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JonathanMilbank
Joined: 14 Apr 2012
Posts: 391
Location: Aberdeen area
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 Posted: Tue Oct 13, 2015 2:02 am Post subject: Rotax 912S mixture vs mag drop |
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I never cease to amaze myself with how much I've forgotten, or never knew in the past 48 years since I started flying for a living. Admittedly most of my flying was powered by gas turbines, but I have been flying my Europa Classic for 18 years and keep having to "re-invent the wheel."
For reasons I won't bore you with, it came about that I needed to fiddle with the carburettor needle positions and after each experimental move of the needles by one notch, I took the engine up to full temperature and performed repeated "mag" drop checks along the way.
The long and the short of this is that the leaner the mixture (needles moved down) the greater the "mag" drop and vice versa. If we had different engines with separate mixture control in the cockpit, then this would be glaringly obvious. But I've become so accustomed to single lever engine control, that I've mentally obliterated such out-dated complications.
So if you think that your "mag" drop check gives borderline (too much drop) results, consider moving your needles one notch richer. It's dramatic how greatly the results get improved, to well within book figures. Here's a quote and a link:
"The leaner the mixture, the more mag drop you'll see on one mag, and that's normal." http://www.advancedpilot.com/articles.php?action=article&articleid=1844
Obviously I never had enough flight time in proper old "heavy metal" aircraft, which teach you a lot.
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Tue Oct 27, 2015 7:03 am Post subject: Re: Rotax 912S mixture vs mag drop |
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Hi Jonathan,
The 912 series engines are designed to be fairly tolerant to a wide range
of stoichiometric fuel ratios. I surmise this because of the absence of a
manual mixture control and the fact that the Bing carbs do close to nothing
in terms of altitude adjustment of mixture as I documented here many years
ago. Note that the Rotax advertisements no longer claim to be altitude
compensating. If anything, in my experience and in chatting with Rotax
and Lockwood tech people over the years is that the engine is generally
running rich and it gets worse after takeoff.
The key purpose of the mag check during run-up is to assess the status
of the ignition system to assure that one has two functioning ignitions.
The first thing to consider in the differential diagnosis of a large RPM drop
on one side of the ignition would ignition problems, e.g., bad or dirty
spark plug, bad cable, bad ignition module, etc. Again, my opinion, but
I would suggest that fiddling with the carb needles when your base
airport is near sea level would be low priority and a low yield procedure.
This is not intended as a criticism as you obvious have substantial
experience, I just think that many reader here, especially the growing
numbers of second or even third generation owners are still just
beginning their Rotax education.
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 391
Location: Aberdeen area
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| Posted: Fri Oct 30, 2015 2:02 am Post subject: Re: Rotax 912S mixture vs mag drop |
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From:
"Erich Trombley" <erichdtrombley>
Date:
Feb 27, 2009
Subject:
large mag drop on both mags, 912uls
While I can't speak to the 912ULS (I have a 914), I can share this story with you. I too had mag excessive drop (600 + rpm) but only on one mag. I tried everything I could think of including new spark plugs, plug wires, swapping coils, purchasing a new coil, and overall conducting a very methodical diagnostic test program. All to no avail. I could not figure out what was going on with the engine. To make matters worse the problem seemed to have resolved itself without me changing anything. I was pulling my hair out. As silly as this is going to sound, it turned out to be mixture. Yes, mixture.
Interesting it was a function of ambient temperature. All of my testing was conducted on the trailer in an unheated garage with the garage door wide open and it was cold. It was only after the ambient temps increased that the problem seemed to have gone away, however, I hadn't made that connection yet. Not realizing what was going on and with the problem having seemed to have fixed itself I made arrangements with a flight instructor to conduct a flight with me the following morning. Well, as you can probably see where this is going, we got to the airport in the morning and put the plane together. I started up the plane and conducted a mag check. The problem was back, yet I hadn’t changed a thing. Also, I might add that even with the excessive mag drop the engine ran very smooth. As such, the instructor suggested we take it up and see how it performed in the pattern. There were no issues on the take-off roll and we continued with the flight. After one circuit in the pattern, I performed a mag drop on downwind... perfect not even 100 rpm drop. The engine was running perfectly, although I did not know why.
It was during the Rotax class I learned why. The instructor asked if anyone had experienced significant mag drop. I raised my hand, “Yes, yes, I have”, I proclaimed. The instructor proceeded to walk me down a path asking other questions such as, was the first time I noted the problem in the winter season or was it year round, “winter”, I replied. I also noted that the drop was much worse on one mag. He then asked if it was a 914, "yes" I stated. The problem surfaced on the first flight of the winter season. In the four years prior to this incident the problem never surfaced. However, the summer preceding this winter flight I installed an inter-cooler. As ambient temps drop the air becomes denser, and with this density change the engine gets leaner. It is the lean condition that causes the excess mag drop. The inter-cooler only served to exacerbate the situation, further cooling the inlet air.
The 914 differs from the 912 on the arrangement of the mags. On the 914 the top plugs of all cylinders are connected one mag and the bottom plugs to the other mag. On the 912 it is my understanding that one mag is connected to two top plugs and two bottom plugs (opposing cylinders) with the other mag controlling the other two top and bottom plugs. The cylinder heads of both the 912 and 914 are such that only one of the two plugs per cylinder is optimized for ideal flame propagation/combustion (top I believe). Hence with the 914 when you isolate the top plugs via a mag drop you will note a greater mag drop than with the bottom circuit offline. With the 912 would see equal mag drops since two top plugs and two bottom plugs are taken offline with each mag, thus cancelling out the differences in the head design. With a lean mixture the drop is more pronounced. The fix, change the needle valve setting to enrich the mixture. Problem solved. Good luck with diagnosing the problem.
Please keep us posted on your progress.
Erich Trombley N28ET Classic Mono 914
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Fri Oct 30, 2015 4:47 am Post subject: Re: Rotax 912S mixture vs mag drop |
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Yes, and you might see the increased air density effect at Furnace Creek
(L06) as well with a airfield altitude of -220 feet. (But probably not in
summer, I've seen OATs in excess of 120F there).
Keeping with the western paradigm, please allow me to share an aphorism taught in medical schools: "When you are in cabin without windows on the high plains of Colorado and you hear the massed hoof beats of a herd
outside, your first thought should not be of Zebras!"
Altered air mass and cylinder charge effects occur but I thought it relevant
to point out that they are rarely the life safety issue of a faulty ignition
system. Remember another great aphorism in the immortal (but
probably apocryphal) words of Willy Sutton (1901-1980): That's where
the money is!"
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