[1flyingyogi]

I have a slightly modified (high compression, cylinder work by Lycon), fuel injected (AFP) O-320 with full inverted oil and fuel in my RV4. If I stay inverted for extended amounts of time (over 15-20 seconds), the engine hesitates very badly AFTER rolling back upright. It has never quit on me, but the engine hesitates so badly that it feels like it's just on the verge of quitting. It runs perfectly while inverted. If if I don't stay inverted for very long (less than 10 seconds), no problems. Anyone have an idea what might be happening??

 

[AZSteve053]

Is it possible you have an issue with a Flop tube? Aged, stiff, leaking ???
Do you regularly inspect ?

 

[planenutz]

Have you ever done any experimentation with the fuel system to see how long the engine runs without a continuous fuel supply? You might find that the engine runs longer than you expect. My point here is - when you're inverted for extended periods of time, is your inverted fuel pick-up actually able to remain submerged in the fuel? If the flop tube is no longer floppy due to age it might be stiff enough to remain clear of the fuel while inverted.

 

[RV8Squaz]

I have a slightly modified (high compression, cylinder work by Lycon), fuel injected (AFP) O-320 with full inverted oil and fuel in my RV4. If I stay inverted for extended amounts of time (over 15-20 seconds), the engine hesitates very badly AFTER rolling back upright. It has never quit on me, but the engine hesitates so badly that it feels like it's just on the verge of quitting. It runs perfectly while inverted. If if I don't stay inverted for very long (less than 10 seconds), no problems. Anyone have an idea what might be happening??

Have you tried it with the boost pump on? I agree with the others; check the flop tube.

 

[1flyingyogi]

The flop tube was a new installation (about 2 years ago). I verified that it flopped just fine before putting the tank back on the wing. I'm pretty sure the flop tube is working as it should. Without flop tube, I would not be able to stay inverted for more than 7 seconds without running out of fuel (engine hesitating badly). This has happened to me repeatedly and is the reason why I installed a flop tube. There's no way the engine would continue running for 20 seconds or more without a flop tube.

The other reason why I think it's not the flop tube is that even longer periods of inverted flight (over 30 seconds) are fine while inverted. The problem always comes on AFTER rolling upright. And actually after rolling upright, it runs fine for about 3-5 seconds, and then it starts to hesitate. I'm thinking it has something to do with the vent lines being full of fuel and blocking air from entering the tanks and making it hard for the pumps to pump fuel to the engine??

I've tried it with both electric pump on and off and I *think* it's improved with the pump on?? I was in the process of trouble shooting this and then ended up buying a Pitts so I didn't feel the need to do aerobatics in my RV anymore, and stopped thinking about it. But now, I want to solve this problem. I want to approach it very carefully though b/c I do NOT want to have the engine actually quit on me!

 

[JonJay]

What’s your fuel pressure doing when it stumbles?

 

[A1AVIATON47]

What’s your fuel pressure doing when it stumbles?

I have a slightly modified (high compression, cylinder work by Lycon), fuel injected (AFP) O-320 with full inverted oil and fuel in my RV4. If I stay inverted for extended amounts of time (over 15-20 seconds), the engine hesitates very badly AFTER rolling back upright. It has never quit on me, but the engine hesitates so badly that it feels like it's just on the verge of quitting. It runs perfectly while inverted. If if I don't stay inverted for very long (less than 10 seconds), no problems. Anyone have an idea what might be happening??

what is oil press when inverted ?

 

[grumman41]

If you have left and right tanks (sorry don’t know the 4 systems) have you tried it on the opposite tank?

 

[Saber25]

Buying the Pitts for acro was a good choice. Problem solved.

 

[Daida]

maybe the floptube gets stuck/hung up?

on the -8 plans (DWG16A) you are ordered to move the fuel-level sender one bay out and build two “anti-hang-up guides”.
i can imagine that the -4 plans dictate similar.

the floptube was put in rather recently. Could you confirm that those precautionary measures were done during the installation?

 

[TS Flightlines]

maybe the floptube gets stuck/hung up?

on the -8 plans (DWG16A) you are ordered to move the fuel-level sender one bay out and build two “anti-hang-up guides”.
i can imagine that the -4 plans dictate similar.

the floptube was put in rather recently. Could you confirm that those precautionary measures were done during the installation?

'MAYBE' during the roll back to normal flight, the flop tube doesnt respond, and the counterweight is actually NOT in fuel but the air space between fuel and the tank. So its actually cavitating. Just because you changed flop tubes 2 years ago, doesnt mean the rubber hose still flops properly.

 

[lr172]

The flop tube was a new installation (about 2 years ago). I verified that it flopped just fine before putting the tank back on the wing. I'm pretty sure the flop tube is working as it should. Without flop tube, I would not be able to stay inverted for more than 7 seconds without running out of fuel (engine hesitating badly). This has happened to me repeatedly and is the reason why I installed a flop tube. There's no way the engine would continue running for 20 seconds or more without a flop tube.

