Fuel Flow Discussion, Moved from Float Mounting

[Bcone1381]

The image below is more invasive than the image in post #94. It shows a vent line running from the right outboard tank across and passed the left tank. Its pilot proof. If parked on a slope with full tanks fuel would only drain out of the lowest fuel cap.

A tank can be isolated with shutoff valves (Like installing one in the fuel tank interconnect vent). Although valves like that are far from pilot proof they should be installed so the are accessible in flight.
Screen Shot 2020-02-07 at 5.19.39 AM.png

[Mark Goldberg]

One data point to mention is the size of the vent hole on Bob's fuel caps. He designed the hole quite small so that insects are unlikely to block them. I say unlikely because there are a lot of planes out there using Bob's fuel caps, and I have never heard of a case of insect blockage. But it is a possibility. Icing up the vent holes would seem like a bigger problem. Mark

[JJohnston]

Wouldn't the Cessna 177 system prevent spillage from parking on a slope? The vent from each tank crosses over to the other side of the opposite tank before turning down; plus I think that little short branch connecting the two crossvents must be a "siphon break". On a slope, the fuel from the low side would only go into the vent until it was at the same height as the high side. Or am I way off? I'm a civil engineer, but structural; my hydraulics are weak.

[Battson]

Until you park on a slope and then you have an automatic fuel drain, but I can't see any way around this.
I like the cap vents and the cross vent between fuel gauge lines

[Battson]

I can't think of any way to use check valves on the cross vent line. If you prevent cross feeding in both directions, then you prevent the pressure difference from equalising too.

Funny how the issues with a fuel system design are not obvious until you think about it critically, next minute they stare you in the face.

[Bcone1381]

Something like this is pretty non-invasive. This suggests fabricating and installing a cross vent between the top sight tubes, adding a vent like Whee's fuel cap nozzle on the cross tube and placing it at the wing root faring, plus adding short vent lines between the rear fuel supply lines and the cross tube.

Alternatively the atmospheric vent for the tank interconnect vent could be run outboard of a tank and down through the bottom wing skin as long as the the atmospheric vent line rises above the highest point in the tanks.
Screen Shot 2020-02-06 at 9.14.45 PM.png

[Bcone1381]

This is all getting quite theoretical and esoteric, it makes it hard to discuss!

Battson, what is not theoretical and esoteric to me is your fuel system in your aircraft has proven itself to be dependable. You fly in BOTH and you know what a normal fuel balance is. In the future if you see an abnormal fuel imbalance, then a fuel venting problem may caused the imbalance. Be suspicious of the fullest tank. Switching from BOTH to the LOW tank might be prudent.

[Archer39J]

Sure the engine isn't gravity fed but the pumps are, verses pumps that are sucking fuel. The pressure differential in the tanks isn't more than the pressure head at the fuel selector. There's no difference between a gravity fed pump and a gravity fed engine at that point, and one clogged vent shuts off that tank after a while. I don't know what would happen with a pump that is sucking fuel, but I think it could only be worse.

[svyolo]

Cessna fuel systems are pretty Googleable. It is amazing how they evolved over the years. They didn't get simpler. I guess over the years the fleet got so many hours they kept finding a new corner of the envelope where the system didn't quite work.

I think I do want some additional venting. Right now I think I will just go with the cross vent in the middle, maybe with a shutoff valve. Mostly because it is easy to implement. I am not sure it is the best. I really like the Cessna 182S venting, but at this point it looks too hard to do. Maybe next time.

[whee]

I think I fixed the pic.

Theses "snorkel" style caps use either a cork or rubber gasket and their function is indeed to provide positive pressure inside the tanks.

I'd offer to induce the failure in my own airplane and video record it but since I have Continental fuel injection it would be of little value to anyone and the failure would probable get attributed an incompatibility with my FI. This summer I'll be testing an LSA. If I get feeling brave maybe I'll induce the failure in it and record it. But it is scratch built...

[Battson]

The caps were ram air pressurized. Same function as the Bob caps just a different form. The fuel system was built exactly as Bob specified except Bob caps were not available so a conventional cap was used.

Picture seems to have failed?

Did they also have the o-ring seal to ensure the tank could develop positive pressure? As the air rushing over the tank could theoretically develop suction, if the vent blocked.

This is all getting quite theoretical and esoteric, it makes it hard to discuss!

