. 44 drill for 4-40 bolt holes. It is best to use a drill press and the
fixture you made (or drill-press vise) here. You can buy a good drill
press for less than $40, and they are good investments; you will use
one for many years in your modeling.
Do not use oil to lubricate while tapping the threads; the fiberglass
contains enough carbon to lubricate the tap. Some oils can weaken the
mount material, causing the threads to break or “strip out.”
Using the hole you drilled and tapped, remount the engine, check
to make sure that everything is still positioned correctly, and then
mark the remaining three holes. It is best to drill and tap one hole at a
time, remount, and then mark the next hole. This is not essential, but
it can prevent cumulative errors because each hole may be drilled
slightly off center.
You use the same mounting procedure with both remaining types
of mounts. For independent I-Beam mounts, attach one I-Beam to
your fixture, ensure that it is square, clamp the engine to it, and attach
the other I-Beam to the fixture. Then drill and tap the holes as in the
preceding. When you are using adjustable fiberglass mounts, slide
them together per the instructions, attach to the fixture, and drill and
tap.
With the engine properly and securely mounted on the airplane, you
are ready to start the break-in procedure. Well, not just yet. You’ll
need fuel, the right propeller, a glow plug, a glow-plug igniter, and a
starter—electric or hand. Glow-plug igniters and starters will come
later, as will detailed glow-plug and fuel selections. For now, assume
that you have the best of each.
However, break-in propellers are important. The size of propeller
used during break-in depends on the engine type—ringed or ABC
(AAC). For ringed engines, use a propeller that is an inch less in
diameter than will be used in flight. ABC engines need the same
propeller as will normally be flown. The propeller’s construction—
wood, fiberglass, etc.—should match for ABC engines but is
noncritical for ringed engines.
ABC engines should be broken in exactly as they will be flown,
except for the high-speed mixture setting. In an ABC type, the
cylinder’s bore (diameter) tapers from a larger diameter at the bottom
to a smaller diameter at the top. The piston has a constant diameter that
is almost equal to the cylinder’s diameter at its bottom.
As the piston travels upward, the bore becomes smaller until, at the
top of its stroke, the piston is slightly larger than the cylinder’s
diameter. However, the piston and cylinder react to the heat generated
when the engine runs by expanding differently; the cylinder expands
more than the piston.
Since the piston is larger than the cylinder at the top in an ABC
engine, break-in involves the cylinder’s wearing away to become an
exact fit to the piston when both parts are hot. But most ABC engines
are built with the cylinder slightly too tight. Therefore, when the
engine is first run and heats up, the cylinder remains too small. During
the break-in, the cylinder loses material until it fits the piston exactly
when hot.
How much wear occurs depends on the engine’s rpm and propeller
load. Using the same propeller for break-in and normal running
ensures that the initial wear pattern will match the run pattern. The
only difference is that the engine will be run slightly richer than
normal during break-in for extra cooling and lubrication. ABC engines
normally have short break-in periods averaging five to 10 flights.
Ringed engines do not need to turn the same rpm during break-in
as during flight, but they do need to run cooler than normal. Thus
ringed engines require a richer fuel mixture during initial flights.
Using a propeller that is an inch less in diameter reduces the engine
load, and heat generated, while allowing the engine to achieve
enough rpm for break-in on the ground with a rich mixture. Ringed
engines usually require more break-in time, averaging 15-20 flights.
66 MODEL AVIATION
Rich full-throttle mixture is best way to break in ringed engines.
A few drops of raw fuel should be noticeable.
Idle needle-valve adjusters that regulate fuel-air mixture below
half throttle can be screws or actual needle valves. High-speed
needle valve is not very effective at less than one-third throttle.
Some engines use small hole in carburetor’s front to adjust idle
mixture. Start with adjustment screw covering half of the hole, as
shown.
68 MODEL AVIATION
Before running any engine, use common
sense and take every precaution. The airplane
must be immobile, the propeller must be
tight, all obstacles must be cleared, do not
smoke, and do it outside. Wear eye and ear
protection, and never stand to the side in the
propeller arc or make adjustments from in
front of the engine. Do not reach around the
spinning propeller to make needle-valve
adjustments, remove the glow driver, or for
any other purpose! Make all adjustments
while standing in the rear of the engine.
Please!
I have taken far too many friends to
hospitals through the years, watched too
many microsurgeries, and hoped far too
many times that they could reattach nearly
severed fingers not to warn anyone reading
this to be careful. There is no reset button
once that propeller hits you. This goes for any
type of propeller turned by any type of engine
or motor.
Break-in procedures for ringed engines
vary by individuals, but consider the
following. Open the high-speed needle valve
a half turn more than the engine directions
state. Have the throttle wide open and the
model properly secured. Prime the engine by
holding one finger over the venturi, hold the
propeller securely, and rotate it
counterclockwise until fuel moves through
the fuel line and nearly into the carburetor.
Do not have the glow driver attached.
Connect the glow driver, making sure that
any wire is clear of the propeller arc, and start
the engine. Remove the glow driver. The
engine will run at full throttle, but at an
extremely rich setting. If the engine falters,
close the needle valve (while standing behind
the engine) just enough to ensure a steady
run. The engine should be spitting raw,
unburnt fuel from the muffler and running
roughly 2,000 rpm slower than normal. Run
the engine this way for five minutes, and then
shut it down and let it cool.
Repeat this procedure twice more. On the
third run, let the engine run rich for two
minutes, and then “lean” the mixture; turn the
needle valve clockwise or close it until the
engine sound changes from a low-pitched
tone to an alternating low-pitched/highpitched
sound. Stop there and let it run for 30
seconds, return to the rich setting for two
minutes, and then stop it again and let it cool.
Restart and then lean the mixture to
achieve that alternating sound, and let it run
there for one minute. Richen the mixture
again (open the needle valve), but only to a
half turn less than the initial rich setting. Now
the engine speed should be approximately
1,500 rpm lower than normal.
After one minute of rich running, lean to
the alternating sound point and run for one
minute. Continue alternating the needle-valve
settings for five more minutes. Stop and let
the engine cool. Restart and set the needle
valve to the alternating sound point. Run the
engine at this point for three to five minutes.
If the engine holds rpm and doesn’t seem to
slow down, it is ready to finish the break-in
while flying. Install the flying propeller. Total
ground time is usually 30 minutes.
Before flying, the idle mixture needs
adjusting. Most .40-size engines use a separate
idle needle valve. The idle adjustment screw
or needle valve meters the amount of fuel that
flows into the carburetor during idle. Before
adjusting the idle mixture, make sure this
valve is set per the engine’s instructions.
Clockwise adjustments lean the idle mixture
and counterclockwise turns richen it.
Some engines use an air-bleed hole located
in the carburetor’s top front section. A screw
meters the amount of air admitted through this
hole at idle, adjusting the idle mixture.
