two fuel feed lines

Hey Guys

I recently heard about a guy who's thinking of running a second fuel feed line from the oposite end of his wetwing fuel tank, that is to say, at the cabin end he already had his fuel feed and return lines, and now would like to add a second fuel feed line at the oposite end of the tank. His idea is to reduce the chances of engine starvation occuring when flying in slip.

I'm just wondering what you guys think about this solution. Disadvantages, advantages. This first thing that pops into my mind is maintenance. If anything goes wrong with that fuel line at the other end of the tank he'll have a hard time getting to it and fixing it. Furthermore, wouldn't installing surge tanks be easier, to avoid engine starvation in slip.

What do you guys think?

Grubac

completely unneccessary

I think with well over 7000 RV's flying if this was an issue, it would have been addressed. Not only is this a bunch of extra work, I think you have a real probability of introducing air into the fuel system.

These are tanks with ribs that serve as baffles. The short time you'd be in a slip would not give enough time for the fuel to completely leave the bay where the pick up line does it's work.

This is a solution for problem that doesn't exist.

grubac said:

Hey Guys

I recently heard about a guy who's thinking of running a second fuel feed line from the oposite end of his wetwing fuel tank, that is to say, at the cabin end he already had his fuel feed and return lines, and now would like to add a second fuel feed line at the oposite end of the tank. His idea is to reduce the chances of engine starvation occuring when flying in slip.

I'm just wondering what you guys think about this solution. Disadvantages, advantages. This first thing that pops into my mind is maintenance. If anything goes wrong with that fuel line at the other end of the tank he'll have a hard time getting to it and fixing it. Furthermore, wouldn't installing surge tanks be easier, to avoid engine starvation in slip.

What do you guys think?

Grubac

Click to expand...

Two feed lines can work, if the fuel system uses gravity to create the flow from tank to engine (or from tank to a header tank). But, on RVs the fuel flow is created by a pump sucking on the line. If one of the feed lines sucks air, that is what the engine will get.

Try and experiment with some vinyl tubing and a T fitting. Hook a piece of tubing to each leg of the T. Put two pieces of tubing in a glass of water, and suck on the third piece - everything works OK. Now pull one of the pieces of tube out of the glass so it is in air and suck again - all you get is air.

Grubak, my Air Tractor and all the Cessna ag airplanes have header tanks so you can only select "Both/on" or off.

The belly of these airplanes is lower than the bottom of the tanks, so the header tanks stay full and work well. Since our RV's all have the belly even with the wing tank bottoms, there's no good way to incorporate a header tank.

Best,

Two comments

1 - because of the angle of the tanks (dihedral) the outboard pickup will be sucking air in level flight before the tank is empty.

2 - When I discussed installing tip tanks with Van in 1996 while I was attending a builder's class at his facility in North Plains, OR (at least that's the name I remember) he said I should consider installing a header tank between the instrument panel and the firewall.

Bob Axsom

gentleman, thank you for your responses.

first of all, sorry for not going into more detail. My friend, in fact, doesn't have a van's aircraft. His aircraft is high wing, with a gravity feeding a header tank, located below the wings. A small booster pump, on the floor of the fuselage then sucks fuel from the header tank and sends it to the motor. One would think, that with a gravity fed system you don't need a boost pump, but he's a bit paranoid and wants to avoid starvation at any cost.

So he's thinking of installing this feed line at the other end of the tank and running it to the header tank. I'm also afraid that this could introduce air in the system.

Grubac

If you deliberately leave out relevant info, you can't expect to get relevant responses.

An extra gravity feed line from the outboard end of the tank in a high wing aircraft to the header tank should be OK.

But, every extra fuel line adds several new places to have a fuel leak. And, the fact that there is a header tank means the original fuel system would be fine for short duration slips, as the fuel in the header tank will continue to feed the engine even if there is an interruption in the fuel supply from the wing tank to the header tank.

Assuming the header tank has large enough capacity, I think adding addition fuel feed lines only increases the risk of fuel leaks, without adding any safety in typical side slips.

A better solution to this "problem" may be to use a fuel selector with a Both position. The fuel will be pushed to the inboard end of one of the tanks during a side slip, and that tank will feed the header tank.

RV builders please note that fuel selectors with a Both position are a very, very bad idea on our aircraft, as the fuel pump will suck air as soon as one of the pickups is not in fuel.

Besides, he's already solved the fuel starvation problem with the sump tank, right? Unless he's planning on max-slip cross countries.

I'm curious; what plane is it? The only one I'm familiar with that had that kind of issue was the BD-4. It has no dihedral.

Charlie

My solution...

I don't slip until short final, land on downwind tank and keep at least 7 gallons in that tank.