Fuel Flow Discussion, Moved from Float Mounting

[jaredyates]

I need to add a story here from the "not particularly proud to tell it" category. But I'm going to fess up because it may save someone else for doing the same.

A couple of months ago I took the Bearhawk on a solo trip the lake to tow paragliders. It's a 3-hour drive or a 1-hour flight. It's a great airplane mission, because the weather demands of paragliders are much more stringent than GA planes, so weather isn't usually much of a concern. I secured permission to tie down at a tiny airport with no attendance or services at all. It's very much like the airport where we keep our airplane, and best of all, it is really close to the boat ramp we use for the towing. The morning flight over was easy. I knew I was going to have to fuel before getting home that evening, but I didn't fuel in the morning, and parked the plane with around 12 gallons on board. That's not much, but it is certainly enough to fly a few miles to the big self-serve airport on the way home.

Having snacked/worked through lunch, we had a great day and were pounding away some much needed calories at the Mexican place. I happened to check the radar image on my phone, and there was a giant line of rain that was headed right for us. The line was parallel to my course home, and probably 40 miles upwind. I didn't have to get home, and knew that. But I also had a hunch that if I hustled over to the airport and got on my way, I'd have a good chance to make it home. It was safe in the sense that the line was barely moving, so I could always make a right turn and be in clear blue sky. If the rain moved over the home airport, I might have an adventure, which is no big deal.

The hop over to the big airport was uneventful, but I knew that time was not on my side. I figured, why not just fill up the right tank? That would be 17 more gallons than I'd need to get home, and over the course of the flight, it would be able to drain itself to the left tank. I'd save the time of having to move the ladder over to the other side, and be on my way 5 minutes sooner.

Skip ahead 15 minutes or so. I was on the radio with Greer approach, flying on course and parallel to an ominous looking line to the left, with a perfect day on the right. I could see the rain falling hard to the left, right at the Blue Ridge Escarpment. I was feeling pretty good about surfing the updraft curling around from that upflow, set at my usual 8gph cruise power and getting a bonus 20-25 knots of IAS over the usual cruise. (note! This technique is not recommended for beginners. Or really for anyone else. It's not the first time I've done it, but it's not without risk.)

I did a cursory and typical check of the fuel sight gauges and got quite a surprise. The left tank was as empty as I've ever seen it, and the right tank was still completely full. This is not what I was expecting, since I'd had some time for the tanks to balance. The left tank level was actually going down, not up!

At this point, my brain was showing the little hourglass symbol. Thankfully and perhaps luckily, I came up with a hypothesis. What if, in my slightly rushed refueling, I forgot to replace the cap? The left tank would have positive pressure, the right tank would have at best ambient pressure, and maybe low pressure. The corrective action that I needed, perhaps very quickly, was to select the right tank only. The pressure in the right tank was lower than the pressure in the left tank, but not so low that the engine wouldn't run. To confirm this, I selected the right tank with a close watch on the fuel flow, and thankfully it never changed. Everything was still good, and I pressed on towards home.

As I got closer, I could see that the rain had not reached home yet. I landed in strong gusty winds, which is also not recommended for beginners or anyone else. But I put the plane away and checked the right tank. Sure enough, the cap was gone.

So while I don't expect that I'll be adding a cross vent in the near term, I do want to point out that I encountered a scenario where the designed system was not tolerant of a particular series of errors- that is, fueling only one tank, and failing to recap the tank. It is totally fair that the system does not need to be tolerant of these errors. It may very well be that the engine would have kept on running. I wasn't in a position to test it.

As I said in the beginning, this isn't a story of great piloting. But I thought it was worth sharing.

 

[zkelley2]

Thank you for sharing. The fuel system as designed is tolerant of this error, provided you don't run it on both.

I've had a leaking fuel tank before, on a previous airplane, on a long leg with no fuel until destination. It's a really good idea, IMO, that if you suspect you're losing fuel from a tank, to immediately switch to that tank to try to burn as much as you can before you lose it.

 

[Bcone1381]

I would like to elaborate my post #114. It seems to me that fuel cross-flowing through the fuel selector from one tank to another reduces the amount of fuel flow available for the engine. So therefore, I am wondering if this safety notice or limitation might not be prudent....

"When a significant fuel imbalance exists do not place the fuel valve to BOTH, rather place the selector to either the L or R tank as appropriate."

 

[robcaldwell]

I've thought long and hard about cross venting, but have decided to simply be disciplined about fueling and maintaining both sides as equal as possible without ever going below 1/4 indicated.

 

[svyolo]

There is more head pressure to feed to carb or FI pump, then to cross-feed to the other tank. The head between the two tanks is only the difference between the fuel levels, 1-8 inches. The head to the carb or pump is 4 feet.

To my mind, I think the flow goes to the engine before it goes all the way up the other fuel line to the opposite tank.

 

[Battson]

I confess to having done exactly the same thing, and I agree the system *MADE THE WAY BOB DESIGNED IT* is very tolerant to issues. I must disagree with zkelley2 above, as I always fly on Both and this was no exception.
We flew for a long time without one fuel cap and I hardly lost a drop of fuel overboard. I was able to tape over the open tank and fly back and get the missing cap (it was a short flight and the closest option).

 

[Battson]

I would like to elaborate my post #114. It seems to me that fuel cross-flowing through the fuel selector from one tank to another reduces the amount of fuel flow available for the engine. So therefore, I am wondering if this safety notice or limitation might not be prudent....

"When a significant fuel imbalance exists do not place the fuel valve to BOTH, rather place the selector to either the L or R tank as appropriate."

The trouble is, you need to know why the imbalance occurred, if you select the fullest tank - you are selecting the tank most likely to have a venting issue or negative pressure in flight, assuming the imbalance occurred unintentionally. Technically you should select the emptier tank and refuel as soon as possible.

****

Statistically speaking, playing with the fuel selector gets more pilots killed than tanks without a cross-vent. I feel it's important to remember that fact, we are only human.

 

[Bcone1381]

We are thinking now! That is good.

Bob designed the fuel system for a R-Both-L selector. If I fear moving it we have a problem that needs to be vetted out.

 

[zkelley2]

I think you can make a good arguement he designed it only fot L/R even if he didnt know it, considering there isn't a cross vent. And I think that argument has been made.

 

[Battson]

I think it's worth remembering that Bob designed and recommends a gravity feed system to a carb engine without any fuel pumps.
In Bob's fuel system, operating with both tanks selected in any fuel configuration will be safe at all times.

Introducing fuel pumps and injection brings some benefits, as well as many new design considerations and operational considerations. As a community, I think we are still learning how to manage those risks, there is no one size fits all.

