SeamusG

I'm posting this thread in the Beginner's forum because I think that some awareness by a beginner glow flyer has some value.

I was under the impression that ya basically used whatever glow plug came with the engine. If your plug fails, pretty much put whatever you have available back in the engine and see if it works. If it does, cool - time to fly. Yea, I did the multiple glow plug break-in with a Super Tigre G-90 - the original, a long idle bar plug from Fox and finally an OSF-type plug. Just followin' orders SIR! Little or no understanding about what I was doing.

While reading a thread in the Glow Engines forum about the pros and cons of specific plug I found some interesting information about the role of the glow plug and my engine. I'm posting an edited version of information that Ipicked up at "Hooked-on-RC-Airplanes.com".

Not a quote but the information was derived from info from the site
" ...
RC glow plugs control the timing where ignition occurs. The goal is to have the air/fuel mixture ignite just prior to TDC.

Methanol is critical to the ignition process. The chemical reaction between the fuel’s methanol and the plug’s platinum filament with a catalyst of cylinder pressure keeps the filament hot. To control the timing of our engine you must have the correct RC glow plug that corresponds to the methanol content in your fuel. If your fuel has a higher methanol concentration (less oil and nitro) you will want a glow plug that will react “less” to the methane during the catalytic reaction in order to have the coils of the plug to produce the necessary heat to reach ignition temperature at the correct time. Plugs that react “less” to the methane are called “cold” plugs. “Hot” plugs react more with the methane causing the plug to get hotter more quickly.

Notes:

[*]Higher cylinder pressure will result in a greater reaction between the methanol and platinum and will result in a hotter plug filament thus igniting the fuel earlier in the cycle.[*]Fuel with less methanol (more oil and nitro) will result in later ignition and fuel with more methanol (less oil and nitro) will result in earlier ignition.[*]Rich air/fuel mixture will result in later ignition and a lean mixture will result in earlier ignition.[*]With everything else equal a “hot” plug will ignite the mixture earlier in the cycle and a “cold” plug will ignite the mixture later.[/list]Big issue is determining when ignition is early or late … If the engine speed decreases significantly when the glow driver is removed you could have a bad glow plug. If you are absolutely sure the glow plug is good, you may want to switch to a hotter plug or switch to a fuel with more nitro (less methanol). If the coils of the glow plug continuously fail or break over and over then you may consider changing to a colder plug. If the engine seems to backfire constantly, you may want to change to a cooler plug or switch to a fuel with less nitro (more methanol). Again, unless you have a legitimate reason, stick with the glow plug that comes with the engine.
..."

My question - how does a relative newbie know if the engine's timing is off? There's a bunch of information and (pretty) well understood techniques for getting the air/fuel mixture squared away - high speed needle rich/lean, low speed needle (idle) rich/lean for all of the popular makes-n-models.

But how do you take the next step to get the timing down?

Hope that you can add to my goal of having my (sometimes expensive) engines perform the way they're supposed to.

BTW - any altitude adjustments will be greatly appreciated as I fly at 6000 feet.

TIA

Charlie P.

There are also long, regular and short plugs.  A short plug lowers the compression (less metal taking up airspace) and that also delays the ignition.  Long plug increases compression, advancing ignition;, but you have to be certain there is piston clearance. 

So, how do you tell if your compression is needing adjustment?  Start with the plug the manufacturer recommends.  Tune the engine as best you can.  Swap plugs and see if there is an improvement or not. 

da Rock

He doesn't, and until he's taken some pretty firm steps to test different heat ranges of plugs in his engine, he won't have a hope. Just as he won't know how to do a blender until he learns a lot more about flying. But your discovery has started you on one of those journeys.


yup, and you've got a start. Now find a single plug manufacturer who sells heat-range labeled plugs and buy plugs in at least 3 heat ranges and start testing. And test with some way to measure rpm.

Down? You might need to move it up. But you'll have to have plugs in different heat ranges to find out.

And your altitude problem isn't unique. Guys have worked on it for years.

