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Fuel return line.

N395V

N395V

Well Known Member
All the production planes I have flown with FI have a return line to the fuel tanks.

My F1 doesn't and has had no problems.

What exactly are the benefits of the return line and what are the dangers/problems associated with not having one?
 
Mornin' Milt

As I recall, most FI systems provide way more fuel flow than is needed, so it's returned. On my Cessna 310, it was returned to the tips so you had to be sure and run enough fuel out of them or the returned fuel would go overboard if they fill up while on the mains.

The IO-520's we ran in the Agtrucks also returned and they had a hot-start problem. You could lean the mixture, run the fuel pump, returning the heat-soaked warm/hot fuel in exchange for cooler fuel from the tanks to ease hot-starting woes.

BTW, are you about ready to fly the Rocket again?

Not sure how your non-return system is plumbed.

Regards
 
BTW, are you about ready to fly the Rocket again?
Not sure how your non-return system is plumbed.
Click to expand...

Probably have 4-5 months to go but I am at the point of running the plumbing to the tanks before I close the wings. Thus my question. My concern is my engine failure was due to vapor lock or more correctly fuel vaporizing from heat and a sudden reduction in fuel pressure during throttle back.

Currently in my F1 and in the Radial the fuel system is an AFP electric pump system at the fuel tank sump to the engine driven pump then into the AFP fuel controller.

I am wondering if I could have avoided the problem by teeing the fuel line at the controller and running the fuel past the engine instead of just into it.

My 414 Cessna was plumbed like your 310 and sop was boost pumps on "lo" all the time.


The F1 has the standard Multiport FI. On the Radial I just replaced the pressure carb with the AFP single point fuel controller.

The M14 community with pressure carbs is split as to need for a return line some have it some don't. No problems have been reported by either group. Unfortunately there are only a few of us running the AFP system and I do not know what they are doing. Mostly in competition aerobats.

Anyway once I am back in the air the Louisville Huddle House is high on my early trip list.
 
There is good stuff in the archives about this.

Don R. at Airflow Performance sums it up well here.

The thread from which the above came is here.

The answer to the original question in this thread is, it depends on which system.
 
AlexPeterson said:
There is good stuff in the archives about this.

Don R. at Airflow Performance sums it up well here.

The thread from which the above came is here.

The answer to the original question in this thread is, it depends on which system.
Click to expand...


Thanks Alex,

I have spent almost an entire day reading those threads and others and they really did not answer my question. Most of the discussion is on routing, placement of components of the system not muh substantive on purpose of the system.

My F1 Rocket has an AFP, no purge valve, no return line so doesn't fit Don's categorizations. The Radial is a different animal. I talked to Don about it on the first install and he was ambivalent about the need.

Perhaps I worded my initial query inncorrectly.

What is the purpose/theory behind return lines? Are these theoretical "nice to haves", convenience items, or absolutely essential to life requirements?

Or, if you will,wif they are nice to haves, what is the benefit derived from their presence? If they are absolutely must haves, why?
 
My understanding for the return line is to maintain a constant pressure to the engine and to help reduce the chance of vapor lock when idling down. Our return line comes off the pressure monitor and returns back to the tank it comes from through a six-port fuel selector valve.
 
FWIW, I know zero about FI systems for aircraft...but I deal with them alot on marine engines so the principals should be similar. That said, the choice of a bypassing system or non-bypassing system is usualy determined by the fuel pump design. Some are designed to be run to a dead head, however most require the use of a bypassing regulator so that the excess fuel is constantaly circulating through the system. It really doesnt matter what motor the system is feeding...the injector servo or rail really doest care as long as it has the proper PSI and flow volume. In theory, the advantage of a bypassing system is that by placing the bypassing regulator as close to the motor as possable, the fuel has spend the least amount of time in the hot area of the engine bay absorbing heat. Also, fuel is always flowing through the fuel pump at a high volume so its is not sitting stagnet in the pump, being heated up, when fuel demand is low. In our marine systems, our pumps typicaly can pump twice the necessary volume at WOT, and at idle the majority of the fuel is being bypassed.
 
