Engines 101 – Part Two

Last month, we 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 engine used in trainers today is the two-stroke, .40 cubic inch displacement “Ole’ Reliable”. This issue, we’ll cover the initial care and feeding of this engine including mounting, break-in and needle settings. Following that segment, we’ll cover propellers, glow plugs, fuel, maintenance and repair. However, except for history and propeller sizes, everything discussed in these sections will also apply to most two-stroke engines, .10 to 2.10 displacements.

The O.S. LA 46 (left) is exactly the same engine as the LA 40 (right) except for having a larger displacement. The LA 46 is more powerful but has higher fuel consumption.

The “forty” two-stroke has been the most popular RC engine for several decades. A logical out growth of UC’s most popular engine of the 1950’s, the “Fox 35”, the “forty” RC offered increased displacement to compensate for the power lost when incorporating a throttle. The first “forty” was familiar to UC pilots transferring to RC, remained easy to hand start, was the same size, weight and power as the .35, and offered good fuel economy. These features made the “forty” popular then, and remain its key advantages to this day.

Today, the old” forty” 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 inches (photo 1). Naturally, the various .45’s, .46’s and “50’s” produce more power that their respective “forties” but also use more fuel and require a larger volume of cooling air to operate. These larger “.40’s” must also have larger propellers that may not offer sufficient ground clearance on smaller “forty-sized” aircraft.

“Forty” engines are offered in either ringed or ABC configurations. The original ringed, sometimes baffled, engines (photo 2) feature low fuel consumption and reliable, cool running The ABC engines (photo 3) are powerful without being temperamental, unless they are solely racing engines and these are definitely outside this article’s scope. Most, but not all, “forty” 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 either its ringed or ABC (also ACC) design.

These reliable, well-used ringed engines are Schnuerle ported and have piston rings. Both the Super Tigre 40 and Enya 45 have been sport engine favorites for many years.

But before the engine can be properly broken-in, it first has to be mounted on the airplane or test stand. Mounting on a test stand is easy; just follow the stand’s directions. Be sure to attach the muffler and tank pressure lines as well. Most all of today’s .40 two stroke engines require muffler pressure to the fuel tank in order 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 while the spray bar is the small brass tube inside the venturi (Photo 4). In order 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 it’s speed.

Before mufflers became common, manufacturers had to make the venturi small in order to increase the in-coming air’s speed. But a smaller venturi restricts the total amount of incoming air and therefore reduces power output. Venturi 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 diversion put pressure inside the tank that forced fuel to flow into the carburetor.

The Super Tigre 45(left) has the smaller, square exhaust port typical of an “ABC” engine, compared to the ringed Super Tigre 40 on the right.

It is easy to see the larger fuel spray bar (right) in this “down the throat” venturi photo. On the left side is the idle mixture adjuster, or needle valve, that controls the fuel/air mixture below half throttle.

While not actually a fuel pump, muffler pressure meant venturi suction was no longer the sole source of the engine’s fuel feed. Therefore, the venturi itself could be made larger without reducing the carburetor’s fuel intake. Making the venturi larger increases an engine’s power output. The larger venturi of today’s engines require that the muffler be attached every time the engine is run to insure the fuel mixture is “rich” enough (has a high 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 the plane can appear daunting, but is actually easy and model pilots will eventually need to know how. While many of today’s Ready-To-Fly (RTF) trainers have the engine already mounted, hard landings may damage the original mount. Almost-Ready-to-Fly (ARF) trainers require the builder to mount the engine. Depending on the airframe, you may need to adjust the engine’s “thrust angle”. That’s the angle between the airframe’s horizontal centerline through the fuselage and the direction, right left, up or down, 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.

The four most common engine mounts. The metal “clamp” mount (center left) does not require drilling mounting holes but is the most difficult to properly align.

There are four types of engine mounts most commonly now in use (See Photo 5). These are the aluminum “clamp on” mount, adjustable fiberglass or solid fiberglass mounts and the independent, twin “I” beam, fiberglass mount. Of these, the aluminum clamp-on mount is the easiest and hardest to use correctly. Easy, because the engine is held in place by two clamps. There is no need to drill mounting holes into the mount.

