X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml Date: Mon, 07 Dec 2009 09:41:58 -0500 Message-ID: X-Original-Return-Path: Received: from web57515.mail.re1.yahoo.com ([66.196.100.82] verified) by logan.com (CommuniGate Pro SMTP 5.3c3) with SMTP id 4005803 for lml@lancaironline.net; Mon, 07 Dec 2009 08:47:50 -0500 Received-SPF: none receiver=logan.com; client-ip=66.196.100.82; envelope-from=casey.gary@yahoo.com Received: (qmail 4110 invoked by uid 60001); 7 Dec 2009 13:47:14 -0000 DomainKey-Signature:a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com; h=Message-ID:X-YMail-OSG:Received:X-Mailer:References:Date:From:Subject:To:In-Reply-To:MIME-Version:Content-Type; b=hLChsPaQjwya2L+asMIAZiVwsEry284JPWrNZnFG/5KXeY2JpdG8IOk+3yHI4usxeF4T3KPJUa5Fizt+AbZZCpzdWNgbO5ya54II8dPLWclNvRDSgTeXrlGbgCjZb3Laza4TEBkxfHOTc7uKDjBlVruwFKqeIu2tq1eZd4FiXQg=; X-Original-Message-ID: <830999.94559.qm@web57515.mail.re1.yahoo.com> X-YMail-OSG: nOD48CQVM1nX2nrCkXf2PHa7Ll3w_YznXtYcUhOwxNVS.rk2QxtpJGST..Ifwzrii09Y8mvRU_I5dDdhWP0dZqkROVgu4p7Gq2pzjiqTrc2390TFdCV3MVk.l9sKJ.9R5lMxUuk3gyDpACRUb1UlWS70FKtIK6Ix_z0Av_nAMcY1VUA5D7WsZfZ01kfA3I5gjKLiQ6OXkLRi_5LYJOVlpj3_prZu9lJ8GRcF0_BU8RCon_5jDITRs9.sAjlDqHwV7czrEpMlPuxNH.pW8Mj1zghTnmm_xwFVhPrco47LhGu.s5YffA6KacQ_vZJ9CgQ4KJ44.Vh6O5JB Received: from [97.122.177.27] by web57515.mail.re1.yahoo.com via HTTP; Mon, 07 Dec 2009 05:47:14 PST X-Mailer: YahooMailRC/240.3 YahooMailWebService/0.8.100.260964 References: X-Original-Date: Mon, 7 Dec 2009 05:47:14 -0800 (PST) From: Gary Casey Subject: Re: Flutter and nose wheel shimmy X-Original-To: Lancair Mailing List In-Reply-To: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="0-2051733245-1260193634=:94559" --0-2051733245-1260193634=:94559 Content-Type: text/plain; charset=us-ascii I have thought about the CG of the nose wheel assembly being behind the pivot center (that would be the axis of the strut) being a consideration in the shimmy event, but I doubt that the fairing itself has much to do with it. However, I did truncate the rear my fairing by cutting off 2 or 3 inches for just that purpose. The tail of the fairing is rather crudely made and is essentially a solid plug of epoxy. I closed off the stub with a thin layer of cloth - much lighter. In order for a given component to contribute to the shimmy it has to have a resonant frequency in the range of the shimmy frequency. Give the side of the fairly a sharp rap and you'll find it really doesn't respond as the resonant frequency is quite high. The fairing could, however aggravate the event by adding to the already large mass of the wheel and tire, which is also behind the rotation axis. I can't remember where, when or on which airplane, but I recall seeing a large lead weight mounted out ahead of a nose wheel. I assume it was to prevent shimmy, but I have no proof. Adding a weight inside the front of the fairing that would balance the weight of the wheel and tire would be difficult and the result would add significant weight. Then I would be skeptical that the strength and stiffness of the fairing would be sufficient. But, would a small amount of weight up front be enough to just improve the balance of the system enough to eliminate the shimmy? It might be. The only negative I see is that the CG of the aircraft would move forward and the empty weight would go up. When looking at shimmy, I have concluded there must be a structural resonant frequency that contributes to, or at least is excited by the shimmy in order to allow it to progress. But what is it? There are a number of contenders: The engine in its mounts, the tail in rotation, the main gear legs in a fore-and-aft motion, and the nose gear strut twisting the mount are some, and there might be more. Then there is the gyroscopic forces created by the nose wheel itself. Is the shimmy event a fixed frequency? I suspect it is fixed or close to fixed, but at what frequency? I'll bet it has never been measured. There are a number of people that have stated that the shimmy has gone away when the brake rotors are replaced or turned, and some that say that the shimmy goes away when the brakes are released. I have always experienced that with various certified airplanes including Cessna Cardinals and Skylane RG's, but also with Piper Cherokees, which don't have tubular gear. I assumed it was because the rake angle increased when releasing the brakes. Is the oscillation of the main gear what these people are calling shimmy and it has nothing to do with the nose gear, or do the brakes just excite nose gear into a shimmy. Too many unanswered questions. Gary ES #157 ________________________________ To: "" Interesting and sounds like a good idea. The wheel assembly itself is also behind the center of rotation (the strut) because of the castor of the wheel mount. And I think shimmy is generally driven by wheel loads rather than aerodynamic ones. That does not mean that ballancing all components can't help. It's not hard to imagine that adding mass to whatever wiggles may cause it to get worse. Mark Ravinski 360 ----- Original Message ----- From: "Pete Sinclair" To: lml Sent: Friday, December 4, 2009 4:28:30 PM GMT -05:00 US/Canada Eastern Subject: [LML] Flutter and nose wheel shimmy So if we balance ailerons and elevators, why not balance the nose gear fairing by adding weight in the front until it is neutrally balanced around the axle? Would this not help reduce nose gear "flutter"? I understand that the real center of vibration of the nose gear is around the strut axis, but balancing the wheel pant would at least bring the center of balance farther toward this axis. Comments? Pete --0-2051733245-1260193634=:94559 Content-Type: text/html; charset=us-ascii
I have thought about the CG of the nose wheel assembly being behind the pivot center (that would be the axis of the strut) being a consideration in the shimmy event, but I doubt that the fairing itself has much to do with it.  However, I did truncate the rear my fairing by cutting off 2 or 3 inches for just that purpose.  The tail of the fairing is rather crudely made and is essentially a solid plug of epoxy.  I closed off the stub with a thin layer of cloth - much lighter.  In order for a given component to contribute to the shimmy it has to have a resonant frequency in the range of the shimmy frequency.  Give the side of the fairly a sharp rap and you'll find it really doesn't respond as the resonant frequency is quite high.  The fairing could, however aggravate the event by adding to the already large mass of the wheel and tire, which is also behind the rotation axis.  I can't remember where, when or on which airplane, but I recall seeing a large lead weight mounted out ahead of a nose wheel.  I assume it was to prevent shimmy, but I have no proof.  Adding a weight inside the front of the fairing that would balance the weight of the wheel and tire would be difficult and the result would add significant weight.  Then I would be skeptical that the strength and stiffness of the fairing would be sufficient.  But, would a small amount of weight up front be enough to just improve the balance of the system enough to eliminate the shimmy?  It might be.  The only negative I see is that the CG of the aircraft would move forward and the empty weight would go up.

