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Magneto tacho signal

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Bob Barrow

Joined: 11 Jan 2006
Posts: 29

Posted: Fri Oct 16, 2009 5:23 pm Post subject: Magneto tacho signal

I have a magneto serving my bottom plugs and LightSpeed EI serving my top plugs.

  

 Because I wanted to be able to read engine speed at run-up with either the magneto or EI turned off I installed a transducer that screws into the accessory pad (where the mechanical tach sender would normally attach). This gives me a tachometer signal at all times to send to the glass Engine Management system.

  

 But now some-one is telling me that the magneto continues to provide a tacho signal even when it is switched off. Is that true. Have I wasted my time and money installing a tachometer transducer.

  

 cheers Bob Barrow

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bicyclop(at)pacbell.net
Guest

Posted: Fri Oct 16, 2009 10:27 pm Post subject: Magneto tacho signal

Bob, 

 

 Got the same setup and have an Electronics International tach which looks at the signal from both ignitions and displays the one still turned on during ignition checks. That way I can see the RPM drop on both sides. Don't know who makes your glass, but you might want to check with them and see if their device can do that, too. When we got the plane, the tach only read the Lightspeed. The tach dropped to zero when the Lightspeed was switched off. It wasn't optimal, but sorta got used to not seeing the drop. I listened for it, instead. I eventually sent the tach back to the factory and had it re-rigged to display both. If I had to choose between the electronic ignition signal and the one from a transducer plugged into the tach cable hole on the engine, I'd take the electronic ignition - it's going to be more accurate.

 

 I'm not sure what signal there is to sense while the mag is grounded. With the P-lead grounded by the switch, the primary coil in the mag doesn't get it's magnetic field suddenly collapsed when the points open and doesn't induce a huge current into the secondary coil and thus doesn't fire the plug. I think the signal the tach is looking for is sudden drop in voltage on the P-lead as it is suddenly disconnected from ground by the opening points. With the switch closed, everything that the primary generates goes straight to ground and the lead from which you would get your tach signal is also grounded.

 

 To change the subject slightly, I've always wondered about having the backup ignition be less reliable than the main. Sure, a magneto doesn't need the electric system to keep working - until it quits working right, anyway, like mine did yesterday. Did the 500hr inspection on it less than a hundred hours ago and the brand new condenser took a crap and took out the points. My next plane will have dual Lightspeeds. Ours hasn't changed a degree in timing in the 6+ years we've been flying it. Say that about a magneto.

 

 Pax,

 

 Ed Holyoke

 

 Bob Barrow wrote: [quote]   I have a magneto serving my bottom plugs and LightSpeed EI serving my top plugs.

  

 Because I wanted to be able to read engine speed at run-up with either the magneto or EI turned off I installed a transducer that screws into the accessory pad (where the mechanical tach sender would normally attach). This gives me a tachometer signal at all times to send to the glass Engine Management system.

  

 But now some-one is telling me that the magneto continues to provide a tacho signal even when it is switched off. Is that true. Have I wasted my time and money installing a tachometer transducer.

  

 cheers Bob Barrow

   

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Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Sat Oct 17, 2009 5:38 am Post subject: Magneto tacho signal

Hi Bob B.,

  

 I also have one LightSpeed electronic ignition and one magneto.  I  have a Grand Rapids engine information system.  I wired the engine  information system to accept either the RPM signal from the electronic ignition  or the RPM signal from the magneto, controlled by a toggle switch*.

  

 I never use the toggle switch and use the RPM signal from the electronic  ignition as the only input.  As you said, that means my RPM goes to zero  when I briefly switch off the electronic ignition during the pretakeoff runup  .  That has never been a problem for me because I rely on exhaust gas  temperature on each cylinder to confirm that all spark plugs powered by each  ignition system are firing.  I believe that is a better method than  measuring the RPM drop on a single ignition system, which was probably used  before airplanes had EGT readings on each cylinder.  

