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Plumbing fuel return lines through fuselage just in case

Draker

Well Known Member
For those of us who are putting off the engine/fuel injection system decision until later in the build:

I read a whole bunch of topics here which suggested adding 3/8" fuel return fittings to tanks while building them, to future-proof in case you end up going with a FI system that requires return lines. The rationale is it's much easier to do this during the tank build rather than after the fact. If you end up with a fuel injection system that does not require a return line, you can simply cap off the lines at the tank. Also noted that most systems do not require return plumbing.

Now, same question, but during the fuselage build! For those of us who still have not made up their minds, does it make sense to drill extra holes in the side skins, and plumb (potentially un-needed) return lines through to the valve area? Does the same rationale apply, that it's easier plumb these now, instead of when the fuselage is put together?

Keep going upstream: Why not install a duplex fuel selector valve (at extra cost) now, "just in case"?

How much future-proofing makes sense and at what point does the extra system complexity, potential leaks, and extra cost stop making sense? I'd like to avoid re-work if I eventually choose, say, an EFII or similar system, and I'd rather do plumbing while I have great access inside the cabin.

EDIT: Also, not trying to debate the merits of different FI systems here. Just whether or not all this future proofing is worthwhile.
 
Ryan,

Given the fair amount of work (time) to implement return lines ?just in case?, which could be wasted by later picking one of several systems which do not require them, I would suggest making a decision now.


Or, before the wings are on it may not be ?that bad? to add lines. At least visualize how you would run them so you don?t put other stuff in the way.

dave
 
yes

I can say that plumbing a duplex valve is not a small task. It is not just "the return line".

At some point you will need to decide...
 
I use a return line to flush hot fuel out of the circuit after a heat soak. Glad I had it the one time I used mogas. It was about 100* an I was having major vapor lock issues after a 15 minute stop.

I run it through a .040" orifice to keep flow under the pumps capacity.

Larry
 
I added a fitting in the tank but had a strong feeling I wasn't going to go with return line due to engine choice. I agree with some others, you probably need to make a choice on what engine you are going with. I decided to keep it stock and go with the M1B from Van's. I like the idea of having somewhat more clear direction from Van's than if I had chosen something wild. Long story short, I put a fitting in the tank but nothing in the fuselage.
 
I understand your dilemma, but I think you?ll save yourself a lot of time, money, and aggravation if you decide on a particular fuel system now, during your build. The EFI/EFII systems aren?t difficult to plumb during the build and can certainly be retrofitted later on if you choose, but I think you?ll be way ahead in the game if you decide on the type of fuel system you want now. Tom, (TS Flightlines) makes a nice fuel line kit to install the return line setup for the EFI/EFII systems if you decide to go that direction. I would say at the very least go ahead and install the return line fittings in your fuel tanks and cap them off.

Mark
 
I can say that plumbing a duplex valve is not a small task. It is not just "the return line".

Yes... the lines in the fuselage and all that (including the duplex valve) are not fun.

Putting the fittings into the wings isn't that bad at all, and easier done during your tank build. If you even think you might be using a return system, do it then. The cost (in time and dollars) is low.

For the fuselage, you have a little while to decide (you can flip the canoe, do your aft upper skins and some other work first). I'm at the point now where I need to start running those lines, so I've been putting in a lot of skull sweat and CAD time trying to fit the duplex valve in without using hard 90 fittings or drilling rivet holes into the spar web. It's cost me a lot of time. But, I decided a long time ago to go with an SDS EFI system.

Just my opinion as the guy working through that right now, but you can wait up until your fuselage is flipped and you've run your flap control and done some of your pitch/roll control installation. From there you start working forward upper structure/panel and your closeouts and you need to make your decision by then.
 
I put return line in early in the build. Fuel system does not require it but I?ll still be able to flush heat soaked fuel before a hot start.
Plumbing these lines on a completed -8 would be a major nightmare. If you think they might be used, do it now.
 
