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#11
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Fuel System Musings, comments encouraged
On Oct 20, 8:17*am, Copperhead wrote:
On Oct 20, 8:43*am, flybynightkarmarepair wrote: On Oct 19, 5:31*pm, Jerry Wass wrote: wrote: Have you looked at an Ercoupe fuel system? Two wing tanks in common (IIRC), pumping with a mech. pump, continuously to a small (5 gal.?) header tank. Header tank overflows back to a wing tank when it's full. Header tank feeds the engine via gravity flow. When the wing tanks are empty, the header tank float & wire gauge begins to drop. At the moment, you know exactly how much fuel is left. Works excellently and is very simple. If the pump fails, you still have the header tank. Rich S. I wonder about paralleling the pumps, instead of series.---If you run both for takeoff/landing you get double the pressure..(may flood engine) In series, if one stops up w/trash, there's no route around it. there's a very small leak back orifice to prevent Heat-expansion/flooding. *Jerry RE the Ercoupe example: I don't like header tanks from a crash safety standpoint, and another vent, and a return line souunds like MORE plumbing to me. *Plus, this is a VW conversion, and if I used a mechanical fuel pump it would be on the TOP of the engine, plus they are not sealed like aircraft mechanical fuel pumps are, so this is a no-go. Paralleling the pumps also introduces more fittings. *There are finger strainers in the fuel tanks to keep out the big crap, plus the fuel pumps only see fuel coming FROM the gascolator, so that failure mode - jamming due to FOD - seems to unlikely to plan around, IMHO. *I'm more concerned about an electrical fault, or the failure of the pump itself due to some internal fault, thus Series makes more sense to me. *And the pumps already have more than enough flow for full throttle. Thanks for the feedback, it keeps me thinking, the point of the exercise.- Hide quoted text - - Show quoted text - Ryan, I fully understand your dislike of a header tank, but have you considered a small fuel cell instead. Yes, I have. They are heavier than a tank without their features would be, and the "stock", relatively inexpensive ones don't have the vents and outlets arranged in a particularly convenient way, plus I don't really have the space for them. Also insofar as fuel pumps are concerned what are the possibilities regarding the use of an electric fuel pump? I guess you mean the failure possibilities. First, I want to clarify one key point. Only one pump operates at a time. The primary pump is also wired so that it ONLY operates when there is oil pressure. The secondary pump is wired seperately, perhaps even to a secondary battery, and is direct, and controlled by a big, boldly marked switch in the panel. My understanding is that these pumps are a solenoid driven by an oscillator driving a power transistor. The oscillator circuit board sometimes fails due to the usual reasons electronics fail. That's about the only failure mode worth mentioning. A few links, that are on my blog, but this discussion seems to have a life of it's own, so: http://www.flycorvair.com/601Sep2004.html Scroll down to see the Model System I'm riffing off of, and a discusion of why the mechanical pump on this installation was removed. There is a pretty complete description both of the original arrangement WITH a mechanical pump and the final version WITHOUT one. http://www2.cip1.com/PhotoGallery.as...27%2D025 %2DG Stock VW fuel pump. Push-on hose barbs, all sealed up, so no way to safety internals. Not particularly confidence inspiring. http://www.pilotfriend.com/experimental/build_17.htm Great article by Lyle Powell on fuel systems, originally published in Sport Aviation, and recommended by the Ellison people. Some of his conclusions can be VERY difficult to implement, such as no tanks without sumps. |
#12
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Fuel System Musings, comments encouraged
"Copperhead" wrote in message ... Also insofar as fuel pumps are concerned what are the possibilities regarding the use of an electric fuel pump? Electrical fuel pumps are very reliable things, electrical systems are unfortunately somewhat less reliable. The first is useless without the other. I had an alternator quietly die on a cross country in a rental airplane a year or two ago. I noticed the problem when my low voltage light winked on. Shortly after that, radios and other electrical gizmos started to die in spite of my power conservation efforts. Since it was a gravity feed Cessna, it sure was nice to not have to wonder when the engine was going to stop. If I can't have gravity feed, give me the combination of a mechanical pump backed up by electrical boost pumps. Vaughn |
#13
