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<< Lancair Builders' Mail List >>
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Fred,
I have tried a second transducer with identical results. See the further
discussion in the next section.
I would guess you are running a Continental engine while Brent (I/O 540?) and
I are running Lycomings. Continental's injection system returns excess fuel
back to the fuel source. Some of the greater-than-actual usage may be due to
this and perhaps Continental publishes the rate at which it returns fuel.
Maybe Continental systems require two flow transducers, out-to-divider and
return-to-tank, where the difference would result in a more accurate reading.
In any event, read on--
Brent, thanks for breaking down the possible sources of inaccuracy -- Maybe
those with accurate readings could respond information about their
installation. Let's look at the items you mentioned:
1) Turbulence - Although you mentioned vibration, my horizontal, padded,
engine mount frame installation should not experience much vibration. I am
using Teflon, SS braid, fire sleeved lines and 6" of the input line is
straight while the exit line uses a 90 degree elbow to get the fuel up to the
spider. However, because the spider is fed with smaller fuel lines, there
are fitting reducers used on the transducer and these could introduce
turbulence. I would have thought that turbulence would reduce the flow
indication.
2) Temperature - This might be significant for me! My front mounted throttle
body sends the fuel back under the engine to the transducer which is mounted
in the hot, dead air behind the engine and then up thru the back baffle so
that only the last foot is in cool incoming air. Heat indeed! I am cooking
the fuel from the time it enters the cowling until it reaches that last foot
before the spider. I will try a blast tube on the transducer (for what it's
worth) and start to think about rerouting some fuel lines.
A friend with a pusher Cozy MkIV and VM1000, reverse cools the engine (air
exits the top) so that all the fuel components (including the pumps, lines
and transducer) are in the cool incoming air. I will ask him if his
flow/totalizer is accurate.
3) True Density - Where do you buy your light weight fuel? I calibrated mine
by volume and find the probes to be very accurate, even the short one in the
header tank. I would have thought the VMS computer would account for any
flow/density question using the third-party sensor.
4) Tolerance - I agree there might be a tolerance band, but my limited
experience with two would indicate that the band is very narrow, as it should
be.
Philosophical or Metaphysical view, sometimes described by the letters IMHO:
In my now ex-Skymaster (Continental powered), fuel flow was indicated in
pounds per hour (pph) except at full power where the indication in red was in
pounds per square inch (psi) and I believe this analog gauge was measuring
pressure all the time, displayed in weight (works if you know the orifice
size), and then mentally converted to gallons per hour (using the inaccurate
6#/gal). Thank God I wasn't in Canada using Imperial Gallons! The rear
engine always indicated 3-4 pph more than the front and, sure enough, after a
long flight, the rear tank (right wing) always took a couple of gallons more
as measured by the "very accurate" FBO fuel truck flow meter. Since the fuel
quantity gauges were notoriously unreliable, I always relied on flight time
-- my mental totalizer, backed up by the fuel truck and checked by the "flow
gauge." The flow gauge was pretty accurate for an analog needle gauge and it
was mainly used to logically lean the engine or adjust the fuel flow during
climb. Good enough for its' implicit accuracy band.
I have different expectations about digital gauges and computers. I expect
to see a very accurate display of sensor collected data. I expect engine
parameters (temperatures and pressures) to be within 2 or 3 percent, why not
flow?
I will not regress and fly primitive equipment (172) in order to fully
appreciate modern, digitally displayed misinformation.
I will take your suggestion to determine the offset (after some above
mentioned experimentation) and have VMS cut a new EPROM. In a discussion
with a VMS rep at Oshkosh, he suggested the same for the warning on fuel
levels -- I want the warning indication switched from 4 gals to 0 gals for
the wing tanks. Too bad I don't have more control over the parameters or I
would fix them myself.
Modern systems should provide that parameter driven controls are changeable
thru the device itself. For example, the Garmin 430 was not producing enough
sidetone and I thought I had to return to the installer for some internal
adjustment -- not so, a special power-on sequence gets you to a series of
pages which control internal functions, one of which is the output level for
the sidetone. Voila!!
Scott Krueger
N92EX
The 'E' is for Experimental.
The 'X' is for the slashes from Occam's Razor.
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