One of the weak points of glow engines is the glow plug itself. It is the one thing that can ruin a great stunt ship in an instant. I was wondering how often contest fliers change plugs? I have flown plugs for a few years and they seem ok, no sag when removing the battery and good engine runs. I have also had times when all seemed great and then as soon as I flipped inverted - silence! Luckily, I haven't had one quit like that in an overhead eight or at the top of the wingover.
So the question for the group is what signs other than RPM sag when removing the battery indicate the plug is on its way out? Should we just replace the plug once a year before contest season? Is there a maintenance we can do to keep a plug in useful condition longer (i.e. giving it a voltage shot at say 1.7V from the power panel every few months)?
Depends on fuel. If you use SIG or any armor-all or other silicone in your fuel, you can get down to as little as 20 flights before the taters set it. It is generally a much bigger problem with larger engines. We never noticed any consequential problems until we got to the 61, although you could occasionally see something. Everything I see tells me that at least inside the engine, they run much hotter. We also never routinely melted pipes until we got to the 61, despite the fact that we run them much richer than say, a 40VF.
It also varies a lot from individual engine to individual engine. The first PA61 I used was Bill Fitzgerald's, and it would clearly lose power in about 15 flights, and then start having real problems just running and starting at about 25-30. First time I changed the plug, it had a tater about the size of the cavity in which the element sits. It was always in trouble when I started running it and I couldn't see why everybody thought it was so great, I was having all sorts of problems just getting it to fly the same airplane I was using with a 40VF. Changed the plug and the first flight darn near pulled my arm out of the socket. A later example of the same engine configured exactly the same way would get through about 50-60 flights with no real problem and only a small tater.
For the few times I ran the RO-Jett on SIG, it seemed to be doing about the same, and the plug was tatering up after about 20 flights, although I couldn't feel the power difference yet. Other people tell me they get the same sort of variation from engine to engine, and that the 67LS seems to be worse than the 61. I don't know about the PA75, we have long since stopped running SIG, so I don't know if David has even tried it. My guess is it would be even worse based on the heat theory, but it's so powerful that no on is running it at more than about 20% of capability, so it can degenerate for a while and still be plenty good enough.
The solution, which seems so simple that I am surprised people still resist it, is to use Powermaster. I have never gotten anything like a tater, only a light frosting and maybe a black discoloration of the element right near the weld - which I expect is carbon/varnish that probably appears after the flight due to heat soakback - see Lauri Malila's solution! - using Powermaster of any variety. I have probably a few hundred flights on individual glow plugs and usually change it once a year out of habit to avoid the problem David had at the 96 NATs. Usually second flight on Thursday at the NATs or immediately after so I can at least run a flight or two before Top 20 day.
BTW, there was a theory floating around out there that the "randy goop", AKA Aero-1 additive, was causing this. But I have seen absolutely *no* evidence of this, and I was looking for it. I don't regularly use it any more, but I never had any detectable difference in the tatering whether or not it had Aero-1. It was absolutely traceable to SIG fuel and seemingly associated with Armor-All, but there was no association with Aero-1 at all. I think most of the people with that theory were also running it in - wait for it, SIG - and jumped to a wrong conclusion.
BTW, I am not attempting to "trash" SIG, and I used it for years. Aside from the time of the 1995 NATS (where they clearly had a problem with a batch or two and there were mysterious clear globs of something floating around in it - which recombined if you added about 1/2 ounce of acetone but still ran bad), it always ran fine. But switching to Powermaster completely resolved the tatering problem for me and all the locals.
Back when it was still happening to me, I would frequently scrape them with an Exacto knife until it was shiny, and it worked about as well. I did try burning them off with extra current but I think you would have to wipe them off since they tend to melt.
BTW, beware new plugs, too. ALWAYS test them before trying the wingover, usually I do a few inside round loops, which is usually where it will show a problem. David does inside rounds followed by outside squares, but if it is going to be a problem it's almost always on inside turns. Same thing when you switch fuel. Also, the first run or few minutes may be much different than the rest, usually faster with the same needle, which goes away about halfway through the flight. Go easy on tweaking it slower on the first flight, because it may slow down later.
Brett
Topic: How often do you change glow plugs? (Read 2828 times)