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Fuel System Musings, comments encouraged
wrote ... Two low-wing tanks require left/right control to avoid sucking air from an empty tank. No Both position, or you'll have trouble, especially if they have separate vents and the tank pressures aren't exactly equal. Great thread! I've always believed the above also, but the certified Beech Skipper has low wing tanks with individual (45 degree cut tube) venting and a single Both/Off selector. Each tank feeds thru an inline check valve into a common header tube below the cabin floor, then single outlet to On/Off valve then to gascolator. The design does try to keep common tank pressure via a tube interconnecting the top of each individual vent tube. But unequal feeding is one of the most common complaints. |
#14
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Fuel System Musings, comments encouraged
On Oct 20, 6:43*am, flybynightkarmarepair wrote:
RE the Ercoupe example: I don't like header tanks from a crash safety standpoint, and another vent, and a return line souunds like MORE plumbing to me. *Plus, this is a VW conversion, and if I used a mechanical fuel pump it would be on the TOP of the engine, plus they are not sealed like aircraft mechanical fuel pumps are, so this is a no-go. The Ercoupe design has worked safely and well for over fifty years. The lack of a header tank does not guarantee crash safety - nor does it's presence necessarily increase the danger. The one pump in that design can either be mechanical or electric, it matters not. There are no vents involved in the design. The float gauge holes in the caps provide adequate venting. If you are forced to park outside in the rain, you simply put a cap over the cap. There is one line connecting the two tanks which are located at the wing roots. If one wing is parked, or flown, a little low it makes no difference. There is a tee in the line that leads to the inlet of the pump. The outlet of the pump goes to the header tank. The overflow of the header tank goes to one wing tank. IMHO, you are making a mountain out of a molehill. This ain't rocket science and you aren't building a space shuttle. Gravity flow to a carburated engine is the safest and most reliable fuel feed possible, until you are attacked by Feherenghi using anti-grav phasers. Listen carefully - putting any pump, electric or mechanical, in the fuel line to the carb will restrict or prevent fuel flow in the event of pump failure. Be safe - use gravity. My 2 cents worth. Rich S. |
#15
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Fuel System Musings, comments encouraged
On Oct 20, 5:45*pm, wrote:
On Oct 20, 6:43*am, flybynightkarmarepair wrote: RE the Ercoupe example: I don't like header tanks from a crash safety standpoint, and another vent, and a return line souunds like MORE plumbing to me. *Plus, this is a VW conversion, and if I used a mechanical fuel pump it would be on the TOP of the engine, plus they are not sealed like aircraft mechanical fuel pumps are, so this is a no-go. The Ercoupe design has worked safely and well for over fifty years. The lack of a header tank does not guarantee crash safety - nor does it's presence necessarily increase the danger. I've read an accident report where 2 POB died in an otherwise survivable Ercoupe crash due to the fuel tank splitting and soaking them in gas. Ignition happened, and they became human wicks. Others have noted ways of mitigating this via fuel cells, and while you make a pretty good argument, I'm not convinced. Fred Weick, the designer of the Ercoupe (although perhaps not this part of it...) also had significant input to the most numerous low wing production design, the Piper Cherokee, and it DOES NOT use a header tank. Strictly circumstantial, to be sure, but draw your own conclusions. The one pump in that design can either be mechanical or electric, it matters not. There are no vents involved in the design. The float gauge holes in the caps provide adequate venting. If you are forced to park outside in the rain, you simply put a cap over the cap. I don't like this at all. I don't like relying on the nut between the headphones to keep me safe from a significant hazard (water in the gas) in an airplane with as small a fuel capacity as the one I'm building. I want a screened vent of at least 3/8 in. dia. that exits the BOTTOM of the wing, but vents the TOP of the tank. There is one line connecting the two tanks which are located at the wing roots. If one wing is parked, or flown, a little low it makes no difference. There is a tee in the line that leads to the inlet of the pump. I recall a LONG argument on this forum (or maybe it was on a Zenith 601 forum - LOTS of that design fly with a system VERY much like you're advocating) about