Initially the screw should cover just half of the
air-inlet hole (see photo). This may be too
rich, but you can lean the idle mixture by
turning the screw clockwise. Turning the
screw past the hole continues to adjust the idle
mixture, despite appearances.
There is little purpose in adjusting the idle
mixture on the test stand since fuel pressure,
air-intake volume, and airflow will be
different once the engine is installed in the
airplane. The idle setting will have to be
readjusted again.
Mount the engine in the airplane if you
have not already done so. Run the engine at
full throttle, and set the needle valve slightly
leaner than the alternating sound point. Stop
the engine, attach the glow driver, and restart it.
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Slow the engine to approximately 3,000
rpm (a tachometer helps here). Watch the
rpm. If the engine gradually slows and then
stops, the mixture is too rich. Once the engine
stops, lean the idle mixture one-quarter turn
and restart. If the engine rpm increases, the
mixture is too lean. Richen the idle mixture,
again once the engine is not running, onequarter
turn.
Check each new setting by running the
engine at full throttle and then reducing to
3,000 rpm. This “clears” the previous
incorrect idle setting. Even if the engine does
not quit but needs final adjustment, stop it
before making idle changes. Take every
opportunity to stay away from a spinning
propeller with your hands or screwdriver.
Continue adjusting until the engine holds a
steady 3,000 rpm. Disconnect the glow driver
and make any final idle adjustments. Why
have the glow driver connected during the
initial idle settings? Incorrect idle mixtures
often dampen an unconnected glow plug so
quickly that there is no time to determine what
is wrong with the setting. Keeping the plug
“lit” helps ease the adjustment process.
After the initial settings, disconnect the
glow driver, idle the engine for 30 seconds,
and then quickly advance the throttle. If the
engine stops, richen the idle mixture
slightly. If the engine stumbles and quits,
won’t accelerate, or accelerates
exceptionally slowly, lean it a bit.
During the first few flights, 3,000 rpm
provides a reliable idle for most engines.
Slower idle settings are possible but run the
risk of the engine’s quitting because of the
high internal friction during break-in. Set
the initial throttle trim on the transmitter for
a 3,000 rpm idle at full “up” throttle trim,
and full “down” throttle trim stops the
engine.
Landing patterns are flown at high idle.
Once the field is “made” (the model can
glide to the runway without engine power),
reduce the trim to half. If the engine quits,
landing is no problem. If it runs more
slowly, you’ll make a pretty landing. This
half-trim setting will be roughly 2,200-
2,400 rpm and is the target idle speed once
the engine is fully broken in.
Breaking in an ABC engine is somewhat
easier. Only one ground run of 10-15
minutes is required, using the flying
propeller. Set the high-speed needle valve
to the most open setting cited in the
instructions. Start the engine at full throttle.
The exhaust sound should be slightly
lean of the alternating low- and highpitched
sounds. If you hear only a highpitched
sound, richen the mixture. If you
hear only a low-pitched sound, lean the
mixture to just past the alternating point.
Run the engine for five minutes, alternating
between full and half throttle.
Run the engine for another five minutes
at a slightly leaner mixture setting, again
alternating between full and mid-throttle.
During the final five minutes, lean the highspeed
mixture until rpm peak and start to
drop. Immediately richen the mixture to
1,000 rpm less than that peak (roughly a
half turn). This is the initial flying highspeed
mixture. Adjust the idle mixture just
as for ringed engines.
After approximately 10 flights for ABC
engines and 20 flights for ringed engines,
the high-speed mixture can be leaned to 500
rpm less than peak. Never run leaner than
this. A trainer’s engine turns approximately
500 rpm faster in flight than on the ground.
The mixture tends to lean as rpm increases.
In steep climbs and while inverted, fuel feed
rates are reduced. Most important, fuel
pressure drops as the tank empties, even
with muffler pressure, as the weight of the
fuel pushing itself into the fuel outlet (tank
head pressure) gets lower.
The slightly rich ground mixture
compensates for all these possible
problems. A setting of 500 rpm rich is the
leanest run without a fuel pump, but 600 is
better and will greatly lengthen engine life.
Next month “From the Ground Up” will
look at fine-tuning propeller, fuel, glowplug,
and engine-size choices. I’ll also show
you some of the differences with fourstrokes.
MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866

LAST MONTH I outlined the types of model
engines, highlighting performance and design
differences. But of all the available types, sizes,
and variations of model engines, the most
common kind used in trainers today is the twostroke,
.40-cubic-inch-displacement “Ol’
Reliable,” or “forty.”
In this installment I’ll cover this type of
engine’s initial care and feeding, including
mounting, break-in, and needle settings. Following this segment I’ll
cover propellers, glow plugs, fuel, maintenance, and repair. Except
for history and propeller sizes, everything I will discuss in these
articles will apply to most two-stroke engines from .10 to 2.10
displacement.
The .40 two-stroke has been the most popular RC engine for
several decades. A logical outgrowth of CL’s most popular engine
of the 1950s—the Fox .35—the .40 RC offered increased
displacement to compensate for the power that was lost when
incorporating a throttle.
The first .40 was familiar to CL pilots who were transferring to
RC; remained easy to hand start; was approximately the same
physical size, weight, and power as the .35; and offered good fuel
economy. These features made the .40 popular then and remain its
key advantages to this day.
Today, the old .40 format comes in many displacements (the
volume of the cylinder the piston travels). The same-size crankcase
(the aluminum engine “block” containing all the moving parts
except for carburetion) now varies from the original .40-cubic-inch
displacement all the way up to .51 cubic inch.
Naturally, the various .45s, .46s, and .50s produce more power
than the .40s, but they use more fuel and require a larger volume of
cooling air to operate. These slightly larger-displacement engines
usually swing larger-diameter propellers that may cause groundclearance
problems on normal “40-size” aircraft. You may have to
adjust the landing-gear length to accommodate them.
The .40s are offered in ringed or aluminum-brass-chrome (ABC)
configurations. The original ringed, sometimes baffled, engines
feature low fuel consumption and reliable, cool running. The ABC
engines are powerful without being temperamental, unless they are
solely racing engines—and those are definitely outside this article’s
scope.
Most, but not all, .40 engines sold today are Schnuerle ported
(have extra fuel-intake ports inside the engine) for more power.
Whether Schnuerle ported or not, the engine’s break-in procedure is
determined by its ringed or ABC (also AAC, or aluminumaluminum-
chrome) design.
Before the engine can be properly broken in, it has to be
mounted on the airplane or test stand. Mounting on a test stand is
easy; just follow the stand manufacturer’s directions. Be sure to
attach the muffler and tank pressure lines as well.
Almost all of today’s .40 two-stroke engines require muffler
pressure to the fuel tank to get sufficient fuel into the carburetor.
Why? Without muffler pressure the engine must create a vacuum in
the fuel feed line to draw fuel from the tank into the carburetor. It
does this by drawing air into the carburetor through the venturi
opening and then past a small hole (the spray bar) that mixes fuel
into the incoming air.