 

[Bcone1381]

The Patrol Book specifies a four way L-Both-R-Off fuel valve.

 

[Battson]

short story; Running on "Both" without having interconnecting tanks. Pressure differential between the tanks caused fuel flow stoppage.

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.

 

[JimParker256]

The Rotax powered RANS S-6ES I'm currently flying was designed with two wing tanks (each with a sight gauge and a set of fore and aft fuel pickups) plumbed to a single "on/off" fuel valve, then to a gascolator, and finally to the engine-driven fuel pump. It's functionally very similar to Bob's design, if you were feeding from "BOTH" tanks all the time.

It is very common to hear reports about one wing tank draining much quicker than the other tank – sometimes to the point where there is no longer any fuel visible in the sight gauge on one side while the other is indicating nearly full. The owners that report this issue say that it's always the same tank that "empties" first – which would seem to me a clear indication that their wing tank vents are producing different "pressures." The designer's recommendation was to check the length of the vent and to compare the angle of the cutoff. (The vent line starts at the top of the tank, and runs back and down, ending in a tube that sticks out below the wing, near the jury strut, with a 45º forward-facing cut-off. If the vent is closer / farther from the jury strut, or if the extension of the tube below the wing surface is different, or if the cutoff angle was different, then the "pressure" due to air flow might also be different.)

However, there have been NO actual reports of fuel starvation resulting from this setup. The "going assumption" is that the pressure head from the fuel in the "mostly full" tank is apparently enough higher than the pressure head from the "air" in the lines and tank on the other side that the engine-driven fuel pump does not "suck air" in those situations. Even those who say one of their tanks "always empties" well before the other say they have no issues utilizing the entire useful fuel load in flight.

I did make a change to my S-6's fuel system design, however, in keeping with RANS' latest design changes to the S-20 and S-21. I added a small (2.75 gallon) header tank behind the seats, and redirected the wing tank outlet lines to flow into the header tank. The header tank vents only through the wing fuel lines. There's a "float switch" (connected to a low fuel warning light on the panel) that triggers when there is 2.5 gallons remaining (roughly 20-30 minutes) of fuel remaining – that's my "Final warning, you idiot! Get this plane on the ground NOW!" light...) The header tank's single outlet flows to the fuel cutoff valve (On/Off), the gascolator, and the engine. At some point, I'll probably add an electrical fuel pump inline between the header and fuel valve, just in case the engine-driven pump ever fails. (Rare, but it can happen.)

With this change, I see very even fuel burn from my two wing tanks – I rarely see anything more than a 1/2 gallon difference in the sight gauges. I'm happy with the setup.

 

[zkelley2]

The Rotax powered RANS S-6ES I'm currently flying was designed with two wing tanks (each with a sight gauge and a set of fore and aft fuel pickups) plumbed to a single "on/off" fuel valve, then to a gascolator, and finally to the engine-driven fuel pump. It's functionally very similar to Bob's design, if you were feeding from "BOTH" tanks all the time.

It is very common to hear reports about one wing tank draining much quicker than the other tank – sometimes to the point where there is no longer any fuel visible in the sight gauge on one side while the other is indicating nearly full. The owners that report this issue say that it's always the same tank that "empties" first – which would seem to me a clear indication that their wing tank vents are producing different "pressures." The designer's recommendation was to check the length of the vent and to compare the angle of the cutoff. (The vent line starts at the top of the tank, and runs back and down, ending in a tube that sticks out below the wing, near the jury strut, with a 45º forward-facing cut-off. If the vent is closer / farther from the jury strut, or if the extension of the tube below the wing surface is different, or if the cutoff angle was different, then the "pressure" due to air flow might also be different.)

However, there have been NO actual reports of fuel starvation resulting from this setup. The "going assumption" is that the pressure head from the fuel in the "mostly full" tank is apparently enough higher than the pressure head from the "air" in the lines and tank on the other side that the engine-driven fuel pump does not "suck air" in those situations. Even those who say one of their tanks "always empties" well before the other say they have no issues utilizing the entire useful fuel load in flight.

I did make a change to my S-6's fuel system design, however, in keeping with RANS' latest design changes to the S-20 and S-21. I added a small (2.75 gallon) header tank behind the seats, and redirected the wing tank outlet lines to flow into the header tank. The header tank vents only through the wing fuel lines. There's a "float switch" (connected to a low fuel warning light on the panel) that triggers when there is 2.5 gallons remaining (roughly 20-30 minutes) of fuel remaining – that's my "Final warning, you idiot! Get this plane on the ground NOW!" light...) The header tank's single outlet flows to the fuel cutoff valve (On/Off), the gascolator, and the engine. At some point, I'll probably add an electrical fuel pump inline between the header and fuel valve, just in case the engine-driven pump ever fails. (Rare, but it can happen.)

With this change, I see very even fuel burn from my two wing tanks – I rarely see anything more than a 1/2 gallon difference in the sight gauges. I'm happy with the setup.

That's how my rotax powered airplane was plumbed to. I would suggest getting that electric backup in there asap. The fuel pump on the rotax has gone through something like 4 service bulletins "recommending" replacement of the earlier design. They are not the strong point and the engine quits without fuel pressure despite being carb.

 

[Bcone1381]

What questions should be asked to lead this towards a scientific based solution. Does this issue need a leader, spokesperson, authority, or a facilitator? Who should that person be? Has anyone plumbed altimeters into upper sight gage plumbing to measure data? Does anyone have a aircraft/circumstances that can make this a e repeatable event?

 

[svyolo]

I think running a tank dry and killing the engine while the other is full is primarily a fuel management error, even the there is also a fuel flow problem. Two problems simultaneously.

Sometimes I will try new stuff, even if it is different than convention. Sometimes it works, sometimes not. Sometimes I actively seek out wisdom from the "herd". Despite being a pilot for work for over 30 years, I am one of the very least experienced here with light aircraft, and their systems.

Sometimes logic is wonderful, but sometimes it can't identify threats or the risk of threats because of external "unknowns". That is where the "herd" is king. Experience in the real world. That includes external threats, and the experience of how often they happen.

Since I chose EFI and that is a choice of the vast minority (going against the herd) I kind of did of a deep dive into fuel systems in light aircraft. Not theoretically based, but I wanted to see what the "herd" did. Luckily Cessna built thousands of high wing aircraft with injected Continentals for me to see what they did, including their change and development over the decades. Piper built more than a couple but I think they were mostly carburated.

Bob's stock fuel system is perfectly acceptable. It is redundant. You have 2 sides. If you run one dry, whether or not one stops feeding, you failed to monitor your fuel state properly. If it results in a mishap, that fact will become at least a "contributing factor" if not a "causal factor".

