mjunior

Listen that guys,

I started flying BAE Hawk 1:5 scale. I have o 14kg (31lb) turbine on it. The manufacturer sells a kevlar fuel tank but I bought (don't ask me why...) two Dubro tanks. After three flights I notice the fuel tanks were loosen inside the reserved compartment and they 'grow' or 'inflate' a lot after refueling.
I disassembled them and I confirmed they are clearly different from the original shape when I installed them first time. The tank walls (side, upper, bottom) are really flat. Originally they were a bit rounded as all plastic fuel tanks.
I think they shrinked due to heat inside the compartment.
Have somebody else already see that?
Is my theory right?

Mucio

sideshow

My Feedback: (11)

First, let me say.....ZOMFG A REAL MODELING THREAD.

...and second and third, what's your vent situation like? There are a lot of really great (WAY better than me) modelers that I'm sure can help you out, they just may need some more information....

lightningmcnulty

My Feedback: (11)

You aren't going to get a whole lot of heat in front of the engine so it won't be that, most likely the vent line is not a suitable internal diameter for the turbine you have. also don't defuel plastic fuel tanks at the end of the day, they will collapse.

cmjets

Hello, I know the Kevlar tanks are expensive, but the fiber glass tanks not, I make all my tanks in fiber glass and run perfect. And the best thing, are cheap.
I think the pressure and depressed of the tanks running make the change of original shape, but not believe that would be a real problem, only a aesthetic issue.
regards from Spain.
Carlos.
_______________________
www.cmjets.blogspot.com

mjunior

Carlos,

Thanks for the reply. I really like the idea of making my own tanks in fiberglass.

Mucio

afterburner

My Feedback: (18)

What diameter fuel tubing are you using to the tanks, vent and UAT?

DrScoles

My Feedback: (18)

Out of curiosity, why aren't any of the tanks made with carbon. Do we want them to flex a little? I used to build kite boards, carbon is a lot more expensive when you're making something big, but for fuel tanks, the price increase would be negligible. it would look oh so much cooler in carbon…. ;-)

mjunior

Sequence

- Tygon 1//8 vent line (around 12" long)
- 1/8" brass pipe
- Tank, smaller (last to fill, first to deplet)
- 5/32" brass pipe
- Tygon 5/32
- 5/32" brass pipe
- Tank, bigger (first to fill, last to deplet)
- 5/32" brass pipe
- 6mm clear tube
- UAT (BVM)

bri6672

My Feedback: (6)

Plastic tanks are fine and have been used for years! The vent line is probably a bit small for your application... Even fiberglass tanks are susceptible to this problem and can actually crack. My Comp Arf flash had this problem, the vent line from the first to the second tank was to small and when filling to fast the main tank (last in line when drawing fuel) swelled and cracked causing a mess. I went to a larger vent line and problem solved. Change the vent line through the system and the shrinkage will go away..

lightningmcnulty

My Feedback: (11)

You have a long and thin vent line, I would suggest that this is the biggest part of the problem.
on all of my jets I run the biggest tygon I can even if this means drilling out the aluminum parts of the stoppers on the tanks that way you can be sure that you are not pulling vacuum when running your engine which is easier on the fuel pump etc.

you should run your engine at full throttle with the canopy off and see if the tanks are really sucking down under power, if they are then you really need to do something about it, if they are just sucking down a little then you can probably leave it, whatever you decide the solution is to enlarge all of the piping and brass tubes after the one that goes to the UAT