Pressurising tanks in lieu of 2nd pump

G'day folks.

Was lost in thought last week routing the pitot/static lines for the Dynon and it occurred to me that the Dynon pitot has the second port to provide a reference source of pitot pressure at high nose attitudes might prove useful for "other" things as well.

That night I was browsing a Kiwi Cri-Cri builder's log and saw that he's using a very light pressurisation of the fuel tank to deliver fuel to his tiny twin engines. And also pondered if building a Cri-Cri was the cheapest way to get 500 Multi...

Now, I've got an H2AD donk to hang on the front of my -9A and won't be running the engine driven fuel pump and it occurred to me that if I was to install a second pitot tube on the left wing and feed the ram air pressure to the vent lines of the tanks, the resulting pressurisation of the tanks would ensure positive fuel flow to the Rotec TBI at any speed above the stall.

Another builder of a LongEz has thoughtfully published a Pitot pressure vs KIAS table and that shows you won't reach so much as 1PSI until 205KIAS, but even at 50 knots you've got above the 0.05PSI minimum required for the Rotec TBI.

It seems this route would alleviate the 2nd pump and if you wanted true redundancy the 2nd battery for said pump as well.

What are your thoughts?

I can see where it would be a valid backup most of the time! I have picked up a bug in my pitot in flight, and I believe this would not only stop pressurization, but stop venting, killing the fuel flow altogether, even with a working fuel pump. I also doubt that the pressure on climb out (slowest speed, highest head, tank to engine) would be sufficient to push fuel uphill. I would expect there are some pretty good reasons that more RVers aren't doing just what you suggest. The fuel system is the biggest culprit in early flight engine failures, and certainly one of the most important systems on the airplane. Personally, I wouldn't go there. Pumps aren't as expensive as airplanes.

Bob

videobobk said:

I can see where it would be a valid backup most of the time! I have picked up a bug in my pitot in flight, and I believe this would not only stop pressurization, but stop venting, killing the fuel flow altogether, even with a working fuel pump.

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That's something I'd considered, and figured the Dynon pitot, with it's Pitot and AoA ports, would nullify that particular failure mode. If you've tagged something big enough to block both your pitot and AoA ports, well, it's just not your day. Though I do remember an article in one ofthe RVators that said a Facet pump will pull enough of a vacuum to actually collapse a tank in the event of a blocked vent line.

videobobk said:

I also doubt that the pressure on climb out (slowest speed, highest head, tank to engine) would be sufficient to push fuel uphill. I would expect there are some pretty good reasons that more RVers aren't doing just what you suggest.

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Odds are there are valid reasons there, but that could also be applied to those with Sube engines, or O-360's in a -9,

Using old mate's table, and unless I've got the numbers asre about (and there's every chance of that), with a specific gravity of 0.71, Avgas will give a head of 9 inches at .23 PSI at 100 KIAS for example. If the fuel pressure drops to the point that the engine starts to run lean the're always the Facet.

I'm not convinced the odds of a fuel pump failure occuring at a low enough altitude and speed are significant enough to not continue to investigate the possibility of implementing this into my -9. Time will tell of course.

You may want to measure the pressure drop in your fuel system. Between the lines, selector, fittings, bypass of the electric, and gascolator, the drop may be more than 1PSI. In fact it is very likely that the pressure drop is more than 1 PSI at the required flow rate for cruise power ~7-8 GPH. Otherwise it seems like an interesting premise.

most vent tubes are angled into the wind to create a slight positive pressure

How about running the vacuum pump outlet into the tank? Blow it up like an old Airstream trailer water tank.

aerhed said:

How about running the vacuum pump outlet into the tank? Blow it up like an old Airstream trailer water tank.

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And risk getting fuel vapor back up the line.

Any discussion of positive pressure on a fuel tank has to address where small leaks go. Atmospheric or slightly negative pressure doesn't promote the migration of fuel liquid and vapor into in unwetted parts of the wing as a pressurized tank could.

Not enough pressure

I don't think that here's enough pitot pressure to get the fuel to the engine. Stagnation pressure at 60 knots Is about 2 inches water gauge, so at 120 knots it'd be 8 inches, or enough to lift a column of fuel about 10 inches. If your tanks were full it might be enough to get fuel to your engine, but not to do anything with it when you get there, and at lower speeds or with empty tanks you' be in trouble.

Dave