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Tip: Electric fuel pump setup

frankh

frankh

Well Known Member
Ok here is the text describing the installation and operation of Fuel sysytem.

The usual disclaimers.....This is how i did my system..I have no idea of your skills at creating your own system so I want it understood loud and clear that In no way accept any responsibilty for you attempting to do something similar..



I have an electric pump set in each wingroot with no mechanical pump. This was an attempt at providing a ?hydraulically correct? pumping solution for use with auto fuel, maybe even auto fuel containing ethanol. This system overcomes the potential issue with Vapour Lock by placing the pumps at the low pressure point of the system and also avoiding heating of the fuel by heat conducted through the engine block or by placement in a hot environment, namely on the engine side of the firewall.

My system is for a FI?d engine, but a much simplified version can be used with a Carb by simply placing a Facet fuel pump in each wingroot. In this case no pressure reducing valve would be required.

The carb version of this system has been running in my Zodiac for 500 or so hours and the FI?d in the RV7a for almost 300 without failure.

FI'd system:

Each pump set is comprised of a NAPA pump (Part #2P74028), Pressure relief valve from Airflow Performance, a filter (upstream of the pump), a manifold welded together from fittings from my local hydraulics shop (you can also find fittings to thread together, welding is a more compact way to achieve the same thing). A check valve (Andair from Vans) is placed inside the cabin in each feed line to the "Vans selector valve".

Note even though I had QB tanks it was not difficult to plumb the extra returns to the tanks?I plumbed the pressure relief back to each tank, and the purge valve discharge back to the right tank. In order to fit everything in the wing roots I put flop tubes in both tanks?Oh Darn, guess I?ll have to do aerobatics now..:) . But you can imitate the same thing with solid tube if you really don?t want to hang in the harness.

The selector valve uses the 4th port so it becomes a simple on/off valve. The two feedlines from each wingroot pump set join together in a Tee..directly below the on/off ?selector valve?. In practice the valve is never oprated.

For TO and landing both pumps are run together, and tank switching is done by running the appropriate pump?Leaving the opposite pump off?Its dead simple in practice.

Make sure each pump is wired independently!!! That means a separate switch and separate fuse/wiring for each pump?No muti function switches which would be a single point of failure.

The two Andair check valves prevents cross feeding from one tank to the other.

The FI system is all AFP but will work equally well where the servo is OK with constant fuel pressure.

The following message will contain the pics. The only thing you can?t see is that I tapped out the ends of the fuel pump to 7/16ths UNF (for a standard #4 ORB fitting) this replaces the standard #3 ORB (at 3/8ths UNF)?.This is easily done on a drill press. Make sure to stuff some shreds of paper towel down the pump to stop shards of metal getting in the pump.

My filters are the cheap paper kind (metal jacket) but I will probably replace those with a sintered bronze filter (Summit racing) if I have to switch to ETOH fuel.

I also have a final filter in each feedline but to be honest this is overkill..well as long as a pump does not break a chunk of rotor of it is.

Thats the basic system and I'm delighted with it.

Frank

Pics to follow
 
Hi Frank,

at the time you wrote this I was about to send you PM about details of your system. I wonder why this didn't get more comments even on other threads I've seen some interest towards your system -- which you flawlessly seems to promote. :D

I didn't catch everything from the text/pics you wrote so I wish you would have time for some more details. Let others see what I've in my mind as well in case some others are lost as well. The engines (as I've said many times) are probably most weakest part of knowledge of building RV and therefore I'm keen to understand the choices and reasons why they are done.


You said that you didn't have mechanical pump at all. Does this mean that either one of your electric pumps is running all the time? Do you have dual battery? If electric pump fails (pressure drops) does engine keep running until you have time to switch another pump on?

As you have no mechanical pump I assume your only return to the tank comes from relief valve at the root of the wing? Just referring to the RV installation pictures of ECi engines at section 7.1 http://www.eci.aero/exp/RV6,7,9.pdf. Would you have similar plumbing diagram of your system?

I didn't catch the fuel valve thing as well as pictures doesn't show them at all. As you say that it's practically not used at all so why have it in first place? But it is located between T-tie and throttle body if I've understood correctly.

If one tanks runs empty and both electric pumps are turned on what goes to the engine (air or fuel)?

which brand and model of engine did you have? You already mentioned that FI was AFP's.
 
