[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]

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]

Your getting there.

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]

Oh

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

Frank

 

[frankh]

Nope

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]

slight variation in the thread

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]

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]

Consensus?

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

 

[frankh]

Sure

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.

My personal position is that the head is too much to overcome with a mechanical pump period!..bypass valve or not!

OK that needs some explanation..A cool morning with a cold engine running 100LL is a very different situation compared to a hot day at high altitude running mogas. If one to add an additional pressure drop of pulling thru a non return valve, then I would suggest this is not a good idea.

The whole reason to move away from a mechanical pump is to provide a "hydraulically correct solution"..I.e the pump is put in a cool location at the lowest point of the fuel system.

The mechanical pump is just wrong from a hydraulic perspective..i.e its in a HOT location (hot fuel boils much more easily than cold fuel), and uphill.

What your suggesting is planning for a double failure...think of it as having three magnetos..Whats more the mechanical pump is a poor solution from a VL perspective.

I did have the ried vapour pressure of Mogas at one point...Its pretty high from memeory, in other words you don't have to suck very much to boil it.

What is the MTbf of an electric pump?..I personally have no idea..I've only flown 1000 hours behind electric pumps with zero failures. Don Rivera will tell you his (Automotive) pumps last for either 2000 hours or 50..In other words most failures appear to be infant mortality failures.

Note Airflow performance do sell a dual electric setup i believe.

My pumps came from NAPA and you can see a descritpion of my system if you search under my name.

Yes the pumps HAVE to be 100% duty cycle rated..they HAVE to be run together for take off and landing.

Good luck in your search for enlightenment..or.. Welcome to the Dark side..


Frank

 

[frankh]

Speak to Rocketbob

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

He was designing such a solution and I vblelieve it has merit..I was even thinking of a slight variation to that idea..I.e put one pump in each wingroot and two pumps on the floor.

Its still 4 pumps and it still retains the elegant "Switch pumps to switch tanks" soultuion.

The nice thing about this setup is that you will probably be able to such fuel through a dead wingroot pump and STILL have enough pressure to supply the FI servo.

Note this sounds like the oppositie argument that I have been making all along..But think of it this way....

You have a dead pump in the wing root...Well you still have 3 pumps in series feeding fuel form the opposite tank..So you take off as normal, get to altitude and swicth to the other full tank with the dead pump..with the reduced flow requirements of cruise flight you will probably be OK and have full access to both tanks.

i think this system has a lot of potential.

I don't know what the presure requirements of the Bendix system are, the AFP is OK between 14 and 90psi I believe..So 2* 11.5 PSI will certainly work..


Frank

 

[rocketbob]

Two Facet 40185 pumps in series will put out 23psi, and will cost you about $100 for the pair. On my F1 I have a pair in each wing root, no mechanical pump. Those are the only pumps. Left/Both/Right DPDT switch controlling the pumps with two backup switches for each side (DPDT, ON-OFF) to eliminate the switch as a single point of failure. I'll have a dual battery setup, with pumps and dual ignitions running on an essential bus. Haven't flown the rocket with this setup, but a friend is soon going to replace his Dukes pump in his rocket with a pair of the 40185 Facets and a check valve to bypass the pumps when they're off.

With this system there is NO single point of failure possible, with the exception of a broken fuel line to the engine.

About a year or so ago I spoke to an engineer at Tempest about mechanical pump failures, and he told me his own opinion was that he thought the mechanical pump ideally should have a bypass.

 

[rocketbob]

You have a dead pump in the wing root...Well you still have 3 pumps in series feeding fuel form the opposite tank..So you take off as normal, get to altitude and swicth to the other full tank with the dead pump..with the reduced flow requirements of cruise flight you will probably be OK and have full access to both tanks.

i think this system has a lot of potential.

I don't know what the presure requirements of the Bendix system are, the AFP is OK between 14 and 90psi I believe..So 2* 11.5 PSI will certainly work..

Frank

You hit precisely on my thinking. Even if one of the pumps is dead, at 11.5psi it may not put out the required pressure for wide open throttle to run a FI system, but it should certainly be enough to run the system at partial power.

 

[mbriese]

Do the Facet 40185 pumps have an internal bypass? If you have 2 or more in Series can the other pumps pull or push fuel through the dead pump?