More redundant is usually better. Not always, especially if it increases complexity or workload. It also may not help long-term if it makes the pilot complacent.

The "herd" went with a cross-vent in high wing aircraft with a fuel valve "both" setting. It is also specified in rules of thumb in a bunch of fuel system textual descriptions for the same application. In this case, I am going with the herd.

 

[Mark Goldberg]

If you fly along in a slight slip or skid - one tank will drain first. Very easy to do. My point is that it is not different pressures in the tanks that might make one drain faster than the other - but how we are not flying wings level. Mark

 

[JimParker256]

Note that your "herd" (Continental engine power) also designed the fuel return to always go to a single tank, thus saving money over a full-duplexed fuel valve and additional return line to the other tank. I think that fact alone would argue for cross-venting the tanks. BWTHDIK?

 

[Nev]

Cessna identified another cause many years ago - internal fuel line pressure differences coming from the slight variations in fuel line plumbing (different radiused bends etc). These sometimes led to vapor locks on one side, and caused an uneven fuel feed rate. So while closely monitoring your tanks is always a great idea, in some cases it can be treating a symptom without realizing there’s a cause.

While Cessna were unable to replicate the exact problem consistently, they did find that tank cross-venting largely eliminated this particular issue. They also speculated that very specific atmospheric conditions were playing a part in the vapor-loc scenario, meaning you could fly without incident for a long time while always having the potential issue lurking.

 

[Nev]

This is probably the most common cause Mark. We used to sometimes balance tank quantities when needed by opening a cross feed valve and deliberately flying out of balance to get fuel across to the other tank. In aircraft without a cross feed valve the fuel can be free to flow through the fuel selector in BOTH to the other tank.

 

[Battson]

In our case, we have two Bearhawk 4-place's in the hanger at the moment, and we flew both yesterday. The first machine sucked 60 L (15 gal) out of one tank and didn't touch the other. Mine draws perfectly equally on both tanks, as always.

Same two pilots flew in both machines at once, same day, almost identical setups apart from:
- the plumbing from the tanks to the selector - one has many more fuel drains and lots more AN hardware
- use of aux tanks vs no aux tanks (with gas cap vents)

Literally everything else about the fuel system is the same. Same gascoalator, same pumps, same injection system, same engine, same venting.

It seems to be very fiddly to pin down why the tanks draw at different rates. Perhaps a cross vent would help THAT symptom.

 

[jaredyates]

I wonder if swapping the caps would make any difference?

 

[Nev]

It’d be interesting to disconnect the fuel lines and compare flow rates L vs R after the selector, before the pump.

 

[Bcone1381]

Fuel Cap position....would the pressure differential exist between each cap's vent if each is placed slightly differently at the wing, or the alignment is not exact.......is one cap slightly higher, or slightly more forward slightly angled?

The only explanation I can see at this point is at best theoretical, but might be worth to investigate.

Lets say the L vent pressure is 4 units and the R is a 6 in cruise and causes an imbalance. Then one slows for landing, and extends flaps....it would seem to change the pressure distribution on the wing, and maybe change the distribution at each caps vent, inducing crossflow of fuel thru the selector valve. Would not the crossflow then limit flow to the stem of the valve's "tee" going to the engine?

 

[Battson]

Good questions - we tried swapping fuel caps, it appeared to make a difference at first, but then it transpired that it made no difference whatsoever.

We haven't tried swapping fuel caps on the aux tanks yet.

 

[Ed.Meyer]

Having followed this thread and then seeing the safety notice from Bob regarding fuel pumps with his fuel system design I decided to do a test.

As some of you know, we have a Patrol with the EFII ignition and injection system which includes fuel pumps (main and secondary) to boost fuel pressure up for the injectors. Also, return lines were installed to return the excess fuel from the continuously running pump. All the details of our system installation in the Patrol are in a Beartracks article I wrote a while back.

Not knowing positively whether un-porting of the fuel outlets on one tank would allow air to enter the fuel system and starve the injection system,a concern expressed in this thread; I decided to do a test:

I flew up over my home airport about 4500 AGL with about 9 gallons of fuel in each side. I was using a cruise power setting with about 8GPH fuel flow showing. I turned the fuel valve to the right tank (I usually run both) and then put the right wing low in a pretty hard slip/skid to intentionally un-port the fuel outlets. I expected that fuel pressure would quickly drop and the engine would quit. After about 10 seconds or so, the fuel pressure did drop and the automatic switching to the secondary fuel pump occurred but the pressure stayed low. At this point I knew I had succeeded in un-porting the fuel outlets on that side. I then went back to normal coordinated flight and the fuel pressure slowly recovered in about 10 - 15 seconds. Much to my surprise, the engine never missed a beat through this.

Following this, I switched the fuel selector to both and again did the right wing low slip/skid. I held this for probably about two minutes monitoring the fuel pressure closely. It never wavered in the slightest.

My conclusion from this is that there is apparently enough fuel flow capacity feeding from only one tank via 3/8" lines to supply all the fuel the engine needs plus whatever amount the fuel regulator returns to the tank even with the other tank feed open to the vent. I recognize that this is not an exhaustive test but it was enough to boost my confidence in the fuel system as installed.

 

[Nev]

This is an interesting experiment Ed. I think that in theory the way you have put the aircraft into a slip, both fuel out-let’s should have un-ported and the engine should have stopped. Given that it didn’t stop, it may be still running fuel from a gascolator or header (I didn’t look at the details of your system), albeit at a reduced pressure. I suspect that the gascolators may be acting as a mini header tank, along with larger diameter fuel lines where used.

It would be interesting to see the same experiment, but instead of an out of balance situation, try an extended steep climb or steep descent. This should unport one outlet, and give the pump an opportunity to suck air in preference to fuel. It may take quite some time before the engine stops because it has to burn all the fuel stored in the lines and gascolator first.

The aircraft that have experienced engine stoppages so far, what we don’t know is how long the fuel stopped feeding before the engine stopped. It may have been several minutes. This effect has implications where the gascolator is plumbed between the fuel selector and the engine. There have been accidents where a pilot has started the engine with fuel selector in the OFF position, and subsequently become airborne, only to have the engine stop at low altitude on climb out. Many fuel systems were designed to prevent this by only having a very limited amount of fuel “stored” in the system (cause the engine to stop before it had a chance to become airborne), but there’s an obvious trade off between that and having sufficient fuel to cater for a un-porting event.

 

[Mark Goldberg]

I have always thought that an engine would start from the OFF position of the fuel valve - maybe. But taxying out and during run up the engine would quit before the plane flies. I have seen this before a few times. But you are probably correct that all planes are different. Mark

 

[zkelley2]

Your thoughts are what I have experienced Mark. I have started with the fuel selector off on purpose. It does start, and it'll idle for a few minutes, but putting power to it for taxi I can't get 100ft.
If I started the airplane on the runway in position I don't think I could put full power to it before it'd stumble.