Answers

The engine is an IO360..A Mattituck to be precise...But to be honest the engine doesn't care...Both the AFP and the Bendix fuel systems are happy with constant fuel pressure and this is what this system provides.

Your right in as much as the electrical design of the pumping system is as critical as the plumbing...I.e the pumps must be wired independantly. I also have a duel alternator setup..I have an SD8 on the vacuum pad..Single Battery (My old carbed Zodiac used a dual battery with the same set up).

Why is there a valve?...No other reason than to provide a placebo for the DAR..Well thats a bit harsh,..I would turn it off prior to crashing!..Other than that its not used. Its plumbed as an on/off valve..it is NOT a selector valve.

The only return in practice is the return at the wingroots, except for the purge valve to flush hot fuel back to the right tank prior to a hot start..Actually works pretty well!

What happens if you run a pump dry?..I don't know I've never done it..Nor do i intend to!..Those roller vane pumps use fuel as a lubricant so running them dry sounds like a bad idea. An airlocked pump won't pump air, simply because the pressure it will generate will be no where near what the opposite pump will do when pumoping fuel..I highly doubt you would get any air going to the engine.

Remember you run BOTH pumps for TO and landing, the fuel pressure does not change if you run one or both pumps.

The engine quits pretty instantaneously if you turn both pumps off, the Dynon alarms maybe half a second before everything goes quiet.

Your questions are a little curious to me as I can't see how you would ever be in a stuation where both pumps are not running..The only unknown for me in flying this system is a freak lightening strike..I don't honestly know what would happen in this case.

Yes one or both pumps are running all the time just as it is in your car..

If you were cruising along and the pump quit...so will the engine..But the engine will never stop turning unless you pull it up into a stall, so the enigine quits, flip on the other pump and carry on flying.

If a pump quits on TO/landing you won't even know.

Experience has shown that a full vapour return like that shown on the Titan engine is not required from a fuel vapourising perspective.

Hope that helps.

Frank
 
Update

As mentioned in the system description at the last annual, I did change my pump inlet filters to the sintered bronze style supplied by Summit Racing. I did this in case any extra water carryover with the ETOH caused the paper elements to swell.

It is generally not considered a good idea to use paper fuel filters for this very reason, but as I had two independant filters I did not worry about it until forced to run ETOH blended mogas.

System still running flawlessly.

Frank
IO360 now featuring 10% ETOH
 
Frank, with the relief line plumbed in after the pump, there's enough backpressure there to get 25psi at the relief valve? I would have thought there would have to be some restriction in the relief line back to the tank. Evidently those Napa pumps put out quite a bit of volume.

The only thing I would be concerned about is what happens if you're empty on one tank, and you're running on the other and the pump quits. Most twins with complex fuel systems have cross-feed pumps, but I'm all for being simple, so I wouldn't want that.

I like the idea of eliminating the selector and no mechanical pump, but going this route would require some additional electrical system redundancy and complexity which I'm not crazy about either.
 
rocketbob said:
The only thing I would be concerned about is what happens if you're empty on one tank, and you're running on the other and the pump quits.
Click to expand...

The same thing would happen here as would happen if you had a standard fuel system, were empty on one tank, running on the other, and the MECHANICAL fuel pump quits. No advantage there...

I'm all for playing "what if" to find gremlins before they bite you - but reality needs to check into the game occasionally. If you're still in the air with one tank completely empty, it better be because you're already on final and screaming at yourself for being stupid and allowing your fuel state to get this low. If you switch tanks every 30 minutes and run one empty, you've got (at most) 30 minutes of fuel remaining. If you're not on the ground (or on short approach) at that point, you're screwing up in a serious way. This is a good point to remember that the FAR's specify 30 minutes fuel reserve PLANNED for daytime VFR, taking into account wind and weather for the entire route at the time of departure.

If you've got more than 30 minutes fuel in one tank, and you're empty on the other, you're either on a maintenance, ferry, or test flight - or you're practicing really really bad flying habits, in which case no level of redundancy is adequate.
 
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Pumps

Thanks for your question Bob,

The pumps on the bench put out about 35GPH at 35psi. This means the pressure drop through the return to the tank is negligible.