My current plans are for a Facet 40185 in each wing root with a check valve, T then 2 more Facet 40185's or possibly the AFP Electric pump setup. Question is: without a fuel selector, in the event of a failed wing root pump, how do you determine which tank to draw fuel from? Will air from an empty tank get picked up if both wing root pumps are off and one tank is empty?

Thanks,

 

[rocketbob]

Do the Facet 40185 pumps have an internal bypass?
Thanks,

In essence, yes, but it does not relieve itself which is why plumbing more than one in series allows the summing of pressure with series pumps. It does not have the "positive shut off" feature listed here:

http://facet-purolator.com/index.php?option=com_content&task=category&sectionid=7&id=16&Itemid=31

Regards,
Bob

 

[rocketbob]

My current plans are for a Facet 40185 in each wing root with a check valve, T then 2 more Facet 40185's or possibly the AFP Electric pump setup. Question is: without a fuel selector, in the event of a failed wing root pump, how do you determine which tank to draw fuel from? Will air from an empty tank get picked up if both wing root pumps are off and one tank is empty?

Thanks,

I wouldn't set it up this way for the reason you describe. My design is to have two pumps in each wingroot, and those are the only pumps in the system.

 

[frankh]

Eh?

Do the Facet 40185 pumps have an internal bypass? If you have 2 or more in Series can the other pumps pull or push fuel through the dead pump?

My current plans are for a Facet 40185 in each wing root with a check valve, T then 2 more Facet 40185's or possibly the AFP Electric pump setup. Question is: without a fuel selector, in the event of a failed wing root pump, how do you determine which tank to draw fuel from? Will air from an empty tank get picked up if both wing root pumps are off and one tank is empty?

Thanks,

yes but you would never purposely run it this way of course so thats a non issue...You simply don't allow the level to get so low that you empty a tank.

This is the disadvantage of a wingroot system..i.e you have to leave some fuel in each tank

Frank

 

[Danny7]

yes but you would never purposely run it this way of course so thats a non issue...You simply don't allow the level to get so low that you empty a tank.

This is the disadvantage of a wingroot system..i.e you have to leave some fuel in each tank

Frank

what happens if you start to get down to the last 3-4 gallons in each wing and have to maneuver, even uncoordinated, and the pickups are uncovered?

 

[frankh]

I have

what happens if you start to get down to the last 3-4 gallons in each wing and have to maneuver, even uncoordinated, and the pickups are uncovered?

Only been that low once (3 gallons in each side) and there was about a 10kt x wind. Did not notice anything different.

If you think about it even in uncoordinated flight one of the pickups will be covered and it simply means the other one will not. The chances of the on pump you need going TU right at that moment would be most unfortunate but you can recordinated the airplne and go around...simply do a crab or otherwise come in high.

With both pumps functioning it was a non issue.

Frank

 

[frankh]

Dry pump

will not pump air against the pump that is pumping fuel...dry pumps make almost no pressure.

Frank

 

[mbriese]

He was designing such a solution and I vblelieve it has merit..I was even thinking of a slight variation to that idea..I.e put one pump in each wingroot and two pumps on the floor.

Its still 4 pumps and it still retains the elegant "Switch pumps to switch tanks" soultuion.

The nice thing about this setup is that you will probably be able to such fuel through a dead wingroot pump and STILL have enough pressure to supply the FI servo.
...
You have a dead pump in the wing root...Well you still have 3 pumps in series feeding fuel form the opposite tank..So you take off as normal, get to altitude and swicth to the other full tank with the dead pump..with the reduced flow requirements of cruise flight you will probably be OK and have full access to both tanks.

i think this system has a lot of potential.

Frank

It sound like the pumps on the floor in this setup are on the firewall side of the T. Is that correct? (only way to have 3 pumps in series if 1 wing root pump fails that I can see.). Can you discuss the Pros and Cons of this setup from the perspective of how thing would be operationally different from the standard setup in various situations such as low no fuel in one tank, failed wing root pump, failed floor pump, clogged filter, etc?

While the vast majority of the time we all plan to have fuel left in each tank, occasionally one might want to run one dry on a Max range flight (30 min + left in the other tank for landing.)So Any ideas on what would happen in the worst case as far as a wing root pump failure on the tank with fuel, the other tank assumed to be empty or nearly so? I plan on testing this during phase one so there are no surprises latter on.

I'm not try to knock the design by any means. I full intend to install an all electric pump setup of some kind, I am just trying to get all the info available to make the best decision for my mission. IE Mogas & some extended range flights.

