Fuel Flow Discussion, Moved from Float Mounting

[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/displa...=280695&row=19

[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 !

[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.

[Battson]

For anyone new to Bearhawk and reading this thread:

I think it's important to keep sight of the fundamentals:

Fuel systems should be designed by experts.
The system designed by the expert is safe, changes to the approved design should be consulted on with the expert designer. We are allowed to experiment, but generally we are not experts, and don't fully understand what we are doing.

The best fuel system is simple.
We know that complex fuel systems have killed competent pilots on many occasions. The less you touch your fuel system in flight, the better.

Having a system which can always draw from both tanks is desirable.
If there is fuel in the plane, the engine keeps running.

[schu]

Ed.Meyer, did you do this with less than 10 gallons in the airplane?

[Sir Newton]

Battson, This entire thread has struck a nerve with me. I 100% agree with all your statements.

[Ed.Meyer]

schu I'm sure there was more than 10 gallons but was able to confirm un-porting by selecting the un-ported tank and watch fuel pressure to zero.

[Nev]

Battson makes some very good points above.

Other than during flight testing, it's a very good idea to always fly in balance (ball centered), and keep fuel in both tanks. (One possible exception being a brief side slip to land).

However, a situation can occur the other way around, where unintentionally flying slightly out of balance over time causes a tank to run dry as a result. It's illustrated here starting with half fuel and the ball out significantly for clarity. In reality the ball only needs to be out by a small amount.

If I got airborne and flew around for a couple of hours unintentionally slightly out of balance here's how the flight might proceed (I saw this beginning to develop on several flights during my own flight testing) :


93030A0E-F990-425F-96A6-BDF5CCF0947A.jpg



90E122F2-5FF8-4503-9563-93D2381F9BEE.jpg


F8D0652C-E62D-440D-AC8B-D4D11A3BA404.jpg




There are a number of reasons why the aircraft could be unintentionally out of balance more to one side than the other. The most common one for me was flying at an airspeed above or below the speed at which the vertical stab and rudder are set for a trimmed condition, and not being used to flying with the ball centered.

This is just to show the importance of maintaining balanced (coordinated) flight - the same for any aircraft type, not to suggest that any changes need to be made.

[AZBearhawk272]

Nev,
I enjoy your commentary and graphics spot on on the issue!

Jon,
Clearly stated, KISS is best.
systems often become complex when political solutions are applied to fix a technical non problem.

Enjoyed the comprehensive discussion

Kevin D
# 272
KCHD

[Nev]

MattS I was wondering if auxiliary tanks were fitted in PR-ZJO ? (I assume they were for a flight of this length).

[MattS]

Yes we have the auxiliary tanks.

[Nev]

MattS do you recall was the remaining fuel measured after being drained, or was the remaining quantity read from the sight gauges ?

It was just a random thought, but I wondered if there any chance that fuel was becoming trapped in the right auxiliary tank ? You made mention early on of the right wing sitting lower.

One thing that has me stumped is that the airplane always sits with the right wing much lower than the left (8 to 10 inches at the wing tip).

After the mishap I saw that 10 litres was found in the left tank and 65 litres in the right. If both mains held 10 litres each at the end of the journey, then the additional fuel in the right side is approximately the quantity that one auxiliary tank would hold.

[MattS]

So almost 4 years after my engine out and hard landing we have some more information. We overhauled the engine and had some wing repairs to complete. After 12 hours or so of flying after getting it back together, I had a couple times when the engine sputtered. It always came back strong but could tell something wasn't correct. It's a Cont IO360 and therefore has a pretty extensive procedure for measuring and adjusting the fuel pressure. Doing this test, we could not get the correct pressures. Wouldn't quite hit the full PSI requirement and was fluctuating alot. We started troubleshooting and found out if we removed the electric boost pump from the system, we could get the correct pressures and it was stable.

