RCM&E Nov 09

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Converting Large glows to spark ignition by Brian Winch

Repent ye sinner!


Oops... wrong call. I was thinking of 'repentance and conversion' but I should have been thinking simply 'convert'... let's start again.


There, that's better. I really wasn't on a religious crusade with that 'repent ye sinner' heading (perish the thought - Sundays are for flying models) - I was thinking about converting glow ignition engines to spark ignition. I've had a considerable number of enquiries about this recently from Australian and UK modellers, and Just Engines have sent me a very nice spark conversion unit for testing. Must be an international collusion - give Winch the spark!

Saito have had their ear to the ground as (at time of writing) they have three very successful four-stroke petrol engines on offer. I've tested all three, and each one is a winner in its capacity range. There's certainly a drift towards using petrol for larger engines - it really is a bit of a hot topic at the moment - and I'd say that there are a number of reasons why. First is the question of fuel: glow engines of 20cc capacity and more need big fuel tanks - really big - to satisfy their thirst. This can be quite expensive, even when mixing your own fuels; and feeding a 35cc glow engine with ready-made fuel can do real damage to your wallet! You can rapidly deplete a 1 litre tank with a single-cylinder, large capacity engine; add more examples of the same engine in a multi installation and you'd think your tank had a large hole in the bottom! Imagine a large model with four x 30cc glow engines. Back-up the fuel tanker and melt the credit card - we're getting into jet turbine engine consumption figures here! Petrol engines, on the other hand, are far more economical. For example, I recorded 59.75 seconds on just 15ml of fuel with a 26cc J'EN petrol engine running at 8,800rpm on a 16 x 6" propeller.

Another factor is reliability. Generally speaking, glow engines are very reliable, but not as good as petrol engines. With an adequate battery power supply, well filtered fuel and an occasional plug check, the petrol engine is just about as reliable as the car in which you carry yourself and your model. Want a few more reasons? Well, the fuel tank position isn't critical - it can be placed wherever's convenient in the model, as the majority of petrol engines have a pump built into the carburettor. And once the engine's tuned there's no variation in tuning or rpm, from full tank right down to the last drop of vapour. You can forget internal corrosion from fuel and by-products of consumption left in the engine, too, as the fuel / oil mix is actually very good at preventing corrosion; unless you're going to store the engine for a very long time, after-run storage oil isn't so necessary (provided you use a high grade fuel oil that has corrosion inhibitors included).

Starting can be a lot easier than a glow engine - particularly in very cold weather. Due to the controlled spark the engine will fire up at a much lower rpm than is necessary for a glow engine, and petrol vaporises at a much lower temperature than methanol. Vaporisation is required in all engines, and more so in a two-stroke in order to get a fuel / gas charge up the transfer ports from the crankcase to the combustion chamber. Actually, about 10% petrol added to glow fuel is a great aid for starting in very cold weather. It vaporises, the engine fires it, and the glow fuel is more inclined to move up as the engine warms up.


Whilst we're talking about petrol fuel, let's consider the fuel / oil mix - often referred to as petroil - and the one great advantage it has over methanol and oil. Petrol has reasonable lubricating properties, whereas methanol has virtually none; as such, the higher lubrication value of petrol means that much less oil is needed in the fuel. Less oil, less mess and more power from the fuel mix. In fact there's no oily fuel residue on a model when using a petrol engine - a few black dots here and there at worst, and these rub off very easily.

Props and Starting

Looking at all three engine types - diesel, glow and spark - there's a consideration as regards the size of propeller used. For a diesel, in my opinion the only caution required is that a small propeller - small for the engine capacity - must not be used, purely for the sake of your fingers. A diesel with a small prop is a bitey little bas... er... beast, and it'll get you with a whack that will make the lightning-fast paw swipe from an angry cat seem like slow motion - and the diesel will hurt a lot more. However, if you persist, the engine will run and it will rev with a banshee howl. On the other side of the coin you can fit incredibly oversized propellers and the engine will still run quite well, albeit at reduced rpm. The reason you can do this with a diesel is that the compression is adjustable, and that adjustment alters the timing to suit the load on the engine. The load on an engine is that which stops the engine from increasing the rpm to the point of destruction, unless the timing has a cut-off point by design for the rpm (very simple explanation). If you fit a flywheel to a model engine it will run probably twice as fast as it does with the default propeller. A flywheel is a diminishing load - it requires power to develop momentum then it becomes a negative load - or, if it's large, an assistance - to the continued running of the engine. As an example of the difference a flywheel makes to the load factor of an engine, Saito produced a modified .30 aircraft engine for car use, and it was capable of 22,000rpm. The engine was modified, and I was keen to see what modifications had been done to obtain the high rpm without the engine vaporising itself, so to speak. I eventually got hold of one for examination and found that the modification consisted of nothing more than a change to the crankshaft for fitting a flywheel, and a different carburettor to suit model car use. Other than that, the engine was a stock-standard aircraft unit. Just for interest I fitted a similar flywheel to my aircraft version of the Saito 30 and, using a blower to keep it cool, I ran it for the same rpm. The propeller load was almost halving the speed potential of the engine.

