Fuel tank fails to feed

[Hank Jarrett]

Re: Fuel tank fails to feed

The condoms are just the first step to find out if your system is air and fuel tight. Mine passed that test with flying colors (I had to rock my wings and establish flow from the wing to the main prior to flight too). Once I got the system air tight the flow was still very slow so I went on to the next steps where I inspected all the lines and valves. It was in the pressure tests I found my main fuel valve was not actually shutting off. The slow transfer was finally traced to a kink in the fuel line where it went from the horizontal section between the wing tank to vertical down the "A" pillar. The line had to turn 90* to go through the cabin leading edge piece and then 90* again to turn down the "A" pillar. There was a kink completely hidden up in the corner where the two 90s were. When I tried to pull thins to the side to inspect the area the fuel line snapped in two at the kink. I put all new lines and a new main valve in and put in two 90* fittings through the leading edge cockpit piece. It is now air tight and the flow is fine. Still need to do a flight test.
Hank

[Gerald McKibben]

Re: Fuel tank fails to feed

I'm still in the 'diagnostic' stage. I'll report back when (if?) I find my problem. I have about decided that having to rock the wings to start the transfer is not so bad after all! But I would like to get to the bottom of the problem.

The condoms are just the first step to find out if your system is air and fuel tight. Mine passed that test with flying colors (I had to rock my wings and establish flow from the wing to the main prior to flight too). Once I got the system air tight the flow was still very slow so I went on to the next steps where I inspected all the lines and valves. It was in the pressure tests I found my main fuel valve was not actually shutting off. The slow transfer was finally traced to a kink in the fuel line where it went from the horizontal section between the wing tank to vertical down the "A" pillar. The line had to turn 90* to go through the cabin leading edge piece and then 90* again to turn down the "A" pillar. There was a kink completely hidden up in the corner where the two 90s were. When I tried to pull thins to the side to inspect the area the fuel line snapped in two at the kink. I put all new lines and a new main valve in and put in two 90* fittings through the leading edge cockpit piece. It is now air tight and the flow is fine. Still need to do a flight test.
Hank

[astjp2]

Re: Fuel tank fails to feed

Trivia for all of you experts, how many inches of water water column = 1 PSI? How tall is the wing tank outlet to the header tank inlet?
Tim

[astjp2]

Re: Fuel tank fails to feed

Another trivia question.... Does a water tank that is 10' in diameter and 50' tall have the same water pressure as a 2" line full of water that runs to the top of a tank that is 50' tall?
Tim

[Hank Jarrett]

Re: Fuel tank fails to feed

You multiply the inches times .036 to get PSI.

Hank

[astjp2]

Re: Fuel tank fails to feed

Hank, that did not answer the question.... c for effort though.

[Larry Lyons]

Re: Fuel tank fails to feed

The answer is, of course, both are identical. The shower/drenching just lasts longer from the 10' diameter tank. when you break a fitting at the bottom!

L

PS: You can multiply psi by 2.3' also to get height of water column. Just so happens I use the 2.3 far more often in my work. We used mercury manameters to measure water levels in Hawaii back in the 70's, just don't remember the conversion. In Hawaii they still moniter the potable water level, under the islands, to the 1/10th of an inch as fresh water is only slightly lighter then salt water.

[astjp2]

Re: Fuel tank fails to feed

Correct!

The answer is, of course, both are identical. The shower/drenching just lasts longer on the 10' diameter tank. when you break a fitting at the bottom!

L

[Hank Jarrett]

Re: Fuel tank fails to feed

Sorry, 27.71". I am just so used to plugging the numbers into the equation I didn't bother to run it backwards for the actual answer.
Hank

[Gerald McKibben]

Re: Fuel tank fails to feed

Could be a clogged line. Then you wouldn't need a condom...... baritone sax h

I think I've narrowed it down to the cap. I don't understand it; the surface on top of the filler neck is smooth enough, and the fit between it and the cap gasket is plenty tight. I added a second gasket made from the 1/32 in material that is fuel proof. That really made the cap hard to tighten. But when I blew through the tube it still leaks down fast. When I cover the filler neck (I used a rubber glove) it holds pressure. So I guess I'll just keep rocking the wings to get the left tank to start flowing. It's really not that big a problem.

