You need to have a system if you want to monitor performance based on plug condition. All of the things the guys have mentioned so far are definitely part of a good system.
1. Once you get to the point that you have a specific plug type (Brand and part number) that works well in the engine and plane, you now have a baseline. If you have a lot of engines and different plugs, write down which one is the right one for that engine. A little spiral notebook in your field box is handy.
2. When the baseline changes, such as when it runs poorly in some aspect, and you have not changed anything else, consider the plug.
3. If the engine starts, but drops significant rpm when the battery is removed, change to another plug from the same manufacturer and of the same part number as your baseline. However, don't just fire the engine and yank the clip. Give it a few seconds to build some heat in the engine, and then pull the clip to make that determination. Consistency of operating procedures is what gives you a better idea if something is starting to go wrong.
4. If the engine starts missing in places that make you nervous during a flight, consider the plug. If you are using a different fuel, with different oil package, then expect the plug and heat range to differ. These all work together, so randomly changing one item will impact other things. And makes you chase a "new baseline." Without notes and if doing a lot of experimenting, you get lots of "chasing."
5. Keep a rough idea of how long you have been using the plug (How many runs.)
6. If your engine is going over the hill and making some metal--but you're running it anyway--expect the plug life to be short. And keep another one handy. Fix the engine problem instead of worrying about which plug might work better.
7. If you get a lean run, where it seems like it goes forever, watch out for the plug. It might take that abuse, or it may not.
8. If you are running a "stuntified setup" with a plethora of head gaskets, or a magical "Hemi mod," you are less likely to blow a plug, but the engine also becomes more critical to slight drops in plug performance. You need a hotter plug at the start (fresh battery) and may have problems in cold weather. And compounded with the use of high oil content.
9. Running lots of oil requires the use of a hotter plug. If you run an upright or inverted engine, and find that on sharp turns the engine stutters, you may be throwing oil onto the plug and putting temporarily coating the element with glop which has to clear before it runs clean. If your setup routinely does this, it is hard on the plug. I have an igNoblARF that does this and it eats plugs.
10. If you are running a 4-2-4 setup really slow you are going to need a strong plug. Probably the hottest one you can get your hands on.
11. A plug that has been running fine, even though it has lots of flights on it, but now seems questionable will generally have a frosty look to the element. You can assume that the catalytic capability has been reduced.
12. If your engine was running fine, then not so good, so you replaced the plug and are now happy--Don't throw the plug back into the box "for future use." I've flown with guys that do this and it means that they aren't at the field flying, they're just there to retest bad plugs. Sitting in the box doesn't really rejuvenate the things.
13. If the element is not centered, and you are buying a good quality plug, then it has been damaged. If you only install plugs that have centered elements, then you know if you take one out that is leaning over to the side that it has been cooked. It may still work for a while, but it has been compromised. You can carefully straighten an element with a pin, but don't complain about the damaged piston, rings, and sleeve when the element goes thru the engine and eventually out the exhaust port.
14. There has been a lot of discussion how certain fuels, which had a batch here or there that produced globs on the plug element. This is universally accepted to be bad. If you have this problem, go research "taters." You could get a PhD in taters, so be forewarned.
15. Using tired, damaged or questionable plugs is not the thing to do with a plane you like. On the other hand, if you have a junker that you just want to go around in circles, then have at it.
16. If you think the plug in your diesel engine has gone bad and it won't start, turn the plug a bit counterclockwise, open the needle valve up and keep flipping. Continue until it starts, or go get some fresh fuel. Once it starts, remove the glow clip and throw the unit in the trash. Be careful, because the batteries will be really, really hot. And, this is important, never tighten the glow plug in a diesel all the way. It ruins the plug.
17. If you are using an adjustable glow driver/power panel, be conscious of the fact that too much input power will make the engine sputter and the prop will cycle back and forth without starting. The element does not need to glow super-bright to work. And that judgment can be even more subjective due to the lighting conditions. What looks pretty bright in your shop may look nearly dead on a bright sunny day on the circle. In a similar vein, be aware that banana jacks can get grimy and make poor contact giving high resistance and low current to the plug. And the wire connection on a clip can give problems too. Either too many broken strands from flexing or loose or contaminated lugs. Ditto with issues with tired, loose, wobbly glow drivers.
18. Half-A engines seem to be particularly sensitive to weak batteries. It may not be the plug. This is compounded if you are trying to run lower nitro fuel than they like. Flippitty, flippitty, flippitty....
19. Using a short reach plug in a long reach engine means the element is buried further into the head. It will behave different from your baseline, and if your baseline was any good, you're not going to like the new setup. Trying to put a long reach plug into a small engine may be even less acceptable. Thunk.
20. Switching back a forth between idle bar plugs and non-idle bar plugs may affect your baseline.
21. There are plugs out there that leak. The post seal is bad. So it may light up fine and may show the right amperage (if you are monitoring that) but won't run right, or in the later stages of decay, won't run at all. Most I have seen leaked when new. I have experienced very, very few of these. Most anecdotal evidence of leaks, at least the stories I hear, go way back to much earlier seal designs. Still, it may explain why one plug that looks fine just doesn't work.
22. If you change out the plug but don't seem to be back to your good baseline performance, check to make sure you didn't end up with two gaskets. Sometimes the old one sticks to the head and you put the brand new one on the plug before you screw it in....and, well, it is embarrassing when you finally figure it out. I find this happens when 3 or 4 club members have gathered around to "help" with fixing my motor and I'm not paying attention to what I'm doing.
23. My all time favorite though, is when the plane comes down after an inconsistent run even though it was working perfectly on prior flights that day. So I hooked up the clip to ground run it and the clip wouldn't stay on. The plug was missing.... Which leads to the more likely case: the plug is loose and the compression is now off and the ignition is delayed and it runs poor and dies perhaps before you run out of fuel. Or it just won't start. This is pretty common on Cox engines that use glowheads.
Once you start tracking what you are getting, you can decide if your baseline setup is working ok for you. Some setups eat plugs. If you run controlled experiments you can likely improve that and save money. Randomly trying stuff without writing it down or constantly using whatever plug that your buddies will loan you means that your results will be unpredictable. There is a reason that guys that like to pick certain engines that they have figured out how to get to work and stick with just those. And all the others are just for screwing around.
So work up your system. It may include more ideas that work for you. And, you need to decide your tolerance for screwing around with plugs vs. the cost of replacing them when straightforward troubleshooting tells you that it's time for that one to go.
The Divot
Topic: Color of glow plug filament, indicative of condition and/or performance? (Read 449 times)