Fuel Starvation With A Full Tank

Discussion in 'Firewall Forward / Props / Fuel system' started by GESchwarz, Jun 19, 2018.

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  1. Jun 19, 2018 #1

    GESchwarz

    GESchwarz

    GESchwarz

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    Why are airplanes designed to allow a pilot to starve his engine when he has another tank that is full???

    My design has two wing tanks thet feed into a common sump. No selector valve.
     
  2. Jun 19, 2018 #2

    BJC

    BJC

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    I’ll bet that even your design has a fuel shutoff valve.


    BJC
     
  3. Jun 19, 2018 #3

    narfi

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    Easier control of how much fuel you have. (weight)
    Controlling CG. (either a heavy wing in case of wing tanks, or fore/aft in case of multiple belly tanks)
    Preventing cross feeding into a downhill tank when parked or with a flat tire.
    Saving enough fuel to land safely in case one tank springs a leak.

    Like most things, there are compromises which you have to choose for your own design.
    However, I would be cautious to think your own way is superior to all others if you haven't tried to understand them and their advantages as well as drawbacks.
     
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  4. Jun 19, 2018 #4

    TFF

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    Low wing can cross drain to low wing if bank too long. If you have some one way valves you trust, you can try that. Just hope one does not lock out. Also many low wing planes will use the tanks to trim roll. One tank gets light, feed on the other as it trims the other way; then back again till destination. Lots of ways to skin the cat. Pilots doing their job should be just as important as design.
     
  5. Jun 19, 2018 #5

    BBerson

    BBerson

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    C-150 cross feeds on a side slope. Just live with it.
    I would go with no fuel valves at all, if possible. My car has none.
     
  6. Jun 20, 2018 #6

    Voidhawk9

    Voidhawk9

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    If you are flying out of balance...
     
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  7. Jun 20, 2018 #7

    Marc Zeitlin

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    Depends on the plane. On high wing planes, where there's substantial head pressure between the tanks and the engine fuel inlet a "both" setting is reasonable - even with slightly different fuel levels in the tanks, the head pressure will ensure the fuel always flows to the engine.

    But on low wing planes, where the fuel tank can easily be below the engine fuel inlet, if you have a "both" setting and due to slips or variations in vent pressure into the tanks, one tank runs dry, you then lose the ability to "suck" fuel from the tank that still has fuel, as air is easier to suck than fuel. This has happened to more than one EAB airplane with low or mid-tanks when the builder thinks they know better than the designer (or the designer doesn't know what they're doing).

    So it all depends on the configuration of the aircraft, with respect to engine/tank position and vent design. Some airplanes can get away without a "L" - "R" valve - others can't. Fuel systems can get very complicated...
     
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  8. Jun 20, 2018 #8

    Rockiedog2

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    The Zenith 701 plans fuel system has 2 wing tanks feeding into a common sump like yours. ON OFF valve only. I built mine IAW the plans and it was supposed to be gravity feed but mine wouldn't reliably feed a 912S at full throttle with the nose up in a steep climb. So I assumed it wasn't gravity feed period and installed a backup electric pump. It had most of the problems mentioned by TFF and narfi. The tanks wouldn't feed evenly and since it wasn't reliable gravity flow whenever one of the tanks got low I had to land for fuel with maybe an hour or more unusable fuel on board. And yeah all the crossflow and trim problems etc. So I designed an OFF L R system that would gravity flow and kept the engine driven pump as well as the backup electric pump. End of problems. Total control. Van's has L R OFF as well...he's got it figured out. The Zenith system could have easily been designed as a good fuel system...the only apparent reason it wasn't was gross oversimplification.
    I liked TFF said the pilot doing his job is as important as system design. Personally, I would build a system that I could control instead on one that does what it wants to.
     
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  9. Jun 20, 2018 #9

    GESchwarz

    GESchwarz

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    My design is a low wing. Both wing tanks gravity feed through their own filter and check valve to the centerline sump tank, which is lower than the two main wing tanks. All three tanks are vented. An electric boost pump draws fuel from the sump and sends the fuel to the mechanical pump. This is how the T-34 Mentor works. The check valve is a simple free-swinging flap door that allows fuel to flow into the sump tank, but not out of the tank. So it is impossible for the sump to run dry before both tanks run dry. Each wing tank is a wet-wing type, between the spars, occupying 4 rib bays. The inboard most internal tank rib features the same sort of free-swinging gate check valve as there is in the sump tank. This ensures that the inboard most bay always has a high fill level. It is from this bay that the fuel exits to the common, centerline sump tank. The sump tank will always be full.

    Does anyone see an issue with my design?
     
  10. Jun 20, 2018 #10

    Marc Zeitlin

    Marc Zeitlin

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    Sounds basically OK, but you might want to evaluate what happens with the flapper valves, depending upon how they work, when in a prolonged slip. The other thing I might caution, although the design SEEMS immune to this issue, is that differential vent pressures can do weird things, and possibly override the pressures that the flappers are meant to deal with. I'd recommend manifolding the vents together so that all three tanks see exactly the same vent pressure, whatever that might be. This way, the tanks won't be TRYING to be at different levels, even if you're protecting against it, which it sounds like you're trying to do.

