Fuel flow calibration and sight gauge issues

[Nev]

I've now got around 27 hours on my 4-place machine, and thoroughly enjoying it. Already we've made quite a few small changes in many areas. I'm still chasing down a few issues one of which is inconsistent fuel readings.

After some flights, I appear to have burnt more fuel from the left tank than the right. Other flights burn equal amounts, so I suspect it's more my own stick and rudder skills, and to a small degree, changes in the rudder rigging, springs etc. I'm continuing to refine this. You may have seen my posts relating to yaw etc, so there is definitely a possibility of imbalance. But I've been videoing the screens quite alot on recent flights and the ball is usually centered, so I'm no longer thinking that an out of balance scenario can explain everything.
I'm also checking that the fuel cap vents are facing forward as evening as is possible on each flight.

Today I leveled the wings in the hangar. This isn't as straightforward as I would have thought and due to quite a large amount of movement in the suspension the wings can sit at a range of angles quite happily. Once I had the wings level (and supported in that position), I selected BOTH on the fuel selector and left it for an hour. The fuel found its own happy level, and I became aware of a small error in the individual tank calibration. Thinking back, I think I had a wing low when I did the calibration without realizing it. So now I think what is happening is that when I check the sight gauges, it indicates an imbalance when one doesn't necessarily exist. I've now mostly corrected for this.

However, there is also an actual imbalance on occasion too, I know this because when filling the tanks I've had to pour up to 25 liters more into one tank than the other. (We don't use gallons down here, they don't work properly below the equator).

I'm posting this because some of you may have had similar experiences and may already have a solution or some helpful ideas. In the meantime I'm not keeping significantly higher reserves.

[predragvasic]

We don't use gallons down here, they don't work properly below the equator

Just brilliant!!!

[robcaldwell]

I have had the same results with my fuel levels and sight gauges. I really never thought much about it. As with the yaw discussion, I cannot ever expect the Bearhawk to fly in a perfect balanced attitude 100 percent of the time. Sometimes I need to rest my right foot on the right rudder pedal. Other times I do not. I'm no engineer, but it seems to me that with an open system of fluid that is connected on both left and right sides, and is permanently maintained outside of the principal 3 axes with unperceived forces always in motion, unbalance is always going to occur. I think of a water level.

[schu]

I've now got around 27 hours on my 4-place machine, and thoroughly enjoying it. Already we've made quite a few small changes in many areas. I'm still chasing down a few issues one of which is inconsistent fuel readings.

After some flights, I appear to have burnt more fuel from the left tank than the right. Other flights burn equal amounts, so I suspect it's more my own stick and rudder skills, and to a small degree, changes in the rudder rigging, springs etc. I'm continuing to refine this. You may have seen my posts relating to yaw etc, so there is definitely a possibility of imbalance. But I've been videoing the screens quite alot on recent flights and the ball is usually centered, so I'm no longer thinking that an out of balance scenario can explain everything.
I'm also checking that the fuel cap vents are facing forward as evening as is possible on each flight.

Today I leveled the wings in the hangar. This isn't as straightforward as I would have thought and due to quite a large amount of movement in the suspension the wings can sit at a range of angles quite happily. Once I had the wings level (and supported in that position), I selected BOTH on the fuel selector and left it for an hour. The fuel found its own happy level, and I became aware of a small error in the individual tank calibration. Thinking back, I think I had a wing low when I did the calibration without realizing it. So now I think what is happening is that when I check the sight gauges, it indicates an imbalance when one doesn't necessarily exist. I've now mostly corrected for this.

However, there is also an actual imbalance on occasion too, I know this because when filling the tanks I've had to pour up to 25 liters more into one tank than the other. (We don't use gallons down here, they don't work properly below the equator).

I'm posting this because some of you may have had similar experiences and may already have a solution or some helpful ideas. In the meantime I'm not keeping significantly higher reserves.

Nev,

How many airplanes have you flown with a BOTH fuel selector valve? What you are seeing with fuel imbalance isn't really uncommon at all. In order for both tanks to feed perfectly evenly you need to have pretty much the identical fuel layout on both sides, absolutely identical venting on both sides, and fly the airplane perfectly straight. As you have sorted out, that's not really going to happen.

So, the question becomes more of limiting this than eliminating it.

You have already taken the time to absolutely confirm that accuracy of your fuel gauge, so about the only other thing I can think of is to add the cross vent between the tanks. That will take the area above the fuel level and even out the pressure (if any). If one vent is working slightly better than another, both tanks should see the effect equally.

At that point you would have what Cessna drivers have, but it's still not going to be perfect. You could work really hard to make sure that the fuel line runs on each side are a mirror of each other, but there is only so much you can do there.

Personally, I just eliminated the both valve and am going to do the piper thing and select left or right. Modern avionics have the ability to alert you every 15 minutes to balance fuel. If the engine acts up, I switch to the other tank. When I'm landing I switch to the fuller tank and I don't need to think about one tank being empty while the other with 1/4 when the valve says both. I should note that I also have larger vertical fuel lines and I'm also running pumps with an IO, so my system is designed to work this way.

Part of the decision for me is I don't want to bother with the cross over vent, and because even my Cessna 170 has been placarded to run single tank operation at cruise above 6000 because of unexplained engine stoppages.

NOTE: To others reading this, this is my opinion, I'm in the minority, and you should absolutely confirm that your system will feed a big engine with a single tank before you try it.

[Nev]

Thanks Schu. That's largely what I was after - to know whether what I'm seeing is normal. I plan to run a tank dry at altitude when weather next permits, to ensure that it restarts fine, then run on BOTH with a dry tank at altitude to ensure no other gremlins. That should then give me the confidence I need to resume normal fuel management.

To answer your question, I've actually only flown one aircraft with a BOTH selector prior to the Bearhawk, a C172. In those days we were blissfully unaware of these issues, and I believe it didn't have sight gauges, just the usual wobbly fuel tank indicators. So we would have assumed an equal fuel burn and written our 30 minute fuel remaining notes accordingly. On many occasions we were out to sea doing fish spotting at 600ft, well out of sight of land.........

[svyolo]

Nev;
I am glad you are here, to answer the questions I would have had to ask myself. LOL My last GA airplane flight was 1995 and that was maybe 5 hours. The other 80 hours was 1980-82. No TW time yet.

The 5 hours in 95 weren't pretty, but got me an ATP. I doubt 27 years later will be any better.

[Nev]

Yeh fuel management on the big Boeings was so much easier.....turn all the pumps on and wait until a message told you to turn them off