Fuel Flow Requirements

[Archer39J]

Something I've been mulling over a bit now. If you can count on both tanks for max flow at takeoff attitude with minimum fuel, does it make sense to combine them at the fuel valve into a 1/2" line? By my numbers it seems that one 1/2" line can be nearly equaled by two 3/8" lines (1.988). I'm considering doing this at the fuel valve since it seems a waste of capacity to not.

My goal is to provide enough flow to a fuel injected engine so as to not require a fuel pump, if that's possible. Thoughts?

[schu]

That won't work. Fuel injection needs 20ish psi to work (injectors need pressure behind them, to inject....) so you will need a fuel pump, and you should have two. Install a engine mounted mechanical pump, then also add an electric boost pump for redundancy.

Once you have pumps, you don't need anything larger than 3/8", and you don't want it either, because it's much harder to work with.

[Archer39J]

Well there you go, thanks.

[whee]

I went with 1/2" lines from the rear port on my tanks to my electric fuel pump. 3/8 from there forward. I don't have "Both" in my fuel system so I wanted good flow in a single line at minimum fuel and no wing mounted pumps. Really there is no need for 1/2" lines in almost every case and like schu says, you have to have pumps somewhere with FI.

[Battson]

Almost all the common hardware is designed for 3/8. I shared your idea of using 1/2" line back when I was at this fuel system stage, but found planning a 1/2" system forward of the fuel value both difficult and fruitless.

The common parts were designed from the ground up to work well together, and the common configurations were designed to manage the risks of powered flight. Of course the homebuilder can do whatever they want, but it doesn't mean that they should Tens of thousands of planes flying with a system which works reliably and is easy to maintain are a safe bet to copy.

Each common device (gascolater, transducer, filter, selector valve, standby pump, etc) offers a restriction to flow anyway, so the larger lines don't help much overall unless you up-size those parts too. You can source larger devices, but then you run into the "Law of Unintended Consequences".... like things not fitting in the spaces provided, or things being incompatible with avionics, plus the unforeseeable etc. etc. etc.

By the by, when designing a system from scratch the designer needs to think about flow rate and pressure independently, they are not proportional to each-other. So both flow and pressure are independent considerations for a fuel injected engine, as noted by Schu. Bendix for Lycoming needs 15psi for normal Ops, 12psi to idle, not more than 45psi at any time. Improvements which improve pressure don't necessarily improve flow rate.

[Archer39J]

Yeah I was just fixating on the flow rates and didn't think about pressures like you said. Plus I'm seeing what you mean about common parts being 3/8, seems to be more trouble than it's worth.

Thanks for the info all

[Rollie]

I ran 3/8 for the whole thing and just did a flow test before my first engine start, I disconnected the fuel line where it goes into the fuel servo, forward of the firewall and checked multiple times how long it took to get 16 oz (a pint) and it was 18 seconds each time on either tank and 17 seconds on both tanks. My my math that comes to 25 gallons per hour on one tank. That is on just gravity flow and I think that is plenty for my IO-360. I'm sure that with the boost pump running it would be more GPH but since 25 GPH is plenty for my engine, I didn't bother checking it with the pump running.

As far as running a fuel injected engine without a pump, please don't!! I only say that based on having had a mechanical pump fail on an Aztec I used to have. Fuel injection requires pressurized fuel. When the mechanical pump failed, the engine quit. I turned on the electric pump and the engine came back on. That redundancy is pretty important. It turned out the mechanical pump had a small hole in the diaphragm, it would work for awhile and then fail. I ended up flying it for awhile like that before I figured it out and got several chances to find out that the engine won't run without pressurized fuel. One of those things where having two engines and two fuel pumps on each engine covered for my ignorance. Had it been a single engine with no backup pump, I may not have had the chance to live and learn.

Airflow performance makes a nice electric fuel pump with a serviceable filter that is not expensive compared to certified pumps. Even an automotive pump would be better than no pump.

[jaredyates]

I was just reading about a statistic where inflight power loss is much more common on twins than singles, and I think it is in part because people are of a mindset to take off with less-than-perfect conditions when they know there is a second engine.