Fuel Selector Valve Testing

The plan is to install a fuel tank selector valve, inspired by Ron Stacey’s very nice Velocity. As Ron says – if you think about it, there are no certified aircraft that don’t have fuel selectors. Another advantage is that a selector valve provides a handy place to hook up an optional auxiliary fuel tank.

The options break down as follows:
A) Position the selector valve close to pilot, probably in the side of the keel. This requires routing fuel from the strakes up to the valve and then back to the sump tank. Because of the long runs, 1/2″ tubing is the minimum practical size (IO-550 engine). Thats lots of fuel line, and there is not much space in the keel for three parallel runs, especially back at the whale tail.
B) Position the selector valve somewhere behind the whale tail. This requires running some type of linkage (cables + pulleys or shafts + u-joints) between the selector knob and the valve itself, but eliminates the long runs of fuel tubing.

Another potential issue with option A is that when the aircraft is in a climb, the selector valve may be higher than the fuel tank. The sump tank is still below the fuel, so this represents a siphon condition – the fuel must run uphill to the valve, and then back downhill to the sump. This should be fine as long as there are no bubbles in the fuel line.

To test this, I set up the fuel valve with the correct distance from and with varying height with respect to a fuel tank, which was the correct height off the “floor”, and could be filled with varying heights of “fuel”. I used water for the tests, and converted the measured flow rates to estimated fuel flow by scaling with the viscosity ratio. The simulated sump tank was a graduated bowl, and for each simulation scenario I measured the time it took for one liter of water to flow into the sump.

The measurements indicate that the siphon works OK, and the maximum expected fuel flow rate (full tanks, level attitude) into the sump was about 46 GPH, in excess of what is required for an IO-550 at full power. This flow rate was only slightly decreased when changed to a steep climb. However, with the fuel tank at 1/4 full, the flow rate into the sump decreased to about 28 GPH, which is marginal for a high power setting. The most worrisome aspect was that introducing a bubble into the feed line from the fuel tank to the valve caused the flow to stop. This always happened if the attitude was climbing, and even sometimes if the attitude was level. The only way to reestablish flow was to change the attitude to a significant dive.

The bottom line is that putting the fuel selector valve forward is very risky without also installing a pump that can re-prime the fuel lines (or run all the time!) to prevent the loss of siphon. Not worth it…