A glow ignition engine will run on a diminished load such as a small propeller, and the rpm will increase considerably to the point of inefficiency - unless you're using a propeller designed for extremely high rpm. The best example of this is a pylon racing prop, which is small, light, and has plenty of pitch. The load of the pitch is greatly reduced due to the small diameter - the load is quite a small area. As an example, in dark days of yore when I was keen on control-line speed flying, I used a toothpick-style (very slim and narrow ends) wooden prop of 8" diameter and 12" pitch on a 10cc engine. Those props drew blood every time you hand-started the engine - I still have the scars to prove it!
Okay, off we go to the other end of the scale - a large propeller on a glow engine. But what is classified as a large prop? All model engines have a default size propeller - a propeller size stipulated by the manufacturer that gets the best all-round performance from a given engine. It's not the smallest prop in the range, and is usually listed as the propeller size that gives the rpm and power stated in the engine specifications. An example (from way back) is what I call the '10 x 6' syndrome. The .40 was the most common engine size from the mid 60's into the early '80s - and it's still popular today. The (almost mandatory) recommended prop for that engine was a 10 x 6", and all .40 engines were judged on that prop. Problem was that the 10 x 6" was seen to be 'the' size for any '40' engine between .40 and .49 (the same-size prop for different capacities?) Much the same for the (then) popular .60 - the unwritten rule was that it had to be run on an 11 x 7" prop - but that was the start of the education.

In the swing

Modellers started experimenting with propellers and discovered that engines would indeed swing different sizes. This 'coming of age' was greatly assisted by the introduction of mass-produced plastic (composite) propellers that were both inexpensive and reasonably tough - they didn't break so readily as the common wood props.
But with this experimentation came a little problem. Due to the fixed timing of a glow engine, if the load was increased too much the engine would pre-ignite... ping... with the result that internal components broke, engines would overheat or, worst of all, propellers would fly off. When this happened there was a fair chance that the engine would 'shaft run' - a dreadful sound. Having no load (or air cooling) whatsoever, the engine would continue running ever-faster until it disintegrated internally and often externally, as inside bits forced their way out of the super hot 'oven'. Added to this catastrophe was the safety factor. If an engine was overloaded it could kick the propeller off during the starting procedure, and human flesh was often the target. This is not only the past - it's still happening but, fortunately, less frequently.

Modellers with a bit of engine experience will have a good knowledge of the default propeller size for the range of engines they use. I have a mental listing of the most common default props, and I use this as a guide when repairing engines or providing information to other modellers. For example, a .90 four-stroke should swing a common style 14 x 6" prop between 9,500 and 10,000rpm, whilst a 1.20 four-stoke will do around the same with a 15 x 8" prop, and so on.


If you listen and observe you can tell when an engine is approaching its stress point (propeller load too big) and that's the warning. Due to the (almost static) ignition timing of a glow engine the warning signs are pinging, overheating and reduced response to tuning the high end. Another warning that you sometimes hear is a sound like cold, wet food dropped into a fry pan that has hot oil in it. This is a type of pinging, which you can usually remedy by fitting a slightly colder plug - retarding the ignition a little. Other than changing the plug, to alter the timing of a glow engine you need to reduce the compression ratio, and this is done by fitting head shims. The higher you go, within reason, the large the propeller you can fit. Fitting an extra washer under the plug also helps; this sets the plug up a bit higher, which slightly retards the timing and does a little bit towards reducing the compression ratio. Believe me, there's not much free space in a (say) 20cc engine combustion chamber, so the protrusion of the plug has to be counted as a space reducer, and reduced space is advanced ignition due to increased compression.

To give you an example of the difference a prop load has on an engine, I'll quote some comparative figures from my testing reviews. When testing the RCV 130, I detected distress when using a 17 x 8" propeller; my review listed the maximum propeller as a 17 x 6" at 8,000rpm. If a larger propeller was required then the load would have to be reduced, achieved by reducing the pitch. As such the engine would probably swing an 18 x 5" (narrow blade) or an 18 x 4" without showing signs of distress. When testing the Saito 20cc petrol engine I used a 17 x 6" to record a figure of 8,201rpm, and no sign of distress whatsoever. A larger propeller could certainly be used but then we get into gyroscopic loads and overloaded crankshaft rear bearings. The reason the large propeller was no load on the Saito was due to the variable timing provided by the CDI unit. As a point of interest, while we're on variable timing, when testing the Enya .41 I used a 16 x 6" propeller to record 5,700rpm, and the engine ran as sweet as a well-oiled clock. Obviously an engine of this type wouldn't be used for hot-dog type flying with rapid and numerous changes of direction; with a 16" prop it would be in an 'old timer' type model, flying sedately and reasonably easy on directional changes, so the gyroscopic load wouldn't be a great problem.

Obviously the above is a very simplified coverage of load factors on engines, but it will give you a lead-in when considering the propellers to use for various model applications.

Just Engines Online | Ignition Systems | Magazine Articles and Reviews of our Ignition Systems |  RCM&E Nov 09