K3 Fuel Sight Gauge and Vent Line

[bbryan]

Got to install a fuel level sight tube on side of 13gal. tank. From other pics looks like it doesn't matter if you install sight tube tilted front or back, but if I install high point of tube toward front spar can I tee vent line with top of sight tube or do I need to tap a separate hole for vent line to header tank.

[cap01]

the model IV has another hole and fitting besides the sight guage fittings for the vent . looks like youll probably have to drill and tap one .

[Geowitz]

I have seen some fuel occasionally go back up the vent line which would make it harder to determine fuel quantity if it's pouring into your sight gauge. If it were me I would tap another line. You can build up a 1/2 inch or so cylinder of fiberglass on the outside of the tank to drill and tap so you have more material for the fitting to bite into.

[HighWing]

I don't think there would be a problem with using a T on top - using the same port for the vent and top of the sight gauge. I designed a low fuel indicator system using the vent line and have been watching the vent line for many years.

What I have found: When the air first shows on the top of the sight gauge - Model IV - there is approximately 9 gallons of fuel left in the tank. For the first four gallons of fuel burn, the sight gauge and the fuel vent will be essentially full. After the air begins to show it will be only air in the vent/sight gauge. I doubt the air movement in the vent line - and yes the fuel level does move - would affect the sight gauge as far as accuracy. I redesigned the Low Fuel sensor for this reason as well as to make it Ethanol resistant.
Lowell

[bbryan]

Great Info, I went ahead and built up three areas for the fittings. Highwing, how did you make your low fuel switch. I been working on that too with a prox switch, tubing and a metal float but haven't perfected it yet.

[GWright6970]

When I get closer in my build, this is what I will be installing:
http://www.highwingllc.com/lowfuel.html

Cheers!
Grover

[HighWing]

Bob,
It is not a very easy project, but doable. I got the idea from Ray Volk, one of the original Desert Fox guys.

Regarding the system in the vent line. I found that there was enough movement of the fuel in the vent that it would trigger false positives. For example, if I was flying uncoordinated with the left wing low, it would cause a downward fuel movement in the vent line and the float would drop. It would start the red light flashing. It didn't bother me because I knew what was happening and it gave a neat system check when I wanted one. Occasionally in turbulence, it would flash as well.
Lowell

[jtpitkin06]

I suppose you can slant your sight tubes either forward or aft. There is an advantage to slanting the top of the sight tube toward the leading edge as it will give more accurate readings with changes in pitch. That is, of course, assuming your sight tube fittings are located near the center of the tank. A tube slanted to the rear will give erroneously high readings with high pitch attitude.

Depending on the design of the rest of your fuel system, adding a T fitting for the vent may, or may not, create a problem. Those systems with two wing tanks, a header tank, and a single shut off valve between the engine and the header will often experience fuel going up or down the vent tube. When the tanks are filled on the ground the vent tube allows purging of air from the header tank. Once the air is out, the fuel level in the vent line will be approximately the same as the sight tube. However, fuel is free to transfer from left to right side through the header tank and travel up the supply tube into the vent line. Transfer may occur during continuous wing low flight, uncoordinated flight or intentional slips. That transfer could flow into your sight tube and give a false reading.

If you have only a single wing tank and a fuselage tank, there is no transfer. Therefore a single wing tank system should not have any problem with a vent line T at the top of the fuel sight tubes.
John

[bbryan]

I went ahead and installed a separate vent line. I was going to use 1/4 I.D. for the vent and sight tube but is seems a little fat to make the bend, Do you guys think a 1/8 I.D. line is big enough for the vent line ? By the was, that's a nice float level switch.

[HighWing]

I don't think a smaller line would be an issue, but then again several thousand Kitfoxes have the quarter inch line and problems hav'nt surfaced over the years. The vent line is in place to allow air from the header tank to escape so that header tank volume is dependably consistent. From my recollection - and the panel tank guys might be able to shine some light on this - the early Kitfoxes didn't have a vent line. It was a closed system and was referred to as a siphon system. There were issues that developed where in certain circumstances air could be sucked into the system with no escape route. There were some reports of fuel starvation - Fix? add a vent line.

Regarding the accuracy of the sight gauges. Most guys, in my experience - probably 200 or so hours in flights of six to ten airplanes - fly with one wing slightly low. In the loose formations we flew in you could definitely see it. Fuel transfer is to be expected in this situation. On one such flight, one of the guys - running behind a Franklin - after landing in remote Idaho refused to fly again because he was convinced that he had a fuel line blockage from one tank. We checked his system thoroughly to the point of undoing the carburetor fitting and measuring the fuel flow from each wing tank - exactly the same. Some with this symptom added fuel valves on each wing tank to better control fuel balance, the rest just left it alone. Fuel level definitely will vary with pitch especially evident with conventional gear on the ground. The sight gauges are only for monitoring relative fuel quantities - on my Model IV, the first bubble at the top of the gauge shows 9 gallons remaining. With my first Model IV, I had double scales on the gauges. One for three point and one for cruise attitude - low and high were the same, only the middle ranges varied by a couple of gallons. The detailed placarding is not in my second Model IV as it was of modest value, actually not worth the effort of adding a gallon at a time and making a mark. My personal opinion is that the Kitfox, irrespective of the evolution, was designed as a simple airplane. Guys tend to personalize things based on prior experience and what makes them feel better. I doubt the personal modifications from the basic plans makes the airplane much safer.
Lowell