Back up 5 years to when we first overhauled the engine and put it on floats. We had the engine sent off to an OH shop and when it came back I had moved 1,000 miles away and wasn't on-site. The team installed the overhauled engine, but when they went to start it, the electric boost pump was leaking. They found another pump in the hanger, hooked it up, it provided the correct pressures to prime and seemed to work. Looking back, we didn't have a history on this pump and we should not have used something this critical without knowing more about it. This engine does not use the electric boost pump for anything except starting and emergency procedures- so 99% of the time it is just a pass through. However we believe that it was intermittently affecting the fuel flow and this is what caused the engine out. With a new/correct pump installed, we were able to complete a positive fuel pressure test and after 10 flights (albeit staying close to the airport!) the fuel flow and pressures are much more stable (with no sputtering!)

Sorry for all the confusion over this situation. I was hesitant to post this and possibly drag up more junk, but also wanted to be very transparent, hopefully set the record straight, and maybe someone can learn from my mistakes. Blessings!

[kestrel]

Please do not be sorry! Objective data is far more useful than unsubstantiated conjecture and can help all of us. Thank you!

[kestrel]

I have really appreciated this forum over the years as sometimes I feel "alone" down here. My goal is to provide accurate information to the group. I am not trying to convince anyone of what happened.

Until September 29, 2019 I flew with the assumption that in a high wing plane flying in BOTH was the safest mode....similiar to what others have said here. I believe I did NOT unport a tank. I would explain it as this; flying uncoordinated caused a pressure differential between the two tanks. Instead of fuel flowing (at the selector) forward through the firewall to the engine driven pump, the pressure differential was enough to cause the fuel to flow back up to the other tank instead. Basically it overcame gravity and starved the engine. Jon told me he was able to replicate this twice. You will not be able to replicate this on the ground unless you have some way to cause a pressure difference in the tanks. Additionally this will not occur just because one tank is dry as air will not flow downhill when there is fuel in the other tank that will push it back up (when running on BOTH).

When I purchased the plane the fuel selector only had "R", "L" and "closed". I accept full responsiblity because I made a modification (installed a selector with BOTH) without doing my homework to see how this modfication would affect everything (again, I was trying to make the plane safer and made a few posts about this modification on the forum trying to make the correct decision). To me this is the rule I did not follow and paid for it:
If two or more tanks
have their outlets interconnected they shall be
considered as one tank and the air space in the tanks
shall also be interconnected to prevent difference in
pressure at the air vents of each tank of sufficient
magnitude to cause fuel flow between tanks.

Thankfully I was the only one in the plane. I was not hurt. A small seatbelt mark on my neck (install shoulder harnesses!). My daughter asked my how I felt soon after the accident. I replied "Blessed and pissed!". We are in the process of repairing the plane. If anyone would like to volunteer to come down, I am looking for some good rivet bangers. I'll throw in a free flight over the Amazon rainforest in a RV6!

Back when I first read this, I was not convinced that it was the correct explanation. I don't believe that fuel will flow "up" to the lower tank causing starvation of the engine.

I do believe that there is a very simple failure mode where a blocked fuel cap vent will result in the fuel for that tank not being available. A cross vent would provide a secondary vent for that tank. I don't believe that is what happened here.

My guess was that there was some other factor that had not yet been identified. While there is still room for reaching an incorrect conclusion, the new data about the fuel pump feels much more like a correct explanation.

It is my opinion that a number of engine failures are pointed at and blamed on mystical boogie men when there is actually a much simpler explanation that is missed because some fact about the system or circumstances hasn't been identified. Maybe in this case that missing fact has been found.

[Nev]

Very interesting fuel and engine failure discussion with Alaskan Bush Pilot Will Johnson.
If it starts at the beginning, skip to 58:30.

https://m.youtube.com/watch?v=1yx1d_2qntM&t=3510s

[Bissetg]

I think there is valuable information in this. As he described the unporting referred to is a function of the fuel sloshing to the other end of the tank. The mitigation being to keep more fuel in the tanks, both selected (I’m not sure if he had the option of selecting both) and the ball on the middle I guess. I still try and keep an hours fuel up my sleeve for the unexpected, such as having to provide assistance to someone else or act as a comms relay in the event of an emergency of another aircraft.

I’ve been really impressed with the fuel system on the B model and that the gascolator seems to get fuel from either tank when at relatively low levels.

I’ve also found the stronger rudder pedal springs have made it a lot easier for folks to keep the ball in the middle and not over control on the rudder causing said sloshing.

