How to prevent low-wing fuel system to draw air?

Discussion in 'Firewall Forward / Props / Fuel system' started by Lucas Delgado, Oct 10, 2019.

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  1. Oct 13, 2019 #21

    wsimpso1

    wsimpso1

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    Your check valves have to operate at the very low pressures you have from that gentle slope and a gravity feed. Opening at a half inch of water is less than 0.05 psi. I went with slosh doors, and they can have issues too.

    Look here - https://bandc.com/product-category/alternators/experimental-alternators/

    The SD-8 makes 8 amps, is 2.9 pounds, and goes on a vacuum pump pad of Lycomings. They have other options. Trying to keep weight down is not much of an issue. Likewise, having two batteries instead of one is way less of an issue now with a variety of lightweight batteries. If your weight budget won't support two of something, then a wobble pump might be an option.

    Decisions, decisions.

    Billski
     
  2. Oct 13, 2019 #22

    BJC

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    I seem to recall seeing some used in ultralights, but I haven’t seen one used in E-AB aircraft.


    BJC
     
  3. Oct 13, 2019 #23

    Dan Thomas

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    That's because they are operated off crankcase pulsation, a feature of two-strokes. Four-strokes won't run a pulse pump.
     
  4. Oct 13, 2019 #24

    Vigilant1

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    Au contraire. The small 4-cycle singles and V-twin industrial engines (Briggs & Stratton, Kohler, etc) use pulse pumps to feed the carburetor. But, I haven't seen them used as transfer pumps. I'd think it would work fine.
     
  5. Oct 14, 2019 #25

    wrmiles

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    I signed up just to address this. I am not too familiar with the details on small, low wing airplanes, but here are some ways that some certified airplanes handle this issue:

    Best compromise of tank shape and pickup location, possibly with flapper valves on the feed bay to handle short term manuevers, and possibly material such as foam to displace fuel in locations where it would be unusable. live with the result, which could vary considerably depending on aggressiveness of the unusable fuel test maneuvers.

    Multiple pickup locations with float valves to prevent air suction when a pickup location is uncovered. Some airplanes require a periodic functional test.

    A feed bay or sump with flapper valves that is kept full by pumps. This somewhat equivalent to the header tank mentioned above. Turbine engine aircraft use jet pumps, which require a source of motive flow, but the Facet pumps mentioned above could work. Thid id used in a considerable number of airplanes with tip tanks as main tanks. Again, shape and location could be considered to minimize penalty if pump quits. Some aircraft provide a failure warning, others don't.

    Multiple pickup points with a check valved Facet pump on each one. (Just threw this in, not aware of a certified airplane that uses it.)
     
  6. Oct 14, 2019 #26

    Dan Thomas

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    That version relies on the intermittent suction through the carb as one or two cylinders draw air. More cylinders smooth the flow and reduce the pulsation.
     
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  7. Oct 14, 2019 #27

    Lucas Delgado

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    Both seems as good redundancy systems, I would look into it
     
  8. Oct 14, 2019 #28

    Vigilant1

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    Maybe there are pumps like that, but that's not what I've seen on these industrial engines. From the B&S web site:
    They are just simple diaphragm pumps. Here's a picture of a V-twin engine mounted in an SD-1 aircraft. The fuel pump is the black plastic item at the top. It receives intermittent pulses through the line (at 6 O'Clock) that leads from the valve cover. Fuel comes into the pump from the tank through the black line that crosses under the pulse line. Fuel leaves the pump and goes to the carb through the short line that goes left from the pump. The "lumpy" pressure in the crankcase of a V-twin engine makes a pump like this practical.
    C[​IMG]
     
    Last edited: Oct 14, 2019
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  9. Oct 14, 2019 #29

    TiPi

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    These pumps only work on 4-strokes if the piston arrangement is causing a volume change in the crankcase. Won't work on multi-cylinder with piston movements opposing each other (generally 3+ cylinder engines with even cylinder spacing).
     
  10. Oct 14, 2019 #30

    pictsidhe

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    They also work on one carb per cylinder 4 strokes from the intake runner. May also work on a multi cylinder single carb with a downstream pointed 'ejector' tube.
     
  11. Oct 14, 2019 #31

    wsimpso1

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    Pump in these types of systems must reliably move significantly more fuel per minute than you consume to keep the header from being drained. Usually, this means 150% of flow at takeoff and climb power. Remember also that crankcase pressures drop as altitude is gained, and the pressure fluctuations that make these pumps work scales with the atmospheric pressure, so make sure that you have enough flow to maintain the header tank level when the size of the fluctuations drop with altitude.
     