I see 35 psi on the left tank and 40 on the right...Of course when they both run together I see 40psi. The difference is in the return valves as i have swapped from one side to the other and the pressure change goes with the valves. No big deal as the AF system is good between 25 and 90 psi (from memory). The pressure is measured just aft of the firewall.

You have hit on the one downside of the system and that is you have to keep some fuel in each tank....So I never suck a tank dry...But even with 3 gallons you still have half an hour of flying on a single tank.

Typically I only get down to 3 gallons in each tank when arriving back home with plenty of airports to refuel at on the way.

The fuel pumps are very reliable too so lets say you foolishly run a tank dry..The chances of the other pump failing during that last 30 minutes of flight on one pump is statitically unlikely..But I wouldn't do it...:)


As to electrical complexity..Certainly you have to do what your comfortable with. On the RV it was easy because its an IFR bird so building redundancy into the electrical system was a no brainer..I have an SD8 backup alternator and the dual buss architecture from Bob Knuckolls..

For the VFR Zodiac I simply added a second battery that gave half an hour of flight by itself and charging from the main alternator via a diode....It worked well and was bone simple.
 
airguy said:
The same thing would happen here as would happen if you had a standard fuel system, were empty on one tank, running on the other, and the MECHANICAL fuel pump quits. No advantage there...
Click to expand...

No that's not correct. In the standard system you have a selector, a mechanical pump, and and electric boost pump. If the mechanical pump quits, you just turn on the boost pump which will pump thru the mechanical pump, and with the selector, you can switch to either tank.

In this setup, there is no selector, no mechanical pump, only pumps at the tanks. If the pump quits on one side, effectively the fuel in that tank becomes unusable. Unless you had crossfeed pumps which will pump fuel to the side with a good pump.
 
rocketbob said:
No that's not correct. In the standard system you have a selector, a mechanical pump, and and electric boost pump. If the mechanical pump quits, you just turn on the boost pump which will pump thru the mechanical pump, and with the selector, you can switch to either tank.
Click to expand...

Valid point, I concede to you. I also still say you should not find yourself in a situation with one tank dry as a normal course of flight. If you change tanks every 30 minutes, then you should be on the ground or very close to it by the time one tank is running dry. If you don't change tanks every 30 minutes, you are running a fuel imbalance between the tanks which could be very inconvenient in the event of an unexpected crosswind landing - not to mention the possibility of unporting your one and only operable fuel pickup in uncoordinated flight.
 
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frankh said:
T
The fuel pumps are very reliable too so lets say you foolishly run a tank dry..The chances of the other pump failing during that last 30 minutes of flight on one pump is statitically unlikely..But I wouldn't do it...:)
Click to expand...

I agree the likelihood of this being a problem is low, and I've had a mechanical pump failure and no electric pump failures, which makes me believe extra complexity to cover this situation is probably not warrranted. In my rocket since I now have dual electronic ignitions I am going to go with dual batteries, dual master switches and contactors, but with only one alternator. I've designed a unique OV protection circuit to work with an internally-regulated alternator, that is stone simple, but I need to test it first with a failed alternator turning on my test setup in my lathe. With a crappy old alternator :)
 
airguy said:
Valid point, I concede to you. I also still say you should not find yourself in a situation with one tank dry as a normal course of flight. If you change tanks every 30 minutes, then you should be on the ground or very close to it by the time one tank is running dry. If you don't change tanks every 30 minutes, you are running a fuel imbalance between the tanks which could be very inconvenient in the event of an unexpected crosswind landing.
Click to expand...

While I agree with you, for me I've had two occasions where weather and after hours fuel availability have dictated pushing things a bit, of course reluctantly and with great concern. I just always think about that Murphy guy in those situations, which would make a so-so situation much worse.
 
There you go

rocketbob said:
. In my rocket since I now have dual electronic ignitions I am going to go with dual batteries, dual master switches and contactors, but with only one alternator. I've designed a unique OV protection circuit to work with an internally-regulated alternator, that is stone simple, but I need to test it first with a failed alternator turning on my test setup in my lathe. With a crappy old alternator :)
Click to expand...