When looking at shimmy, I have concluded there must be a structural resonant frequency that contributes to, or at least is excited by the shimmy in order to allow it to progress.  But what is it?  There are a number of contenders:  The engine in its mounts, the tail in rotation, the main gear legs in a fore-and-aft motion,  and the nose gear strut twisting the mount are some, and there might be more.  Then there is the gyroscopic forces created by the nose wheel itself.  Is the shimmy event a fixed frequency?  I suspect it is fixed or close to fixed, but at what frequency?  I'll bet it has never been measured.

There are a number of people that have stated that the shimmy has gone away when the brake rotors are replaced or turned, and some that say that the shimmy goes away when the brakes are released.  I have always experienced that with various certified airplanes including Cessna Cardinals and Skylane RG's, but also with Piper Cherokees, which don't have tubular gear.  I assumed it was because the rake angle increased when releasing the brakes.  Is the oscillation of the main gear what these people are calling shimmy and it has nothing to do with the nose gear, or do the brakes just excite nose gear into a shimmy.  Too many unanswered questions.
Gary
ES #157

To:
"" <LML@yahoo.com>

Interesting and sounds like a good idea.

The wheel assembly itself is also behind the center of rotation (the strut) because of the castor of the wheel mount.

And I think shimmy is generally driven by wheel loads rather than aerodynamic ones.  That does not mean that ballancing all components can't help.

It's not hard to imagine that adding mass to whatever wiggles may cause it to get worse.

 

Mark Ravinski

360



----- Original Message -----
From: "Pete Sinclair" <pete@leapfrogventures.com>
To: lml
Sent: Friday, December 4, 2009 4:28:30 PM GMT -05:00 US/Canada Eastern
Subject: [LML] Flutter and nose wheel shimmy

So if we balance ailerons and elevators, why not balance the nose gear fairing by adding weight in the front until it is neutrally balanced around the axle?  Would this not help reduce nose gear "flutter"?  I understand that the real center of vibration of the nose gear is around the strut axis, but balancing the wheel pant would at least bring the center of balance farther toward this axis.

Comments?

Pete


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