Product Review - 2004/05

72 MODEL AVIATION
Dean Pappas
P r o d u c t R e v i e w
1144 Estates Blvd., Hamilton Square NJ 08690
Mintor 3M 1.40
Pros:
• Linear, predictable throttle response.
• Light weight.
• Horsepower competitive with other
engines in class.
• Jewlerylike finish—too pretty to fly!
Cons:
• Confusing importation network.
• Jewlerylike finish—too pretty to fly!
IN THE SIX or seven years since the FAI
eliminated the engine-displacement limit in
the RC Precision Aerobatics event,
commonly called Pattern, participants have
been looking to the natural horsepower-toweight
advantage of the tuned-pipeequipped
two-stroke engine as an
alternative to the popular and eminently
suitable YS 120 supercharged four-stroke
engine.
Until recently, the four-stroke has
maintained its hegemony with a modest
boost in displacement to 1.40 cubic inches.
The first two-strokes to enter this market
were also of 1.20 displacement and quickly
evolved into 1.40s and 1.45s.
A great deal of development on the part
of the engine manufacturers has gone into
giving the two-stroke the tractability and
predictable throttleability necessary for
Aerobatics competition. Those
characteristics would be appreciated in
many other areas, such as Scale or 3-D
aerobatics.
The engine I will review came from a
manufacturer that is new to the
aeromodeling market, not to mention the
demanding competition segment. Its efforts
have produced an excellent entry to this
highly competitive market. The Mintor
Corporation of Bergamo, Italy, is an
established firm that has recently turned its
experience in precision machining to
making model engines under the name 3M.
The first 3M product is a 1.40
displacement, front-intake, rear-exhaust
two-stroke that fits squarely in the middle
of the existing market. This is the engine I
will review. It is accompanied by a rearintake
version and, more notably, a similar
but larger 1.70 version which was
introduced during the period when this
review engine was being flown.
This review will not be a standard bench
First impressions are often accurate. The 3M is a beautifully built piece of machinery
inside and out. The entire engine is machined from aluminum blocks.
Dean pressed his trusty old Dr. Jekyll Pattern model into service as a flying engine test
bed. It logged more than 75 flights before Dean wrote this review.
May 2004 73
Photos courtesy the author
test; a total of approximately 10 gallons of
fuel were run through the engine, and
approximately 65 flights were made in the
engine test bed: a Dr. Jekyll II modified
with thicker wings, weighing close to the
maximum 5 kilograms (11 pounds)
permitted under competition rules.
Performance and rpm readings were
taken on the airplane rather than on the test
stand. This is more representative of actual
running conditions because the engine
mount typically used for this type of
airplane has some effect on performance.
The engine was flown as much to evaluate
its “table manners” as to determine its
performance levels.
Engine Construction: The 3M is
completely machined from bar-stock
aluminum, and the crankcase and all other
aluminum parts are anodized for corrosion
Carburetor is machined from aluminum with machined metal
control arm. Fuel-metering curve was almost perfect!
Typical for Aerobatics competition engines, 3M has backplatemounted,
crankcase pressure-driven fuel pump.
The Mintor’s carbon-fiber tuned pipe and O-ring-style header are extremely light, and
the pipe is well tuned to the engine’s timing.
You don’t really need 5,200 pounds of van to keep the test stand
from moving, but the Mintor is certainly strong!
You can easily see generously sized, deep fins on head and
cylinder. Machining rather than casting permits this.
May 2004 75
protection and appearance. The result is an
engine that resembles fine jewelry as much
as it does a piece of machinery. The purple
anodized head and propeller drive washer
add to the effect.
When the review 3M made its debut at
the home flying field, attached to a break-in
test stand, the local club members’ reaction
was that I should not run such a beautiful
piece of machinery (let alone fly it!).
Machined bar-stock aluminum can
produce lighter components than all but the
most exotic casting processes. As a result,
and the careful machine work done to
remove unnecessary bits of metal from
every component, the 3M weighs a mere
780 grams, which checked out at just less
than 27.75 ounces on my scale.
The anodized crankcase has an
additional benefit. The typical Pattern
model uses a soft engine mount, and those
often require the use of a nose ring to
support the front of the engine, just behind
the drive washer; the constant rubbing of
the rubber ring against an aluminum casting
usually creates a messy, gray dust. The
anodized aluminum does not do this.
As is typical for the market segment, the
3M is equipped with a crankcase-pressuredriven
fuel pump mounted in the backplate.
The pump is of the type first introduced by
Perry Aeromotive (now Varsane). The
pump permitted the tank to be placed at the
model’s center of gravity, with no fueldelivery
problems. The mounting flanges,
bolt pattern, and rear needle-valve
placement are compatible with the YS fourstroke
and O.S. 140 RX.