The venturi is that big hole in the carburetor that opens as the
throttle is advanced, and the spray bar is the small brass tube inside
the venturi. To get enough fuel suction, the incoming air must be
moving quickly through the venturi. For proper fuel suction, the
volume of moving air is not as critical as its speed.
Before mufflers became common, manufacturers had to make
the venturi bore small to increase the incoming air’s speed.
However, a smaller venturi restricts the total amount of incoming air
and therefore reduces power output. Venturi bore size had to be a
compromise between power and reliable fuel feed.
The advent of mufflers allowed manufacturers to divert some of
the exhaust gases into the fuel tank itself. This rerouting put pressure
inside the tank that forced fuel to flow into the carburetor.
While not actually acting as a fuel pump, the addition of muffler
pressure meant that venturi suction was no longer the sole source of
the engine’s fuel feed. As a result, the venturi bore diameter could
be made larger without reducing the carburetor’s fuel intake.
Making the venturi bore larger increases an engine’s power
output. Today’s engine’s larger venturi requires that the muffler be
attached every time the engine is run, to ensure that the fuel mixture
is “rich” enough (has a high enough fuel-to-air ratio) to lubricate and
cool the engine. This is especially important during break-in,
whether the engine is mounted on a test stand or in an airplane.
Mounting an engine in a model may seem daunting, but it is easy,
and model pilots eventually need to know how to do it. Although
many of today’s RTF trainers’ engines are already mounted, hard
62 MODEL AVIATION
Turning,
Turning,
Turning
by Frank Granelli
The O.S. .46 LA (L) is exactly the same as the .40 LA (R) except
for its larger displacement. The .46 is more powerful but has
higher fuel consumption.
landings may damage the original mounts. An ARF trainer requires
the assembler to mount the engine.
Depending on the airframe, you may need to adjust the engine’s
“thrust angle,” which is the angle between the airframe’s horizontal
centerline through the fuselage and the direction—right, left, up, or
down—in which the engine is pointing in relation to that centerline.
Remounting in a slightly larger mount is usually the best way to make
thrust adjustments, especially if the engine is cowled.
There are four types of engine mounts most commonly in use
today: aluminum “clamp-on” mounts; adjustable fiberglass or solid
fiberglass mounts; and independent, twin I-Beam, fiberglass mounts.
Of those, the aluminum clamp-on mount is the easiest and the
hardest to use correctly. It’s easy because two clamps hold the engine
in place; there is no need to drill mounting holes into the mount. It’s
difficult to ensure that the engine is centered and aligned inside the
mount.
Clamp-on mounts are larger than the engine’s crankcase, allowing
the engine to be mounted too far to one side or twisted between the
mounting beams. Both situations affect the engine’s thrustline and
consequently the airplane’s handling characteristics—never for the
better. Compounding the alignment problem is that most trainers and
sport ARFs have right and/or downthrust built into the firewall (the
wood faceplate to which the mount is bolted).
The firewall’s offset means that it is impossible to align the engine
inside the mount by measuring from any point on the airframe, unless
you are a surveyor or mathematician. If you are not, all measurements
must be done in relation to the mount itself.
The initial step is to determine how far forward in the mount the
engine needs to be. If your model has a cowling and spinner, make
sure there is at least 1⁄16 inch clearance between the front of the
cowling and the rear of the spinner. A photo shows what happens
without this clearance. If the engine is not cowled, make sure the
propeller will clear the fuselage side plates.
Once you have established the engine’s fore and aft placement,
make a mark at the rear and front of the engine’s mounting plate.
Measure the mount’s outside width at the front and the rear of the
marks.
Measure the width of the engine’s mounting plates. Subtract this
number from the mount’s width, and the result is the total extra side
space at the front and rear of the engine’s position. Divide this extra
space—front and rear each—by two, measure in from the outside of
the mount by this amount at the proper locations, and mark. Draw a
line between the two marks on each side. Aligning the outside of the
engine’s mounting plates to these two lines centers the engine in all
directions inside the mount.
Lightly clamp only one side of the engine. Ensure that the engine
hasn’t moved by checking the reference line on the unclamped side,
and—just to make sure that everything is straight—mount the
propeller.
Make a mark in the top middle of the mount’s faceplate (the rear
mount part that holds the aluminum mounting beams), and measure
from this center mark to each propeller tip as a check. The distances
should be the same. If not, they will not be too different and can easily
be adjusted without moving the engine sideways.
Do not use this check measurement without centering the engine in
the mount first. If you do, it is possible to have the engine too far to
one side. Equal propeller-tip distances will then ensure that the engine
is twisted inside the mount.
Once everything checks out, install and tighten the second clamp,
and then secure the first clamp. It takes longer to read this than to do
it.
You can use the same method to position the engine in a solid
May 2004 63
“The .40 two-stroke has been the
most popular RC engine for several
decades.”
These reliable, well-used ringed engines are Schnuerle ported
and have piston rings. SuperTigre .40 (L) and Enya .45 (R) have
been sport-engine favorites for many years.
SuperTigre .45 (L) has smaller, square exhaust port typical of
ABC engines, compared to ringed SuperTigre .40 (R).
64 MODEL AVIATION
It’s easy to see larger fuel spray bar (R) in “down the throat”
venturi photo. On left is idle mixture adjuster, or needle valve,
that controls fuel/air mixture below half throttle.
The most common engine mounts. Metal “clamp” mount (second
from left) requires no mounting holes to be drilled but is most
difficult to align properly.
Make sure there is at least 1⁄16 inch between spinner backplate
and cowling. Flexible (soft) engine mounts require at least 1⁄8
inch spacing.
Marking front and rear of engine’s mounting plates is first step in
aligning engine in mount wider than its crankcase.
Inexpensive ($10-$15) dial micrometer is best way to measure
mount’s beam width, but small engineer’s ruler also works well.
Measure front and rear marks; there is a difference.
Same dial micrometer makes it easy to measure engine’s width
(which is 2.42 inches here). This measurement is hard to make
without micrometer but is usually printed in engine’s instructions.
Photos courtesy the author
fiberglass mount that may be too large for it. However, if you have
good karma and eat healthy, this type of mount usually fits the engine
securely and may even have the beams spread slightly apart to
accommodate it. In this case, only the engine’s fore/aft position needs
to be determined and the mounting holes drilled.
Drilling perfect mounting holes used to be tough and once served to
“build character” in a modeler. But now, several companies sell tools
that make this job so simple, fast, and troublefree that some of us have
to find other ways to become “characters.” A photo shows the Great
Planes Dead Center engine-mount-hole locator in use, and several
other manufacturers make almost identical tools.
To properly use this tool, you need to make a mount fixture or
possess a drill-press vise. The fixture is easy to make. Join two pieces
of 1⁄2 plywood (approximately 6 inches square) with epoxy and screws
so that they are perpendicular. You will use this fixture throughout
your entire modeling career, so make sure it is correct and well braced.
