Now if I I use 5,280' in a mile and divide that by 60 to get Feet per minute, I get 88 FPM for one mile an hour, divide our 317.14 FPM velocity by 88, we get 3.6 Miles per hour.
I would think under the right conditions that might be enough velocity to suck some water up the tube.
Rick, Mike
I have looked at all common components and cannot find an air leak or loose
fitting anywhere. If it is intermittent I don't know how to troubleshoot,
but it has not happened again!
My son came up with a possibility that has a high probability of causing the
problem. The only hang-up is that it happened on both vent lines; that seems
to lower the probability.
Here is the theory. The 1/4" tube from the NACA scoop takes a 180 degree
1" uphill bend. The flex tube from the NACA scoop to the tank vent line then
goes uphill another 1" to the fuel tank vent line. The 1/4" vent line goes
downhill to the tank by the camber of the wing. The PROBLEM: a 1" uphill
metal tube, a 1/2" downhill flex tube then 1.5" uphill to the tank vent
line. Think of a "P" trap under your sink.
I took the wing tips off and sure enough that is exactly what I had. Because
I built everything symmetrical, both wing vents were the same. Not only
that, there was clear evidence that there had been something in the P trap
at some time because the flex tubing was discolored in the low spot. It was
very evident in the right wing and not so evident in the left wing. I
naturally fixed the problem by replacing the tubing with a length 1" longer
than 1/2 of the old length.
How did water get into the P trap? As I said, it is the only time I
squeeged the wings parallel to the cord. There was enough moisture that it
was running down the wings in places. (I need to do the experiment of
drizzling water down the edge of the wingtip to see if it collects in the
NACA scoop.) If there was water in the scoop, then it could have been sucked
into the trap from start-up prop wash, taxing, or on takeoff.
Why both tanks? My standard starting procedure is to prime and start on
the left tank and run on it until the Chelton system boots up. Then I switch
to the right tank for taxi, run-up and takeoff. No boost pump until takeoff.
Soooo--
It is possible for both the left and right traps to get water in them from
prop wash or taxi and maybe at takeoff. The boost pump would suck hard
enough to overcome the small amount of water in the trap, but it did not
suck it into the tank. Since there was a small vacuum in the tank, when I
shut off the boost pump the fuel flow and pressure would immediately go to
zero and the engine quit. This assumes that the mechanical fuel pump could
not overcome about 1/2" to 1" water pressure. Before I landed the water had
evaporated and on the ground and ever since I could not/cannot repeat the
problem.
The best reasonable explanation I have come up with!
Yes, I was running LOP as I always do. I have more time than money. I
usually run 12.5 with no wind and 13 to 13.5 against the wind. I tested the
vent lines and neither had a restriction except for the 1/4" diameter. No I
have not drained the gas filter because I hoped to repeat the problem. Now
that I have a "high probability" solution I will sump the filter.
Eugene Long
Lancair Super ES