X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 05 Jul 2009 22:28:38 -0400 Message-ID: X-Original-Return-Path: Received: from imr-da03.mx.aol.com ([205.188.105.145] verified) by logan.com (CommuniGate Pro SMTP 5.2.14) with ESMTP id 3745761 for lml@lancaironline.net; Sun, 05 Jul 2009 15:48:37 -0400 Received-SPF: pass receiver=logan.com; client-ip=205.188.105.145; envelope-from=PTACKABURY@aol.com Received: from imo-ma03.mx.aol.com (imo-ma03.mx.aol.com [64.12.78.138]) by imr-da03.mx.aol.com (8.14.1/8.14.1) with ESMTP id n65JloYq023464 for ; Sun, 5 Jul 2009 15:47:50 -0400 Received: from PTACKABURY@aol.com by imo-ma03.mx.aol.com (mail_out_v40_r1.5.) id q.bf7.60adaed7 (29678) for ; Sun, 5 Jul 2009 15:47:49 -0400 (EDT) From: PTACKABURY@aol.com X-Original-Message-ID: X-Original-Date: Sun, 5 Jul 2009 15:47:49 EDT Subject: Re: [LML] Nose shimmy theory X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1246823269" X-Mailer: AOL 9.1 sub 5006 X-Spam-Flag:NO -------------------------------1246823269 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en Fred: Interesting theory--so maybe a multi grade strut oil is a consideration. However, I think your theory must be able to explain the= fixed gear ES shimmy problem too. I have always related the ES problems to its over= ly heavy wheel pant and its effect on dampening--and therefore incorrectly= figgered shimmy was not as great a concern in the LIV. paul In a message dated 7/5/2009 11:39:36 A.M. Pacific Daylight Time, frederickmoreno@bigpond.com writes: Sorry to hear of another nose wheel shimmy problem. We have been over this ground many times. I have a further thought to add to the comments= already contributed. The nose wheel shimmy is controlled by internal damping using the oleo hydraulic fluid bleeding through an orifice as the strut rotates from sid= e to side. Too little fluid or too much clearance, too little damping, and destructive oscillations set in. Consider: The viscosity of the strut oil varies dramatically with temperature. When retracted, the nose gear gets heat soaked in the hot= air under the engine which is roughly 150F above ambient in normal cruise condition= s. The strut and oil get hot, and the oil viscosity drops =E2=80=93 a LOT.= Then you drop the gear and get a nice (comparatively) cold blast across= the strut that cools it and the oil inside. The oil is in the annular sp= ace between piston and cylinder, and probably cools fairly rapidly as the external surface is exposed to the air blast. Without doing the heat tr= ansfer calculations for flow around the strut, my guess is that the time to cool= the oil in a 100 knot air blast is a few minutes. So here is the thought: if the nose strut is truly heat soaked, and the= gear are extended only 1-2 minutes prior to touchdown, the oil may still= be warm to hot, and the ability to damp shimmy is therefore substantially reduced compared to a cold damping test in the hangar. So here is the proposition: shimmy may well correlate with time between= gear extension and touchdown. If in doubt, lower the gear early, and ext= end downwind. This is pure supposition. Other thoughts? Fred Moreno **************It's raining cats and dogs -- Come to PawNation, a place where pets rule! (http://www.pawnation.com/?ncid=3Demlcntnew00000008) -------------------------------1246823269 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en

Fred: Interesting theory--so maybe a multi grade strut oil is= a consideration. However, I think your theory must be able to explain= the fixed gear ES shimmy problem too. I have always related the ES probl= ems to its overly heavy wheel pant and its effect on dampening--and therefore incorrectly figgered shimmy was not as great a concern in the LIV.

paul

In a message dated 7/5/2009 11:39:36 A.M. Pacific Daylight Time, frederickmoreno@bigpond.com writes:

Sorry to hear of another= nose wheel shimmy problem. We have been over this ground many times.&nb= sp; I have a further thought to add to the comments already contributed.

The nose wheel shimmy is= controlled by internal damping using the oleo hydraulic fluid bleeding= through an orifice as the strut rotates from side to side. Too little flui= d or too much clearance, too little damping, and destructive oscillations set= in.

Consider: The viscosity of= the strut oil varies dramatically with temperature. When retracted, th= e nose gear gets heat soaked in the hot air under the engine which is roughly= 150F above ambient in normal cruise conditions. The strut and oil get= hot, and the oil viscosity drops =E2=80=93 a LOT.

Then you drop the gear and= get a nice (comparatively) cold blast across the strut that cools it and the= oil inside. The oil is in the annular space between piston and cylinde= r, and probably cools fairly rapidly as the external surface is exposed to the= air blast. Without doing the heat transfer calculations for flow= around the strut, my guess is that the time to cool the oil in a 100 kno= t air blast is a few minutes.

So here is the thought: if= the nose strut is truly heat soaked, and the gear are extended only 1-2 minu= tes prior to touchdown, the oil may still be warm to hot, and the ability to= damp shimmy is therefore substantially reduced compared to a cold damping tes= t in the hangar.

So here is the proposition= : shimmy may well correlate with time between gear extension and touchdown. = If in doubt, lower the gear early, and extend downwind.

This is pure supposition.&= nbsp; Other thoughts?

Fred Moreno

It's raining cats and dogs -- Come to PawNation, a place where pets ru= le!

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