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Post  Iceberg on Sun Mar 10, 2019 6:19 pm

What is the general consensus. How many hours will a glowhead keep working? Using 25% nitro and 10% castor. My estimate seems to be about two hours max? Does that sound correct? Curious to hear what others have found.
Thanks
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Post  944_Jim on Sun Mar 10, 2019 9:31 pm

Ummmm, I think you need to check your fuel. Is that 10% castor the ONLY oil in the mix?

These guys were built to run on roughly 20% oil. And the wife's tale I go by is at least 1/2 the oil is castor and the other half synthetic.

But, back to your question. I've blown more by hooking them up to the 12 volt ports in my power panel lately, so it is purely my stupidity or old age, not hours.
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Post  balogh on Mon Mar 11, 2019 1:16 am

It varies of course and depends e.g. on nitro content and engine compression set with cylinder shims. I had several high compression TeeDee heads working for many hours with the filament already bent and the head internals almost black from castor shellack. Some others surrendered after only a few runs.

The Merlin drop in replacements and Nelson turbo plugs are even improving the engine performance and seem to also last longer than stock COX heads.
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Post  Iceberg on Mon Mar 11, 2019 2:42 am

Hi Balogh
I lived in Budapest for almost two years. Loved the place.

Yes I have Merlins as well as Fireball screw insert style. I agree they seem more consistent than the original Cox. However some of the originals were awesome lasting many many hours. then some maybe like 1-2 hours. I spun on the lathe about a dozen burnt out cox and made heads that accept the screw in fireball insert. The actually work real good. I'm impressed.

Thanks!
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Post  Ken Cook on Mon Mar 11, 2019 4:21 am

Glow plug life is like playing the lottery. A new engine is more likely to take out a plug over one that's well run in due to metal contamination. Inadequate or loose engine mounting can also vibrate the plug into submission. This is typical of Cox planes that utilize the product and postage stamp backplates. The fit isn't very snug within the firewall area. While prop balance can also play a factor, so can prop faces that are not parallel to each other. This usually occurs with molded props vs wood, but I have seen the through hole on wood props not square to the hub face. Many times, I have seen a plug that has it's lower coils frosted in which is causing a lot of resistance and offers poor performance or just engine shut offs. Therefore, a plug that still glows isn't always a suitable one to use.

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Post  smooth_bill on Mon Mar 11, 2019 2:39 pm

I've often wondered if minute glow driver voltage levels, can cause premature failure of glow plugs?

For instance the 1.5 dry cells, versus 1.25 Ni-cads?

My Gilbert glow heads are particularly sensitive to higher voltage, and burn out instantly on what I consider normal power panel settings? Now, I always connect a spare Gilbert head to my glow panel with the control knob backed all the way off, and then increase it slowly until a reasonable glow shows. The knob setting and power panel meter is usually half the setting for modern glow plugs! Granted these are special cases where plugs have suffered poor storage for decades, and the setting for the 07's are actually lower than the 11's.

Yes, the starter jacks on a power panel will fry any plug instantly, and I've been known to blow two in a row before catching my mistake! Now, I always leave my starter ground wire connected to the panel starter ground jack. Can't plug glow plug leads into only one jack, though I'll probably try.

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Post  franzkleber on Mon Mar 11, 2019 3:59 pm


Yes, the starter jacks on a power panel will fry any plug instantly, and I've been known to blow two in a row before catching my mistake!
Bill[/quote]


This happened also to me … I heard a strange sound "bop" and immediatly understand my mistake after removing the plug , no coil and noticing from the hole of the head something strange on the piston … a small "ball" … the remain of the molten spiral !!
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Post  pkrankow on Mon Mar 11, 2019 8:20 pm

1 run to many many weekends of fun. Too many factors to know for sure.

To isolate things inspect your plug. I do not care the brand. Inspect for contamination. Use a magnifying glass and make sure the wire is where it should be. Use a T pin or similar to gently adjust if necessary.

I have read about using a T or hat pin and testing the ends.
https://coxengines.ca/engine-and-fuel-guides.html
See the document
Gibeault Mouse Race Program

Test the plug at 1.5v in a darkened space. A cupped hand under a tool box lid is usually dark enough. A drop of fuel then shaken out of the coil cavity will briefly smoke under power.

