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  • #31
    I imagine there are several reasons why some FI systems need return lines and others don't. With the EFII system we are using one benefit is that there are zero vapor lock or flooded hot start issues. With the fuel contantly circulating when the system is powered up, there is cool fuel all the way to the injectors all the time. Even on a hot day, and hot engine restart, it starts immediately and runs smooth. I know the same cannot be said for some FI systems...

    Comment


    • Bdflies
      Bdflies commented
      Editing a comment
      Hey Ed, it sure would be great if you'd do a write up about your experience with the EFII system. Installation issues? Good documentation? Good support? Teething pains initially? Any quirky operating requirements? A Beartracks article would be great, or a thread on the forum.
      Systems like the EFII are cutting edge (for recip airplane engines) and it would be great to hear your thoughts about your experiences.

      Bill

    • Ed.Meyer
      Ed.Meyer commented
      Editing a comment
      I will work on a write up. It is somewhat different than the norm.

  • #32
    I believe the purpose of the fuel return was always to eliminate vapor lock. Cars used to always use it. I believe most have gone away from a return in the last decade or so for environmental reasons. Most now have an in-tank pump, and no fuel return. The tank is run at a slight negative pressure to reduce evaporative emissions.

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    • #33
      Excellent discussion, and I'm learning a lot. And it's also stimulated my thinking on this subject. So, even though I am 99% likely to go with a carbureted engine for the simplicity of it, I'm still trying to think through the injected design side...

      An FI system that "recirculates" the excess fuel would seem to either need a header tank (which I don't like for various reasons) or it would need one (or more) return lines to the fuel tank(s) themselves. I've heard some "interesting" stories from a friend with a Bonanza that had a total of 7 tanks (main, aux, and tip in each wing, plus a "baggage" area tank), but had a fuel system that aways returned the fuel to the left main tank... Fuel management seemed nightmarish to me, since he had to keep coming back to the left main to ensure it didn't overflow the returned fuel... Based on that, if I were using such an FI system, I would probably use a full-duplex fuel selector – plumbed such that the return fuel goes back to the same tank that is selected. Yes, that would require two return lines (one to each wing tank), but the additional weight would almost certainly be lower than the weight of a header tank, gauge, etc.

      In Zzz's case, if a 1/2 inch fuel line is what is required to supply the "gravity-only" fuel flow rate to prevent "sucking" fuel with the pump, then I would use 1/2 inch fuel lines connected to both the forward and aft fuel pickup bungs. And agreeing with Zzz's (or was it Whee's?) about using the "T" or "Y" fitting for the returning fuel, it would probably be prudent to add an additional bung to the upper side of the fuel tank for the returned fuel.

      But in any case, I would NOT do away with the "dual" fuel pickup points, for the reasons discussed earlier.
      Jim Parker
      Farmersville, TX (NE of Dallas)
      RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)

      Comment


      • #34
        I agree with Jim's comment and would that I would NOT do away with the forward tank pickups. We plumbed the fuel lines as Bob designed all the way to the gascolator. The dual fuel pumps (redundant) and filters are after that. We put in a duplex fuel valve and return lines to both tanks. Of course had to add fittings to the tanks which we placed at about the center of a rib lightening hole for easy access and about the center fore and aft. I don't think it much matters whether high or low.

        An interesting observation occurred yesterday and I had noticed i before but this thread came to mind this time. When I first powered up for start and the fuel pump came on, I could hear bubbles in the fuel tanks for a couple seconds as air was being purged out of the system.

        I gave many hours of tought and some lost sleep over many of the concerns and ideas, including header tank, expressed here before deciding on the final design.

        As to the concern regarding sucking air from an unported tank outlet, I worried about that as well. Apparently it does not suck hard enough to overwhelm gravity. Tested this before first flight by measuring fuel flow in hard nose up attitude with only 5 gallons in each tank. Was the same whether selector was on both or either individual tank. This is with 3/8 lines throughout.

        As others have said, interesting thread...

