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fuel flow trouble http://t18.net/thorpforum/viewtopic.php?f=8&t=6810 |
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Author: | BobMoe [ Wed Jul 18, 2012 1:49 am ] |
Post subject: | Re: fuel flow trouble |
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 |
Author: | SHIPCHIEF [ Wed Jul 18, 2012 11:07 pm ] |
Post subject: | Re: fuel flow trouble |
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? |
Author: | Rich Brazell [ Wed Jul 18, 2012 11:19 pm ] |
Post subject: | Re: fuel flow trouble |
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 |
Author: | BobMoe [ Thu Jul 19, 2012 6:08 am ] | ||
Post subject: | Re: fuel flow trouble | ||
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
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Author: | Bill Williams [ Thu Jul 19, 2012 6:43 am ] |
Post subject: | Re: fuel flow trouble |
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. |
Author: | James Grahn [ Fri Jul 20, 2012 7:36 pm ] |
Post subject: | Re: fuel flow trouble |
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. |
Author: | Rich Brazell [ Fri Jul 20, 2012 10:23 pm ] |
Post subject: | Re: fuel flow trouble |
Now you're starting to sound like an old retired transport pilot ! RB |
Author: | James Grahn [ Sat Jul 21, 2012 7:29 pm ] |
Post subject: | Re: fuel flow trouble |
I wish I was retired! I've got too much stuff to do. This working thing keeps getting in the way |
Author: | Rich Brazell [ Sun Jul 22, 2012 9:24 pm ] |
Post subject: | Re: fuel flow trouble |
If you think when you're retired you will have more time for other "projects" think again partner ! RB |
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