Dual electric fuel pumps ... pipe in series or parralell?

[Guest rocketdriver]

I have to replace the two electric fuel pumps on the KR2 with the carburetted Subaru motor due to one of the pumps leaking badly. Currently they are piped in series.

 

The blurb for the new pumps (made in NZ) says that they can pull thru a dead pump if need be.

 

The new pumps are rated at 1.5 lpm (more than adequate) and 4 to 6 psi.

 

Should I pipe them up in series or parralel? Each has its own electrical circuit and fuse and can be switched independantly.

 

cheers

 

RD

 

 

[Guest Maj Millard]

Be very carefull plumbing elect fuel pumps in series !.. They should always be parallel, with a one-way check valve. The Rotax set up using the Pierburg pump, and Rotax check-valve is a good example of how to do it, and is shown in their parts manual for the 912.

 

A very tragic and fatal accident up here in the North a few years back, with a Subaru -powered 601 highlighted the dangers of series fuel-pump installation. Owner had installed two Facet elect pumps in series, in the cockpit area, because of a supposed fuel supply problem to the engine.

 

The aircraft crashed in flames during a cross-country flight, and the pilot did not survive his massive pre-crash burn injuries. The fire damage to the cockpit area indicated an intense in-cockpit fire, possible intiated by a fuel line separating.

 

When you plumb in series, you boost the line fuel pressure by double, if both pumps are on. Not a good idea for either fuel lines, hose clamps, or carbureters..The addition of a fuel pressure guage plumbed in just before the carbureter is a wise investment also, as most carbs have an upper delivery pressure limit...If you have a faulty or suspect fuel pump, you replace it with a new one, not fit a second to 'pull' fuel through it ...........................................................................................................Maj...

 

 

[facthunter]

All good advice from Maj there. You add the pressures, (in series) EXCEPT where the final pump is the common car type of engine driven pump where the diaphragm is pulled against a spring ( which determines the pressure the pump delivers), by a cam operated lever. If you turn on a "boost" pump upstream the pressure acts to keep the diaphragm at full stroke position and IT does not move from that position untill the boost pump is turned off AND some fuel is used. Have a fuiel pressure gauge and use the lowest pressure that will do the job 4 feet of pressure "head" would be enough and that is under 2 psi. (At the carb inlet). At some point of the aircraft's build/service you should have determined the flow rate of the system to make sure it is more than adequate to keep up the fuel to the engine in all circumstances. I wonder if this is always done? Nev

 

 

[Guest rocketdriver]

Hi Maj and FH

 

Thanks for the advice ...parralel was my instinct too, but the a/c arrived with them in series.

 

Now to find room for the rearranged system .... I want to put a fuel drain both sides of the pumps too especially as I have only one tank outlet to use and the pumps are non return ...

 

cheers

 

RD

 

 

[bones]

Also a REAL good idea is to have a separate fuel filters for each pump, because even if you have 2 pumps and the filter is blocked turning another pump on may not do anything, simply just before each pump put a inline filter, job done, that way if the duty pump stops, either electically of fuel blockage, you KNOW you have a clean filter and pump to try.

 

Edit; remember to change the second pump filter each service also, paper filters do degrade in time, so no good thinking you got a good filter and pump if it hasnt been changed for awhile and its blocked too

 

 

[Guest Maj Millard]

Good stuff Bones and facto, The Rotax one-way check-valve, although at Rotax prices, is a nice unit and should last a long time. The Magni Gyros come with a two-pump set up in parallel, and have the two one-way valves standard..Thats a good recommendation on the filters Bones.......................................Maj...

 

 

[Guest ozzie]

Major, If you have float type carbies a pressure regulator valve would be a good idea i think, so as to not over ride the carby needle and flood the carby. Was this the problem with that Suby? And then plumb the excess fuel back to the tank. I remember the AUF mag asking members for a solution for this. Pretty sure it was in relation to that in flight fire incident.

 

A good filter is one that has a pop up valve that will trip if the filter becomes blocked it may mean some crap getting to the carby but if it rapidly blocks in flight the engine will keep running for a period to allow a safe forced landing while the engine is still producing power.

 

Ozzie

 

 

[Guest Maj Millard]

Ozzie, I didn't have much to do with that aircraft prior to his accident, other than meeting him once at Ingham, and I believe he had some general engine problems on the way back to his base. The final accident occured not long after that. Lee Ungerman was the one who did the actual crash investigation at the time..

 

I spend a lot of time telling people and new builders, to keep the fuel system as simple as possible, bare minimum to supply fuel to the engine, period. The more you complicate the system, the more chance of problems, or failure.........

 

For instance if you have an engine operated (mechanical) pump, I fail to see why you need two boost pumps behind it. One as a boost/back-up should be more than sufficient.......

 

There are many good examples of simple, effective, and safe fuel systems out there. The Lightwing is one,and Tecnam, Savannah and Slepcev Storch are others...............................................Maj...