        Comment


        • #35
          I am using Bob's design with 3/8 tubing for my FI engine except I am only using the forward bung on the left tank where the return goes. I have a lot of time in Bonanzas with the return to the left main only and am very comfortable with the operation. Just takeoff and land on the left main. If there is a problem, the left main always has some fuel in it. Low wing planes do not flow any fuel without a pump and do fine with 3/8 inch tube. I have an Titan engine and understand the return is less than a couple gallons an hour, similar to the Bonanza.

          Newer RANS aircraft have a header tank under the baggage compartment and skip the forward tank drawoff.

          Comment


          • Ed.Meyer
            Ed.Meyer commented
            Editing a comment
            Not clear to me but sounds like you are using the left forward bung for the return. Interesting idea. I would think though that if you select the left tank only then you feeding from only one bung, the left rear. Wounldn't you then risk sucking air to that one feed with low fuel and nose low or left slip? I would think it would be better to select both for critical operations.

          • jim.mclaughlin924
            jim.mclaughlin924 commented
            Editing a comment
            I welded in an extra bung for the return line.near the top of the tank.

        • #36
          I am starting to like Whee's idea of 1/2 inch lines as well. If nothing else, extra capacity, stored in the lines itself. Sort of a built in small header tank in each line.

          Lots of great responses. I learned a lot.

          I still haven't quite come to closure over forgetting how loading works in a coordinated turn. 30 years of flying for a living hasn't taught me much I guess. LOL

          Comment


          • #37
            Originally posted by svyolo
            I am starting to like Whee's idea of 1/2 inch lines as well. If nothing else, extra capacity, stored in the lines itself. Sort of a built in small header tank in each line.
            I’d wait till empirical data suggests my solution is suitable. Hopefully within the month I’ll be performing actual flows tests with a complete system. Testing the fuel system will be part of my phase one testing.
            Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

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            • #38
              I chose to have Left, Off, Right. With the FI system And my header tank with two 1/2 “ vents back to the wing tanks. I placed one vent on the rear left and the other front right., no matter what attitude your in you always have a vent facing up for any air to escape back to the main tanks. The Header tank will fill up super fast with the two 1/2 vents. No matter if you slip for an extended period with low fuel it will not cause fuel starvation or air to enter the FI system. I only added the header tank and two 1/2 “ vents to what Bob shows on the plans.
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              • #39
                This has been a funny discussion, kind of like "Who's on first." We're not talking about the same things.

                EFII systems are much more akin to car electronic fuel injection, which utilize a pressurized fuel rail to provide supply to the electric injectors. If you decide to go down this road, you have to throw what you know about the Bob fuel system out the window. Like schu said, it is great for a carb or mechanical drip fuel injection. But when you go with a system that utilizes high pressure fuel pumps to pressurize your fuel rail to 35 psi, there is no consideration of "will my engine continue to run on gravity feed alone?" It will not. So why worry about it? At that point you focus on the reliability and failover of your electric pumps.

                The fuel return is mostly to keep fuel cool that is running over the top of, or along side hot cylinders. You could also use a header tank to accomplish that, but you need a minimum volume tank to effect the cooling. I have chosen to return to the tanks because I just don't want to use a header tank in my aircraft.

                You cannot return fuel to both tanks at once using a BOTH selector. You have to use a duplex valve and operate on LEFT or RIGHT; supply and return are set to one tank at a time. The header tank can get you around this limitation, so that's a plus for the header tank. But if you've flown old Pipers or Bonanzas, you get used to managing your tanks. No big deal.

                Those of us choosing to go with EFII aren't modifying our fuel systems just for the hell of it. I would stick with the Bob design if I was using a carburetor. But if you commit to EFII, it is a different approach and so you modify the fuel system to support the paradigm of electronic everything. The guidance on the fuel system design has come more from the system engineers, and is the same for RVs, Cubs, etc.

                To Kevin D: I'm not sure I understand your claim that an electric fuel pump doesn't "suck." ? With fuel upstream and downstream of the pump regulated to 35 psi, how can you avoid drawing fuel at an equivalent flow rate? There's always going to be a positive pressure, given the tanks are above the pumps (in the Bearhawk), so it's not fighting gravity.
                Last edited by Zzz; 05-04-2018, 12:59 PM.