 

[Ed.Meyer]

Airplanes that have carburetors have a float bowl that holds enough fuel for a short idle. Ours will not start with the fuel valve off because the pump will not build pressure. Screams loudly in protest. I think the gascolator probably was a factor in supplying a bit of fuel.
I agree that this could be further tested with climb/decent and lower fuel levels. Given what I learned so far, I think that fuel coming from a single port would be enough since each port is a 3/8” line and even with both ports feeding from a single tank, they tee into a single 3/8” line. Doing this test at real low fuel levels I an not willing to do.

 

[zkelley2]

I think we know, or we all should know that a single 3/8 port and line can feed enough fuel because we've all done the fuel test recommended by the AC and even asked for by most DARs/FAA people.

 

[Nev]

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

The issue is what happens once a pump is added to the system, and one line then becomes un-ported.
Will the pump suck fuel from the other line, or will it suck air in preference to fuel from the un-ported line, thereby resulting in a stoppage.

 

[Bcone1381]

Having followed this thread and then seeing the safety notice from Bob regarding fuel pumps with his fuel system design I decided to do a test.
...........
My conclusion from this is that there is apparently enough fuel flow capacity feeding from only one tank via 3/8" lines to supply all the fuel the engine needs plus whatever amount the fuel regulator returns to the tank even with the other tank feed open to the vent. I recognize that this is not an exhaustive test but it was enough to boost my confidence in the fuel system as installed.

I think you are right, Ed. You experiment gives good evidence that the first fuel pump in your system is supplied by gravity. Will a change in power change your fuel flow demand? I don't think so, but not sure. I seems to me like your system has a fixed rate of fuel demand out of the tank regardless of the power setting, and system returns unused fuel. Am I right?

 

[Ed.Meyer]

You are right Brooks that the system returns unused fuel. When I first turn it on prior to engine start, I can hear a short burst of bubbling in the tanks as air is purged. I am not sure if the volume pumped stays the same. Fuel pressure varies some with manifold pressure. Higher MAP has higher FP. The same fuel pumps drive the EFII system for an O540 setup so I think the volume being pump is somewhat higher than the engine demand.than our O360.

 

[svyolo]

ED;
Do you return to a header, or the main tank(s)?

 

[Ed.Meyer]

Return is to main ranks via a duplex fuel valve.

 

[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.

 

[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.

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.

 

[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

 

[Nev]

Very good point.

 

[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.

 

[Nev]

Brooks mentioned earlier in this thread that he wondered if fuel could be flowing laterally through the fuel selector when in the BOTH position. I also got to wondering further about the whole "can a fuel pump suck air" issue. Previously I'd assumed it could. But I now have an issue with this - the fuel pump is flooded from the gascolator. The gascolator would have to empty first, before the pump could "suck" air.

In the BOTH position, at least one tank should be keeping the gascolator topped up.

Could a lateral fuel flow through the gascolator in the BOTH position create a Venturi like condition ? With a fuel pump "pulling" from the front, and a lower pressure at the back, could fuel vapor form? Could it even empty the gascolator back into the high tank ?

I realize that I'm going right off the reservation, but I'm not certain that the issue was ever resolved conclusively.

Some of you have a fluid dynamics or fuel system design background and can probably shed light on this for me.

 

[jaredyates]

The gascolator doesn't deliver from the bottom, so it wouldn't need to be empty in order to deliver air. It would only need to lose enough fuel to unport the fittings.

 

[Nev]

That's very interesting. I had also thought that it might act as a mini header tank, but in this case it wouldn't.

 

[schu]

I should comment that engines that feed the majority of the fuel back into the tanks (EFI) can for sure suck fuel faster than can be placed back into tank and settle enough to be picked up again. This is why I don't think EFI is a good option with the bearhawk, and why I think a header tank is mandatory in case of EFI.

With an IO, your fuel consumption at the gascolator isn't any different than a carb, it's just under pressure. Returnless mechanical FI is pretty nice in this regard.

 

[Sir Newton]

Thanks for posting, this information Matt.

 

[Sir Newton]

100% it's worth discussing. The loss of one plane is to many planes.

 

[Sir Newton]

yes, 100%
This stuff scares me during my pillow time!

 

[Sir Newton]

Thank-you
I am reading the thread now. I would have missed this completely had you not moved it.

 

[Sir Newton]

FYI, I have lots to say regarding fuel systems designs & choices of pump types. Different types of pumps will act differently in given situations. This is VERY common knowledge in oil & gas processing industry.

 

[Sir Newton]

Glad to hear this story. Thank-you.

 

[Sir Newton]

hey man, I just found this thread today.

 

[Redneckmech]

There is a lot of great information in this thread and I have learned a lot about fuel systems in the process of reading through it. I have a slight background in fluid dynamics as that was one of my main focuses when I was in engineering school, having said that, I am far from an expert.

Looking through the history of this conversation, there has been a lot of discussion on whether a cross vent at the top of the tank was absolutely necessary when using the selector in the both position.

I may have missed someone else pointing this out, but much of the discussion was related to whether there could be a pressure difference between tanks when both were vented to the open atmosphere. The assumption that goes along with that, is that both tank vents are seeing the same pressure (atmospheric) which may not always be the case when the aircraft is in flight.
As a wing produces lift, it creates a pressure differential between the upper and lower wing surface with a corresponding pressure gradient that changes based on the angle of attack (see attached photo). If there are rigging differences, the wings are not perfectly aligned, or the aircraft is slipping/skidding, there will be a variation in the pressure that is seen by the vent since the pressure will change relative to the velocity of air over the surface. When that happens, the pressure will want to equalize between tanks and will do so through the easiest path.

If there is an upper vent hose, air will flow through it to the lower pressure tank (in through the higher pressure vent, and out through the lower pressure vent). If there is no upper vent, and the pressure difference is great enough, the only path that exists is from the higher pressure tank, down through the lines to the fuel selector (when on both) and back up the line to the lower pressure tank.

This sets up the scenario that may have happened to the OP where the higher pressure (from the vent) from the empty tank was great enough to push air down through the fuel selector back up the line to the lower pressure tank (that was full of fuel) preventing the fuel in the full tank from being able to reach the engine.

Hopefully this helps,

Bill

 

[Nev]

I've just re-read post 37 by MattS and I believe this is what he was saying in that post that caused his engine stoppage.

 

[Bcone1381]

Troy,

Do you think a faulty seal in one of the fuel caps might cause a pressure differential to exist between the two tanks? Then I wonder if it is combined with an airspeed change how it might effect things.