unporting a tank at low fuel levels and sucking air, that I don't want to rehash, but I'll just note that I'm firmly in the LEFT/RIGHT/OFF fuel selector camp for low wing planes with wing tanks. Gravity flow to a carburated engine is the safest and most reliable fuel feed possible, I agree, I agree, to the extent that I'd rather build a high wing airplane if plans for one that meets my mission requirements were available when I was first looking. The Aerosport Quail is the only HIGH wing all metal VW powered single place homebuilt aircraft I'm aware of, and it's not clear you can actually get plans, although a source iin Oregon is rumored. For a low wing plane with fuel in the wings, which I'm well convinced is safer than fuel in the fuselage, pumps are a necessary evil, and since they can fail, I'll take two please. William Wynne has done more engine installations by far than I have, and he's going this way; so am I. ================================================== ============ Anybody have anything to say about the fittings? That was my initial interest. Should I ditch the pumps I have and just bite the bullet and go with AN fittings per the exemplar? Do all those unions make sense?(heavy little suckers! Think of a ball of solid brass the diameter of a quarter for the 1/8" NPT size, and the diameter of a Susan B. Anthony dollar for the 1/4" NPT size, and they lighten the wallet to the tune of $13-20 per at the same time they increase empty weight) - given that NPT fittings don't seal anyway metal-to-metal, and you can "clock" them with SOME degree of freedom as long as they're at least slightly more than finger tight, relying on Loctite Pipe Sealant to keep them leak free? |
#16
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Fuel System Musings, comments encouraged
Survivable Ercoupe crashes where the occupant(s) burned to death (NTSB
Identifiers, no more static URLs for crash reports): SEA00LA136 NYC98FA107 CHI96FA034 Not clear this one was survivable, absent the fire MKC82FCG17 Pilot lived a few weeks, died of burns. NYC85FA222 "water and sludge throughout the fuel system" This IS not to conclude the Ercoupe is unsafe. Reading accident reports is always instructive and sobering, but it would take more analysis than I have time or energy for on a school night to reach solid conclusions. Drunks seem to be drawn to Ercoupes is one extremely shaky conclusion drawn from my reading of 50 fatal accidents since 1962, out of 482 accidents in the same time period. |
#17
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Fuel System Musings, comments encouraged
On Oct 20, 5:45*pm, wrote:
This ain't rocket science and you aren't building a space shuttle. Gravity flow to a carburated engine is the safest and most reliable fuel feed possible, until you are attacked by Feherenghi using anti-grav phasers. ------------------------------------------------------------------- Agree. As with most engines, the VW fuel pump is driven at cam speed but it is NOT driven off the cam shaft, a significant difference with regard to its mechanical durability. It is driven off the crankshaft via its own gear-train. The pump is a two-chamber type in which the mechanical input serves only to extend a compression spring. The spring is then free to retract, the amount of retraction determined by the quantity of fuel drawing into the lower chamber during the extension cycle. In effect, the pumping action is performed by the spring rather than by the engine. Mechanically, the pump is isolated from the engine by a push-rod that is actuated by a pinion gear. The flexible diaphragm that makes up the floor of the lower pumping chamber has a useful service life in excess of 1000 hours, the push-rod slightly more than that. Output pressure is typically between 8 and 16 ounces, depending on the spring. Any flow-control valve, such as the ball-valve in the stock Solex carburetor, which typically has a pressure between 8 ounces and two pounds is sufficient to shut off delivery. However, any fracture- failure in the delivery circuit will cause the pump to spray gasoline in all directions. Wear in the push-rod is reflected by its length which may be meausred by removing the pump (two studs). Volkswagen provided three pump push-rods of different lengths. Failure of the flexible diaphragm offers a bit of visual warning in the seepage of fuel at the parting line of the pump. This is only valid for the early model, rebuildable fuel pumps, which is what I prefer to use on my VEHICLES. In an airplane I'm more comfortable with a gravity-fed system, even if that includes a header tank in the fuselage. The push-rod actuates a bell-crank in the base of the pump body, which is a white-metal casting. The bell-crank's pivot bears on the casting without provision