 

[frankh]

No pumps north of firewall

Correct! The pumps would be in the cabin North of the Tee.

If you have a dead pump you only have two pumps in series. I.e the two pumps on the floor in the cabin. You take off with the good wingroot pump, plus the two in series...Climb to altitude, reduce fuel flow and turn off the wingroot pump.

The issue now is, where will the two pumps suck the fuel from?

To make this work you would need a selector valve with a "both" position in place of the tee or a single on/off valve in each line feeding the tee.

I know the Vans valve can be plumbed as an on/off valve using the 4th port. But i don't know if this means it can be plumbed as a left/right/both though.

So if you had such a valve, normal ops would be leave the valve set to both, never in normal ops would you use the selector.

you switch tanks by switching pumps as in the wingroot system.

Now lets say the left WR pump dies...Ok you shut down the left pump and run off the right..Reduce cruise power and shutdown the right pump.

Now fuel is being sucked from both..If the engine is happy you now swap tanks to left by using the selector.

For landing (or subsequent take off's) simply select "Both" and run the right WR pump.

For a failed floor pump, no biggie, just operate as normal. For a plugged fuel filter, thats the same as a wingroot pump failure.

i think you really need to plan on having fuel in each tank. Now with the above system you could drain a tank (the Facet pumps are much more forgiving running them dry than the FI style pumps) and then simply draw off the tank with fuel. The reason for this is that you still have three pumps that provide the required redundancy.

Frank

 

[frankh]

Be careful with carbs

My thoughts so far have assumed a fI'd motor. Using a carb however it would be difficult to find a pump wtih a low enough pressure to avoid over pressurising the carb with pumps in series.

For a carb situation you would use a Facet in each wingroot and a single floor pump. a 40106 makes about 6psi so you would only run the floor pump in the event of a WR pump failure.

Frank

 

[islandmonkey]

Keep 'em Coming

I am pleased with the responses to my initial post and am grateful to frankh for his words of wisdom and experience. I am definitely coming round to the idea of the one electric fuel pump per tank idea.

I do want to keep the weight of my project (RV3) down to the minimum. Installing two Odyssey SBS J 16 batteries at 15.4 lbs a piece plus the weight of a standby altenator, for example the B&C SD20 at 5.75 lbs makes an additional 21 lbs I have to carry around for having redundancy. I will probably use duel P Mags so some weight will be saved in this area. Combined with the weight saving of installing an AFS EFIS and engine monitor with a backup steam ASI, I want to aim for a maximum empty weight of 750 lbs. Any reduction of this would be a bonus. Does anyone have any ideas where weight could be saved? Carbon fibre, paint, for example.

On another note, the more redundancy thats built in, then there is more likelihood of a failure. Think about it. Having two electric fuel pumps installed, increases the chances of one pump failing by 100% !!!!
This means procedures for identifying and securing a failure so that further safe flight operations can continue have to be documented and learned.

 

[rv7charlie]

redundant redundancy

This is a bit of thread drift, but do you really need 2 batteries *and* 2 alternators? What's the likelihood of having an alternator failure and a battery failure on the same flight?

The 2nd alternator is likely to be lighter but more expensive and complicated to install.

The 2nd battery is likely to be simpler & cheaper, but heavier.

Doing both seems kinda like adding a 3rd magneto: both heavy and expensive.

Charlie

 

[airguy]

In an aircraft that is electrically dependent, the idea is that you might lose either the alternator OR the battery - and you don't know which one, so you have to be prepared either way. In some cases you may lose the entire primary bus if a main wire shorts the bus to ground. For most builders who put together a dual battery, dual alternator setup, the second alternator is a smaller accessory pad-mounted unit that will only put out several amps, not enough to run everything onboard. The idea is that this emergency electrical bus is used ONLY to run the flight-critical "absolutely gotta have it" items that enable continued flight to a safe landing at the nearest available field. The other items (like big displays, flaps, second nav/comm, all that other non-critical stuff) does not have connectivity to the emergency bus and will rely on having enough spare juice in the primary battery to run it if you want it.

An RV can land on almost ANY runway without flaps - so the flaps don't get juice from the E-buss. Landing lights will, nav lights won't. GPS will, Dynon panel won't (it has it's own battery backup internally). Transponder will, backup nav/comm won't. Fuel pumps will and ignition will, XM radio won't. You get the idea....

