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Bobby, I've looked into wide band O2 sensor a bit.
As you know, the principal difference over the narrow band is that the sensor
output voltage curve is much more linear and extends over a wider range of
voltage (from approx 0.5 - 2.5 vice 0 - 1 for the narrow band).
It is not uncommon to have the air/fuel indication vary a
bit. I would not expect each combustion process to be exactly the same as
the one occurring the moment before (but, should be close). So the amount
of oxygen left over in the exhaust gas from each combustion event is
likely to vary (at least a small amount). This in turn would show up as
small variations in the air/fuel ratio. Also RPM might have some effect,
at lower rpm the combustion events are further apart than at higher rpm.
Another possibility is as you mentioned is the possibility
that the sensor is indicating a slight misblance between the two rotors.
If that is the case I would expect the RATE of variation you see to vary with
rpm and be fairly consistent at different air/fuel ratios and rpms.
Ah, such fun.
Ed
Sent: Monday, March 21, 2011 10:18 AM
Subject: [FlyRotary] Re: Forced Landing.
Steve,
The wideband may be picking up a
small fuel imbalance between the two rotors. Or is could be perfectly normal for
all widebands to jump around. I would expect an increase in EGT’s with a gradual
partial blockage. A high EGT alarm would give some indication. I would
also expect a steady but higher than normal temp with a build in restruction.
But with a sudden blockage at high power? The answer may not be of any value
since only a pressure sensor would tell the pilot to stop trouble shooting.
Bobby
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Steven W. Boese
Sent: Sunday, March 20, 2011 8:20
PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Forced Landing.
Bobby,
I have not done any
testing with a wideband O2 sensor. Unless the engine were operating right
on a threshold of a large untuned injector pulse width change (staging is the
only thing that comes to mind), it seems unlikely that the variation in O2
sensor would be associated with injector delay.
Within the limits that
were tested, the EGT increased with increased exhaust back pressure due to
partial blockage. The mixture was somewhat rich and no change in O2 sensor
was seen. Lean mixtures were not investigated at that
time.
Steve
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Bobby J.
Hughes
Sent:
Sunday, March 20, 2011 7:46 PM
To: Rotary motors
in aircraft
Subject: [FlyRotary] Re: Forced Landing.
Steve,
Thanks for the offer. Actually I think it
would be great if you could conduct a test. I had thought about a block off
plate that could be pulled into place with a long wire. This would allow a
high power test run with a sudden blockage. It’s the only way I can come up with
to create a not so dramatic reinactment. Your test stand would be a better
platform then the airplane. The engine was popping and shaking when the failure
occurred. As best I can remember my wideband O2 was off the scale lean but I’m
not certain. Speaking of wideband O2’s. My wideband has always jumping around a
few 10ths or more. My EM3 get’s a narrowband output from the wideband. The EM3’s
mixture bar appears stable. Could the wideband be seeing a injector bounce
associated with the injector delay? As an example when leaned for cruise, say
13.2 it’s often switching rapidally between 13.1 and 13.3. At this point the
engine seems reasonable smooth. The larger the bounce the less smooth the engine
becomes. Have you done any of your testing with a wideband?
Bobby
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