X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from willowsprings.uwyo.edu ([129.72.10.31] verified) by logan.com (CommuniGate Pro SMTP 5.2.14) with ESMTPS id 3747030 for flyrotary@lancaironline.net; Mon, 06 Jul 2009 18:48:47 -0400 Received-SPF: none receiver=logan.com; client-ip=129.72.10.31; envelope-from=SBoese@uwyo.edu Received: from ponyexpress-ht1.uwyo.edu (ponyexpress-ht1.uwyo.edu [10.84.60.208]) by willowsprings.uwyo.edu (8.14.2/8.14.2) with ESMTP id n66Mm5u9024994 (version=TLSv1/SSLv3 cipher=AES128-SHA bits=128 verify=FAIL) for ; Mon, 6 Jul 2009 16:48:06 -0600 (MDT) (envelope-from SBoese@uwyo.edu) Received: from Boesexps (172.26.4.5) by uwmail.uwyo.edu (10.84.60.198) with Microsoft SMTP Server id 8.1.375.2; Mon, 6 Jul 2009 16:48:05 -0600 From: sboese To: "'Rotary motors in aircraft'" Subject: Muffler Date: Mon, 6 Jul 2009 16:48:02 -0600 Message-ID: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0019_01C9FE59.87189750" X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.6626 Importance: Normal X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 ------=_NextPart_000_0019_01C9FE59.87189750 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Some of you may remember that I returned home from the Rotorfest at the = end of May to find the internal baffle of my muffler broken. I theorized = that the broken part may have partially blocked the outlet and caused a power drop compared to what I should have been producing. Pictures of the = broken parts and repair were posted on the list last month. =20 When the weather and my time off coincided recently, I gave the repaired muffler a flight test after a ground run to be sure things were working normally. The flight lasted 9 minutes. The plane was much lighter than when leaving Texas, but the density altitude was about 9000 ft which is normal for here. The plane would barely climb, would not accelerate = over 110 kts, and the oil and coolant temperatures were abnormally high. = This was quite similar to the data that I posted last month which was a = record of the take off and climb upon leaving the Rotorfest. =20 Since the repair to the muffler was not a fundamental change in design, blaming the lack of performance on the damaged muffler was probably incorrect. So what was happening? With the engine shut down and still = warm, turning it over slowly with the prop produced a faint clicking noise in = a regular pattern. All indications were that the noise was coming from internal to the engine. Compression was fine on all compression strokes when turning with the prop. I thought that maybe the engine had = developed chatter marks in the rotor housings to the extent that the apex seals = would make the clicking noise when turning slowly while also losing = compression when turning fast enough that the apex seal could not follow the uneven surface of the housing. Pretty remote possibility, I know, but grasping = at straws makes one come up with some pretty weird ideas. This was investigated by removing the exhaust headers and looking into the = chambers. No chatter marks were seen. Everything I could check short of a = complete engine teardown seemed to be normal. =20 =20 To make a long story short, replacing the spark plugs brought things = back to normal as demonstrated by a subsequent flight test. So the data posted = last month is most likely not a record of operation with increased back = pressure as I had indicated, but rather one of operation with spark plug SAG. = This also is in line with the observation during the trip to the Rotorfest at = one point where I fully opened the throttle for a short time and observed = the RPM to drop off slightly. I was under the impression that when SAG = occurs, it is sudden and unmistakable. Apparently with my system, at least, it = can be somewhat insidious. The SAGGED take off when leaving from Texas = didn't seem unusual since I'm used to the lower power levels produced at the = high density altitudes here. The trip around the pattern after SAG = developing gradually during time right around liftoff was memorable. =20 Oh, the clicking noise? It turned out to be due to the slight lash in = the crank angle sensor gear when a sensor point was magnetically pulled into alignment with the pickup. Being a little more hard of hearing could = have saved me a lot of work.=20 =20 Steve Boese =20 =20 =20 ------=_NextPart_000_0019_01C9FE59.87189750 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

Some of you may remember that I returned home from = the Rotorfest at the end of May to find the internal baffle of my muffler = broken.  I theorized that the broken part may have partially blocked the outlet and = caused a power drop compared to what I should have been producing.  = Pictures of the broken parts and repair were posted on the list last = month.

 

When the weather and my time off coincided recently, = I gave the repaired muffler a flight test after a ground run to be sure things = were working normally.  The flight lasted 9 minutes.  The plane was = much lighter than when leaving Texas, but the density altitude was about 9000 = ft which is normal for here.  The plane would barely climb, would not accelerate over 110 kts, and the oil and coolant temperatures were = abnormally high.  This was quite similar to the data that I posted last month = which was a record of the take off and climb upon leaving the = Rotorfest.

 

Since the repair to the muffler was not a fundamental = change in design, blaming the lack of performance on the damaged muffler was = probably incorrect. So what was happening?  With the engine shut down and = still warm, turning it over slowly with the prop produced a faint clicking = noise in a regular pattern.  All indications were that the noise was coming = from internal to the engine.  Compression was fine on all compression = strokes when turning with the prop.  I thought that maybe the engine had = developed chatter marks in the rotor housings to the extent that the apex seals = would make the clicking noise when turning slowly while also losing = compression when turning fast enough that the apex seal could not follow the uneven = surface of the housing.  Pretty remote possibility, I know, but grasping at = straws makes one come up with some pretty weird ideas.  This was = investigated by removing the exhaust headers and looking into the chambers.  No = chatter marks were seen.  Everything I could check short of a complete = engine teardown seemed to be normal. 

 

To make a long story short, replacing the spark plugs brought things back to normal as demonstrated by a subsequent flight = test.  So the data posted last month is most likely not a record of operation = with increased back pressure as I had indicated, but rather one of operation = with spark plug SAG.  This also is in line with the observation during = the trip to the Rotorfest at one point where I fully opened the throttle for a = short time and observed the RPM to drop off slightly.  I was under the impression that when SAG occurs, it is sudden and unmistakable.  Apparently with my system, at least, it can be somewhat insidious.  = The SAGGED take off when leaving from Texas didn’t seem unusual since = I’m used to the lower power levels produced at the high density altitudes = here.  The trip around the pattern after SAG developing gradually during time right = around liftoff was memorable.

 

Oh, the clicking noise?  It turned out to be due = to the slight lash in the crank angle sensor gear when a sensor point was = magnetically pulled into alignment with the pickup.  Being a little more hard of hearing could have saved me a lot of work.

 

Steve Boese

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