If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Check the cutaway picture and info at http://www.ppavionics.com/FacSolidState.htm Notice that there is only one model that has a positive shutoff and a built in check valve to prevent backflow. The others depend on the valve built into the inlet. Ordinarily I would say that the inlet valve would prevent backflow but I have seen two cases where it did not. One resulted in a fuel starvation crash. Apparently the inlet valve can hang open too easily to bet your life on it. I recommend that a seperate check valve be used for safety on the models that do not have one built in. I also would not consider an application that depends on reverse flow through the pump. Bob In article , says... Are you really sure fuel will flow backwards through the Facet pump? I may be wrong as it has been a long time since I worked with one of the se pumps, but I remember that the design did not allow flow in the opposi te direction. I have seen a number of fuel setups that used two pumps in parallel. With one pump "on" the other pump, the pump that was not energized, did not allow any back flow. There was no need for a check valve in the system as the pump acted as its own. Greg Reid wrote: I understand that the simple Facet pumps have no check-valve so this should be possible. But is it harmful to such pumps? Greg |
#12
|
|||
|
|||
I have two of the 40108 pumps listed at that web site. I just hooked
them up opposed to each other. Neither could force gasoline backwards through the other. They may not be leak free when exposed to reverse pressure...the design may not be a full check valve design, but they certainly do not flow backwards enough to allow filling a tank. sleepy6 wrote: Check the cutaway picture and info at http://www.ppavionics.com/FacSolidState.htm Notice that there is only one model that has a positive shutoff and a built in check valve to prevent backflow. The others depend on the valve built into the inlet. Ordinarily I would say that the inlet valve would prevent backflow but I have seen two cases where it did not. One resulted in a fuel starvation crash. Apparently the inlet valve can hang open too easily to bet your life on it. I recommend that a seperate check valve be used for safety on the models that do not have one built in. I also would not consider an application that depends on reverse flow through the pump. Bob In article , says... Are you really sure fuel will flow backwards through the Facet pump? I may be wrong as it has been a long time since I worked with one of the se pumps, but I remember that the design did not allow flow in the opposi te direction. I have seen a number of fuel setups that used two pumps in parallel. With one pump "on" the other pump, the pump that was not energized, did not allow any back flow. There was no need for a check valve in the system as the pump acted as its own. Greg Reid wrote: I understand that the simple Facet pumps have no check-valve so this should be possible. But is it harmful to such pumps? Greg -- Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL Homebuilt Aircraft Newsletter" | Publishing interesting material| | on all aspects of alternative | | engines and homebuilt aircraft.| *------------------------------**----* \(-o-)/ AIRCRAFT PROJECTS CO. \___/ Manufacturing parts & pieces / \ for homebuilt aircraft, 0 0 TIG welding While trying to find the time to finish mine. |
#13
|
|||
|
|||
How about a reversible positive displacement pump that won't allow fuel to
seep past it while stopped? A gear pump will do that if closely fitted. That way you could pump fuel in either direction until you were trimmed and just shut it off. Bill Daniels "Greg Reid" wrote in message om... I've installed a 12-gallon fuel cell aux tank in the tailcone of my 4-place conventional low-wing plane -- intended for go-fast trimming when flying solo more than for its extra fuel capacity. As you know, a typical 4-place is terribly nose-heavy with only front-seat passengers and no baggage. The tank would be emptied if flying with a full load of passengers and baggage. Now I'm considering how to plumb it. My engine is gasoline, not fuel-injected. The simplest approach would be to vent the tank it to the outside, and run a single 3/8 hard tube between my left main and the aux tank. The aux tank is positioned towards the ceiling of the turtledeck so there's a fair amount of gravity-feed available for it in normal flying attitude to drain back into that same tank. I'd have a single electric pump to pump from the main uphill to the aux tank, with a shut-off valve in the line to keep it there. Opening the valve would let it gravity-feed back into the main tank ... slowly. That is, if it's OK to allow it to gravity feed "backwards" through the fuel pump. (I need to consider the possibility