More nitro gets more oxygen into the combustion chamber, which is what high altitude air doesn't. And a more open venturi can help if your engine's base timing can make use of it. And different compression might help. The problem is that almost every engine, not just brand of engine, but every individual model, responds to hop up (which is what you're trying to do) depending on how "average" it is in timing, gas passing capability, response to nitro, compression, and a few others.

da Rock

But first step is to get a tach and the selection of glow plugs.

First off, try one brand of fuel with two different amounts of nitro. Decide which give you the best engine handling and that you're ok with it's power. Choose a fuel you know you can find next time you need some.

It'll help to do some prop tests before the plug tests. Get a couple of different props in different sizes that your engine mfg recommends for your engine. Tach them all on the ground and record the rpms. Fly 'em and see if the engine can pull them over the top of a loop without struggling. Pick which size does the best and the size that does next best.

Now forget about the heat range of plugs thing and fly. You've made the choices that'll make the most improvements. If you've discovered your engine still won't do what you want it to do, buy a bigger engine. It's what most guys at high altitudes have found over the year is the most cost effective solution. If you read enough engine test articles you'll find the very few that try different plugs and seen that plugs seldom cause significant change. But......

Testing the fuel and props and using the tach has moved you out of the rank newbie category. It's prepared you with the tools and experience to actually test the plugs to see what your engine at your altitude on your prop on your model actually does.

Oh yeah, if you crash your testbed model during this time, you gotta start the testing with the new model from square one. But that's all right. This stuff is fun to do. Awesome fun to do.

Lnewqban

Check attachment

SeamusG

lnewqban - I gotta grin from ear to ear plastered all over my face - many thanks.

jaka

Hi!
The best way to learn how to set-up and engine and to get to know how things work is the get involved in pylon racing!

SeamusG

With the info provided by lnewqban I would have an advantage. You've heard the old racing adage "I taught him everything he knows, but not everything I know" haven't you? [&:]  I also know through racing experience that until you get to the opposition's skill level it's tough to differentiate between operator's skill and equipment preparation. Am I down on power or am I just a putz? I like road racing on closed course driving because it's all about the "stop watch" - test-n-tune is sooooo much easier that way.

Montague

If the engine runs well, doesn't eat plugs all the time, transitions well, doesn't throw props at you, and generally does what it should, then for sport flying, you've got the right plug. .

Racing or anything where engine size is limited, but power is important can make you learn to look for a few hundred RPM here and there, and plugs is one place. (I fly combat, so I've been there, looking for power, not to the extent the pylon guys do though).

Oh, sometimes you can hear a bit of a frying egg sound when the engine is running. That's your pre-detonation or "pinging" at the RPMs our model engines turn. So if you hear that, you might want to see about retarding the timing a little (pick your method of choice).

oldtyme

There is a MONDO thread going on right now about glow plugs........particularly OS #8 glow plugs.......in the Glow Engines section of RC Universe

da Rock

Actually the best test setup ever made for modelers is Control Line. You can very accurately time the speed, and you can actually feel what's happening. You can even do consumption testing very accurately.

Unfortunately, there is a world of difference in CL and RC, and little carryover.

If you're interested in finding a set of plugs that cover the heat range, O.S.Max sells 4 presently that do that.

#6 A3 they rate as HOT
#8 ~~ they rate as MED.HOT
#10 A5 they rate as MED.COOL
~~ R5 they rate as COOL (it is sold for RC car engines).

SeamusG

and to add a bit to the confusion - from Tower's site ...

#7 - medium hot (no reference to length)
#8 - medium length - medium plug (no reference to hot or cold - guess that "it's JUST RIGHT".)

But - da Rock - you make an excellent point - one manufacturer, 4 plugs ranging from cold to hot with a couple of steps. It would make for an excellent "tuning"kit for a field test - same plane, same day, quick turn-around!

Thx ...

jeffie8696

If you want a super selection of plugs and heat ranges try Merlin Plugs. http://www.merlinglowplugs.com/Aircraft.html

SeamusG

I really like the site - using glow plugs as browser buttons - nice touch.

Being unfamiliar with the various brands as I am - are Merlin plugs high end, mid range and economy?

Didn't notice the first time Ilooked at the site - the glow plug icons are red for hot, green for medium and blue for cold.

da Rock - Merlin has CL-specific plugs.