So, we now know the plug is "good" right now without much question.

Next we need to not cause premature failure by overcompressing the engine.

Good fuel prevents buildup in the element. Regular cleaning of the engine and prevention of debris entering the combustion chamber also helps.

The last concern is that many plugs the wire is plated and not solid. Use will remove platinum plating eventually.

Best of luck. These things are like incandescent light bulbs, they work now, but may not work next time.

Phil
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Post  wmazz on Tue Mar 19, 2019 2:49 am

I was surprised that nobody directly blamed detonation. Detonation destroys.
Cox engines have combustion chambers that doesn't provide any protection
to the engine from detonation.

Other problems are heat, load, vacuum leaks, and a very adjustable fuel screw
settings. Who knows when "heat soak" kicks in and the engine fails to properly
dissipate heat.


Bill M.
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Post  GallopingGhostler on Tue Mar 19, 2019 12:38 pm

wmazz wrote:I was surprised that nobody directly blamed detonation. Detonation destroys. Cox engines have combustion chambers that doesn't provide any protection to the engine from detonation.

wmazz, I gather you're speaking from your practical competition experience with 2 cycle marine and motorcycle engines, and modelling engines. Probably reason why you don't hear much is that there's been much knowledge pool experience with the Coxes that people more or less follow tried and true methods and less experimenting out of the box. Those diehard intimates with competition would know more.

I've replaced a head with a season or two of flying. This is with production engines using production heads, gasket and standard 1.5 V dry cell starting battery. What I have found is that the nichrome element erodes. They look brittle toward the end of life and don't glow as good as they did when new. Anyway, that's my take being a non-competitive sport user. Don't know what contributes to the element degrading this way, unless caused by heat from combustion.

Nice thing back in the old days was ready availability of inexpensive Cox parts to include heads in department stores and hobby shops.

This was on 15% to 25% nitro fuel with 10% oil content (all Castor).

Other problems are heat, load, vacuum leaks, and a very adjustable fuel screw settings. Who knows when "heat soak" kicks in and the engine fails to properly dissipate heat.

Haven't really had this problem with the Cox production tank engines. Perhaps that is why they were very popular, one didn't have to mess with tank plumbing. Also more than not, at least with the half-A kits I've built, RC, CL and FF, most of the time engine cowling was simple slab sided cheek cowls or no cowl at all. Thus they afforded maximum cooling in flight.

I gather that Leroy Cox and company had done a good job on debugging the engines early on that stock production engines on recommended fuel were very fool proof, which contributed to their long history.
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Post  balogh on Tue Mar 19, 2019 1:44 pm

wmazz wrote:I was surprised that nobody directly blamed detonation. Detonation destroys.
Cox engines have combustion chambers that doesn't provide any protection
to the engine from detonation.

Other problems are heat, load, vacuum leaks, and a very adjustable fuel screw
settings. Who knows when "heat soak" kicks in and the engine fails to properly
dissipate heat.


Bill M.

Bill you may be right, COX engines may be susceptible to detonation triggered by e.g. early ignition (too high compression ), leading to high running temperatures and even to engine overheating - in my humble theoretical assumption because of the high frequency waves generated by the detonation, that increases turbulence  and speed of gases swirling inside the combustion chamber, and thus intensifying convective heat exchange (heat transfer coefficient is exponentially proportional to turbulence in fluid, manifested in high Reynolds and Nusselt numbers that heat transfer coefficient is a function of) of gases with the metal parts confining the chamber, the glow filament included. I have a few COX high compression heads, and Merlin type drop in inserts used in my high performance TD and Killer Bee engines,that have serious discoloration because of castor  caking on the piston top and glow head dome, and in such a head a stressed filament is also often observed...

