Fuel Flow Discussion, Moved from Float Mounting

[Battson]

I think it’s well established that a single 3/8 line will provide sufficient fuel under gravity feed conditions.

I think this is true for a carburettor engine with a fuel system built as Bob designed it.

We did the above, then added an electric pump with a bypass built-in, and a Red Cube fuel flow transducer. We couldn't get 150% flow with those things in the line, however we did get the 125% required for a continually pumped fuel system (engine driven pump with electric backup).

So I agree - 3/8 line is more than enough for what most people need. If someone was installing a carb engine with an electric pump only, and a red cube transducer - they might have issues.

[zkelley2]

You're not supposed to use the red cube on a gravity feed system. Gold or I guess black.

[Battson]

Yes, Gold is recommended. By the way, the cost of the Gold cube is as the name suggests...
I did wonder whether the transducer alone would make all the difference. The EFII pump system claims it doesn't impede flow when in bypass, but....

[zkelley2]

I'm not sure why someone building and wants gravity fed carb wouldn't just go 1/2 lines. Cost is minimally different and when you're installing it, it makes no difference. The AN fittings cost a couple dollars more each. But there's no way you won't get 150%. You don't have to redesign anything.

[Mark Goldberg]

Would 1/2" lines really make a difference with the 1/4 NPT fittings of the gascolator and outlets of the tanks? Mark

[zkelley2]

That's actually a really good question as the 1/4 npt would be the smallest diameter at that point. The reason I think it would help is because 3/8 lines can flow plenty of fuel, we know that, but it's usually us, the builders that put some combination of too many bends or too many AN fittings with sharp corners in there that restricts the flow. The AN8 fittings and 1/2 bends will flow more. That's my logic behind the 1/2" lines. But given a straight pipe out the fitting and nothing else, then yes, the 1/4 NPT would absolutely be the choke point and there would be no point.

Actually the fuel valve would be the smallest diameter, because though most of them use 1/4 NPT, the routing inside the valve is incredibly restrictive. Something like 1/3 to 1/4 the area of a 1/4 NPT ID. When I saw that I wondered how much better flow I could get with a better flowing valve.

[Nev]

Vapour-lock
I’ve been rereading this thread with interest after a chat with a friend and a discussion of the early Cessna fuel management recommendations above 5000ft (already mentioned previously but I’ll attach below).

The Cessna issue is with vapour-lock as opposed to un-porting, and probably pertains to a gravity fed system with no pump. Cessna recommended selecting either L or R above 5000ft altitude.

Density Altitude
Assuming that Cessnas recommendation of 5000ft would be density altitude I ran a few calculations.

With atmospheric conditions of high temp/low pressure/high humidity etc, it’s not difficult to get a density altitude above 5000ft when flying at 1000-2000ft AMSL. Flying in the tropics could easily result in fuel line vapor-lock at a very low altitude. This could also be the case in other areas when flying on very hot/humid/low pressure days, or with an engine fuel line routing that is prone to heating the fuel. Tight bends in the fuel tubing might cause a localised low pressure too.

Viewed another way, it could be possible to experience vapour-lock issues at very low altitudes when the atmospheric conditions align, or when our individual fuel system routing and design create those same conditions, even though most of us are following essentially the same fuel system design.
This may explain why some Bearhawks have experienced a problem and other similar ones haven’t.

Fuel Pump
Additionally, by adding a fuel pump to the system (as I have) in order to maintain system fuel pressure, in certain conditions the pump itself may cause a decrease in fuel line pressure aft of the pump, when combined with any of the issues already high-lighted throughout this thread (fuel transfer between tanks due to tank pressure differential etc) leading to a fuel line vapour problem.

Still very interested in thoughts and discussion on this, and I’m open to the idea that there may be several contributing factors to this issue, or perhaps more than one issue with the same end result.

D347B639-D6BD-4EF8-BFB6-FB53174DE74A.jpeg

[zkelley2]

Vapour-lock
I’ve been rereading this thread with interest after a chat with a friend and a discussion of the early Cessna fuel management recommendations above 5000ft (already mentioned previously but I’ll attach below).

The Cessna issue is with vapour-lock as opposed to un-porting, and probably pertains to a gravity fed system with no pump. Cessna recommended selecting either L or R above 5000ft altitude.

Density Altitude
Assuming that Cessnas recommendation of 5000ft would be density altitude I ran a few calculations.