Ryan, putting the return fittings in the tanks is the easy part, especially if you havent built them yet. Even if you have, Vans was nice enough to make VA141 flanges with NPT threads for fittings. Saves making a hole for a bulkhead fitting and having to use a AN914 lock nut on the back side---especially if the tank is already sealed up.

The cabin plumbing is different. Duplex valve, extra plumbing, so if its a future consideration (maybe) then you might want to hold off until that time comes. You're going to have alot to do anyway changing systems.

Tom
 
I've already added the return lines to my tanks, and have them capped off. I'm also strongly leaning towards a standard, by-the-plans XIO-360-M1B from Van's, and will very likely not get creative with the engine system. This discussion has convinced me to not horse around with extra work inside the fuselage unless I find I actually need it.
 
Go for it!

I would strongly encourage installation of a 3/8” fuel return fitting in the tanks during tank build, even if you end up flying with a plug in it. I’m in the position now of needing to install return lines in a completed 7A without removing wings or tanks, and it will be a real challenge. The cener of the raised circle of the inboard rib, forward of the sending unit installation, offers an ideal location.

After considerable study and thought, I have concluded that all high-pressure boost pumps that re-circulate bypass fuel to the pump inlet, either externally or internally, are inherently dangerous. This opinion will no doubt generate vociferous disagreement as many available systems are set up this way, but I’ll be making detailed arguments in its favor in a VAF thread before too long, along with documentation of my retro-fit installation.

Much better to have the fitting and not need it than need it and not have it.- Otis
 
I've already added the return lines to my tanks, and have them capped off. I'm also strongly leaning towards a standard, by-the-plans XIO-360-M1B from Van's, and will very likely not get creative with the engine system. This discussion has convinced me to not horse around with extra work inside the fuselage unless I find I actually need it.

Ryan,

That?s great that you?ve already installed the return fuel fittings. I think you?ll find that having a way to to return fuel will come in VERY handy, regardless of what type of fuel system you ultimately decide to go with. I?m installing the full EFI/EFII setup in my build, so I needed the full duplex/return line setup. My buddy is running an Ellison T.B. on his -7. After experiencing a bad case of vapor lock, he plumbed in a 1/8? return to one of his tanks. All he does is turn on his electric pump and open a little bypass valve which ?sweeps? out all of the hot fuel. The return line is small enough that it doesn?t effect his fuel pressure, so even if he were to forget the bypass valve open there?s no danger in getting into a low fuel pressure situation. The electric or manual fuel pump can hold sufficient pressure with the bypass valve open so there?s never a safety issue. This little modification has completely eliminated his vapor lock problem. So, once again, I think you?re already ahead in the game for installing those return line fittings.

Mark
 
I ended up needing a return line to prevent vapor lock on my Rotec TBI. Used 1/4" lines instead of 3/8. Had to add the lines later. Adding the fitting to the tank was more work than running the line through the fuselage.

I see from the previous poster that 1/8" would probably have been big enough.
 
After considerable study and thought, I have concluded that all high-pressure boost pumps that re-circulate bypass fuel to the pump inlet, either externally or internally, are inherently dangerous. This opinion will no doubt generate vociferous disagreement as many available systems are set up this way, but I?ll be making detailed arguments in its favor in a VAF thread before too long, along with documentation of my retro-fit installation.

That's a pretty bold statement, given the number of such systems installed in innumerable aircraft...I look forward to reading your detailed arguments soon.
 
Does anyone have any pictures of how and where you installed your return lines in your tanks? Size also?
 
lines

If you are considering an EFII system any time in the future, the return lines should be 3/8", same as the supply lines...
 
That's a pretty bold statement, given the number of such systems installed in innumerable aircraft...I look forward to reading your detailed arguments soon.


The biggest problem is that this type of return introduces air bubbles to the incoming fuel. Return lines are best if you need a return fuel flow.
Curious as to what aircraft are running this type of return? Are they certified?
 
