Okay this is a question for an answer I should know - but I don't so here goes.

Let's say you have started up your engine without any hassles. Is it possible for you glow plug to die in flight? Does your glow plug have any major impact on your engine in flight. I know a glow plug gives that initial spark for the engine to fire. But after that...

Glow plugs have a big impact on your engine's performance when starting and during flight. In fact, it's like the "heart". A bad glow plug equals bad or no performance at all. And yes I've had my airplane just shut off in mid air due to a bad glow plug. Even without on-board glow, they stay ignited the entire time during flight. So to answer your question glow plugs has everything to do with the overall performance of your engine.

A lot of people think the glow plugs in model engines are just like the ones used in large Diesels engines (i.e. trucks). They are not the same. In a Diesel, the glow plug is energized when the engine is cold to pre-heat the combustion chamber for easy starting. After that, it is shut off and plays no role. In a glow model engine, things are different. There is generally a coating of platinum on the heating coil. The battery gets the coil hot initially for starting but the coils stays glowing after that. The platinum acts like a catalyst, just like it does in the catalytic converter on your car. This promotes fuel burn which creates heat and keeps the coil glowing red hot the entire time that the engine is running. This makes a model engine a "glow ignition" engine rather than "compression ignition" (Diesel) or "spark cognition" (gasoline).

So to answer your question, yes the glow plug plays a huge role while in flight. They can fail in two ways:

The first is a burnt open coil. You usually discover this when connecting the battery and trying to start the engine. The coil just won't glow and the engine won't start.

The second is a degraded or contaminated coil. There is still a metal coil there so it will glow with the battery connected and the engine may start. As soon as the battery is gone though, it can't sustain the catalytic reaction very well. This results in misfires and poor running just like a bad spark plug in your car would. You may very well notice this in flight as a really noticeable problem or maybe the engine just doesn't seem "right".

In smaller engines that use glow heads instead of plugs the coil can get coated with different fuel additives and look like they are glowing fine but wont stay ignited after the ignitor is removed, it can get confusing to find.....only seen one glow head die mid air and that was in a race....Rog

How long do glow plugs usually last for? I've got an engine that hasn't been run in over 10 years, but it only had about two flights on it since new. Will the plug be shot?

How long they last depends on the engine they are in, fuel used, plug quality and so-on. I've had them last a couple years of use in some 4-strokes. If yours didn't get contaminated with something, there is a good chance it will work even after sitting that long.

Well you really won't know until you fire it up I've flown a single glow plug in an engine through almost 5 gallons of fuel. This was over the course of 2 flying seasons (2 years). Versus some glow plugs didn't even last a few tanks. But you say it's been sitting 10 years? It's hard to say, because it will depend on the brand and type of glow plug, as well as the engine and fuel type. I typically replace a glow plug whether or not it's bad after a couple gallons of fuel. Because if my engine is running well and suddenly it starts running crappy, I'd usually replace the glow plug before I start re-adjusting stuff. Because 80% of time that's what it is. So I guess the worse thing in your case if it's gone bad, you'll have to pay a whole 5-7 bucks to replace it [8D]

They can last for years or seconds. Most of mine are years old and just fine. When I pull an old engine out that was sitting for a long time I have had them go pop as soon as I put the juice on them. You can have nasty old castor that has gone hard in the coil and it will burn out. A number of things can happen. All you can really do is try it and see what happens. I keep a lot of spares on hand in my flight box, they are just one of thos must have items. Don't need them often but if you do need one and don't have any it can end your day fast.

When I have a problem with my engine, one of the first things I do is change the glow plug. I do this because it's easy and relatively cheap. I then throw the old plug into a medicine jar. To date I have accumulated half a dozen of various sizes, temperatures and condition. Is there any way to test these old plugs or should I just pitch them?

you can hook them to a battery and see if they glow or not but that only tests one aspect. The only way to completely test them is to install in an engine and see how it runs.

Also I believe the number/type of glow plug fitted to a engine, hot or cold, will determine the ignition timing on a engine when running.</p>

Glow plugs can be made many different ways. Platinum an Rhodium are used in the very best. And yes heat range has a significant impact on the performance. You can pick up several hundred rpm with the right plug and fuel combo.
Also the right plug will affect your idle and transition, hotter is usually better if you have an engine loading up on you. Idle bar plugs work by keeping the splash of the intake charge off the element while it is running at low speed to keep it hotter for when it is compressed enough to fire.
Ihave had a Merlin 2003 HD plug last 2 years of hard use in a variety of engines.

