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Engine basics => Engine set up tips => Topic started by: Larry Wong on November 18, 2009, 02:28:58 AM
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Any one know what might cause my P/A 40 UL on a pipe to start to quit off an on at the end of a flight, this just started lately, before it would just lean out a ???bit then a clean stop, nothing has change on fuel or prop?
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Larry,
Any chance the tank could have shifted position?
Jim
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Jim the tank is in tight inside of fusalage. n~ the flight run is right on, the pipe is the same.
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Have you shifted the leadouts recently?
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Check for a leak. It don't take much. I had one leak which wound up being a take apart filter. With silicone tubing I have even had new tubing leak.
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Any one know what might cause my P/A 40 UL on a pipe to start to quit off an on at the end of a flight, this just started lately, before it would just lean out a ???bit then a clean stop, nothing has change on fuel or prop?
HI Larry
This is 99% of the time fuel delivery problems
either a hole in the tubing, filter ,a small leak, sometimes a crack will appear inside the tank that will call this, or a leak either fuel or air.
So look for these things
Randy
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Randy surely knows his engine business, I dont question him, but I have had a similar situation when I changed the trim on one of mine, I have to second Alan's question, have you changed the leadout position? If the plane is flying in a slightly different Yaw position, it can change the fuel pickup characteristics.
But as Randy said, check all your fuel system as a baseline ,,
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If the rear end of the pickup pipe is soldered to the inside of the tank, it could have come loose. I remember having that happen years ago. As a matter of routine, we used to unsolder the back of every new tank, just to check that joint and see if the pipe was in the center and almost to the end of the tank. We also cleaned the crud out of there.
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Thanks guys I will start at the fuel system as Randy said and go from there. #^
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Randy surely knows his engine business, I dont question him, but I have had a similar situation when I changed the trim on one of mine, I have to second Alan's question, have you changed the leadout position? If the plane is flying in a slightly different Yaw position, it can change the fuel pickup characteristics.
But as Randy said, check all your fuel system as a baseline ,,
Not only moving the leadouts, but also CG, rudder or engine offset can change the engine shutdown. The model is flying yawed out, then the engine shuts off, so yaw reduces. As the yaw reduces, fuel flows back to the engine, and it refires. This makes the yaw increase, so the engine shuts off again, and the cycle continues...
Beware of yaw changing trim changes! It can change the way your engine runs, stops, and your launch rpm... This is not to say that you shouldn't make those trim changes, but make you aware that you need to watch for secondary results, good or bad. :o Steve
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Any one know what might cause my P/A 40 UL on a pipe to start to quit off an on at the end of a flight, this just started lately, before it would just lean out a ???bit then a clean stop, nothing has change on fuel or prop?
Your second post added that you see absolutely no other problems. That would make me lean more towards the Mark S. and Steve H. thinking. Any tank issue other that a subtle yaw issue would almost certainly show up during the run, not just in the last few drops in the tank.
The fine line on model yaw, tank skew, is discussed in some depth here.
http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=257774&mesg_id=257774&listing_type=search#257909
One lesson here is a fraction of a degree can cross the line where residual fuel remains in the front of the tank instead of the rear of the tank, causing the three-lap-burp-dance. The change can be small enough you didn't knowingly change anything in trim -- weight gain from fuel, unnoticeable warp that causes no other problem.
Many planes do end the run with a little residual fuel remaining in the front of the tank, and the flyer is never the wiser -- until minor yaw creep moves it closer to the line, creating a burp dance from the other side.
Larry Fulwider
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Many things can affect the end of the run, I have seen a plane that burped over and over again be cause the pilot took the plane up to 45 or 50 degrees at the end of the run..when he brought it down to 10 ft off the ground the tank emptied normally without much burping at all.
So there is another thing to try if it applies to how you end the run.
I have seen others that as soon as the engine starts to burp at the end of the run, the pilot just goes up to 45 degrees and does a cutoff loop
Randy
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If the fuselage is wide enough cant the rear of the tank out about 1/8 inch. Have a couple four strokes that cutoff is scary clean. No warning at all they just shut off like a switch..
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Since Larry W. didn't comment about my question on changing the leadout position, I assume he didn't. However if he recently changed the CG a bit through the trimming process, that also can affect the yaw if the leadouts weren't adjusted at the same time.
That end of flight sputtering can drive you crazy!