Header tank design and location

[Bcone1381]

I am doing MFI with Bob's original fuel system design due to the issues I am reading about here. My conclusions mirror almost exactly with what schu described. But please allow me to offer what I think might be a reasonable design improvement....way off from Bob's design. So its worth what you are paying for it.

I am installing my fuel system in accordance with Bob's Patrol Book. The electric fuel pump on my installation will between the gascolator and the firewall. I am not running EFI. I am not installing a header tank. My term "Design Improvement" was a poor choice of words. I intended to offer something to consider for those who are installing EFI with its large volume pump/fuel return. A header tank installed under a seat concerned me.

For those who are using EFI, be sure that its simple to operate without having special operating limitations.
My preference at this juncture in time would be to use a single 5 gallon header tank located aft of the cargo area with an on-off fuel valve.

Bill, its been decades since I last flew a low wing injected aircraft. I sense I may be missing something due to your post...

[Bdflies]

So Brooks, I take it that you're planning on an electric pump at the rear header tank. That pump will run on low at all times, except high boost for starting - maybe takeoff/landing also.

A tank behind the baggage bulkhead will be well below the engine driven pump, on takeoff. You have to treat such an installation as one would view a low wing, injected aircraft.

Bill

[svyolo]

Brooks;
Aft of what bulkhead? The cargo bulkhead? I am going to think about your idea.

[Bcone1381]

Schu;
I like to make rational decisions instead of what I want. Usually rational wins out. In the case of MFI vs EFI, I think MFI is good enough for me, but I am much more comfortable working with EFI. For me MFI is a box of parts I don't understand. The cost of MFI and dual EI's is similar to EFI/EI.

I think I could have been happy with either, but in the end I am more I EFI's box of parts more than MFI's. Electrically I don't mind a few extra wires as i am good at that part and I will make it reliable. I only don't like the extra plumbing. More precisely, I don't like plumbing!!!!!

No problem, svyolo., I reference Post #55... Realize, that making rational decisions is resulting in a compromise....the header tank being placed under the seats.

I am doing MFI with Bob's original fuel system design due to the issues I am reading about here. My conclusions mirror almost exactly with what schu described. But please allow me to offer what I think might be a reasonable design improvement....way off from Bob's design. So its worth what you are paying for it.

I would feel more comfortable if a +5 gallons header tank was placed aft of the bulkhead with a "Not full" sensor in it, gravity fed by only the aft fuel tank bungs. Highly unlikely that a 5 gallon header tank could be emptied during descent to landing with the aft tank bungs unported....plus it increases fuel capacity. It can have 1 return line running back to it and have a simple on-off fuel valve. it removes fuel lines, removes fittings, and connections. It becomes almost a C-150 fuel system from the operators point of view. Sucking air problem is eliminate, its in a safer location.

Doing pattern work on almost empty main tanks would be no worry. Every landing would fill the header back up in seconds. If one ran the main tanks dry, VFR fuel reserves are in the still in the header.

Note this popular maxim, from Saint-Exupery:

..perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away...

I look up to you who go the EFI, but I am not there yet.

[svyolo]

Schu;
I like to make rational decisions instead of what I want. Usually rational wins out. In the case of MFI vs EFI, I think MFI is good enough for me, but I am much more comfortable working with EFI. For me MFI is a box of parts I don't understand. The cost of MFI and dual EI's is similar to EFI/EI.

I think I could have been happy with either, but in the end I am more I EFI's box of parts more than MFI's. Electrically I don't mind a few extra wires as i am good at that part and I will make it reliable. I only don't like the extra plumbing. More precisely, I don't like plumbing!!!!!

[Peter Bishton]

Matt you are lucky to be alive. Looking at the blog and photos the aircraft destroyed. God has other plans as you said. Peter b
4 place scratch build Australia

[svyolo]

Right now I am leaning towards a header tank under the front seats. Maybe just under the right seat, with the fuel pumps under the left. I get access to all fuel components, plus the elevator cable adjustment. It is contained within the fuselage, at the strongest place on the whole fuselage.

[svyolo]

Scshu;
Your comment on SDS's individual fuel trim? Your thoughts about why you don't like it?

MFI and a couple of electronic ignitions is just as expensive, or more, than full EFI/EI. Rebuilt carb and 2 rebuilt magnetos are cheap to buy, but the mags would need to be rebuilt 3 times to get to TBO, and the carb probably at least once, unless you fly a BUNCH. Running LOP should save 3-4000 gallons between TBO's. Rebuilt carbs and magnetos are cheap to buy, but I think they are the most expensive over the course of the life of the engine. By far.