You can probably get by with just a second battery that powers the E-buss for X number of minutes to allow you to land if you only fly VFR - but adding the standby alternator can greatly increase that X number of minutes for the scenario of running at night, or in weather, and needing to use the nav/comms and possibly the autopilot for shooting an approach. It all comes down to what level of redundancy and safety you are willing to trade for dollars and weight. My airplane will be primarily a business travel airplane, flying in all kinds of conditions and locations - so I am willing to trade the dollars and weight for that level of redundancy. YMMV.

 

[frankh]

For me

I went the dual alternator single battery route.

I was amazed at how much that little SD8 vacuum pad mounted alt would put out..If your OK with going down to 12V (I am) I found that I could drive pretty much everything I needed including the GNS 430W.

My bet is that it puts out nearer 12Amps at 12V

My airplane is setup to fly IFR and by using a Pmag I had enough to run everything I needed.

Frank

 

[airguy]

I went the dual alternator single battery route.

I was amazed at how much that little SD8 vacuum pad mounted alt would put out..If your OK with going down to 12V (I am) I found that I could drive pretty much everything I needed including the GNS 430W.

My bet is that it puts out nearer 12Amps at 12V

My airplane is setup to fly IFR and by using a Pmag I had enough to run everything I needed.

Frank

Perfectly valid - and I may end up in the same boat after some testing and number crunching. The Dynon panel will run itself on backup power, and if you kill power to everything else you can rely on 11-12 amps constant from the SD8 and any surge capacity above that (like radio transmissions) can come from the battery reserve. It's a judgement call and "warm fuzzy" decision.

 

[islandmonkey]

This is a bit of thread drift, but do you really need 2 batteries *and* 2 alternators? What's the likelihood of having an alternator failure and a battery failure on the same flight?

The 2nd alternator is likely to be lighter but more expensive and complicated to install.

The 2nd battery is likely to be simpler & cheaper, but heavier.

Doing both seems kinda like adding a 3rd magneto: both heavy and expensive.

Charlie

If you going to just have electric pumps suppling fuel pressure then some kind of redundant power source is required, so its not really drifting the thread which I did start. A standby battery and alternator is required with the capacity to drive the fuel pumps for a reasonable time in order to continue flight to the closest airfield in the event of a failure of the main battery. A B&C SD20 alternator weighs 5.75 lbs and cost $695. Not much extra weight or dosh for a 20 amp alternator. I would prefer more positive criticism, suggestions and answers to my questions. However this is an open forum so feel free to respond.

Where would you fit a third magneto?

The idea is to have an essential buss and a non essential buss. The fuel pumps would be part of the essential buss along with radio, transponder and combined EFIS/engine monitor. Manual trim and flaps would be used. There would be warning lights for low fuel pressure from each pump and low volts from each battery/alternator along some means of measuring the current . EFIS and GPS would be backed up with internal batteries. Maybe the SD20 is overkill. The SD8 at half the cost and weight might suffice. Comments welcome.

 

[frankh]

If you going to just have electric pumps suppling fuel pressure then some kind of redundant power source is required, so its not really drifting the thread which I did start. A standby battery and alternator is required with the capacity to drive the fuel pumps for a reasonable time in order to continue flight to the closest airfield in the event of a failure of the main battery. A B&C SD20 alternator weighs 5.75 lbs and cost $695. Not much extra weight or dosh for a 20 amp alternator. I would prefer more positive criticism, suggestions and answers to my questions. However this is an open forum so feel free to respond.

Where would you fit a third magneto?

The idea is to have an essential buss and a non essential buss. The fuel pumps would be part of the essential buss along with radio, transponder and combined EFIS/engine monitor. Manual trim and flaps would be used. There would be warning lights for low fuel pressure from each pump and low volts from each battery/alternator along some means of measuring the current . EFIS and GPS would be backed up with internal batteries. Maybe the SD20 is overkill. The SD8 at half the cost and weight might suffice. Comments welcome.

A standy battery AND an standby alt are a bit overkill (my humble opinion). You are into a dual redundancy thig with this setup.

I.e you are assuming you will have a battery failure and an alternator failure at the same time. The reality the chance of that happening is infinitesimally small.

I.e we normally plan for a single unit to fail in any single flight. Add to that batteries just really never fail..May become disconnected sure but an actual failure these days is un heard of.

So planning on a single unit to fail.like an EI, fuel pump, radio, GPS, battery or alternator etc.

So lets assume your battery becomes diconnected..well then your alternator will continue to make juice and you can wire the SD8 to self excite (I didn't).

