wing tank interconnect?

Guys I'm placing this here in hopes people will spot it and comment. I also plan to post it in the RV-10 forum to see what people think of the idea. Background; I plan to use an alternate engine system with electronic fuel injection. (not here to argue the point this is a completely seperate idea suggested on another forum that would work with Lyc or alternate engine)

Fuel management and systems are a frequently dicussed item in all sorts of forums. Poor fuel management leads to engine outs and sometimes crashes. At the very least momentary panic as we struggle to restart the engine knowing there is still plenty of fuel in "at least one of the tanks" ((famous last words?) Rather than a selector valve, someone suggested why not run a big tube between both tanks and make them one big tank? I know that in long turns fuel could slosh between wings, and other problems but even in single wing tanks we will baffle to keep the fuel at the wing root. Why not put in a non-structural rib about 6" from the inside wall on both sides with inward facing trapdoors. This would trap about 5 gallons around the center pickup, almost like a header tank. It should keep the pickup covered in all but serious aerobatics. Comments? One line from the tank/s, one return, no selector valve. couldn't be simpler. Redundant pumps could still be used, the plumbing would just be simpler. The more I think about it the more I like the idea.
Bill Jepson
Rotary10-RV

One thing that particularly concerns Van's is when people mess with their tried and proven (4000+ flying) fuel system design.

The design you outlined may work quite well, but think about what you're giving up:

If a fuel drain fails, you don't lose the fuel from one tank, you lose fuel from both tanks. Believe me, fuel drains do fail.

You no longer have separate fuel sources should one of your tanks become contaminated.

Finally (most important), due to the way fluids and air flow, you'll probably get more flow from one tank than the other. Maybe because you're flying slightly uncoordinated... Due to this, one tank empties before the other and your fuel system sucks air instead of gas. Hello, glider pilot...

Don't forget that your fuel system modification adds time and weight to your project in addition to making your airplane a freak if you ever need to sell it.

Kyle Boatright said:

One thing that particularly concerns Van's is when people mess with their tried and proven (4000+ flying) fuel system design.

The design you outlined may work quite well, but think about what you're giving up:

If a fuel drain fails, you don't lose the fuel from one tank, you lose fuel from both tanks. Believe me, fuel drains do fail.

You no longer have separate fuel sources should one of your tanks become contaminated.

Finally (most important), due to the way fluids and air flow, you'll probably get more flow from one tank than the other. Maybe because you're flying slightly uncoordinated... Due to this, one tank empties before the other and your fuel system sucks air instead of gas. Hello, glider pilot...

Don't forget that your fuel system modification adds time and weight to your project in addition to making your airplane a freak if you ever need to sell it.

Click to expand...

Kyle, The fuel drain is the only real concern here. If you discover fuel contamination in flight you are likely to be gliding anyway. Preflight checks for sure! As to the drain issue, I'm talking about a 2" ID tube here, no restriction side to side. It could be done for about 1-1/2 pounds, some of which you would get back by eliminating the fuel selector valve and attendant plumbing. The system could be converted back if you felt it neccessary for sale.
Bill Jepson

Where are you going to put the fuel pick-up? In that pipe?

Rotary10-RV said:

Kyle, The fuel drain is the only real concern here. If you discover fuel contamination in flight you are likely to be gliding anyway. Preflight checks for sure! As to the drain issue, I'm talking about a 2" ID tube here, no restriction side to side. It could be done for about 1-1/2 pounds, some of which you would get back by eliminating the fuel selector valve and attendant plumbing. The system could be converted back if you felt it neccessary for sale.
Bill Jepson

Click to expand...

Fuel caps sometimes leak in the rain. In Van's standard system, if one cap leaks water into your tank, the other tank isn't contaminated. In the system we're talking about here, one fuel cap leak could contaminate all of your fuel.

As to the 2" line going interconnecting the tanks... I'd think a 2" fuel line running through the fuselage is a heck of dangerous thing to do. People work awfully hard to keep fuel out of the cockpit, and you're thinking about runnig a 2" PIPE through there?

Again, the design shown in the plans is a well proven system. In addition, it is a flight critical system. Think about that before you alter it...

I wouldn't go the pipe route. I'd leave the system stock except I'd use 2 fuel pumps, one on the line from each pick-up. You'd need a valve in each line with a sensor to detect whether you're getting fuel or air. If it detected air, the valve would be closed.