                Comment


                • Ed.Meyer
                  Ed.Meyer commented
                  Editing a comment
                  The duplex valve we have does have a 'both' setting for supply and return. Fly set to both most of the time.

                • JimParker256
                  JimParker256 commented
                  Editing a comment
                  Edited to move it to a "stand-alone" comment for better thread flow...

              • #40
                Originally posted by Ed.Meyer
                The duplex valve we have does have a 'both' setting for supply and return. Fly set to both most of the time.
                It's certainly possible, I was advised to not do it because it's not reliably metered between the tanks, and you can fill one more than the other. You have the SPRL?

                Comment


                • Ed.Meyer
                  Ed.Meyer commented
                  Editing a comment
                  Yes. SPRL 4. It is possible that it returns more to one side than the other when set to both. If so, it has not been an issue.

              • #41
                Aircraft designers generally do not like homebuilders modifying the design. While some things are obviously not going to effect safety or airworthiness, nothing is usually more serious to "redesign" than the fuel system. While I know that some you builders posting in this thread are quite smart and capable people, I still urge caution changing the basic fuel system from what Bob designed - which has worked and proven safe. If you do, then you become the test pilot. Mark

                Comment


                • PaulSA
                  PaulSA commented
                  Editing a comment
                  I'm with you, Mark. With 2 pick-ups per tank I really don't see the need to modify anything. Ok, with a FI system, you need to put in a secondary electric pump but the basic system is sound. I know it is called "Experimental" for a reason, but highly complex FI systems that require dual batteries and return lines etc don't really - in my mind- seem to have a place in what, is after all, a bush aeroplane........

                  Of course, in the end, it's your aeroplane so do whatever you want.

              • #42
                Any fuel system, carb, MFI, or EFI, needs to be "fed" fuel. Since Bob's design works, I will start with that. Downstream of that, fuel will enter the fuel pump to be pressurized. But any fuel system needs to be reliably "fed". No type of fuel injection changes that.

                There will only be a short section of pressurized fuel line aft of the firewall. Maybe 18 inches, and 2 fittings. The rest of the pressurized fuel system will be ahead of the firewall.

                Comment


                • #43
                  Let's consider this: One of the areas where experimental amateur built aircraft fall significantly short of their certified counterparts is in the category of "Fuel Starvation" related incidents. That's where the aircraft had fuel available, but the engine quit operating because the fuel could not get to the engine. Some of these fall into the "stupid pilot tricks" category, where fuel tank selected by the pilot was empty when the engine quit. But a not-insignificant number of them occurred because even though there was fuel in the tanks, there was no way that fuel could possibly be delivered to the engine. These typically get classified as "pilot error", but should really be classified as "builder error" (or in rare cases, "designer error"). In the came of all three Bearhawk designs, Bob has designed a fuel delivery system that eliminates the "designer error" part of the equation, leaving it up to us as builders to either follow his design or go design our own system. If we opt to deviate from Bob's design, we're effectively designing our own system. And unless we're professional engineers competent to design our own fuel systems, we probably should tread cautiously in this area...

                  For a CERTIFIED airplane that uses a fuel pump to deliver the fuel to the engine, the manufacturer is REQUIRED by 14 CFR 23.995 to demonstrate that the fuel system can deliver fuel at a rate equal to 125% of the engine's maximum full-throttle fuel consumption. Gravity-only fuel systems (with neither an electric boost pump or an engine-driven fuel pump) would need to be able to flow 150% of the max full-throttle fuel consumption.

                  Both "pump" and "gravity" types are required to demonstrate they can meet the stated fuel flow capability while in the most critical flight attitude for that aircraft design, and the testing must be done with the tanks at very low "fill level" – only the "unusable" fuel plus whatever is needed to perform the test. My reference for these statements is 14 CFR 23.995, which can be found at https://www.law.cornell.edu/cfr/text/14/23.955.