[Bcone1381]

I agree with Grant....valuable information. I want to know what happens when fuel sloshes. I have experimented a bit with clear tubing in the shop. What follows is a bit of data, experimentation and art of imagination to maybe help me understand this dynamic..

Cessna's gravity system on C-172 and C-182 shows the designers made changes over time. The venting of the fuel lines that exit the tank seemed to be an interest.. The creation of an air bubble must be happening inside a fuel line when fuel sloshes and unports a tank bung. The bubble is an invisiable, silent, disappearing restriction to fuel flow. I see Fuel flow being hitting the bubble restriction and slowly descending around it. A long bubble slows flow considerably.

The Bubble wants to rise up inside a vertical section of the tubing but the fuel flowing down the line tends to keep the bubble suppressed inside of the fuel line. Its worse in a horizontal section of the fuel line. Venting gives the bubble an escape route out of the tube. I imagine the Cessna designers found getting that bubble restriction to move out of a line that is horizontal also to be more difficult than getting it removed out of a line that is vertical. Look where they put the bubble escape route.

This is a C-172N fuel system diagram.

Screenshot 2024-10-28 at 6.59.54 AM.png

[TimTall]

I accidentally left my fuel valve in the both position while parked on a cross slope the other day. I ended up with about 12 gallons of fuel in the left tank and about 3 in the right. I figured it was a perfect opportunity for a test.

I climbed out on the left tank to 5000' agl and set up an orbit overhead the field. I then switched to the right tank. In level flight 3 gallons of fuel doesn't even register in the fuel gauge, so I'm running below what I would ever want to see in real world conditions.

I wanted to see what it would take to unport the lines and cause the engine to quit and to see how long it would take to re-light. I did a few orbits on the single tank at cruise settings simulating an out of trim condition. I was right wing low by enough that it was feeling pretty uncomfortable. I held this for about 5 minutes with no signs of fuel starvation.

I was more interested to learn what would happen in an aggressive slip in a landing configuration though. Suppose I'm low on fuel at the end of a flight and I have to slip to get in and I've forgotten to select both tanks for landing. How long would I have before the engine quits, would there be warning signs, and how long would it take to make power again?

I slowed down to approach speed, 60mph, full flaps, idle power. I did a few shorter slips and climbs back to altitude until I got it to quit. It took almost 30 seconds in a maximum slip to starve the engine. That's a very long slip. At least 500 vertical feet. I tried a few at higher power settings to simulate a situation like lifting a wing for a passenger to take a photo where I'd be purposefully out of trim with power on. At a slow cruise setting it took about 15 seconds to quit.

In all cases the engine quit abruptly but smoothly. There was no noticeable stutter or surging. During the idle test it barely even got quieter. As expected, EGT's dropped off the scale and fuel flow went to zero but nothing else changed.

It took what felt like forever for the engine to re-light, in reality it was around 20 seconds each time after returning to level flight. Not a problem at 5000' feet but far too long at 100'. The engine restarted smoothly with just a little bit of surging for the first few seconds. Turning on the boost pump as soon as i rolled level helped by several seconds but it made the restart rougher.

By this point i didn't have much gas left in the almost empty wing. I would like to test if turning to the fuller tank would speed up the re-light. The higher head pressure may help. I was down to almost no fuel by the last test. An extra 6" of head pressure isn't much but it might make a difference. Although in my scenario, reaching for a fuel valve on short final, on what is most likely about to be a deadstick landing, is probably not a good idea.

I'm running a different fuel system to most of you but the fluid dynamics should be similar. The air in the lines needs to be replaced with fuel. In my case that takes about 20 seconds apparently.

Overall I learned that i'll continue to take-off and land on Both tanks but there is some margin for a mistake. 30 seconds is a long slip. Even in the mountains there's not many situations where that would be necessary in a Bearhawk. It slips very well but it can also descend like a rock in a straight line if it needs to.

[Nev]

I wanted to see what it would take to unport the lines and cause the engine to quit and to see how long it would take to re-light.

I love this sort of testing, excellent work and thanks for sharing.

Did you notice any difference with the ball out to the side of the low tank (remaining fuel at far end of tank and unporting) compared with the ball out to the side of the fullest tank (low tank well ported but possibly completely empty) ?