Crankcase and Crankshaft and Bearings:
As I described, the crankcase is fully
machined from aluminum, clear anodized,
and every bit of excess material is
machined away, contributing to the
engine’s light weight.
It is a two-piece affair, with the cylinder
and crankcase portions bolted together
using a large-diameter O-ring seal. This
facilitates the machining of the gas
passages and their close match to the ports
in the cylinder liner.
The four bolts are angled for access. If
overtightened, they may distort the bottom
portion of the cylinder and cause running
problems. The factory specification for
these is 4.6 N-M or 3 pound-feet of torque.
If you must disassemble the engine and do
not have a torque wrench in this range, grab
an “L”-shaped Allen wrench by the short
end and use only your thumb and
forefinger.
The crankshaft is of one-piece
construction, made from hardened steel,
with a 15mm-diameter gas passage. This is
a relatively small-diameter passage, in
keeping with the engine’s intended rpm
range and the need to keep gas velocities
high for good throttleability.
The aluminum drive washer has large
lightening holes and is tied to the crank
with a tapered brass collet. The 8 x 1mm
crankshaft threads proved to be adequately
long for even the largest propeller hubs and
a spinner backplate.
The crankshaft bearings showed no
signs of wear or corrosion during the test.
Amsoil MP was used as an after-run
corrosion preventative after each flying
session.
Piston, Ring, Liner, and Connecting Rod:
The top end of the 3M is all fitted relatively
tightly, trading off ease of break-in for
longevity and good running behavior in
summer heat. I consider this an excellent
tradeoff because predictable throttleability
is often the first victim of hot conditions.
The aluminum piston has a single castiron
ring, which is keyed to prevent
rotation. The steel cylinder liner was a tight
slip-fit to the crankcase cylinder. This is no
doubt done in the interest of heat transfer.
The 3M has a 32mm (1.26-inch) bore
and a 28.5mm (1.12-inch) stroke. As is
typical for the new generation of two-stroke
aerobatic engines, the 3M is not a longstroke.
As it turns out, the greater vibration
of a long-stroke engine (unless the
connecting rod is made long and therefore
heavy) is undesirable. Also, a loss of
intake- and exhaust-port area results from a
smaller-circumference piston.
The exhaust has a modest 140° duration,
and the bypass timing is 120°. The resulting
blow-down period is short, meaning that
the 3M benefits greatly from the use of a
tuned pipe. It also means that the tuned pipe
does not need to be of the “long-chamber”
design to get flexible operating
characteristics from the engine. (If you are
as big as the Hulk, the 3M would work
beautifully as a Control Line Stunt engine!)
The connecting rod is bronze bushed at
both ends, and the edges of the rod are
chamfered to improve gas flow from the
crankshaft passage. The big-end-tocrankpin
clearance on the test engine was
measured at 0.005 inch (0.127mm).
Although this is a normal figure for rod-tocrankpin
clearance, it is not particularly
tight considering that this engine is
intended for low-rpm use.
Since the 3M is intended for running in
the neighborhood of 8,000 rpm, it could
easily be fitted as tight as 0.003 inch
(0.076mm). As a consequence, it was
decided to use a fuel containing a high film
strength lubricant including 2% castor oil
for break-in and normal running. S&W
15% Sport Blend was used after a break-in
on a similar 10% blend. No undue wear
was observed after the test period.
Cylinder Head: The bright-purple
anodized head has twin glow plugs. The
design has “evolved” from a single central
plug design, much like that of the O.S. RX,
with a raised lip around the glow plug. The
second plug is mounted behind and angled
aft. In contrast, both of the larger 3M 1.70s’
plugs are mounted vertically and spaced
equal distances from the center of the
combustion chamber.
The manufacturer recommends O.S. A5
plugs in both holes, and this combination
ran well. K&B 1L glow plugs ran equally
well, with slightly better longevity.
Carburetor and Pump: As I described,
the 3M is equipped with a backplatemounted
pump and regulator. The
backplate is O-ring-sealed to the
crankcase; no paper gaskets here! A fine
mesh fuel filter is highly recommended
between the tank and fuel pump. The
carburetor is of the rotary metering barrel
and slot type, as is typical of this class of
engine.
The idle-mixture adjustment was
authoritative and strongly affected the
midrange. The manufacturer’s instructions
included with the engine recommend that
the idle mixture be set rich initially to
avoid overleanness at half throttle during
break-in. Please heed this advice.
The needle valve is remotely mounted
to the rear of the engine; its position,
along with the engine-mounting bolt
pattern, makes the 3M compatible with a
prior YS four-stroke or O.S. 140 RX
installation.
