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#31
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"Rich S." wrote
I went back and re-read the entire thread and, you're right, I failed to identify my airplane and to describe the fuel system configuration. [Long, careful, and complete description deleted] Thank you. As I suspected, you have a system that I always knew COULD suffer from vapor lock. Insufficient head pressure for reliable gravity feed, and a pump inline, mounted on the engine and thus sucking fuel. There's nothing inherently wrong with that - every design is a compromise - but there is an awful lot of FUD associated with the autogas vapor lock issue. I know people who have had vapor lock problems. I also know lots of people who use autogas in high wing Cessnas and high wing Pipers, and NONE of those people have EVER experienced vapor lock - even when running autogas in high temperatures while crossing the Rockies, as I have. Clearly this is a design issue. Emeraude, powered with a Lycoming O-320-E3D and having a mechanical fuel pump on the engine. There is a Bendix electric fuel pump mounted on the engine side of the firewall, downstream of a gascolator. I think these are the key points. Your pumps are in the engine compartment, past the restrictions. Thus they are sucking fuel, not pushing it, and are creating a point in your fuel system where the pressure is lower than atmospheric ambient. There is a fuel pressure gauge teed in at the carburetor. Given the system, I think that's a fine idea. Generally, you do not see fuel pressure gauges on gravity-fed carbureted systems. Generally you DO see them on pump-fed systems. It's worth considering WHY this is. There are two 20 gallon fuel tanks, one located ahead of the instrument panel and the other behind the seat. The nose tank has about 15" of head above the carb inlet when the aircraft is level. There is a tank selector valve mounted on the left kick panel. The rear tank is almost level with the carb. The tank vents exit the bottom of the fuselage at 90° to the slipstream and received no pressure "boost" from it. I have no idea how much the restrictions in the fuel line (filters, check valves in the pumps, fittings, etc.) affect the head pressure or fuel flow. A fuel filter (if it's an actual filter and not a screen) will cost you about about a foor or two of head pressure. Not sure about the check valves. The fittings are basically irrelevant. Just FYI - these things can be calculated. There are published data. A sophmore-level textbook of fluid mechanics for chemical or mechanical engineers will contain all this information. On the day I experienced vapor lock and loss of power, the selector valve was set on the nose tank and it was at least 2/3 full, adding another 10" to the head distance. We were at 9,500' and the OAT was 85°. The electric pump was turned off in cruise and the fuel pressure gauge had been reading just above red line at ~2 lbs. Just curious - is this normal for your installation? It seems very low to me. In other words - it sounds like either your fuel filter was clogged, or gas bubbles had already started to form in your fuel system, causing pressure drop. I turned on the electric pump and switched to the rear tank. Nothing. I find this unsurprising - unless the vapor lock was between the electric pump and the mechanical one, there would be no effect from 'sucking harder' when your problem was vaporization of the fuel in the line. Since you have carefully insulated the fuel lines inside teh engine compartment, there is no reason to believe the gas bubbles would form there. I admit that I had forgotten to try the manual prime pump. I doubt strongly that it would have helped. I'm sorry to be so long-winded about this, but you asked for details. And I'm glad you provided them. Especially details of the fuel system. Feel free to criticize my plane or my actions, but try to be kind. Actually, I think all the actions you took were very reasonable. My experience and ratings are similar to yours (though I racked up my 1500 hours in a little over a decade, and some of them in gliders and light twins) and I doubt I would have done anything differently. As for your plane - I will make this comment. The fuel system you have is very similar to what one might expect to see on older certified light aircraft, with one significant difference. An older aircraft is most likely not going to have a real fuel filter. Instead, it's likely to have a gascolator with a settling bowl and a fuel screen - what's often referred to as an elephant catcher. Adding a REAL fuel filter does provide additional protection for the engine, but this protection is not free. If there's one thing I've learned from my years as an engineer, it's that everything is a tradeoff. That fuel filter protects your engine from dirt and sediment, but it is a major source of pressure drop. Worse, the more necessary that fuel filter is (meaning the more contamination in the fuel) the more pressure drop it causes. I suspect that if you had a simple wire screen, you would not have had a problem. But that's 20/20 hindsight. I'm just letting you know what may happen if you persist in the belief that your airplane is *immune* to vapor lock. Actually, I no longer own that TriPacer. But I do believe that a plane with a properly designed gravity fed fuel system is going to be immune to vapor lock. I believe this based on the calculations (posted elsewhere in this thread) and based on the field experience of those who have been doing it. I also believe that any plane with a suction-pump fuel system is at risk for vapor lock. That doesn't mean we should not build such planes (everything is a tradeoff!) but that we should be aware of the issues. To me, this is just one more issue where the design of the systems makes a difference in what constitutes acceptable operation. Trust me in this, though. You don't ever want to hear the person you love screaming and sobbing in fear. Never. I'll take your word for it. Michael |
#32
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I have been mulling over several different questions regarding fuel system
plumbing in the last year or so - even up to the last resort of an extra one gallon (or so) tank with a mechanical pump piped to the intake manifold after the carb. Was a modification that was made to some aircraft in the hostile hills of PNG at one time and known as the missionary fuel system. A pump or shutoff valve to supply fuel equal to about 80% power into a jet in the inlet manifold below the carby, so that if the carby jet blocked there was a fuel option that might get one home..... Would require the carby butterfly to be opened up to match and if the jet was partly blocked, the fuel to the carby would be shut off manually to prevent flooding the motor. A small reserve tank may also be a good idea to go with it. Considering same as a backup to my aircraft when I build in a couple of years. Not much weight penalty and the nice warm fuzzy feeling of an emergency fuel backup system...... Hope this helps, Peter |
#33
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"Bushy" wrote in message
... A small reserve tank may also be a good idea to go with it. Considering same as a backup to my aircraft when I build in a couple of years. Not much weight penalty and the nice warm fuzzy feeling of an emergency fuel backup system...... Hope this helps, Peter Peter....... I remember as a teenager, sitting on the fender of a hot rod, trying to dribble gas into the carb to get us home after a fuel pump went West. Rich S. |
#34
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Barnyard BOb -- wrote:
I remember as a teenager, sitting on the fender of a hot rod, trying to dribble gas into the carb to get us home after a fuel pump went West. Rich S. ++++++++++++++++++++++++++++++++++++++++++++ Rich, I once had a young cousin who did that. He was out of the hospital in less than 6 months.... after major skin grafting and agonizing therapy. Seems the engine backfired and the gas stream then burned back to the jug he was holding over the carburetor. Youngsters destined to be firemen are a lucky lot? g Barnyard BOb - that which does not kill us makes us stronger? He should have put the gas in a Windex bottle and stood back to squirt it into the carb. I watched a guy show and engine to a potential buyer while the engine sat in the middle of his yard using this technique. Now, how do you hunker over the cowling to spray that bottle into the carb while on downwind? Maybe use a garden sprayer type system? -- ----Because I can---- http://www.ernest.isa-geek.org/ ------------------------ |
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