Fuel return line

The benefits of a preventive method are hard to quantify.
We do know that a great many engine failures cannot be explained for lack of evidence.Vapor lock is exactly one of the failures that leaves no evidence
behind.
You may fly without a return line for 98% of your flying without a problem but one day you will be idling when temps are 100F while waiting for take off and cook the almost stagnant fuel in the overheated engine compartement.The ensuing take off will be the most likely point and time for the engine to spudder. Just when you need it most!

A constant flow fuel return line ensures that a constant flow of cool fuel is introduced to the fuel system and thus prevent the fuel from boiling.The fuel return line is more a function of the fuel system rather than a component of a certain kind of fuel injection system.
My 8 has an AFP injection system and since it already has a return line for the purge valve it was very easy to plumb a constant flow return line into this line. I teed off the engine driven fuel pump with a #70 orifice which returns 9GPH .(as tested a@25psi) The outflow from the Fuel pump is plumbed back behind the fire wall where fuel flow and pressure is measured just before entering the fuel meter.
I believe vapor lock can be prevented
 
OK it all makes sense. I really think vapor lock or boiling fuel is what knocked me out of the air last year and I really do not want to go through that again..

A couple of Murphy Moose operators have teed off the engine driven fuel pump thru a number 30 drilled orifice into an inverted metal gascolator the back to the tanks. Does this sound like a reasonable thing to do? (bearing in mind it is a pressure carb or single point FI not multiport FI)
 
Fuel return with FI

N395V said:
OK it all makes sense. I really think vapor lock or boiling fuel is what knocked me out of the air last year and I really do not want to go through that again..

A couple of Murphy Moose operators have teed off the engine driven fuel pump thru a number 30 drilled orifice into an inverted metal gascolator the back to the tanks. Does this sound like a reasonable thing to do? (bearing in mind it is a pressure carb or single point FI not multiport FI)
Click to expand...

Milt,

I think they used a .030 orifice to run the return fuel back to the tank not #30 (which is .128 dia.). With a .030 orifice teed into the outlet of the engine driven fuel pump, this should circulate around 6-8 GPH at 25 PSI fuel pressure and 8.5-10 GPH at 50 PSI through the pump in addition to what the engine is using. The additional 6 GPH should take a fair amount of heat out of the pump, and should help keep the engine driven pump from vapor locking if the boost pump is not on and the engine is at low power. There should be no problem with pump capacity at high RPM?s but you will have to make sure the engine driven pump can keep up the pressure at idle, since you will be siphoning off an additional 6 GPH of the pump capacity at idle. Obviously you will now have some fuel management issues to deal with if you are returning the fuel to one tank and you have multiple tanks in the aircraft. This should not be a problem if you know what?s going on. Email me off line at our address if you have more questions on this subject.

Don
 
Placard

Don thanks for the very informative post.

Milt: I'd do exactly what Don described but I'd also add a placard.."Use left tank for 30 MIN first before switching to right tank" or similar , in order to make room for the returned fuel. In the future, someone else may own the airplane and it may also serve as a reminder to you that if this instruction is not followed, your fuel will be going overboard (fire?) if it's returned to the left tank and your range will be miscalculated as well.

Regards,
 
pierre smith said:
Milt: I'd do exactly what Don described but I'd also add a placard.."Use left tank for 30 MIN first before switching to right tank" or similar , in order to make room for the returned fuel. In the future, someone else may own the airplane and it may also serve as a reminder to you that if this instruction is not followed, your fuel will be going overboard (fire?) if it's returned to the left tank and your range will be miscalculated as well.

Regards,
Click to expand...

You should use a six-port fuel selector valve which will automatically put the fuel back in the tank you are taking it from. Doing it any other is like playing Russian roulette. There is no way to accurate calculate your fuel flow rate to the opposite tank. Or you could have two fuel selector valves. One for the fuel to the engine and one for the fuel back to the tanks.
 
Spent several hours on the phone yesterday 1st with the M14 importer then Don at AFP and derived much good and interesting info.

