Question: What gives with Model Airplanes Engines? What's a 2 stroke? What's a 4 Stroke? What do you mean by Glow as compared to Gas? Now there are electric motors being adapted to replace engines... how the heck do I choose what is right for my model?

Answer: Well, answers to this can take a bit of explaining. Here is an overview.

2 Stroke (sometimes called 2 cycle)

• Is what the majority of current modelers mean when they say "Model Airplane" or "Glow" Engine
• Noisy screaming type of sound that must be carefully muffled and never sounds like the real thing.
• Uses Model Airplane Fuel (alcohol, oil, nitromethane and additives)
• Uses a Glow Plug hence the term "Glow" Engine.
• Generally the cheapest, lightest, most powerful engine for the money.
• Referred to by displacment in hundreds of cubic inches (or in cubic centimeters = cc)
• Imperial measure is cubic inches and a slang has developed
  • .40 cubic inches is referred to as "a 40"
  • .90 cubic inches is referred to as "a 90"
  • 1.20 cubic inches is referred to as "a 120"
• Generally range in size from .049 to 1.80 although some smaller and bigger engines exist.

4 Stroke (sometimes called 4 cycle)

• Has become more popular since the 90's with those wanting quieter more realistic engine sound for use in semi-scale or scale models.
• Quieter more familiar engine sound when in the air. Much more realistic for scale flying models. Sounds much more like the real thing.
• Uses Model Airplane Fuel (alcohol, oil, nitromethane and additives) although special blends with more or less oil and more or less nitro may be required for some engines.
• Uses a Glow Plug hence the term "Glow" Engine can be used for either 2 stroke or 4 stroke engines
• Generally costs more, weighs more and gives less power for more money than a 2 stroke engine of similar size.
• Referred to by displacment in hundreds of cubic inches (or in cubic centimeters = cc)
• Imperial measure is cubic inches and a slang has developed
  • .52 cubic inches is referred to as "a 52"
  • .91 cubic inches is referred to as "a 91"
• Generally range in size from .25 to 2.50 although some smaller and bigger engines exist.
• To get the same power as a 2 stroke, add on about 30-50% in size. This is a very rough rule and does not consider weight and torque issues but it is a good guide.
  • A model calling for a 2 stroke .46 would require roughly a 4 stroke of size .60-.70
  • A model calling for a 2 stroke .61 would require roughly a 4 stroke of size .80-.90

Gas (sometimes called Ignition Engine)

• Has become more popular for use with big models.
• Quieter more familiar engine sound when in the air. Much more realistic for scale flying models. Sounds like the real thing  because it pretty well is the real thing!
• Uses Gasoline. Special oil or additives may be added to the gas in some cases. Gas is 25-50% cheaper than Model Airplane Fuel.
• Uses a Spark Plug hence the term "Ignition".
• Often adapted by the manufacturer from some other purpose such as a weed eater, leave blower etc. Although the engines end up quite different with regards to fuel supply, ignition, prop adapter and flywheel etc, they usually started with an existing product and adapted from there.
• Seldom used on smaller models and nearly always used on big models due to the higher weight of the engine and ignition system.
• Generally costs more, weighs more and gives less power for more money that a 2 stroke engine of similar size. May cost and weigh more than a 4 cycle Glow engine.
• Referred to by displacement in cubic centimeters (cc) (or in cubic inches but almost universally now in cc)
• Metric measure is cubic centimeters (cc) and the cc is nearly always noted in referring to these engines
  • for example "30 cc"
• Generally range in size from 20cc to about 65cc although some smaller and considerable numbers of bigger engines exist.

Electric Motors (both brush type and brushless type)

• Have become more popular since 2000. Increasing in popularity
• Just as a combustion  Engine needs and is succesful depending on the type of fuel it burns, an electric Motor needs batteries.
• As battery technology put more and more power (run time) into batteries of a certain weight (we call this energy density) electric powered flight is starting to take off (literally)
• Generally brushless motors are taking over. They can be very expensive when coupled to a suitable electronic speed control and a good set of NiMh or LiPo batteries.
• Main advantage is quieter and no mess. No fuel to buy.
• Main disadvantage is price and flight time per charge.
• They will continue to improve and be more popular in time.
• The rating system is not yet standardized so it is hard to compare apples with applies. Generally if a model calls for a .40 two stroke engine, modelers need to select an electric motor that can provide similar power in order to have similar flight characteristics. This can vary widely by motor type (brushless is better), battery type, speed control and wiring.

Please note that to this point this article makes no attempt to advise on if a particular model can be adapted to accomodate a different type of power system. For some considerations related to fitting 4 stroke engines into a model please search on "Four Stroke" for further assistance.

Question: Can I use a 4 Stroke engine on my VMAR ARF model?

Answer: In most cases, Yes.

Better Answer: There are some things to consider here. First of all if the model has a cowl it will usually have a power module consisting of a removeable forward firewall and a set of engine mounts. These engine mounts can be oriented in many different ways so you can select the orientation and separation of the mounts to suit your engine. It is also very easy to work on with this setup because the forward firewall is completely removeable from the model. So... VMAR models with a cowl and power module probably will be able to accomodate a 4 stroke engine. Conversely... models without a cowl and power module usually have a factory installed engine mount and a fixed firewall only and may be more problematic when trying to fit them out with a 4 stroke engine. It is safe to say that if the model does not have a cowl and power module there may be significant work and mucking around in order to install a 4 stroke engine.

4 stroke engines are not as standardized as most 2 stroke engines... some have the carburetors at the front, some at the back. Some have valve pushrods running up the front of the crankcase... some up the back. As a consequence, 4 stroke engines vary quite a bit in length (i.e. the distance from the prop shaft to the back of the engine) within a particular size. Although for example a 2 stroke .46 from one supplier will be very similar in external size to that of a 2 stroke .46 from another supplier... it is not necessarily the case when comparing 4 strokes. A .52 four stroke from one manufacturer can be quite different from a .52 four stroke from another manufacturer. There is no way we can be aware of all the different sizes and shapes of four stroke engines in the market and modelers should be checking with the engine manufacturer for sizes before assuming that the engine will fit into any particular model. Once you know the length of the four stroke engine that you are considering, place the cowl on your model. Then measure from where the back plate of the spinner will be to the front face of the removeable power module firewall. Subtract about 1.25" from this distance and then compare the result with the length of your four stroke engine. Engines that exceed this length will significantly protrude from the front of the cowl and should be trial fitted carefully before purchase.

In some cases, we are pre-cutting cowls to make installation of our recommended 2 stroke engine go as quickly and easily as possible. We do stock blank (not cut) cowls for those who wish to use a differnet 2 stroke engine size (for example a .61 instead of a .46) or a 4 stroke instead of a 2 stroke or want to go with an inverted installation or other orientation when we may have assumed a 90 orientation when pre-cutting the factory cowl.

Lastly, we have some models that do not use a cowl per-se and do not have a power module but may look like they do. For example the F4U Phantom and Skyhawk and Arrow Tiger have a shaped and fitted nose section but not a cowl and we have not recommended 4 stroke engines for these models at all. Again, if we do not use a cowl and power module on a model... we may be significant work and mucking around in order to install a 4 stroke engine.

Please note that to this point this article makes no attempt to explain the differences between the different types of engine systems. For some of the pro's and con's of 2 stroke and 4 stroke engines please the article included below: