Gas or glow: Which engine is right for you?

Model Airplane News, Mar 2000 by Patrick, Dave

Gasoline-powered aircraft have become very popular over the years, and for good reason. A gradual shift toward larger models has occurred, and this, in turn, has stimulated the popularity of gas-burning powerplants, For a while, this niche was filled by largedisplacement glow engines. I can remember when a .60-size glow engine was considered huge; then there was the .90, the 1.08, and so on. But today, we have many fine examples of gas engines designed and manufactured exclusively for RC.

There are many things to consider when you choose an engine type. The following are only my opinions; I'm sure you'll find exceptions. Glow engines are generally lighter, simpler, more powerful, cooler running and less prone to generate radio interference from the ignition system. Glow engines are also usually less expensive-not a bad case for using glow engines, right?

Wait; not so fast. If you want a really big glow engine, there is a price to pay. Compared with gasoline engines, glow engines are very fuel thirsty. For example, when running on glow fuel, my Infinity 140cc, in-line, twin-cylinder engine requires two, 32-ounce fuel tanks, and that was barely enough to make it through a complete flight at the TOC. In fact, a couple of times, I had to land with only one cylinder firing because of a lack of fuel. Executing the same maneuvers while running on gasoline, however, my 3W* 140 requires only one, 32-ounce tank, and it often lands with about a third of the fuel remaining.

I have been told that, all things being equal, the fuel flow for a glow engine is 2 1/2 times greater than it is for one that burns gasoline. Add to that the fact that glow fuel costs seven to 10 times more than gasoline, and it really begins to add up. By comparison, today's RC gas engines are quite powerful, economical and reliable.

Availability of certain parts is also an issue, as far as some of the bigger glow engines are concerned. Most parts for gasoline engines enjoy a relative "off-the-shelf" status, especially cylinders and pistons.

Excellent carburetors, such as those made by Walbro and Tillitson, are also available for gas engines. These were designed for use with chain saws and have a built-in diaphragm pump. A pumper carb is great for model aircraft; you can place the fuel tank almost anywhere.

On smaller airplanes, however, the benefits of using a gas engine may not be worthwhile. Because of its ignition system, a gas engine requires a larger (and heavier) battery pack. Weight is always More critical with smaller airframes, and a cross-over point must be determined at which a certain size of gas engine is more favorable than a glow engine. This decision is an individual one and must be based on one's personal need for performance and economy.

I have two Cermark*, Signature Series 1/4-scale S-2B Pitts biplanes; one is powered by a YS* 1.20 4-stroke glow engine, while the other is powered by a Zenoah* G-23 gas engine. Both aircraft are very reliable, and both are excellent aerobatic performers. I can say that I enjoy flying both setups very much, but for competition, the YS 1.20 is clearly the more powerful engine. The Zenoah uses inexpensive automotive gas and has a great smoke system. The G-23 doesn't pull the model in a vertical line as well as the YS, but I love using it to fly fun maneuvers such as Lomcevaks and rolling circles (especially with that smoke system).

Another consideration is that the ball bearings in a glow engine tend to rust more easily than in a gas engine because the alcohol in glow fuel attracts moisture and requires the use of an after-run oil to prevent corrosion. A gas engine does not have this problem, as the oil mixed with the gas provides adequate protection. Also, because glow engines run cooler than gas engines, they do not generate as much smoke as their gas-fed cousins.

So in a nutshell, my advice is: the bigger the plane, the more reason you'll have to consider a gas engine.

CARBURETOR LINKAGES

As I mentioned earlier, most gas engines are equipped with either a Walbro or a Tillitson carburetor. These carbs weren't originally designed for RC plane use. They have a self-contained fuel pump and an atmospherically controlled regulator. The pump system is very clever and works very well but sometimes can cause some grief. A small hole in the carb's front cover allows the pump to sense ambient air pressure so it can properly adjust the fuel-flow pressure to the needle valve. Sometimes, as the model is flying, the oncoming airflow will blow over this hole and cause the regulator to work incorrectly, and this makes it very difficult to properly adjust the needle valves. Also, if the engine is inside a cowl, the internal cowl pressure can change during different maneuvers, further complicating the situation. I once had a TOC plane whose engine would go very rich whenever I added right rudder in a rolling circle. Go figure!

If you experience these kinds of problems with your carb, you may need to isolate the pressure variations. Sometimes, a simple cover placed over the hole is all that's needed to prevent the problem. In more extreme cases, modelers have found that the only way to solve the pump-pressure fluctuation is to carefully install a pressure fitting in the small hole and then vent the carb to the static air pressure within the fuselage with a length of fuel line. Thankfully, my new 3W 140 is very happy when it's left alone, but all engine installations differ.