I'm thinkin' that these guys are into glow plugs big time.

jeffie8696

Iknow Aldon Kelly the owner , just spent a half hour on the phone with him today in fact. He is super knowledgable about everything glow.
His plugs are made by hand and individually tested.
He is also an OEM supplier to at least one well known big time RC manufacturer.

SeamusG

Thx Jeffie - Time to try something new.

downunder

[link=http://www.modeltechnics.com/glowplug/firepower.html]This[/link] site by a glow plug manufacturer also has some good information about how plugs work and what heat range is. Probably of more interest are the test figures on different engines/fuels showing how peak revs and head temps change with varying heat ranges. Essentially it shows that almost always the best plug is the one giving the highest revs and this is true regardless of prop size, fuel used or any other variable.

TimBle

I'm not sure the bit in bold is correct.

The higher the methanol content, the hotter the plug needs to be to initiate the combustion.
Nitro is a combustion improver and provides the activation energy for the methanol to burn after it (the Nitro) is initiated by compression (which provides the activation energy to set off the nitro). Hence the more nitro in the fuel the colder the plug. the less nitro the hotter the plug.

TimBle

Idid exactly this with my GMS47. Itried put the OS #8, #7 and #6 against the Modeltechnics M7, M6 and M5

the M7 got the nod. Ggreat plugs those. Next best was the OS #7

iskandar taib

My Feedback: (5)

AND he's a long time CL Speed and Racing flier! Used to make those green fiberglass props racing people used to use back in the 1970s and 80s.

Iskandar

iskandar taib

My Feedback: (5)

Actually, nitro is quite hard to ignite. Top Fuel dragsters use a lot (I've heard 70 degrees) of spark advance to get the nitro to burn sufficiently by the time TDC comes, and it's still burning when the exhaust valves open.

That said, I don't agree with a lot of what is said on that web site. What they say is that a cold plugs is actually BETTER at igniting methanol, and is needed when the concentration of methanol is less. I think it can be demonstrated that this is false - using too cold a plug with low nitro fuel will cause late ignition. Too cold and the engine quits when you remove the leads. If the cold plug (with it's thicker element) were better at igniting methanol then this wouldn't happen.

Racing engines use cold plugs for two reasons, I think - they're tougher, and the extra heat caused by the burning of copious nitro advances ignition, and you need then to retard ignition with the colder plug.

Another thing that web site mentions that I KNOW is not correct is this myth of plated elements. No such thing. The wire in the plug element is solid Pt with 10-30 Rh and some Ir in the case of racing plugs. Cox glowheads used Pt - Ru but I think they were the only ones. At current Pt prices, the tiny piece of wire would cost 60 cents to a dollar, depending on the plug.

Iskandar

Sport_Pilot

I agree with you taib.  I suspect the glow plug may actually ignite nitro as well or better than methanol.  Or else those old C/L speed guys would have had to put batteries on those planes with all nitro fuel.  Actually a stoichastic mixture of nitro is not that hard to ignite.  However nitro is a monopropellant, that is it can burn with no added air.  So the top fuel dragsters and funny cars burn extremely rich mixtures.  When that rich it is hard to ignite.

opjose

That's not necessarily true however.

You could simply have too much fuel feeding into the engine, particularly at idle.

One technique I use in tuning glow engines is to listen for any drop in RPM's at idle when I remove the glow driver.
If there is a drop I lean the engine until I get none, or almost none.

ONLY if I can't get to that point, I'll switch to a different or new plug. Usually that clears up idling problems and the engine cutting out.

Also if pulling the driver out has such an affect at higher speeds, you may also have the wrong type of plug for the engine/fuel/prop combo you are using.

Sport_Pilot

If you swap several plugs, run them up to full speed, adust high speed mixture, then the plug with the highes RPM is the right plug.  If after that the idle drops off after removing the battery, then you know the idle mixture is too rich.

opjose

That's a good way to determine best heat range, but it didn't go to the point that a significant drop of RPM's after removing the glow driver does NOT necessarily indicate any problems with the plug.

You don't HAVE to swap plugs as well, particularly if you already know the proper plug for an engine...

However novices seeing the text about drop in RPM's will immediately think they have a bad plug, when it is likely more a rich setting problem. I've seen them do this all to many times.