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  12. Oct 14, 2019 #32

    Dan Thomas

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    I have an ancient Koehler engine here that has a pulse pump on the carb that has no crankcase line to it. It relies on venturi suction pulses to operate the diaphragm.
     
  13. Oct 14, 2019 #33

    Dan Thomas

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    I'd also be concerned with the length of tubing from the engine to the pump. Long tubing has drag that would seriously reduce the pulsation amplitude at the pump. Larger tubing would offer less drag, but its extra volume also dampens the pulse.
     
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  14. Oct 14, 2019 #34

    SVSUSteve

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    Honestly, they are not that hard to scale down to size. I think that would be the simplest way to address the problem in terms of both ease of design and weight. It would also be the safest way from a crash survivability standpoint. If you can stand the weight penalty there is also the anti-slosh foam used in racing fuel cells and military aircraft.
     
    Last edited: Oct 14, 2019
  15. Oct 14, 2019 #35

    Dan Thomas

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    It would be rather easy, as you say. They're not necessary in high-wing airplanes, since outlets can be installed in both the front and rear of the tank to get fuel in any normal maneuver, and since most simpler high-wing airplanes have either a Both position on the fuel selector, or the lines are all teed together just upstream of the fuel shutoff valve. Air reaching the engine is pretty much impossible with setups like that, and for injected engines (which, unlike a carburetor, cannot tolerate any air in the fuel), a header tank just aft of the firewall vents air back to one of the tanks. High-wing airplanes don't need engine pumps except in higher-power applications or if they're injected, and only need a boost pump if an engine pump is also installed.

    Even so, running the fuel level too low could cause engine stoppage. A surge tank wouldn't help much there. It's difficult to eliminate all the ways an engine can get air instead of fuel. One can only design the simplest, efficient, light and safe system he can. Adding multiple pumps and other stuff can introduce more failure points, and more low places for water to accumulate and maybe freeze, so more drains are necessary.

    High-wing airplanes are also a lot easier to get out of when they end up on their backs. It's one thing that always bothered me with my Jodel.
     
  16. Oct 14, 2019 #36

    SVSUSteve

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    Of course, you could avoid that issue by being on the ground with no less than an hour of fuel (or a quarter of a tank for lower endurance aircraft).

    The trade off is that if they impact upright, the fuselage tends to accordion downward (because of the mass of the wings and fuel) causing severe head injuries before it pops back into a reasonable approximation of its original shape. It wasn't until NASA started drop testing aircraft at Langley that it became clear that this was as common as it was. It had kind of a "urban legend" status until then because of the aircraft manufacturers denying it happened and investigators finding blood/hair/brain matter on the roof above a restrained victim. That's one reason I argue for roll bars in both high- and low-wing designs.
     
    Last edited: Oct 14, 2019
  17. Oct 15, 2019 #37

    delta

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    It would seem that flop tubes are well understood within the aerobatic community. Why are they not incorporated more where possible venting is a concern?
     
  18. Oct 15, 2019 #38

    Dan Thomas

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    Because they really aren't necessary. A well-designed non-aerobatic system can easily do without them. Furthermore, they have their limitations, such as how much of a tank they can actually reach, the life of the tubing, and how does one prevent it from picking up water from the sump? That tube has to be pretty flexible, and ordinary fuel hose is rather stiff.

    We should ask BJC about that. He flies aerobatics and might have a flop-tube fuel system (along with inverted oil). Does that tube merely reach the top or bottom of the tank, or does it get into odd corners too?
     
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  19. Oct 15, 2019 #39

    BJC

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    I would not rely on a flop tube to met the OP’s objective because the transitions from climb to level to gliding flight is typically so benign that a flop tube may not always follow the fuel.

    In the Pitts, the tank is an oval cylinder running fore and aft in the forward fuselage.The flop tube enters the tank through the forward end, centered left to right, and a few inches up from the bottom. Fully extended, it almost hits the aft end of the tank. It will pick up fuel down to a couple of cups under positive g. It gets uncomfortably quiet with three gallons or less at 45 degrees nose up while inverted.


    BJC
     
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  20. Oct 15, 2019 #40

    BJC

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    Early PA-18s use small “header” tanks, as does my Sportsman. I could silence the engine of the Super Cub flying wing on my A152 by descending under power long enough to run the Cub’s header tank dry. Great fun.


    BJC
     

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