Actually you have all the extra electrical complexity you need to run the wingroot system right there....Just wire one of the pumps to run from your second battery..Easy
 
I'm mulling changing things over. I'm actually using a PS-5C pressure carb on my Rocket, for a number of reasons which I wont go into here, and I can buy low cost Facet pumps that will deliver 11.5 PSI, so I wouldn't have to run expensive pumps or relief valves. Simple and cheap, the more I think about it the better I like it...
 
Exactly

The little square solid state pumps (although I don't know of one that will make 11.5 psi) are internally relieved so no pressure relief valve is required..This is what I used on my carbed zodiac and the pumps made about 6psi.

Whats more i did not put filters in front of these pumps as they apparently pumped all kinds of stuff quite well. Thus they were even better than the rollervane pumps form a vapour lock perspective.

Depending on what pump you end up with you might put the filter before or after the pump.

A nice simple and very compact solution.

Frank
 
You'll

Probably find one will make moe pressure than the other..In my Zodiac this resulted in one of the pumps doing pretty much all of the pumping, so switching pumps was required to balance the fuel load.

Frank
 
Bob I used one of those high pressure pumps to pump oil on the smoke system that I built. Used it in Mark's F1 worked great. Cheap and light weight.
 
rocketbob said:
And I can't think of a reason not to run both at the same time all of the time, unless you run a tank dry...
Click to expand...

Running them both all the time will not give you warning of an in-flight fuel pump failure. You'll just be going merrily along until your out-of-balance wing condition becomes obvious to you, and then you've got control issues while trying to land. Running only one pump at a time while in cruise will make you instantly aware of a dead pump when you try to change tanks. At that point you're still far enough away from a serious wing imbalance to make a relatively easy landing.

The pumps are constantly pushing against their respective pressure relief before T-ing into the single line going forward, and if you have both pumps operating, the one that has the pressure relief set at the lower value (what are the odds they will be exactly the same?) will be pumping all the fuel, resulting in one light tank and one heavy tank.
 
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After a bit deliberation I've decided to go this route, and am going to pull the fuel selector out and not use a mechanical pump on the engine. Since I'm running the PS-5C pressure carb (poor man's throttle body injection), I need 9-14psi. I ordered some Facet 40185 pumps, which are rated at 9-11.5psi. These are the high pressure versions of the cheap and super reliable Facet boost pumps used in carb'ed installations. The plan is to install two of these cube pumps in each wingroot. These pumps have a built-in check valve, but I am adding additional check valves just like how Frank has his set up since one open check valve can cause a system failure. One or both pumps will be running for normal cruise, the other two are boost/backup pumps. These will be wired to the e-bus which is schottky-diode isolated from all power sources, so that in the event of a battery short/failure or main bus short power is still available and power is available to both ignitions and the pumps. Sounds complicated but it really is dirt simple the way I'm going to have things wired.

I like this system alot now that I think about it because it gets rid of the troublesome mechanical pumps, gets rid of the fuel selector, and all of the mickey-mouse plumbing required. One other nice with the cube pumps is they only draw 1.6A.
 
airguy said:
Running them both all the time will not give you warning of an in-flight fuel pump failure. You'll just be going merrily along until your out-of-balance wing condition becomes obvious to you, and then you've got control issues while trying to land.
Click to expand...

Greg I've flown my RV a handful of times with one tank full, other tank empty, to empty the tank before removing the inline filter on the empty side to clean it. There's no control issue, it does require full aileron trim and a little bit of continuous stick pressure. No big deal at all.
 
rocketbob said:
Greg I've flown my RV a handful of times with one tank full, other tank empty, to empty the tank before removing the inline filter on the empty side to clean it. There's no control issue, it does require full aileron trim and a little bit of continuous stick pressure. No big deal at all.
Click to expand...

Not having had the opportunity to try that, I'll have to bow to the voice of experience on that one. I did try it once in a 172XP with about 12 knots crosswind component on landing, and it was a little more than I wanted to deal with on a regular basis.
 
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rocketbob said:
After a bit deliberation I've decided to go this route, and am going to pull the fuel selector out and not use a mechanical pump on the engine. Since I'm running the PS-5C pressure carb (poor man's throttle body injection), I need 9-14psi. I ordered some Facet 40185 pumps, which are rated at 9-11.5psi. These are the high pressure versions of the cheap and super reliable Facet boost pumps used in carb'ed installations. The plan is to install two of these cube pumps in each wingroot. These pumps have a built-in check valve, but I am adding additional check valves just like how Frank has his set up since one open check valve can cause a system failure. One or both pumps will be running for normal cruise, the other two are boost/backup pumps. These will be wired to the e-bus which is schottky-diode isolated from all power sources, so that in the event of a battery short/failure or main bus short power is still available and power is available to both ignitions and the pumps. Sounds complicated but it really is dirt simple the way I'm going to have things wired.