Does anyone have any pictures of how and where you installed your return lines in your tanks? Size also?

Here are a couple of shots of mine, RV-7. Not saying others should do it this way, but it's how I did it.

Inboard bay, showing return line, fitting, vent line, and wire for capacitive sender:

IMG_0024-M.jpg


I ran them all the way to the outboard-most bay, where the caps are:

IMG_0018-M.jpg


Here's the fitting on the outside of the inboard tank rib, capped off because given my engine choice I'll likely not need the return line after all!

IMG_0036-M.jpg
 
Thanks Ryan. I'm building an RV9A.
Would it be OK to just install a bulkhead fitting in the access panel, without running a line all the way out to the filler neck?
 
Thanks Ryan. I'm building an RV9A.
Would it be OK to just install a bulkhead fitting in the access panel, without running a line all the way out to the filler neck?

Marshall,

I think it’ll be just fine to install a return line fitting in the access panel. The main advantage to extending the return line to an outside bay is to dump the warm return fuel as far away from the pick-up/supply fuel inlet as possible. This gives the warm return fuel time to mix with the rest of the fuel in the tank and get cooled down before it’s picked back up again.

With a fully circulating fuel system like what’s used in the EFI/EFII setup, I’d be curious to know the actual temperature of the returning fuel?? I could be wrong, but I think as fast as that fuel is moving around the fuel rail that it doesn’t have time to pick up an excessive amount of heat.

What is your primary reason for wanting to install a return system? Are you planning on an EFI/EFII setup where a return line is mandatory?....or are you going with a traditional aviation type of fuel system architecture (fuel injected or carbureted) where the return line is there primarily just to “sweep” out the hot fuel to prevent vapor lock?

Regardless of the type of fuel system you’re installing and/or the reason you want/need a return line, if your tanks are already built I’d think that an access panel mounted return fuel line fitting would be just fine. If you’re still building your tanks, than extending that return fuel line so the warm fuel is dumped into an outside bay certainly wouldn’t hurt anything.

Mark
 
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Thanks Ryan. I'm building an RV9A.
Would it be OK to just install a bulkhead fitting in the access panel, without running a line all the way out to the filler neck?

I did not have access beyond the first bay without rebuilding the entire tank, so dumped in the first bay. She has not flown so if this is a bad idea please chime in.
 
I ran mine out to the outboard bay because I was building the tanks anyway and there was no difficulty to get it out there. I don?t know if I?d have even bothered with a return fitting if my tanks were already done. My personal opinion is that it becomes an ?only if I really end up needing it? mod once the tanks are sealed. I don?t want to open those dang things up ever again!
 
Return line

"With a fully circulating fuel system like what?s used in the EFI/EFII setup, I?d be curious to know the actual temperature of the returning fuel?? I could be wrong, but I think as fast as that fuel is moving around the fuel rail that it doesn?t have time to pick up an excessive amount of heat."


I just reviewed the Fuel Flow Report for the last EI/EFI system I installed (SDS system). Fuel flow tested was 40 gallons per hour through the system & measured just ahead where it would dump back into the tank. That flow rate would suggest it takes approx 22 seconds for 1 quart to pass through any one spot in the plumbing. If the fuel lines FWF are fire sleeved (insulated), I doubt if the return fuel picks up more than a few degrees while passing through a well installed system. My un-scientific test was to hold the return line plumbing under the seat while the engine ran, it was not much different in temp than the feed line.

In my opinion, installing the return tank fitting on the access cover is perfectly fine. If you are concerned about air bubbles near the fuel pick up, use a 90 degree bulkhead fitting & aim the returning flow to the front of the tank.

Don't make the tank return more complicated than it has to be.
 
Marshall,

I think it?ll be just fine to install a return line fitting in the access panel. The main advantage to extending the return line to an outside bay is to dump the warm return fuel as far away from the pick-up/supply fuel inlet as possible. This gives the warm return fuel time to mix with the rest of the fuel in the tank and get cooled down before it?s picked back up again.

