Feeding from both tanks at the same time

BGordon · #1 11-15-2015, 01:26 PM

Good evening everyone,
I am sitting here about to pull the trigger on an Andair fuel selector and am wondering...I see a lot of traffic concerning switching tanks at set times, but why don't people just buy a selector that allows the engine to feed from both. Would there be a problem from installing a "left, right, both, off" selector?

Thanks in advance

Brandon

Mike S · #2 11-15-2015, 01:31 PM

Yes.

This has been hashed out more than a few times here.

For a low wing aircraft, you do not want to use a "both" selection.

If one tank goes empty, you will be sucking air, which makes the engine real quiet.

pierre smith · #3 11-15-2015, 02:01 PM

Quote:

Originally Posted by Mike S

Yes.

This has been hashed out more than a few times here.

For a low wing aircraft, you do not want to use a "both" selection.

If one tank goes empty, you will be sucking air, which makes the engine real quiet.

Our Air Tractor crop dusters have an "On or Off" fuel selector. They run both tanks ALL the time because they have a header tank that's below the wings, in the fuselage and it'll never suck air with one tank dry. The engine is fed from the header tank. With our RV tanks even with the floor, we can't have a header tank.

Best,

BGordon · #4 11-15-2015, 02:04 PM

I don't understand, if I have a line open between the two tanks wouldnt the fuel seek a common level and be the same in each tank?

judoka5051 · #5 11-15-2015, 02:06 PM

Check your PMs

N941WR · #6 11-15-2015, 02:09 PM

If you are in uncoordinated flight, then one tank would drain before the other. Also, our planes don't have a balance line between them.

You can get away with this on high wing planes because gravity causes the fuel to flow down hill and if one tank goes empty before the other, the tank with fuel in it will continue to feed.

Do this, get two glasses, fill one with water and put just a little bit in the other. Take two straws and stick them together so that they form a Y. Put one leg of the Y in each glass and start drinking. As soon you empty the one glass, you will only get air.

One other thing, please don't change the fuel system. Most EAB accidents have to do with nonstandard fuel systems. Keep it simple, keep it safe!

RV3bpilot · #7 11-15-2015, 02:14 PM

I have such a system in my aircraft. I do draw from both tanks at the same time.
All semi-trucks have been doing this for years. The older trucks just used one cross over line in between the 2 tanks that would siphon the fuel back and fourth to keep it equal in both tanks. In fact most older trucks just fed the engine out of one tank and the cross over line kept them equal.
In an aircraft you must have a cross over line that can be shut off in case you catch a bird in one of the tanks.
Also you need to be able to draw from either the right, left or both selection so in case of the hole caused by a bird in one tank you can simply switch the fuel lever to the other tank to draw fuel.
You can safely do it... It was one of my goals to have an airplane that was always easy to fly and that included not having to mess with the fuel. I know others have done it too.
The cross over line must be big enough to flow more fuel than being used by the engine, also the cross over line must be right on the floor at the lowest place possible in the aircraft so it will not trap air in it.

rvbuilder2002 · #8 11-15-2015, 02:35 PM

There are things that can be done to make a both system work in a low wing airplane..... most of the time.

Keep in mind that airplanes aren't trucks.
They maneuver in 3 dimensions and experience vertical and/or lateral G forces for extended periods of time.

To put it simply, there is a reason that pretty much all certificated low wing airplanes have separately selected fuel tanks.
The only one I am aware of that had a both selection was the Varga Kachina, and it has had a high proportion of fuel starvation accidents.....
Maybe a connection?