If your alternator dies..then you got the SD8 and the battery.

The point about the thrid magneto was to illustrate the dual redundancy mentality..I.e you don't plan on two magnetos failing on any one flight, the fuel pump/battery/alternator is the same thing.


Oh one downfall of the lectric system..I don't know what would happen during a lightening strike..Nor do I wan't to find out..


Frank

 

[mbriese]

Compromise ?

Correct! The pumps would be in the cabin North of the Tee.

If you have a dead pump you only have two pumps in series. I.e the two pumps on the floor in the cabin. You take off with the good wingroot pump, plus the two in series...Climb to altitude, reduce fuel flow and turn off the wingroot pump.

The issue now is, where will the two pumps suck the fuel from?

To make this work you would need a selector valve with a "both" position in place of the tee or a single on/off valve in each line feeding the tee.

I know the Vans valve can be plumbed as an on/off valve using the 4th port. But i don't know if this means it can be plumbed as a left/right/both though.

So if you had such a valve, normal ops would be leave the valve set to both, never in normal ops would you use the selector.

you switch tanks by switching pumps as in the wingroot system.

Now lets say the left WR pump dies...Ok you shut down the left pump and run off the right..Reduce cruise power and shutdown the right pump.

Now fuel is being sucked from both..If the engine is happy you now swap tanks to left by using the selector.

For landing (or subsequent take off's) simply select "Both" and run the right WR pump.

For a failed floor pump, no biggie, just operate as normal. For a plugged fuel filter, thats the same as a wingroot pump failure.

i think you really need to plan on having fuel in each tank. Now with the above system you could drain a tank (the Facet pumps are much more forgiving running them dry than the FI style pumps) and then simply draw off the tank with fuel. The reason for this is that you still have three pumps that provide the required redundancy.

Frank

This is what I was thinking. While this may work just fine for people who know their system and took the time to research or design it, it may be confusing to another pilot / owner.

What do you think of the following compromise? 1 Wing root fuel pump for each tank, L/R/OFF fuel selector, 2 pump in series on the floor and of course no mechanical pump. You would only run the 2 floor pumps under normal conditions and use the fuel selector to chose the tank. For take-off, Landing or in the now unlikely event of vapor lock, you switch on the appropriate wing root pump. (or may be both and let the fuel in the unselected tank return to it via the pressure a pressure relief valve.) Since the root pumps are not in constant use may be less chance of failure as well? Of course a fuel pressure indicator / alarm would also be a good idea to warn you of a failed "floor pump" A redundant electrical system is a given.

I would think this setup would be "pretty good" from a vapor lock perspective, however not as good as all pumps in the wing roots. It also allows the pilot to switch tanks using the "normal method" and provides even better vapor lock protection / redundancy when needed. You also have access to all of your fuel and I don't believe it would be overly complicated or weight significantly more than boost & mechanical, and using Facet pumps the cost and electrical draw are both relatively low.


Just some thoughts,

 

[rv7charlie]

If you going to just have electric pumps suppling fuel pressure then some kind of redundant power source is required, so its not really drifting the thread which I did start. A standby battery and alternator is required with the capacity to drive the fuel pumps for a reasonable time in order to continue flight to the closest airfield in the event of a failure of the main battery. A B&C SD20 alternator weighs 5.75 lbs and cost $695. Not much extra weight or dosh for a 20 amp alternator. I would prefer more positive criticism, suggestions and answers to my questions. However this is an open forum so feel free to respond.

Where would you fit a third magneto?

The idea is to have an essential buss and a non essential buss. The fuel pumps would be part of the essential buss along with radio, transponder and combined EFIS/engine monitor. Manual trim and flaps would be used. There would be warning lights for low fuel pressure from each pump and low volts from each battery/alternator along some means of measuring the current . EFIS and GPS would be backed up with internal batteries. Maybe the SD20 is overkill. The SD8 at half the cost and weight might suffice. Comments welcome.

My apologies; my previous post was meant to be a constructive suggestion with a bit of humor thrown in, but I made an obviously poor attempt at both.

Frank's post explained more succinctly what I was trying to say, that you should be able to get the desired redundancy by picking either dual alternators or dual batteries & save some money and some weight.