                  While that requirement does NOT technically apply to Experimental Amateur Built aircraft, I think we would all agree that this fuel flow test is a really good idea. (And as a side note, my local DAR will NOT sign off an EAB aircraft that has not had this test performed.) You'll also note that the requirement to perform the test at the most critical aircraft attitude would generally mean that the airplane would be in a very nose-high attitude, since max-performance takeoffs are generally the most critical situations for fuel delivery.

                  But if you're leaving off the forward pickup point for the fuel tanks, I would suggest that in addition to the nose-high attitude test, you might want to consider that your actual "most-critical attitude" might well be a nose-down attitude approximating the final approach profile. That's because a go-around would be initiated from that attitude, and if you're been in a nose-low descent for a while, it is possible that the aft fuel pickup point has been "unported" for however long that nose-low flight has gone on. Translation: The fuel line from the aft pickup point to the fuel pump could be bone dry when your engine quits due to fuel starvation. You might have sufficient fuel on board to be able to restart, but if you're flying using "Both" tanks, it's entirely possible that both lines could be bone dry. How long would it take to refill those lines so the engine could be restarted? If you're on final approach when this happens, would you have enough altitude remaining to maintain a level (or better yet, nose-up) attitude so the aft pickup point would again be awash in fuel and begin to fill the line?

                  So, far from being a misunderstanding about differences between FI and Carbureted engine fuel delivery systems, my concerns are about whether or not your altered fuel system design can still safely meet the criteria of 14 CFR 23.995, regardless of the fact that it may not be required to do so... We kind of like having you around, Zane, and don't want to read about about you in an NTSB report!
                  Jim Parker
                  Farmersville, TX (NE of Dallas)
                  RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)

                  Comment


                  • #44
                    Just to fan some air across the coals of this
                    thread.

                    There had been a comment about pumps and sucking a while back that needs some clarity.

                    Most positive displacement pumps are not
                    capable of operating successfully without a flooded or pressurized inlet. They do not suck well and basically quit pumping once they
                    cavitate or drop below vapor pressure at the inlet. If pumps sucked well every water well with a submersible at the bottom of the hole
                    would be unnecessary.
                    The typical engine driven diaphragm pump is designed to suck but the pressure capability
                    is minimal and the durability questionable.

                    With respect to feeding fuel from the tanks to
                    the engine. Half the conversation is unporting a line feeding the engine.
                    Liquids are funny things, they only go where you guide them. Many different factors determine how they get down the pipe.
                    one thing is sure. After unporting a feed line,
                    air has to come out for fuel to go in.
                    Hence the dual feeds from each tank.
                    The line with the highest static pressure becomes the feed, the other becomes the vent. No vent, fuel does not like to flow down where air is heading up.

                    Comment


                    • #45
                      Originally posted by JimParker256 View Post
                      Let's consider this: One of the areas where experimental amateur built aircraft fall significantly short of their certified counterparts is in the category of "Fuel Starvation" related incidents. That's where the aircraft had fuel available, but the engine quit operating because the fuel could not get to the engine. Some of these fall into the "stupid pilot tricks" category, where fuel tank selected by the pilot was empty when the engine quit. But a not-insignificant number of them occurred because even though there was fuel in the tanks, there was no way that fuel could possibly be delivered to the engine. These typically get classified as "pilot error", but should really be classified as "builder error" (or in rare cases, "designer error"). In the came of all three Bearhawk designs, Bob has designed a fuel delivery system that eliminates the "designer error" part of the equation, leaving it up to us as builders to either follow his design or go design our own system. If we opt to deviate from Bob's design, we're effectively designing our own system. And unless we're professional engineers competent to design our own fuel systems, we probably should tread cautiously in this area...

                      For a CERTIFIED airplane that uses a fuel pump to deliver the fuel to the engine, the manufacturer is REQUIRED by 14 CFR 23.995 to demonstrate that the fuel system can deliver fuel at a rate equal to 125% of the engine's maximum full-throttle fuel consumption. Gravity-only fuel systems (with neither an electric boost pump or an engine-driven fuel pump) would need to be able to flow 150% of the max full-throttle fuel consumption.

