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Mark Phillips in TN
Joined: 10 Jan 2006
Posts: 431
Location: Columbia, TN
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 Posted: Sun Oct 05, 2008 2:15 pm Post subject: Unhappy Instrumentation on Jabiru 3300 |
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(Long description- delete now if uninterested!)
The folks that build Light Sport Jabirus at my home field at Shelbyville TN recently asked me to look at an annoying problem which appears on certain of their completed composite airplanes using the Jabiru 3300 six-cylinder and Grand Rapids EIS engine monitors tied to GRT Sport EFIS displays. These engines have permanent-magnet alternators built into their flywheels and a very basic solid-state rectifier/regulator that comes with the kits from Australia.
Except for this one problem, the basic electrical system, although not designed to Aeroelectic principals, appears to work fine in the 100+ airplanes they have produced using it, and it only appears on perhaps one out of three aircraft completed in the past 6 months or so. The most excellent folks at Grand Rapids have no solution, saying they are not seeing this on other installations of their EIS. Units have been returned to them for testing with no anomalies found.
Problem: The EIS engine monitors are annunciating random "phantom" alarm messages, indicating a monitored parameter has exceeded a set limit. These include high or low EGT&CHT alarms, voltage levels, pressure levels etc. Of particular interest is that on the EFIS engine monitor page there is a graphic representation of CHT/EGT readings on a time scale (similar to a seismograph trace). This shows a trace history of these temps as continuous lines advancing across the screen. On the trace lines of the EGTs in particular there are obvious spikes (plus and minus). The duration of each spike is virtually instantaneous and indicate an increase or decrease in EGT of from 50-200 degrees, assuming they were accurate. The spike frequency is random, sometimes occurring 10-15 seconds apart, often much closer- frequency appears to be highest after engine start, and less frequent as the battery recharges. These spikes appear to be systemic as the alarms being generated are not limited to EGT/CHT (the only data shown as traces) as noted above.
At first is seemed to be caused by the Right magneto, as grounding it eliminates the spikes, but many attempts at relocating the P-leads and even disconnecting them entirely made no improvement. P-leads are 18 gauge shielded, single conductor using the shields to complete the circuit to a L/R/Start type keyswitch. Any electrical wiring or components near the mag were relocated with no improvement. Suppressor wires and plugs were tried with no change. Replacing the mag also did nothing.
Suspecting the rather basic rectifier/regulator, I checked the Z-figures and notes relating to the Jabiru and experimented with a 30,000uF 30VDC electrolytic capacitor between the regulator output and ground. This clipped the tops of the highest of the spikes shown on the EGT traces, but had little other effect. Another experiment was to disconnect the alternator output leads from the regulator and run strictly on the battery, but the spikes persisted. (All connections to the regulator are butt-spliced, soldered & shrink-wrapped) This leads me to believe it has nothing to do with the charging system and is specific to the Right magneto.
I've run out of ideas- all suggestions welcome!
I'd still like to experiment with a better rectifier/regulator, but unsure where to hunt one down- perhaps others have found reliable substitutions? (The B&C PRM1C-14 is limited to 14 amps continuous- 30-35 is required)
Mark Phillips
Columbia, TN
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selwyn
Joined: 12 Mar 2008
Posts: 102
Location: Australia
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| Posted: Sun Oct 12, 2008 10:11 pm Post subject: Re: Unhappy Instrumentation on Jabiru 3300 |
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These seem to work and have been credited with solving some radio noise problems on Jabirus. I have no idea if they will have any effect on your EIS problems.
http://www.powermate.com.au/
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Cheers, Selwyn
Kit 66 VH-ELZ |
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Mon Oct 13, 2008 7:59 am Post subject: Re: Unhappy Instrumentation on Jabiru 3300 |
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Spikes as you describe indicate a serious deficiency of inductors
and capacitors in a switching circuit. I have no idea whats in the Jab
regulator, but for sure, it's non-linear and creating switching transients.
On the other hand, it seems likely that a lot of engine sensor monitors
are designed by people who do not seem aware of Shannon's Sampling
Theorem. Dedicated EGT / CHT input lines should be heavily prefiltered
before digitizing since there is no real signal above 0.5 Hz or so. In a
properly designed Engine Monitor, the front end should completely filter out these transients, even if they do exist. You could post process the
data, but with the presence of transients like you see, the output would
be garbage from aliasing. Really guys, this is first year EE stuff, and you
should complain to GRT about why their design is sensitive to this noise
when it cheaply could be insensitive.
Ira
(I'm not an EE, but I played one for 4 years of Grad School)
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