Glow plugs are the norm on all "glow" engines used with methanol-based fuels. The glow plug is made up of a steel "plug" with a platinum wound wire element. When inserted into the head of the engine, the plug forms the uppermost portion of the combustion chamber. In operation, there is a catalytic reaction between the alcohol and the platinum element that when combined with the right compression, it causes the alcohol to burn. It is common to attached a 0.5-1.2V battery to the glow plug to cause it to glow, and remove this battery once the engine is running and the element continues to glow through the catalytic reaction. But for the stronger, race-spec, airplane, or heli plugs, a glow starter up to 4.8V may be needed.
Glow plugs are commonly referred to as "hot" or "cold" plugs. Both of these terms refer to the heat range of a glow plug, and it is easiest to notice the difference at an idle. In general, the hotter the plug, the richer the mixture can be at idle and the engine will continue to operate. The colder the plug, the mixture will need to be leaned out in order to operate properly. Check your engine's specs to determine the best plug to use.
Assessing the Condition of Your Glow Plug
Glow plugs on the market today are designed to provide good service to the user and may last a long time or a short time, depending upon the way you choose to operate your engine.
Physical indications that you might need to change the glow plug are:
- Twisted or mangled glow plug element. This is usually caused by too high a compression ratio.
- Small "bumps" are attached to the glow plug element. This will generally be noticeable during the break-in process. These are actually tiny pieces of aluminum that have attached to the element and these will severely hinder the operation of the glow plug.
- The glow plug element is no longer shiny, but dull a almost white powder color. This comes with age and is a by product of the catalytic reaction. The shinier the wire, the better the catalytic reaction will be.
Operating indications that you need to change your glow plug are:
- The glow element will not light with a charged glow igniter. This indicates a physical short or breakage in the element wire itself.
- Glow plug lights but the engine will not stay running once the battery is disconnected. This is usually an indication if the microscopic particles we discussed earlier.
- Glow plug lights, engine runs, but there is a noticeable loss of rpm at full throttle when the battery is disconnected. This is an indication that the white powder residue is building to the point that the catalytic reaction of the glow plug is no longer anywhere close to being optimum.
2-Stroke, 4-Stroke, Gas or Glow?
If you want to get off the ground, you have to have an engine. The most common powerplant for radio control airplanes has been a 2-stroke engine burning glow fuel - a methanol/nitro-methane/oil mixture. But depending on the model you're looking to fly, you can also choose a 4-stroke engine, or an engine fueled by regular gasoline. And with today's electronic enhancements, you can even use an electric brushed motor or brushless motor to power your plane.
The size and type of engine is determined by the size and type of model. An important factor is to keep the engine in proportion to the plane. Most trainers, for example, use a .40 size 2-stroke engine. You can boost power by moving up to a .46 or even .48 size, but too much power can be as bad as too little, so it is always best to stick within the recommendations that come with your kit.
As an alternative to the commonly-used 2-stroke, many modelers prefer a 4-stroke engine. A 4-stroke engine has a different rhythm in its operation and produces a distinctive sound (think sports car rumble vs. motorcycle whine). The more "realistic" aircraft sound produced by 4-stroke engines, like Saito, is especially prized by scale modelers.
Giant scale modelers can also choose engines fueled by regular gas instead of glow fuel. More economical to run, they are also large and powerful. Zenoah, the top name in gas engines, offers sizes ranging from 2 horsepower/1.37 cubic inches to a whopping 6 horsepower/4.45 cubic inches. These are definitely built for the "big birds".
Not as commonly seen yet, but jet turbine engines with their monster power and thrust are known to make there way into the extreme hobbyists' rigs.
Here's a few more RC Engine pages: