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Fuel Tank Cross Feed

I was wondering if anyone has tried running a second fuel line from the left tank to the tunnel and putting a "T" fitting off the right fuel tank line (through a shut-off valve and a check valve) to allow the Right tank to cross feed into the left tank? This would allow us low-wing guys to fly with a "both" setting and reduce fuel management issues. The idea would be this: upon reaching cruise; you select the left tank and turn on the cross feed valve. This would allow the right tank to drain into the left tank and keep both tanks relatively level. The check valve would keep the left tank from going the other way and the separate fuel line from the left tank to the tunnel would prevent air-lock. Thoughts?
 
Sounds like alotta work, cost and weight for no benefit really.

I would prefer to think a little and keep things light, cheap and simple.

Besides, this way you can run one tank totally dry to maximize your endurance if necessary.

My two...

:rolleyes: CJ
 
I don't mean to sound sarcastic here, but what do you really gain besides more complexity, more weight and not much benefit?

Using a "both setting" on low wing aircraft isn't as simple as either hooking both tanks together, or even with your crossfeed solution.

Here's the issue. How are you going to get the crossfeed rate to exactly be 1/2 of the fuel consumption of the engine (thereby keeping both tanks draining at the same rate)? You can't just simply turn on a pump and feed fuel to the other tank, because you'd need to keep a way from it filling all the way up then pumping the gas over the vent line.

So, in the end, if you need to "manage" the crossfeed, why not just chuck all that extra plumbing and manage the fuel selector? Meaning, if you need to worry about valves and switches for the crossfeed, I'd just suggest turning the fuel selector instead.

Just my 2 cents as usual..and maybe not overly polite, but I'm not one to usually mix words. I'm all for experimenting...just not with fuel systems. Increased complexity = decreased reliability.

Cheers,
Stein.
 
HA! We said the same thing, Stein!

I just had less typing to do so I beat you to the punch!

...what he said!

:D CJ
 
I wan't going to use a pump but rather just let gravity keep the two tanks level. The added weight is one line from the left tank to the tunnel, a check valve, an on/off valve and a "T" fitting. The added benifit is no longer having to switch tanks every 15-30 minutes. Just seems like a reasonable thing to think about doing.
 
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I know of two RV4s that use a similiar system and they have been flying for over 12 years. They never have to trim in flight as the fuel is always balanced. It is quite simple, and no, there is no way this will unport an intake, and leave you high and dry the way that they did it.
 
terry_georgia said:
I wan't going to use a pump but rather just let gravity keep the two tanks level. The added weight is one line from the left tank to the tunnel, a check valve, an on/off valve and a "T" fitting. The added benifit is no longer having to switch tanks every 15-30 minutes. Just seems like a reasonable thing to think about doing.

At first glance and thought you are quite right, it's not much weight, but here's another way to look at it that represents the increased complexity and increased failure points.

1 "T" Fitting
1 B nut/flare & connectionon the fuel line that the "T" Fitting is inserted in.
1 B nut/flare & connection on the other side of the "T".
1 B nut/Flare and connection on the tank line to the T fitting
1 B nut/flare and connection on the line hooking to the "valve"
1 Valve
1 more B nut/flare on the other side of the valve
1 more B nut/flare on the line to the tank
1 Bulkhead fitting to plumb it back into the tank
1 "Check Valve" (no fittings if inserted directly onto the T or on/off valve).

This equates to 6 more connections in your fuel system, all inside the cockpit along with 2 additional valves (On/Off and "check"), a T fitting and a new tank connection.

That being said it's still not that simple. In fact, using gravity is an even less desirable way of doing it. Yes, there may be some plane somewhere that has done it for a million years, but a couple out of 4,000 flying RV's doesn't mean it's right or will work in your installation. It's designed the way it is not just because Van's did it that way, but because he did it that way for a reason.

Here's the problem. Unless the venting system of both tanks is 100% equal, the the laws of physics take over and one tank ends up being ever so slightly having an increased pressure over the other. What then happens is all the fuel will be pushed from one tank to the other side, until it's full, then maybe out the vent until the other tank is empty. This is a very well known and proven phenomenon on low wing airplanes. Unless the vents are interconnected, the plane is flying perfectly level, etc... you run the rist of all the fuel ending up in one tank. Heck, even a few select high wingers have this same problem. Some have solved this problem by interconnecting the vent lines, but then here you go with yet more plumbing to solve a problem that really isn't there until you create it.

