REFLECTOR: Odd fuel pressure Problem

[Ronnie Brown]

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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