With a fully circulating fuel system like what?s used in the EFI/EFII setup, I?d be curious to know the actual temperature of the returning fuel?? I could be wrong, but I think as fast as that fuel is moving around the fuel rail that it doesn?t have time to pick up an excessive amount of heat.

What is your primary reason for wanting to install a return system? Are you planning on an EFI/EFII setup where a return line is mandatory?....or are you going with a traditional aviation type of fuel system architecture (fuel injected or carbureted) where the return line is there primarily just to ?sweep? out the hot fuel to prevent vapor lock?

Regardless of the type of fuel system you?re installing and/or the reason you want/need a return line, if your tanks are already built I?d think that an access panel mounted return fuel line fitting would be just fine. If you?re still building your tanks, than extending that return fuel line so the warm fuel is dumped into an outside bay certainly wouldn?t hurt anything.

Mark[/QUOTE



I bought a started project and am building my tanks now. I "studied" the fuel system, the valve and all the lines. It doesn't look like it requires a return line. FI is a totally new concept to me. Looks like I may not need to bother with return lines. The project came with a "0" time Lycon IO320 B1A engine. Hence the reason for the dumb/silly questions.
 
Marshall,

I think it’ll be just fine to install a return line fitting in the access panel. The main advantage to extending the return line to an outside bay is to dump the warm return fuel as far away from the pick-up/supply fuel inlet as possible. This gives the warm return fuel time to mix with the rest of the fuel in the tank and get cooled down before it’s picked back up again.

With a fully circulating fuel system like what’s used in the EFI/EFII setup, I’d be curious to know the actual temperature of the returning fuel?? I could be wrong, but I think as fast as that fuel is moving around the fuel rail that it doesn’t have time to pick up an excessive amount of heat.

What is your primary reason for wanting to install a return system? Are you planning on an EFI/EFII setup where a return line is mandatory?....or are you going with a traditional aviation type of fuel system architecture (fuel injected or carbureted) where the return line is there primarily just to “sweep” out the hot fuel to prevent vapor lock?

Regardless of the type of fuel system you’re installing and/or the reason you want/need a return line, if your tanks are already built I’d think that an access panel mounted return fuel line fitting would be just fine. If you’re still building your tanks, than extending that return fuel line so the warm fuel is dumped into an outside bay certainly wouldn’t hurt anything.

Mark[/QUOTE



I bought a started project and am building my tanks now. I "studied" the fuel system, the valve and all the lines. It doesn't look like it requires a return line. FI is a totally new concept to me. Looks like I may not need to bother with return lines. The project came with a "0" time Lycon IO320 B1A engine. Hence the reason for the dumb/silly questions.
Hello Marshal,
I have return fuel lines in my 9A. If you have questions let me know. The type of FI system will determine whether you need return lines or not. As for the questions about running them all the way out to the outboard bay, well, that is where mine are. However, after I built mine per the ECI plans, they later determined returning them to the root bay was adequate. Of course ECI is now Continental so who knows what they have to say about it now.
 
I ran mine out to the outboard bay because I was building the tanks anyway and there was no difficulty to get it out there. I don?t know if I?d have even bothered with a return fitting if my tanks were already done. My personal opinion is that it becomes an ?only if I really end up needing it? mod once the tanks are sealed. I don?t want to open those dang things up ever again!

Roger that.
You might need to update your sig. line...and for good reason!
 
Return fuel at first bay no problem, hot starts no problem. 650 RPM idle no problem.
 
Off the top of my head I'd be hesitant to dump the return fuel at the outer bay due to the torturous path it takes to get back to the pickup. In a min fuel situation the time it takes for the return fuel to flow through all those proseal clogged ribs might mean the difference between making the runway or ending up in the trees. I think the primary requirement is to ensure the return flow does not churn up the fuel at the pickup. That level of separation is available in the first bay.
 
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