Header and Tuned Pipe: The engine
provided for testing came with two
different-height headers. The lightweight
header is coupled to the engine with twin
Viton O-rings of ASTM (American
Society for Testing and Materials) “dash-
211 size.” The O-rings lasted the entire
test period and were replaced before
further running. Depending on the engine
mount used, they will easily last 100
flights.
The 3M carbon-fiber tuned pipe is
light, at 150 grams, and attractive in
finish. The pipe is reinforced with a
section of aluminum tubing at its input, to
prevent crushing by the clamp used to
retain the silicone rubber header coupler.
The aft end of the pipe is machined from
aluminum and provides a machined
retention groove for a rubber exhaust
elbow.
As with any carbon-fiber muffler or
pipe, heavy mounting-clamp forces are to
be avoided. With the exhaust system
assembled at “stock” length, the end of the
exhaust stinger was only 35.5 inches (90
cm) from the propeller mounting face.
This makes the exhaust system one of the
shortest tuned systems available.
78 MODEL AVIATION
Break-In: The 3M 1.40 I tested was fitted
tightly, as stated before. The engine was
broken in with the stock tuned pipe and
header, lengthened by leaving a 1-inch gap
(2.5 cm) between them. This helped the
intentionally rich engine transition
properly. It required nearly a full gallon
before the 3M would consistently hold a
near-peaked needle setting. Since then, the
factory is fitting the engine more
conventionally.
I have observed several more recent
examples of the engine, and several
tankfuls now suffice for break-in with no
difference in running characteristics. I used
10% S&W Sport mix fuel for the first
three-quarters of a gallon and 15%
afterward. It is important to richen the idle
mixture, as directions that came with the
engine state. The idle mixture cam can
easily make the engine overly lean in the
midrange with the lengthened pipe. In
general, it is best to start rich and work your
way toward the ideal setting.
The engine was mounted in the
airframe, and the pipe was restored to
“stock length”: with only a .375-inch
(9mm) gap between the header and pipe.
The first baffle in the pipe was then 22.5
inches (57 cm) away from the center of the
combustion chamber, as measured through
the center of the header pipe.
Flying Setup: During the first few minutes
of running, at the stock pipe length, the 3M
agreeably turned 17 x 12 and 17 x 13N
APC propellers. I selected the 17 x 13N for
initial running; it turned 7,700 rpm at a
flyable (slightly rich) needle setting.
After a minimal amount of fiddling, the
idle mixture was set for a snappy idle-tofull-
throttle transition, and the mixture was
checked at half throttle by briefly pinching
the fuel-feed tubing. The half-throttle
mixture was close to peaked. The idle
mixture cam was almost perfectly centered.
After flying several aerobatic schedules,
the decision was made to slightly richen the
idle mixture to fatten the midrange. The
symptom was a slight leanness when
throttling up from half power at high
airspeed. It was not noticed on the ground.
There was no change in the idle-to-fullthrottle
transition. After a second gallon, a
reliable 1,500 rpm idle was obtained. This
is in keeping with a properly set-up
example of the four-stroke competition.
This setup provided excellent quiet on
the ground and in the air, as required by the
Pattern-event rules. The ground-noise levels
were 92 decibels at 3 meters, measured on a
screen-topped table. This number is
approximately 1 or 2 decibels lower than
that to be obtained over a reflective hard
surface.
In the air, the 17 x 13N showed no
propeller-tip “buzz” at full throttle and
provided good downhill braking. It is
apparent that the 3M and its exhaust system
are designed for this rpm range. The pipe
length was set at 22.5 inches (57 cm) from
glow plug to first baffle.
80 MODEL AVIATION
An APC 17 x 13 (standard blade) turned
the same 7,700 rpm on the ground and
braked slightly better, with no changes in
carburetor or pipe. The full-profile 17 x 13
is my normal flying propeller.
Changing to an APC 17 x 12, the engine
required that the pipe be shortened
approximately .25 inch, leaving a minimal
gap between the stock pipe and header. This
slight change in length eliminated a
tendency to richen in prolonged, shallow,
full-throttle dives. The pipe length was
22.25 inches (approximately 56 cm) from
glow plug to first baffle.
This 17 x 12 turned at 7,900 rpm static,
and acceleration in the air was slightly
improved. I observed minor propeller
buzzing, but this combination could easily
be called “quiet” at 93 decibels when
measured on the same sound table.
When I tried an APC 17 x 11, the
relationship between pipe length and
midrange mixture became apparent. With
the stock exhaust set for minimal gap, the
ground rpm was 8,200 and the idle mixture
required richening to prevent overleanness
in the midrange.
