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Dave12

Well Known Member
During the one gallon fuel flow test I am having trouble getting the gallon of gas pumped out within the 180 seconds. The last half gallon slows the process and it requires another 30 seconds to pump out. Battery voltage is not the issue. I removed the bowl from the gascolator and by bypassing the screen and hose the test is within specs. The screen is clear. What do the pro's have to say about this? Anything to be worried about? One more thing, How about those NAVY S.E.A.L.s, the CIA and those ARMY Helicopter Crews? Was that slick or what? My son is a Navy FMF Corpsman with the 2nd Marines. He called last night prouder than a Peacock.
 
Hi Dave - -

Hate to say it, but the small passages in the lower corners of the tank baffles are critical. Would seem you got a little too much sealant near them, and they are not letting enough fuel thru to the outlet.

John Bender
 
Fuel flow

The size of the passageways through the tank baffles is important when the fuel level is low. If the passageways are too big, then during a steep climb, the fuel will run through the passageways to the aft of the tank and the fuel pump will suck air. If the passageways are too small, then the engine will use all of the fuel around the finger screen and more fuel will not be able to run through the passageways fast enough to supply the engine. When the fuel level is very low, a prolonged steep climb could result in all of the little remaining fuel running to the aft end of the tank.
Joe
 
Add more fuel!

Dave try it with 4 gallons of fuel in the tank and see what happens!
 
I just completed some more testing. I pulled the screen and it was clear. I poured 1.5 gallons in and in 2:45 I had 1 gallon. The last .5 gallon took 1:30. I tried different amounts and actually found that it is the very last quart that is slow. I know these are famous last words, but during the tank build I was very careful to leave these openings clear as I do understand their purpose. Looking for some advice from the smart guys.
 
Yes, it does seem to be functional. It does not leak when closed and allows fuel to flow.
 
Within limits?

If my math is correct, your fuel flow is within limits. 1/2 gallon took 1:30 minutes or 90 seconds. Double that time for a whole gallon: 90 x 2 = 180 seconds, right at the limit. Or is my logic faulty? In one hour, 20 gallons will be pumped. For reference, it took 120 seconds for my tank to empty.
Does the pump speed up when the tank is almost empty? If so, that indicates the pump is sucking air. I think that is normal with an almost empty tank.
Joe
 
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Yes, the pump does suck air during the last half gallon. Your logic is welcome as I had neglected to view the problem the way you just did. This is why I ask the smart guys. Thank you. Just waiting to see what the other pro's say. Truthfully, I hope to never have to depend on the last gallon but I never say never.
 
Joe's point is good - -

If you put in the gallon, the pump should be able to get ahead of the 'drain-in' rate and give you a different sound indicating it is running out of gas to get into the pump.

Another silly question ( maybe ), did you take the drain completely out of the gascolator ?

John Bender
 
I just knew it - -

I'd be sorry for jumping in - - out of ideas ! ! ! !

When it is quiet, with no gas, does the pump sound like it will vibrate itself out of the plane ? ? ?

John Bender
 
Gotcha!

Hi Dave,
You are not the first to have this problem. See this thread:
http://www.vansairforce.com/community/showthread.php?t=65401&referrerid=8455
I put aside my fix until I had the aircraft finished. When I pull the top I want to put in a moeller gauge, Currently I manage the problem by using only the top half. But the new gauge and fresh proseal arrived this week, so its time to solve the problem.

I?m pretty unimpressed with the PAP test. It forces you to remove the tank and take the top off, at the end of the building process, if your tank fails. Why not a bench test before the lid goes on?

How are the pre-fabricated fuel tanks that Vans sells, tested?
http://www.vansairforce.com/community/showthread.php?t=66013&referrerid=8455

I bet Vans does not just trust to luck when they sell a tank. So my first step will be to ask Vans for the details of their bench test.
Rod
 
For what it's worth...

Your situation is exactly how my fuel test went. During the last half gallon or so the pump sucks air and cavitates. That seems quite normal to me, since you are drawing fuel out of the tank at a faster than normal rate than you would in actual flight conditions. Approx 20gpm as opposed to 5 or 6.

In real life, in what circumstance would you be using the last half gallon of fuel anyway? Only in an extreme emergency, and by then you would be preparing for an emergency landing!

IMHO your tank is just fine, and I wouldn't spend any more time worrying about it. Build on!
 
Good idea, Rod

Why not a bench test before the lid goes on?
Click to expand...
That is a good idea. The tank could be filled with water to check for leaks and to wash out any debris. A test can be developed to measure flow rate at the tank outlet without using a pump, just gravity. Then if the flow rate is not adequate, the passageways can be cleaned out or enlarged easily before the tank lid is installed.
Joe
 
I asked Vans tech staff about a bench test for fuel tanks, and what they did with their prefabricated tanks. Ken Scott replied:
?We don't have such a procedure. We know the tanks are built to
plan, so the flow from the tank will be adequate. There is no
procedure for the builder to test the tank before installation --
the PAP test is the real proof, because it tests the entire system,
rather than just the tank.?

Fair enough. My response was:
?I do admit to being surprised there is no bench test. I agree that the
definitive test is the one specified. In my case, I was certain that I
had double checked everything, before closing the tank. Some human
error has crept in there somewhere. I will let you knowq what I find
when I take the top off the tank?

So Dave, if I find the holes are clear when I take the top off the tank, I will have to work out my own bench test. I have a few ideas to base that test on. I would like to think that the engine would supply full power to the last drop of fuel at the most inconvenient moment. Such a moment might be a forced go-around. Food for thought.

Rod
 
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