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Thread: Mag Drop

  1. #11
    Co-Pilot Martin Watson's Avatar
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    Roger, are you saying that an indicated rpm drop on the tachometer can be a sign of a dodgy stator, even when the true engine rpm is not changing?
    What is physically going on there? I can't see how that would work, but I'm keen to learn.

    And yes. I agree that any sudden change in behaviour must be a cause for concern.
    Martin
    BMAA 5370
    Fixed wing instruction, examinations and revalidations in Norfolk.


  2. #12
    Captain Roger Mole's Avatar
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    That was my experience Martin. It concerned me so much that I began a steady climb with very slightly increased revs but with rev counter slowly falling until I got to a height that I thought I could get down from safely if the engine eventually stopped, which it didn't.

    My point is that this 'thing' is producing your spark without which you have no engine. If you ignore the symptoms 'because it always cures itself' what'll you do when it does fail which sod's law will make happen when you're over tiger country?

    You must take any symptoms like this seriously. Bully for you if you find it's a dodgy plug or lead (yeah right...) but if at the end all those other checks turn out negative go for the stator. 195€ is a small price to pay for not being dumped in the proverbial just when you least expect it, let alone when on final or just after take off.

    Martin, I can't explain it because I'm no electronics expert. Maybe it's due to one or more windings intermittently breaking down so they're progressively sending fewer/weaker pulses than they should be. If so, possibly the spark on one cylinder is becoming weaker at the same time but the current/voltage(??) is still enough to give a spark albeit one of progressively poorer quality. I dunno.
    Last edited by Roger Mole; 10-09-19 at 21:38 PM.

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  4. #13
    Airfield Ops tomshep's Avatar
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    Yep. I have had a 447 stator land me in the New Forest which wasn't on my travel plan.

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  6. #14
    Co-Pilot Peter Twissell's Avatar
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    It is quite possible for the tachometer to show a stator problem before it has a significant effect on the engine.

    There are two types of electrical tachometer, digital and analogue (in operation, not readout).

    A digital tachometer has an internal electronic clock. It uses this to count the number of ignition pulses per second, calculating engine rpm and moving the indicator needle.
    This type will have an electrical supply from the battery as well as the connection to the stator and will either read the correct rpm, nothing at all, or in borderline cases may jump around if some of the stator pulses are too weak to trigger the counter.

    An analogue tachometer is powered by the impulses from the stator and uses them to directly drive the indicator needle, similar to a voltmeter. If the stator pulses are weak, the rpm indication will fall.

    If your tachometer continues to work with the engine running and main power switch off, it's an analogue type.
    G-BZNP Still not dead

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    Martin Watson (11-09-19), Roger Mole (11-09-19), unwind-protect (11-09-19)


  8. #15
    Captain Roger Mole's Avatar
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    Peter you're a gem and explained so simply that even electrical dumboes like I can understand. My take is that the standard tachos on Rotax 2 strokers are analogue.


  9. #16
    Co-Pilot Sean Dougan's Avatar
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    Pretty much yes, Roger. Although there's two types depending on if it's an ignition power coil tapped one with the Ducati system, or one driven off a generator coil with the Bosch points ignition.
    The Aviasport gauge used with most 912s is also voltage sensitive, rather than a pulse counter. It can be affected by the gap between the sensor and trigger wheel, and possibly any corrosion in the wiring connectors.

    That Hobbs gauge trigger you made is actually a digital tachometer in it's base form. With a bit of extra code and some connections it could output the engine speed to an LCD, or drive a stepper motor or servo mounted into a gauge.
    We got a mechanical contrivance getting in the way of a good time, and the worst kind of mechanical contrivance - a German Engineered mechanical contrivance.

