X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Wed, 27 Jul 2005 15:45:50 -0400 Message-ID: X-Original-Return-Path: Received: from imo-m17.mx.aol.com ([64.12.138.207] verified) by logan.com (CommuniGate Pro SMTP 4.3.6) with ESMTP id 616654 for lml@lancaironline.net; Wed, 27 Jul 2005 11:55:55 -0400 Received-SPF: pass receiver=logan.com; client-ip=64.12.138.207; envelope-from=Mquinns@aol.com Received: from Mquinns@aol.com by imo-m17.mx.aol.com (mail_out_v38_r4.1.) id q.67.49aab21d (15887) for ; Wed, 27 Jul 2005 11:55:06 -0400 (EDT) Received: from mblk-d44 (mblk-d44.mblk.aol.com [205.188.212.228]) by air-id08.mx.aol.com (vx) with ESMTP id MAILINID82-3e0f42e7ae5a1c4; Wed, 27 Jul 2005 11:55:06 -0400 X-Original-Date: Wed, 27 Jul 2005 11:55:06 -0400 X-Original-Message-Id: <8C760DAB16ABF1A-E28-1ED6@mblk-d44.sysops.aol.com> From: mquinns@aol.com References: Received: from 207.126.196.16 by mblk-d44.sysops.aol.com (205.188.212.228) with HTTP (WebMailUI); Wed, 27 Jul 2005 11:55:06 -0400 X-MB-Message-Source: WebUI X-MB-Message-Type: User In-Reply-To: X-Mailer: AOL WebMail 1.1.0.13360 Subject: Re: [LML] Re: Counterbalance Content-Type: multipart/alternative; boundary="--------MailBlocks_8C760DAB16ABF1A_E28_1E47_mblk-d44.sysops.aol.com" MIME-Version: 1.0 X-Original-To: lml@lancaironline.net X-AOL-IP: 205.188.212.228 X-Spam-Flag: NO ----------MailBlocks_8C760DAB16ABF1A_E28_1E47_mblk-d44.sysops.aol.com Content-Type: text/plain; charset="us-ascii" Rob, I understand your explanation but need to ask a related question. Your explanation deals with the balancing mass equalizing the weight on both sides of the hingeline (I think). Is there any importance to the weighting (balancing) of the rudder top to bottom? I am asking the question because my rudder needs to have some balancing weight removed and it is weighted both at the top and bottom. My preference is to remove weight at the bottom of the rudder which has a shorter moment arm. The shorter moment arm of the lower weight will allow more to be remove to lighten the total weight of the rudder. BUT, will this have any impact on the strength, balance, bending of the rudder versus removing the weight at the top of the rudder (with the longer moment arm)? Mark Quinn Daytona Beach 360 MK II ---Original Message----- From: RWolf99@aol.com To: Lancair Mailing List Sent: Sun, 24 Jul 2005 22:38:19 -0400 Subject: [LML] Re: Counterbalance Why do you need to balance a rudder to prevent flutter? This is a very-often-heard question, even within an airplane design company, so don't feel bashful about asking. Here's my version of the answer ... The vertical fin deflects under load, just like a wing or horizontal tail. We are interested in dynamic effects here -- not the steady state loads that you'd get in a steady side slip. Something applies a side load to the vertical fin (a gust or a rapid rudder input) and the fin bends towards one side. If the center of mass of the rudder is behind the hinge line, the rudder will lag behind the deflection of the fin. This means that the rudder is deflecting in the direction to INCREASE the side load on the fin, thus amplifying the deflection. The fin overshoots it's steady-state deflection (any moving thing will do that) and starts to move back towards the steady-state deflected position. As it changes it's direction the rudder once again lags behind, thus providing a driving force to move the vertical fin farther than it otherwise would have gone. If the center of mass is ahead of the hinge line, the nose of the rudder will lag behind the fin deflection, thus the rudder applies an aerodynamic force which REDUCES the side load and therefore the tendency of the fin to deflect. The key here is not that we're trying to make the nose of the surface hang downwards (well, we are, but gravity is not the issue), but rather than we're trying to move it's inertia (it's center of mass) to be forward of the hinge line, so deflections of the structure to which it's attached make the surface move to generate aerodynamic forces which DECREASE the load which made the structure deflect in the first place. It's the same for a wing/aileron, a stabilizer/elevator, or a fin/rudder. We just use gravity to help us measure where the center of mass is. It doesn't matter whether the surface is used horizontally (ailerons and elevators), vertically (rudders), or otherwise (ruddervators on a Bonanza, for example). As an exercise to the curious, why don't we balance flaps? - Rob Wolf LNC2 80% (see you at Oshkosh -- I'm taking the airlines) ----------MailBlocks_8C760DAB16ABF1A_E28_1E47_mblk-d44.sysops.aol.com Content-Type: text/html; charset="us-ascii"
Rob,
   I understand your explanation but need to ask a related question. Your explanation deals with the balancing mass equalizing the weight on both sides of the hingeline (I think). Is there any importance to the weighting (balancing) of the rudder top to bottom? I am asking the question because my rudder needs to have some balancing weight removed and it is weighted both at the top and bottom. My preference is to remove weight at the bottom of the rudder which has a shorter moment arm. The shorter moment arm of the lower weight will allow more to be remove to lighten the total weight of the rudder. BUT, will this have any impact on the strength, balance, bending of the rudder versus removing the weight at the top of the rudder (with the longer moment arm)?
 
Mark Quinn
Daytona Beach
360 MK II
 
 
---Original Message-----
From: RWolf99@aol.com
To: Lancair Mailing List <lml@lancaironline.net>
Sent: Sun, 24 Jul 2005 22:38:19 -0400
Subject: [LML] Re: Counterbalance

Why do you need to balance a rudder to prevent flutter?  This is a very-often-heard question, even within an airplane design company, so don't feel bashful about asking.  Here's my version of the answer ...
 
The vertical fin deflects under load, just like a wing or horizontal tail.  We are interested in dynamic effects here -- not the steady state loads that you'd get in a steady side slip.  Something applies a side load to the vertical fin (a gust or a rapid rudder input) and the fin bends towards one side.  If the center of mass of the rudder is behind the hinge line, the rudder will lag behind the deflection of the fin.  This means that the rudder is deflecting in the direction to INCREASE the side load on the fin, thus amplifying the deflection.  The fin overshoots it's steady-state deflection (any moving thing will do that) and starts to move back towards the steady-state deflected position.  As it changes it's direction the rudder once again lags behind, thus providing a driving force to move the vertical fin farther than it otherwise would have gone.
 
If the center of mass is ahead of the hinge line, the nose of the rudder will lag behind the fin deflection, thus the rudder applies an aerodynamic force which REDUCES the side load and therefore the tendency of the fin to deflect.
 
The key here is not that we're trying to make the nose of the surface hang downwards (well, we are, but gravity is not the issue), but rather than we're trying to move it's inertia (it's center of mass) to be forward of the hinge line, so deflections of the structure to which it's attached make the surface move to generate aerodynamic forces which DECREASE the load which made the structure deflect in the first place.  It's the same for a wing/aileron, a stabilizer/elevator, or a fin/rudder.  We just use gravity to help us measure where the center of mass is.  It doesn't matter whether the surface is used horizontally (ailerons and elevators), vertically (rudders), or otherwise (ruddervators on a Bonanza, for example).
 
As an exercise to the curious, why don't we balance flaps?
 
- Rob Wolf
LNC2 80%
(see you at Oshkosh -- I'm taking the airlines)
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