Product Review
Dean Pappas
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.
- Jewelry-like finish — too pretty to fly!
Cons:
- Confusing importation network.
- Jewelry-like 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-to-weight advantage of tuned-pipe-equipped two-stroke engines as an alternative to the popular and eminently suitable YS 120 supercharged four-stroke engine.
Until recently the four-stroke maintained its hegemony with a modest boost in displacement to 1.40 cubic inches. The first two-strokes to enter this market were of 1.20 displacement and quickly evolved into 1.40s and 1.45s.
A great deal of development by engine manufacturers has gone into giving the two-stroke the tractability and predictable throttleability necessary for Aerobatics competition. Those characteristics are appreciated in many other areas, such as Scale or 3-D aerobatics.
The engine reviewed here comes from a manufacturer new to the aeromodeling market but not new to precision machining. Mintor Corporation of Bergamo, Italy, 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. It is accompanied by a rear-intake version and a similar but larger 1.70 version, which was introduced during the period this review engine was being flown.
This review is not a standard bench test: 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 and 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 affects performance. The engine was flown as much to evaluate its "table manners" as to determine its performance levels.
Engine Construction
First impressions are often accurate. The 3M is a beautifully built piece of machinery inside and out. The entire engine is machined from bar-stock aluminum; the crankcase and all other aluminum parts are anodized for corrosion protection and appearance. 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 allows lighter components than most casting processes. Careful machine work removes unnecessary metal from each component, and the 3M weighs a mere 780 grams (just less than 27.75 ounces) on my scale.
The anodized crankcase has an additional benefit. Typical Pattern models use a soft engine mount that often requires 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 resists this.
As is typical for this market segment, the 3M is equipped with a crankcase-pressure-driven fuel pump mounted in the backplate. The pump is of the type first introduced by Perry Aeromotive (now Vansarone). The pump permitted the tank to be placed at the model’s center of gravity with no fuel-delivery problems. The mounting flanges, bolt pattern, and rear needle-valve placement are compatible with the YS four-stroke and O.S. 140 RX.
Crankcase, Crankshaft, and Bearings
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 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 bolts is 4.6 N·m (3 pound-feet) of torque. If you must disassemble the engine and do not have a torque wrench in the range, grab an L-shaped Allen wrench by the short end and use only your thumb and forefinger.
The crankshaft is one-piece, made from hardened steel, with a 15 mm-diameter gas passage. This relatively small-diameter passage suits the engine’s intended rpm range and keeps 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 1 mm crankshaft threads proved 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 fitted relatively tightly, trading off ease of break-in for longevity and good running behavior in summer heat. This is an excellent tradeoff because predictable throttleability is often the first victim of hot conditions.
The aluminum piston has a single cast-iron ring keyed to prevent rotation. The steel cylinder liner is a tight slip-fit to the crankcase cylinder for improved heat transfer.
The 3M has a 32 mm (1.26-inch) bore and a 28.5 mm (1.12-inch) stroke. As is typical for the new generation of two-stroke aerobatic engines, the 3M is not long-stroke. Greater vibration of a long-stroke engine (unless the connecting rod is made long and heavy) is undesirable. Also, a smaller-circumference piston reduces intake- and exhaust-port area.
The exhaust has a modest 140° duration and the bypass timing is 120°. The resulting blow-down period is short, so the 3M benefits greatly from the use of a tuned pipe. It also means the tuned pipe does not need to be of the long-chamber design to get flexible operating characteristics from the 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-to-crankpin clearance on the test engine was measured at 0.005 inch (0.127 mm). Although normal for rod-to-crankpin clearance, it is not particularly tight considering 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 (0.076 mm). As a consequence, it was decided to use 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 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 evolved from a single central-plug design (much like the O.S. RX) with a raised lip around the glow plug. The second plug is mounted behind and angled in. In contrast, both plugs on the larger 3M 1.70 are mounted vertically and spaced equally from the center of the combustion chamber.
The manufacturer recommends O.S. A5 plugs in both holes; this combination ran well. K&B IL glow plugs ran equally well, with slightly better longevity.
Carburetor and Pump
The 3M is equipped with a backplate-mounted pump and regulator. The backplate is O-ring-sealed to the crankcase; no paper gaskets are used. 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 recommend that the idle mixture be set rich initially to avoid overleaning 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 prior YS four-stroke or O.S. 140 RX installations.
Header and Tuned Pipe
The test engine came with two different-height headers. The lightweight header is coupled to the engine with twin Viton O-rings of ASTM "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 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, making the exhaust system one of the shortest tuned systems available.