The real solution would have been to
shorten the pipe approximately .375 inch
(9mm), which likely would have raised the
ground rpm another 200 or 300. I could not
bring myself to cut the header to perform
this test because the 17 x 11 propeller was
already noisy in the air, and greater rpm
would only have worsened the situation.
For non-Pattern applications, this setup
would produce excellent power and still be
moderately quiet.
For its intended application, the 3M is
best run with a load such as the 17 x 12 or
17 x 13, in the 7,700-8,000 rpm range.
Four-blade 141⁄2 x 11 or 15 x 11 propellers
would also suit the 3M. For non-Pattern
use, the engine’s construction would
certainly stand up under higher-rpm use.
Handling Characteristics: As I stated, the
3M displays a reliable idle at 1,500 rpm
with the recommended pair of O.S. A5
glow plugs fitted and “standard” 15% fuel.
The engine’s table manners in flight are
excellent; the problems you often encounter
with piped two-strokes used for Aerobatics
competition are nonexistent. This is no
doubt because of the engine’s large, deep
fins; tight liner-to-crankcase fit; and dualplug
design. It is also claimed that the barstock
crankcase and cylinder head reject
heat better than a casting. It works ...
The challenges that tuned-pipe use often
present in Aerobatics are undertaken in
exchange for a sizable power boost and
excellent exhaust muffling. The action of
the resonant chamber greatly attenuates the
harsh high-frequency content of the exhaust
note—better than all but the most restrictive
mufflers.
Horsepower utilization in Aerobatics
competition has evolved because of the
tremendous power levels available. Wideopen
throttle (WOT) is now used only
during vertical climbs and the climbing
portions of looping maneuvers. Level flight,
whether at baseline altitude or at the top of
a maneuver such as a Square Loop, is
performed at approximately half throttle. A
slow idle with good compression braking is
vital in the diving portions of maneuvers to
preserve a constant speed presentation.
The basic problem is one of midrange
throttle hysteresis. When throttling back
from WOT to midthrottle, the engine may
stay “on the pipe” with the rpm remaining
elevated. This tends to happen after a long
AMA Academy of Model
Aeronautics
ARF Almost Ready to Fly
BEC Battery Eliminator Circuit
CAD computer-aided design
cc cubic centimeter
CD contest director or
compact disc
CG center of gravity
CL Control Line
cm centimeter
cu. in. cubic inch
DT dethermalizer
EPP (foam) expanded polypropylene
ESC Electronic Speed Control
FAI Fédération Aéronautique
Internationale
FCC Federal Communications
Commission
FF Free Flight
LCD Liquid Crystal Display
LE leading edge
LED light-emitting diode
Li-Poly Lithium Polymer
mA milliamperes
MA Model Aviation
mAh milliampere-hours
MHz megahertz
mm millimeter
Nats AMA Nationals
Ni-Cd Nickel Cadmium
NiMH Nickel Metal Hydride
RC Radio Control
rpm revolutions per minute
RTF Ready to Fly
SASE self-addressed, stamped
envelope
TE trailing edge
ModelAviation’s
Frequently Used Abbreviations/Acronyms
climb in warm weather or if even slightly
lean.
Alternatively, the pipe boost may cease
abruptly and leave the flier with less
horsepower at the same stick position. This
can happen if the engine is slightly rich or
cooled slightly from recent low-throttle
running. The problem worsens if the
throttle is advanced from idle to the same
midstick position.
The lack of predictability in delivered
horsepower at midstick can be maddening
while trying to negotiate an Aerobatics
schedule. This hysteresis problem can be
alleviated by lengthening the tuned pipe—
but at a price.
If the tuned pipe is set longer than
optimal, elevated airspeed (such as a WOT
diving run into a maneuver) will cause the
engine to richen excessively. This can even
extinguish the fire. Please note that by
optimal I mean for the purposes of in-air
handling—not for maximum ground rpm.
The best-flying pipe length is often a half
inch or so (approximately 1.5 cm) longer
than that which produces maximum ground
rpm.
The 3M displayed no throttle hysteresis.
The pipe lengths quoted caused no
richening in prolonged shallow WOT dives.
The idle mixtures that produced good midto
full-throttle transitions in the air
transitioned well from idle. The
manufacturer got the metering curve right
on this carburetor!
In cold weather, prolonged ground
idling would cause the engine to stumble
cold (appearing rich) on throttle-up to
takeoff. This is not criticism, but a hint to
change the plugs to suit the weather. Hotter
glow plugs alleviated this, and the
recommended O.S. A5s worked properly in
a wide range of weather conditions.