I like this system alot now that I think about it because it gets rid of the troublesome mechanical pumps, gets rid of the fuel selector, and all of the mickey-mouse plumbing required. One other nice with the cube pumps is they only draw 1.6A.
Click to expand...

For sure it will work mechanically. From the electric point of view, are you going with 2 batteries? Not sure I understand "....the e-bus which is schottky-diode isolated from all power sources...."

With the Lycoming install, I decided to keep the 2 electric pumps from the Subaru system but got rid of a 15 pound battery and its associated wiring by keeping the mechanical pump. I guess it is a trade off. I learned today the mechanical pump becomes almost inaccessible once the exhaust system is installed. I forgot to hook up the drain line from it until today and it was some job getting the fitting and hose in.

Going electric has hits appeal but I will tell you it is not bullet proof. I had one engine shut down in flight with the Subaru due to a tripped poly fuse on EXPBUS and a friend had the same thing happen a couple weeks ago. In both cases the engine restarted by going to backup battery which uses different circuits but it sure gets your attention. I am presently looking forward to flying with an engine that will keep running if the electric system goes TU and am about 3 days away from firing the beast up. :)
 
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David-aviator said:
For sure it will work mechanically. From the electric point of view, are you going with 2 batteries? Not sure I understand "....the e-bus which is schottky-diode isolated from all power sources...."


Going electric has hits appeal but I will tell you it is not bullet proof. I had one engine shut down in flight with the Subaru due to a tripped poly fuse on EXPBUS and a friend had the same thing happen a couple weeks ago. In both cases the engine restarted by going to backup battery which uses different circuits but it sure gets your attention. I am presently looking forward to flying with an engine that will keep running if the electric system goes TU and am about 3 days away from firing the beast up. :)
Click to expand...

Ok a couple of points

1) Nothing is bullet proof no matter what it is. And as I pioneered this system I of course am biased..:)

2) This sytem uses two pumps running at the same time for take off an landing. If I remember correctly from the Egg system it used a duty/standby pump..I.e one pump running and if it quit a second one would start. I personally don't think this is a desirable solution because that that split second delay in a critical moment (like near the ground) can lead to sucking a lot of seat cushion!

3) You never rely on a single point of failure with this sytem...well you could argue the battery buss is a single point I guess but hard to see what the failure mode is. My system uses a switch for each pump and a seperate fuse of course and two alternators,..mainly because it flys in IMC.

4) Are the risks of vapour lock more than total electrical failure?..Good qustion, personally when running autofuel I think they are.

When I really looked at this I came to the conclusion that running a mechanical pump with two wingroot pumps added extra failure modes and complexity...and with high vapour pressure fuel was undesirable anyway.

remember the electric only pump solution is only as "risky" as a Lycoming running two electronic ignitions...if your electrical system goes TU you'll come down just as fast with two Ligthspeeds as you will with 2 electric fuel pumps. There are plenty of airplanes flying about this way and nobody seems to have a problem with it.

Frank
 
frankh said:
Ok a couple of points

1) Nothing is bullet proof no matter what it is. And as I pioneered this system I of course am biased..:)

2) This sytem uses two pumps running at the same time for take off an landing. If I remember correctly from the Egg system it used a duty/standby pump..I.e one pump running and if it quit a second one would start. I personally don't think this is a desirable solution because that that split second delay in a critical moment (like near the ground) can lead to sucking a lot of seat cushion!

3) You never rely on a single point of failure with this sytem...well you could argue the battery buss is a single point I guess but hard to see what the failure mode is. My system uses a switch for each pump and a seperate fuse of course and two alternators,..mainly because it flys in IMC.

4) Are the risks of vapour lock more than total electrical failure?..Good qustion, personally when running autofuel I think they are.

