Roughness when at idle in flight...

[SwimmingDragonfly96]

Hey everyone,

When I first purchased my flying RV earlier this year, I had the characteristic popping on downwind when going to idle. Since then, I have taken it easy with pulling on the throttle, and I no longer get the popcorn noise. However, a different symptom has taken its place the last 50 hours or so (might have occurred after I wiggled the exhaust once and was amazed at how freely it moves).

I now get a sort of thump when I come to idle, kind of a bang that I can feel through the floorboards that repeats itself until advancing the throttle a bit, but it's not a symptom that I have seen much of on the forums. There was one person who described the same thump, but isolated it to a non vented cap on a smoke tank, which I do not have. It doesn't seem to be getting worse. 7a, o360-a1a, CS prop. Their description of the feeling, which was sort of spot on, is the thump you get in a commercial jet when they just start moving for taxi. EGT/CHT, OP, OT and all other engine parameters do not detect anything.

Any ideas?

 

[lr172]

cars use air pumps to send air into the exhaust during warm up. They do this because the rich mixture during warm up leaves raw gas in the exhaust and if the air is mixed in close enough to the exh valve, it will mix with the fuel and self combust. This makes the tree hugger's happy. However, they introduce th air in a controlled fashion, so it burns instead of combusts.

Most folks on downwind have the mixture fully in, so filthy rich and dumping lots of fuel into the exh. If your exh gaskets are not sealing well, you are getting the same thing happening. Now, when you dump the throttle, you send a large slug of gas out the exh and it goes POP inside the pipe once it gets that rush of air being sucked in past the leaking gasket (think Bernoulli). The first thing to look for when getting pops on rapid deceleration is exh manifold gasket leaks. This seems consistent with your observation of a loose exh system. The carbs do a very sloppy job of metering fuel and don't behave like the more sophisticated computerized injection systems and therefore waste a lot of fuel on rapid deceleration. You don't see this on cars anymore, as the computer actually stop the fuel flow on heavy decel. This saves fuel AND prevents issue like the one you are having.

Larry

 

[Carl Froehlich]

A thump like metal on metal? If so, you already identified that you have an exhaust system support problem. I recommend you investigate and start by lookin for any exhaust pipe to airframe contact. Exhaust pipes should move a little, but not flop around.

Carl

 

[SwimmingDragonfly96]

A thump like metal on metal? If so, you already identified that you have an exhaust system support problem. I recommend you investigate and start by lookin for any exhaust pipe to airframe contact. Exhaust pipes should move a little, but not flop around.

Carl

Hey Carl,

I wouldn’t say it sounds like metal on metal, feels more like a pop. I’d say before it was popcorn, now instead of multiple pops, it feels like one larger one and feels more like someone kicked the plane.

 

[agent4573]

Hey Carl,

I wouldn’t say it sounds like metal on metal, feels more like a pop. I’d say before it was popcorn, now instead of multiple pops, it feels like one larger one and feels more like someone kicked the plane.

Are you full rich when this happens? Does it continue to happen if you lean the mixture out?

 

[SwimmingDragonfly96]

Are you full rich when this happens? Does it continue to happen if you lean the mixture out?

I am indeed full rich when this happens. I’ve never tried leaning the mixture being that I’m pretty task saturated so I usually just add a bit of power and all is well.

 

[wcalvert]

It would be well worth the effort to do some trials to define whether or not this happens when leaned vice full rich. If you're too busy when coming into the pattern, do it at a medium altitude like 3k', lean it to cruise and pull the power as if entering the pattern. You'll be able to answer your question right away...

When I enter the pattern the mixture is as lean as it can be and stays there during the entire idle descent to touchdown. Go around requires mixture full rich then throttle up. Something worth practicing but not a procedure you won't be able to master.

Then pop the hood and do the checks the others have mentioned, or maybe do the checks first.