I highly recommend the 3M 1.40. Its table
manners are excellent, and the delivered
performance is on par with its
contemporaries. The company has already
answered the never-ending call for more
power with its larger-displacement 1.70. It
also makes a rear-intake version of the
1.40. I believe that is the ideal format for a
Pattern engine, although I didn’t test that
variant.
If you purchase this engine, just be
prepared to listen to your flying buddies tell
you that it is too pretty to run. MA
Manufacturer:
Mintor Corporation
Via A. Volta, 13—24060—S. Paolo
D’Argon
Bergamo, Italy
[email protected]
www.mintor3m.it
Importer:
AeroSlave
[email protected]
www.aeroslave.com

72 MODEL AVIATION
Dean Pappas
P r o d u c t R e v i e w
1144 Estates Blvd., Hamilton Square NJ 08690
Mintor 3M 1.40
Pros:
• Linear, predictable throttle response.
• Light weight.
• Horsepower competitive with other
engines in class.
• Jewlerylike finish—too pretty to fly!
Cons:
• Confusing importation network.
• Jewlerylike finish—too pretty to fly!
IN THE SIX or seven years since the FAI
eliminated the engine-displacement limit in
the RC Precision Aerobatics event,
commonly called Pattern, participants have
been looking to the natural horsepower-toweight
advantage of the tuned-pipeequipped
two-stroke engine as an
alternative to the popular and eminently
suitable YS 120 supercharged four-stroke
engine.
Until recently, the four-stroke has
maintained its hegemony with a modest
boost in displacement to 1.40 cubic inches.
The first two-strokes to enter this market
were also of 1.20 displacement and quickly
evolved into 1.40s and 1.45s.
A great deal of development on the part
of the engine manufacturers has gone into
giving the two-stroke the tractability and
predictable throttleability necessary for
Aerobatics competition. Those
characteristics would be appreciated in
many other areas, such as Scale or 3-D
aerobatics.
The engine I will review came from a
manufacturer that is new to the
aeromodeling market, not to mention the
demanding competition segment. Its efforts
have produced an excellent entry to this
highly competitive market. The Mintor
Corporation of Bergamo, Italy, is an
established firm that has recently turned its
experience in precision machining to
making model engines under the name 3M.
The first 3M product is a 1.40
displacement, front-intake, rear-exhaust
two-stroke that fits squarely in the middle
of the existing market. This is the engine I
will review. It is accompanied by a rearintake
version and, more notably, a similar
but larger 1.70 version which was
introduced during the period when this
review engine was being flown.
This review will not be a standard bench
First impressions are often accurate. The 3M is a beautifully built piece of machinery
inside and out. The entire engine is machined from aluminum blocks.
Dean pressed his trusty old Dr. Jekyll Pattern model into service as a flying engine test
bed. It logged more than 75 flights before Dean wrote this review.
May 2004 73
Photos courtesy the author
test; a total of approximately 10 gallons of
fuel were run through the engine, and
approximately 65 flights were made in the
engine test bed: a Dr. Jekyll II modified
with thicker wings, weighing close to the
maximum 5 kilograms (11 pounds)
permitted under competition rules.
Performance and rpm readings were
taken on the airplane rather than on the test
stand. This is more representative of actual
running conditions because the engine
mount typically used for this type of
airplane has some effect on performance.
The engine was flown as much to evaluate
its “table manners” as to determine its
performance levels.
Engine Construction: The 3M is
completely machined from bar-stock
aluminum, and the crankcase and all other
aluminum parts are anodized for corrosion
Carburetor is machined from aluminum with machined metal
control arm. Fuel-metering curve was almost perfect!
Typical for Aerobatics competition engines, 3M has backplatemounted,
crankcase pressure-driven fuel pump.
The Mintor’s carbon-fiber tuned pipe and O-ring-style header are extremely light, and
the pipe is well tuned to the engine’s timing.
You don’t really need 5,200 pounds of van to keep the test stand
from moving, but the Mintor is certainly strong!
You can easily see generously sized, deep fins on head and
cylinder. Machining rather than casting permits this.
May 2004 75
protection and appearance. The result is an
engine that resembles fine jewelry as much
as it does a piece of machinery. The purple
anodized head and propeller drive washer
add to the effect.
When the review 3M made its debut at
the home flying field, attached to a break-in
test stand, the local club members’ reaction
was that I should not run such a beautiful
piece of machinery (let alone fly it!).
Machined bar-stock aluminum can
produce lighter components than all but the
most exotic casting processes. As a result,
and the careful machine work done to
remove unnecessary bits of metal from
every component, the 3M weighs a mere
780 grams, which checked out at just less
than 27.75 ounces on my scale.