 

 

[fly_tornado]

So would you recommend having the fuel pump in the fuel tank? My BMWs have immersed fuel pumps and I can see that it would make the plane a lot safer as a leaking fuel pump wouldn't pose a great fire risk. My only problem is how to get it into and out of the fuel tank, I was thinking of some sort of cradle.

 

 

[Guest rocketdriver]

Hi Fly ...

 

Sugest you go to a wreckers and have a look at a late model commodore. These have the pump in the tank. When I did my sports/racing car, I mounted it the same way, if I remember correctly, through a removable hatch in the top of the tank.

 

cheers

 

RD

 

 

[facthunter]

That's how they are done.(Through a large cover). I always have a bit of an aversion for having electric motors immersed in fuel. They rely on having too rich a mixture to ignite, but what if the tank is empty and the fuel pump is left on? Bit like the 44 gallon drum and the angle grinder. Nev

 

 

[Guest ozzie]

Ozzie, I didn't have much to do with that aircraft prior to his accident, other than meeting him once at Ingham, and I believe he had some general engine problems on the way back to his base. The final accident occured not long after that. Lee Ungerman was the one who did the actual crash investigation at the time..I spend a lot of time telling people and new builders, to keep the fuel system as simple as possible, bare minimum to supply fuel to the engine, period. The more you complicate the system, the more chance of problems, or failure.........

 

For instance if you have an engine operated (mechanical) pump, I fail to see why you need two boost pumps behind it. One as a boost/back-up should be more than sufficient.......

 

There are many good examples of simple, effective, and safe fuel systems out there. The Lightwing is one,and Tecnam, Savannah and Slepcev Storch are others...............................................Maj...

Yes major this sounds like the one Lee asked for a design for the fuel system for the Subby engines as there where a few problems. several where submitted all were return to tank using a pressure regulator to stop the carby from flooding. I would not use any hose and clamps on a fuel system that was over gravity feed. ir i decent pump is being used it would be seamlees tube and fittings only. be carefull about using fuel pumps from late model fuel injection cars some run rediculus pressures. most system s can have three pumps. one in the tank, one for the injection system and one to return.

 

 

[eightyknots]

Bit like the 44 gallon drum and the angle grinder. Nev

Tell us more about this one, facthunter.

 

 

[facthunter]

Plenty of examples of long empty drums being blown sky-high and often fatal results. Nev

 

 

[fly_tornado]

I think you would be more worried about getting electrocuted than sparks.

 

 

[Guest Maj Millard]

I'm not a big fan of electrical pumps in fuel tanks either, some large jet transports use a venturi type pump (jet pump) which is submersed in the fuel. Once it is operating it requires no further power to draw the fuel out of the tank.

 

Fuel pumps shouldn't leak fuel, a well designed system will have a pressure return line back to the tank, to vent off excess fuel pressure above that required by the carb.

 

The after-market copies of the Facet fuel pumps possibly from an unspecified country, have been know to leak from around the square body. These should be avoided at all cost, and when buying a Facet square 'ticker' pump, make sure it is a genuine Facet pump. The copies will be cheaper in price, but any monies saved at purchase will be wasted when the damn things leak !.....Also as Ozzie mentioned the pumps from fuel injected autos will produce way more pressure than we need on our aircraft, in most cases...................................................................................Maj...

 

 

[HEON]

Have a look at Rotax 914 instalation manual as they use two electric pumps and it is a proven system.

 

Note well statement there that they MUST BE COMPLETELY INDEPENDENTLY WIRED

 

 

[Guest ozzie]

Fill the drum with water, then cut.

 

Ever seen fuel tanks being welded whilst still containing fuel? They do it

 

 

[Guest ozzie]

Have a look at Rotax 914 instalation manual as they use two electric pumps and it is a proven system.Note well statement there that they MUST BE COMPLETELY INDEPENDENTLY WIRED

More than makes sense, bit silly having redundency then wiring them up to the same fuse. Bit like some dual magneto common drives.

 

Quite a lot of aircraft with immersed fuel pumps. immersing them does several things, it lubricates them and keeps them cool. built and maintained correctly electrical devises in explosive atmospheres are quite common. I have never heard of a pump being the cause of a explosion, running wire thru the tanks on the other hand. 747 off east coast USA blew apart due to fraying wires.

 

 

[HEON]

Ozzie my emphasis on independently wiring is that I had a factory built aircraft where a common earth return was used by error.

 

Do I have to say which wire shorted out?

 

Result one very quiet landing!

 

 

[Guest ozzie]

Ozzie my emphasis on independently wiring is that I had a factory built aircraft where a common earth return was used by error. Do I have to say which wire shorted out?

Result one very quiet landing!

I have never been a fan of bonding earth wires to the airframe or linking earth wires together from several componets or relying on the componets case to provide an earth. I prefer to isolate the componet from the airframe and then running a decent gauge wire to the earth bus or directly to the battery.