Even apparently simple things, like starting an engine, often assume a good bit of inter-related background knowledge. Of course, the problem for the newcomer is acquiring that know-how quickly. So, I want to look at the underlying fundamentals one by one and then go though a typical and practical start-up procedure step-by-step. Buckle up tight in the back now!
We’ll be starting a standard sport glow engine in the modelling equivalent of the missionary position. That is, sited in an orthodox, upright, ‘hanging out in the breeze’, uncowled installation. This is the sort of set-up you might find in a traditional sportster or ARTF trainer. For this article, we’ll use an O.S. 15FP glow engine (albeit with an Enya silencer), one of the sweetest motors known to mankind. The beauty of choosing this engine is that its scaleable design means that all its bigger sisters have identical starting and running requirements.
To run satisfactorily, a glow engine needs the following:
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- Correct fuel
- Correct tank and pressurisation
- Correct plumbing
- Correct propeller
- Correct glow plug
- Correct tank height
- Correct needle valve setting
Intimidated by that lot? Don’t be. In a moment we’ll work through this list one by one, so that you can safely eliminate any bloopers before you start. Once you’ve got the hang of starting your engine, you can progress to other techniques, fuels, plugs and props.
MODEL AND ENGINE MATCH
Getting an engine to run is often about the relationship of the model to the engine. Most of the basic conditions above will normally be met automatically in the layout of a good sport model. A good model will have its tank height set so that, half full, the fuel head will automatically be level with the fuel intake to the engine’s carburettor. It will also have easily accessible tubing runs for filling and venting the tank, and for sending pressure back to the tank from the exhaust nipple on the silencer.
For a beginner with a .15 glow engine I would recommend a commercial fuel with a minimum of ten percent nitromethane. I use Model Technics 10% nitro, Optifuel 10% nitro and my own budget favourite, Southern Modelcraft 10% nitro. These medium-nitro fuels will put a few starting factors in your favour as a beginner, though five percent nitro from such suppliers, would also suffice, if that is all you have.
I’d avoid straight fuel (no nitro) until you know what you’re doing. Most glow engines, except the very small ones, will run on straight fuel but a bit of nitro puts the odds in your favour. First of all, the fuel should be fresh. That is, bought recently, say this season, and always kept in an airtight container. This is because fuel is hygroscopic and can absorb water if not correctly stoppered. Diluted fuel is not what you want. Fuel needs to be kept clean and a clunk-type sintered brass filter in the delivery line, inside your fuel bottle, is the very minimum requirement. These days with much improved synthetics, I’m happy to use castor or synthetic oil based fuels.
Tank issues account for many mysterious problems with glow engines. I would recommend a commercial SLEC tank (or similar square-section design) since they seat correctly and cannot easily move to choke off the fuel in flight, as a cylindrical tank can. The SLEC tank design also has a slanted front, which stops the fuel, vent and pressure tubes from being crushed against the engine firewall in an abrupt arrival. The length of the flexible feed tube to the clunk weight inside the tank is critical. It should not be too short and it should not be so long as to touch the rear of the tank. Before filling, it’s a good habit to hold the model upright and give it a firm shake. This shock should untangle the clunk weight.
There are one or two fundamentals to absorb for most sport engine installations. You can’t pump fuel into a solid walled tank unless you’re simultaneously letting trapped air out. Similarly, you cannot suck fuel from a tank if air cannot get in at the same time. So, you need to vent the tank to air (to atmospheric pressure) as you pump in fuel. This is part of a larger process explained fully below. Transparent silicon tubing is great stuff for plumbing your fuel installation, and you should always take a spare length with you to the field. Silicon tubing lets you observe if fuel is present in a line. However, watch out for nicks, tears and those internal blockages from almost invisible slivers of translucent tubing and diaphanous grass seed husks. Also, colour coding your feed and pressure lines makes good sense: one colour for fill, one for vent, one for pressure etcetera.
TWO OR THREE LINE?
Essentially, you have two main tank plumbing choices:
1. The three line solution, where you have a tank with three tubes:
a.) One tube to fill the tank with fuel.
b.) One tube to bring pressure back from the silencer pressure nipple to the tank.
c.) One tube to feed fuel from the tank to the engine’s carburettor fuel intake.
Just note this: as you fill the tank, the air inside will be pushed out, and thus vent to the atmosphere, via your silencer pressure line (b). Once you’ve filled the tank and before you start the engine, you’ll need to be able to seal off the ‘fill’ tube (a) with a bung. This ensures that, when running, slight pressure from the silencer actually pressurises the tank and does not bleed away, as it would if there were no bung in place.