Cheers

 

[SwimmingDragonfly96]

It would be well worth the effort to do some trials to define whether or not this happens when leaned vice full rich. If you're too busy when coming into the pattern, do it at a medium altitude like 3k', lean it to cruise and pull the power as if entering the pattern. You'll be able to answer your question right away...

When I enter the pattern the mixture is as lean as it can be and stays there during the entire idle descent to touchdown. Go around requires mixture full rich then throttle up. Something worth practicing but not a procedure you won't be able to master.

Then pop the hood and do the checks the others have mentioned, or maybe do the checks first.

Cheers

I understand what it means if it stops doing it when it's lean, but what does it mean when it continues doing it while leaned out?

 

[agent4573]

I understand what it means if it stops doing it when it's lean, but what does it mean when it continues doing it while leaned out?

That it's not a fuel mixture issue. Just trying to eliminate a common possibility. My pops really bad if I richen it up in the pattern. I have to leave it super lean to not get this noise on decent and in the pattern.

 

[Kyle Boatright]

Interesting. My RV-10 pops really badly at low power settings and leaned out. Like on descent from a cruising altitude. I have to substantially enrichen the mixture to stop it, which is pretty much a waste of fuel since I end up burning 8-10 GPH in descent with the power way back, compared to 10.5 GPH in cruise.

I'm open for suggestions. I've already looked for vacuum leaks pretty thoroughly, but...

 

[rv8ch]

Interesting. My RV-10 pops really badly at low power settings and leaned out. Like on descent from a cruising altitude. I have to substantially enrichen the mixture to stop it, which is pretty much a waste of fuel since I end up burning 8-10 GPH in descent with the power way back, compared to 10.5 GPH in cruise.

I'm open for suggestions. I've already looked for vacuum leaks pretty thoroughly, but...

Kyle, are you running a carb or FI? I have the FM200 which is mechanical FI, and I do not have any of the problems listed in this thread - is this just a carb issue?

 

[Kyle Boatright]

Kyle, are you running a carb or FI? I have the FM200 which is mechanical FI, and I do not have any of the problems listed in this thread - is this just a carb issue?

The -10 is fuel injected.

 

[lr172]

Interesting. My RV-10 pops really badly at low power settings and leaned out. Like on descent from a cruising altitude. I have to substantially enrichen the mixture to stop it, which is pretty much a waste of fuel since I end up burning 8-10 GPH in descent with the power way back, compared to 10.5 GPH in cruise.

I'm open for suggestions. I've already looked for vacuum leaks pretty thoroughly, but...

exh popping is most typically caused by too lean of a mixture. You can get random misfires (generally unnoticeable as it is one or two revolutions) and that dumps raw gas into the exh and the next proper cycle sends superheated gas that causes the that raw gas to combust. I had a motorcycle that would do this, but usually only on the decel near idle. The fix was new exh gaskets. In your case, I would be looking for any signs that the exh gaskets are compromised. Most assume that the gaskets will cause the exh to push out and this is only partially true. The pressure and the velocity in the pipe is constantly changing and some times it is pushing gas out and other times it is sucking outside air in. Introducing fresh air into the exhaust can cause the popping in certain cases, either lean or rich. What MAP/RPM and GPH are you using on decent when this happens.

If it only happens below 1200 RPM it could be that the idle mixture is too lean.

 

[Kyle Boatright]

exh popping is most typically caused by too lean of a mixture. <snip> Introducing fresh air into the exhaust can cause the popping in certain cases, either lean or rich. What MAP/RPM and GPH are you using on decent when this happens.

If it only happens below 1200 RPM it could be that the idle mixture is too lean.

Typically, I'm cruising at 10.5ish GPH and 2300 RPM, maybe 20-22 inches of MP, altitude dependent.

On descent, I pull the throttle back to ~18", reduce RPM to maybe 1800-2000, and start getting the popping at anything much under 8 GPH, which seems like a pretty high fuel flow for what's supposed to be a low power descent. Maybe I'm doing something wrong in my descent profile. Suggestions?