Screw the engine mount to this fixture, making sure that it is level and
square.
Position the engine, hold it in place, and use the tool to drill one
small, shallow mark in a mounting beam. Mark only one hole for now.
Remove the engine and drill the hole. What size hole? You should use
the largest hardened socket-head machine bolt that will fit inside the
May 2004 65
With the proper measurements established, it is simple to draw a
straight line on the mount with a ruler.
Loosely clamp engine between lines using clamp on side
opposite line. Once adjusted, tighten one clamp enough to
prevent engine movement.
Install second clamp, check final alignment. Engine must be
centered before using this measurement to double-check
alignment.
There is no easier way to mark engine mounting holes when
drilling is required. Mark one hole, drill and tap, remount engine,
mark remaining holes.
First, mount separate beam mounts to engine on fixture. Once
mounted, complete engine/mount assembly can be positioned on
“firewall” and mounting holes drilled.
engine’s mounting holes. The screws that came with your engine
mount are okay, but hardened steel bolts are stronger and easier to
install. Most .40 engines use 4-40 or 6-32 bolt sizes.
After you have drilled the hole, tap matching threads into it.
Fiberglass is softer than metal, so use a drill that is one size smaller
than what is printed on the tap. Use a No. 37 drill for 6-32 bolts and a
No. 44 drill for 4-40 bolt holes. It is best to use a drill press and the
fixture you made (or drill-press vise) here. You can buy a good drill
press for less than $40, and they are good investments; you will use
one for many years in your modeling.
Do not use oil to lubricate while tapping the threads; the fiberglass
contains enough carbon to lubricate the tap. Some oils can weaken the
mount material, causing the threads to break or “strip out.”
Using the hole you drilled and tapped, remount the engine, check
to make sure that everything is still positioned correctly, and then
mark the remaining three holes. It is best to drill and tap one hole at a
time, remount, and then mark the next hole. This is not essential, but
it can prevent cumulative errors because each hole may be drilled
slightly off center.
You use the same mounting procedure with both remaining types
of mounts. For independent I-Beam mounts, attach one I-Beam to
your fixture, ensure that it is square, clamp the engine to it, and attach
the other I-Beam to the fixture. Then drill and tap the holes as in the
preceding. When you are using adjustable fiberglass mounts, slide
them together per the instructions, attach to the fixture, and drill and
tap.
With the engine properly and securely mounted on the airplane, you
are ready to start the break-in procedure. Well, not just yet. You’ll
need fuel, the right propeller, a glow plug, a glow-plug igniter, and a
starter—electric or hand. Glow-plug igniters and starters will come
later, as will detailed glow-plug and fuel selections. For now, assume
that you have the best of each.
However, break-in propellers are important. The size of propeller
used during break-in depends on the engine type—ringed or ABC
(AAC). For ringed engines, use a propeller that is an inch less in
diameter than will be used in flight. ABC engines need the same
propeller as will normally be flown. The propeller’s construction—
wood, fiberglass, etc.—should match for ABC engines but is
noncritical for ringed engines.
ABC engines should be broken in exactly as they will be flown,
except for the high-speed mixture setting. In an ABC type, the
cylinder’s bore (diameter) tapers from a larger diameter at the bottom
to a smaller diameter at the top. The piston has a constant diameter that
is almost equal to the cylinder’s diameter at its bottom.
As the piston travels upward, the bore becomes smaller until, at the
top of its stroke, the piston is slightly larger than the cylinder’s
diameter. However, the piston and cylinder react to the heat generated
when the engine runs by expanding differently; the cylinder expands
more than the piston.
Since the piston is larger than the cylinder at the top in an ABC
engine, break-in involves the cylinder’s wearing away to become an
exact fit to the piston when both parts are hot. But most ABC engines
are built with the cylinder slightly too tight. Therefore, when the
engine is first run and heats up, the cylinder remains too small. During
the break-in, the cylinder loses material until it fits the piston exactly
when hot.
How much wear occurs depends on the engine’s rpm and propeller
load. Using the same propeller for break-in and normal running
ensures that the initial wear pattern will match the run pattern. The
only difference is that the engine will be run slightly richer than
normal during break-in for extra cooling and lubrication. ABC engines
normally have short break-in periods averaging five to 10 flights.
Ringed engines do not need to turn the same rpm during break-in
as during flight, but they do need to run cooler than normal. Thus
ringed engines require a richer fuel mixture during initial flights.
Using a propeller that is an inch less in diameter reduces the engine
load, and heat generated, while allowing the engine to achieve
enough rpm for break-in on the ground with a rich mixture. Ringed
engines usually require more break-in time, averaging 15-20 flights.
66 MODEL AVIATION
Rich full-throttle mixture is best way to break in ringed engines.
A few drops of raw fuel should be noticeable.
Idle needle-valve adjusters that regulate fuel-air mixture below
half throttle can be screws or actual needle valves. High-speed
needle valve is not very effective at less than one-third throttle.
Some engines use small hole in carburetor’s front to adjust idle
mixture. Start with adjustment screw covering half of the hole, as
shown.
68 MODEL AVIATION
Before running any engine, use common
sense and take every precaution. The airplane
must be immobile, the propeller must be
tight, all obstacles must be cleared, do not
smoke, and do it outside. Wear eye and ear
protection, and never stand to the side in the
propeller arc or make adjustments from in
front of the engine. Do not reach around the
spinning propeller to make needle-valve
adjustments, remove the glow driver, or for
any other purpose! Make all adjustments
while standing in the rear of the engine.
Please!
I have taken far too many friends to
hospitals through the years, watched too
many microsurgeries, and hoped far too
many times that they could reattach nearly
severed fingers not to warn anyone reading
this to be careful. There is no reset button
once that propeller hits you. This goes for any
type of propeller turned by any type of engine
or motor.
Break-in procedures for ringed engines
vary by individuals, but consider the
following. Open the high-speed needle valve
a half turn more than the engine directions
state. Have the throttle wide open and the
model properly secured. Prime the engine by
holding one finger over the venturi, hold the
propeller securely, and rotate it
counterclockwise until fuel moves through
the fuel line and nearly into the carburetor.
Do not have the glow driver attached.
Connect the glow driver, making sure that
any wire is clear of the propeller arc, and start
the engine. Remove the glow driver. The
engine will run at full throttle, but at an
extremely rich setting. If the engine falters,
close the needle valve (while standing behind
the engine) just enough to ensure a steady
run. The engine should be spitting raw,
unburnt fuel from the muffler and running
roughly 2,000 rpm slower than normal. Run
the engine this way for five minutes, and then
shut it down and let it cool.
Repeat this procedure twice more. On the
third run, let the engine run rich for two
minutes, and then “lean” the mixture; turn the
needle valve clockwise or close it until the
engine sound changes from a low-pitched
tone to an alternating low-pitched/highpitched
sound. Stop there and let it run for 30
seconds, return to the rich setting for two
minutes, and then stop it again and let it cool.