For what it's worth, some of the all-electric (fuel, ignition, everything) alternative engine guys I know size their batteries to give at least an hour of 'essential bus' operation if they lose the alternator, and assume unlimited duration from the alternator if they lose the battery. This technique gets them up to their personal 'comfort threshold' of redundancy. These are guys that started with dual batteries, etc & after hundreds of hours in the air (and experience with some electrical failures), decided that they didn't need the extra weight to remain safe.

I hope this is useful info.

Charlie

 

[frankh]

Thoughts

This is what I was thinking. While this may work just fine for people who know their system and took the time to research or design it, it may be confusing to another pilot / owner.

What do you think of the following compromise? 1 Wing root fuel pump for each tank, L/R/OFF fuel selector, 2 pump in series on the floor and of course no mechanical pump. You would only run the 2 floor pumps under normal conditions and use the fuel selector to chose the tank. For take-off, Landing or in the now unlikely event of vapor lock, you switch on the appropriate wing root pump. (or may be both and let the fuel in the unselected tank return to it via the pressure a pressure relief valve.) Since the root pumps are not in constant use may be less chance of failure as well? Of course a fuel pressure indicator / alarm would also be a good idea to warn you of a failed "floor pump" A redundant electrical system is a given.

I would think this setup would be "pretty good" from a vapor lock perspective, however not as good as all pumps in the wing roots. It also allows the pilot to switch tanks using the "normal method" and provides even better vapor lock protection / redundancy when needed. You also have access to all of your fuel and I don't believe it would be overly complicated or weight significantly more than boost & mechanical, and using Facet pumps the cost and electrical draw are both relatively low.


Just some thoughts,

One of the advantages of the electric only fuel pump setup is that you do away with the fuel selector..you switch tanks by switching pumps..its elegantly simple.

If you go back to the selector (L/R/OFF) then you have lost some of this simplicity..also (and I gurantee this will happen) one day you will have the left tank selected on the selector and the right wingroot pump only running.

Now you are sucking through a dead pump at high flowrate (takeoff) and the associated risk of vapour lock.. For this reason I would limit any selector (and I'm not a great fan of them anyway) to dead pump ops in low power cruise, and that would require a "Both" position.

I mean one of the beauties of the WR system is that you have 100% of the fuel flowing from each tank..WR pump dies on TO and you won't even know it.

Frank

 

[mbriese]

That makes sense. Other than the fuel selector being an added complication in your opinion, the real problem may be a vapor lock possibility from pulling fuel through the wing root facet pump when not running? They must create some flow restriction and not truly free flow then. Would a bypass on each pump then be in order? (from a vapor lock perspective.)

Looks like the system may be becoming too complicated to justify "simple" access to all the fuel in the event of a wing root pump failure.

Thanks Frank for all your great ideas. It looks like I will be keeping all my pumps in the wing roots

 

[L.Adamson]

One of the advantages of the electric only fuel pump setup is that you do away with the fuel selector..you switch tanks by switching pumps..its elegantly simple.

If you go back to the selector (L/R/OFF) then you have lost some of this simplicity..also (and I gurantee this will happen) one day you will have the left tank selected on the selector and the right wingroot pump only running.

Now you are sucking through a dead pump at high flowrate (takeoff) and the associated risk of vapour lock.. For this reason I would limit any selector (and I'm not a great fan of them anyway) to dead pump ops in low power cruise, and that would require a "Both" position.

I mean one of the beauties of the WR system is that you have 100% of the fuel flowing from each tank..WR pump dies on TO and you won't even know it.

Taking things to the extreme......... with no fuel switch

I've crashed! The plane is inverted or nose down. Fuel is dribbling into the cockpit from a ruptured line at the firewall; as I attempt to knock out the canopy and free myself from the seat/shoulder harness.

I can't stop the flow from both fuel tanks, as I have no off switch.... ???

*************************************************

Or...............what about the ability to shut off the fuel flow, when you know you're about to pile in? Just as you'd do with the master switch.

Or............... fire in the engine compartment

L.Adamson ---- RV6A

 

[frankh]

There is fuel shut off

If you look at my system I kept the fuel selector but it is plumbed as a simple on/off valve, precisely for the reason of isolating fuel prior to a forced landing.

There is a 4th port on the standard Vans selector that allows it to be plumbed this way.

Note that for normal ops however the fuel "selector" is never touched, tanks are switched using the fuel pumps.


Frank

 

[frankh]

One other thought

Make sure that each individual tank/pump line is filtered...See the thread entitled "worm in my fuel system"..Yes I did plug a filter once and now have larger filters sitting on my bench awaiting installation.

Frank