Need Help: What is this sound?

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3 weeks 2 days ago #3502 by Nathan Wolfe
Replied by Nathan Wolfe on topic Need Help: What is this sound?
Steve,

Sage advice. Return to first principles. Suspect the things you touched.

We were back at it today and believe we've deduced where the problem is by running a couple different tests on the ground and in flight. We'll validate tomorrow.

We've ruled out the entire hot side of the turbo system! YAY! The problem appears to be on the cold side (Airbox, connections etc) Which makes sense since that stuff was touched during the Carb pull and replace.

The MP thing appeared to be a false lead as, after a sleep and a think about what you said *(Steve) I was able to get the airplane to make the sound (in flight) at engine MP settings that shouldn't be engaging the turbo at all. (6000' descending at idle).

I'll update tomorrow.

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3 weeks 3 days ago #3500 by STEVE ELLS
Replied by STEVE ELLS on topic Need Help: What is this sound?
Hi Nathan;
I seems from your description that you have tested the turbo system, and induction systems. Found no faults
As always, when a new problem pops up after MX, you go back to the last site of the last MX, and very carefully check the systems for correct installation, security and rigging.

The high pitched sound could easily be a piece of metal rubbing on another metal piece. (Since I can't hear it, that's obviously a guess)

I would look closely at the cowling, cowling installation, and baffling to see if you can find any evidence of metal to metal contact.

Push and pull and move the prop side to side to see if you can feel any movement or hear odd noises.

Check the rigging of the turbo/throttle control cable and connections. It's complex and easy to miss something.

The timing of the noise starting is weird. Why would it wait 15 minutes to start? Why is it related to MAP?

Check to see if the exhaust down pipe is rubbing on the cowling--that there's plenty of clearance from the cowling cutout.

I don't know what your glareshield is like, but when it happens again, push and/or pull on the glareshield--just a little bit of rubbing will make noise.

Could it be speed related? Doesn't seem likely. But an air leak past a door seal or wing to fuselage fairing might be the culprit.

Throwing out ideas.

Good luck, Let me know what you find,
See you at OSH

Steve

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3 weeks 3 days ago #3499 by Troy Whistman
Replied by Troy Whistman on topic Need Help: What is this sound?
Glad to hear you're close to Gary Main and his excellent shop... as I read your description of the operational circumstances that cause the chirping, I wondered if the turbocharger main bearings may be going.

How many hours are on your turbo? It may be worth having Gary check that out, too.
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3 weeks 3 days ago #3498 by Nathan Wolfe
Replied by Nathan Wolfe on topic Need Help: What is this sound?
HI Troy,

It happens under the following conditions:
- Engine is fully up to temp (Read it doesn't happen on initial departure. Only after it's been running under flight conditions for 20 mins or so)
- MP is above what the engine can produce NA (So... somewhere between 6-7K' Density altitude.

So if I take off and climb to 7500' (with DA 9000) and set up for cruise at 25" and 2200 RPM about 15 mins after I setup, it will start that sound. It will continue till I pull the power back down to 22(ish) inches or less. If I power up again (as I did in the video I posted) it will come back.

Pitch attitude has zero effect on it.

I can't tell where it's coming from. All i can say is that we are talking about a many KHz frequency sound and try to think about what could possibly be making a sound at that frequency.

An additional Update:
My shop spoke to Main Turbo in Visalia, CA. They discussed the symptoms and after checking the turbo bearings and play (turbo is in great shape, recently overhauled), came up with the hypothesis that it might be the popoff valve starting to fail and "whistling".

We took it off. Fortunately I am close enough to Main Turbo that I took it over there to be tested. It tested perfectly... so it ISN'T the Pop-off valve "whistling".

My Shop also did an inductance test with house air and soapy water, the system is solid.

Not sure it matters, but the carb was just taken off and sent out for rebuild and put back in during the annual. So was the Dual Mag. I IRAN the mag, So quite a lot of accessory stuff was removed and put back recently.

I'm stumped and I hate to fly a plane that started making a strange new sound right after maintenance.

This is one of those irritating problems that happens in flight but we can't get it to happen on the ground. Ugh!

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3 weeks 3 days ago #3497 by Troy Whistman
Replied by Troy Whistman on topic Need Help: What is this sound?
Nathan,

You might try sending that video to to see if they can identify it. I'd be concerned for sure if you're hearing a bearing squeak/chatter.

Troy

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3 weeks 3 days ago #3496 by Troy Whistman
Replied by Troy Whistman on topic Need Help: What is this sound?
Nathan,

I can hear it in your video. Does it only happen when adjusting MP? If you leave things in a steady state once it starts, does it stop or continue? Are you able to localize which direction the sound is coming from? Have you changed aircraft pitch/attitude when this sound starts, to see if it changes?

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