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  11. #17
    Captain Randombloke's Avatar
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    Quote Originally Posted by Roger Mole View Post
    Definitely the stator, it's a known perennial 582 problem. Yours hasn't 'gone' but it's on the way out. If you're handy with a spanner you can change it yourself. Buy a replacement from Loravia - gone up a little since I bought mine off them but still the cheapest in Europe AFAIK. Don't try and kid yourself by taking measurements that yours hasn't gone/is on the way out. It is as soon as you get a big difference in mag drops you know.
    I agree with almost everything you say but I'm going to pick up on the not taking measurements, which is the one way you'll avoid going down the wrong path. I'll also flag up that when ours was on the way out the audible difference in mag drop wasn't that large but the difference in indicated RPM drop in the gauge became very big. As Sean says, it has something to do with the way tapping occurs, and if the tap is before or after the shorted turns, as that's the way I think they fail.

    I speak from my own experience with a stator failure on a 582. I spoke to a skilled mender who was able to quote the figures for the resistance off the top of his head, and said that they wouldn't start much below 240 Ohms, although Rotax may have quoted 230.

    Quote Originally Posted by Peter Twissell View Post
    An analogue tachometer is powered by the impulses from the stator and uses them to directly drive the indicator needle, similar to a voltmeter. If the stator pulses are weak, the rpm indication will fall.

    If your tachometer continues to work with the engine running and main power switch off, it's an analogue type.
    This is the standard 582 arrangement, AFAIK.

    Quote Originally Posted by Sean Dougan View Post
    Pretty much yes, Roger. Although there's two types depending on if it's an ignition power coil tapped one with the Ducati system, or one driven off a generator coil with the Bosch points ignition.
    As I understand it, all 582s are CDI so no Bosch points systems, that's 447/503 land, and some of them are CDI too.

    I agree with everything Roger says with the exception of measurement, but the most important thing is that this is not a can that can be kicked down the road. Other than that his description of the whole thing is what I experienced, and had to do to fix it. The problem sometimes self heals but the lower resistance can still be measured. I think this is because a tiny number of shorted turns allow functioning, and measure low, but then the intermittently failing ones take the measurements way down. When failing one of my coils measured 210, it must have been starting on the other one, but when the mag was properly dead it measured 180.

    Another give away is that an engine which started on the button becomes temperamental and takes forever to fire up. It's a common problem with a certain generation of blue top 582, not sure about the silver, if you find the resistance measurements on T'Internet you'll see newer stators are higher resistance.

    Roger's most valuable piece of advice is about how sudden the onset is. Don't ignore this.
    Steve U.
    PG, HG & microlights
    "Weekend bimbler, day to day car driver & genuinely undeserving Southern oik who has never done anything of any worth"

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  13. #18
    Co-Pilot jetlag's Avatar
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    I accept the combined wisdom but my situation has not been answered by this conversation. For the past 300+ hours, when cold, one mag shows a drop from 3000 to 2000 but once warmed up, no differences. The stated drop on the tacho is not real, witnessed by the closed eyes test, which I only suggested for the ground tests and not in the air. I have checked the coils and all seems to be within norms. Plugs have been changes, leads renewed and all checked over at the strip down for the rebuild 30+ hours ago. It has been so since the first day I bought her and never changed, starts easy and plugs have a nice colour when changed. ... Confused dot com
    F-JRIB LF1751 Corme Ecluse

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  14. #19
    Co-Pilot Peter Twissell's Avatar
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    Jetlag, the symptoms suggest that one of the stator coils is producing significantly lower voltage than the other. It is quite possible that both are still producing enough to keep sparking at the plug and run the engine, but that much difference indicates something needs attention.
    Presumably, in 300+hours there have been several services, new plugs and other system components and the stator gaps have been checked.
    As a test, swap the wires connecting the two mags to the tacho and see if the rpm drop remains associated to the same mag. If it doesn't, it could be something as simple as a poor connection in the wiring.
    G-BZNP Still not dead

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  16. #20
    Co-Pilot Martin Watson's Avatar
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    Excellent stuff, guys. Thanks to all.
    Martin
    BMAA 5370
    Fixed wing instruction, examinations and revalidations in Norfolk.


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