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-ma06.mx.aol.com ([64.12.78.142] verified) by logan.com (CommuniGate Pro SMTP 5.2.14) with ESMTP id 3745749 for lml@lancaironline.net; Sun, 05 Jul 2009 15:30:25 -0400 Received-SPF: pass receiver=logan.com; client-ip=64.12.78.142; envelope-from=Sky2high@aol.com Received: from imo-da03.mx.aol.com (imo-da03.mx.aol.com [205.188.169.201]) by imr-ma06.mx.aol.com (8.14.1/8.14.1) with ESMTP id n65JTj8U000526 for ; Sun, 5 Jul 2009 15:29:45 -0400 Received: from Sky2high@aol.com by imo-da03.mx.aol.com (mail_out_v40_r1.5.) id q.bcf.4c5942b0 (48552) for ; Sun, 5 Jul 2009 15:29:44 -0400 (EDT) From: Sky2high@aol.com X-Original-Message-ID: X-Original-Date: Sun, 5 Jul 2009 15:31:08 EDT Subject: Re: [LML] Nose shimmy theory X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1246822268" X-Mailer: AOL 9.1 sub 5006 X-Spam-Flag:NO -------------------------------1246822268 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en Fred, Right on! On a flying 360/320 with a pretty good seal betwixt the engine= compartment and the nose wheel well, there is still an upper section of = the strut being bathed in the hot air from engine cooling. If the gear is dropped at 45 degrees from the numbers on the downwind, there is not enoug= h time to cool the strut down. Think about most instrument approaches wher= e the gear comes down at the FAF, yielding 5 or 6 miles of flight before touchdown. I use the gear to help slow down upon entering the airport are= a (about 4 miles out) and still the strut has been heated enough to expand= the nitrogen enough to have added an effective 3/4" to the strut length. I once was advised to use heavier motorcycle fork oil and found I could = barely get the nose wheel to turn when taxiing on a cold winter's day. Ah, the yin and yang of it............... Grayhawk In a message dated 7/5/2009 1:39:36 P.M. Central 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 **************An Excellent Credit Score is 750. See Yours in Just 2 Easy= Steps! (http://pr.atwola.com/promoclk/100126575x1222887319x1201497660/aol?redir= =3Dhttp://www.freecreditreport.com/pm/default.aspx?sc=3D668072&hmpgID=3D62= &bcd=3DJul yExcfooterNO62) -------------------------------1246822268 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en
Fred,
 
Right on!  On a flying 360/320 with a pretty good seal betwixt= the engine compartment and the nose wheel well, there is still an upper sectio= n of the strut being bathed in the hot air from engine cooling.  If the ge= ar is dropped at 45 degrees from the numbers on the downwind, there is not enoug= h time to cool the strut down.  Think about most instrument approaches where= the gear comes down at the FAF, yielding 5 or 6 miles of flight befo= re touchdown.  I use the gear to help slow down upon entering the airpor= t area (about 4 miles out) and still the strut has been heated enough to expand= the nitrogen enough to have added an effective 3/4" to the strut length.
 
I once was advised to use heavier motorcycle fork oil and found I cou= ld barely get the nose wheel to turn when taxiing on a cold winter's day.
 
Ah, the yin and yang of it...............
 
Grayhawk
 
In a message dated 7/5/2009 1:39:36 P.M. Central 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


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