red ignition coil primary resistance as a maintence item ch

[pilotyoung]

I'm new to a Onex with an AeroVee engine. I bought it about 2 months ago and have flown it 22 hours. I have been flying with both ignitions on all the time.

Is it common practice to turn of the secondary ignition during ground ops and at cruise?

Thanks.

John

[Rynoth]

I'm new to a Onex with an AeroVee engine. I bought it about 2 months ago and have flown it 22 hours. I have been flying with both ignitions on all the time.

Is it common practice to turn of the secondary ignition during ground ops and at cruise?

Thanks.

John

I would say it is fairly common, particularly on the ground. The 20A alternator produces very little/if any actual current at ground RPM settings, while the secondary ignition still pulls quite a few amps (6-7 in my experience) on the ground. Basically while on the ground you're going to be constantly draining the battery due to the alternator not being up to speed, and the secondary ignition is probably your biggest current draw that is easy to shed.

Cruise is probably less common but I'm sure folks do it. I sometimes do it because my secondary ignition timing is a bit advanced... this can give me an advantage in how much I can lean the mixture at the cost of a bit higher CHT (giving me slightly better fuel economy.) But if my CHT gets warm I can turn the secondary off, run a bit richer on the primary ignition only and CHT comes down.

[bvolcko38]

I will have to experiment with that. Watch the ammeter as i slowly hand prop.

[Rynoth]

I will have to experiment with that. Watch the ammeter as i slowly hand prop.

I'd be curious what you find, just be careful since you run the risk of the spark plug firing, any residual fuel vapor could ignite, as unlikely as that might be.

[n502pd]

thanks for all the info and replys! what I still need, at the most, is a known bad coil, a suspected bad coil, and of course a new unused coil. I can design bench top methodoligy that can make the coils fire without the on engine module, and this way I can look at the voltage output, spark color, spark 'fatness' and primary current draw. i Feel some data, namely the quiesent primary resistance, and possibly current draw, could give us a way to quickly assertain the 'health' of the coil. All of this effort, of course, eventually might help prevent surprises by a sudden coil failure if we could identify a trend of changing resistance and current draw on a failing coil before it actually does fail. and, of course this method of monitoring, either current draw, or resistance, most certianly will not catch 100% of impending failures.

Yet another idea; many years ago, some conventional point/condenser ignition systems used an 8 volt primary coil, with a ballast resistor in the 13.5 volt power to the coil, that would limit the primary current under engine run conditions, and that resistor would be shorted out by the ignition switch for starting function only, and would give a way more energic spark for starting, partictularly in cold weather. this could be adopted fairly easily to our situation to limit high rpm current with only small reduction if secondary energy, but this idea isnt my main direction at this time. Just something else to think about! Geeze, there I go again....thinking! I do have to watch that!!

thanks again for all the comments!

[n502pd]

about running with one ignition off, I have not done that, even in the 172 over 23 years, and now with the aero vee with 45 hours tt on it. my numbers have been very stable since breakin is pretty much done. i am not an advocate of changing something that is working fine while airborne unless there is a really good reason to do so. motto here is 'if its aint broke, dont ,'fix' it"...just one mans opinion!!

[lakespookie]

LOL you know what is broke that 20 Amp Generator

[n502pd]

OK, I am greatly interested in all these comments! thank you all for them!

I have come up with another question, a pretty simple one to answer: on the face of the red coils supplied by sonex, the end facing the prop, what value of resistance is printed there, if any? On one set I have measured recently, I didnt recognise any marks, but I didnt specifically look, either. the sonex site web store has a pix of what they sell, ACV-A01-25, and it is labeled '3 OHM'. I would like to see how many are using the 3 ohm coils and have had failures, suposedly due to heat, and those who may have a 5 ohm coil fail too. If possible, would some of you look to see what is in use?

As always, thank you for any input!

[n502pd]

I hate it when this happens! I have to admit that I am wrong on the printing of ohms value on the red sonex supplied coils: I said on the prop side of the mounted coil. Well, on N502pd, those values are printed on the firewall side of the coil, and would require removing the coil to see them. I dont want others to actually disassemble their planes to just accomidate my needs for info, so, maybe I should have directed my request to those who have not mounted the coils, or wired them in yet, or have very easy access to them to dismount them and look. Sorry for my first mistake, this year! Yeah....sure...!!

[n502pd]

From what I understand about these ignition systems, Ryan gets my vote, with the understanding that the only way the coil can generate the HV is with a changing curent thru the primary of the coil, and if the engine isnt turning, no CHANGE in current will happen, and no HV output. This change comes when the trigger interrupts the current in the coil at the proper time, and causes the magnetic field formed by that current flo in the primary, to collapse rapidly, and that collapse in the magnetic field is what induces the the secondaary to furnish the HV to the plugs. If engine isnt rotating, all one gets is dc current flow thru the coil, and that will eventually damage the coil due to internal ohmic heating. this same heating occurs when engine is running, but is relative to the engine rpm per unit of run time. If , IMHO, one were to fly the coils over many hours, non stop, eventually, IMHO, the heat damage will catch up and damage the coil at an inopportune time. This is the whole basis of what I have been considering, and maybe internal, compounded damage could be identified as a gradual decrease in primary resistance, with corresponding increase in current flow thru the coil.

Further info I have found, from motorcycle blogs in several different areas, with several differend models of engines, have indicated there are coils with resistances as low as0.5 ohms, running in varrious increments to 5.0 ohms. I also found a number of statements indicating that if one had initially a 2.5 ohm coil and trigger assembly, and the coil went south, one could replace that 2.5 ohm coil with a 3 ohm or higher resistance coil, and provided the trigger assembly wasnt damaged, and have no detrimental effects, other than a slight lowering on the HV output. However, going the other way, 3 ohm to 2,5 or lower ohm coil, will do damage to the trigger as it wasnt made to handle the increased coil current flow. I agree with this senario. Basically, if you have a 3 ohm coil, it should be fine to use a 5 ohm coil, but not a coil less than the original, provided the trigger assembly is not damaged.

Today, I dug into Sirpeedee and found I do have the sonex coils, and are 3 ohm coils, and they measure 3+ ohms on my little ohm meter. I did disconnect one primary wire and then left both wires on, with mag switches and secondary switches in the off position, and found no difference in the resistance, 3+ ohms in both cases. this may make it easier to monitor the coil resistance if my idea goes forward as another item on the condition inspection.

And lastly, I still need some of you who are using these coils to send on to me, either here or via PM, measurement data (stamped ohmic value, and measured value) on your coils primary winding, and if you have a dead coil, can you donate it to my program to determine how it failed. I would greatly appreciate any help!

as always, thanks!