You need to search the forum out about "taters" on glow plugs!! One reason I like idle bar plugs is that it give carbon and other junk, like 'taters" a place to attach that can be cleaned off besides getting into the element. I have seen lots of carbon but never the "taters" that Brett and some others say they see when using certain fuels. They can be scraped off the idle bar also. With the scarcity of glow plugs these days, coupled with the cost, that is kind of important now.
Taters are a real problem. The first time I had heard anything about them was from combat guys, using some sort of weird Ucon synthetic. But we all more-or-less forget about it for years, running 40/46VFs and the smaller PAs. When people starting running PA61s, the power would routinely degenerate over the period of 10's of flights, and when inspected, the glow plugs had blobs of something apparently melted to the element. Replace it (when they were 79 cents a piece) or scrape them, and it would work again, then degenerate. Everyone was running SIG at the time. To solve the issue we started experimenting with fuel, and quickly found that Powermaster (any grade) never developed taters, even after hundreds of flights.
The power loss was absolutely enormous - I first started by replacing my 40VF with a PA61 that Bill Fitzgerald loaned me. First few flights were very disappointing, it clearly flew much better and had more line tension with the VF, and it was not a hard call. I was about to switch back when someone suggested I switch the plug. I took the current one out, and you almost couldn't see the wire, almost the entire cavity was filled up with this tan crap. Replaced and the next flight damn near pull my arm off. I went to some Team Trials with it, and after the first practice session it was back to no power again.
The mechanism is unclear but in all cases, extreme heat was implicated. Combat planes obviously run at full power, but PA61s and above seem to run very hot in places - we never melted any pipes or had taters on 40VFs, but almost everyone who switched to PA61s immediately started having all sorts of heat-related problems. I assume it is due to the size. It is inevitable that smaller engines have larger surface areas per unit volume and 1/2As need high-nitro to even keep running most of the time and run very cool/inefficient. The PA61 is just idling in a stunt plane, but, it also is much harder to get the heat out of them due to the area/volume issue.
The offending material causing the taters is also unknown. Randy tells us that it was the castor oil, but SIG and Powermaster have essentially the same amount of castor and one does it and one does not. For a while, "Aero-1" additive was suspected, but they form with SIG whether you use Aero-1 or not. My best guess is that it is the "anti-foaming agent" in SIG is the culprit. I base this on the fact that people who *add* Armor-All or other silicone-containing things like Rain-X to Powermaster suddenly start getting taters. If you look closely, the tater looks like a black plastic blob on the element, usually starting about halfway between the coil and the weld (and growing from there). If you scrape it, you can scrape off the black and underneath it is a (usually smooth) tan blob that looks like plastic has melted onto the wire.
The theory is that it is getting hot enough to polymerize something in the fuel into a kind of plastic. On the coil itself, it gets hot enough to burn off, next to the body, it's too cool to form the polymer, in-between it has the ideal conditions and forms.
The simple solution is to use Powermaster or some other fuel that doesn't cause the issue. I don't think we have to worry about SIG any more, but I have no idea if the off-brands have the same issue or not.
Brett
Topic: How glow plugs work (Read 4545 times)