The other reason why I think it's not the flop tube is that even longer periods of inverted flight (over 30 seconds) are fine while inverted. The problem always comes on AFTER rolling upright. And actually after rolling upright, it runs fine for about 3-5 seconds, and then it starts to hesitate. I'm thinking it has something to do with the vent lines being full of fuel and blocking air from entering the tanks and making it hard for the pumps to pump fuel to the engine??

I've tried it with both electric pump on and off and I *think* it's improved with the pump on?? I was in the process of trouble shooting this and then ended up buying a Pitts so I didn't feel the need to do aerobatics in my RV anymore, and stopped thinking about it. But now, I want to solve this problem. I want to approach it very carefully though b/c I do NOT want to have the engine actually quit on me!

Who did the flop tube installation and did they review the plans? You need more than a flop tube for inverted flight. You also need a hinged door that helps to keep the fuel in the inner bay. I am guessing this was not done during the retrofit (would be very difficult to do through the 7" hole) and very possible it is the reason that you are having issues. I do not do inverted flight, so have never thought about this much. However, I have done reseals on tanks with flop tubes, so understand the design elements of this configuration. I believe that the primary purpose of the door is to allow fuel to rush back into the inner bay after transition FROM inverted. Without this, it would take a good bit of time for fuel to get through the standard, tiny baffle holes. There is also a guide bar that must be installed. It helps to keep the tube from hanging up on certain parts as it transitions from inverted to upright. Speculate that this wasn't installed either.

Fuel in the vent lines DOES NOT inhibit proper venting in any way.

Larry

 

[1flyingyogi]

Who did the flop tube installation and did they review the plans? You need more than a flop tube for inverted flight. You also need a hinged door that helps to keep the fuel in the inner bay. I am guessing this was not done during the retrofit (would be very difficult to do through the 7" hole) and very possible it is the reason that you are having issues. I do not do inverted flight, so have never thought about this much. However, I have done reseals on tanks with flop tubes, so understand the design elements of this configuration. I believe that the primary purpose of the door is to allow fuel to rush back into the inner bay after transition FROM inverted. Without this, it would take a good bit of time for fuel to get through the standard, tiny baffle holes. There is also a guide bar that must be installed. It helps to keep the tube from hanging up on certain parts as it transitions from inverted to upright. Speculate that this wasn't installed either.

Fuel in the vent lines DOES NOT inhibit proper venting in any way.

Larry

Yes, we installed the hinged door, guide bar to prevent the flop tube from hanging up, etc. Bruce Hill helped me with this project - he actually did most of the hard stuff. Everything was done exactly according to plans. And I know that the flop tube is working as it should because I can stay inverted much longer than before the flop tube install. Before, the engine would hesitate while inverted after about 7 seconds or so. But I think I know what the problem is. Now that I think about it, the problem is always worse when I fly with very little fuel in the tanks. What is happening, it seems, is that while inverted, the fuel gets drained out of the inner bay (the door helps to keep the fuel in, but it's not a perfect seal, so after prolonged inverted flight, if there is little fuel in the tank, it will eventually all drain out anyway). And when I roll back upright, there is no more fuel in the inner bay and it takes time for it to come back, which is what's causing engine hesitation for a few seconds before enough fuel gets drained back into the inner bay to be picked up by the flop tube and sent to the engine. I'm quite sure this is it! I will go and test to confirm this and report back. Thanks!

 

[lr172]

Yes, we installed the hinged door, guide bar to prevent the flop tube from hanging up, etc. Bruce Hill helped me with this project - he actually did most of the hard stuff. Everything was done exactly according to plans. And I know that the flop tube is working as it should because I can stay inverted much longer than before the flop tube install. Before, the engine would hesitate while inverted after about 7 seconds or so. But I think I know what the problem is. Now that I think about it, the problem is always worse when I fly with very little fuel in the tanks. What is happening, it seems, is that while inverted, the fuel gets drained out of the inner bay (the door helps to keep the fuel in, but it's not a perfect seal, so after prolonged inverted flight, if there is little fuel in the tank, it will eventually all drain out anyway). And when I roll back upright, there is no more fuel in the inner bay and it takes time for it to come back, which is what's causing engine hesitation for a few seconds before enough fuel gets drained back into the inner bay to be picked up by the flop tube and sent to the engine. I'm quite sure this is it! I will go and test to confirm this and report back. Thanks!

Did you cut a larger hole in the baffle under the door? The door is open while inverted. It only closes when upright. The large hole is to let a rapid rush of fuel from the 2nd bay to flow into the inner bay as you transition to upright, I believe. The baffle holes are all on the bottom, so when inverted, fuel should move outward.