But it is difficult to insure that the engine is perfectly 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 inside the mounting beams. Both situations affect the engine’s thrust line and therefore the plane’s handling characteristics, never for the better. Compounding the alignment problem is that most trainers and sport ARF’s have right and/or down thrust built into the firewall, the wood faceplate that the mount bolts to.

The firewall’s offset means it is not possible 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, then all measurements must be done in relation to the mount itself. The first step is to determine how far forward in the mount the engine needs to be. If your plane has a cowling and spinner, make sure there is at least 1/16” clearance between the front of the cowling and the rear of the spinner. Photo 6 shows what happens without this clearance. If the plane is not cowled, make sure the propeller will clear the fuselage side plates.

Always make sure there is at least a 1/16 in. space between the spinner back plate and the cowling. Flexible (soft) engine mounts require at least 1/8 in. spacing.

Once the engine’s fore and aft placement is determined, make a mark at the rear and front of the engine’s mounting plate (photo 7). Measure the mount’s outside width at both the front and rear of the marks (photo 8). Then measure the width of the engine’s mounting plates (photo 9). Subtract this measurement from the mount’s widths and the result is the total extra side space at both 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 make a mark. Then just draw a line between the two marks on each side (photo 10).

Aligning the outside of the engine’s mounting plates to these two lines centers the engine in all directions inside the mount (photo 11). Lightly clamp one side of the engine only. Insure that the engine hasn’t moved by checking the reference line on the unclamped side and, just to check that everything is exactly straight, mount the propeller. Then make a mark in the top middle of the mount’s faceplate, the rear mount part that holds the aluminum mounting beams.

Marking the front and rear of the engine’s mounting plates is the first step in aligning an engine in a mount wider than the engine’s crankcase.

An inexpensive ($10-15) dial micrometer is the best way to measure the mount’s beam width, but a small engineer’s ruler also works well. Measure both front and rear marks, as there is a difference.

Measure from this center mark to each propeller tip as a check (photo 12). The distances should be exactly the same. If not, they will not be too different and can be easily adjusted without moving the engine sideways. DO NOT use this check measurement alone, without centering the engine in the mount first. If this is done, it is possible to have the engine too far to one side. Equal prop tip distances will then insure the engine is twisted inside the mount. Once everything checks out, Install and tighten the second clamp then secure the first clamp as well. It actually takes longer to read this than to do it.

The same dial micrometer makes it easy to measure the engine’s width, 2.42 in. in this example. This measurement is difficult to make without a micrometer, but this dimension is usually printed in the engine’s instructions.

Now it is just as easy as drawing a straight line with a ruler.

Loosely clamp the engine between the lines using the clamp on the side opposite the line. Once adjusted, tighten the one clamp enough to prevent engine movement.

Install the second clamp and then check the final alignment. The engine must be centered before using this measurement to double check alignment.

The same method can be used to position the engine in a solid fiberglass mount that may be too large for the engine. However, many times, if you have good karma and eat healthy, this type of mount securely fits the engine 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 a tough job and once served to “build character” in a modeler. But now, several companies sell tools that make this job so simple, fast and trouble-free that some of us have had to find other ways to become “characters”. Photo 13 shows the Great Planes “Dead Center” engine mount hole locator in use, but several other manufacturers make almost identical tools.

To properly use this tool, you first need to make a mount jig, or possess a drill press vise. The jig is easy to make. Just attach two pieces of ½” plywood, about 6” x 6”, together with epoxy and screws so that they are exactly perpendicular. You will be using this jig for your entire modeling career so make sure it is correct and well braced. Screw the mount to this jig, making sure it is level.

Position the engine, hold it in place and use the tool to drill one small, shallow mark in a mounting beam (photo 13). Mark just one hole for now. Remove the engine and drill the hole. How, what size hole? As to size, you should use the largest hardened socket head machine bolt that will fit inside the engine’s mounting holes. While the screws that came with your engine mount are OK, hardened steel bolts are stronger and easier to install.

There is no easier way to mark the engine mounting holes when drilling is required. Mark only one hole, drill and tap, remount the engine and then mark the remaining holes.

