How does the vintage FF instrument work

[MV Aviation]

Hi folks,

while inspecting the fuel system on my 65E for the installation of a JPI FF sensor, I noticed that the main fuel line from the throttle body goes up in-between cylinders 1&3 into the distributor. From there all four cylinders are fed. So technically, the best location for the FF sensor would be in the hose from the throttle body to the distributor. Googling for pictures of other installations brought up similar solutions.

Now to the part I don't quite understand. Another (6th) line comes out of (or goes into??) the distributor. This hose goes through the firewall and, as far as I can see, into the analog FF/MP gauge. But, (again) as far as I can see, there is no return line. So, my question is: how does the analog FF gauge work (physically)? Is it correct that only one fuel line goes into it. I'd expect that, in order to measure a FLOW, there would be one IN and one OUT line necessary.
Anyway, the point why I'm investigating this is that if the FF from the throttle body to the distributor is NOT the net FF to the cylinders (since some fraction of the flow goes somewhere else), the JPI FF reading would not be correct.

Any thoughts?

Thanks,

Marco

EDIT: further research on the web brought to light that the old FF gauges are correlating FF from fuel pressure. In that case, only one line into the gauge would make sense. AND this would mean that no actual flow is leaving the spider through that line. Hence the JPI would indeed read the net flow to the cylinders. Can anyone confirm this?

Edited September 2, 2017 by MV Aviation

[kortopates]

You are correct, old legacy FF gauges only measured fuel pressure, not flow at all. And these days we would never bring a fuel pressure line into the cockpit, nor an oil pressure line. Instead, using fuel pressure for an example, you'd take the hose coming off the fuel divider and take back to the firewall where there is less vibration and mount a pressure transducer on the end of the hose and just route the electrical wires through the firewall - the advantages of modern electrical instrumentation over your OEM analog gauges. The STC FF documentation will tell you exactly where to install the FF. I haven't seen any that weren't installed inline between the fuel pump and the servo. But refer to your documentation. Mooney also provided drawings for the J model when they were installing them so it might be worthwhile to check a J IPC to see how Mooney physically installed them since your setup should be pretty similar, but main requirement is no bends in the line close to the fuel flow transducer.


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[carusoam]

Marco,

Add this to your knowledge base...

From an engineering perspective.  FF can be derived from the pressure drop across the device.  As long as the flow geometry is simple, doesn't change, and doesn't have another unknown varying restriction to flow downstream... it generally only takes a single pressure sensor to make work. It is just a fuel pressure indicator with its gauge calibrated appropriately in gph...

Some Mooney's got FF devices, most got FP devices...

The early O's got only a FF device, but no FP device.

FF gauges since the 90s are generally measuring the flow of fuel in the fuel line going to the engine, using a paddle wheel with an imbedded magnet that rotates in the stream.  The FF instrument is calibrated with precision using the adjustable K factor...each time the paddle wheel rotates, a sensor counts the magnet passing by.  A small computer counts the magnet pulses over a period of time, then displays this data as gph...

MP, manifold pressure, comes from the engine intake, even though it is in a shared indicator... MP will not be derived from the fuel lines...

In 1965, it was normal to bring oil, fuel and MP lines to the cockpit to convert to OilT, FuelP, and MP.  By the 90s the thirty year old lines were breaking and falling off causing leaks of hazardous fluids in places they don't belong.  Modern electronics and/or digital technology makes these hazards completely a non-issue...

An interesting observation is the size of the fuel line going to the cockpit. It looks a least as large as the the lines feeding a cylinder.  If it is only delivering the hydraulic pressure it wouldn't need to be very large.  Fuel isn't known to be compressible and there isn't any flow. (Things that add error to a pressure reading)

Look to JPI, EI, and others for sensor technology that would be a way to eliminate these hazards in your plane.  Modern FL gauges are starting to be adopted as well...  search for CEIS for this.

Eliminating fuel and oil lines to the instrument panel is a good idea.  Other fuel lines running through the cabin Go from the tanks to the selector switch/valve.  They have some rubber Seles and connectors that are worth some attention while you are modernizing your ship.

Hope this helps.  PP ideas only, not a mechanic.

Best regards,

-a-