Fuel system testing and unporting tests.

[Nev]

Yesterday I spent 2 hours at 8000 ft trying different fuel scenarios.
Firstly I ran the Left tank dry by running only from the left tank. It took quite some time to run the fuel level down as I was running LOP.

The engine first surged several times with approximately 5-8 liters in the tank, so I think it was unporting at this stage. I then selected the right tank, and the engine restarted within around 5 seconds with the fuel pump off.

At this stage I could still see fuel in the left sight gauge, so I repeated the same process a number of times. Each time I was surprised at how long the engine would run for before stopping again. I was also able to keep it running by putting the aircraft out of balance (and flooding the ports )such that the fuel pickups were flooded. Eventually this was no longer possible and at that point I could no longer start the engine from the the left tank. One point that was clear is that with little fuel in the tank, being out of balance away from the ports very quickly unported the tank and stopped the engine.

During this time on several occasions I selected BOTH, and observed that the engine restarted each time in around 5 seconds and ran well. Each time the engine stopped it was preceded by the fuel pressure dropping approximately 10 seconds beforehand and as a result I've now set the low fuel pressure alarm to come on at 20 psi, which just gives enough warning to change tanks. The fuel pressure could also be seen rising just before the engine restarted.

Once the left tank was completely dry, I selected BOTH again, the engine restarted, and I flew for approximately 45 minutes at a cruise power setting. Then I put the aircraft well out of balance and flew for 5 minutes before repeating the out of balance to the opposite side. The engine continued to run normally. At various times I turned the electric fuel pump on, but there was no difference.

Finally, I descended to 2000ft (still on BOTH, with the Left tank empty). I then set climb power and climbed through several thousand feet with a pitch attitude of 10-15°. The engine continued to run normally. I landed with 30 liters in the right tank, and the left tank empty. I then refuelled and noted that the left tank took 103.6 liters to fill.

This exercise has given me alot more confidence on my aircraft to run in the BOTH position, even with a large fuel imbalance. It has also made me reluctant to select an individual tank when it has a low fuel quantity. When running LOP my fuel burn is around 37-42 LPH, so 30 mins reserve is only 20 liters, or 10 aside. I'm still getting my head around this, but I would think that on my aircraft the BOTH position would be safest as Bob intended.

I'm posting this here as another data point regarding possible unporting of tanks in other threads. Bear in mind that I'm not a test pilot, and all of our aircraft have differences. Mine is a Bravo 4 place model, with an IO540, electric fuel pump, no return lines, and a fuel selector with L,R, BOTH, OFF.

I've attached a short video showing one part where I had the left tank selected before it was completely dry, and I was flying out of balance. After the engine surges several times, then stops, I selected the BOTH position and it restarts with the fuel pump off after about 5 seconds.

Incidentally the monitor sitting on the dashboard is a carbon monoxide monitor, which I've now installed permanently on the right wing root panel. It works much better than the card type, has an audio alarm, and a digital readout. It costs NZD$53, which is a cheap insurance in my books and I've learnt that the cards don't often register a low but continuous CO level. As a result I've also installed a small gurney flap ahead of the gascolator fuel drain which has cut the CO ingress a lot when running ROP. When LOP the levels are zero.

Here a link to the video showing testing of my fuel system, and my experience with carbon monoxide.

[jaredyates]

Fantastic work Nev, it is great to get real testing results and know your machine!

[Mark Goldberg]

Very interesting Nev. Thanks for taking the time to methodically look at this. Real world testing is worth a lot. Granted - this was just your plane with the way it was built and its fuel system set up.

Your courage also needs to be commended. I could never be up there and killing my engine repeatedly. My heart would likely fail itself from high blood pressure - or at the very least I would need a change of pants & underwear. Mark

[marcusofcotton]

Excellent work and report!

[alaskabearhawk]

Respect...thanks!

[Ed.Meyer]

Thanks you Nev for this. Your tests were much more extensive than mine in my Patrol which were posted here sometime ago and Jared included in a Beartracks issue. I, like others, did not have the nerve to take it to engine stopping several times, but did enough to convince myself that unporting one tank would not create fuel starvation to the engine when running on both. Your tests further increase confidence for me as well.

[robcaldwell]

Wow! That takes some serious huevos Nev! You're in the Bob Hoover club now!

I tested my tanks in the Left, Right and Both position at 3/4 full at 5,000. That was about as brave as I could get! Though highly unlikely, I imagined a vapor lock / hot start scenario while trying to maintain best glide and looking for a patch of grass.

[Utah-Jay]

I like the fan turning when I am AGL

[schu]

Good work Nev.

I’d like to point out that this appears to be an IO with an electric and mechanical pump. These results do not apply to those running a carb and gravity, and as Mark said, could be very specific to this airplane even if you do have pumps.

[Nev]

Thanks Schu. Yep that's correct, actually exactly why I did the tests to see if having a pump(s) in the fuel system changed the outcome.

