I have a problem with one of my planes that I think can be blamed on the fact that the tank sits lower than the engine. I have been told that the clunk should be level with carb. On mine the clunk sits about 3 inches below the carb. Maybe even more than that. The problem with it is that when ever I pull any tight manuver or a "hard G" the engine will die or starve out. Also I seem to have mixtures that are never the same. It seems to change tune, sometimes in mid-flight.If, in fact, I am right about this being my problem, will a pump fix the problem? I have a Perry VP-20 pump that I used on another plane with great results. I have changed engines from a Royal 46 with a perry carb to a MDS-48 with the stock carb. They both do the same thing but the MDS does it a whole lot worse. I think that comes from the difference between a two needle carb and a air bleed carb. Any advice would be appreciated. This is getting real irritateing but I think I am about to perfect the dead-stick landing!

The general rule of thumb is that the center line of the tank (not the clunk) should be level with or slightly below the spray bar of the carb.

jess

Another "general rule of thumb" is for the top of the tank to be level with, or three-eighths (3/8) inch above the spray bar. I would think a pump would help here, but can't advise which one. Are you using muffler pressure?

Actually unless using uniflow the center of the tank should be 1/4" to 1/2" below the spraybar. This is for a 60 sized plane and needs to be larger or smaller depending on the size of the engine and tank.

i never have had a problem with the tank being too LOW . my problems have always when the tank is too HIGH. (fuel drips) I think it's more than the tank is too low . I agree it sounds like a fuel delivery problem , but there are MANY things that can cause that problem

I've never understood that. Don't you want the center of the tank in line with the spray bar, so you get the same fuel mixture when the plane is upright or inverted?

Tom

Tom what you say makes sense but doesn't actually hold true. Almost without exception better fuel draw occurs when tank is lower than spray bar, Definitly more problems occur when tank high. Guess reason here is tank is pressurized from muffler therefore maintains good flow at all attitudes. Inverted engines of all types often have problems due to tank too high. Tank location in his case is it's far too low for good fuel draw. He could raise tank or use a pump.

Don't you want the center of the tank in line with the spray bar, so you get the same fuel mixture when the plane is upright or inverted? [/quote]

You won't get the same fuel mixture, it will be leaner when upside down. When right side up the clunk is at the bottom and vent at the top, there is no restriction to the vent. Imagine pressure as inch's of fuel. The clunk is at the bottom and the spray bar is at the center lets use a round number for muffler pressure of 10 inchs of fuel, that is a bit low but it is very round. Lets pretend the tank is 4" deep from top to bottom. So when the plane is upright the tank pressure is the same as the muffler pressure because of the unrestricted vent and the fuel pressure is 10"of muffler pressure plus the 4" of fuel on the clunk minus the 2 inches from the spray bar to the clunk which comes to 12". Now when upside down the vent is at the bottom and is now restricted by the fuel in the tank, so the tank pressure is now the 10" of muffler pressure minus the 4" of fuel in the tank or 8". So the fuel pressure is 8" + 4" - 2" = 10" so the engine will run slightly leaner because the fuel pressure is lower. In this case the best location would be 1" below the spray bar. Then when upright the pressure would be 10" of tank pressure + 4" from top of fuel to clunk - 3" from clunk or 11". When upside down the pressure would be 8" + 4" - 1" or 11" which is the same pressure. Actually the rule of thumb should be that 1/4 of the tank should be above the spraybar, but 1/2" is close enough for most installations. In fact even centered is often not noticeable.

Doubt the problem is where the tank is located. The engines run fine pointed vertical when the tank is is 8" below the carb...people here make alot of fuss about tank position for some reason? However 3" low when the plane is level is a lot and probably richens the engine when inverted but it shouldn't cause the engine to quit. I'd look for bad fuel lines or crap in the carb, maybe try tuning a little richer..

I got it. Thanks Sport Pilot.

Tom

It is not uncommon for people flying control line precision aerobatics to move a tank up or down the thickness of a playing card.

I think I can relate too. I am going to look further in the direction of bad lines or crap in my clunk. Man there is nothing worse than crap in your clunk. Problem is that I had a problem with weak gear and beefed up the mount to the gear. Gear is real solid now but you ain't getting that tank out without a saw! Oh well. Sure hope I find something in there. Seems like I remember there being a filter in the clunk and my dad said that back in the day they had alot of problems with that type of clunk stopping up. When you would refill it would clear it untill you pulled fuel back through in flight. Thus giving you a "wandering fuel mixture". Well it sounds like a good explanation any-way. I'll let you know what happens. Thanx guys

I wonder if engines are more tolerant of the tank position with (Tettra/Jett) bubbless tanks. Pylon racing enthusiasts seem to be the only ones using them. I'm guessing it's probably best to line bubbless tanks up with the spray bar.