2. The two line solution. In this case, you only have a fuel feed line from the tank to the carburettor, along with a second pressure line from the silencer to the tank. You fill the tank by slipping the fuel tubing off the carburettor inlet barb, and push your fuel bottle nozzle into that line. As you fill with fuel, the displaced air leaves the tank via the silencer. Since the engine is not running, the silencer is still at atmospheric pressure and it all works very well. Mind you, it’s best to tilt the engine back as you fill, so that any overspill can be seen exiting the silencer, and not just filling it up!
This two line solution neatly deals with one of the main drawbacks of the three line solution, i.e. when filling the tank you’re inevitably pumping fuel into the engine supply line as well as the tank and could be flooding the carburettor. The simple solution with a three line set-up is to gently nip off the carburettor feed line with haemostat scissors, or a simple clip, whilst filling. This will stop fuel from flooding the carb. Once full, you remove this fuel line constrictor before starting the engine. The downside of the two line solution is that you need ready assess to the carburettor feed line for re-fuelling, and a means to temporarily stop it off. This can be tricky in some cowled models.
Some authorities claim that pressurising a tank is unnecessary. All I will say is that this has not been my experience. I find that a pressurised tank delivers a full tank run and steadier tune.
BEWARE! LOOSE SILENCER
A less obvious gremlin, often missed, is a silencer which is blowing slightly and not tightly bolted to the cylinder head. This can lead to poor starting and erratic running, because the expected pressurisation for your chosen needle valve setting is incorrect. It would certainly explain an engine that mysteriously needs constant re-tuning.
NEEDLE VALVE SETTING
A glow motor needs the correctly rationed amount of fuel to start. The needle valve on the engine meters the fuel. Strictly, such fuel is actually a mixture of atomised liquid glow fuel plus air, and not just the liquid glow fuel itself. The needle valve atomises the fuel into an aerosol. Until it is set up correctly, a glow engine may need a slightly different needle valve setting for starting, compared to that for efficient running. In other words, a glow engine needs a needle valve setting that produces a starting fuel mix with relatively more liquid fuel in it to start, than to run. This is referred to as a rich setting – rich in liquid fuel and roughly analogous to the choking of a car engine, which briefly enrichens the fuel mixture with relatively more petrol to start the engine.
Once it has started under these rich conditions, a glow engine will need a different fuel mixture to lean out and achieve its maximum power for flight. We lean out the mixture for maximum revs manually by turning the needle valve in (clockwise) from its start setting.
As we slowly turn in the needle click-by-click we’re rewarded by an increasing engine note. As we carry on clicking and pausing to hear the effect, we will soon notice that the engine will cross over from a rich, slower, lower pitched, ‘four-stroking’ setting, to a clearer, higher note. It miraculously breaks into that marvellous free-revving region, where a two-stroke engine liberates itself, and becomes fully leaned out. Aaaah! You can keep yer Pavarotti. Gimme a Japanese two-stroke fully on song, any day!
Now, if we had been tracking the revs against pitch, we would have seen the curve climb steadily. However, soon the engine will hit its sweet spot, where no more revs are available. If we carry on turning the needle valve in, then the revs will fall off a cliff and the engine will start to slow down dramatically. The remedy is to re-open up, click by click, until the maximum revs are re-achieved. Then, we do something else. We back off the needle valve one or two further clicks. This will slightly enrichen the mixture again and ensure that the engine doesn’t suffer the dreaded lean run when the model is in the air.
Lean Run? When a model is flying properly, the load is reduced on the engine and the revs set on the ground, can quickly be exceeded. This can mean that the engine is now straining, and perhaps running too hot whilst running on less fuel. Since fuel brings with it lubrication and cooling, as well as power, we don’t want a lean run.
Left to over-rev, an engine may damage itself and even seize, so a lean run should always be avoided. Mind you, there is a simple check we can make on the ground. Once started and tuned for best flying revs, we can put the nose of the model upwards, say at about 80-degrees, and then adjust the needle valve for maximum revs before backing it off two or three clicks. If we get it right, the engine will continue to pull strongly with its nose in the air and will not lean out, falter, or fail to pull through enough fuel.
Of course, this top-end tweaking depends on first finding the correct start setting. With a new engine, your first port of call should be the manufacturer’s handbook. This will give you the datum for the correct start setting. Most of my engines will start at about two-and-a-half turns open on the needle valve. This just means two and a half turns anti-clockwise from fully screwed in.