With atmospheric conditions of high temp/low pressure/high humidity etc, it’s not difficult to get a density altitude above 5000ft when flying at 1000-2000ft AMSL. Flying in the tropics could easily result in fuel line vapor-lock at a very low altitude. This could also be the case in other areas when flying on very hot/humid/low pressure days, or with an engine fuel line routing that is prone to heating the fuel. Tight bends in the fuel tubing might cause a localised low pressure too.

Viewed another way, it could be possible to experience vapour-lock issues at very low altitudes when the atmospheric conditions align, or when our individual fuel system routing and design create those same conditions, even though most of us are following essentially the same fuel system design.
This may explain why some Bearhawks have experienced a problem and other similar ones haven’t.

Fuel Pump
Additionally, by adding a fuel pump to the system (as I have) in order to maintain system fuel pressure, in certain conditions the pump itself may cause a decrease in fuel line pressure aft of the pump, when combined with any of the issues already high-lighted throughout this thread (fuel transfer between tanks due to tank pressure differential etc) leading to a fuel line vapour problem.

Still very interested in thoughts and discussion on this, and I’m open to the idea that there may be several contributing factors to this issue, or perhaps more than one issue with the same end result.

D347B639-D6BD-4EF8-BFB6-FB53174DE74A.jpeg

That's a good POH snipet. Engine power irregularities of any sort are almost always fuel. Pump on, switch tanks. From memory followed by a checklist.

[svyolo]

I'm not sure why someone building and wants gravity fed carb wouldn't just go 1/2 lines. Cost is minimally different and when you're installing it, it makes no difference. The AN fittings cost a couple dollars more each. But there's no way you won't get 150%. You don't have to redesign anything.

The fuel valve manufacturers sells a couple of different size fittings for their valves. I also noted the size of the internal passages. The valves also have sharp corners internally. All that being said, they seem to work in most Bearhawks, but if you return fuel to the main tanks with either EFI or Continental, you might overtax the valve.

I think I have seen marine Groco valves in a few (Bob's?) Bh's. They are much cheaper, heavier, but some come with much bigger internal passages.

I am tired of changing my mind on things, so I am going to fly with the fuel valve that I have installed. 3/8 lines are more than acceptable for 180 hp. Maybe just adaquate for 260 hp, but they have a good track record. More than that, or you are flowing more fuel(EFI or Conti) to return to the main tanks, my choice would be 1/2". I am returning fuel to a header tank, so I am going with 3/8".

I think if you are "sucking" fuel though the Newton valve you might run the risk of exacerbating turbulence and allowing some vapor to come out of suspension. Sharp 90's might do the same. I think it is better to "push" the fuel through with gravity.

[Nev]

I have gone full circle on this and I'm not convinced that interconnecting the tanks will actually prevent this; having the benefit of clarity via Bob.
I am really sorry to "flip-flop" on this.
Having had the event at our home airfield I got very interested and I cannot understand some of the logic.

The facts are:

• This issue seems to only affect engines with a fuel pump.
• Any fuel pump will rather suck air than fuel, if air is available.
• Low fuel in one tank is universal in the incidents discussed.
• If the engine sucks for air long enough it will stop, whether the selector is on both or L/R

Occam's razor: It seems much more likely that sucking air bubbles due to prolonged unporting or running a tank dry caused these incidents. A cross vent will not prevent that.

The whole mysterious scenario of one tank robbing fuel from the other just seems unlikely. Sorry to the supporters of this idea.... but nobody has offered an adequate explanation, the necessary pressure difference is too great, and the experts don't support it.

Also the hypothesis of any plane with a "Both" selector needing a cross vent, we haven't provided evidence. I THINK the reason for the cross vent is probably as a backup in case one of the external vent blocks. That way, you can leave the selector on both and if there is fuel you're likely to get it, even if a vent blocks. That was the whole point of the both selector, avoiding accidents due to fuel mismanagement.

I understand Bob is considering providing some additional guidance about fuel system design.

Could it be that those who fly their aircraft in balance consistently don’t get to see this problem?

We used to balance fuel in a Metroliner by opening the cross-feed valve and flying out of balance. It only took a couple of minutes usually, and fuel would happily flow from one tank to the other, with no fuel pumps assisting.

A Bearhawk fuel system with the fuel selector in Both is the same, kind of like an open cross-flow valve.

But their are other ways to induce a cross feed as well. Having a pressure differential between tanks will do it. One fuel cap facing forward, the other not etc.

[svyolo]

One of the numerous rabbit holes I have been down in my build is fuel systems. I thought I would just use Bob's and be done with it but my choice of EFI complicated the issue. This is not my opinion, but "convention". Everything coming out of the FAA and stuff I read says if you are sucking fuel, you shouldn't do that on BOTH on fuel selector. If you are gravity feeding in BOTH on a high wing airplane, the two tanks should be vented together. You can actually google the history and layout of Cessna fuel systems over the decades. I can probably take a screenshot or two and post them here.