Most .40 engines use 4-40 or 6-32 bolt sizes. After the hole is drilled, matching threads will be tapped into it. Fiberglass is softer than metal so use a drill one size smaller than the size printed on the tap. For 6-32 bolts, use a No. 37 drill or a No. 44 drill for 4-40 boltholes. It is best to use a drill press and the jig you made (or drill press vise). Good drill presses are now sold for less than $40 and are good investments, as you will use one for many modeling years. Do not use oil while tapping the threads as 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 one hole you made, remount the engine, check that everything is still positioned correctly and then mark the remaining three holes. It is actually best to drill and tap just one hole at a time, remount, and then mark the next hole. This is not necessary but can prevent cumulative errors as each hole may be drilled slightly off center.

The same mounting procedure is used with both remaining types of mounts. For independent “I” beam mounts, attach one “I” beam to your jig, insure it is level, clamp the engine to it and attach the other “I” beam to the jig (photo 14). Then just drill the holes as above. When using adjustable fiberglass mounts, slide them together per instructions, attach to the jig and drill.

Separate beam mounts should first be mounted to the engine on a jig. Once mounted the complete engine/mount assembly can be positioned on the “firewall” and the mounting holes drilled.

With the engine properly and securely mounted on the plane, we are ready to start the break-in. Well, not just yet. You’ll need fuel, the right propeller and a glow plug, also a glow plug igniter, and starter – electric or hand. Glow plug igniters and starters will come later as will detailed glow plug and fuel selections. For now, just assume you have the best of each.

Break-in propellers however, are important. The size propeller used during break-in depends upon the engine type – ringed or ABC (AAC). For ringed engines, use a propeller one inch less in diameter than will be used in flight. ABC engines need the exact same propeller as will normally be flown. Type construction, wood, fiberglass, etc. should also match for ABC engines but is not critical 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 engine, 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 diameter of the cylinder at its very top. As the piston travels upwards, the bore becomes smaller until, at the very top of its stroke, the piston is almost the same size as 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 nearly the same size as 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 exactly fits the piston 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 insures 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 5-10 flights.

Ringed engines do not need to turn the same rpm during break-in as during flight, but do need to run cooler than normal. Therefore, ringed engines require a richer fuel mixture during initial flights. Using a propeller one inch less in diameter reduces the engine load, and heat generated, while allowing the engine to achieve enough rpms for break-in on the ground with a rich mixture. Ringed engines usually require more break-in time, averaging 15-20 flights.

Before running any engine, use common sense and take every precaution. The plane must be immobile, the propeller tight, all obstacles cleared, no smoking and do this outside. Wear eye and ear protection and never, repeat never, stand to the side in the propeller arc or make any adjustments at all from in front of the engine. DO NOT REACH AROUND THE SPINNING PROPELLER TO MAKE NEEDLE ADJUSTMENTS, TO REMOVE THE GLOW DRIVER OR FOR ANY OTHER PURPOSE. Always make all adjustments standing in the rear of the engine. ALWAYS, PLEASE.

I have taken far too many friends to hospitals over the years, watched too many micro-surgeries and hoped far too many times that they could re-attach nearly severed fingers to not to warn anyone reading this to be careful. There is no “reset button” once that propeller hits you.

Break-in procedures for ringed engines vary by individuals but you should consider this one. Open the high-speed needle valve one-half turn more than the engine directions state. Have the throttle wide open and the plane properly secured. Prime the engine by holding one finger over the venturi, hold the propeller securely and rotate it counter-clockwise until fuel moves through the fuel line and nearly into the carburetor. Do not have the glow driver attached.

A very rich full-throttle mixture is the best way to break-in ringed engines. A few drops of raw fuel should be noticeable.

Connect the glow driver, making sure any wire is clear of the propeller arc, and start the engine. Remove the glow driver. The engine will run full at throttle but very, very rich. If the engine falters, close the needle valve, (while standing behind the engine), just enough to insure a steady run. The engine should be spitting raw, un-burnt fuel out of the muffler and running about 2,000 rpms slower than normal (photo 15). Run the engine this way for 5 minutes then shut down to cool.