So, to get the start position, just turn in the needle valve gently until it stops and is fully closed, then open two and half full turns anti-clockwise from there. This will be your start datum. Once you get to know your engine, and its settings, you’ll be able to fuel up and fly it at its running position, without touching the needle valve between flights.
CORRECT GLOW PLUG
The first requirement for a correct glow is to fit the correct glow plug. If I start a sport .15-size engine then I unhesitatingly choose an O.S. F plug, or a Taylor’s LR (long reach) sport plug with idle bar. The Enya No.3 is also an excellent choice, though the big pommel on the plug terminal may not work reliably with some glow starts. These three have stood the test of time although please note that, a four-stroke engine will usually need a dedicated four-stroke plug.
Now, it’s almost embarrassing to say, but a good glow needs lots of electrical juice. I would never sally forth without at least two fully charged glow starts, and a 12V flight panel in my flight box to back ‘em up. By the way, that friendly ammeter on your glow panel is a superb diagnostic aid. It can show a dead short, a flat battery, a blown plug, or a poorly connected plug at a glance.
You want between 2 and 4 amps on that meter and a warm orange glow if you check the plug. Yellow / white is far too much, dull red is too little. Also, glow starts with meters are preferable to those without. Check your glow clip for accidental shorts, and don’t just drop it into damp grass. You should only use thick, flexible copper wire, as short as practical, for your glow clip leads and remember to route your glow lead safely away from the prop. Note also that a faulty plug, even if apparently glowing, may sometimes cause erratic starting and running. The quick answer is to substitute a new one.
This looks easy. Choose a prop of the correct pitch and diameter as recommended by the manufacturer. That’s all well and good, however, certain brands of prop perform better than others of the same nominal size and pitch. Also, although it is less the case with modern engines, choosing a heavier prop can work wonders on worn engines with poor compression as it will increase the flywheel effect. Note, also, that the manufacturer will often recommend larger diameter running in props, so it pays to follow those instructions with a new engine.
Right then, just a few quickies before we leave props:
- Don’t use an electric propeller on a glow engine, and vice versa.
- Make sure both your prop and your spinner are balanced. Discard any prop with chips or dings.
- Avoid wooden propellers whilst you’re learning.
- Thicker bossed, sturdier, composite props tend to distort less with full revs and often deliver better power.
PERFECT YOUR FLICK
Make sure you’re flicking anti-clockwise and make sure that when the engine is coming up to compression (it gets harder to turn the prop is coming around to about one o’clock. (See photo). This means you’re not wasting effort and the full effect of your leverage will drive the prop smartly over compression.
Flicking has to be smart and snappy, not wimpy and languorous. Don’t be in a rush. Make sure that each flick is a good one. Practice makes perfect and patience is certainly a virtue. Use a chicken stick or rubber finger stall even when starting a .15 two-stroke.
If using an electric starter, always check first that you can flick the engine over by hand, smoothly. In that way you avoid a hydraulic lock in the engine, i.e. where the up-going piston is vainly trying to compress a liquid (too much raw fuel), which we all know from our schoolboy physics, nature will not allow!
If it is locked then you have too much fuel in the upper cylinder. In small engines, with open ports, you can simply blow it out. Bigger engines with silencers need to be tipped backwards, so that the excess fuel can drain out through the silencer as you carefully tease the engine over to pump out the fuel.
If that will not work, then you’ll need to shut down the needle valve tight, unscrew the glow plug and spin the motor over quickly to expel all the excess fuel from the glow plug hole.
I’ve assumed a modicum of common sense and safety awareness but, needless to say, always, always restrain your model before starting the engine.
You should move round behind the model to remove the glow lead clip and tickle the needle valve just as soon as the engine has started. The annals of model flying are filled with tales of practitioners who decided to tweak the engine settings from in front and misjudged the diameter of the prop arc. In this respect it’s good practice to paint the prop tips.
COP THE CHART
As you can see, there are quite a few interconnected variables when it comes to reliably starting and running an i.c. engine. These can seem daunting to the beginner but really it’s just a case of looking out for the obvious things.
To speed your progress I’ve designed an accompanying Glow Engine Mind Map fault finding chart which you can find on RCM&E’s website at www.modelflying.co.uk. It attempts to link symptoms to faults and then suggests possible solutions. The scope extends a little beyond this article yet it’s not exhaustive.
Take your time with your engines, if you get a problem then the cause is nearly always far simpler than you think.
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