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General control line discussion => Open Forum => Topic started by: Terry Caron on October 27, 2015, 06:48:59 PM
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Having some grasp of the how and why of a uni-flo tank's MOL constant fuel delivery, and understanding that a chicken hopper can be either standard vent or uni-flo, in a standard vent chicken hopper it seems that, with MOL constant suction and atmospheric pressure on it's confined "available fuel" volume, it would achieve much the same effect as uni-flo - why doesn't it?
Terry
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After mulling it over, and recalling a blinding realization achieved in the thread on uni-flo tanks (that we're working with a dynamic fuel system) I'll try to answer my own question (someone please correct me if I'm wrong!):
In any standard vent tank,including chicken hoppers, while suction and atmospheric pressure may not vary, during flight the fuel pressure at the pick-up does vary as the pressure applied by the fuel in the reservoir diminishes.
And applying muffler instead of atmospheric pressure to a non-uni-flo system doesn't change that.
Speaking of MOL constant-speed engines of course.
Right?
Terry
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What's MOL?
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MOL = More or less.
Terry
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Save grief and weight and go with a clunk uniflow. If I go back to ic I will never use metal tanks again.
Warren
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I do fly clunk uni-flos Warren, along with standard vent, and having learned more (in another thread) about them since opening this thread I may go to them exclusively.
Terry
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One reason muffler pressure can help is that it reduces the percentage change in the head pressure of the fuel(pressure due to the difference in pressure due to the height of the fuel above the fuel outlet). A standard vented tank will always gradually lean out as the fuel runs out because the depth of the fuel changes. It's only a pressure of less than an 3-4 inches of water, but that is enough to change the engine run from rich at the beginning of a flight to lean as it runs out of fuel. Muffler pressure increases the the pressure on the fuel from nearly nothing to a couple of pounds per square inch, or the equivalent of about 50-60 inches of water. That makes the fuel pressure change due to the 2 inches of fuel in the tank only 3-4% or so of the fuel head change, instead of a 100% change. The fuel pressure goes from a fairly constant couple of psi above normal air pressure down to 0 psi above normal air pressure just as the engine runs out of fuel.
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Thanks so much for your reply Phil - I didn't know muffler pressure was that significant.
So my conclusion was partly correct and your point is well taken.
Terry
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Me too, thanks Phil, that helped my understanding also. I run muffler pressure on most of mine regardless of std or uniflow.
Rusty
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Phil the only problem with mufler pressure is you can't get a 4-2-4 break like you can with a uniflow and keep a constant lap time at the same time.
Warren
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Phil the only problem with mufler pressure is you can't get a 4-2-4 break like you can with a uniflow and keep a constant lap time at the same time.
Warren
Are you referring only to standard vent tanks or to uni-flos also, Warren?
Terry