Hi

I have been wondering. Flying this weekend I did a couple of maneuvers. When I doa, don't know if it's called an inverted loop, but I turn the plane inverted in level flight and then do a loop. When I pull the plane up into the air - just as I get to the first quarter at the top - the engine stutters. It doesn't die it just stutters and as I go down again, exiting the loop, it's fine again.

What could this be the result of? I can pull the plane vertical and let it run without it doing it and I can do a normal loop fine.

Sounds like a fuel tank or  mixture issue, check the tanks lines,clunk,and make sure everything is properly installed, then state the engine and full throttle it and hold it inverted see if this fixes it... if not then lean or richen the engine as needed</p>

I believe that in that loop the fuel goes to the top of the tank, but it also moves to the top-front and top-back, depending on acceleration of the plane during the loop, leaving the clunk dry during deceleration.

If your engine is too sensitive to bubbles, or tuned on the lean zone, it will show that is not happy.
The situation will worsen for a half full tank.
Try the loop with a full tank.

Does it do it after flying straight inverted for a while?
If so, the clunk may not be moving freely to the top of the tank.

If I understand you correctly.....you start in level flight and pull up into the loop (inside loop) with no problems....but inverted and push? into the loop (outside loop) starting at the bottom???????is that correct? the inside loop the fuel will stay in the bottom of the tank...the outside loop fuel will move to the top of the tank....sounds like most probable the clunck is sticking to the back of the tank not allowing free movement in the loop to the top of the tank......or the tank is too big and you don't have enough head pressure in the tank from the muffler......let us know what you find</p>

This video shows what happens to the fuel inside the tank during aerobatics:

http://richter-lackierung.de/aktuali...amefuntana.wmv

Some times the clunk cannot follow that dancing around and aspires air.

Yeah great video... it completely disproves the "fuel sticks to the back of the tank on downlines, Myth".

As stated it is likely that the clunk is not travelling with the fuel when the plane is inverted and heading nose up.

Usually this is because the fuel line is too long...

In effect the clunk gets uncovered for a prolonged period of time.

That has to be the best video Ihave ever seen to instruct someone how the fuel is effected during flight. Ialways thought the G forces would have kept the fuel to the back of the tank, so much for that thinking stuff. Inever thought the clunck was out of the fuel that much.
I'm now thinking a lot more about the bladder type of tanks.
Thanks for that.

Yeah and it also shows that it is a good idea NOT to let your fuel run down to 1/3 of a tank or less.

Even during approaches the clunk ends up uncovered... ouch!

That is a very informative video. Like someone also mentioned - I also always thought the G forces would "press" the fuel to the back. Guess I was wrong lol.

Yes JetMech - I think you understand correctly. I will take the tank out tonight and see what the clunk does. I'll also see if I can't start the engine and somehow simulate the stunt while holding the plane in my hands in such a way that I can see the back of the tank and see what it does. Haven't had time yet this week to check into this matter...but tonight I'll get to it - I hope.

Thank you

What a great vid. Got me thinking now...

geez be careful....moving a running aircraft around while you're trying to look at the back of the tank....get help.....
the clunk cannot touch the back of the tank.....at anytime

I will - I'm going to ask a friend to help me out. I know it's dangerous - I'll first make sure I can do it without endangering anyone - else I'm not going to do it.

Yeah and the 1/4" recommended clearance is a very good idea.

Under higher "G" loading the tubing to the clunk can stretch, causing it not to move up or down as the plane is violently thrown about.

Found this out the hard way.

Ionly go 1/8 and I also use a cut off wheel and open up the clunk slit more and cut in another one, an X . My LHS now carries the fuel line like the stuff that comes with the new tanks, very flexable instead of me rebuilding tanks with standard fuel line. I'm using Tygon in my gassers and have to reline every year or it gets too stiff to move around. Iplan on going to the lawn mower shop and buying some of the line they use in brush cutters that stays limp over time. None of my LHS carry it, Ikeep hearing that line, we will order some but never see any.
Ithink this year I will retank with the bladder type of tanks in my glow planes. Bit more work fueling but it may be worth it in the long run. Ihave used them for racing and they work pretty well, no foaming or bubbles, you just need to fill them then remove the air out of the bladder, a bit more extra work.