of the main tank gravity-feeding back into the aux tank in a prolonged steep climb. I'll need to remember to shut off the valve whenever a fuel transfer isn't wanted.) I'm wondering about installing a second electric pump in series at the aux tank end, pointing back towards the main tank, to considerably speed up the draining. Only one pump would be run at a time of course -- either to fill or to drain the aux tank. But when either pump was on, it would be pumping fuel "backwards" through the other one. I understand that the simple Facet pumps have no check-valve so this should be possible. But is it harmful to such pumps? I've got a more elegant approach (a little complicated to draw here) that would use a single pump and a double-stacked selector valve as used on a fuel-injected engine. The selector valve makes it so that in the "fill" position, the pump would pump fuel from the main to the aux, and in the "drain" position, that same pump would pump fuel from the aux to the main. This is the slick set-up, but is considerably more complex/expensive/heavy to implement. The fourth approach would use separate fill and return lines, with separate pumps, check valves, and shut-off valves. This would avoid venting overboard. But it's even more complex, and I hope to not have to go there. Surely others have solved this simple plumbing challenge before me. I welcome your ideas. :-) Greg |
#14
|
|||
|
|||
"Greg Reid" wrote in message om... I've installed a 12-gallon fuel cell aux tank in the tailcone of my 4-place conventional low-wing plane -- intended for go-fast trimming when flying solo more than for its extra fuel capacity. As you know, a typical 4-place is terribly nose-heavy with only front-seat passengers and no baggage. The tank would be emptied if flying with a full load of passengers and baggage. Greg, Thats a bunch of weight and complexity to add for trim fiddling. I can see the benefits but think that a fixed/removable ballast bar bolted to somewhere around the rudder post weighs less, is less complex, gets you 90% of what you are looking to achieve and has no failure modes that would be critical to flight. Make a bar for solo, one for 2 up. Some certificated gliders and helicopters do it this way. But I'm afraid it might be too simple.......... Scott |
#15
|
|||
|
|||
Hey guys, thanks for all of the commentary/concern ... and sorry I've
been remiss in responding until now. Thanks in particular to Bruce for going the extra mile and a half in actually hooking up and testing to see if you can pump backwards thru a typical Facet pump (nope). A simple "gear motor" mentioned elsewhere is what I need, I suppose, so that the single pump could pump both ways (with polarity reversed). I could add an on/off valve (manual or electric solenoid) to complete the single-pump-and-single-tube hookup. Such gear motors are typically found in cheap hardware-store siphoning pumps that let you drain or fill your auto crankcase via the dipstick tube (turned by an electric hand drill). I wonder if I can find one of aircraft quality somewhere. I did find the ppavionics site mentioned. That's the link I mentioned having found (and then temporarily misplaced) earlier. They've got a Facet pump model that contains internal valving to prevent flow in either direction when off. So they could be used for a two-line and two-pump setup for "fill" and "drain". I found some other stuff on their site that seems "interesting", and wrote them (no response as yet). (And no, I have no financial or other interest.) I just want to assure you all that even when the aux tank is full, I'll still be within rear CG limits ... barely. I wouldn't really CHOOSE to take-off or land (or do intentional stalls) at that rear limit, but it WILL be possible to do so safely. I'm currently leaning towards the obvious two-line and two-pump arrangement, but am still on the lookout for a lighter/simpler alternative using a single line and single pump. As I mentioned earlier, it's easily possible to use a single line and a single pump to fill and drain (with the pump always running in its normal mode) by using a stacked selector valve as used with a fuel-injected engine, and a bit of fancy cross-plumbing of the valve ports. This is basically trading the expensive selector valve for the second pump and line. It might be a little lighter and somewhat less complicated. Someone else suggested much earlier using a single pump to fill and a "bypass valve" to drain by gravity. That's an excellent and simple solution -- if I could trust that gravity alone will drain the tank within a reasonable length of time. I need to do a simple experiment to find out. Thanks again for all the comments, Greg |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Hot weather and autogas? | Rich S. | Home Built | 33 | July 30th 03 11:25 PM |