Model Airplane Engine Types

There are a variety of engine types available to us and every few years a type we never had before becomes available.  I can't predict what will be next, but there are already many engine types to choose from.

Actually, I predict that electric power is going to become more popular than internal combustion.  The technology has improved to the point of having lightweight batteries that provide strong power for reasonable flight times.  Electric planes have power comparable to glow to include extreme speed planes.

In coming years, the price for what is now considered to be high-end equipment should come down to where it more affordable.  The quieter propulsion systems, comparable performance and lack of exhaust mess all over the airframe should make electric much more enticing.  In fact, in some places, E-power may be mandatory in order to keep flying fields.

If you are a beginner and you plan to fly a gas-powered trainer, then my recommendation is a two-cycle glow engine for your first plane because almost anyone at your local club will be able to assist you with it.  However, you may want to go straight to electric power.  That way you will not have spent money on support equipment for glow and then have to go back and purchase the equipment for electrics.

Engine Types

• Two-Cycle Glow

  2-Cycle engines are by far the most popular type of model aircraft engine.  They are simple, inexpensive, powerful and reliable.  These engines are capable of turning very high RPM and are the engine of choice for propeller driven speed aircraft.  Ducted fan aircraft depend on high fan RPM to produce their thrust and only a two-cycle can produce these kinds of RPM.

  Also see

  • Calculating Speed and Propeller Efficiency

• Four-Cycle Glow

  4-Cycle engines started becoming popular in the 1980's and have become well-established in the R/C community.  They were originally billed as sounding realistic (which they don't).  They don't sound anything like full scale aircraft engines, but they are much more pleasant to listen to.

  Four-strokes have more torque than two-stroke engines while turning lower RPM.  That means they are better for aircraft that require strong vertical climbs and more consistent airspeeds.  Four cycle engines are as reliable as two-cycle engines but are more complicated with more moving parts.

  Lastly, four-stroke engines are more fuel efficient than two strokes.  A given size 4-stroke will run much longer on a tank of fuel than the same size 2-stroke.

  For many of our applications, a two-cycle is used when a four-cycle would be a better choice.  In fact, it is my opinion that with the exception of models that need high rpm and trainers, a four-stroke is always a better choice.  However, a comparably sized four-cycle engine is more expensive.

• Two-Cycle Gas

  When large aircraft first started appearing at flying fields they were powered by converted chain saw and weed-eater engines.  Now there are a several manufacturers building gas engines specifically for R/C aircraft.  The fuel for these engines is much less expensive than glow fuel which (in theory) helps offset the cost of flying larger models.

  Personally, I think that gas engines are the most realistic sounding engines other than turbines.  They have that deep, throaty growl that reminds me the old Stearmans.

  While any engine can inflict serious bodily harm, these engines in particular can remove limbs and should be treated with extreme caution.

• Diesel

  You could fly model aircraft for your entire lifetime and never see an aircraft with a diesel engine at your field.  For whatever reason they just are not popular.  I have no practical experience with these engines, but they sound like they have a lot of advantages over other types.

  For example, they are capable of swinging larger propellers which would be of great benefit on scale models of early types such as WWI aircraft.  Additionally, they do not require the use of a glow driver and battery.

  I would really like to use a diesel on a small Rubber to R/C conversion.  However, the only available choices are far more expensive than a comparable two-cycle and there usually is not anyone around who can provide guidance to those of us who have no experience with these engines.  Nevertheless, I am going to have one at some point in the not too distant future.  I'll let you know what I learn.

• Wankel

  As far as I know there is only one Wankel engine produced for model aircraft.  That is the .30 Wankel manufactured by O.S. Engines.  I have one and was pleased with its performance in every area except fuel consumption.  It burns fuel so inefficiently that I have decided it is strictly a novelty engine and I display mine in a glass case.

  Update: Actually I really like this engine and it is in regular service.

  Also see

  • Photo of Wankel mounted to My Stik 30

  • Inner workings of OS Wankel

• Gas Turbine

  A true turbine engine in a miniature size.  These engines are very powerful and sound exactly like their full-size counterparts.  Although very expensive, they are becoming more popular among those who like fast aircraft or build scale models of jet aircraft.

• Pulse Jet

  These engines are very powerful and loud.  They are generally only used for speed models as they have no way to throttle them.  Additionally, they can be very dangerous and are outlawed by most R/C clubs.

  PulseJetEngines.com has a lot of information about pulse jets as well as plans, videos, a forum and other information.  I know jack about pulse jets so I can't say if the information is good quality but they seem to know what they're talking about.

Many multi-cylinder piston engines are being manufactured. They are available as horizontally opposed , Inline, V and radials.  Generally these engines have less vibration than a comparably sized single, but they also have less power for the same total displacement.

Also see

• Beautiful Radial Engines

Selecting the "Right" Engine for Your Model Aircraft

Normally I choose the engine at the top of the range recommended by the manufacturer.  For example, if the kit suggests an engine from .25 - .40 then I will use a .40 or .45.  The only real exception to this are 1/2A kits that I will normally put a .10 or a .15 in if I think the model can handle it.

Because a comparably sized 4-stroke engine is less powerful than a 2-stroke, the rule of thumb is to use a 2-stroke of about 2/3 to 3/4 the size of a 4-stroke.  For example, if a plane is designed for a .45 2-stroke, a .60 4 stroke can be substituted.  If a plane is designed for a .90 4-stroke, then a .60 2-stroke can be substituted.  4-strokes are more fuel efficient so if you make a swap like this, the tank size can remain the same.

I also buy name brand engines.  They're all good.  Aircraft engines are like cars.  Some people swear by Ford and others by Chevy, but they all work the same.  What I don't buy is store-brand engines or clones.  The reason for that is I don't know who actually made them and they all look suspiciously like O.S. engines.

What that means to me is that O.S. spent the money to develop the engine and then someone else bought one, took it apart and measured it and made a clone.  For them, the development cost is a fraction of what OS paid and I just don't think it is ethical and it might even be illegal.  Regardless, I just don't buy them, but that is my personal attitude and you can take it for what it is worth.

I generally don't use tuned mufflers — commonly called tuned pipes which are closely related, but different.  The cost of a tuned system plus the engine is generally same as or more than the cost of the next size engine.  I don't see the point in adding the headaches and complexity of a tuned system to an aircraft when I can just buy the next size engine instead.  If I already have the engine, then I may purchase a pipe for it — especially if a larger engine will not fit the airframe.

Exceptions to my philosophy might be a competition event where engine size is limited.  Another exception might be if you are building a speed-demon.  Speed comes directly from RPM so a larger engine is not going to get a lot more speed, but it will give a better climb and more power for hovering, etc.  But if you want to go fast, there is no substitute for RPM — exactly what a tuned system is designed for.

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Copyright © 2003 Paul K. Johnson