Original Cox GlowHead Life in Hours Discol10


Sorry for the boring theoretical assessment of the impacts of detonation on engine parts temperature, I gather you are interested in this part of the "model engine science" ...Long story short, detonation, IMHO contributes to engine overheating and thus shortening the glow head life
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Post  GallopingGhostler on Tue Mar 19, 2019 7:34 pm

Did someone say Heat Soak?  lol!
Original Cox GlowHead Life in Hours Mccoy314

(Actually, in reality I think previous owner used a non-heat resistant aftermarket spray paint, not engine enamel.) Huh... Tired w/ Coffee Read

I remember problems with pre-detonation or spark knock on car engines, when US government mandated use of lower octane non-leaded petrol back in the 1970's. It sounded like marbles rattling inside the engine compartment upon strong acceleration. One would retard ignition, but sometimes by doing so would cause the engine to run hotter.

Manufacturers came up with all sorts of solutions. My 1970 Mazda 1800 (resembled a BMW, a really nice car actually, very comfortable and smooth riding, a touch larger than a Toyota Corona) had dual point ignition. During idle, the second set of points was activated, which ran the engine at idle a number of degrees after top dead center. Another one was to lower the cylinder's compression ratio, which also reduced the power of the engine. That Toyota 4 banger that was making over 100 HP before now was making only 85 HP.

https://www.savvyanalysis.com/articles/detonation-and-pre-ignition

Savvy Analysis wrote:Engines can tolerate detonation for substantial periods of time, but there is no engine that can survive for very long when pre-ignition occurs. The engine will not run for more than a few seconds with pre-ignition. If you see a piston crown that looks sandblasted or a ring land that has fractured, it was probably caused by heavy detonation. If you see a hole melted in the middle of the piston crown, it was probably caused by pre-ignition. Other signs of pre-ignition are spark plugs with melted electrodes or insulators spattered with molten metal. Figure 5 shows an example of extreme damage caused by pre-ignition.
Original Cox GlowHead Life in Hours 05-fig10

This is from seriously prolonged, heavy pre-detonation.
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Post  Iceberg on Tue Mar 19, 2019 7:56 pm

I have one black widow that seriously screams. Early model. It detonated without two head gaskets. Also one td .051 that needs the head gaskets to avoid detonation. However different heads perform differently some needing more takers or less. Just need to test and try. Thanks
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Post  JennyC6 on Tue Mar 19, 2019 8:28 pm

GallopingGhostler wrote: Another one was to lower the cylinder's compression ratio, which also reduced the power of the engine. That Toyota 4 banger that was making over 100 HP before now was making only 85 HP.

That's the path Ford went with their big inch I6's. My '85 F150 only makes 120HP out of 300 cubic inches/4.9 liters in part because it only has 8:1 compression and deeply dished slugs. On the converse, those things run fine on pretty much anything vaguely flammable; I actually drove it for a week on a tank of 80% gas 20% diesel and it didn't care beyond being a fair bit harder to start in the morning.
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Post  wmazz on Tue Mar 19, 2019 10:25 pm

GallopingGhostler wrote:
wmazz wrote:I was surprised that nobody directly blamed detonation. Detonation destroys

wmazz, I gather you're speaking from your practical competition experience with 2 cycle marine and motorcycle engines, and modelling engines.

I was surprised. The modeling experience was Giant Scale airplanes running
25% nitro and McCoy#9 glow plugs.

(I believe, but I do not know) Glow plugs are so fragile that they can be
easily damaged and continue running; because the heated element doesn't
require an electrical current to continue glowing while the engine is running.


Bill M.
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Post  706jim on Wed Mar 20, 2019 2:02 pm

JennyC6 wrote:
GallopingGhostler wrote: Another one was to lower the cylinder's compression ratio, which also reduced the power of the engine. That Toyota 4 banger that was making over 100 HP before now was making only 85 HP.

That's the path Ford went with their big inch I6's. My '85 F150 only makes 120HP out of 300 cubic inches/4.9 liters in part because it only has 8:1 compression and deeply dished slugs. On the converse, those things run fine on pretty much anything vaguely flammable; I actually drove it for a week on a tank of 80% gas 20% diesel and it didn't care beyond being a fair bit harder to start in the morning.

My 81 Ford had that engine. Not particularly powerful, but durable. It rattled like crazy particularly when cold but it never did fail.
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