Restart and then lean the mixture to
achieve that alternating sound, and let it run
there for one minute. Richen the mixture
again (open the needle valve), but only to a
half turn less than the initial rich setting. Now
the engine speed should be approximately
1,500 rpm lower than normal.
After one minute of rich running, lean to
the alternating sound point and run for one
minute. Continue alternating the needle-valve
settings for five more minutes. Stop and let
the engine cool. Restart and set the needle
valve to the alternating sound point. Run the
engine at this point for three to five minutes.
If the engine holds rpm and doesn’t seem to
slow down, it is ready to finish the break-in
while flying. Install the flying propeller. Total
ground time is usually 30 minutes.
Before flying, the idle mixture needs
adjusting. Most .40-size engines use a separate
idle needle valve. The idle adjustment screw
or needle valve meters the amount of fuel that
flows into the carburetor during idle. Before
adjusting the idle mixture, make sure this
valve is set per the engine’s instructions.
Clockwise adjustments lean the idle mixture
and counterclockwise turns richen it.
Some engines use an air-bleed hole located
in the carburetor’s top front section. A screw
meters the amount of air admitted through this
hole at idle, adjusting the idle mixture.
Initially the screw should cover just half of the
air-inlet hole (see photo). This may be too
rich, but you can lean the idle mixture by
turning the screw clockwise. Turning the
screw past the hole continues to adjust the idle
mixture, despite appearances.
There is little purpose in adjusting the idle
mixture on the test stand since fuel pressure,
air-intake volume, and airflow will be
different once the engine is installed in the
airplane. The idle setting will have to be
readjusted again.
Mount the engine in the airplane if you
have not already done so. Run the engine at
full throttle, and set the needle valve slightly
leaner than the alternating sound point. Stop
the engine, attach the glow driver, and restart it.
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70 MODEL AVIATION
Slow the engine to approximately 3,000
rpm (a tachometer helps here). Watch the
rpm. If the engine gradually slows and then
stops, the mixture is too rich. Once the engine
stops, lean the idle mixture one-quarter turn
and restart. If the engine rpm increases, the
mixture is too lean. Richen the idle mixture,
again once the engine is not running, onequarter
turn.
Check each new setting by running the
engine at full throttle and then reducing to
3,000 rpm. This “clears” the previous
incorrect idle setting. Even if the engine does
not quit but needs final adjustment, stop it
before making idle changes. Take every
opportunity to stay away from a spinning
propeller with your hands or screwdriver.
Continue adjusting until the engine holds a
steady 3,000 rpm. Disconnect the glow driver
and make any final idle adjustments. Why
have the glow driver connected during the
initial idle settings? Incorrect idle mixtures
often dampen an unconnected glow plug so
quickly that there is no time to determine what
is wrong with the setting. Keeping the plug
“lit” helps ease the adjustment process.
After the initial settings, disconnect the
glow driver, idle the engine for 30 seconds,
and then quickly advance the throttle. If the
engine stops, richen the idle mixture
slightly. If the engine stumbles and quits,
won’t accelerate, or accelerates
exceptionally slowly, lean it a bit.
During the first few flights, 3,000 rpm
provides a reliable idle for most engines.
Slower idle settings are possible but run the
risk of the engine’s quitting because of the
high internal friction during break-in. Set
the initial throttle trim on the transmitter for
a 3,000 rpm idle at full “up” throttle trim,
and full “down” throttle trim stops the
engine.
Landing patterns are flown at high idle.
Once the field is “made” (the model can
glide to the runway without engine power),
reduce the trim to half. If the engine quits,
landing is no problem. If it runs more
slowly, you’ll make a pretty landing. This
half-trim setting will be roughly 2,200-
2,400 rpm and is the target idle speed once
the engine is fully broken in.
Breaking in an ABC engine is somewhat
easier. Only one ground run of 10-15
minutes is required, using the flying
propeller. Set the high-speed needle valve
to the most open setting cited in the
instructions. Start the engine at full throttle.
The exhaust sound should be slightly
lean of the alternating low- and highpitched
sounds. If you hear only a highpitched
sound, richen the mixture. If you
hear only a low-pitched sound, lean the
mixture to just past the alternating point.
Run the engine for five minutes, alternating
between full and half throttle.
Run the engine for another five minutes
at a slightly leaner mixture setting, again
alternating between full and mid-throttle.
During the final five minutes, lean the highspeed
mixture until rpm peak and start to
drop. Immediately richen the mixture to
1,000 rpm less than that peak (roughly a
half turn). This is the initial flying highspeed
mixture. Adjust the idle mixture just
as for ringed engines.
After approximately 10 flights for ABC
engines and 20 flights for ringed engines,
the high-speed mixture can be leaned to 500
rpm less than peak. Never run leaner than
this. A trainer’s engine turns approximately
500 rpm faster in flight than on the ground.
The mixture tends to lean as rpm increases.
In steep climbs and while inverted, fuel feed
rates are reduced. Most important, fuel
pressure drops as the tank empties, even
with muffler pressure, as the weight of the
fuel pushing itself into the fuel outlet (tank
head pressure) gets lower.
The slightly rich ground mixture
compensates for all these possible
problems. A setting of 500 rpm rich is the
leanest run without a fuel pump, but 600 is
better and will greatly lengthen engine life.
Next month “From the Ground Up” will
look at fine-tuning propeller, fuel, glowplug,
and engine-size choices. I’ll also show
you some of the differences with fourstrokes.
MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866

LAST MONTH I outlined the types of model
engines, highlighting performance and design
differences. But of all the available types, sizes,
and variations of model engines, the most
common kind used in trainers today is the twostroke,
.40-cubic-inch-displacement “Ol’
Reliable,” or “forty.”
In this installment I’ll cover this type of
engine’s initial care and feeding, including
mounting, break-in, and needle settings. Following this segment I’ll
cover propellers, glow plugs, fuel, maintenance, and repair. Except
for history and propeller sizes, everything I will discuss in these
articles will apply to most two-stroke engines from .10 to 2.10
displacement.
The .40 two-stroke has been the most popular RC engine for
several decades. A logical outgrowth of CL’s most popular engine
of the 1950s—the Fox .35—the .40 RC offered increased
displacement to compensate for the power that was lost when
incorporating a throttle.
The first .40 was familiar to CL pilots who were transferring to
RC; remained easy to hand start; was approximately the same
physical size, weight, and power as the .35; and offered good fuel
economy. These features made the .40 popular then and remain its
key advantages to this day.
Today, the old .40 format comes in many displacements (the
volume of the cylinder the piston travels). The same-size crankcase
(the aluminum engine “block” containing all the moving parts
except for carburetion) now varies from the original .40-cubic-inch
displacement all the way up to .51 cubic inch.