 

[schu]

After re-reading this thread I have removed my previous post because I was wrong. This entire thread was about someone that had fuel starvation with a pump, I forgot that the float plane had a Continental in it.

Here is an updated summary:

There seems to be two modes of failure for the fuel system:

• Unporting a tank and sucking in air instead of fuel.
• Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

The first failure can happen with any pump but is greatly exaggerated by a return to tank fuel system such as in the case of EFI because it's pumping a lot more fuel because not all of it is being used.

The second failure has appeared on one gravity fed o-360 bearhawk without a cross vent, and also the IO-360 Continental in this thread, which if memory serves uses a return unused fuel to tank fuel system.

I am unaware of any failures with returnless/bendix/afp fuel injection systems, however if the imbalance was the issue in the case of the float plane, and enough to stop the continental pump, there is no reason to believe that it wouldn't also stop the lycoming IO fuel pump.

For me the take-away is:

• If you have a significant imbalance between your tanks, then there is obviously some pressure difference between them, and probably best to avoid the both setting.
• The vent between the tanks almost certainly makes it nearly impossible to have the above mentioned imbalance, even if your venting system has an issue.
• If you are returning a significant amount of fuel to the tank, then a header tank is probably best. Those are much harder to unport, even if you are cycling 100GPH.
• Pilot awareness is super important. Monitor the fuel system.

 

[Redneckmech]

Hi Brooks,

That is a great point! It absolutely could, since the pressure is relative to the velocity distribution of the air over the wing and the velocity of the air will approach zero as you reach the skin due to the boundary layer, there will be a pressure change relative to the height above the wing skin. The flow of air around the fuel cap will also be disrupted and that could create an even greater pressure differential due to the turbulence of that disrupted flow.

The below photo shows the velocity profile of the boundary layer. Velocity and pressure have an inverse relationship, as the velocity decreases, the pressure will increase.



As I said in my earlier post, I am by no means an expert and I haven't really revisited fluid dynamics in about five years. I know there are others here with far greater knowledge and experience in aerodynamics that could provide much greater detail and insight but hopefully this helps to clarify the general fluid dynamic mechanisms that are in effect.

Bill

 

[Redneckmech]

After re-reading this thread I have removed my previous post because I was wrong. This entire thread was about someone that had fuel starvation with a pump, I forgot that the float plane had a Continental in it.

Here is an updated summary:

There seems to be two modes of failure for the fuel system:

• Unporting a tank and sucking in air instead of fuel.
• Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

The first failure can happen with any pump but is greatly exaggerated by a return to tank fuel system such as in the case of EFI because it's pumping a lot more fuel because not all of it is being used.

The second failure has appeared on one gravity fed o-360 bearhawk without a cross vent, and also the IO-360 Continental in this thread, which if memory serves uses a return unused fuel to tank fuel system.

I am unaware of any failures with returnless/bendix/afp fuel injection systems, however if the imbalance was the issue in the case of the float plane, and enough to stop the continental pump, there is no reason to believe that it wouldn't also stop the lycoming IO fuel pump.

For me the take-away is:

• If you have a significant imbalance between your tanks, then there is obviously some pressure difference between them, and probably best to avoid the both setting.
• The vent between the tanks almost certainly makes it nearly impossible to have the above mentioned imbalance, even if your venting system has an issue.
• If you are returning a significant amount of fuel to the tank, then a header tank is probably best. Those are much harder to unport, even if you are cycling 100GPH.
• Pilot awareness is super important. Monitor the fuel system.

Great summary,

There are an incredible number of variables that are in play and that is likely why one of the leading causes of experimental accidents is related fuel system issues.
Since each our systems are in effect, one-off creations tailored to our own needs, fuel line routing, line size, selector location, engine type, etc., will all be slightly different and will produce different results in fuel flow.

It is still a very long time until I start on my fuel system, but this thread has been incredibly thought provoking and brought up many different considerations that I had not contemplated until they were mentioned.

Many thanks to everyone who has contributed.

Bill

 

[Bcone1381]

After re-reading this thread I have removed my previous post because I was wrong. This entire thread was about someone that had fuel starvation with a pump, I forgot that the float plane had a Continental in it.

Here is an updated summary:

There seems to be two modes of failure for the fuel system:

• Unporting a tank and sucking in air instead of fuel.
• Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

The first failure can happen with any pump but is greatly exaggerated by a return to tank fuel system such as in the case of EFI because it's pumping a lot more fuel because not all of it is being used.

The second failure has appeared on one gravity fed o-360 bearhawk without a cross vent, and also the IO-360 Continental in this thread, which if memory serves uses a return unused fuel to tank fuel system.

I am unaware of any failures with returnless/bendix/afp fuel injection systems, however if the imbalance was the issue in the case of the float plane, and enough to stop the continental pump, there is no reason to believe that it wouldn't also stop the lycoming IO fuel pump.

For me the take-away is:

• If you have a significant imbalance between your tanks, then there is obviously some pressure difference between them, and probably best to avoid the both setting.
• The vent between the tanks almost certainly makes it nearly impossible to have the above mentioned imbalance, even if your venting system has an issue.
• If you are returning a significant amount of fuel to the tank, then a header tank is probably best. Those are much harder to unport, even if you are cycling 100GPH.
• Pilot awareness is super important. Monitor the fuel system.

I use to believe and shared something I now believe was incorrect…. It had to do with sucking air if the front tanks port is uncovered.

I can’t recall who shared this…..maybe it’s in this thread…

I Now I believe if the front port is uncovered (like when slowed way down with a tank that is 20% full) then gravity will keep the the front fuel line essentially full by way of that tanks TEE (prior to the fuel selector). I can’t see an EFI with very significant fuel return changing this. The tank and the associated fuel lines would need to be empty before the pump would have the chance to suck air.

Here is a diagram of the fuel system from Bob's Patrol Book to help visualize this.

 

[schu]

I Now I believe if the front port is uncovered (like when slowed way down with a tank that is 20% full) then gravity will keep the the front fuel line essentially full by way of that tanks TEE (prior to the fuel selector).

Sure, but what if your EFI system is cycling 100GPH of fuel? Do you think that gravity will keep up and keep the inlet flooded? Or do you think that the fuel will be cycling fast enough for the pump to pull in some air when either side is unported?

I suspect the second one, and heard about an EFI install on a cub in Alaska that had an unport which caused fuel starvation from someone I trust. It's anecdotal, but it makes sense to me that in the case of EFI the second that there is air exposed to a port that is drawing fuel, you will get a bubble, while if you have a port exposed that is draining fuel, it might not matter that much. Given the bearhawk gravity system is only good for around 40-50GPH, I think that makes both of your ports drawing fuel not simply draining it.

