Twin Electric Fuel Pumps (again maybe?)

[islandmonkey]

There have been a number of recent threads on this and other forums where builders have mentioned backing up an electric fuel pump with another electric fuel pump. When I first read this, being a traditionalist, I thought thats a stupid idea and a recipe for disaster. However, I have pondered and thought about this more and I think maybe its not such a bad idea, provided that the installation is totally redundant. Lets look at the pros and cons.

The traditional way is to backup the mechanical fuel pump with an electric fuel pump.

Pros:

Redundancy maintained with dissimilar types of pump that are diversely powered.

Tried and tested over many years of implementation.

Cons:

Should the electric pump fail, the mechanical pump is prone to vapor locks especially when using mogas because of the distance of the pump from the tanks and the greater head when the aircraft is in the climb.

Fuel tank selector required to change tanks. This increases the likelihood of potential leaks because of the increased number of pipe unions. Also the tank selector is a potential if unlikely source of failure. It has been known for tank selectors to fail during mid selection causing total fuel starvation and the inability to select any tank.


Lets now look at the idea of twin electric pumps, each pump pulling fuel from its own tank. So you have a left pump and a right pump. By the way this is not my idea and I do not want to take any kudos for it.

System Description:

Two electric fuel pumps each capable of 100% duty cycle are installed. The pumps would be from the same manufacturer and would have a known MTBF measured in hours that would exceed 2000 hours by a significant order of magnitude. Each pump draws fuel from its own individual tank and there is no mechanical fuel selector installed. Tank selection is achieved by switching on the appropriate pump. Both pumps are selected on during critical phases of flight such as take off, landing, aerobatics or stall spin training/practice. An engine driven mechanical pump is installed also. In the event of an electrical fuel pump failure, there is a method of transferring fuel from one tank to another either using transfer pumps or by a crossfeed pipe that is normally closed but in the event of a fuel pump failure on either side a valve can be opened to allow fuel to crossfeed under gravity. There is a mechanical fuel on/off valve downstream of the pumps to turn the flow of fuel off to the engine compartment in case of a fire.

Pros:

Vapor locks are eliminated.
Tank selection is easy as long as there is no confusion on electrical switch operation.
The number of pipe unions is potentially reduced, thereby reducing the likelihood of leaks.
Mechanical fuel selector eliminated, making it easier to select a tank (remember John Denver).
Fuel in failed pumps tank is available if some sort of crossfeed system is installed.

Cons:

Reliance on 100% availability of electricity.
Crossfeed procedures must work if the fuel in the failed tank is required to complete the flight.
Increased weight of redundant systems. (two batteries, two altenators and three fuel pumps.
__________________________________________________ _____


Now that I have written all this down, I am wondering if it is such a good idea. Okay, thats it. Please feel free to comment or make suggestions and also please let me know if I have missed anything.

[Mike S]

Originally Posted by islandmonkey

T
System Description:

Two electric fuel pumps each capable of 100% duty cycle are installed. The pumps would be from the same manufacturer and would have a known MTBF measured in hours that would exceed 2000 hours by a significant order of magnitude. Each pump draws fuel from its own individual tank and there is no mechanical fuel selector installed. Tank selection is achieved by switching on the appropriate pump. Both pumps are selected on during critical phases of flight such as take off, landing, aerobatics or stall spin training/practice. An engine driven mechanical pump is installed also. In the event of an electrical fuel pump failure, there is a method of transferring fuel from one tank to another either using transfer pumps or by a crossfeed pipe that is normally closed but in the event of a fuel pump failure on either side a valve can be opened to allow fuel to crossfeed under gravity. There is a mechanical fuel on/off valve downstream of the pumps to turn the flow of fuel off to the engine compartment in case of a fire.

Please feel free to comment or make suggestions and also please let me know if I have missed anything.

O.K., I will

I suggest that if you do something like this, set it up so either pump can be operated from either buss. SPDT center off switches would do the job. Up for normal use, off in middle, down for backup electric source.

You didnt actually say so, but I got the impression that the pumps were each powered by a dedicated electric system.

In the event of a electric failure of the system powering the tank that still has fuel----assumes one tank dry----it would be nice to access the other electric system, rather than use a transfer pump. The issue here is time.....

By the way, how are you going to power the xfer pump?? One feed from each buss, diode blocked would work.

[Shockwave]

You might want to look into FrankH's setup. I believe he has 2 pumps per tank, with 1 pump capable of fully supplying the fuel, and redundant electrical systems.

[frankh]

But you have confused yourself.


The reson for having two electric pumps is because the mechanical one can't be assumed to work during a critical phase of flight, i.e a hot day at high altitude, using mogas.

This means with a traditional system you essentially have a single pump system..i.e if the electric one fails on take off, the mechanical thing could instantly vapour lock and this is likely to spoil your whole wekend!

So in other words there is zero reason to install a mechanical pump with two electric pumps, especially if the mechanical pump cannot suck through one of the electric pumps.

Save your self the weight and expense and leave the mechanical pump out.

You also have two choices with this system.

1) put the pumps in the wingroots and potentially have limited access to the fuel, but you get the best VL protection and no selector.

2) Put both pumps next to each other on the floor in the traditional location..Not quite as good of VL protection (but still superior to the mech pump) and you stick with the selector for access to all the fuel.


There are two reasons the mech pump are bad hydraulically...It is sucking uphill but the pump is also heated by the engine block..Both very bad things when pumping a high vapour pressure liquid.