  

 The electronic ignition has been 100% trouble free so far, but if it failed  in flight, I would lose my tachometer reading.  I have enough  experience with my airplane by now that I don't think I'd have any difficulty  continuing flight to my planned destination without a tachometer.

  

 Best,

 Dennis

 Lancair Legacy, nearly 300 tach hours

  

 *The Grand Rapids EIS must be configured correctly for the type of tach  signal that it receives.  I configured mine for the signal produced by the  electronic ignition, which is different than the signal produced by the  magneto.  So if I ever switched the toggle switch to display RPM from the  magneto, the actual number would be way off, but by a knowable amount.  In  nearly 300 hours of flying, I've never actually used the toggle switch to select  RPM from the magneto and if I had it to do over again, I would leave it out of  my design.  Higher parts count, additional complexity, another point of  failure.  

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Oct 17, 2009 6:44 am Post subject: Magneto tacho signal

At 02:05 AM 10/17/2009, you wrote:

  	  Quote: 	
		  Bob, 

 

 Got the same setup and have an Electronics International tach which looks at the signal from both ignitions and displays the one still turned on during ignition checks. That way I can see the RPM drop on both sides. Don't know who makes your glass, but you might want to check with them and see if their device can do that, too. 	


    Good idea. I think most electronic/glass tachs

    offer dual sensing capability.



  	  Quote: 	
		   When we got the plane, the tach only read the Lightspeed. The tach dropped to zero when the Lightspeed was switched off. It wasn't optimal, but sorta got used to not seeing the drop. I listened for it, instead. I eventually sent the tach back to the factory and had it re-rigged to display both. If I had to choose between the electronic ignition signal and the one from a transducer plugged into the tach cable hole on the engine, I'd take the electronic ignition - it's going to be more accurate. 	


    Actually, the "accuracy" of tachometer signals

    is dead-on irrespective of technology. These

    are integer pulse signals of so many pulses per

    engine revolution and are not subject to errors

    of calibration. Now, reading a p-lead offers some

    challenges with respect to lots of short spikey

    signals riding on the signal of interest but that's

    a simple filtering issue that does not affect the

    accuracy of the signal of interest.



 

  	  Quote: 	
		  I'm not sure what signal there is to sense while the mag is grounded. 	


    For a p-lead sense tach, this is right-on. Some models

    of tachometer have a vent-plug in the side you can

    replace with a hall or electro-magnetic transducer

    that watches poles of the magneto's magnet fly by.

    This sort of "mag pickoff" is functional whether or

    not the magneto is operating.



  	  Quote: 	
		  To change the subject slightly, I've always wondered about having the backup ignition be less reliable than the main. Sure, a magneto doesn't need the electric system to keep working - until it quits working right, anyway, like mine did yesterday. Did the 500hr inspection on it less than a hundred hours ago and the brand new condenser took a crap and took out the points. My next plane will have dual Lightspeeds. Ours hasn't changed a degree in timing in the 6+ years we've been flying it. Say that about a magneto. 	


    "Reliable" or "service-life"? Had this mag been

    functioning as-expected for 500+ hrs? I'll suggest

    there's value in making a distinction between

    COMPONENT reliability/service-life and SYSTEM

    reliability. System reliability is calculated

    by plotting all critical failure modes on a

    "reliability tree" and performing the math

    that predicts probability of total loss of system

    (the whole airplane) function when viewed through

    the relatively narrow window of one flight cycle.



    System reliability is enhanced when you have dual

    sources for critical functionality . . . i.e. dual

    ignition. The question to be pondered is, "what is

    the liklihood that I loose BOTH systems in any one

    tank full of gas?"  I'll suggest further that

    your experience with the magneto has been pretty

    much on a par with the rest of the folks in the

    industry. Changing it out would not mathematically

    improve your system reliability.



    With respect to "mag drop" . . .



