Dual Clunks in Fuel Tank

Dual clunks are often used in a 3 line system, similar to this picture

but with the fuel fill also going to a clunk. This is usually done for a cowled engine where you can't easily get to the carb fuel line for fueling. Don't know what the engine being inverted would have to do with it.

Dave

So, what would be the purpose of putting a clunk on the fill line? It sure isn't obvious - whether the engine is under a cowl or not...

Putting two pickup clunks in a single tank presumes one clunk can't do the job

- which implies there are some aircraft attitudes that the single clunk wouldn't be submerged in fuel. If that was actually true, adding a second clunk doesn't seem like it would help - as the pressure on the tank would simply escape through the path of least resistance - the clunk that isn't submerged.

Beyond that, one would have to assure the two pickup clunks don't interfere with each other, lest they wrap around each other. That could be difficult if their respective pickup tubes emanate from the same stopper.

Single pickup clunks have worked for every imaginable model aircraft type, if care is taken to implement a proper tank design....

/daytripper

The clunk on the fill line is to aid in draining the tank.

Yes - *that* actually makes sense - not for "fueling", as above, but for "draining" - as long as it doesn't interfere with the pickup clunk. I've never built a model that the carb line wasn't somehow reachable, but I can see how that could be the case.

Cheers

/daytripper

On Mon, 03 Sep 2007 19:49:14 -0400, daytripper wrote in :

So you can drain the tank at the end of the day without taking off the cowl to take the line off of the carb.

Marty

Another possibility, frequently used in heli and control line setups:

Here is the "Uniflow" two clunk setup:

Construction photos at: Supposedly, the fuel flow will not vary depending upon how much fuel is in the tank, as described in this newsletter article I Googled: "The air pressure at the top of the tank will be reduced by an amount equivalent to the height of the fuel in the tank. E.g. 2 in. of fuel has a pressure of 0.05 lb./sq. in. If the ambient air pressure is 14.7 lb./sq. in., the air pressure at the top of the tank will be

14.65 lb./sq. in. With 1 in. of fuel, this pressure will be 14.675 lb./sq. in. Uniflow compensates for changes in fuel level in the tank, but it can?t deal with changes caused by the position of the tank. If your aircraft generally flies straight and level it will hold the mixture more constant than the exhaust pressure method."

I hope this helps.

Actually what he is referring to is most likely a uniflow tank setup. I remember using uniflow tanks back in my control line days. Look here for an explanation:

IFSC

ifly has the correct answer. Duke Fox of Fox engines recommended that setup - probably because some Fox engines were notorious for leaning out in flight. An inverted engine with the tank higher than the carb will have fuel dripping out of the carb with standard setup, but not with the "Uniflow" setup.