[whee]

I understand your machine had some other kind of vented cap, where the vent is not pressurised by ram air. It was unwritten, but I have assumed that unpressurised fuel cap must have played some part of the cause of the issues with your last plane. Set me straight if you see fit.

I think we've established that with a gravity system conforming with Bob Barrow's design and with his design of fuel cap, the risk is negligible. As ever, it appears Bob is exactly right, let's be clear on that. We have no evidence of failure in a system entirely as Bob intended it. Bearhawk Aircraft has always been very clear that we diverge from Bob's approved design at our own risk. So here we are.

With a conventional design, gravity feed system, it appears the chances of a failure seem almost impossible when you run the numbers. The engine needs so little fuel pressure to run and one of the tanks is always under some pressure, which reduces the risk further.

In post #81, I drew the conclusion that fuel injection means you need to install this vent. I think that was jumping two steps ahead when I meant to just take one. I think all we can say for certain is, some fuel injection systems are at-risk of this failure, the high pressure ones. We actually have no evidence to suggest Bendix systems are at risk.

Similarly, I didn't say that ONLY fuel injection systems are at risk. There are a million ways to design a dangerous fuel system, and only a small number of ways to make an inherently safe system.

The caps were ram air pressurized. Same function as the Bob caps just a different form. The fuel system was built exactly as Bob specified except Bob caps were not available so a conventional cap was used. See pic of the conventional cap style cap.



U286116__92249.1368114694.1280.1280.jpeg

[Archer39J]

The check valve idea is interesting, and I am especially tempted because the cross vent would need a check valve anyway.
However, I am not sure whether I would trust a check valve in my main fuel line - seems to introduce another potential engine-stopping failure point.
In the vent line, a failure of the check valve is less risky.

Yeah I was thinking a check valve in a separate vent line that terminates inside the wing with a weep hole just in case, I described it in a comment a bit ago. That would solve the clogged cap issue but the commercially available ones I've found have opening pressures slightly too high for a reversed vent case so I'm considering making my own.

Here's the one I was looking at https://www.mcmaster.com/5492k51

ETA: I think the risk for my system is low enough for the reverse vent case though, fleet history being what it is in addition to my analysis. My main concern is icing since two clogged vents will take you out of the sky.

[Battson]

It affects both gravity feed carburetor systems and fuel injection systems. Matt’s failure was with Continental Fuel injection. The two times I experienced the failure it was in a Lycoming O360 powered plane. Bendix fuel injection might actually be more tolerant of this issue because of the types of pumps used.

I understand your machine had some other kind of vented cap, where the vent is not pressurised by ram air. It was unwritten, but I have assumed that unpressurised fuel cap must have played some part of the cause of the issues with your last plane. Set me straight if you see fit.

I think we've established that with a gravity system conforming with Bob Barrow's design and with his design of fuel cap, the risk is negligible. As ever, it appears Bob is exactly right, let's be clear on that. We have no evidence of failure in a system entirely as Bob intended it. Bearhawk Aircraft has always been very clear that we diverge from Bob's approved design at our own risk. So here we are.

With a conventional design, gravity feed system, it appears the chances of a failure seem almost impossible when you run the numbers. The engine needs so little fuel pressure to run and one of the tanks is always under some pressure, which reduces the risk further.

In post #81, I drew the conclusion that fuel injection means you need to install this vent. I think that was jumping two steps ahead when I meant to just take one. I think all we can say for certain is, some fuel injection systems are at-risk of this failure, the high pressure ones. We actually have no evidence to suggest Bendix systems are at risk.

Similarly, I didn't say that ONLY fuel injection systems are at risk. There are a million ways to design a dangerous fuel system, and only a small number of ways to make an inherently safe system.

[Battson]

My solution is a check valve on each tank, ones that open at a pressure well below the pressure head of the fuel. Air gets in if your cap vents get clogged, doesn't let fuel out, doesn't remove your ability to isolate tanks on a slope, and won't let fuel settle in a low point for those wanting to go outboard side to outboard side with their cross vent (dihedral isn't shown in any of those fuel diagrams).

Not 100% on this solution but it's where I'm leaning.
​​​​​​

The check valve idea is interesting, and I am especially tempted because the cross vent would need a check valve anyway.
However, I am not sure whether I would trust a check valve in my main fuel line - seems to introduce another potential engine-stopping failure point.
In the vent line, a failure of the check valve is less risky.