Repeat this procedure once more. On the third run, let the engine run rich for 2 minutes, then begin to “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-pitch / high-pitch sound. Stop there and let it run for 30 seconds. Then return to the rich setting for 2 minutes and then stop it again.

Restart, then lean to that alternating sound and run for 1 minute. Then richen the mixture again (open the needle valve) but only to one-half turn less than the initial rich setting. Engine rpms should now be about 1,500rpm lower than normal. After 1 minute running rich, lean to the alternating sound point and run for 1 minute. Continue alternating the needle valve settings for 5 more minutes. Stop and let cool. Restart and set the needle valve to the alternating sound point. Run the engine at this point for 3-5 minutes. If the engine holds rpm and doesn’t appear 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 forty size engine use a separate idle needle valve (photo 16). 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 as per the engine’s instructions. Clockwise adjustments lean the idle mixture while counter-clockwise turns richen it.

Idle needle valve adjusters that regulate the fuel/air mixture below half throttle can either be screws or actual needle valves. The high-speed needle valve is NOT very effective below 1/3 throttle.

Some engines use an air-bleed hole located in the carburetor’s top front section (photo 17). A screw meters the amount of air admitted through this hole at idle, adjusting the idle mixture. Initially, the screw should cover just half the air inlet hole (see photo). This may be too rich but the idle mixture can be leaned by turning the screw clockwise. Turning the screw past the hole continues to adjust the idle mixture, despite appearances.

Some engines use a small hole in the carburetor’s front to adjust the idle mixture. Start with the adjustment screw covering just half the hole as in this photo.

There is little purpose adjusting the idle mixture on the test stand since fuel pressure, air intake volume and airflow will be very different once the engine is installed in the airplane. The idle setting will just have to be readjusted again. Mount the engine in the airplane if that has not already been done. Then run the engine at full throttle and set the needle valve just slightly leaner than the alternating sound point. Stop, attach the glow driver and restart.

Slow the engine to about 3,000 rpm (a tachometer helps here). Watch the rpms. If the engine gradually slows, then stops, the mixture is too rich. Once the engine stops, lean the idle mixture one-quarter turn. If the engine rpms increase, the mixture is too lean. Richen the idle mixture, again once the engine is not running, one-quarter turn. Check each new setting by running the engine at full throttle then reducing to 3,000 rpm. This “clears” the previous incorrect idle setting. Even if the engine does not quit, but needs final adjusting, always stop the engine before making any idle adjustments. Take every opportunity to stay away from a spinning prop with 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 just 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, then quickly advance the throttle, If the engine just stops, richen the idle mixture just slightly. If the engine stumbles and quits, won’t accelerate or accelerates very slowly, lean it a bit.

During the first few flights, 3,000rpm provides a reliable idle for most engines. Slower idle settings are possible, but run the risk of the engine’s quitting due to the high internal friction present during break-in. Set the initial throttle trim on the transmitter for a 3,000 rpm idle at full “up” throttle trim, while full “down” throttle trim stops the engine.

Landing patterns are flown at high idle. Once the field is “made” (the plane 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 very pretty landing. This half-trim setting will be around 2,200 to 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 given in the instructions. Start the engine at full throttle. The exhaust sound should be slightly lean of the alternating low and high-pitched sounds. If only a very high-pitched sound is heard, richen the mixture. If only a low pitched sound, lean the mixture to just past the alternating point. Run for 5 minutes, alternating full throttle and half throttle. Continue running for 5 minutes at a slightly leaner mixture setting, again alternating full and mid throttle. During the final five minutes, lean the high-speed mixture until rpms peak and start to drop. Immediately richen the mixture to 1,000 rpms less than that peak (about ½ turn). This is the initial flying high-speed mixture. Adjust the idle mixture just as for ringed engines.

After about 10 flights for ABC engines, 20 flights for ringed engines, the high-speed mixture can be leaned to 500 rpm less than peak. Never run leaner than this. In flight, a trainer’s engine turns about 500 rpms faster than on the ground. The mixture tends to lean as rpm’s increase. In steep climbs and while inverted, fuel feed rates are reduced. Most importantly, 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. 500 rpms rich is the leanest run setting without a fuel pump. 600 is better and will greatly lengthen engine life.