    The engine slows down slightly when deprived of

    half of the sparks for ignition due to an apparent

    retardation of spark. This happens because mags

    are fixed timing to accommodate worst case

    operations (sea level, full power). When you install

    one or two electronic ignitions that watch

    manifold pressure and appropriately advance the

    spark, you don't get as pronounced (if any) mag

    drop when shutting one ignition off. The the only

    reason NOT to fly when presented with no (or

    un-symmetrical) mag drop is to investigate a

    "shift in timing". Such events suggest sloppy

    work by the mechanic who last tightened the

    mag attach hardware.



    It's far more important to simply know that

    each ignition source functions independently

    of the other irrespective of how much the

    tachometer reading changes for each condition.



    Bob . . .    [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Oct 17, 2009 7:03 am Post subject: Magneto tacho signal

<snip>

 	  Quote: 	
		  The electronic ignition has been 100% trouble free so far, but if it 

failed in flight, I would lose my tachometer reading.  I have enough 

experience with my airplane by now that I don't think I'd have any 

difficulty continuing flight to my planned destination without a tachometer.


	


    Aha! I think you could do it too. Our flight instructors

    never gave us lessons in what was NECESSARY for low risk

    flight, what was USEFUL for flight and what one could

    do to cope loss of one or more instruments.



    I used to have guys come to 1K1 to rent my airplanes

    and when going for a get-acquainted ride with them,

    it was not uncommon to have a prospective renter who's

    eyes were glued to the instruments . . . or at least

    spent very little time outside the cockpit.



    One fellow was so concentrated on watching the needles

    I borrowed one of our instructors set of block-out

    patches and covered EVERYTHING on one of our

    C-150's and invited him to go flying with me. I was

    able to demonstrate comfortable operation of the airplane

    without knowing what ANY of the numbers were. But

    that operation was based on staying well inside

    the airplane's limits by referencing things I had

    explored about visual cues for pitch and power simply

    based on where the throttle was and what the engine

    sounded like and position of the nose with respect

    to the horizon.



    I got to go retrieve the company A36 when one of

    our pilots left the airplane on the Dodge City

    airport because the "tach had died". He didn't know

    enough about the airplane to even check the tach

    cable fittings (the one on the engine had come

    loose). But even if we hadn't fixed the tach

    before bringing the airplane home, it would have

    been no big deal to fly it back with a dead tach.



    One of the really nice things about the OBAM aviation

    community is that builders are inherently more

    knowledgeable about their airplanes but many

    of us still drag a lot of the "padded cockpit"

    environment with us. Airplanes don't crash

    because a gage quit, airplanes crash because

    the pilot lost his connection with the machine.

    You can't loose something you never had.I would

    encourage everyone, starting with their fly-off,

    to explore flight operations with all manner of

    degraded instrumentation.



        Bob . . .



         ---------------------------------------

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        ( appearance of being right . . .       )

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bicyclop(at)pacbell.net
Guest

Posted: Sat Oct 17, 2009 8:54 am Post subject: Magneto tacho signal

Howdy,

 

 Robert L. Nuckolls, III wrote:  	  Quote: 	
		  At 02:05 AM 10/17/2009, you wrote:

    "Reliable" or "service-life"? Had this mag been

    functioning as-expected for 500+ hrs? I'll suggest

    there's value in making a distinction between

    COMPONENT reliability/service-life and SYSTEM

    reliability. System reliability is calculated

    by plotting all critical failure modes on a

    "reliability tree" and performing the math

    that predicts probability of total loss of system

    (the whole airplane) function when viewed through

    the relatively narrow window of one flight cycle.

