X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Tue, 07 Jul 2009 08:02:06 -0400 Message-ID: X-Original-Return-Path: Received: from mta31.charter.net ([216.33.127.82] verified) by logan.com (CommuniGate Pro SMTP 5.2.14) with ESMTP id 3747334 for lml@lancaironline.net; Mon, 06 Jul 2009 22:41:31 -0400 Received-SPF: pass receiver=logan.com; client-ip=216.33.127.82; envelope-from=troneill@charter.net Received: from imp10 ([10.20.200.10]) by mta31.charter.net (InterMail vM.7.09.01.00 201-2219-108-20080618) with ESMTP id <20090707024053.NEOO2647.mta31.charter.net@imp10> for ; Mon, 6 Jul 2009 22:40:53 -0400 Received: from axs ([75.132.241.174]) by imp10 with smtp.charter.net id Cqgs1c00J3mUFT705qgsuH; Mon, 06 Jul 2009 22:40:53 -0400 X-Original-Message-ID: From: "terrence o'neill" X-Original-To: "Lancair Mailing List" References: Subject: Re: [LML] Re: Nose shimmy theory X-Original-Date: Mon, 6 Jul 2009 21:40:57 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0014_01C9FE82.72243970" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.5512 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 This is a multi-part message in MIME format. ------=_NextPart_000_0014_01C9FE82.72243970 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable One suggestion; balancing the nosewheel might help. Terrence N211Al L235/320 ----- Original Message -----=20 From: PTACKABURY@aol.com=20 To: lml@lancaironline.net=20 Sent: Sunday, July 05, 2009 21:28 Subject: [LML] Re: Nose shimmy theory 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 overly heavy wheel pant and its effect on dampening--and = therefore incorrectly figgered shimmy was not as great a concern in the = LIV.=20 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.=20 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 fluid or too much clearance, too little = damping, and destructive oscillations set in.=20 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 conditions. The strut and oil get hot, and the oil viscosity = drops =E2=80=93 a LOT.=20 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 cylinder, 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 knot air blast is a few = minutes. =20 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.=20 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.=20 This is pure supposition. Other thoughts? Fred Moreno -------------------------------------------------------------------------= ----- It's raining cats and dogs -- Come to PawNation, a place where pets = rule! ------=_NextPart_000_0014_01C9FE82.72243970 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable =EF=BB=BF
One suggestion;  = balancing the=20 nosewheel might help.
Terrence
N211Al L235/320
----- Original Message -----
From:=20 PTACKABURY@aol.com
Sent: Sunday, July 05, 2009 = 21:28
Subject: [LML] Re: Nose shimmy=20 theory

Fred:  Interesting theory--so maybe a multi grade strut oil = is a=20 consideration.  However, I think your theory must be able to = explain the=20 fixed gear ES shimmy problem too.  I have always related the ES = problems=20 to its overly heavy wheel pant and its effect on dampening--and = therefore=20 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,=20 frederickmoreno@bigpond.com writes:

Sorry to hear of = another nose=20 wheel shimmy problem.  We have been over this ground many = times. =20 I have a further thought to add to the comments already contributed. =

 

The nose wheel shimmy = is=20 controlled by internal damping using the oleo hydraulic fluid = bleeding=20 through an orifice as the strut rotates from side to side.  Too = little=20 fluid or too much clearance, too little damping, and destructive=20 oscillations set in.

 

Consider: The = viscosity of the=20 strut oil varies dramatically with temperature.  When = retracted, the=20 nose gear gets heat soaked in the hot air under the engine which is = roughly=20 150F above ambient in normal cruise conditions.  The strut and = oil get=20 hot, and the oil viscosity drops =E2=80=93 a LOT.

 

Then you drop the gear = and get a=20 nice (comparatively) cold blast across the strut that cools it and = the oil=20 inside.  The oil is in the annular space between piston and = cylinder,=20 and probably cools fairly rapidly as the external surface is exposed = to the=20 air blast.   Without doing the heat transfer calculations = for flow=20 around the strut, my guess is that the time to cool the oil in a 100 = knot=20 air blast is a few minutes. 

 

So here is the = thought: if the=20 nose strut is truly heat soaked, and the gear are extended only 1-2 = minutes=20 prior to touchdown, the oil may still be warm to hot, and the = ability to=20 damp shimmy is therefore substantially reduced compared to a cold = damping=20 test in the hangar.

 

So here is the = proposition:=20 shimmy may well correlate with time between gear extension and=20 touchdown.  If in doubt, lower the gear early, and extend = downwind.=20

 

This is pure = supposition. =20 Other thoughts?

 

Fred=20 Moreno

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