Break-In
The 3M 1.40 I tested was fitted tightly. The engine was broken in with the stock tuned pipe and header, lengthened by leaving a 1-inch (2.5 cm) gap 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, start rich and work toward the ideal setting.
The engine was mounted in the airframe and the pipe was restored to stock length, with only a 0.375-inch (9 mm) 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 from the center of the header pipe.
Flying Setup
During the first few minutes of running at the stock pipe length, the 3M agreeably turned APC 17 x 12 and 17 x 13N propellers. I selected the 17 x 13N for initial running; it turned 7,700 rpm at a flyable (slightly rich) needle setting.
After minimal fiddling, the idle mixture was set for a snappy idle-to-full-throttle transition, and the mixture and throttle were checked at half throttle by briefly pinching the fuel-feed tubing. The half-throttle mixture was close to peaked and the idle-mixture cam was almost perfectly centered.
After flying several aerobatic schedules, I slightly richened the midrange because it tended to lean when throttling up from half power at high airspeed. This was not noticed on the ground. After a second gallon, a reliable 1,500 rpm idle was obtained, in keeping with a properly set-up example of the four-stroke competition engine.
This setup provided excellent quiet on the ground and in the air, as required by Pattern-event rules. Ground-noise levels were 92 decibels at 3 meters, measured on a screen-topped table (approximately 1–2 decibels lower than over a reflective hard surface).
In the air, the 17 x 13N showed no propeller-tip buzz at full throttle and provided good downline braking. It is apparent that the 3M and its exhaust system are designed for this prop range. The pipe length was set at 22.5 inches (57 cm) from glow plug to first baffle.
An APC 17 x 13 (standard blade) turned the same 7,700 rpm on the ground and braked slightly better with no changes to carburetor or pipe. The full-profile 17 x 13 is my normal flying propeller.
Changing to an APC 17 x 12 required shortening the pipe approximately 0.25 inch, leaving a minimal gap between the stock pipe and header. This change in length eliminated a tendency to richen in prolonged, shallow full-throttle dives. The pipe length was then 22.25 inches (approximately 56 cm) from glow plug to first baffle.
The 17 x 12 turned 7,900 rpm static and acceleration in the air was slightly improved. I observed minor propeller buzzing, but this combination was still "quiet" at 93 decibels measured on the same sound table.
With an APC 17 x 11 and the stock exhaust set for minimal gap, 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 0.375 inch (9 mm), which likely would have raised ground rpm another 200–300 rpm. I did not shorten the header 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 while remaining moderately quiet.
For its intended application, the 3M is best run with a load such as a 17 x 12 or 17 x 13 in the 7,700–8,000 rpm range. Four-blade 14½ x 11 or 15 x 11 propellers would also suit the 3M. For non-Pattern use, the engine's construction would stand up under higher-rpm operation.
Handling Characteristics
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 often encountered with piped two-strokes in Aerobatics competition are nonexistent. This is due to the engine's large, deep fins; tight liner-to-crankcase fit; and dual-plug design. It is also claimed that the bar-stock crankcase and cylinder head reject heat better than a casting, and that appears to work.
The tuned pipe offers a sizable power boost and excellent exhaust muffling. 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 tremendous available power. Wide-open throttle (WOT) is now used only during vertical climbs and climbing portions of loops. 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 diving portions of maneuvers to preserve a constant-speed presentation.
The basic problem is midrange throttle hysteresis. When throttling back from WOT to mid-throttle, the engine may stay "on the pipe" with rpm remaining elevated, especially after a long climb in warm weather or if 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 from recent low-throttle running. The lack of predictability in delivered horsepower at mid-stick can be maddening while negotiating an Aerobatics schedule.
Hysteresis 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 and may even extinguish the engine. By "optimal" I mean optimal for in-air handling, not for maximum ground rpm. The best-flying pipe length is often about a half inch (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. Idle mixtures that produced good mid-to-full-throttle transitions in the air also transitioned well from idle. The manufacturer got the metering curve right on this carburetor.
In cold weather, prolonged ground idling could cause the engine to stumble on takeoff (appearing rich) on throttle-up. This is not criticism but a hint to change plugs to suit the weather. Hotter glow plugs alleviated this, and the recommended O.S. A5s worked properly across a wide range of conditions.
I highly recommend the 3M 1.40. Its table manners are excellent and delivered performance is on par with its contemporaries. The company has already answered the call for more power with its larger-displacement 1.70, and it also makes a rear-intake version of the 1.40 (which I did not test but believe to be an ideal format for the Pattern engine).
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
Transcribed from original scans by AI. Minor OCR errors may remain.