 	
 Reliable? Functioning as expected? Well, yes.... if it is expected to foul plugs and deliver rough mag checks requiring more lean running at high RPM to clear up before taking off. Granted, that doesn't happen every time as we operate very lean on the ground to try and minimize this kind of behavior. Requiring much more maintenance? I guess that's expected, too. Cleaning and gapping plugs takes an inordinate amount of time. All that lead that gets stuck down by the insulator doesn't come out easily. I just buy new $3 plugs every year for the Lightspeed and done (and the old ones are never lead fouled, anyway). The timing always changes between annuals because of wear on the points and must be reset. And no, it didn't work for 500hrs this time. At the 500hr inspection (yet more maintenance not required on an electronic ignition), the wear items (points, condenser, bearings, seals - none of which exist on the crank sensor Lightspeed) were replaced and this was only about 80hrs before failure. By the way, this is the Bendix mag. It's meant to be rebuilt as many 500hr cycles as you want, unlike the throwaway Slick mag. 80hrs is way too soon for problems to occur, yet sometimes they do.

  	  Quote: 	
		  

    System reliability is enhanced when you have dual

    sources for critical functionality . . . i.e. dual

    ignition. The question to be pondered is, "what is

    the liklihood that I loose BOTH systems in any one

    tank full of gas?"  I'll suggest further that

    your experience with the magneto has been pretty

    much on a par with the rest of the folks in the

    industry. Changing it out would not mathematically

    improve your system reliability.

 	
 Changing it out with another mag certainly wouldn't improve system reliability. Comparing my experience with the magneto to my experience with the electronic ignition, I think my system reliability would improve, measurably, by replacing it with an EI. The EI never fouls plugs, never (so far) requires unscheduled maintenance. The maintenance that it does require, such as checking timing and replacing plugs at annual, is quick, cheap and easy. I could even re-use the plugs. They're always in great shape after a year and a couple hundred hours of use.

 

 If the only reason to have 2 ignitions was for backup - maybe. The problem is that these engines with big ass pistons really need 2 functional ignitions to get the flame spread over with before the moment has passed to make useful power. The 25 degree BTDC timing (fairly standard, although there are exceptions) is a compromise designed to maintain a margin of safety from detonation at full power and sea level. The EI fires at the same timing under those conditions. At high altitude (read low manifold pressure), 25 degrees is way too late for best power and there is no danger of detonation, so the EI advances to as much as 40 degrees BTDC. This puts the moment of maximum cylinder pressure at the crank position of greatest leverage (about 19 degrees ATDC). The magneto fires 15 degrees later than the EI and is not much help. The role of the magneto under these conditions is truly as a backup system. The EI also puts out a much hotter spark fired over a larger spark plug gap. At lean mixtures especially, this ignites the mixture more reliably. Klaus claims another 5% improvement in fuel economy with a second EI. If much of your flying is in cruise, I'd think this is probably about right. It'd probably take a while to pay for the new ignition that way, though.

  	  Quote: 	
		  

    With respect to "mag drop" . . .

   

    The engine slows down slightly when deprived of

    half of the sparks for ignition due to an apparent

    retardation of spark. This happens because mags

    are fixed timing to accommodate worst case

    operations (sea level, full power). When you install

    one or two electronic ignitions that watch

    manifold pressure and appropriately advance the

    spark, you don't get as pronounced (if any) mag

    drop when shutting one ignition off. The the only

    reason NOT to fly when presented with no (or

    un-symmetrical) mag drop is to investigate a

    "shift in timing". Such events suggest sloppy

    work by the mechanic who last tightened the

    mag attach hardware.

 	
 

 Asymmetrical mag drop is one thing. As you said, this is to be expected with 2 different types of ignition. Rough running on one ignition is something else and indicates fouled plugs or another, possibly worse, failure. This is definitely a good reason not to fly until cleared up as you are now down to one good ignition. This raises the odds that you'd have 2 failures on 1 tank of gas to an unacceptable level.

  	  Quote: 	
		  

    It's far more important to simply know that

    each ignition source functions independently

    of the other irrespective of how much the

    tachometer reading changes for each condition.