One more point:

There was a bearhawk that had an engine stoppage with EFI. I'd take a look at that NTSB if you are planning on EFI so see if there is something to be learned. Also, I don't think EFI is any more fuel efficient than AFP given how our systems tend to remain at static RPMS. I think the only reason for EFI is the ability to run garbage ethanol mogas fuel, which is for sure cheaper than avgas.

schu

 

[svyolo]

I believe that is correct. But if the fuel system is feeding correctly by gravity, the gascolator should be full, and providing about 1 psi boost from the gravity feed if I remember right. But feeding from the top, not a header tank or reservous of any kind.

 

[svyolo]

I spent a bunch of time trying to figure all this out a couple of years ago. The best info, was Cessna's several decades of evolution of their fuel systems on MFI Continental engines, which also flow extra fuel and return fuel. I mostly used the kit stuff, but added a cross vent, and a small header tank (the local DAR, retired Boeing fuel guy) referred to it as a "collector tank". I return fuel to the small tank(2L) under the front seats. The fluid logic is what Cessna went to after 30-40 years with injected Continentals. I have never flown one of those planes, and don't know if my implementation is the same, but the system logic is as close as I could get it.

If one front or aft line is unported, I am pretty sure gravity will try to refill the other line, one way or another. Will it refill the other line, to the feed detriment of the fuel pump? Not sure.

I will say the Newton valve is not that impressive other than low weight and cost. Lots of sharp edges internally, and I think it is the biggest constriction in the fuel lines.

 

[Bcone1381]

Schu, I had not thought about 100 gph. More to consider.

 

[Nev]

There seems to be two modes of failure for the fuel system:

• Unporting a tank and sucking in air instead of fuel.
• Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

A couple of thoughts:

Unporting a tank and sucking in air instead of fuel.
My feeling is that this is more likely to occur when a selector is in either the L or R position, and the aircraft is being flown out of balance. My understanding is that the aircraft is designed to be normally flown in the BOTH position to prevent this.

Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.
My current thinking is that it's not having a severe imbalance that's causing the issue, rather the other way around. Flying out of balance when in the BOTH position is leading to a severe fuel imbalance, and the fuel flowing through the selector may be causing an issue at the gascolator/selector. Many years ago I flew Metroliners and if we ever had a fuel imbalance we would simply open the cross flow valve (similar to our BOTH position) and fly out of balance for a few minutes. The fuel would transfer from one side to the other really quick. Sometimes we forgot to close the valve for a few minutes and would quickly develop an imbalance to the opposite side.

My own experience is that I found the Bearhawk particularly difficult to fly in balance initially. More recently I am getting much better at keeping it in balance, and as a consequence I seldom get more than a couple of liters of fuel imbalance between refueling. I mention this not to knock the Bearhawk, but to reiterate that it did get flown out of balance alot when I was new to it. Other pilots found the same difficulty when flying my aircraft. When I refueled I would always find that I pumped far more gas into one tank than the other - though not always the same tank. MattS mentions in post 27 that his Bearhawk with floats was more demanding to fly in balance, and that after the mishap they found 55 liters in the Right tank and only 10 liters in the Left tank. This was after feeding off the Right tank for 30 mins in an attempt to balance the fuel.

I think it's probably very important to maintain balanced flight (side slipping excluded ), and (for my own bearhawk), to use the BOTH position of the fuel selector whenever possible.

 

[Ursa Buteo 272]

Very interesting thread and I have been following and re reading for a considerable time.
Just about to perform my fuel flow tests and was wondering what tests I can do while I am in the testing mode to gather additional information about modes and scenarios.

Configuration is: Two main tanks, gravity feed down to 2 gascolators, one for each tank located at the lowest point on the fuselage bottom just inboard of each wing strut.
Gascolators feed fuel to the Newton Fuel Valve. R, L, B, OFF. OFF is off with no cross feed. L,R isolate. Fuel valve feeds a pass thru electric boost pump to the Continental engine driven mechanical pump. Mechanical pump has a Vapor / Excess fuel return to the left tank via a check valve.

That said, I am making a test matrix that isolates various system elements, vents, gascolators, Tank levels, Mechanical fuel level sensors, Redcube fuel flow transducer.
With all the data collection through the Dynon I should be able to have a pretty good picture of baselines and influences of changes.


Fringe technical comments: With respect to aircraft and most fuel systems in general. Very few fuel pumps are designed to " Suck or have Suction" on the inlet to the pump. The desired baseline is to have a flooded inlet with positive head pressure on the inlet of the pump. " Suction" at the inlet will cause fuel to vapor lock and create bubbles and possibly associated cavitation. The pumps are designed to move a volume of liquid from inlet to outlet. Any air, entrained or otherwise will cause a substantial decrease in the pumps ability to move fuel from inlet to outlet.
Pressure seen at the outlet of the pump is the result of trying to force the liquid fuel through a downstream restriction. Pumps do not make pressure, pressure is from a restriction to flow. If there is not flow, there is no pressure.

With respect to slipping with low fuel causing the fuel to move away from the tank outlets and pickups becoming un-ported.
When slipping to landing, fuel burn is ounces per minute. Very little flow consumed by the engine. At altitude performed numerous full cross controlled slips, engine idle with less that 1\4 tank
and the engine never faltered while in BOTH. With the un-ported tank selected it generally took 3 minutes of sustained slip before engine roughness and it smoothed out within 7 seconds of wings level coordinated flight.

Now it gets interesting, this is a ground based test you can safely perform yourself. Have the left tank less than 1/4 full. Select left tank, and taxi at a nice fast walking pace in a fairly tight right turn... take a Dramamine first and bring a lunch. When the engine finally quits, stop the plane and immediately try to start the engine. If it does not start wait a minute and try again. If it still does not start, switch to BOTH, If it still does not start, walk back to the hangar and get the truck and a tow strap......... Been there. Done that. Now the question is why would it not start with no airfow over the wings and exactly the same tank air vent pressures and conditions??????

More will be revealed..........

Kevin D
[NODE="272"][/NODE]
KCHD

 

[Nev]

Very interesting Kev. What is happening in the out-of-balance test to stop the fuel from either or both tanks re-establishing normal feed to the selector ?