When I really looked at this I came to the conclusion that running a mechanical pump with two wingroot pumps added extra failure modes and complexity...and with high vapour pressure fuel was undesirable anyway.

remember the electric only pump solution is only as "risky" as a Lycoming running two electronic ignitions...if your electrical system goes TU you'll come down just as fast with two Ligthspeeds as you will with 2 electric fuel pumps. There are plenty of airplanes flying about this way and nobody seems to have a problem with it.

Frank
Click to expand...

The vapor lock issue is the #1 reason I prefer electric pumps with one running continuously. That area from the firewall to the mechanical pump is most vulnerable. Pressurized fuel coming through the firewall really secures that area. The early Subaru install had the pumps on the forward side of the firewall out of concern for a pressure line failure in the cabin but that risk soon paled after a couple guys had vapor locks where the fuel came through the firewall. Pumps are now in the cabin. I still don't like pressure lines in the cabin but it is IMHO less of a risk than vapor lock, and as you say, especially with mogas.

You are absolutely correct about the plumbing complexity with electric and mechanical pumps in series. The mechanical pump can not suck fuel through the electric pump so a by-pass with a check valve has to be in place.

I am not knocking your creation....just coming off the totally electric operation and currently feeling good about it. :)
 
David-aviator said:
The vapor lock issue is the #1 reason I prefer electric pumps with one running continuously. That area from the firewall to the mechanical pump is most vulnerable. Pressurized fuel coming through the firewall really secures that area. The early Subaru install had the pumps on the forward side of the firewall out of concern for a pressure line failure in the cabin but that risk soon paled after a couple guys had vapor locks where the fuel came through the firewall. Pumps are now in the cabin. I still don't like pressure lines in the cabin but it is IMHO less of a risk than vapor lock, and as you say, especially with mogas.

You are absolutely correct about the plumbing complexity with electric and mechanical pumps in series. The mechanical pump can not suck fuel through the electric pump so a by-pass with a check valve has to be in place.

I am not knocking your creation....just coming off the totally electric operation and currently feeling good about it. :)
Click to expand...


Nope totally understood and 900 hours of faultless operation is testament to the fact it works...I can understand your reluctance to go all electric again...but..

Jan Eggenfelner eventually threw me off his feedback forum when I was trying to point out design flaws with his system..

One of those reservations was this duty/standby pump setup...I.e I think it was a flawed system. (the other was the pumps north of the firewall)

back then I had several years of success with my carbed zodiac and this system. I suggested a redesign of the sube system that looks very much like mine does now..Jan dismissed it immediately..mainly i think because he did not understand it..Supposition on my part. But this is where the design for the FI'd setup came from..yes I was considering an Egg engine at the tim.

But it was a wingroot mounted pump with the pressure relief at the pump..The challenge with the soob was this need for the continual return from the firewall..Well I still don't think that is necessary and the Lycoming seems to have proven that to be the case...Would also avoid the need for the 6 port Andair selector valve.

Anyway having two pumps that can run continuously is a far better solution I think..as I guess you discovered..Not that a pump failing at altitude won't get your attention, but at least its not dangerous.

Frank
 
David-aviator said:
Not sure I understand "....the e-bus which is schottky-diode isolated from all power sources...."
Click to expand...

The system is two Odyssey PC-680's, two master contactors, two battery master switches, one alternator. Then onto the main bus fuseblock, a switched avionics fuseblock, and an essential-bus fuseblock. The e-bus, which only has fuel pumps and ignitions, is fed off of the main bus via one side of a Ixys DSS2x121-0045B schottky rectifier. The e-bus alternate feed switch is an SPDT switch with both sides of the switch going to each battery, with another Ixys rectifier (which has two rectifiers in one package) in between the switch wires going to either of the two batteries.

So if I had one battery dead and the master off, the e-bus rectifier will isolate it, and prevent back-feeding it.
 
Thanks for all the effort it took to share this. I'm very pro-auto fuel and leaning toward all electric so some day way too far in the future I'll be looking at a system like this. It seems very nicely done and thoroughly what if'ed within reasonable failure modes.

Question, hydraulically speaking, how much safer is it from a VL perspective to have the pumps in the root vs the tunnel forward of the stick?? How much higher would the pump sit, ie how much higher a hill would the fuel have to be sucked up to reach the pump if located in the tunnel. The concern being of course VL pre-pump.