That is not my opinion but seems to be "convention". I don't have enough experience designing light aircraft fuel systems to have an opinion.

I think there is enough real world data and experience to say that a BH fuel system, as designed, will properly feed a 260 hp engine, either carborated or fuel injected (returnless)if the fuel injection doesn't return fuel to the main tanks, including running the fuel valve in BOTH. There is a question mark if 3/8 inch fuel lines can handle higher fuel flows that happen with returning fuel (Conti or EFI) without the fuel pump sucking on the fuel lines. I can't say one way or another, but chose to return fuel to a header tank, and vent that header tank to a cross vent between the two main tanks.

[Nev]

Fuel balance

During my first 15-20 hours I noticed that fuel was typically being used unevenly from each tank. This manifested itself each time I topped the tanks up. To mitigate the risk (when I was getting used to the aircraft) I simply kept the tanks closer to full than empty. I was also having the usual difficulty getting used to flying in balance after many years of yaw dampers doing the job for me. My observation is that the large rudder to vertical stab ratio on the Bearhawk B also makes it difficult initially (but it's great in a crosswind !). This was reiterated to me when I had a couple of experienced tail-dragger pilots who also found it very sensitive in yaw. Depending where the aircraft sits in the cruise (L or R yaw) can have quite an effect.

I did get used to it, and as others suggested to me on the forum it became second nature. A small rudder trim tab helped and I have it set for neutral at around 115kts IAS.I found that my fuel burns became incrementally more balanced. I now also select the left tank every time I shut down to prevent a fuel transfer in the ground. The undercarriage doesn't always sit evenly if there's a weight difference (imbalance) so it can transfer on level ground, and as it transfers it exacerbates the problem and causes even greater fuel transfer. Easily prevented by moving the selector out of "BOTH" when parked.

Typically my tanks are now within 2 liters now when I refuel (measuring the fuel pumped in). It's still early days for me and still very low time on the aircraft, but I've gone from having a fuel imbalance of up to 20 liters (gulp!) to 2 liters, (and quite often an imperceptible amount).

I guess this doesn't mean that this is the only cause of a fuel imbalance issue, but I thought I'd post this here as one more data point.

[Battson]

I now also select the left tank every time I shut down to prevent a fuel transfer in the ground. The undercarriage doesn't always sit evenly if there's a weight difference (imbalance) so it can transfer on level ground, and as it transfers it exacerbates the problem and causes even greater fuel transfer. Easily prevented by moving the selector out of "BOTH" when parked..

Any reason not to turn the fuel off, instead of leaving one tank ported to the engine?

Before anyone comes up with the old anecdote of taking off with the fuel selector "off".... first - please try priming and starting a Bearhawk with the fuel selector "off"... then we can talk.

[Nev]

Any reason not to turn the fuel off, instead of leaving one tank ported to the engine?

No reason at all - that would work equally as well to prevent fuel transfer

I have previously tested the scenario of starting the engine with the fuel selector off - it won't even pressurize, let alone start. So I agree, for a fuel injected Bearhawk the risk should be mitigated. Also for a carbureted aircraft with the correct sized carb bowl.

However one day I'll be in a different aircraft, so I try to develop habits that hold good for that scenario too. This is partly from losing a relative many years ago in this exact situation.

[whee]

Some fuel valves, such as the valves found in many Cessnas, connect the left and right tanks together in the off position. If your goal is to prevent fuel transfer it would be worth checking the function of the valve you installed in your airplane.

[Nev]

Very good point.

[zkelley2]

Any reason not to turn the fuel off, instead of leaving one tank ported to the engine?

Before anyone comes up with the old anecdote of taking off with the fuel selector "off".... first - please try priming and starting a Bearhawk with the fuel selector "off"... then we can talk.

Like whee said, in a lot of fuel valves, off is just both with the outlet pointed backwards, which is in this scenario the same effect as both.

[Battson]

Does this apply to any valve commonly used in the Bearhawk? Or are we having a trivia moment

[zkelley2]

Does this apply to any valve commonly used in the Bearhawk? Or are we having a trivia moment

I'll have to go back and look at some drawings to confirm, but I was under the impression that the SPRL valve most people use is that way.

[Battson]

I have one of the SPRL valves and it won't crossfeed in the off position.

[alaskabearhawk]

Same here. I checked my Newton SPRL valve and it doesn't crossfeed when in the OFF position.