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Operation Notice from Bob about Fuel Tanks on Systems with Fuel Pumps

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  • #16
    I guess I'll comment, hopefully it helps someone:

    For years we were told to build to plans and NOT to modify the fuel system, and that it was dangerous to do so, but many of us knew that the bearhawk fuel system isn't adequate for every engine installation, specifically for any type of fuel injection that required a return.

    Now we see this notice, but it's not really clear, and doesn't explain why. Someone asked if it's ANY fuel pump, to which Mark says yes, and given that Mark no doubt asked Bob, that is the official answer, but I think it might help to go through the different types of fuel systems and explain why design choices might make the engine quit.

    Carb:

    A carb doesn't need any fuel pressure, it just needs fuel in the float bowl, and the amount of fuel it needs is exactly whatever the engine uses. For this reason gravity works fine. The FAA says that a gravity fed fuel system must flow 150% of the fuel needed by the engine at a nose up attitude. So, you simply raise the nose, do a flow test and that's it.

    Here are the gotchas: The FAA also states that when you use a fuel selector valve with the 'BOTH' position to tie the two tanks together, you also need to tie the vents together so that the pressure above and below the tanks stays the same. The bearhawk doesn't have this. In theory, with both tanks vented to atmosphere, they should flow the same, and this shouldn't be a problem, but there has been two reports of engine stoppage when one tank is very full and the other isn't when the pilot was using 'BOTH'. It's pretty obvious that in this condition for whatever reason, one tank is feeding the other and staving the engine. If I was running a carb I would have a cross vent, or I wouldn't use the 'BOTH' position. Even my Cessna 170 is placarded to use single tank operation at cruise, and it has a cross vent. All that said, many have been flying for years with no cross vent on both. Given that the bearhawk doesn't have hard points and exact plans to mount the fuel system, every single one is slightly different, so nobody knows why some work great, and others have had problems.

    Bendix (returnless) fuel injection:

    The Bendix system needs fuel pressure in order to work, so it requires the use of a fuel pump, and due to redundancy, two fuel pumps. Most people run an electric boost pump that can flow fuel when it's off, along with an engine driven pump. They operate at around 20-30psi. Because the fuel system doesn't have a return, the fuel pumps cycle fuel internally to regulate the pressure. What this means for the rest of the fuel system is that fuel delivery to the pump only really needs to be whatever GPH the engine is consuming. For this reason I believe the bearhawk fuel system works fine for returnless fuel systems. The flow of fuel down the door posts isn't really any different with returnless FI vs a carb.

    Gotchas: You must manage the boost pump and turn it on when you need it. Also, like with a carb, it would be wise to use single tank or add the cross vent, but I don't think this is as big of a deal when fuel pumps are involved as flooded input port at the pump would almost certainly cause fuel to be pumped into the engine. The biggest issue with returnless FI (other than additional weight) is the fact that you have an engine driven pump which gets hot with fuel circulating in it which also gets hot. This can cause vapor lock if you aren't running a fuel that is very resistant to vapor lock (avgas). There are no certified airplanes with returnless fuel injection that have STC for mogas.

    Return fuel injection (EFI and continental):

    In the case of EFI, they need a bit of fuel pressure to work, and all of the ones I've seen pump the fuel through the injector rails, then to a regulator which bleeds the excess fuel back to the tank. This means that the fuel demand at the tank is whatever the pump consumes, which for some pumps can be quite high, like 80gph-100gph high. Of course, whatever the engine isn't using is put back into the tank through a duplex valve, but what if a tank becomes unported? Well, that pump is going to suck air pretty fast because it's consuming so much fuel. One way to deal with this is to try to never unport, so always use the fuller tank or only use one port (which you would want in the center), or use a header tank and return the fuel there. Personally, I would absolutely be using a header tank if I was going to run EFI. The continental runs at a lower pressure and doesn't return as much fuel, so it might work, and we have one member here that is doing it, but he put in 1/2" fuel line on the aft door post and also uses a L/R/Off valve.


    Because I didn't want a carb due to the uneven fuel distribution, and also because I didn't want to completely redesign the fuel system, I'm going to run airflow performance with the factory fuel system and a valve that is L/R/Off. Landing will require selecting the fuller tank and turning on the boost pump. I don't expect any problems, but it is more complicated than a carb. The pro is that I'll be able to use LOP at low power cruise to cut down on my fuel costs, and I no longer have any issues with carb ice.


    This not the official word from Mark or Bob, but hopefully it fills in the blanks for someone trying to work through what to do.
    Last edited by schu; 11-13-2020, 12:31 PM.

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    • schu
      schu commented
      Editing a comment
      Well, there is something new I didn't know. I assumed they cycled the fuel like a boost pump. Either way, if there isn't any flow, the fuel can get hot in there.

    • zkelley2
      zkelley2 commented
      Editing a comment
      If by boost pump you mean the facet pumps that most people use, they also do not cycle fuel within themselves.

      https://www.youtube.com/watch?v=Dtf66p8M11g

    • svyolo
      svyolo commented
      Editing a comment
      I looked into a bunch of fuel system schematics and cutaways several months ago. All the diagrams I saw for Bendix style engine driven fuel pumps show an internal bypass to return excess fuel from the outlet back to the inlet.

      The EFII and AFM boost pumps have to return fuel somewhere. The are constant displacement electric pumps. It is built into the external manifolds they come with in addition to a check valve to allow them to be bypassed. Same with the expensive certified boost pumps.

  • #17
    Thanks for the info Schu.

    So, I have a standard fuel injected system with a back-up electric pump and no cross vent. Would you therefore recommend running LEFT/RIGHT in the cruise and fuller tank for T/O and Landing or BOTH for T/O and Landing?

    Comment


    • schu
      schu commented
      Editing a comment
      I'm going to run left/right and use the fuller tank because there are less ports to unport, because I'll certainly have much more fuel in one tank instead of both being low, and because I don't like how the bearhawk lacks the cross vent. I'll also probably use mogas in cruise from one side and land/takeoff with avgas from the other.

      Battson runs BOTH, doesn't have a cross vent, and has hundreds of hours of trouble free operation.

      I'm not sure what would work best for you. My solution requires much more fuel management, but I also plan on using fuel sensors with my EFIS as well as 30 minute alerts to check fuel to warn me if I'm being dumb. Battsons solution is dead simple, and very likely doesn't have issues with the lacking cross vent because as long as the pump inlet is flooded, it will pump fuel, even if there is some imbalance in the tanks.

      Do you want to trade fuel management to have less unporting with a single, but fuller tank? That's up to you.

      You could call Bob and talk to him too. I'm not the engineer. He may tell you you need a header tank....

  • #18
    It seems prudent to me to have a limitation the says "Don't select BOTH if a significant fuel imbalance occurs". This sets the fuel system up to crossfeed from one tank to the other tank via the fuel selector. A crossfeed situation would not allow 100% fuel flow to the engine.

    It also seems prudent to me to have little concern if both tanks quantities are essentially equal. Run it any way you desire as long as that limitation is honored and fuel is being supplied to the engine.
    Brooks Cone
    Southeast Michigan
    Patrol #303, Kit build

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