Tom

Well I am at a loss now. I finally got my tank out and went in just knowing I was going to see my problem. I can't see any thing wrong at all. No holes in the lines, no screen in the clunk, tank is perfectly clean, all lines are where they should be. The tank can only go one place and in one position. The plane is a Fun-World 40 made by World Models. I just can't beleive they designed it wrong so I will leave the position as is. Other than that newly designed acess hatch I just cut in it. I will, I think, try the Perry VP- 20 pump on it. Back to asking questions again. Old school one here. Vent up, clunk down, fill tube down. Someone tell me I got that right. By the way, I put a vacum pump on it. Plugged the other lines and it is still holding 20 pounds after 30 minutes. Any one got any ideas? I don't have much hair left and it is getting thinner by the minute.

"I just can't beleive they designed it wrong ... so I will leave the position as is."

Believe it. It will function flying straight and level so it does 'work'. It just is not positioned optimally.

Terry in LP

Quite a few ARFs suffer from improper tank positon.

jess

I can believe they did now but don't know what to do about it. the tank is at the upper part of the fuse but the engine is even higher. It didn't look centered when I put it together but it had to go on the mark to line up with the cowling. If in fact that is the problem, using a pump should solve the problem right? I think I'll try it but uf it still screws up I am going to a four-stroke. From what I have seen they tend to pull fuel better and run a little more consistant. HOPEFULLY

WM did set that engine high. Can you rotate the engine and get a new cowling. Complain (nicely) to WM in Livermore CA, I bet they send you a new one if they have one. They like happy costumers.

James,


As some already wrote, the correct level for the fuel-tank, is so its center-line is at the same level as the nozzle in the carburettor.

Hugh (Sport Pilot) wrote, that it should be mounted a bit lower, to compensate for the vent location (the vent being 'in the drink', when the model is inverted)...
This is true, but the effect of 2-3", or so, of fuel above the vent, with the tank full, becomes less and less significant, as the fuel is used up during flight.

Eventualy, as your plane comes in for an 'inverted landing'; nose high and tank nearly empty, the vent tube will no longer be submerged in fuel...


If you just abide by the simple center-line - nozzle relationship, portrayed in the first line of this post, you'll be alright.


The problem is that most kit and ARF manufacturers, seem not to intend for the model to actually fly... They instruct the builder/owner to place the fuel-tank, nowhere near to where this relationship will be maintained... I began [link=http://www.rcuniverse.com/forum/m_4960623/anchors_4960623/mpage_1/tm.htm#4960623]a thread about this[/link], over two years ago.

It is up to you to 'remodel' the fuel-tank compartment and/or the engine's mounting, to acheive this objective and for your engine to run right...

This subject came up in a Clarence Mather column once. The reader asked (and I'm paraphrasing), "Why don't kit manufacturers take the proper engine/tank relationship into account?"

Clarence's response was something like, "Kit manufacturers know planes; they don't necessarily know engines."

Tom

I can relate to the manufacturers not being worried about tank position. My problem was indeed tank being lower, about 4-5inches , carb to clunk. I thought about inverting the engine but I am not a big fan of inverted engines so I put a perry pump on it and problen is solved. I only have a slight transistion problem thqt I think I can solve by tweaking out the pump pressure. I did like the idea that freakingfast had when he said to write World Models and get a new cowling but not for a new cowling but to let them know that we as the consumer would like a little more attention to this problem in the first place. If I had to find something good about this whole ordeal it would have to be that I think I can dead-stick one down better than any-one else in my club. Practice makes perfect!

Quote:

ORIGINAL: James c harrell

...My problem was indeed tank being lower, about 4-5inches , carb to clunk.

James,


You're making that incorrect assumption again... and missing the point.

It does not matter how high the clunk in your fuel-tank is!
The only thing that matters is the level of the fuel.

This is the reason people tell you to set the tank, so its center-line is at the same level as the carburettor's nozzle.
...And no-one but you even mentions the clunk...

Suppose the clunk is all the way at the bottom of a 4.5' tall barrel (with a large enough diameter fuel-line that would not impede flow), filled almost to the brim with fuel, to which the engine is mounted. As long as the level of the fuel is no higher than 2" above the carburettor's nozzle, to no lower than 2" lower than the same; the engine will get a reliable fuel-delivery.

Pascal's Law determines that all that matters is the level of the fuel.

So, if you set the average level (the tank's center-line), to the same level as the fuel's point of entry into the engine, you will get the minimum variation in the amount of fuel delivered, at each throttle setting.

The pic are copied from instruction manual, the pic tell more than 1000 word. See in the pic how the tank are placed in right height with the spraybar in the carburator.

Jens,


The photo you posted is from the manual for the OS LA engine...

Probably because this engine targets modelers' first plane (usually their trainers), even the line drawn does not coincide with the spray-bar (it is higher than it is), but it also places the tank's center-line about 3/8-1/2" below the level of the nozzle.


This is not that critical for a model that would probably spend no more than 1-2% of its flight time inverted (and then it would richen-up a bit).

I still think the same practice of 'tank center-line at nozzle level', should be applied to first planes too...

A little closer: