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Yakman
Joined: 04 Feb 2012
Posts: 10
Location: Auckland New Zealand
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 Posted: Fri Sep 14, 2012 7:37 pm Post subject: Yak18t air leak |
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I realise this topic is probably worn thru with you all but Im relatively new to the Yak and a little hesitant to go rushing in. When I went up to the plane a couple of days ago I did the usual start up procedure and noticed a hissing sound coming from the front somewhere. I also noticed the air pressure dropping very slowly. Opened up the cowls and on the left-hand side of the firewall is a device Approx 250mm long, grey in colour and I presume it’s a blow off relief valve for the compressor. It has 3 pipes connected to it, one from the compressor, one exhaust which exits to the bottom of the fuselage and another pipe which disappears thru the fire wall. The hissing sound is coming from the exhaust at the bottom of the fuselage and when I put my finger over it, the air then escapes from around the plate which is screwed to the grey device (I suspect the relief valve for when the compressor exceeds the required pressure). I started the engine to let pressure build up and its stays up to where it should be in flight, however after engine shut down the pressure drops away after about an hour. I presume this valve assembly is the problem, what do you all think? If I dismantle the unit, what am I likely to find and what is the likely fix? Is it an O ring failure, seal etc? Your help would be much appreciated….AJ
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Rob Rowe
Joined: 10 Jan 2006
Posts: 124
Location: Berkshire, UK
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| Posted: Sat Sep 15, 2012 7:14 am Post subject: Re: Yak18t air leak |
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AJ
The device you're describing sounds like an AD-50 which is now only found on -18Ts (and some early -52s, mostly later removed at overhaul).
As you've surmised its purpose is to relieve the compressor load when the main air system pressure has reached 50 bar and restore charging again when the main air drops below 40 bar.
I've no hands-on experience with this device but fortunately the Russian one-size-fits-all approach means I've found a description from the Mi-8 helicopter that also uses it ... translated (basically) into English courtesy of Google (as per the attached PDF).
Your post-shutdown leak problem suggests the Check Valve (4) on the diagram is either corroded or its seal has deteriorated (or both), so that would be a good place to start looking.
I'm sure the pros on the list will be able to advise further on dismantling tips and spares.
Hope this at least gets you started ... good luck.
Rob R
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