It appears in the last 2 years 5 or 6 M14 powered aircraft (including mine) have gone down due to un explained power loss. The one thing they all had in common was no return line.

After talking to Don and the importer I am convinced the line is essential mainly to provide added fuel flow for cooling of the pump and lines and to decrease the amount of time fuel spends in a hot environment.

My particular installation has only one tank which is the entire leading edge of the one piece wing and the 6" aft of the spar.

So based on Dons advice I am going to run a return line from the pump to a tee and then through 2 .0150 orifices out to each wing tip so as not to get an imbalance. While it is one contigous tank there are several baffles to keep fuel shift to a minimum and I suspect return to only one side will give me a heavy wing.

thanks for all the input.
 
N395V said:
Spent several hours on the phone yesterday 1st with the M14 importer then Don at AFP and derived much good and interesting info.

It appears in the last 2 years 5 or 6 M14 powered aircraft (including mine) have gone down due to un explained power loss. The one thing they all had in common was no return line.

After talking to Don and the importer I am convinced the line is essential mainly to provide added fuel flow for cooling of the pump and lines and to decrease the amount of time fuel spends in a hot environment.

My particular installation has only one tank which is the entire leading edge of the one piece wing and the 6" aft of the spar.

So based on Dons advice I am going to run a return line from the pump to a tee and then through 2 .0150 orifices out to each wing tip so as not to get an imbalance. While it is one contigous tank there are several baffles to keep fuel shift to a minimum and I suspect return to only one side will give me a heavy wing.

thanks for all the input.
Click to expand...

Milt,

Sounds like you've decided what to do and are about to do it.

I know this is late but for what it is worth, a loop return did not work with the FI Subaru because the fuel was not adequately cooled in all circumstances. Even running the loop outside the engine compartment did not provide adequate cooling. The only thing that really works is returning fuel to a tank or tanks.

The system we use is directed by a 6 port Andair valve and returns fuel to the supply tank. Fuel management is stone simple. It can not be screwed up. Returning fuel to a single tank or header tank must be managed as the by pass fuel sometimes exceeds fuel burned, sometimes by a lot. At one point, I measured a by pass of 35 gph with the 4 cylinder engine. If all that fuel were returned to a single tank, it would require attention by the pilot.

I do not know your fuel system so this may be useless information, but it may be worth thinking about returning fuel to the supply tank - maybe that's what you are doing by running it to the tip tanks through a separate valve. The 6 port Andair would eliminate the need for another valve but may not be practical at this point with your airplane.
 
AFP

Hey Milt,

Just a tought but have you considered running two electric pumps and no mechanical pump?

I do this on my IO360 and it has an AFP system (my pumps come from NAPA and I use one of Don's pressure relief valves at the discharge of each pumps that circulate back to its own tanks...My pumps are in the wing roots.

I can run one or both pumps togther and this gives me a constant 30psi or so.

The reason I mention this is that your fuel pressure is going to be dependant on the engine RPM..As you said, pull to idle, fuel pressure goes away and fuel boils.

Seems to me having a guaranteed way of maintaining 30psi under all conditions is not a bad idea.

I think I would be tempted to do this and then insulating the lines North of the firewall would be a lot easier.

Let me know if I can help further...The only return I have is through the purge valve...which is rarely used.

Frank
 
Half isnt always half

Hey Milt, I think you need to rethink this a bit.

Don's post mentioned a .030" return orifice.

You are now talking about two .015" orifices--------half of the size Don mentioned.

Seems logical-----two holes half the size should be equal to the one larger hole, right??

But, in fact when you double a hole, you get four times the area, and theoretical flow. And the smaller size orifice you are working with, the flow drops even faster due to friction loss, so the 1/4 area actually equals less than 1/4 flow.

You probably need to go to .020" or maybe even a tad bigger if you want to keep the same flow as a single .030" orifice. Sorry, I dont know the exact size, (all my flow charts are for fire hose/nozzles) hopefully one of the engineer types on here can help.
 
Two Orifices

Mike S said:
Hey Milt, I think you need to rethink this a bit.

Don's post mentioned a .030" return orifice.