Naturally, the various .45s, .46s, and .50s produce more power
than the .40s, but they use more fuel and require a larger volume of
cooling air to operate. These slightly larger-displacement engines
usually swing larger-diameter propellers that may cause groundclearance
problems on normal “40-size” aircraft. You may have to
adjust the landing-gear length to accommodate them.
The .40s are offered in ringed or aluminum-brass-chrome (ABC)
configurations. The original ringed, sometimes baffled, engines
feature low fuel consumption and reliable, cool running. The ABC
engines are powerful without being temperamental, unless they are
solely racing engines—and those are definitely outside this article’s
scope.
Most, but not all, .40 engines sold today are Schnuerle ported
(have extra fuel-intake ports inside the engine) for more power.
Whether Schnuerle ported or not, the engine’s break-in procedure is
determined by its ringed or ABC (also AAC, or aluminumaluminum-
chrome) design.
Before the engine can be properly broken in, it has to be
mounted on the airplane or test stand. Mounting on a test stand is
easy; just follow the stand manufacturer’s directions. Be sure to
attach the muffler and tank pressure lines as well.
Almost all of today’s .40 two-stroke engines require muffler
pressure to the fuel tank to get sufficient fuel into the carburetor.
Why? Without muffler pressure the engine must create a vacuum in
the fuel feed line to draw fuel from the tank into the carburetor. It
does this by drawing air into the carburetor through the venturi
opening and then past a small hole (the spray bar) that mixes fuel
into the incoming air.
The venturi is that big hole in the carburetor that opens as the
throttle is advanced, and the spray bar is the small brass tube inside
the venturi. To get enough fuel suction, the incoming air must be
moving quickly through the venturi. For proper fuel suction, the
volume of moving air is not as critical as its speed.
Before mufflers became common, manufacturers had to make
the venturi bore small to increase the incoming air’s speed.
However, a smaller venturi restricts the total amount of incoming air
and therefore reduces power output. Venturi bore size had to be a
compromise between power and reliable fuel feed.
The advent of mufflers allowed manufacturers to divert some of
the exhaust gases into the fuel tank itself. This rerouting put pressure
inside the tank that forced fuel to flow into the carburetor.
While not actually acting as a fuel pump, the addition of muffler
pressure meant that venturi suction was no longer the sole source of
the engine’s fuel feed. As a result, the venturi bore diameter could
be made larger without reducing the carburetor’s fuel intake.
Making the venturi bore larger increases an engine’s power
output. Today’s engine’s larger venturi requires that the muffler be
attached every time the engine is run, to ensure that the fuel mixture
is “rich” enough (has a high enough fuel-to-air ratio) to lubricate and
cool the engine. This is especially important during break-in,
whether the engine is mounted on a test stand or in an airplane.
Mounting an engine in a model may seem daunting, but it is easy,
and model pilots eventually need to know how to do it. Although
many of today’s RTF trainers’ engines are already mounted, hard
62 MODEL AVIATION
Turning,
Turning,
Turning
by Frank Granelli
The O.S. .46 LA (L) is exactly the same as the .40 LA (R) except
for its larger displacement. The .46 is more powerful but has
higher fuel consumption.
landings may damage the original mounts. An ARF trainer requires
the assembler to mount the engine.
Depending on the airframe, you may need to adjust the engine’s
“thrust angle,” which is the angle between the airframe’s horizontal
centerline through the fuselage and the direction—right, left, up, or
down—in which the engine is pointing in relation to that centerline.
Remounting in a slightly larger mount is usually the best way to make
thrust adjustments, especially if the engine is cowled.
There are four types of engine mounts most commonly in use
today: aluminum “clamp-on” mounts; adjustable fiberglass or solid
fiberglass mounts; and independent, twin I-Beam, fiberglass mounts.
Of those, the aluminum clamp-on mount is the easiest and the
hardest to use correctly. It’s easy because two clamps hold the engine
in place; there is no need to drill mounting holes into the mount. It’s
difficult to ensure that the engine is centered and aligned inside the
mount.
Clamp-on mounts are larger than the engine’s crankcase, allowing
the engine to be mounted too far to one side or twisted between the
mounting beams. Both situations affect the engine’s thrustline and
consequently the airplane’s handling characteristics—never for the
better. Compounding the alignment problem is that most trainers and
sport ARFs have right and/or downthrust built into the firewall (the
wood faceplate to which the mount is bolted).
The firewall’s offset means that it is impossible to align the engine
inside the mount by measuring from any point on the airframe, unless
you are a surveyor or mathematician. If you are not, all measurements
must be done in relation to the mount itself.
The initial step is to determine how far forward in the mount the
engine needs to be. If your model has a cowling and spinner, make
sure there is at least 1⁄16 inch clearance between the front of the
cowling and the rear of the spinner. A photo shows what happens
without this clearance. If the engine is not cowled, make sure the
propeller will clear the fuselage side plates.
Once you have established the engine’s fore and aft placement,
make a mark at the rear and front of the engine’s mounting plate.
Measure the mount’s outside width at the front and the rear of the
marks.
Measure the width of the engine’s mounting plates. Subtract this
number from the mount’s width, and the result is the total extra side
space at the front and rear of the engine’s position. Divide this extra
space—front and rear each—by two, measure in from the outside of
the mount by this amount at the proper locations, and mark. Draw a
line between the two marks on each side. Aligning the outside of the
engine’s mounting plates to these two lines centers the engine in all
directions inside the mount.
Lightly clamp only one side of the engine. Ensure that the engine
hasn’t moved by checking the reference line on the unclamped side,
and—just to make sure that everything is straight—mount the
propeller.
Make a mark in the top middle of the mount’s faceplate (the rear
mount part that holds the aluminum mounting beams), and measure
from this center mark to each propeller tip as a check. The distances
should be the same. If not, they will not be too different and can easily
be adjusted without moving the engine sideways.
Do not use this check measurement without centering the engine in
the mount first. If you do, it is possible to have the engine too far to
one side. Equal propeller-tip distances will then ensure that the engine
is twisted inside the mount.
Once everything checks out, install and tighten the second clamp,
and then secure the first clamp. It takes longer to read this than to do
it.
You can use the same method to position the engine in a solid
May 2004 63
“The .40 two-stroke has been the
most popular RC engine for several
decades.”
These reliable, well-used ringed engines are Schnuerle ported
and have piston rings. SuperTigre .40 (L) and Enya .45 (R) have
been sport-engine favorites for many years.
SuperTigre .45 (L) has smaller, square exhaust port typical of
ABC engines, compared to ringed SuperTigre .40 (R).
64 MODEL AVIATION
It’s easy to see larger fuel spray bar (R) in “down the throat”
venturi photo. On left is idle mixture adjuster, or needle valve,
that controls fuel/air mixture below half throttle.
The most common engine mounts. Metal “clamp” mount (second
from left) requires no mounting holes to be drilled but is most
difficult to align properly.