 

[Gerhard Rieger]

The Cessna 210 has L&R and a header tank, it was working for Cessna in this advanced aircraft for more than 40 years. For the Bendix system you need large vent tubes by double venting the header tank making sure it will always fill in no time. Having left and right only allows you to isolate a problem, what I did is venting my aux tanks to the main tanks, the header tank is vented to the left and right main tanks by 1/2 inch lines, they are on the upper right and lower left of the header tank both on the upper limit of the header tank. This will also vent out any air from the tank no matter what the attitude of the aircraft is. If the main tank vent block, it will vent from the aux tank. There are one risk to manage, the fuel selector, to make sure that its either on left or right. Taking off with the fuel selector in the off position will allow you to be air born and running the header tank dry, exactly why its important to use checklists and have operating procedures in place, even on a light aircraft like the Bearhawk. The main issue is to have a functional and proven fuel system. We all test our fuel systems to determine flow to the engin, many believe fuel under the floor is a huge risk, no more a risk than your gascolator ripped off in a crash and fuel pouring out by the gallons.

 

[Bcone1381]

Ops....Sorry....Bill, not Troy. (Bill from Troy, ID)

 

[schu]

Unporting a tank and sucking in air instead of fuel.
My feeling is that this is more likely to occur when a selector is in either the L or R position, and the aircraft is being flown out of balance. My understanding is that the aircraft is designed to be normally flown in the BOTH position to prevent this.

I think that unporting a tank and sucking air are two different issues. With gravity and almost certainly bendix/returnless, the system is gravity to the inlet of the pump, and as long as the engine isn't using more fuel than gravity provides, I imagine they are the same, and that unporting a tank isn't a huge deal, if it's brief. In the case of a returning fuel system, it's possible to have the pump pulling more fuel than gravity can supply and I suspect we are now firmly in the realm of sucking air.

Either way, do you think that unporting is less or more likely when using one tank vs two in a slip assuming you have the correct tank selected? What if you have 10Gallons of fuel in the airplane, which is more likely to unport? 6/4 split with both or 9/1 split with the 9 gallon tank selected?

Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.
My current thinking is that it's not having a severe imbalance that's causing the issue, rather the other way around. Flying out of balance when in the BOTH position is leading to a severe fuel imbalance, and the fuel flowing through the selector may be causing an issue at the gascolator/selector. Many years ago I flew Metroliners and if we ever had a fuel imbalance we would simply open the cross flow valve (similar to our BOTH position) and fly out of balance for a few minutes. The fuel would transfer from one side to the other really quick. Sometimes we forgot to close the valve for a few minutes and would quickly develop an imbalance to the opposite side.

My own experience is that I found the Bearhawk particularly difficult to fly in balance initially. More recently I am getting much better at keeping it in balance, and as a consequence I seldom get more than a couple of liters of fuel imbalance between refueling. I mention this not to knock the Bearhawk, but to reiterate that it did get flown out of balance alot when I was new to it. Other pilots found the same difficulty when flying my aircraft. When I refueled I would always find that I pumped far more gas into one tank than the other - though not always the same tank. MattS mentions in post 27 that his Bearhawk with floats was more demanding to fly in balance, and that after the mishap they found 55 liters in the Right tank and only 10 liters in the Left tank. This was after feeding off the Right tank for 30 mins in an attempt to balance the fuel.

I think it's probably very important to maintain balanced flight (side slipping excluded ), and (for my own bearhawk), to use the BOTH position of the fuel selector whenever possible.

Given you evidence that imbalanced fuel load is due to flying in a slip gives a lot of credibility to Bill's theory above. If imbalance is due to one wing flying more or less than the other, then it seems that putting the vent in the cap and not having a vent is for sure a factor.

I stand by my original thought that having a cross vent probably goes a long way to dealing with this, and that single tank operation also gets around this because balancing is explicit and the tanks are discrete systems.

In regard to the metroliner, who knows how that worked. I suspect the vents aren't in the caps, which changes everything.

Thanks for the dialog!
schu

 

[Redneckmech]

All good! After you commented I realized that my sig was confusing so I updated it. I figured out who you were talking to! haha

 

[Nev]

Either way, do you think that unporting is less or more likely when using one tank vs two in a slip assuming you have the correct tank selected? What if you have 10Gallons of fuel in the airplane, which is more likely to unport? 6/4 split with both or 9/1 split with the 9 gallon tank selected?

More likely when only feeding from one tank when lower on fuel. I would think the 6/4 is more likely to unport if feeding from the L tank and the ball well out to the Left. Problem is that eventually the individual tank selected will contain LESS fuel because it's been burn off, and unporting is even more likely if flying uncoordinated.

Using the 9/1 as a more extreme scenario to illustrate, with only the R tank selected and the ball out to the right indicating a skid/slip situation, the fuel in the right tank will be at the right hand (outboard) end of the tank away from the ports so unporting is very likely. Fuel from the left tank in this situation is not available because only the right tank is selected. If the BOTH position was selected, the fuel in the left tank would be at the right hand (inboard) end of the left tank covering the ports, and available to feed the engine.

This effect can be observed inflight by putting the aircraft out of balance. If you look at the sight gauges, one gauge will read very full (it has fuel at the inboard end of the tank), and the other gauge will read very low (fuel is at the outboard end of the tank). Both tanks might actually contain the same amount of fuel. What is happening is that the fuel in both tanks is moving in the same direction as the ball and occupying that end of the respective tanks. Hence the recommendation to normally run with BOTH tanks selected.

In my mind, whether fuel can transfer from one tank to the other is not a question, I've seen it happen often. It results easily from flying in an uncoordinated situation and the fuel will always flow in the direction of the ball. The Bearhawk can be challenging to fly in balance initially so it makes sense to me that this could be happening and would result in more fuel being consumed from one tank, and more fuel remaining in the other tank. But importantly it also causes the APPEARANCE of more fuel in one tank due to the sight gauges indicating a fuel imbalance situation that may or may not exist.

Try it next time you go flying, With about half fuel in each tank, push the left rudder pedal and observe the ball move to the right. Then look at the fuel sight gauges. The left sight gauge will suddenly be indicating alot more fuel. The right sight gauge will suddenly be indicating less fuel. Then put the aircraft back into balanced flight and the fuel sight gauges will indicate the same fuel in both tanks. All that has happened is that the fuel has moved to a different position in each tank, either towards the sight gauges (and ports) or away from the sight gauges (and ports).

Another way to observe this is to park on a slope with say 5 gallons each side. The downhill sight gauge will show empty while the uphill gauge shows full. The engine will start in the BOTH position, and the Uphill tank position. But select the downhill tank, and it'll likely be completely unported. Leave the fuel selector in BOTH while you go and have lunch and see where the fuel is when you come back. It flowed to the downhill tank through the fuel selector and while it did this there was fuel flowing back up the lines and into that tank. Block either tank vent before you go to lunch and the fuel will likely stay exactly where you left it.

When inflight in these out of balance situations, we tend to then select what appears to be the fullest tank in an effort to rebalance the fuel. The tanks may actually contain the same amount of fuel, but by selecting the tank the appears fullest we are lowering the amount of fuel in that tank more than the other side.

