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Strange noise

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Strange noise

Postby Anonymous » Tue Feb 10, 2015 10:10 pm

Posted by: Peter Knight on 22:20:45 10/29/14 Wed

The symptoms are as follows:-
The aircraft is a J430 with 800 hours. On take-off but before I reach 100 ft AGL I occasionally hear a soft 'Thump' sound that seems to come from the rear of the cabin. The effect is not reproducible. This morning I heard it on first take-off but not on any of the subsequent touch and goes even when I stopped on the runway.

Unusual noises are unwelcome in any aircraft so I have investigated carefully and I can state with confidence a lot of things that are not causing the effect.

It is not caused by loose items of the luggage rack in front of the sound curtain.
The trim/elevator control assembly are AOK.
The main undercarriage legs are well secured and there is no movement in the wheel axles etc.
All structural components such as wing spars/flaps etc are secure and correct.
Firewall forward and engine are all secure (but anyway noise seems to be behind me when in P1 position)
I am not over-rotating and striking the tail.

This is getting a bit irritating as I thought I would quickly find the source of the noise. The fact that it is not easily reproducible does not help.

If any one has a bright idea or has come across a similar problem then advice would be very welcome. Until then I will keep taking the aircraft apart and putting it back together again until the problem is solved (or the noise just goes away).

Many thanks
Peter
Anonymous
 

Re: Strange noise

Postby Anonymous » Tue Feb 10, 2015 10:10 pm

Posted by: Gary on 19:11:52 09/09/14 Tue

Hi, i had a noise in the rear of my 450, a soft thumping noise that developed over 85knots, slow down and it went away, turned out to be the wing root plastic trim, it had duct tap on it and this had aged and was tapping the side of the fuz. Could be something as daft as this
Cheers
Gary
Anonymous
 

Re: Strange noise

Postby Anonymous » Tue Feb 10, 2015 10:10 pm

Posted by: Vic on 21:24:21 09/09/14 Tue

Could it be a vacuum effect on the sound curtain making it bow one way or the other ?
Anonymous
 

Re: Strange noise

Postby Anonymous » Tue Feb 10, 2015 10:10 pm

Posted by: Peter Knight on 10:13:42 09/10/14 Wed

Thank you both.
I have had the undercarriage fairings off and polished them, dusted them, removed strange insects etc and they are secure and not the source of the noise.

I think Vic's suggestion that a pressure change may be the origin is a good possibility as a very small change in pressure on either side of the sound bulkhead at the rear of the cabin could cause it to flex and give out the 'thump'.

( I recall that some of the pressure changes can be counter intuitive, with the two seater the cabin was at slight negative pressure even with the vents open and this was sufficient to draw some exhaust gas from under the elevator forward along the inside of the fuselage into the cabin. Knowing of this effect I made the rear bulkhead fairly airtight. Perhaps I over did it and the pressure change becomes significant at certain speeds and configurations of flight).

The other component that might be subject to a small pressure effect could be the rear door. Perhaps it flexes enough to shut itself more securely as a result of lower cabin pressure?

I'll try changing one thing at a time to see if the problem goes away. It would be easier if the effect was less erratic
Peter
Anonymous
 

Re: Strange noise

Postby Anonymous » Tue Feb 10, 2015 10:10 pm

Posted by: BobP on 00:39:10 09/13/14 Sat

Could you temporarily create a small vent in the rear bulkhead (jam it slightly open or whatever) just to prove whether this is the cause of the noise.

Or you could make a simple manometer (length of clear plastic tube, U bend, coloured water) to see if there is any reversal/change in pressure when the noise occurs. It could also be used to see if there is a risk of fumes being drawn into the cabin.

just ideas
Bob
Anonymous
 

Or just take it out...

Postby Anonymous » Tue Feb 10, 2015 10:10 pm

Posted by: Clive on 11:08:10 09/14/14 Sun

Or just take it out...
Anonymous
 

The answer is...

Postby Anonymous » Tue Feb 10, 2015 10:10 pm

Posted by: Peter Knight on 17:42:47 10/28/14 Tue

Eventually I found that a retaining screw at the base of the sound curtain had come loose. This allowed the divider at the back of the cabin to flex a bit like a drum when there was a small pressure change in the cabin as the aircraft started to climb out.
Tighten up the screw to secure the sound curtain and the noise completely disappeared.
Only took 6 weeks to track this one down! Many thanks for all the suggestions and advice.
Peter
Anonymous
 

Re: The answer is...

Postby Anonymous » Tue Feb 10, 2015 10:10 pm

Posted by: Harry on 22:19:07 10/29/14 Wed

It is always down to something very silly ...
Anonymous
 

Re: The answer is...

Postby Anonymous » Tue Feb 10, 2015 10:11 pm

Posted by: Mick on 22:20:45 10/29/14 Wed

yes, quite.
Anonymous
 


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