The anodized crankcase has an
additional benefit. The typical Pattern
model uses a soft engine mount, and those
often require the use of a nose ring to
support the front of the engine, just behind
the drive washer; the constant rubbing of
the rubber ring against an aluminum casting
usually creates a messy, gray dust. The
anodized aluminum does not do this.
As is typical for the market segment, the
3M is equipped with a crankcase-pressuredriven
fuel pump mounted in the backplate.
The pump is of the type first introduced by
Perry Aeromotive (now Varsane). The
pump permitted the tank to be placed at the
model’s center of gravity, with no fueldelivery
problems. The mounting flanges,
bolt pattern, and rear needle-valve
placement are compatible with the YS fourstroke
and O.S. 140 RX.
Crankcase and Crankshaft and Bearings:
As I described, the crankcase is fully
machined from aluminum, clear anodized,
and every bit of excess material is
machined away, contributing to the
engine’s light weight.
It is a two-piece affair, with the cylinder
and crankcase portions bolted together
using a large-diameter O-ring seal. This
facilitates the machining of the gas
passages and their close match to the ports
in the cylinder liner.
The four bolts are angled for access. If
overtightened, they may distort the bottom
portion of the cylinder and cause running
problems. The factory specification for
these is 4.6 N-M or 3 pound-feet of torque.
If you must disassemble the engine and do
not have a torque wrench in this range, grab
an “L”-shaped Allen wrench by the short
end and use only your thumb and
forefinger.
The crankshaft is of one-piece
construction, made from hardened steel,
with a 15mm-diameter gas passage. This is
a relatively small-diameter passage, in
keeping with the engine’s intended rpm
range and the need to keep gas velocities
high for good throttleability.
The aluminum drive washer has large
lightening holes and is tied to the crank
with a tapered brass collet. The 8 x 1mm
crankshaft threads proved to be adequately
long for even the largest propeller hubs and
a spinner backplate.
The crankshaft bearings showed no
signs of wear or corrosion during the test.
Amsoil MP was used as an after-run
corrosion preventative after each flying
session.
Piston, Ring, Liner, and Connecting Rod:
The top end of the 3M is all fitted relatively
tightly, trading off ease of break-in for
longevity and good running behavior in
summer heat. I consider this an excellent
tradeoff because predictable throttleability
is often the first victim of hot conditions.
The aluminum piston has a single castiron
ring, which is keyed to prevent
rotation. The steel cylinder liner was a tight
slip-fit to the crankcase cylinder. This is no
doubt done in the interest of heat transfer.
The 3M has a 32mm (1.26-inch) bore
and a 28.5mm (1.12-inch) stroke. As is
typical for the new generation of two-stroke
aerobatic engines, the 3M is not a longstroke.
As it turns out, the greater vibration
of a long-stroke engine (unless the
connecting rod is made long and therefore
heavy) is undesirable. Also, a loss of
intake- and exhaust-port area results from a
smaller-circumference piston.
The exhaust has a modest 140° duration,
and the bypass timing is 120°. The resulting
blow-down period is short, meaning that
the 3M benefits greatly from the use of a
tuned pipe. It also means that the tuned pipe
does not need to be of the “long-chamber”
design to get flexible operating
characteristics from the engine. (If you are
as big as the Hulk, the 3M would work
beautifully as a Control Line Stunt engine!)
The connecting rod is bronze bushed at
both ends, and the edges of the rod are
chamfered to improve gas flow from the
crankshaft passage. The big-end-tocrankpin
clearance on the test engine was
measured at 0.005 inch (0.127mm).
Although this is a normal figure for rod-tocrankpin
clearance, it is not particularly
tight considering that this engine is
intended for low-rpm use.
Since the 3M is intended for running in
the neighborhood of 8,000 rpm, it could
easily be fitted as tight as 0.003 inch
(0.076mm). As a consequence, it was
decided to use a fuel containing a high film
strength lubricant including 2% castor oil
for break-in and normal running. S&W
15% Sport Blend was used after a break-in
on a similar 10% blend. No undue wear
was observed after the test period.
Cylinder Head: The bright-purple
anodized head has twin glow plugs. The
design has “evolved” from a single central
plug design, much like that of the O.S. RX,
with a raised lip around the glow plug. The
second plug is mounted behind and angled
aft. In contrast, both of the larger 3M 1.70s’
plugs are mounted vertically and spaced
equal distances from the center of the
combustion chamber.
The manufacturer recommends O.S. A5
plugs in both holes, and this combination
ran well. K&B 1L glow plugs ran equally
well, with slightly better longevity.
Carburetor and Pump: As I described,
the 3M is equipped with a backplatemounted
pump and regulator. The
backplate is O-ring-sealed to the
crankcase; no paper gaskets here! A fine
mesh fuel filter is highly recommended
between the tank and fuel pump. The
carburetor is of the rotary metering barrel
and slot type, as is typical of this class of
engine.