Eventually, we will select the opposite tank when it shows a large enough real fuel imbalance. At this point (if we're still flying uncoordinated to the same side) a very large fuel imbalance may have developed that has overcome the tendency for the sight gauges to show the "uphill" tank as being the fullest. Incidentally, if the fuel imbalance had been caused by a blocked/restricted vent issue for example (instead of flying uncoordinated) we would now be selecting the tank that is most resistant to feeding fuel to the engine (if that was how it ended up with more fuel in it). In a more extreme out of balance situation (with BOTH selected) the fullest tank may even have a fuel flow into that tank (up the lines instead of down the lines) as the fuel flows across from the other tank. The question in my mind is what happens if we now select that (fullest) tank - is it able to adequately feed the engine ?

Just to clarify as Kevin mentioned above, we're not talking about a conventional side slip to land here. In that situation the aircraft is normally high on approach, and engine at idle, so fuel demand is very low.

Incidentally, in the case of the Metroliner, it was a twin engine aircraft and the cross feed was a separate line to the engine supply. So having fuel flowing into one tank didn't impede the fuel supply out of that tank to the engine.

 

[Nev]

One fuel flow test I'd be interested to see that had me stumped. Start with an empty L tank. Pour a small quantity of fuel (say 3 gallons) into the left tank. Then disconnect the line where it exits the firewall and select L on the selector. Does any fuel come out ? If not, select both, the L again. Any change ? On mine it only started flowing after making another selection then back to the original tank.

 

[jaredyates]

Nev, in this case, imagine if an air bubble was trapped in the left line near the fuel selector. It was trying to rise up the line and there was enough head pressure from the 3 gallons to keep it trapped from rising, but not enough pressure to purge the air out through the firewall end. Selecting both allowed the head pressure from the right tank to overpower the small pressure on the left, thus burping the bubble back up into the left tank. I haven't experienced this type of thing with the fuel system but have with gravity-fed water systems as we collect lots of rainwater here on the farm.

 

[jaredyates]

I have a question about cross venting. If the plane is parked on a slope and the fuel selector is set to isolate the tanks, what is to keep the high side from draining to the low side through the cross vent? A ball valve?

 

[Nev]

Jared, that's exactly what we think was happening. Eventually after a lot of head scratching, we were able to repeat it consistently on both sides. Specifically we think a bubble was being trapped between the electric pump and the Gold Cube transducer right behind the firewall (we were able to release it by untorquing the AN fittings). Changing the fuel selector to another position cleared it every time and it then flowed from the original position as well. We tested with the opposite tank empty which then probably acted as a vent. When it later contained fuel it probably sufficiently increased the head pressure.

 

[Bcone1381]

"4-8-1-3 green - flight idle - low - beacons on - parallel- starting engine check - clear capped flat on the right - clear capped flat on the left."

 

[alaskabearhawk]

On my installation, the port for the cross vent it at the top and outboard side of each tank. http://www.mykitlog.com/users/displ...a&project=527&category=8690&log=280695&row=19

 

[Nev]

Haha better memory than me!

 

[zkelley2]

Generally nothing.

 

[RatherBFlying]

This was also my impression, after comparing the Newton valve and an Andair valve side by side I decided to use the Andair due to the concerns you've noted.

 

[schu]

More likely when only feeding from one tank when lower on fuel. I would think the 6/4 is more likely to unport if feeding from the L tank and the ball well out to the Left. Problem is that eventually the individual tank selected will contain LESS fuel because it's been burn off, and unporting is even more likely if flying uncoordinated.

Using the 9/1 as a more extreme scenario to illustrate, with only the R tank selected and the ball out to the right indicating a skid/slip situation, the fuel in the right tank will be at the right hand (outboard) end of the tank away from the ports so unporting is very likely. Fuel from the left tank in this situation is not available because only the right tank is selected. If the BOTH position was selected, the fuel in the left tank would be at the right hand (inboard) end of the left tank covering the ports, and available to feed the engine.

This effect can be observed inflight by putting the aircraft out of balance. If you look at the sight gauges, one gauge will read very full (it has fuel at the inboard end of the tank), and the other gauge will read very low (fuel is at the outboard end of the tank). Both tanks might actually contain the same amount of fuel. What is happening is that the fuel in both tanks is moving in the same direction as the ball and occupying that end of the respective tanks. Hence the recommendation to normally run with BOTH tanks selected.

I think a few more distinctions are needed here. I was talking about the likely hood of unporting any port which would probably cause issues when using a fast moving pump, and I think you are talking about the likely hood of having every port unported which is the mode of failure with a gravity fed system.

That leads to the next distinction, what is the source of imbalance? Your example seemed to consider the source being fuel flowing through the valve from one side to the other due to gravity effects of imbalanced flying, when I was more talking about fuel drawing more from one side than the other due to the venting differences between one wing flying more than other with the vent on top of said wings.

From the perspective of pumps, you really don't want anything unported, thus having both selected may make the system more likely to unport because there are more ports, and less fuel over those ports.

I used an extreme 9/1 example in an effort to point out that if you only have 10 gallons in the airplane and a pump pumping fuel, I'd rather it be over two ports that are more likely covered because there is nearly double the fuel over them. Yes, in a slip the wrong way that can lead to fuel starvation, but with 9 gallons in a flat 25 gallon tank, I suspect you would have to be in a very significant slip with the wrong tank selected before it matters.

At the end of the day there are a lot of things to consider, and as I'm not flying yet I'm going to call it good here on this thread, but I do hope my comments helped organize and explore the numerous theories and possibilities as there are a lot of things to consider.

 

[Nev]

Absolutely Schu - your comments all help to explore possibilities - and I think a few of those thoughts are probably close to hitting the nail on the head. The fact that this thread is on it's 12th page shows just how interested we all are in it !

 

[schu]

I have a 170 with a cross vent, if the fuel selector is set to isolate the tanks nothing happens because the vent is on top of the tank where there is just air.

 

[Battson]

Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

The Bearhawk fuel system can supply well over 140 L/hr from one tank to the selector. The IO-540 engine needs less than 90 to run at full power.

For an engine cruising consuming 60 L/hr or less, how can the other tank draw more than 80 L/hr to starve the engine - all while fighting against gravity, against the suction from the engine, and when it's specifically designed to be under slight positive pressure? Remember this suction condition would have to occur gradually, it can't just happen instantaneously.... so the engine would splutter and give plenty of warning signs, before giving up. So we would be seeing partial stoppages more commonly than full stoppages, if this was a real thing.

I doubt this is a risk for the normal Bearhawk fuel system.