redsplatter

Almost done rebuilding a Miss Diamond from sep. 89 Model Airplane News plans for my dad, and I have two possible choices to place the fuel tank, (due to a few changes I made). It will have to mount behind the firewall, putting the center of the tank about 2 in. below center of the carb, or it will have to mount near the balance point putting the center of the tank 3/8 to a 1/2 in. below center of the carb but at a distance of 5 in. from carb to front of tank. I prefer the latter, but am unsure if there may be a problem due to the tank being that far back. The engine is the newer style Fox 15. Anyone have experience with the tank this far from the carb.

Thanks

BMatthews

Sorry, neither of you two options is usable. You've got to sacrifice something to get the tank up where the engine is if you expect to get a consistent run. The tank needs to be as close to the engine as practical with the centerline of the tank no further below the carb than about 1/2 to 3/4 of an inch.

Either one of your options will run on the ground with everyting resting in a static setting but as soon as the model starts to change attitude the mass of the fuel will change it's position in relation to the needle valve by quite a bit. And then you get problems. You can try this yourself to determine how far away you can get away with. Mount the engine and tank to a 2x4 or some plywood or whatever and mount the tank in a position that mimics the model. Start and tune the engine and then play with holding the mounting board in various positions and also shake it up and down semi violently to simulate the rough and tumble of flying. If the engine runs well without changing it's setting you're fine.

In your first case with the tank well low I think you'll have problems getting it to run the whole tank out without having to change the needle valve setting. In the second it'll be much too sensitive to model attitude and other flight loads.

redsplatter

Thats what I was afraid of. Teach me to check things a little closer before getting this far along. If I could get a slimmer tank, (from top to bottom), I could make it work. Seems like a few years ago, someone made a slimline tank, but I don,t remember the size. Anyone know where to get something like this? If not, I,ll probably make my own metal tank, like the control line ones.


Found the tanks, Hayes tanks through Hobby People.

BMatthews

The size of the tank doesn't matter. It's the distance the engine has to draw the fuel.

redsplatter

I didn't mean to use a smaller tank, I wanted to stick to around 4 oz., but a slimmer tank will fit further toward the nose due to a taper in the front compartment which prohibits me from using a standard size, or I should say standard shape. Got to thinking about the film canister tank and figured, why not make my own out of a plastic bottle laying around the house. After searching high and low, I found a 4 oz. bottle of some type of hand soap with the right shape to fit all the way next to the firewall and be 1/2 in. below the carb. Cleaned it and filled with some 25% fuel and will let it sit for a day just to make sure there is not a compatability issue. The neck can be cut off and opened up with a unibit to the right size. Using a sullivan stopper kit, I will convert it to a tank and then toture test to make sure the stopper will hold. I had originally moved the firewall back to accommadate the engine and mount while keeping the prop just past the front of the fuse and reshaped the winshield area. This is where I didn't check to see if the tank would fit. The taper is in the top of the tank compartment causing the tank to hit the sides of fuse well below the carb. If this homemade tank works, I'll pass it along

Thanks

BMatthews

Oh, in that case you're on the right track. I misunderstood what you intended.

Those stoppers put a LOT of pressure on the necks of the bottles. You may be better off, if you don't want to buy a commercial tank, to use the base of the bottle and add in the lines like suggested in the film container thread with fuel tubing for seals and brass tubing forced in to compress the silicone. Just be sure to smooth the ends so it doesn't cut the silicone. If you're burying the tank I'd add some RTV caulking silicone to the through seals just as a belt and suspenders sort of thing.

I remember the Miss Diamond design. She's a cutie. You and your pop have good taste in models.

redsplatter

Homemade tank turned out OK. I did use a stopper, but the plastic cap that fits on the outside of tank was not satisfactory due to there is no protruding neck on my tank and the rubber stopper being pulled too far out of the tank when tightening. Scrounged around and found a sullivan 1 oz. round tank that had a flat cap designed for a tank with no neck and this was the part that brought everything together. No leaks, no stress noticed, and fits like a glove.

jboy381-RCU

you could put a oneway valve in the pressure line to the tank...that way there is constant pressure to push fuel though the line to the carb...i had to do this with my .40 model to keep the fuel from running back to the tank...i don't have that problem anymore...

john