Thorp Air Command - T18.net

Well, I don't think we have discussed this aspect of the fuel system. Let's try the worst case senerio. You have a gravity feed system and a mud dauber plugs your vent line. I'm pretty sure your engine will STOP. Will it stop with a fuel pump? Probably. Will this happen if a spyder builds a nest in the line. I don't know. How about when you are low on fuel? Do you still get 16 gallon fuel flow? There are a lot of variables to consider.

Since Don was kind enough to give the details on the fuel tank pictured, what if he fills the tank and starts a long left turning climb. Will his engine sputter or quit? Probably not if he has a fuel pump. What if another builder copies his design with a gravity system?

I don't think the fuel system, per plans, is as safe as it could be. One well placed bug shouldn't cause a forced landing.

If everyone is satisfied that the fuel system is fine. Turn off those fuel pumps and go let's fly. You know they will fail eventually. If there is a way to improve it, I'd like to hear more.

One thing I've considered is to drill a vent hole in the fuel cap and attach a plastic/mylar flap to the bottom. Kind of a check valve! You have an alternate vent, plus you still have head pressure.

Thanks for humoring me,

Bob MO

Her are a couple of pictures of vents that work: Veiw is looking aft at the floor bottom.



The one on the top is Bill Williams'; bottom is Bob Highley's. The tank vent is on the right looking at it. The left aft facing tube is the scupper drain from the fuel cap overflow area. Bill used 1/4" tube; I used 3/8".

_________________
Bob Highley
N711SH
SN 835
KLAL

N883FF has similar fuel tank vent line as Bob's pictures show, although the 3/8" line sticks down over 3". It had a very slight forward bend and the forward facing 'salami' cut.
Before departing Eastern WA, yesterday, I gave it slightly more forward bend. Although we flew back 1,000 ft higher, we always had 1 psi of fuel pressure with the boost pump running. Zero without, but still full power and the ability to run rich to verify fuel availability.
I can't be certain that this was effected by the bending of the vent tube, and I also wonder?
Does a leaking fuel cap create a suction on the tank at cruise speeds? Is there a vacuum over the fuel filler door?

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

What kind of fuel cap do you have ? Flush mounted ? Now begining to wonder/ponder how old are your fuel lines FWD ? Old age may cause them to fail ? Just thinking out loud !


RB

Bob,

That is Not at all the way I have visualized the vent. Like I said, I have yet to find the line in the prints. I would think your arrangement with the tube straight down would create a vacuum. The ones I remember seeing were swept forward into the airstream, like a pitot tube. I remember some were flared to increase head presure. I did find one pic where the tube appears as I imagined.

Bob MO

N711SH, N30WW,N818LM all three had the vents per Bob's picture. These planes have/had a total of 5500 hours of flying and no fuel problems. The RV's use a AN4 fitting cut at a 45 angle facing forward. One of the differences that our fuel cap is hidden under a hindged door and not the flush filler cap that some use. In checking the difference between Bob's and my vent lines is he used 3/8 and I used 1/4. I think Les used 1/4 also. I would check the finger strainer in the tank, fuels lines(proper size), fuel shut off valve, gascolator( aircraft grade and not a lawn mower) and the fuel lines to the engine, fuel pump itself as they do go bad. I built my fuel system copying a 180 Piper using a Piper select valve, same size fuel lines and a Piper gascolator. I would suspect Vapor lock for the low FP as a pump probably will pump 1 pound of air pressure without gas. Use a direct read mechanical FP gauge.
Another thought.. car gas or 100LL, even my Model A Ford will vapor lock with auto gas.

I was the one who posted about fuel siphoning years ago. My vent tube is welded about 3/4 inch into the top of the tank. Every time the tank is filled to the neck, it siphons fuel. I had the engine quit on takeoff at least a half a dozen times. My vent is per plans. It is located inboard of the right gear. I tried extending the tube into what I thought would be more airflow. No change. I tried drilling (venting) the cap. That was a bad idea. Almost choaked on fuel smell and it did not prevent the engine from quitting. I finally installed a ticker pump in line in between the gas collator and the carb. Worked like a charm. Have not had fuel flow problems since. It is wired to the master. I don't even have to think about it. It just works - which is perfect for an old retired fighter pilot.

Now you're starting to sound like an old retired transport pilot !

RB

I wish I was retired! I've got too much stuff to do. This working thing keeps getting in the way

If you think when you're retired you will have more time for other "projects" think again partner !

RB