ww2birds

My Feedback: (14)

Hi Guys,

I could use some advice from those of you who have done multiple tank setups, especially those employing a belly tank. I have a Yellow F-15, and up to now I've been using a UAT plus the two stock 63 oz saddle tanks in parallel. It's all plumbed with 5/32" brass fittings and tygon. The tanks are connected in parallel, both feed lines and both vents are teed, the vent goes to a Slimline fitting on the belly of the plane, and the feed line goes to the UAT.

So far everything has worked well, the tanks fill and drain at the same rate, and the only peculiarity I notice is the swelling of the saddle tanks when they fill and start to overflow in the taxi tank. I assumed that was the flow resistance increasing when fuel vs. air was going thru the vent line.

In a quest for longer flights, I have added a wet belly tank, 50 oz capacity. I plumbed it in by attaching the previous vent line from the twin saddle tanks to the feed line on the belly tank, and the belly tank vent becomes the system vent. The belly tank is all 3/16" dia brass and Tygon, inside and out.

When filling the new 3-tank system, I do see some swelling of the saddle tanks (they are wide + flat, polypropoylene, so very sensitive to pressure), but it fills up nicely.

The problem comes when attempting to drain the system, using the fueling can to defuel the system. The saddle tanks begin to collapse, seemingly unable to draw fuel from the belly tank.

I am wondering if the height of the fuel draw from the belly to the saddle tank is simply too much, and the pressure difference required is collapsing the saddle tanks. I have not tried to run the turbine and see if the slower fuel draw of the fuel pump would not start to collapse the tanks, assuming something is wrong and this is too risky.

And ideas from others who may have successfully used wet belly tanks?

Thanks
Dave

brad59

My Feedback: (32)

Most external tanks on full scale planes are pressurized for exactly that reason. Bleed air on jets, Vacuum pump exhaust air on the old warbirds. Larger tubes might help, but the column of fluid may be to high. Planes like the P-51 and T-28 run boost pumps 100% of the time just for their main tanks. You may need one between your external and main tanks or find a way to pressurize your external tank.

Shaun Evans

Hi,

I'd run 3/16" on the mains anyway, but the only reason you're seeing this problem is that the main tanks are relatively large for what they're made of. If they were kevlar tanks, you probably wouldn't perceive to puffing and squeezing. Ultimately, I think the only real liability here is that your pump is working a bit harder and thereby drawing a bit more off the battery. What I'd like to know is what tank you used under there? Thanks in advance for your info.

ww2birds

My Feedback: (14)

Thanks, Brad(59) for the comments. I think a boost pump is impractical, but I wonder if facing the vent forward would be enough to give it the boost it needs...

Dave

ww2birds

My Feedback: (14)

Hi Shawn,

It's Dan Gill's (Details-4-Scale) F-16-style tank. Pretty close to the shape of the scale drop tank I got with the plane .. just a little longer and fatter .. but looks pretty good attached. Dan does a very nice job on these tanks with an internal fuel cell and a nice scale-outline outer shell.

Dave

ww2birds

My Feedback: (14)

Well, encouraged by the west coast (time zones are wonderful...) feedback, I went ahead and connected a lab DC supply to the fuel can pump and simulated a 16 oz/min drain rate, and found that the saddles compress/collapse a little (about 1/2") and then stay put .. and the fuel then runs fine.

There was a small variation in pump current draw as the fluids were pumped out, but it was not dramatic. I suspect it is within the parameters that the ECU/pump feedback loop can handle (well at least I hope so...).

Still may think about it a bit before flying it .. but it's looking up. Creepy stuff to be going on in the air as the plane flies...

I thought that the one disadvantage with the Kevlar tanks was they were not happy being over or underpressured, since they don't flex so easily and the seams can crack... I have one of the nice Gary Mueller ones in my Reaction 54 and I take it easy on the fuel fill rate for that exact reason.

Dave

Shaun Evans

Hi,

You'd be shocked how much pressure those plastic tanks can take (both positive and negative). I don't use Kevlar simply because I don't get the advantage for the cost difference. I originally thought they were supposed to be tougher, but they sure as you-know-what don't survive crashes as well as the plastic ones. I will be using the Mueller tanks in the big bug though...

Eddie P

My Feedback: (4)

Plastic tanks aren't bad as long as the plastic blend is flexible and thich enough to be durable. What I do not like is fiberglass tanks, I think Kevlar tanks are a direct result of people getting hosed with tank ruptures from fiberglass tanks. One Fiberglass tank is OK, you get three and four, something is gonna give on the refuel, in my experience, sooner or later.

I've had good luck with Dan Gill's tanks (Kevlar).

I've got the Gary M. Kevlar tank in my R54 too, great tank (kevlar). Replaced the Dubro plastic one.