Another Fuel Flow Indication Mystery...

[AlexPeterson]

Background:

1. AirFlow Performance Injection

2. Floscan mounted on firewall, in between the servo and flow divider. Straight 6" section of tubing on the inlet side of the Floscan "B". Floscan mounted with wires up as per their instructions.

3. Rock solid fuel flow indications cruise, matched totals vs fill ups.

4. No difference ever seen with and without fuel pump on, as would be expected for this Floscan position in the system.

5. 13+ years, 1464 hours on the setup.

6. Engine monitor - Rocky Mountain Instrument.

Since day one, the fuel flow reading has been erroneously high at idle or just above (taxi power), when cold. Indication at idle will be around 5.5 gph, which obviously is bogus. Leaning until just at the rpm drop does not lower indication significantly. Often the indication will drop to expected values by the time I'm ready to take off, perhaps in the 1.2 gph range. After a flight, it reads normally on the taxi in.

I've simply been ignoring the mysterious indication, since I didn't know what to do about it anyway. Over-indication is on the conservative side from a total fuel on board viewpoint.

After following the recent thread something like "Where does the fuel go?", I've become again curious about it. Tonight, while warming up the oil (for an oil change) on the ramp, I paid particular attention to the indications. As expected, it showed about 5 gph at idle. When throttling up to 1500 rpm, it dropped to about 3 gph, probably about right for that rpm. Back to idle - indication back up to 5. I repeated this 4 or 5 times. The settling time between power settings seems to be about 3 to 4 seconds. The ground run was about 15 minutes, and the idle indication never did behave. I did have the cowl off during this run.

I could speculate that, with the cowl off (ambient ~45F), whatever normally warms up (and corrects the erroneous reading) prior to take off did not warm up, hence the persistence of the behavior tonight.

I haven't a clue - any theories? I'll run and get some popcorn now...

[petehowell]

Was your run up and idle done ROP or LOP?

We need to know this before any real analysis can begin.....

[aerhed]

Also, were you in the left or right seat? We need to know this for parallax error.

[rv7charlie]

Vibration? Admittedly speculation, but if idle vibration frequency is causing the sensor or fuel in the line to resonate, it might be tricking the wheel in the sensor into forward/reverse movements large enough to generate extra pulses.

[DanH]

Charlie might be on to something there. Perhaps not anything resonant...just plain 'ole Newton.

Got photos of the installation, and the line from servo to divider?

[scsmith]

Originally Posted by rv7charlie

Vibration? Admittedly speculation, but if idle vibration frequency is causing the sensor or fuel in the line to resonate, it might be tricking the wheel in the sensor into forward/reverse movements large enough to generate extra pulses.

But the OP said that it reads properly once warmed up.

[AlexPeterson]

Thanks guys - I'm working FWF annual right now, so I'll get a picture tomorrow.

Keep the humor coming also!!

[AlexPeterson]

Here's the installation detail:

[jakej]

Originally Posted by AlexPeterson

Here's the installation detail:

Alex

FWIW I'd either rotate the FF unit clockwise slightly (to create a 'head' of fuel) or change the hose to the flow divider so that the fuel is going uphill out of the Flowscan. From my experience it's best to having the fuel flow at least slightly uphill to prevent any 'freewheeling' of the flow. As high frequency vibration can be an issue I always have the cube just included in the hosing, not mounted to any hard surface.

HTH

Jake J

[rhen]

Originally Posted by AlexPeterson

Here's the installation detail:

There is a good chance of capturing air in the flow device, creating a vena contracta and forcinghigher speed flow through the unit, thus showing an over reading, once air disipates, it goes back to normal