[N3UW]

Nev,
Very nice work and writeup.

[Bissetg]

This is really interesting Nev. I’ve heard about injected Bearhawk engines stopping due to Unporting. My understanding is that a fuel pump will rather suck air than fuel if it’s given the choice. Whenever I’ve run a tank out intentionally (haven’t done it in a Bearhawk) on an injected engine, the drop in fuel pressure and change in engine running has given ample time change tanks without the engine stopping. That would seem to be consistent with your results.

My question is then, how or what is different with the Bearhawks that have unported and stopped?

Given that I’m in the process of changing my B Model from a carbureted engine (that did stop but nothing to do with lack of fuel) to injected I’m really keen to understand what creates the risk and what can be done to mitigate it.

[Nev]

My question is then, how or what is different with the Bearhawks that have unported and stopped?

At this stage Grant, I don't think it is peculiar to Bearhawks, and I'm no longer convinced that it is confined to fuel injected engines (with a pump sucking fuel).

PaulSA touched on this in another thread that I didn't understand at the time with regards to whether or not the system will suck air with a pump running. Thinking about it now, with an injected engine, both pumps are forward of the gascolator, and so as long as you have sufficient head of pressure feeding to the gascolator, I think the pump inlet will remain flooded. When I did the fuel flow tests before flying, even a single tank was capable of a flow rate well in excess of the full power fuel demand, so my pump inlets are always flooded (the gascolator would always remain full) and therefore there is no tendency to suck fuel (or air).

There are two situations where this could change though - a blockage in any part of the system up line from the aux pump that slows the flow rate (for instance a partially blocked finger strainer etc), and unporting one tank when the other tank is empty (turbulence or out of balance scenario).

Even with a small amount of fuel in each tank, an out of balance situation would unport one side and flood the opposite, so no issue. But as soon as a tank is empty and the other tank is low enough that it can unport, then I think an out of balance situation that uncovered the ports would cause a stoppage.

Yaw Stability
This was why the yaw stability had my full attention and hence the related thread.

Incidentally, my ailerons had a fairly significant reflex in them initially that gave light and responsive roll control. Two days ago I reduced the reflex on both ailerons by 1.5 turns of the rod end bearings on each side. The roll response is now slightly heavier (but has a nice feel to it), but the yaw appears to be slightly more stable at a forward CG - something I wasn't expecting. I've also changed the rudder pedal springs to what Budd Davidson suggested (5lbs/inch). This also helped a small amount. My rudder trim tab now gives an in trim speed of around 110kts. At lower speeds I need to apply light right rudder (during climb for instance) and at higher speeds a small amount of left rudder.

I think that perhaps a combination of all these factors has improved the yaw stability, and combined with gaining flight time on type, it's much easier now to remain in balance. I've been videoing a number of flights and when I play back the video I notice now that it's nearly always in balance.

So for someone with significant experience or currency in similar aircraft this probably wouldn't be an issue and you'd naturally tend to fly in balance all the time without thinking about it, even if the aircraft was less stable in yaw. But for myself and perhaps one or two others like me either through being less current or new to aircraft that require you to be more proactive to keep the aircraft in balance, it could be very easy to fly out of balance. Particularly if the individual aircraft itself was less stable in yaw during initial flights as you fine-tune the rigging, as mine seemed to be for the first 20 hours.

This could potentially lead to an out of balance fuel situation, run one tank dry, and unport the remaining tank. An easy way to avoid this is just to carry more fuel until the aircraft rigging has been fine-tuned and keep the aircraft balanced.

[Bissetg]

Thanks Nev, your logic seems sound. It would be good to hear from someone who has experienced the problem to see if they can corroborate your proposition. Keep up the good work.

[svyolo]

There are several fuel system threads, and people reporting their fuel flow tests. I came to the conclusion that stock fuel system with 3/8 lines are totally compliant for 200 hp, marginal for much more than that. An MFI injected Lyc 540 doesn't flow any more fuel than a carburated one. I don't see any difference. The big difference is with Conti or EFI injection where they flow excess fuel, and return fuel to the main tanks. If they return to a header or "collector" tank (not my term, but from a Boeing engineer), the fuel flow from the mains is the same. 3/8 lines are not big enough if you return to the mains in those cases. Either increase the size of the fuel lines, or return to a header tank. That was my choice. Whee chose 1/2 inch lines, which also works.

Bob's fuel system is designed to be gravity fed, not suction fed. If gravity isn't enough, I think more gravity is better than more suction. I think a lot have gotten away with a 540 BH with 3/8 lines, no problem. Fuel pumps or not. But I don't think their is a lot of headroom there, although the prototype BH5 has 320-ish hp and 3/8 lines.

Nev, awesome info. Keep it coming. My flying experience, and lack thereof in taildraggers, is similar to yours so I am learning from yours.

[Nev]

That's a very good point about the extra fuel Flow required for a system with return lines. I wonder if there's a link there.