REFLECTOR: Odd fuel pressure Problem

[Wayne]

On a digital oscilloscope they call that "ailiasing"
----- Original Message ----- 
From: "Ronnie Brown" <romott at adelphia.net>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Friday, November 26, 2004 2:26 PM
Subject: Re: REFLECTOR: Odd fuel pressure Problem


>A similar problem is occurring with the Blue Mountains/Dynon's with a
> vibration caused sampling frequency problem.  Attitude indicators are 
> laying
> over sideways at certain rpms.
>
> Since Chuck and others are having a similar problem, I would suggest that 
> a
> snubber/orifice needs to be added to the fuel pressure sensor line.
> Digital/computerized instrumentation samples the data instead of having a
> continuous readout like an analog instrument.  If the pulses from the fuel
> pump are in sync with the digital sampling frequency, then the read out at
> certain times could  be sampling the low pressure part of the readings, 
> and
> hence the indicated low pressure (at certain rpms or other harmonics).
>
> My fuel pressure sensor line has a 1/16" orifice - a fitting was filled 
> with
> silver solder then drilled out with a drill bit.  This should reduce the
> pulses reaching the sensor and improve the accuracy of the fuel pressure
> readout.  I have similar orifices on my oil pressure and MAP lines.
>
> Incidentally, the fix on the Dynons was a change in the solid state gyro
> sampling frequency to avoid the offending engine RPM harmonic.
>
> Ronnie
>
>
>
> ----- Original Message ----- 
> From: "Jack Sheehan" <j.f.sheehan at larc.nasa.gov>
> To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
> Sent: Friday, November 26, 2004 1:09 PM
> Subject: Re: REFLECTOR: Odd fuel pressure Problem
>
>
>> Scott,
>> Here is what I understand. The VM-1000 is a really good instrument.
>> there have been a lot of installation problems but the readings you
>> get from the instrument are correct usually. I have had the same
>> problem with a Lycoming IO-540. It seems that the lycoming fuel pumps
>> do actually fluctuate in pressure. The tolerance on the alarm of the
>> VM-1000 sometimes does not trip but sometimes it does. Visionmicro
>> does have a new software change that permits the delay of the alarm.
>> They also have a recommended fix for the fuel pressure. They
>> recommend the installation of a snubber in the pressure line just
>> before the transducer similar to the one that is in front of the
>> manifold pressure transducer. For some reason I can almost predict
>> the low fuel pressure alarm as I climb through 8500 feet.
>> Seems the digital instruments are telling us things we really didn't
>> want to know and would never see in an old analog gauge.
>> If you need more info I suggest you call Visionmicro systems. ask for
>> Dave he is a really helpful technical support guy.
>> We used to have to make a decision between JPI or Visionmicro for a
>> good digital system but now they are the same company or at least
>> there has been some sort of merger but VisionMicro is still the place
>> to call.
>> Why it took so long to happen on your airplane I do not know but I
>> can also go for weeks without any fluctuation alarms which also does
>> not make a lot of sense. I suppose temperature,pressure etc make a
>> difference.
>>
>> Jack
>> N55XL
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