    Try to think about ALL possible aircraft orientations - nose high, nose low, slips to either sidewhile nose high and nose low, full tanks on one side and empty on the other (and everything in between), etc. Where are the tanks in relation to the engine, where's the boost pump, etc., etc. If you've done all that and can find no situation in which the center sump (you don't say how large) can EVER run dry when there's fuel in either wing tank, even with prolonged non-coordinated flight, then you should be good to go.
     
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  11. Jun 20, 2018 #11

    pictsidhe

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    One way valves all need some pressure. How accurately can you match yours? Any slight imbalance and you are sucking air. Inertia effects can also do this and a few inches of fuel is very little pressure. My point is that it's very easy to get an imbalance and rather tricky to avoid one. A slight slip will also mess with the system.
    If one tank has more fuel, that side of the plane is going to want to be lower. With joined tanks you'll have a very wide and thin tank that will need to be very level to be drained dry.
     
  12. Jun 20, 2018 #12

    Dana

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    If you're gravity feeding the sump tank from the wing tanks and all three tanks are vented, what's to stop the sump tank from overflowing?
     
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  13. Jun 20, 2018 #13

    Toobuilder

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    Yes, a few things.

    The main tanks will have to be a considerable height above the sump tank to allow enough head pressure to make this a viable gravity feed system. As a low wing, I doubt you have this.

    A check valve will need some spring pressure to allow a seal. A free swinging door will allow leakage over time, which will defeats your purpose. A flapper door works in a highly dynamic environment like the oil sump on a road race car where the sump cavity is replenished every few seconds, but is not a long term level control solution like an airplane parked on a sloping ramp or uncoordinated flight.

    Do not underestimate the difficulty in achieving equal vent pressure. Even high wing Cessnas with high head pressure working for them will empty tanks at different rates when "Both" is selected.

    Your system might be the magic one that actually works, but I'd still install a "L BOTH R OFF" selector just in case it doesnt.
     
  14. Jun 20, 2018 #14

    Tiger Tim

    Tiger Tim

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    Thanks for that explanation Toobuilder, I had always wondered why very few light low-wings have a “BOTH” selection while nearly all high wings do.
     
  15. Jun 20, 2018 #15

    Lucrum

    Lucrum

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    I'm going with a single fuselage tank. But it's for simplicity more than being worried about forgetting to switch tanks.
    Maybe a better question might be, why are airplanes designed to allow a pilot to crash them when nothing is mechanically wrong with the plane.
     
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  16. Jun 20, 2018 #16

    Rockiedog2

    Rockiedog2

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    All that stuff just to protect the pilot from himself? Maybe he oughta be doing something else
     
  17. Jun 20, 2018 #17

    pictsidhe

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    I'm going for twin tanks. One per engine. One tank slightly bigger than the other so that fuel exhaustion is staggered a bit. Engine vacuum operated fuel shutoff valves are a maybe.
     
  18. Jun 20, 2018 #18

    GESchwarz

    GESchwarz

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    These one-way flapper valves are a simple flat plate of aluminum on a piano hinge. When closed, it lays across a port on a flat surface. In the main tanks, that flat surface is the web of the wing rib that doubles a tank baffle. In the sump, it is a plate of aluminum with a 1/4” hole in which the AN inlet hose fitting mates.

    This valve opens easily and will allow flow by gravity at even the slightest head pressure. Certainly, they will seep fuel in the opposite direction, but the difference in flow between the two directions is somewhere around 100x, so the concern that these valve can allow significant backflow is unfounded.

    As I mentioned already, the main tanks are baffled by three internal wing ribs. The inboard-most rib features one of these check valves. That ensures that than inboard-most bay is always nearly full. The port of that valve is 1” in diameter. Any slow rocking of the wings would quickly top off that inboard bay. That bay is where the port is that feeds the sump.

    I cannot imagine a scenario in which the sump would ever not be full.

    I will certainly tie all three vents together. Overflow from the vent is accomplished the way Vans does it, with a high loop to just under the canopy rail, then out the bottom of the fuselage.

    The main tanks are self-leveling because the pair of valves in the sump naturally allow greater flow from the side with the higher head pressure, regardless of how slight the difference in pressure.

    I greatly appreciate your generous responses.
     
  19. Jun 20, 2018 #19

    BJC

    BJC

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    Good luck with all of that.


    BJC
     
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  20. Jun 20, 2018 #20

    Turd Ferguson

    Turd Ferguson

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    I'll post a schematic if I remember when I get home but a Cessna Caravan has a similar system. 2 wing tanks flow to a common tank called a reservoir tank (similar flapper valves and all) where it is then fed to the engine. Fuel flow is augmented from the reservoir tank to the engine because of demand but that part is irrelevant. If the plane is parked on uneven ground, fuel will flow to the low wing (through the reservoir tank). Fortunately, Cessna installed an On/Off valve for each wing tank so that can be prevented if recognized ahead of time. If the plane is flown uncoordinated, the wing tanks will become imbalanced. Too much imbalance and you run into an autopilot limitation. Every pilot I have flown a Caravan with blames the airplane for fuel imbalance in flight, lol. So they use the fuel On/Off selectors for a purpose which they were not designed, restrict flow to the reservoir tank to balance the fuel. That could lead to problems in a critical fuel situation which brings us back to the question of why airplanes are designed so the the pilot can run out of fuel with fuel in another tank?
     

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