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MPPalmer(at)aol.com
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 Posted: Fri Jul 11, 2008 3:08 pm Post subject: Vapor pressure and the case of the missing fuel? |
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After blowing the vents out good and hard two days ago and not seeing anything more to fix, I'm starting to button the plane back up. (I'll plan on reverse flushing the transducer next month, after my eye surgery, if this problem still isn't solved. But I'm having a hard time believing the Flowscan is off. That would imply we were burning 7.8 GPH all of a sudden when we've been running in the high 6's, low 7's for years now. I suppose if the cam were worn....but there wasn't metal in the oil filter last time.) I put a new piece of aluminum tubing on the center section fuel vent, extending the assembly a few more inches into the airstream to pressurize the tank better. The old one was down to less than 2 inches from the skin. An interesting observation today.
As you may recall, I topped the tanks on Wednesday. Did it myself. Just for grins, I checked the tanks today. (Friday.) The main tank is down about 3 to 4 gallons! I'm pretty sure I didn't blow 3 to 4 gallons out the vents when I was testing. Where is the fuel going? (Surveillance tape shows no one in the hangar stealing fuel.)
I tightened one fuel cap some more, although I thought it was fairly tight already. I can't smell any fumes from that cap.
The header tank is staying right where it's been for the week. It's only the main that's apparently gone down.
I left a sample cup full of fuel out to see if it will all evaporate by tomorrow. I'll plan to check the level tomorrow in the main and if it hasn't changed, I'll plan to re-top off the tank again, to see if the level really is changing. If it does, then, as Sherlock Holmes would say, it must be a leak in the wing somewhere, no matter how improbable.
[Interestingly, we had a big thunderstorm blow through here last night. I don't know what the baro pressure did for the evening, but I've noticed that pump bottles of epoxy (The West System) routinely dribble epoxy when not in use. I think it's baro pressure changes and a one way valve moving fluid. Exhaling, if you will. I wonder if I have a similar situation with the wing, although I don't know what the one way valve would be. (I don't hear the B rib check valves slap anymore, but there were bypass holes in the ribs anyway.)
One would think I would smell fuel in the cockpit if it were leaking past the spar, since there are holes from the cockpit to each bay in the wing through the ribs for the aileron push tube. But then again, the fuel doesn't seem as stinky as it sometimes is, and maybe I'm not noticing it. (This would serve me right. When I do the annual, I usually take the flashlight and mirror and peer all around the inside of the bays, looking for fuel stains. I didn't do that this time.) When I had the vent tubes disconnected in the cockpit this week, I didn't notice the usual overpowering smell of avgas like I usually do when there's a bona fide leak.
Years ago I tore the airplane apart for a few months, trying to find what smelled like a dead mouse in the wing. Turned out to simply be a highly sulfured batch of AvGas that season! This year, I don't notice fuel vapors at all. Now I'm wishing they'd put the sulfur back!
I'm still looking for that "Eureka!" moment, when I suddenly understand what the problem has been all along regarding this case of the missing fuel. So far, I've not had that. And that bothers me.
Maybe after checking the tank level again tomorrow, I'll know more.
Mike Palmer <><
**************
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grosseair(at)comcast.net
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| Posted: Sat Jul 12, 2008 9:03 am Post subject: Vapor pressure and the case of the missing fuel? |
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When I pressure tested my wing I had leaks that I absolutely could not
find using any of the usual techniques. I finally bought a combustible
gas detector to sniff out the leaks. My tank was dry and I didn't want
to put fuel in it so I pressurized the tank with propane then used the
detector to find the leaks. (I had two in very inaccessible places and
had to cut holes in the wing to fix them, but that's another story.) The
instrument I used was a TIF8800A Combustible Gas Detector that I bought
on ebay for $100. (Mine was new, but used ones are less. Retail is
$200.) It has a probe on the end of a flexible wand that's about 18
inches long so you can get it in some pretty tight spaces. The
sensitivity is adjustable, and it works very well. I was able to
pinpoint the source of my leaks to within an inch or so at pressures
less than 1psi. I'm sure if you have a tank leak you will be able to
find it with this instrument.
Here's a link to one that's on sale on ebay right now:
http://cgi.ebay.com/TIF-TIF8800A-COMBUSTIBLE-GAS-DETECTOR_W0QQitemZ150268327575QQihZ005QQcategoryZ50965QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
John Grosse
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