Fuel selector or two check valves?

Use separate valves if the tanks aren't vented to the same
source. For certified aircraft, any gravity-feed system that uses one
valve or a valve with a "Both" position must have the tanks vented to
the same source to keep tank pressures equal. Many homebuilts have
problems along this line and a few have quit and crashed because the
builder made mods and/or didn't understand the reasons the designer
made things the way he did. Even some designs had shortcomings to start
with. The original Glastar design, for instance, had a vent tube under
each wingtip to feed each tank separately, and small differences in ram
air pressure would cause one tank to drain before the other. If the
pressure differential is large enough, the tank with lower pressure
will not flow at all and the engine will quit when the other runs dry.
The Cessna 150 used a single valve to control the flow from both
tanks, teed together at the valve inlet, and the tanks were both
connected to the single vent under the left wing. You could still get
uneven flow if you flew with one wing a bit low; remember that the
tanks are well apart and a slight bank will raise one above the other
to cause crossflow. Check valves could stop that, but they'd have to be
installed as low as possible so that the small amount of head pressure
will open them, and their springs would have to be very weak. My old
Auster had such check valves but still used two shutoffs because the
tanks were separately vented at the caps.
If I was to build another airplane I'd have shutoffs right at the
tank outlets; maintenance on the system is a pain if you have to drain
the tanks every time you want to fix something, and it would be nice to
have them there in case of a leak lower down in the system while in
flight.

Dan

wrote:

...

You could still get
uneven flow if you flew with one wing a bit low; remember that the
tanks are well apart and a slight bank will raise one above the other
to cause crossflow. Check valves could stop that, but they'd have to be
installed as low as possible so that the small amount of head pressure
will open them, and their springs would have to be very weak. My old
Auster had such check valves but still used two shutoffs because the
tanks were separately vented at the caps.

I'll add that check valves present an unwanted resistance to flow in
the intended direction, minimization of that effect tends to go
hand-in-hand with reduce reliably and completeness of closure when
the pressure is reversed.

If I was to build another airplane I'd have shutoffs right at the
tank outlets; maintenance on the system is a pain if you have to drain
the tanks every time you want to fix something, and it would be nice to
have them there in case of a leak lower down in the system while in
flight.


For that application, would not full-aperture ball valves be
appropriate?

--

FF

For that application, would not full-aperture ball valves be
appropriate?

Yup. I use one in my Jodel now; they work fine, but a weak point
appears to be the o-ring around the shaft. The ball and teflon seats
work OK. Watch that you don't get a really poor quality Asian unit;
there's a wide range of stuff from good to pretty lousy out there.
Badly cut pipe threads and all.

Dan

"Rob Turk" wrote in message news:SN6we.366$in3.124@amstwist00...
Instead of using a left/right fuel selector, could I use two check valves and connect the two tanks into a single
feed?

[LTANK]---|-----T-----|-----[RTANK]

Where | and | are the check valves and 'T' is a T-connector towards the header tank.
The check valves prevent fuel moving from one tank to the other. Any pro's, con's or thoughts?

Rob



What is the pressure drop through the valve and the cracking pressure of the valve? Pretty hard to have 'matched'
valves, so one will most likely flow freer...