Fuel return line.

[pierre smith]

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,

[TSwezey]

Originally Posted by pierre smith

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,

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.

[N395V]

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.

[David-aviator]

Originally Posted by N395V

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.

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.

[frankh]

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

[Mike S]

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.

[Don at Airflow]

Originally Posted by Mike S

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.

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

[frankh]

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

[APACHE 56]

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

[AllanC]

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