Of course that would introduce a lot of extra failure points, and doesn't really simplify your installation.

PJ
40032

Kyle Boatright said:

Fuel caps sometimes leak in the rain. In Van's standard system, if one cap leaks water into your tank, the other tank isn't contaminated. In the system we're talking about here, one fuel cap leak could contaminate all of your fuel.

As to the 2" line going interconnecting the tanks... I'd think a 2" fuel line running through the fuselage is a heck of dangerous thing to do. People work awfully hard to keep fuel out of the cockpit, and you're thinking about runnig a 2" PIPE through there?

Again, the design shown in the plans is a well proven system. In addition, it is a flight critical system. Think about that before you alter it...

Click to expand...

Kyle, On this system you could use one cap and have less to leak, though I would probably still use 2. On the fuel in the cockpit, all of the highly reguarded "standard" RV systems have fuel lines running through the cockpit. Most RV's have at least fuel lines and a transfer valve. Many have the boost pumps in a console position, though some mount them on the firewall.
What I propose is to run a heavy wall tube/pipe directly in front of the short transverse (fuselage) spar, the connections would be made to the tanks at the normal bulkhead using a hose or other flexible connection. In any accident that could crush that tube gas would be the least of your worries.
Comment?
Bill

I like having two tanks. If a pickup clogs or breaks and falls into the tank, or a vent gets plugged, I'll have a backup. With modern engine monitors it seems a pretty easy deal to program some type of reminder to switch tanks.

Steve Zicree
RV4

Well, I'm of the opinion that just because that's the way it has always been done dosen't mean it's the only or best way to do it. Conversely, I also don't believe in change for changes' sake.

Certainly the LEFT-RIGHT-OFF fuel management leaves much room for improvement if that improvement does not introduce complexity or more negatives. Van's makes a great product, but if one is saying nothing should be modified then you should NOT be building or flying an Experimental airplane.

While I myself would not implement this ?joined? tank idea, I would not discount any modification of the fuel system. I myself am toying with this configuration.

http://wcurtis.nerv10.com/9XOther/RV10FuelPlumb.pdf

An Andair valve with LEFT ? MAIN ? RIGHT positions; Facet 40171 check valve pumps and a main tank with about a 10 gallon (60 lb) capacity. In a normal scenario, the valve would remain on the main tank while the electric pumps continuously refill it. If there was an electrical failure or problem with fuel contamination, you would use the selector valve to explicitly select a tank.

Now I just have to figure out where to put that main tank in a nose heavy RV-10.

All *constructive* comments welcomed.

golden rule

I think you should build it per plans.

Pipers have the same setup as RV?s (low wings), Cessna have a both or interconnect (high wing gravity feed).

One problem may be already mentioned, is the fuel from one tank can drain into the other tank on the ground or air. The excess fuel can be pushed out the vent if the pressure head is great enough. With a cross tube and parked on a ramp fuel can drain out the low tank drain. Keep it simple, keep it light, and build it per plans.

There was one RV I looked at that had two pumps, one pump per tank. The pumps "T' into one line to the engine. For take off both pumps on (both tanks feed engine), cruise pumps off and engine feed from both tanks, imbalance turn high tank pump on for a while until balanced. The facet pumps (some models) have reverse check valves. There was no selector valve. There was a single fuel shut off at the firewall and was cable operated to shut off fuel positively. I would not recommend it but it worked. I only mention it because it was interesting. No switching ans as long as one tank has fuel you are good to go. If the fuel is empty on one side the tank with fuel and the pump on will keep air from being drawn into the system. Interesting.

George

In any accident that could crush that tube gas would be the least of your worries.
Comment?
Bill[/QUOTE]

I think you should not be so quick to discount this as being an issue. I have seen photos of many RV accidents in which all injurys were non life threatening but the damaged condition of the fuselage and wings was amazing. Wings move, stretch and sever fuel lines, etc. The cross section area of you 2 inch line is probably at least a magnitude of 20 for the volume of fuel that can flow should it be severed open.

Scott

What about the issue of hydrostatic pressure?

If both tanks are connected, they form one long column of fuel. If you have a sideways acceleration (say, during a sideslip), the fuel at the "bottom" has a lot of fuel "above" it, increasing the hydrostatic pressure above what that tank normally sees.

Do some quick math and convince yourself it's not an issue before you plumb the tanks together.