Make sure there is at least 1⁄16 inch between spinner backplate
and cowling. Flexible (soft) engine mounts require at least 1⁄8
inch spacing.
Marking front and rear of engine’s mounting plates is first step in
aligning engine in mount wider than its crankcase.
Inexpensive ($10-$15) dial micrometer is best way to measure
mount’s beam width, but small engineer’s ruler also works well.
Measure front and rear marks; there is a difference.
Same dial micrometer makes it easy to measure engine’s width
(which is 2.42 inches here). This measurement is hard to make
without micrometer but is usually printed in engine’s instructions.
Photos courtesy the author
fiberglass mount that may be too large for it. However, if you have
good karma and eat healthy, this type of mount usually fits the engine
securely and may even have the beams spread slightly apart to
accommodate it. In this case, only the engine’s fore/aft position needs
to be determined and the mounting holes drilled.
Drilling perfect mounting holes used to be tough and once served to
“build character” in a modeler. But now, several companies sell tools
that make this job so simple, fast, and troublefree that some of us have
to find other ways to become “characters.” A photo shows the Great
Planes Dead Center engine-mount-hole locator in use, and several
other manufacturers make almost identical tools.
To properly use this tool, you need to make a mount fixture or
possess a drill-press vise. The fixture is easy to make. Join two pieces
of 1⁄2 plywood (approximately 6 inches square) with epoxy and screws
so that they are perpendicular. You will use this fixture throughout
your entire modeling career, so make sure it is correct and well braced.
Screw the engine mount to this fixture, making sure that it is level and
square.
Position the engine, hold it in place, and use the tool to drill one
small, shallow mark in a mounting beam. Mark only one hole for now.
Remove the engine and drill the hole. What size hole? You should use
the largest hardened socket-head machine bolt that will fit inside the
May 2004 65
With the proper measurements established, it is simple to draw a
straight line on the mount with a ruler.
Loosely clamp engine between lines using clamp on side
opposite line. Once adjusted, tighten one clamp enough to
prevent engine movement.
Install second clamp, check final alignment. Engine must be
centered before using this measurement to double-check
alignment.
There is no easier way to mark engine mounting holes when
drilling is required. Mark one hole, drill and tap, remount engine,
mark remaining holes.
First, mount separate beam mounts to engine on fixture. Once
mounted, complete engine/mount assembly can be positioned on
“firewall” and mounting holes drilled.
engine’s mounting holes. The screws that came with your engine
mount are okay, but hardened steel bolts are stronger and easier to
install. Most .40 engines use 4-40 or 6-32 bolt sizes.
After you have drilled the hole, tap matching threads into it.
Fiberglass is softer than metal, so use a drill that is one size smaller
than what is printed on the tap. Use a No. 37 drill for 6-32 bolts and a
No. 44 drill for 4-40 bolt holes. It is best to use a drill press and the
fixture you made (or drill-press vise) here. You can buy a good drill
press for less than $40, and they are good investments; you will use
one for many years in your modeling.
Do not use oil to lubricate while tapping the threads; the fiberglass
contains enough carbon to lubricate the tap. Some oils can weaken the
mount material, causing the threads to break or “strip out.”
Using the hole you drilled and tapped, remount the engine, check
to make sure that everything is still positioned correctly, and then
mark the remaining three holes. It is best to drill and tap one hole at a
time, remount, and then mark the next hole. This is not essential, but
it can prevent cumulative errors because each hole may be drilled
slightly off center.
You use the same mounting procedure with both remaining types
of mounts. For independent I-Beam mounts, attach one I-Beam to
your fixture, ensure that it is square, clamp the engine to it, and attach
the other I-Beam to the fixture. Then drill and tap the holes as in the
preceding. When you are using adjustable fiberglass mounts, slide
them together per the instructions, attach to the fixture, and drill and
tap.
With the engine properly and securely mounted on the airplane, you
are ready to start the break-in procedure. Well, not just yet. You’ll
need fuel, the right propeller, a glow plug, a glow-plug igniter, and a
starter—electric or hand. Glow-plug igniters and starters will come
later, as will detailed glow-plug and fuel selections. For now, assume
that you have the best of each.
However, break-in propellers are important. The size of propeller
used during break-in depends on the engine type—ringed or ABC
(AAC). For ringed engines, use a propeller that is an inch less in
diameter than will be used in flight. ABC engines need the same
propeller as will normally be flown. The propeller’s construction—
wood, fiberglass, etc.—should match for ABC engines but is
noncritical for ringed engines.
ABC engines should be broken in exactly as they will be flown,
except for the high-speed mixture setting. In an ABC type, the
cylinder’s bore (diameter) tapers from a larger diameter at the bottom
to a smaller diameter at the top. The piston has a constant diameter that
is almost equal to the cylinder’s diameter at its bottom.
As the piston travels upward, the bore becomes smaller until, at the
top of its stroke, the piston is slightly larger than the cylinder’s
diameter. However, the piston and cylinder react to the heat generated
when the engine runs by expanding differently; the cylinder expands
more than the piston.
Since the piston is larger than the cylinder at the top in an ABC
engine, break-in involves the cylinder’s wearing away to become an
exact fit to the piston when both parts are hot. But most ABC engines
are built with the cylinder slightly too tight. Therefore, when the
engine is first run and heats up, the cylinder remains too small. During
the break-in, the cylinder loses material until it fits the piston exactly
when hot.
How much wear occurs depends on the engine’s rpm and propeller
load. Using the same propeller for break-in and normal running
ensures that the initial wear pattern will match the run pattern. The
only difference is that the engine will be run slightly richer than
normal during break-in for extra cooling and lubrication. ABC engines
normally have short break-in periods averaging five to 10 flights.
Ringed engines do not need to turn the same rpm during break-in
as during flight, but they do need to run cooler than normal. Thus
ringed engines require a richer fuel mixture during initial flights.
Using a propeller that is an inch less in diameter reduces the engine
load, and heat generated, while allowing the engine to achieve
enough rpm for break-in on the ground with a rich mixture. Ringed
engines usually require more break-in time, averaging 15-20 flights.
66 MODEL AVIATION
Rich full-throttle mixture is best way to break in ringed engines.
A few drops of raw fuel should be noticeable.
Idle needle-valve adjusters that regulate fuel-air mixture below
half throttle can be screws or actual needle valves. High-speed
needle valve is not very effective at less than one-third throttle.
Some engines use small hole in carburetor’s front to adjust idle
mixture. Start with adjustment screw covering half of the hole, as
shown.
68 MODEL AVIATION
Before running any engine, use common
sense and take every precaution. The airplane
must be immobile, the propeller must be
tight, all obstacles must be cleared, do not
smoke, and do it outside. Wear eye and ear
protection, and never stand to the side in the
propeller arc or make adjustments from in
front of the engine. Do not reach around the
spinning propeller to make needle-valve
adjustments, remove the glow driver, or for
any other purpose! Make all adjustments
while standing in the rear of the engine.