[astjp2]

Re: Fuel tank fails to feed

My point with those questions is the fact that there is only approximately 1 PSI of differential pressure between the wing and header tanks. If for some reason you are getting more pressure into the header or the wing gets negative pressure due to airflow when flying, the gas will not flow to the header correctly. Tim

Sorry, 27.71". I am just so used to plugging the numbers into the equation I didn't bother to run it backwards for the actual answer.
Hank

[Gerald McKibben]

Re: Fuel tank fails to feed

My point with those questions is the fact that there is only approximately 1 PSI of differential pressure between the wing and header tanks. If for some reason you are getting more pressure into the header or the wing gets negative pressure due to airflow when flying, the gas will not flow to the header correctly. Tim

I was already suspecting that the pressure (even with the pressurized air from the tube in the cap) is marginal anyway, so any little problem can prevent the flow starting.

I appreciate everyone responding though; I learned a lot.

[waltermrich]

Re: Fuel tank fails to feed

Im going to chime in here, I do not suspect the air pressure into a wing tank with the wing tank cap with a air tube is marginal , at air speeds of 90+ going into that tube I would suspect it would rise the pressure at least a few pounds. I know that my wing tanks will hardly drain when the aircraft is parked, but when I open the drain valve from a wing tank in flight , I can watch the nose tank gage rise from near the bottom to upper levels (depending how much gas I have in the wings) in just a few minutes

[astjp2]

Re: Fuel tank fails to feed

Ok, but how do you know if the original poster is not getting pressure into his header tank somehow? Also we do not know the diameter of the tubes on the wing tank caps. I have seen 1/4", 3/16" and a formed metal cover shaped like a scoop. Each one will provide a different pressure into the header tank. Also, how much pressure will you get with a 1/4" diameter pitot tube at 90 mph? You can blow an airspeed indicator apart with your breath. Its not much, you need 230+ mph to get 1 psi. So the original poster is looking at maybe 30 inches of water column, or around 1.2 psi? Hank would probably do the math if we asked him. Tim


knots/mph inches
17.4/20 0.20
26/30 0.44
34.8/40 0.79
43.5/50 1.23
50/57.5 1.63
69.6/80 3.16
70/80 3.20
86.9/100 4.94
100/115 6.56
104.3/120 7.13
110/126.5 7.95
120/138 9.48
130/149.5 11.14
130/150 11.18
150/172.5 14.87
173.9/200 20.04
200/230 26.71
217.4/250 31.63

Im going to chime in here, I do not suspect the air pressure into a wing tank with the wing tank cap with a air tube is marginal , at air speeds of 90+ going into that tube I would suspect it would rise the pressure at least a few pounds. I know that my wing tanks will hardly drain when the aircraft is parked, but when I open the drain valve from a wing tank in flight , I can watch the nose tank gage rise from near the bottom to upper levels (depending how much gas I have in the wings) in just a few minutes

[Guest]

Re: Fuel tank fails to feed

Ok, but how do you know if the original poster is not getting pressure into his header tank somehow? Also we do not know the diameter of the tubes on the wing tank caps. I have seen 1/4", 3/16" and a formed metal cover shaped like a scoop. Each one will provide a different pressure into the header tank. Also, how much pressure will you get with a 1/4" diameter pitot tube at 90 mph? You can blow an airspeed indicator apart with your breath. Its not much, you need 230+ mph to get 1 psi. So the original poster is looking at maybe 30 inches of water column, or around 1.2 psi? Hank would probably do the math if we asked him. Tim


knots/mph inches
17.4/20 0.20
26/30 0.44
34.8/40 0.79
43.5/50 1.23
50/57.5 1.63
69.6/80 3.16
70/80 3.20
86.9/100 4.94
100/115 6.56
104.3/120 7.13
110/126.5 7.95
120/138 9.48
130/149.5 11.14
130/150 11.18
150/172.5 14.87
173.9/200 20.04
200/230 26.71
217.4/250 31.63

FYI - the pressure is only different when the air is flowing into the tank thru the cap vent, ie we have a flow rate. When there is no flow there is not pressure drop. But the situation that we are thinking about is when the fuel is going out and the air is going in to the wing tank.

The nose tank is expelling air out it fuel cap vent as long as the other wing tank valve is closed.

One might expect that the factors effecting back pressure from the nose tank is the same regardless of with wing tank is draining into it with the exception of flow rate.