of bushings or bearings. In a high-time pump it is common to see the bearing-holes worn to an oval. The pivot itself is held in place by a cotter key (early) or snap-ring (late) which is also subject to wear. The pallet of the bell crank calls for periodic lubrication with axle grease, maintenance it seldom sees when the vehicle is maintained by the typical owner. (I believe the service interval was every 12,000 miles or annually but I'd have to check the Factory Service Manual. I do it about once a year and have never had a mechanical failure due to lack of lubrication. I have had the pivot and the fastener break.) In my opinion, each builder is responsible for answering such questions for themselves. I can offer an opinion and may provide an example to support it but ultimately, that's the limit of my interest and responsibility; I can always refuse to ride in the thing. (And have, on several occasions.) Personally, I don't care for pumps and would eliminate them if possible, especially if they are engine- driven. The next best choice would be an electrically driven pump (or any number of them) located where a failure would not douse the engine with gasoline. Even so, at best all this would do is reduce the risk of an IN-FLIGHT fire. Following a crash or bad landing, there is usually some amount of fuel at the scene regardless of where the fuel tank is located and no matter how the fuel is delivered to the engine. -Bob Hoover |
#18
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Fuel System Musings, comments encouraged
flybynightkarmarepair wrote:
My blog has become a way for me to think out loud about my project. Putting things down in a narrative helps me crystallize my thoughts, and give me something to refer back to when the time comes to move from thinking to doing. Anyway, I've been THINKING about my fuel system, and have put down my thoughts, I encourage those with more experience to comment either here or via the blog. http://users.lmi.net/~ryoung/2008/10...tings-and.html Lots of good stuff in the links, especially the Sacramento Sky Ranch ones. I'm in what I hope are the closing stages of a build of a 601XL with a William Wynne built Corvair engine. I have the exact fuel system shown in the photo. Go with the AN stuff. You can get them from Summit Racing and they don't cost that much. For those that suggest a pumping into a header tank. That is exactly what the older 601HD and HDS did. There just isn't room for it in the 601XL if you have any kind of radios behind the panel. |
#19
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Fuel System Musings, comments encouraged
Vaughn Simon wrote:
"Copperhead" wrote in message ... Also insofar as fuel pumps are concerned what are the possibilities regarding the use of an electric fuel pump? Electrical fuel pumps are very reliable things, electrical systems are unfortunately somewhat less reliable. The first is useless without the other. I had an alternator quietly die on a cross country in a rental airplane a year or two ago. I noticed the problem when my low voltage light winked on. Shortly after that, radios and other electrical gizmos started to die in spite of my power conservation efforts. Since it was a gravity feed Cessna, it sure was nice to not have to wonder when the engine was going to stop. If I can't have gravity feed, give me the combination of a mechanical pump backed up by electrical boost pumps. Vaughn That's why we ZenVair people use real good batteries. |
#20
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Fuel System Musings, comments encouraged
On Oct 20, 11:55 pm, flybynightkarmarepair wrote:
Anybody have anything to say about the fittings? That was my initial interest. Should I ditch the pumps I have and just bite the bullet and go with AN fittings per the exemplar? Do all those unions make sense?(heavy little suckers! Think of a ball of solid brass the diameter of a quarter for the 1/8" NPT size, and the diameter of a Susan B. Anthony dollar for the 1/4" NPT size, and they lighten the wallet to the tune of $13-20 per at the same time they increase empty weight) - given that NPT fittings don't seal anyway metal-to-metal, and you can "clock" them with SOME degree of freedom as long as they're at least slightly more than finger tight, relying on Loctite Pipe Sealant to keep them leak free? AN fittings are available in aluminum. They weigh almost nothing. Look for the blue fittings. But watch when you thjread an aluminum pipe thread into another aluminum fitting or tank boss; they'll grab and gall and tear the threads apart if you don't use some decent sealant. We use Seal-Lube, Permatex Aviation Form-A-Gasket, sometimes Loctite hydraulic sealant. Stay away from teflon tape. Dan |
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