I think conventional wisdom is to display unmetered fuel pressure on your engine monitor. At least that's the way mine is set up. That's the number that's listed in the TCM operating limitations. As for the setup using the SID, fuel flow is what you should pay attention to at the top end and use the higher end of the range as the minimum. Since we take off with our low boost on, I set up my engine to be at the upper limit of 27.3 without the boost pump on and I get another 1.~ GPH with the low boost. TCM says the unmetered pressure is just for reference when setting up the fuel injection, that you should use metered pressure and fuel flow.
After some investigation, I believe there's a direct corrrelation between metered pressure and fuel flow and a very close correlation between unmetered fuel pressure and fuel flow. I took several hours of data recorded by my engine monitor and imported the data into Excel and compared the numbers and the numbers move within the accuracy of the measurements. With this correlation, I believe you could set up your fuel flow to the right number and assume your metered fuel pressure is correct. The one big assumption is whether there's any leakage or blockage in the system. By checking the metered and unmetered pressures and fuel flow, you have a way of confirming that all the components are working correctly.
Bottom line, follow the procedure exactly, or get someone to do it for you.
Mike Easley
Colorado Springs
-----Original Message-----
From: Paul Bricker <pbricker@att.net>
To: lml@lancaironline.net
Sent: Tue, Apr 20, 2010 1:17 pm
Subject: [LML] Re: Fuel Pressure Setup
Dan,
Fuel flow is what we really care about. It is roughly proportional to
metered fuel pressure. I installed sensors that measure actual fuel flow so
metered fuel pressure is somewhat redundant. For these reasons I monitor
unmetered fuel pressure. My logic is it may give me a warning of fuel pump
failure or fuel line contriction before it would be reflected in the fuel
flow.
Paul Bricker
N63PB
-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Dan
Ballin
Sent: Monday, April 19, 2010 1:39 PM
To: lml@lancaironline.net
Subject: [LML] Re: Fuel Pressure Setup
While I think this is all good information, the original question Jon
had and I have is how to monitor fuel pressure. The setup procedure
using SID97 is a different issue. I think the more interesting
question is does one monitor metered or unmetered fuel pressure in the
cockpit. It seems that TCM and at least Coumbia (Cessna)
recommend/use metered and that Lancair generally sets up their
aircraft to use unmetered. I haven't gotten a good answer as to why
and what the advantages and disadvanages are. Obviously the values
are different, but they are measuring essentially the same thing.
Can anyone comment on what advantage/disadvantage there is to using a
metered vs unmetered source for you fuel pressure measurement while
flying.
Dan Ballin
LEG2 #286
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