OK I am learning. something I never gave a thought to .<div>
</div><div>What fuel / plug combo can I put in a   O. S. LA 65  to bring it to life.</div>

That really boils down to experimentation. OS has some good info on their site:

http://osengines.com/accys/glowplugs.html

http://osengines.com/accys/choosing-glowplugs.html

Some additional info here:

http://www.scootworks.com/rdrc/gloplugs.html

Lot of good choices on the market, haven't heard of anyone going wrong with the OS #8 plug in that engine. I only burn Power Master 15%, had good luck with there 10% also. Between 10% and 15% you won't see much difference in your LA. I run a lot of different engines, most the time OS and YS four strokes so the 15% just works for me, anything from 1/2A to the 1.40 YS, This way I only have to stock one type of fuel. You may have an LHS that sells another brand. Over time just try the different fuels and see what one you think runs best in your engines.

if have access to methanol (have any race car friends?) you can mix your own fuel for about 7 bucks a gallon useing castor oil from sigmfg.com. no nitro though, although it really doesnt matter for the la and most other engines

Thanx for the info people. Learned a lot now. This question popped into my head the other day when I was going over my plane. It has stood for 2 months now, since our weather was either too hot to fly or the wind was crappy. Well tomorrow looks better so we are heading out very early. Anyway, so I was checking everything again hinges, nuts, servos, starting up the engine etc etc. When I took the glow plug out and hooked it up to the driver to see how the glow is on the plug, I noticed that the top part (first or so twist in the coil) wasn't glowing. I suspect I need to replace the plug or soon?

It may not necessarily need to be replaced. The ultimate test is how easy it starts and how well it runs in the air.

Just don't suspect, test instead.

Besides ChuckW's advice, mixture on the rich end keeps plugs healthy, while mixture on the lean end kills them fast.

Listen for a noticeable drop in the iddle rpm's of the engine when you retire the ignitor; that's a sure sign of plug going bad.

Some glow plugs (like the OS #8) don't glow on the last complete circle of the plug wire. Typically when plugs die they don't glow at all. You'll just have to run the plug and see what happens.

Significant rpm drop when plug ignitor is removed (when engine isn't tuned excessively rich obviously) is also an indication that your plug temperature range is too cold. Move to a hotter plug if it does this on a new plug.

It technically could mean its too hot (detonating), or fuel nitro % is too high, compresion is too high from removing too many shims, etc as well, but those are quite unlikely if you haven't modified the engine and are using proper % fuel, not to mention you'd likely be able to hear pinging.

So I take it then that there is no good way to test the condition of a glow plug on the bench. IOW you need to run it in the engine. (That;s the engineer in me coming out. We like to have validated test procedures).

Hi!
That's correct!
You only see what a plug is good for running the engine.
But!
All sport engines (nearly all engines on the market) work good on OS 8 or Enya 3 glow plugs (except of course all four strokes that should run OS F , same as Y&S plugs).
These are the really good "all time favorites" working with 0-15% nitro fuel.
Using higher nitro contents...or rising the compression, demands a colder plug. Only experimentation can tell you what works best for what plane and prop combination.

An example:
In Q-500 pylon here in Sweden where we use sport .40 engines like Webra GT .40 , OS FX.40 and TT .40 PRO SE, and others in combination with 10x6 props and 80/20 fuel (No nitro) the Nova Rossi 4 or Rossi 3 plug t is slightly superior to the OS 8 and Enya 3 (gives 100-300 more rpm).Only when you race ,at top speed you need that ultimate rpm. Otherwise rely on the "all time favorites"!

Right. And as alluded to while RPM drops point to a problem, this alone may not mean anything is wrong with the plug at all!

e.g. As suggested it may be the wrong temperature plug for a situation, the engine may be too rich, fuel flow may be changing too much, etc.

Normally I tune the engine, and disconnect the ignitor. If the engine slows down, I go leaner until removing the ignitor seems to have no effect.

- IF - no matter what I do I cannot seem to get to this point, or getting to this point results in unreliable operation, only then I may start looking at the plug itself.... and even then, the plug may simply be the wrong type or temp for the particular situation.