What glow plugs

[Mack]

Hi All
Can someone let me know what type of glow plug I have in my heli. The motor is an Irvine36 MK11. How long do these plugs last(in flying time)?
Thanks

[Dedcalm]

I have no way of telling what glow plug you currently have but there are 2 main types used in heli's. OS Number 8 or Enya Number 3. Personally I have been running OS8's in my OS32 and OS50 Hyper since new and have no problems.

As for how long they last, well the one in my OS32 has been in there for over 1 season now. In fact I dont remember when I last changed it! Personnaly I cant fault the OS8. I use Enya 3's in my MDS engined planes but thats another story.

Rob

[Raptorite]

Hi Mack,

As Rob says, without seeing it there is no way of telling what sort of plug is currently in your engine. However, the OS and Enya plugs are slighty different, so you will be able to tell just by looking at it which one it is. The Enya has a fat bodied top, where the glow lead connects, whereas OS's have a thin top, probably only about 2mm in diameter.

If you take the plug out, it should have the brand and type stamped on the side so you can see what number it is.

Most people run with a medium temperature plug with mid range nitro content fuels, somewhere between 10 and 20% is the norm. In this case the most popular choice is either an Enya 3 or an OS No 8.

These plugs tend to last for around 2 to 3 gallons worth of flying time, some fuel manufacturers actually recommend changing the plug after 3 gallons as a matter of course. It also depends on how much and how you run the engine, if for example at the end of a flight you simply let the model drain the tank dry, then the life of the plug will be shortened as it is likely to overheat and burn up the coil when the motor goes lean and dies. Similarly, if you flood the engine or leave the model for long periods at home with the fuel line connected to the carb and open, the nitro will evaporate from the residue in the engine leaving behind a gloopy oily substance, which has a habit of sitting in the plug round the coil. This will also burn the plug out next time you connect the glow to it to start the engine. So you see there are all kinds of reasons why some plugs last longer than others.

The bottom line is if you hear the engine miss or cough slightly in flight, or it seems not to be as powerful as it was, chances are the plug is getting weak and needs to be changed before the next flight.

Hope this helps.

Cheers,

Pete.

[Dedcalm]

Interesting comments on how to keep the engine after use. Personally I either drain the excess fuel and then run the engine till it dies or just let it run till it stops. This way I keep the Nitro levels left in the engine to a minimum and thus the damage done by it. I have no problems with premature failure of my plugs. Of course it is sensible to use after run oil if you are storing the engine for more than a day or so.

Rob

[Mack]

Thanks Pete
I actually had fuel left in the tank and drained it as soon as I read what you had to say. Am I correct if I say the glow plug lights the fuel while the heli is flying? If this is correct what keeps the plug glowing? :?
I get a small leak(of unused fuel I think) between the exhaust and the motor, there is no gasket between these and there was none supplied, can I make one and fit it? Thanks again for the help. This bug has bitten me bad, I am now looking for my 3rd heli. Thinking about scale, any advice on what to go for guys.

[Dedcalm]

The glow plug has a filament which is what ignites the fuel in the engine. You pass a current (about 3A at 1.2v) through the glow plug to start the engine. The combustion then keeps the element hot thus keeping the engine running.
When idling or low rpm the engine is more likely to stop as the element cools more (more time between ignitions) which is why the latest development is to run an inteligent glow feed which heats the plug when the plug cools too much or below a certain throttle position. I am curently testing my own version of this.
The "leak" is more likely to be oil from the fuel. Aluminium gaskets are available (e.g. from QuickUK) which help to seal the exhaust header to the engine. I dont believe that this leakage effects the running of the engine though, just makes more of a mess especially if your fuel has any caster oil content.

Rob

[Raptorite]

Hi Mack,

Rob is spot on in his description of how the glow plug works, so 10 out of 10 to him for that

If you have a slight leak between the exhaust manifold and the engine port, you may find that the bottom or idle needle is difficult to set for nice crisp throttle response from the bottom.

As two stroke engines, particularly glow engines as we all use in our helis rely upon an amount of fuel residue being bounced back from the baffle in the exhaust into the combustion chamber for ignition along with fresh fuel / air mixture from the carb. Having a leak in the exhaust manifold will mean that some of that residue leaks out instead of being sent back to the combustion chamber. The result is the idle needle needs to be set slightly richer than it really needs to be, in order to compensate. This in turn means that the engine will be quite rich in the mid range and can cause it to feel saggy, inconsistent and slightly down on power in and around the lower to mid range which roughly translates to hovering point and when it is in its unlaoded state while the model descends through 0 degrees pitch.

I would recommend you get hold of some high temperature clear silicone sealant and use that on both sides of the alloy gasket to insure all the mating surfaces are completely sealed. Once you have done that you may well find that you need to lean out the bottom needle slightly which will make the engine run much more consistently.

Cheers,

Pete.

[Dedcalm]

Interesting information on the cylinder to exhaust join and how it can effect the bottom end carb settings.
I have always thought that you pump additional air back in through the exhaust port for tuned pipes (a kind of supercharging I guess). I was not aware that this was also relevant with standard mufflers etc.
Interestingly if you look at engine reviews then I think that engines produce more power with no exhaust at all when compared to the stock muffler. Of course tuned pipes increase power but you need to match the pipe with the rpm required.

Rob

[Raptorite]

Hi Rob,

You are sort of there, with a muffler, as you say, it bounces very little back into the engine, therefore relying almost entirely on the fuel / air mixture from the carb. This is why engines with mufflers tend to be easier to set the carb up correctly and therefore run smoother, with a reasonable amount of power. The other benefit is that the engine will be far more consistent from a full tank to an empty one, so it wont be rich while the tank is full and lean out slightly as the fuel level drops.

A pipe on the other hand is a different animal altogether, tuning the lenght as you have mentioned is actually setting the distance between the first baffle and the combustion chamber, therefore dictating the amount and timing of the unburnt fuel / air mixture being deflected back into the engine. This in turn means that the carb settings can be leaner than they would be with a muffler, meaning the engine will develop more power.

The reason pipes tend to have a very narrow power band is because the optimum amout of fuel / air mixture being ignited is quite different at different RPM's, so pipe tuning must be carried out with a very specific RPM in mind to arrive at the optimum combustion mixture. Also, because the fuel / air mixture being supplied by the carb is far more critical due to the amount being deflected back from the pipe, the slightest change in the consistency of supply from the carb will considerably affect the performance and consistency of the engine run. This is why with a pipe the engine will tend to run rich with a full tank and lean out as the fuel level drops, hence in some cases the need for an artificial pump such as a perry pump to maintain the consistency of the fuel supply to the carb.

Cheers,

Pete.

[Mack]

Hi Pete
I picked my heli up and held it verticaly and got quite alot of oil running out the exhaust(almost as thick as honey. What is this stuff? My fuel is Model Technics Contest 10. It it semi synthetic with 16% castor. Is this all unburnt castor oil?

[Dedcalm]

This is the unburnt oil from the fuel that is settling in the exhaust, it is not just castor but in your case a mixture of castor and synthetic. You still get this when running a fully synthetic fuel.

Rob