Sticky valve? Nope! Ignition? Nope! Oil canning? Maybe...

[Brantel]

Originally Posted by bret

Did you fly with the baggage wall out or just removed and reinstalled? it is structural and required. I have seen on here a pic taken where a close inflight shot during aerobatics shows obvious oil canning in the tail cone.

No way would I fly with that bulkhead removed.... just removed it right before Osh to change ELT batteries and perform annual condition inspection items behind there.

[Brantel]

Originally Posted by KC10FE

The first thing I would check is the wings. Look at the wing root fairings and any smoking rivets underneath. It sounds like a wing loading problem in my opinion, I would look hard at attachments for galling or any kind of shifting. From what I understand, the bang appears around 150 kts ( High pressure underwing ) then reappears when slowed to around 70kts nose low ( wing pressure unloading ).....just my 2 cents.

Nothing unusual found....and the sound appears to come from behind the cockpit.

[Brantel]

Originally Posted by MarkW

Hey Brian,
Sorry to hear about all the problems you are having since OSH. Looks like it would be easy to eliminate the oil canning with a temporary lightweight stiffener double side taped to each side for a short flight. I almost tend to lean toward a flap actuator or flap linkage problem.

Already in progress. I checked out the flaps and links and nothing out of the ordinary found.

[penguin]

Take care when adding stiffeners - secure the ends to some structure else the skin may crack at the end of the 'floating' stiffener.

[Chkaharyer99]

When flying formation off a RV-7 I see oil canning in the same area you are depicting in the video's.

[Brantel]

Smoking Gun Found!



So after my last test flight with stiffeners....BAM! Did it again.... RATS!

So I landed almost defeated and as I was taxiing back to the hangar I thought to my self: "Self, what else has changed?" And I answered back to myself "Nothing but the recent smoke system install..." WAIT A MINUTE, HOLD THE PHONE!

You see I have flown many flights since installing the smoke system and did not have any issues like this BUT I never flew with the tank almost empty! You see right before Osh, I knew I would be heavy so I pumped all but about a half gallon of the oil out of the tank!

When I ordered this smoke system, I took the option to have the non vented cap along with the spring loaded check valve/vent.

Is this starting to make sense yet?

The SMOKING GUN is the oil tank! It was oil canning due to the changes in pressure around it. The non vented cap and check valve won't let air out and it takes a few psi of pressure to get the spring in the check valve to open so this was causing the tank to oil can.

The force required to make it oil can is significant and when it does it is loud! I now know why I thought it was an exhaust pop in the beginning!

To simulate what is going on, I used my vacuum in reverse to blow air into the tank. BANG! A very sharp, metallic sounding BANG!, press it back in and BANG! Even with the vac running, it is very loud!

Links to video of issue:

https://goo.gl/photos/arhQXRyvDi3zd5f79

https://goo.gl/photos/eQ8ueiNSmn3nq8A87

I am off now to test fly with the cap loose just to be sure but I am 99.9999% sure this is it! I am sure that the reason it did not show up sooner was because it always was always at least 75% full!

[jibby212]

I was reading through the posts and thinking of my mustang I had that would randomly make a bang sound which I finally figured out that it was the fuel tank collapsing because the previous owner removed the charcoal vent system and plugged it. I was going to post my experience thinking that the possibility of that happening to your fuel tank was very unlikely but wow! pretty close.

[Brantel]

Originally Posted by jibby212

I was reading through the posts and thinking of my mustang I had that would randomly make a bang sound which I finally figured out that it was the fuel tank collapsing because the previous owner removed the charcoal vent system and plugged it. I was going to post my experience thinking that the possibility of that happening to your fuel tank was very unlikely but wow! pretty close.

It can and has happened to RV fuel tanks. Some have collapsed from blocked vents. I had already checked mine for that issue. Never dawned on me that it could be the smoke tank!

For the final test flight of the day I stuffed a rag in the tank fill hole and went flying. All the climbs, decents and speed changes I could muster and there was no bangs! Smoking Gun Confirmed!

Maybe I can sleep now. I have not slept well since leaving Osh thinking about this issue!

[PCHunt]

Reminds me of the time I was in the right seat of a 737 in a descent, and I heard a sharp "crack!" I started looking around furiously for a cracked window, and the Captain said: "What are you looking for?". I said I heard a loud crack noise and was looking for a cracked window pane.

He said, "........... Tha was your water bottle collapsing!"

[erich weaver]

The other common source is the bag of chips you threw in for that mid flight snack exploding at 10,000 MSL.

Erich