If you are building a complicated IFR airplane, the electrical system is the same for EFI or a carb. If you are building a simple VFR airplane (me), EFI increases the electrical cost/complexity. I have a hard time justifying it other than I simply prefer it. But I do prefer it.

[Bdflies]

Glad you're here to relate that episode to us! What you described is the real bugaboo about experimenting with fuel systems. When problems arise, they pop up FAST and at the worst possible times.

[MattS]

We had completed the bulletin and the port was separated from the sight gauge.

[MattS]

This subject has spiked my interest in the last few weeks as I put our Rebel floatplane into the trees on a go-around. The only reason I'm alive is because God must have more for me to do! I walked away but the pastor who was with my broke his femur You can see more at http://thesteidingers.com/blog/2018/05/rebel-accident/). Climbing out from a low pass to check for wires, etc I was turning from cross-wind to downwind. Because the creek was narrow, I had to climb out over the trees. The motor didn't falter, it just stopped dead. What happened? why would a reliable Lycoming O-320 just quit dead? The fuel design in a murphy Rebel has just one port in each tank. Initially it was about in the middle and connected to the fuel sight gauges. A bulletin called for the exit to be separated from the sight gauge and moved farther back. Our Rebel had no header tank, just gravity and with the 150% flow test completed.

On this particular day we had taken off with a little more fuel in the right tank. After talking off, I closed the left tank and was burning fuel from the right to equalize. A storm came up so I descended fairly quickly, did a high pass and then a lower pass about 250 feet off the water to check for power lines running across the river. First mistake, waiting to do my landing checklist until i was in the pattern instead of when I approached my landing site. As soon as it quit I immediately remember the left tank but only had time to open the valve-- I'm guessing I had about 10 seconds to shove the stick forward and land in the forest. No time to try and re-start. I had 30 litters in the left tank and 25 in the right at the time of the accident (verified by sight and our fuel flow meter). I was using my right tank, I was in a left turn and if you are in a coordinated turn (see Mark's post in this thread) the fuel should not flow away from the port, but even if it would have, it would have flowed down to that port. I have flown and done hundreds of landings in this plane with the above fuel scenario. After some research, two things I see were different this time. ONE; I was not using both tanks. I can't remember ever approaching a landing site when I wasn't on BOTH. TWO: This was not a "planned" approach. The storm was coming so I descended steeper than normal. What do I think happened? The fuel tank is integral, so there are quite a few obstacles (ribs, etc) that keep the fuel from "quickly" un-porting the single exit. However, because of my longer, steeper descent, I think the fuel might have had time to flow around / through the obstacles to the front of the tank. I believe it became un-ported and when I powered up and climbed there was air in the line. If this had happened in my other approaches, because of an un-coordinated turn for example, it would not have happened to both tanks. Therefore the tank not un-ported would have pushed the air out of the other line (see Jim's last post). What we've learned. Either have two ports in each tank or a header tank. I'm still not sure how the Cessna 150's get by with only one port (if I remember correctly, the larger Cessna models have two, but I believe the smaller models only have one).

[Mark Goldberg]

There have been ground loops in Bearhawks where the gear collapsed. But I do not recall that anyone has ever sheared off the gascolator on the belly. Mark

[svyolo]

Jim;
I totally agree with the threat of fire, and the gascolator being so vulnerable, but like someone said, light planes are designed to fly, not crash. If you use a true header tank then the normal fuel lines are more than adequate as they are not "feeding" 40 gph. Only what the engine is consuming.

[JimParker256]

Interesting discussion... Hopefully no one is thinking anyone is mad at anyone – we're all just trying to learn together, right?

It doesn’t eliminate the possibility of unporting, it increases it. If you have a fuel pump cycling fuel at 40GPH and gravity can only feed that pump at 30GPH, then you nose down with minimal fuel, the rear port can come unported, and instead of simply not providing fuel like in a carb system, the pump will draw air into the system.

I don't understand that statement. If you're only using the aft pickup point, it will unport in a nose-down attitude anyway. That's kind of the entire reasoning behind having both front and rear pickup points... Yes, one of the other could unport, but not both. And with both front and rear points, when one of them unports, you've still got a column of fuel from the other one to the "y" in the fuel line, and all the way to the pump. Last time I took a fluid dynamics course (admittedly MANY years ago) I understood that the mass of the fuel being greater than the mass of air would result in the pump pulling "fuel" rather than air.