                      Both "pump" and "gravity" types are required to demonstrate they can meet the stated fuel flow capability while in the most critical flight attitude for that aircraft design, and the testing must be done with the tanks at very low "fill level" – only the "unusable" fuel plus whatever is needed to perform the test. My reference for these statements is 14 CFR 23.995, which can be found at https://www.law.cornell.edu/cfr/text/14/23.955.

                      While that requirement does NOT technically apply to Experimental Amateur Built aircraft, I think we would all agree that this fuel flow test is a really good idea. (And as a side note, my local DAR will NOT sign off an EAB aircraft that has not had this test performed.) You'll also note that the requirement to perform the test at the most critical aircraft attitude would generally mean that the airplane would be in a very nose-high attitude, since max-performance takeoffs are generally the most critical situations for fuel delivery.

                      But if you're leaving off the forward pickup point for the fuel tanks, I would suggest that in addition to the nose-high attitude test, you might want to consider that your actual "most-critical attitude" might well be a nose-down attitude approximating the final approach profile. That's because a go-around would be initiated from that attitude, and if you're been in a nose-low descent for a while, it is possible that the aft fuel pickup point has been "unported" for however long that nose-low flight has gone on. Translation: The fuel line from the aft pickup point to the fuel pump could be bone dry when your engine quits due to fuel starvation. You might have sufficient fuel on board to be able to restart, but if you're flying using "Both" tanks, it's entirely possible that both lines could be bone dry. How long would it take to refill those lines so the engine could be restarted? If you're on final approach when this happens, would you have enough altitude remaining to maintain a level (or better yet, nose-up) attitude so the aft pickup point would again be awash in fuel and begin to fill the line?

                      So, far from being a misunderstanding about differences between FI and Carbureted engine fuel delivery systems, my concerns are about whether or not your altered fuel system design can still safely meet the criteria of 14 CFR 23.995, regardless of the fact that it may not be required to do so... We kind of like having you around, Zane, and don't want to read about about you in an NTSB report!
                      Nobody wants to not die more than me, but a life lived with a boring carburetor and nothing to provoke old EAA guys is a slow death of sorts. If you guys think that making modifications automatically means eschewing FAR 23.955, or testing, that MUST be entertaining and get the cushion sweaty.

                      I'm not pushing the envelope of design. Several have come before me implementing similar designs, many RV guys. Even the beloved old 170 only has a single mid-tank pickup. I guess I've never flown one low enough on fuel to make it an issue.

                      My design of rear port only is predicated on avoiding unporting the front pickup. The fuel system will be circulating fuel at 35-45 GPH. What happens when either of the pickups, that are T-ed or Y-ed together downstream, is unported? Will it suck air? If you don't line the word "suck" to describe fuel or air filling the void behind the supply side of a high pressure fuel pump, please suggest another term. It does create a lower pressure region, compressible or incompressible, right?

                      So, a few scenarios:

                      1. For some reason my imagination has failed me for the better and an unported front(or rear) pickup doesn't actually "suck" air. Everything remains primed nicely, the pump never cavitates.

                      2. Air DOES get pulled into the supply line upon unporting or one of the fuel tank pickups. Fuel pump manages to push it through into the fuel rail and the engine dies or runs like crap long enough to vent the air through the injectors, if it even will. I've heard reports where it was a non-event.

                      3. The air is circulated back the tank quickly via the return lines, and is removed from the system. Or the fuel pressure regulator purges it through a bypass.

                      As for in-flight fuel starvation, yeah, it sucks to unport front or rear pickups in any pitch attitude. But climbing steeply and maybe slowly is when I'd least like it to happen. it would suck in a descent too but usually you're carrying an abundance of airspeed. And for approaches,I don't approach any runway or airstrip pitched down aggressively. But one shouldn't have to use any flying technique to compensate for engineering flaws.

                      Any situation where you have unusable fuel is undesirable, though, I agree. I have a few emails out to other high wing EFII builders to see if they've intelligently mitigated the risk of unporting. But as far as I know, you either go header tank or full return with single tank pickup.
                      Last edited by Zzz; 05-08-2018, 01:21 AM.

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