Other reasons for wing root vs tunnel?

Thanks much
 
wingroot vs tunnel

Well not THAT much difference to be honest..The line sizes are relatively large and while there are a few bends and elbows hydraulically speaking the losses would be small. The other advantage to the tunnel is that you could retain the selector valve and suck from either tank.

However...(There is always at least one of those) if you were slow in changing from one tank to the other you could in theory vapour lock one or both of the pumps...

Also the because whenever you suck on fuel you are dependant on not sucking so much that you reduce the pressure on the fuel to below the vapour pressure of the fuel, so if something whacky like you trod on a fuel line for example..you could still end up in a VL situation.

The engineering purist in me says, "If I'm going to do it right..Lets do it right" I.e place the pumps as close to the source as possible.

It also removes the selctor valve which is what I prefer.

But you could certainly design a system where both pumps were more conveniently located...Albeit you now have to run the return lines to the tanks if the engine is fuel injected.

Frank
 
Electric Fuel Pumps continuously running

I have FI using a standard Bendix system on a 200 HP Lycoming. No return line is needed.

I have both a mechanical pump on the engine and NO pump on each wing.

What I have found in 525 hours is that the engine pump will PULL the fuel from each wing. I only use the electric boost pump in take off and landing. Unlike your system, I only have one electric pump and my feed is either left or right tank.

I guess what this all proves is there are more than one ways to skin a cat. My goal was to achieve the simplest system as possible. So there are no cross feeding, or check valves, etc.

At first I thought it wouldn't work without the electric pump running, but I soon found on ground tests, that wasn't the case and with only 5 gals of fuel in the left wing, it (the mechanical pump) pulled the fuel fine and the fuel pressure was 26 psi sitting and running on the ramp.

The only problem I have had so far, is the electric pump (which I don't use all that much..go figure) died after only 400 hours tach time. And it was one of those expensive pumps to boot!

Tom Hunter
 
Your design is the standard setup, it will work just fine, and does every day for thousands of aircraft.

The problem with that setup, that we are trying to avoid, is the vapor lock exposure caused by the pump PULLING the fuel from the tank up to the pump. This produces an area of low pressure on the intake side of the mechanical pump, which also happens to be a high-temperature area due to heatsoak from the engine block. These two factors both combine to push the fuel closer to vaporizing and reduce the margin of safety. To eliminate that issue, you need to either remove the heat source or increase the pressure.

Putting the fuel pumps in the wingroot instead of the engine block accomplishes BOTH and results in the same steady fuel supply to the engine. It also eliminates the need for a fuel valve to switch tanks - some say this is a good thing, some say bad - personal choice there.

Turning on your fuel boost pump will accomplish the same thing as using a wingroot pump - deliver pressurized fuel into the FWF to prevent vaporlock, in addition to covering against a mechanical fuel pump failure at low altitude. Trouble is, as long as you've got the heatsoak exposure from the mechanical pump, your at some non-zero risk of vapor lock. Taking that pump out of the equation just adds more safety margin and brings you back to the same level of complexity as the standard system. Both methods use two fuel pumps, if one fails the other one saves you.
 
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Frank, this is a five year old thread that is still providing excellent information! Thanks for all the time and patience in describing the system. I'll be following your lead for my -10 in the coming years to help mitigate cost, LL worries, and help out the tree huggers!

For the return line, where does it come off of the FWF area? I'm imagining just before the servo for the FI system? Then what does it look like, any need for a check valve or just run another line back to a tank? Should the exit in the tank be low or hi or does it matter?
 
It depends on what type of pump you have, but the majority of electric fuel pumps actually use the fuel itself to cool the sealed electric motor as it flows by. Since the pumps move more fuel than the engine needs, some of it goes forward to the engine and the rest is bypassed via a return line. You don't want to take that return line and just route it back into the suction of the pump, since you'll end up simply recirculating the majority of the fuel during low-power operation and it will get hot as it makes repeated loops through the pump cooling the electric motor each time - that's exactly the problem we are trying to get away from. A return line back to the tank for the bypass flow solves this problem, allowing the return fuel to mix with the fuel in the tank. On my airplane I took that return line and fed it into the second bay in the tank, so that I wouldn't have any problems with splashing or foaming of the fuel in the first bay where the fuel pickup is.
 
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