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Fuel Return line flow rate

engineerofsorts

engineerofsorts

Well Known Member
Can anyone enlighten me on what the fuel return line flow rate should be--or rather, what it will look like. I checked the flow with an endoscope, and observed the return flow was not constant--the fuel level in the return line bobbed up and down, overflowing and putting out a drop or two of fuel every five seconds. I vaguely recall observing this a while back, and in was a constant, though slow flow.
 
On mine, I turned on the master, opened the fuel valve and adjusted the return fuel flow (engine off) until the fuel flow read zero. It varies a little bit over time, so I tried to even it out to get as close as possible to zero indicated flow. Apparently it worked; my computed and actual fuel usage were spot on after a 3-hour flight.
 
Now I'm confused

DaleB said:
On mine, I turned on the master, opened the fuel valve and adjusted the return fuel flow (engine off) until the fuel flow read zero. It varies a little bit over time, so I tried to even it out to get as close as possible to zero indicated flow. Apparently it worked; my computed and actual fuel usage were spot on after a 3-hour flight.
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I'm a bit puzzled by this. What adjustment of the return fuel flow is there? My concern is with getting proper return fuel flow--so that fuel is circulating back to the tank enough to keep vapor locking in check. I was not aware of any control of return fuel flow--I am concerned that I have some blockage in the return path restrictor plug such that I am not getting enough flow back.
 
Ah. Two completely different views of the same thing. :) I adjusted the Dynon return fuel flow parameter to match the actual return fuel flow. There is no adjustment that I know of for the return flow itself.
 
Jetguy said:
Rob, did you check the return flow with the engine running?
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No--with engine off--wanted to know if I was getting any circulation after engine shutdown while keeping the electric fuel pump running--still troubleshooting all avenues of my vapor lock problems--my thinking being that the flow restrictor on the fuel return line is partially clogged.
 
DaleB said:
Ah. Two completely different views of the same thing. :) I adjusted the Dynon return fuel flow parameter to match the actual return fuel flow. There is no adjustment that I know of for the return flow itself.
Click to expand...

I guess I should look again at the EMS setup--I can't recall a fuel return parameter to set unless you have flow meters in both directions--I was hoping there was a constant in the equation so I could subtract out the return flow as a constant, but the only thing I recall from the setup menus is the K factor (multiplier or pulses-per-gallon).
 
engineerofsorts said:
I guess I should look again at the EMS setup--I can't recall a fuel return parameter to set unless you have flow meters in both directions--I was hoping there was a constant in the equation so I could subtract out the return flow as a constant, but the only thing I recall from the setup menus is the K factor (multiplier or pulses-per-gallon).
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We may have different systems... mine is the D-180. I don't know if Skyview has that setting or not. The return flow setting is a lot cheaper and somewhat less precise than having two flow transducers.

The only reason I even know this is thay after doing a software update on the Dynon, my fuel flow read way high - like 3 or more GPH high. It read about 3 GPH with the engine off, as I recall. I found the return flow parameter set it so that the indicated fuel flow was zero, and that fixed it.
 
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I looked at mine with a video scope once after a spurious hi fuel flow alarm. With the electric fuel pump on it looked like a weak water fountain just bubbling over. I think some guys have posted that the flow is pretty strong with the engine driven pump running, but I have no personal observation with the engine driven pump recirc flow.
 
With the electric fuel pump on, the D-180 indicated 1.5 gallons per hour.
With the electric fuel pump off, the D-180 indicated 0 gallons per hour.
Edit: In retrospect, those number do not seem right.
 
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Rob
re the return line making fuel flow on EFIS incorrect.
I had hoped to get acccurate fuel flow measurements till it was pointed out to me why it over read on EFIS.
There are 2 threads I started:

http://www.vansairforce.com/community/showthread.php?t=136571

and

http://www.vansairforce.com/community/showthread.php?t=137075

I am told Tecnam Rotax installation have red cube or Floscsan in engine compartment that takes its reading from beyond the place return line takes off and is thus accurate. Not sure if RV-2 can do same thing.
 
Restrictor hole size

Based on both online and offline responses, I've concluded that my fuel return line is most likely partially blocked. I will be inspecting the return line flow restrictor for booogers. Does anyone know what the diameter of the flow restrictor's hole should be--my guess is that it is smaller than the smallest numbered drill that I have in my collection.
 
Mystery solved

After checking the restrictor and fuel lines, I shoved the endoscope a bit further down into the tank and discovered the return line has a crack right where it bends up into the filler tube. Seems the flow is fine, and I assume shouldn't be a problem for now. I'll repair it next time I have to open up the tank.
 
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