 	
 Hear, hear. Another thing is: the traditional mag check at 1600 or 1700 RPM and full rich doesn't really tell you much about the true condition of the ignition system. My magneto passed this check, yet ran rough in flight, causing me to return to the airport. Not an emergency, also not at all comfortable. A more thorough check can be done at altitude, leaned. This shows up problems you might not notice on the ground check. Unfortunately, I didn't do this before landing on the prior flight.

 

 Pax,

 

 Ed

 [quote]

    Bob . . .    	  Quote: 	
		  



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Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Sat Oct 17, 2009 2:42 pm Post subject: Magneto tacho signal

Well, there is something wrong, as I never have to touch my Bendix mags 

in less than 400 hours, and then only to tweak the timing by a degree or 

so. I don't need to burn off deposits at runup. I do routinely pull the 

bottom plugs at annual and clean them...top plugs every other year, just 

to get rid of the small amounts of lead. Your points should not wear 

that fast. Either the condenser isn't right, allowing faster pitting, or 

the cam isn't properly lubed allowing it to wear. 600-700 hours between 

inspections for new points, etc. is what I have been getting with my 

S200 series 4 cylinder mags. Sure, I would like variable timing, but the 

total improvement with both ignitions optimized is only going to be 

about 5%. Any claim for more is dreaming. You might consider having a 

reputable magneto specialty shop go through that mag. There may be more 

bearing or rotor wear allowing slop in the rotor, or something else wrong.



Ed Holyoke wrote:

the relatively narrow window of one flight cycle.

  The timing always

 	  Quote: 	
		   changes between annuals because of wear on the points and must be reset. 

 And no, it didn't work for 500hrs this time. At the 500hr inspection 

 (yet more maintenance not required on an electronic ignition), the wear 

 items (points, condenser, bearings, seals - none of which exist on the 

 crank sensor Lightspeed) were replaced and this was only about 80hrs 

 before failure. By the way, this is the Bendix mag. It's meant to be 

 rebuilt as many 500hr cycles as you want, unlike the throwaway Slick 

 mag. 80hrs is way too soon for problems to occur, yet sometimes they do.


	


 	  Quote: 	
		   If the only reason to have 2 ignitions was for backup - maybe. The 

 problem is that these engines with big ass pistons really need 2 

 functional ignitions to get the flame spread over with before the moment 

 has passed to make useful power. The 25 degree BTDC timing (fairly 

 standard, although there are exceptions) is a compromise designed to 

 maintain a margin of safety from detonation at full power and sea level.

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_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

peter(at)sportingaero.com
Guest

Posted: Sun Oct 18, 2009 11:30 am Post subject: Magneto tacho signal

Only the E/P-mag (afaik) continues to provide a tach signal with the ignition switch turned off.  A Slick or Bendix mag will stop producing pulses when the ignition is turned off (unless a vent plug pick-up is installed). I suspect you haven’t wasted your money.  

   

Peter  

   

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Oct 19, 2009 8:08 am Post subject: Magneto tacho signal

 	  Quote: 	
		  

If the only reason to have 2 ignitions was for backup - maybe. The 

problem is that these engines with big ass pistons really need 2 

functional ignitions to get the flame spread over with before the 

moment has passed to make useful power.


	


    Understand. But having both ignition systems operating

    all the time is not a safety issue . . . only

    a performance issue.



<snip>



  Not an emergency, also not at all comfortable. A more thorough 

check can be done at altitude, leaned. This shows up problems you 

might not notice on the ground check. Unfortunately, I didn't do this 

before landing on the prior flight.



    Again, a performance issue that does not put

    the flight at serious risk for an off-airport

    return to earth.



    To be sure, there is a host of simple-ideas affecting

    performance and reliability that builders are encouraged

    to consider in crafting his/her design goals.



    System reliability conversation targets risk reduction.

    Many cause/effect features in an FMEA will speak

    to reductions in performance or increases in pilot

    workload. The goal is to maximize probability

    of putting the rubber back on the ground with the

    aluminum smooth and folks aboard smiling.



        Bob . . .



         ---------------------------------------

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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