I'm not knocking your desire to make the plane better, in fact I'd encourage it in other areas....just not the fuel system. Switching tanks 2-3 time per hour isn't that much of a pain in the rear. I still submit that if you build it light, build it simple, and build it fast you're better off in the end.

Once again, the above information is only my personal opinion and carries no more weight than any others so take it for what it's worth - Good luck!

Cheers,
Stein.
 
Maybe fuel management isn't a bad thing...

I like having to switch tanks every now and then. It ensures I keep track of how much fuel I have in each tank. It also allows me to make sure I have (say) four gallons in one tank at landing, rather than two in each two tanks, which might result in unporting.

If it could be automated in a very reliable, high-integrity, visible fashion, I'd consider it. However, I really don't want to design, nor can I afford to buy, such a system for my RV. I'm guessing that it's not as simple as it sounds, or Grumman, Piper, Beechcraft, and Mooney would've done it on their low-wing airplanes.
 
terry_georgia said:
I wan't going to use a pump but rather just let gravity keep the two tanks level. The added weight is one line from the left tank to the tunnel, a check valve, an on/off valve and a "T" fitting. The added benifit is no longer having to switch tanks every 15-30 minutes. Just seems like a reasonable thing to think about doing.

It really isn't necessary to switch that often. My standard procedure is to fly the first 30 minutes on a given tank (including takeoff and climb). This will produce the largest trim imbalance because all the initial fuel usage is from the very outboard end of the tank because of the wing dihedral.
After the first 30 minutes I do 1 hour intervals. The first half of the hour will bring you back to equal (approx.) fuel in each tank, then the second half of the hour will give you a 30 minute burn differential on the oposite tank.
By continuing to switch every hour you never have a fuel quantity differential of more than 1/2 hour between tanks
This difference is easily trimmed out with a small amount of aileron trim.

If you research the NTSB accident report archives you will find that a large # of experimental/amature built accidents are cause by engine stoppage due to fuel delivery issues.
Adding compexity to the fuel system also adds the possibilty for additional problems.
Before doing this type of modification you should probably think about whether the benefits are worth it.

Scott
 
X-feed: Cessna Yes; Piper No?

Why do high wings have a both (x-feed) and low wings left / right only. There is a reason I just cant put my finger on. I do know with fuel on Both in a Cessna, full tanks and parked on an slope you can drain the high tank thru the low tank and out the vent.

The details are a little fuzzy but there have been a few accidents with a tie or X-feed between tanks. It seems like a good idea, never having to do the takeoff and landing procedure step: "SWITCH TO THE FULLEST THANK".


Two other reasons, one of which is standardization and following the plans. The insurance companies are looking at mods to critical systems and the fuel system is one of the most critical. There have been some insurance issues in the past, albeit not the x-feed idea on a low wing as far as I know. John Denver crash brought up the issue of "Non-standard" not per plans modifications to the fuel system to the forefront. The other reason was mentioned, complication and little gain with potential of making more problems. With a Left, Right and Off only we always have (with proper fuel planning and operating procedures) a reserve amount of fuel to switch to. With both, when you're dry, you're dry. Instead of "allow(ing) us low-wing guys to fly with a "both" setting and reduce fuel management issues", it could raise more. George

PS I think someone mentioned it. Fuel awareness and switching tanks is good situational awareness, however you got to check the one item before takeoff and landing, fullest tank.
 
gmcjetpilot said:
Why do high wings have a both (x-feed) and low wings left / right only. There is a reason I just cant put my finger on. I do know with fuel on Both in a Cessna, full tanks and parked on an slope you can drain the high tank thru the low tank and out the vent.

Simple answer, Gravity. There is no need for specialized boost pump on most non complex highwings, the engine pump will always be fed.
 
gmcjetpilot said:
Why do high wings have a both (x-feed) and low wings left / right only. There is a reason I just cant put my finger on. I do know with fuel on Both in a Cessna, full tanks and parked on an slope you can drain the high tank thru the low tank and out the vent.

I have not thought through every scenario in the original question, but as to high wing Cessnas, they have a small header tank, they call the fuel resevoir tank, that's gravity fed by the two wing tanks. This assures that the single fuel line going to the auxiliary fuel pump won't go dry (assuming there is fuel of course).

Low wings as a general rule, don't have room for a header tank that's lower than the wing tanks. If one wing tank on a low wing goes dry, the fuel flow to the pump will stop, using a "both" selection.

This can be demonstrated by sucking water out of two glasses with "two" straws. Dump the water out of one glass, and it's impossible to suck from the other, with both straws in your mouth.

L.Adamson
 
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