You could have a wingrrot system and then have a crossfeed pump setup but these birds can fly an awfully long way on half of their fuel too.


Frank

7a..A professional Engineer who designs pumping systems for a living but does acknowledge having the occasional "stupid idea"...

[frankh]

I don't believe John Denver's selector valve failed..I think it was postioned so he couldn't reach it quickly.

Frank

[frankh]

Originally Posted by Shockwave

You might want to look into FrankH's setup. I believe he has 2 pumps per tank, with 1 pump capable of fully supplying the fuel, and redundant electrical systems.

One pump per tank..but your right the electrical system is equally as critical as the fuel pumps.

Frank

[islandmonkey]

Mike, the fuel pumps power source would be part of a Essential Bus that is fed from either a main or backup source of electrons. I am leaning towards the crossfeed pipe solution rather than pumps because far less complicated and has less parts to go wrong.

Please be aware, I am not thinking of actually implementing this idea yet, I am to much of a traditionalist. There were were some discussions on other threads that IMO were not appropriate to that thread. However I do find the topic very interesting and in the interests of eliminating vapor locks I am trying to get some ideas together to put together a plan of implementation that is thought out and has no weak links. I also want to see if there is a consensus of opinion for or against this idea.

[rv7charlie]

I'm preparing to install a Precision FI system & wondering about the series-connected Facets someone mentioned in another thread. Can two of these 13psi Facets in series provide enough pressure to serve as backup to the standard mechanical pump? If so, that would eliminate the elaborate bypass plumbing required for the automotive pumps some are using as boost pumps.

If it'll work, who sells the higher pressure Facets?

And, so the thread isn't totally hijacked, would 2 pairs of these serve as dual electric with no mechanical pump on the Precision/Bendix system?

Thanks,

Charlie

[islandmonkey]

Originally Posted by frankh

But you have confused yourself.


The reson for having two electric pumps is because the mechanical one can't be assumed to work during a critical phase of flight, i.e a hot day at high altitude, using mogas.

This means with a traditional system you essentially have a single pump system..i.e if the electric one fails on take off, the mechanical thing could instantly vapour lock and this is likely to spoil your whole wekend!

So in other words there is zero reason to install a mechanical pump with two electric pumps, especially if the mechanical pump cannot suck through one of the electric pumps.

Save your self the weight and expense and leave the mechanical pump out.

You also have two choices with this system.

1) put the pumps in the wingroots and potentially have limited access to the fuel, but you get the best VL protection and no selector.

2) Put both pumps next to each other on the floor in the traditional location..Not quite as good of VL protection (but still superior to the mech pump) and you stick with the selector for access to all the fuel.


There are two reasons the mech pump are bad hydraulically...It is sucking uphill but the pump is also heated by the engine block..Both very bad things when pumping a high vapour pressure liquid.

You could have a wingrrot system and then have a crossfeed pump setup but these birds can fly an awfully long way on half of their fuel too.


Frank

7a..A professional Engineer who designs pumping systems for a living but does acknowledge having the occasional "stupid idea"...

Thanks for the reply Frank. Your right, I do confuse myself sometimes.

The initial post was aimed at people like yourself who have implemented a solution like this. Here is some questions for you:

With good bypass valves installed in each electrical pump, is the head to much for the mechanical pump in the climb attitude and would vapor locking occur?

Are your pumps rated for 100% duty cycle and do you know their MTBF?



The mechanical pump was retained in order to maintain fuel pressure in the event of an electrical failure. Its not much of a weight penalty.

John Denver's fuel selector was located in a really unaccessable location, your right it did not break. Electrical switches can be located in very convenient positions avoiding limbo dances or close inspection of ones navel to change tanks. I have heard of mechanical tank selector handles coming away in peoples hand leaving no possibility of tanks selection.

[frankh]

Originally Posted by islandmonkey

Mike, the fuel pumps power source would be part of a Essential Bus that is fed from either a main or backup source of electrons. I am leaning towards the crossfeed pipe solution rather than pumps because far less complicated and has less parts to go wrong.

Please be aware, I am not thinking of actually implementing this idea yet, I am to much of a traditionalist. There were were some discussions on other threads that IMO were not appropriate to that thread. However I do find the topic very interesting and in the interests of eliminating vapor locks I am trying to get some ideas together to put together a plan of implementation that is thought out and has no weak links. I also want to see if there is a consensus of opinion for or against this idea.

You must be joking.... Apart from the fact I recieve about 10 emails a month telling me (a) thats I'm stupid and going to die in the next few flights, b) irresponsible in leading other builders astray and C) Wow...Your STILL alive, you mean a dual electric system really does work!...Oh and d) I was at take off the other day, turned off my electric pump and the engine spluttered and quit..

Well you'll never get consensus...

I developed the idea of the wingroot setup on a Subaru where we did not have the option of a mechanical pump anyway, so it was a questions of where to install the facet pumps..Not if.

that airplane has over 500 hours on it now.

When I came to building the RV I never considered doing anything other than running twin eletric pumps..the idea of a mechanical pump was incredibly stupid to me in fact. I have had the same arguments over and over, and its not that the twin electric system is without design issues, its just that I believe they can be safely overcome and I have a total time of about 1000 hours behind twin electric setups.

I have a few followers of the "no mechanical pump gospel" now but we are an enthusiastic bunch and no one has had an issue so far.


If your looking for consensus you'll never get that, but I gurantee you it will work.

besides, we can't have these alternative engine guys telling us we CAN"t burn anything but 100LL can we..heheh

Frank...Who also burns ethanol laden mogas

Frank