The idle-mixture adjustment was
authoritative and strongly affected the
midrange. The manufacturer’s instructions
included with the engine recommend that
the idle mixture be set rich initially to
avoid overleanness at half throttle during
break-in. Please heed this advice.
The needle valve is remotely mounted
to the rear of the engine; its position,
along with the engine-mounting bolt
pattern, makes the 3M compatible with a
prior YS four-stroke or O.S. 140 RX
installation.
Header and Tuned Pipe: The engine
provided for testing came with two
different-height headers. The lightweight
header is coupled to the engine with twin
Viton O-rings of ASTM (American
Society for Testing and Materials) “dash-
211 size.” The O-rings lasted the entire
test period and were replaced before
further running. Depending on the engine
mount used, they will easily last 100
flights.
The 3M carbon-fiber tuned pipe is
light, at 150 grams, and attractive in
finish. The pipe is reinforced with a
section of aluminum tubing at its input, to
prevent crushing by the clamp used to
retain the silicone rubber header coupler.
The aft end of the pipe is machined from
aluminum and provides a machined
retention groove for a rubber exhaust
elbow.
As with any carbon-fiber muffler or
pipe, heavy mounting-clamp forces are to
be avoided. With the exhaust system
assembled at “stock” length, the end of the
exhaust stinger was only 35.5 inches (90
cm) from the propeller mounting face.
This makes the exhaust system one of the
shortest tuned systems available.
78 MODEL AVIATION
Break-In: The 3M 1.40 I tested was fitted
tightly, as stated before. The engine was
broken in with the stock tuned pipe and
header, lengthened by leaving a 1-inch gap
(2.5 cm) between them. This helped the
intentionally rich engine transition
properly. It required nearly a full gallon
before the 3M would consistently hold a
near-peaked needle setting. Since then, the
factory is fitting the engine more
conventionally.
I have observed several more recent
examples of the engine, and several
tankfuls now suffice for break-in with no
difference in running characteristics. I used
10% S&W Sport mix fuel for the first
three-quarters of a gallon and 15%
afterward. It is important to richen the idle
mixture, as directions that came with the
engine state. The idle mixture cam can
easily make the engine overly lean in the
midrange with the lengthened pipe. In
general, it is best to start rich and work your
way toward the ideal setting.
The engine was mounted in the
airframe, and the pipe was restored to
“stock length”: with only a .375-inch
(9mm) gap between the header and pipe.
The first baffle in the pipe was then 22.5
inches (57 cm) away from the center of the
combustion chamber, as measured through
the center of the header pipe.
Flying Setup: During the first few minutes
of running, at the stock pipe length, the 3M
agreeably turned 17 x 12 and 17 x 13N
APC propellers. I selected the 17 x 13N for
initial running; it turned 7,700 rpm at a
flyable (slightly rich) needle setting.
After a minimal amount of fiddling, the
idle mixture was set for a snappy idle-tofull-
throttle transition, and the mixture was
checked at half throttle by briefly pinching
the fuel-feed tubing. The half-throttle
mixture was close to peaked. The idle
mixture cam was almost perfectly centered.
After flying several aerobatic schedules,
the decision was made to slightly richen the
idle mixture to fatten the midrange. The
symptom was a slight leanness when
throttling up from half power at high
airspeed. It was not noticed on the ground.
There was no change in the idle-to-fullthrottle
transition. After a second gallon, a
reliable 1,500 rpm idle was obtained. This
is in keeping with a properly set-up
example of the four-stroke competition.
This setup provided excellent quiet on
the ground and in the air, as required by the
Pattern-event rules. The ground-noise levels
were 92 decibels at 3 meters, measured on a
screen-topped table. This number is
approximately 1 or 2 decibels lower than
that to be obtained over a reflective hard
surface.
In the air, the 17 x 13N showed no
propeller-tip “buzz” at full throttle and
provided good downhill braking. It is
apparent that the 3M and its exhaust system
are designed for this rpm range. The pipe
length was set at 22.5 inches (57 cm) from
glow plug to first baffle.
80 MODEL AVIATION
An APC 17 x 13 (standard blade) turned
the same 7,700 rpm on the ground and
braked slightly better, with no changes in
carburetor or pipe. The full-profile 17 x 13
is my normal flying propeller.
Changing to an APC 17 x 12, the engine
required that the pipe be shortened
approximately .25 inch, leaving a minimal
gap between the stock pipe and header. This
slight change in length eliminated a
tendency to richen in prolonged, shallow,
full-throttle dives. The pipe length was
22.25 inches (approximately 56 cm) from
glow plug to first baffle.
