Dave Saylor
Joined: 11 Jan 2015
Posts: 210
Location: GILROY, CA
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 Posted: Mon Apr 20, 2015 7:07 pm Post subject: Fuel Selector was: Fuel Return Line Location? |
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Hi Justin,
It sounds like you've put a lot more thought into this than I have. I didn't think about a high wing. You're correct, a BOTH position shouldn't be issue.
I'd still be concerned, though, about overfilling a tank. Alarms are great when they work, but simplicity is better.
The concept of overpressuring a tank with a pump that can put out more than 100 psi is scary to me. Once you overwhelm the vent, what keeps the tank from rupturing? Fuel tanks typically only handle one or two psi before they start to deform.
I can practically guarantee you from experience with numerous fuel systems that if you draw from both and return to both, you will fill one and drain one. I've seen it happen a few times. The only explanation I can offer is that I *think* that a small imbalance in vent pressure will bias one tank enough to allow the levels to become very different. Then you'd be working to balance things out, at who knows what phase of flight. Yikes.
Thank you for flying those big planes. I'm sure that their designers also craved simpler systems.
--Dave
On Sun, Apr 19, 2015 at 9:50 PM, Justin Jones <jmjones2000(at)mindspring.com (jmjones2000(at)mindspring.com)> wrote:
[quote]Dave,
I understand your concern and welcome any ideas and thoughts that may be cause for issues. The issues you brought up have been considered and here is what I have come up with. Note that I have spoken with many others about this as well, but it doesn’t mean that we aren’t missing something.
The normal configuration (covered on the before take-off checklist) would be both return valves open and the fuel valve on both. The transfer of fuel during refueling operations (while the aircraft is on floats) is one reason for this setup, and the other is the ability to draw fuel from both tanks simultaneously.
“You could return to a full tank and waste fuel overboard” - This scenario could present itself, but would cause an over-pressure situation due to the vent size. The digital engine monitor senses fuel pressure and sounds an audio and visual alarm when it is either too high or too low.
“You could unport an empty tank into “BOTH” and starve the engine” - This scenario has also been tested. With one tank empty, one tank full, and the selector to “BOTH”, there was still full fuel pressure. This may be due to the high-wing design and the fuel system plumbing called for by Robert Paisley of EFII.
“You could turn off both return valves and… I don’t know what, but it seems bad” - It would cause the fuel to back up at the valves, and the fuel pressure would increase to the maximum that the GL393 fuel pump would put out. This would be 105 PSI at 10 Gallons per hour. Not sustainable, but not harmful to the injectors or fuel lines for short periods of time. Again, the Engine Monitor will alarm when the fuel pressure begins to increase. The fuel system has been tested to the fuel pump stall pressure of 115 PSI and found to have no leaks.
I fly C-130s and King Air 200s and they both have fuel systems that are far more complicated. Flying aircraft will always require thought. With the duplex fuel valve, the pilot must remember to switch the fuel valves when it is necessary. I like the idea of being able to operate in a normal scenario with the fuel valve on both, and have the fuel is return to both tanks. If the situation arises, I can put the fuel where it is needed and I have a good monitoring system that will alert me if I make a mistake in the positioning of fuel valves. The return valves do not have a position that will cause the engine to quit turning.
Thoughts?
Thanks for your input.
Respectfully,
Justin
| Quote: | On Apr 19, 2015, at 7:30 PM, David Saylor <saylor.dave(at)gmail.com (saylor.dave(at)gmail.com)> wrote:
Hi Justin,
You are of course free to build whatever fuel system you like, but the one you described has some issues.
I think you'd be better off with a L-R-OFF duplex fuel valve that routes return fuel to the same tank you're burning from. You're headed towards a scenario that makes it pretty easy to mismanage the fuel:
You could return to a full tank and waste fuel overboard;
You could unport an empty tank in "BOTH" and starve the engine;
You could turn off both return valves and...I don't know what, but it seems bad.
Please consider a simpler system. Caution never goes as far as we think it will.
--Dave
On Sun, Apr 19, 2015 at 8:00 PM, Justin Jones <jmjones2000(at)mindspring.com (jmjones2000(at)mindspring.com)> wrote:
| Quote: | --> RV10-List message posted by: Justin Jones <jmjones2000(at)mindspring.com (jmjones2000(at)mindspring.com)>
Berck,
I am using an EFII system as well. Just a thought, but I put a normal L/R/Both/Off fuel valve on the supply side (AN6) and an AN6 size return line that Splits to each tank. After the "T" I put a simple Parker petroleum 1/4 turn ball valve on each side just before the tank. Now, I can shut one or the other return valves off, turn on the fuel pump, and transfer fuel on the ground. It can also be done in the air (with caution of course) to trim fuel.
Note: I'm building a Bushcaddy not an rv10.
I can send pics if you would like.
Justin
> On Apr 19, 2015, at 18:10, Berck E. Nash <flyboy(at)gmail.com (flyboy(at)gmail.com)> wrote:
>
> --> RV10-List message posted by: "Berck E. Nash" <flyboy(at)gmail.com (flyboy(at)gmail.com)>
>
> We're planning on EFII, so we need a full-sized return line for both
> sides. Just now starting work on the fuel tank, and I'd like to get
> this sorted before we get too far. I've spent awhile searching, and
> have only managed to find lots of places not to put the port for the
> fuel return line. Apparently it can interfere with both the wing spar
> and the aileron return line. Does anyone have a clear diagram of where
> I can safely put it? A hint about which flange/fittings to buy would be
> greatly appreciated as well. Thanks in advance!
>
> Berck
>
>
>
>
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--Dave
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