You are now talking about two .015" orifices--------half of the size Don mentioned.

Seems logical-----two holes half the size should be equal to the one larger hole, right??

But, in fact when you double a hole, you get four times the area, and theoretical flow. And the smaller size orifice you are working with, the flow drops even faster due to friction loss, so the 1/4 area actually equals less than 1/4 flow.

You probably need to go to .020" or maybe even a tad bigger if you want to keep the same flow as a single .030" orifice. Sorry, I dont know the exact size, (all my flow charts are for fire hose/nozzles) hopefully one of the engineer types on here can help.
Click to expand...

Actually two .0212" diameter holes would be the same as one .030" diameter hole.

And what you would want to do is size the two orifices so that the fuel pressure at idle did not drop below 20-25 PSI. This would allow the largest amount of fuel to recirculate. So it might be possible to use larger bypass orifices than .021".

DOn

Don
 
Yes

An orifice with the mechanical pump will presumably allow the fuel pressure to drop even further when the motor is pulled to idle....Incidently what does the fuel pressure drop to???? Do you have a guage?

The fundamental vapour lock problem in this case appears to be a combination of loss of pressure and heating of the fuel.

A return sort of helps the second one but makes the fuel pressure loss even worse.

I think finding a way to maintain pressure under all conditions is fundamentally a better path to take, together with better insulation.

Of course the mechanical fuel pump is also relatively large so is hard to stop it absorbing heat form the engine as after all it is bolted directly to the engine block.

Thus removing the pump altogether seems a great idea and we know an AFP system works great with constant pressure.

Indeed the AFP servo can be run on pressures up to 90psi....Not sure I'd want it that high but 30-40psi works great.

The downside to my system is that it is a lot of work and your airplane now becomes electrically dependant...But with a backup battery or alternator is quite a managable risk, even IFR

Frank
 
FI return line in QB fuel tank

ISSUE: I will soon have QB wings with fuel tanks completed. What is the best (assuming feasibility) of adding a fuel injection system fuel return line (vapor lock mitigation)?

I am considering pros and cons of adding a fuel return line for all the reasons discussed in the various links and threads on this topic. My query to Vans brought the response that "you can't add return line to QB tanks." I don't think that is true as I've seen some pretty dramatic aircraft mods over the years and adding a fuel return line seems to rank at the bottom of the tech complexity scale. So I'd like to solicit advice/consel as to not only "how" but if it is necessary with a particular brand of FI.
thanks,
Don
 
Don,

All of the Subaru users, myself included, have fuel return lines through the Andair 6 point fuel valve to the respective tanks, and mine is a QB wing. Its very simple. Just an appropriate size bulkhead AN fitting on the outside wing rib, and on the inside, accessible through the fuel sender plate hole, a fuel line extension to the 2nd or 3rd bay. This ensures mixing and cooling of the returned fuel. Of course, this then requires a feed and return line from the valve to each wing. Easily done when your wings arrive and are in their cradle. Just keep the shavings out!!

Allan
 
check out the ECI website

APACHE 56 said:
ISSUE: I will soon have QB wings with fuel tanks completed. What is the best (assuming feasibility) of adding a fuel injection system fuel return line (vapor lock mitigation)?

I am considering pros and cons of adding a fuel return line for all the reasons discussed in the various links and threads on this topic. My query to Vans brought the response that "you can't add return line to QB tanks." I don't think that is true as I've seen some pretty dramatic aircraft mods over the years and adding a fuel return line seems to rank at the bottom of the tech complexity scale. So I'd like to solicit advice/consel as to not only "how" but if it is necessary with a particular brand of FI.
thanks,
Don
Click to expand...
ECI has some detailed directions that include pretty good pictures on how to install the fuel return lines. Their instructions even include a parts list for everything you will need to make it work. As AllenC mentioned, it is not a complicated process as long as the tanks are off the wings. Their design has the return line traveling through the Z-brackets that attach the tank to the spar. It is a clean efficient path that I think will work well with any tank whether quick built or not.

Here is the website you can find that information from ECI http://www.eci2fly.com/exp
 
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