This 17 x 12 turned at 7,900 rpm static,
and acceleration in the air was slightly
improved. I observed minor propeller
buzzing, but this combination could easily
be called “quiet” at 93 decibels when
measured on the same sound table.
When I tried an APC 17 x 11, the
relationship between pipe length and
midrange mixture became apparent. With
the stock exhaust set for minimal gap, the
ground rpm was 8,200 and the idle mixture
required richening to prevent overleanness
in the midrange.
The real solution would have been to
shorten the pipe approximately .375 inch
(9mm), which likely would have raised the
ground rpm another 200 or 300. I could not
bring myself to cut the header to perform
this test because the 17 x 11 propeller was
already noisy in the air, and greater rpm
would only have worsened the situation.
For non-Pattern applications, this setup
would produce excellent power and still be
moderately quiet.
For its intended application, the 3M is
best run with a load such as the 17 x 12 or
17 x 13, in the 7,700-8,000 rpm range.
Four-blade 141⁄2 x 11 or 15 x 11 propellers
would also suit the 3M. For non-Pattern
use, the engine’s construction would
certainly stand up under higher-rpm use.
Handling Characteristics: As I stated, the
3M displays a reliable idle at 1,500 rpm
with the recommended pair of O.S. A5
glow plugs fitted and “standard” 15% fuel.
The engine’s table manners in flight are
excellent; the problems you often encounter
with piped two-strokes used for Aerobatics
competition are nonexistent. This is no
doubt because of the engine’s large, deep
fins; tight liner-to-crankcase fit; and dualplug
design. It is also claimed that the barstock
crankcase and cylinder head reject
heat better than a casting. It works ...
The challenges that tuned-pipe use often
present in Aerobatics are undertaken in
exchange for a sizable power boost and
excellent exhaust muffling. The action of
the resonant chamber greatly attenuates the
harsh high-frequency content of the exhaust
note—better than all but the most restrictive
mufflers.
Horsepower utilization in Aerobatics
competition has evolved because of the
tremendous power levels available. Wideopen
throttle (WOT) is now used only
during vertical climbs and the climbing
portions of looping maneuvers. Level flight,
whether at baseline altitude or at the top of
a maneuver such as a Square Loop, is
performed at approximately half throttle. A
slow idle with good compression braking is
vital in the diving portions of maneuvers to
preserve a constant speed presentation.
The basic problem is one of midrange
throttle hysteresis. When throttling back
from WOT to midthrottle, the engine may
stay “on the pipe” with the rpm remaining
elevated. This tends to happen after a long
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climb in warm weather or if even slightly
lean.
Alternatively, the pipe boost may cease
abruptly and leave the flier with less
horsepower at the same stick position. This
can happen if the engine is slightly rich or
cooled slightly from recent low-throttle
running. The problem worsens if the
throttle is advanced from idle to the same
midstick position.
The lack of predictability in delivered
horsepower at midstick can be maddening
while trying to negotiate an Aerobatics
schedule. This hysteresis problem can be
alleviated by lengthening the tuned pipe—
but at a price.
If the tuned pipe is set longer than
optimal, elevated airspeed (such as a WOT
diving run into a maneuver) will cause the
engine to richen excessively. This can even
extinguish the fire. Please note that by
optimal I mean for the purposes of in-air
handling—not for maximum ground rpm.
The best-flying pipe length is often a half
inch or so (approximately 1.5 cm) longer
than that which produces maximum ground
rpm.
The 3M displayed no throttle hysteresis.
The pipe lengths quoted caused no
richening in prolonged shallow WOT dives.
The idle mixtures that produced good midto
full-throttle transitions in the air
transitioned well from idle. The
manufacturer got the metering curve right
on this carburetor!
In cold weather, prolonged ground
idling would cause the engine to stumble
cold (appearing rich) on throttle-up to
takeoff. This is not criticism, but a hint to
change the plugs to suit the weather. Hotter
glow plugs alleviated this, and the
recommended O.S. A5s worked properly in
a wide range of weather conditions.
I highly recommend the 3M 1.40. Its table
manners are excellent, and the delivered
performance is on par with its
contemporaries. The company has already
answered the never-ending call for more
power with its larger-displacement 1.70. It
also makes a rear-intake version of the
1.40. I believe that is the ideal format for a
Pattern engine, although I didn’t test that
variant.
If you purchase this engine, just be
prepared to listen to your flying buddies tell
you that it is too pretty to run. MA
Manufacturer:
Mintor Corporation
Via A. Volta, 13—24060—S. Paolo
D’Argon
Bergamo, Italy
[email protected]
www.mintor3m.it
Importer:
AeroSlave
[email protected]
www.aeroslave.com