But you're absolutely right that if you have a pump drawing 40 GPH and a supply line that can only deliver 30 GPH, you're going to have an issue. That's why the fuel flow test is required for certified airplanes, and is a REALLY good idea for experimentals. (It's also why it might be a good idea to use 1/2" fuel lines if you really need that much fuel supplied...)

I would way rather have the fuel system inside the fuselage than outside because if it’s outside, it’s WAY more prone to rupture in a crash. Consider builders that run fuel lines or pumps under the tubing in the tunnel. If the airplane has a crash that wipes the gear off, you will have a ruptured fuel system. I’m planning for my fuel pump to go under the pilot seat and nothing extending below the floor. I figure the same cage that is protecting me can protect my fuel system.

I agree with your position on fuel lines below the "safety cage" and am still struggling with the idea of having the gascolator mounted where Bob's design suggests. It seems like the gascolator would be the first thing to break off, and that would unleash a flood of fuel from the lines (assuming they're still intact). My "gravity feed" Citabria has a fuel drain way back behind the baggage area (low point in the fuel supply while on the ground), as well as a gascolator at the bottom of the firewall (engine side), and all the lines run between the floorboards and the "cage"... Seems more "protected" from a gear failure that way.

But the fuel in the wings versus in the fuselage thing is something that reading NTSB reports might change your mind about... I know I changed my thinking on this topic after reading a bunch of them. I wish I still had the reference(s), but it was many years ago, and the link was lost in a hard drive crash... The thing is that fuel is only one third of the equation when it comes to fire. You also need oxygen (which will pretty much always be there) and something to ignite the fuel vapor... Outside the fuselage, there are very few things that can create the spark necessary to start the fire going. Within the fuselage, you have batteries, wires, switches, generators, and all sorts of other things that can spark the fire... The NTSB post-crash fire accident reports where the plane had gas tanks in the fuselage were sobering... Lots of examples of the fuel catching fire and – key point – fatally burning the occupants before they could safely evacuate from an otherwise survivable crash.

By the way, that whole "post-crash evacuation" thing is the key reason I'm building my Patrol with doors on both sides (seaplane version) even though I'll probably never put it on floats. Hopefully, with doors on both sides, I can evacuate away from the fire... Unless I REALLY have a bad day, and manage to rupture both wing tanks, after crashing into a field of flint...

(As total aside – I knew a pilot in Brazil who crashed three airplanes into the jungle, and sheared both wings off the plane each time. Once he even managed to shear both wings and the tail section, just aft of the cabin. All three times, he survived with only bruises and very minor cuts. No fires Amazing...)

Bobs design is great for carbs and for lycoming injection which has no return. It’s probably not sufficient for return type fuel systems with continentals or EFI. Those systems demand much more fuel to be flowing. If I was building one, I’d probably use a duplex valve, return to tank, and build two 1.5”x10” round tubing header tanks with 1/2 on one side and 3/8 on the other, and put one on each line between the front and rear mains and the T under the door jam, then use 1/2” line between the mains and the small header tank. That would hold almost a gallon per side, be very well protected under the door jam, and would be very hard to unport, while maintaining Bob’s simple system.

The pump would have to suck enough fuel to empty two 1.5x10” columns of fuel, along with 3 ft of 1/2 tubing faster than the engine can use it before it sucks air. I think I like that way more than 3/8 lines when dealing with a fast pump and the engine at idle.

Probably why someone else suggested upgrading Bob's design by using 1/2" tubing through the entire system for the non-Lycoming EFI solution. Doubtful that even an IO-540 could use more fuel than a 1/2" line can deliver... (But the fuel flow test would confirm that one way or the other...)

That said, ill probably run lycombing injection, and know that mogas might not work in my airplane.
Schu

So, will you still use the mini header tanks you described under the door jamb area? I'm trying to visualize that, but I'm building a Patrol and it's pretty crowded under there, with rudder cables, flap cables, and aileron cables, not to mention the throttle cable for the back seat, and the fuel lines themselves... Have to ponder that one a bit.

As I said earlier, interesting discussion...

[svyolo]

Schu;
I was thinking about something similar. Build mini header tanks into each gravity fed line. Still thinking about it. I have seen a few EFI installations where I think fuel heating is also much less of an issue. The fuel divider is mounted aft of the engine. Should transfer much less heat to the return fuel.