Please!
I have taken far too many friends to
hospitals through the years, watched too
many microsurgeries, and hoped far too
many times that they could reattach nearly
severed fingers not to warn anyone reading
this to be careful. There is no reset button
once that propeller hits you. This goes for any
type of propeller turned by any type of engine
or motor.
Break-in procedures for ringed engines
vary by individuals, but consider the
following. Open the high-speed needle valve
a half turn more than the engine directions
state. Have the throttle wide open and the
model properly secured. Prime the engine by
holding one finger over the venturi, hold the
propeller securely, and rotate it
counterclockwise until fuel moves through
the fuel line and nearly into the carburetor.
Do not have the glow driver attached.
Connect the glow driver, making sure that
any wire is clear of the propeller arc, and start
the engine. Remove the glow driver. The
engine will run at full throttle, but at an
extremely rich setting. If the engine falters,
close the needle valve (while standing behind
the engine) just enough to ensure a steady
run. The engine should be spitting raw,
unburnt fuel from the muffler and running
roughly 2,000 rpm slower than normal. Run
the engine this way for five minutes, and then
shut it down and let it cool.
Repeat this procedure twice more. On the
third run, let the engine run rich for two
minutes, and then “lean” the mixture; turn the
needle valve clockwise or close it until the
engine sound changes from a low-pitched
tone to an alternating low-pitched/highpitched
sound. Stop there and let it run for 30
seconds, return to the rich setting for two
minutes, and then stop it again and let it cool.
Restart and then lean the mixture to
achieve that alternating sound, and let it run
there for one minute. Richen the mixture
again (open the needle valve), but only to a
half turn less than the initial rich setting. Now
the engine speed should be approximately
1,500 rpm lower than normal.
After one minute of rich running, lean to
the alternating sound point and run for one
minute. Continue alternating the needle-valve
settings for five more minutes. Stop and let
the engine cool. Restart and set the needle
valve to the alternating sound point. Run the
engine at this point for three to five minutes.
If the engine holds rpm and doesn’t seem to
slow down, it is ready to finish the break-in
while flying. Install the flying propeller. Total
ground time is usually 30 minutes.
Before flying, the idle mixture needs
adjusting. Most .40-size engines use a separate
idle needle valve. The idle adjustment screw
or needle valve meters the amount of fuel that
flows into the carburetor during idle. Before
adjusting the idle mixture, make sure this
valve is set per the engine’s instructions.
Clockwise adjustments lean the idle mixture
and counterclockwise turns richen it.
Some engines use an air-bleed hole located
in the carburetor’s top front section. A screw
meters the amount of air admitted through this
hole at idle, adjusting the idle mixture.
Initially the screw should cover just half of the
air-inlet hole (see photo). This may be too
rich, but you can lean the idle mixture by
turning the screw clockwise. Turning the
screw past the hole continues to adjust the idle
mixture, despite appearances.
There is little purpose in adjusting the idle
mixture on the test stand since fuel pressure,
air-intake volume, and airflow will be
different once the engine is installed in the
airplane. The idle setting will have to be
readjusted again.
Mount the engine in the airplane if you
have not already done so. Run the engine at
full throttle, and set the needle valve slightly
leaner than the alternating sound point. Stop
the engine, attach the glow driver, and restart it.
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70 MODEL AVIATION
Slow the engine to approximately 3,000
rpm (a tachometer helps here). Watch the
rpm. If the engine gradually slows and then
stops, the mixture is too rich. Once the engine
stops, lean the idle mixture one-quarter turn
and restart. If the engine rpm increases, the
mixture is too lean. Richen the idle mixture,
again once the engine is not running, onequarter
turn.
Check each new setting by running the
engine at full throttle and then reducing to
3,000 rpm. This “clears” the previous
incorrect idle setting. Even if the engine does
not quit but needs final adjustment, stop it
before making idle changes. Take every
opportunity to stay away from a spinning
propeller with your hands or screwdriver.
Continue adjusting until the engine holds a
steady 3,000 rpm. Disconnect the glow driver
and make any final idle adjustments. Why
have the glow driver connected during the
initial idle settings? Incorrect idle mixtures
often dampen an unconnected glow plug so
quickly that there is no time to determine what
is wrong with the setting. Keeping the plug
“lit” helps ease the adjustment process.
After the initial settings, disconnect the
glow driver, idle the engine for 30 seconds,
and then quickly advance the throttle. If the
engine stops, richen the idle mixture
slightly. If the engine stumbles and quits,
won’t accelerate, or accelerates
exceptionally slowly, lean it a bit.
During the first few flights, 3,000 rpm
provides a reliable idle for most engines.
Slower idle settings are possible but run the
risk of the engine’s quitting because of the
high internal friction during break-in. Set
the initial throttle trim on the transmitter for
a 3,000 rpm idle at full “up” throttle trim,
and full “down” throttle trim stops the
engine.
Landing patterns are flown at high idle.
Once the field is “made” (the model can
glide to the runway without engine power),
reduce the trim to half. If the engine quits,
landing is no problem. If it runs more
slowly, you’ll make a pretty landing. This
half-trim setting will be roughly 2,200-
2,400 rpm and is the target idle speed once
the engine is fully broken in.
Breaking in an ABC engine is somewhat
easier. Only one ground run of 10-15
minutes is required, using the flying
propeller. Set the high-speed needle valve
to the most open setting cited in the
instructions. Start the engine at full throttle.
The exhaust sound should be slightly
lean of the alternating low- and highpitched
sounds. If you hear only a highpitched
sound, richen the mixture. If you
hear only a low-pitched sound, lean the
mixture to just past the alternating point.
Run the engine for five minutes, alternating
between full and half throttle.
Run the engine for another five minutes
at a slightly leaner mixture setting, again
alternating between full and mid-throttle.
During the final five minutes, lean the highspeed
mixture until rpm peak and start to
drop. Immediately richen the mixture to
1,000 rpm less than that peak (roughly a
half turn). This is the initial flying highspeed
mixture. Adjust the idle mixture just
as for ringed engines.
After approximately 10 flights for ABC
engines and 20 flights for ringed engines,
the high-speed mixture can be leaned to 500
rpm less than peak. Never run leaner than
this. A trainer’s engine turns approximately
500 rpm faster in flight than on the ground.
The mixture tends to lean as rpm increases.
In steep climbs and while inverted, fuel feed
rates are reduced. Most important, fuel
pressure drops as the tank empties, even
with muffler pressure, as the weight of the
fuel pushing itself into the fuel outlet (tank
head pressure) gets lower.
The slightly rich ground mixture
compensates for all these possible
problems. A setting of 500 rpm rich is the
leanest run without a fuel pump, but 600 is
better and will greatly lengthen engine life.
Next month “From the Ground Up” will
look at fine-tuning propeller, fuel, glowplug,
and engine-size choices. I’ll also show
you some of the differences with fourstrokes.
MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866