X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 24 Jul 2005 22:38:19 -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 613150 for lml@lancaironline.net; Sat, 23 Jul 2005 23:12:38 -0400 Received-SPF: pass receiver=logan.com; client-ip=64.12.138.207; envelope-from=RWolf99@aol.com Received: from RWolf99@aol.com by imo-m17.mx.aol.com (mail_out_v38_r1.7.) id q.2b.77b08d5a (3924) for ; Sat, 23 Jul 2005 23:11:50 -0400 (EDT) From: RWolf99@aol.com X-Original-Message-ID: <2b.77b08d5a.301460f6@aol.com> X-Original-Date: Sat, 23 Jul 2005 23:11:50 EDT Subject: Re: Counterbalance X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1122174710" X-Mailer: 9.0 SE for Windows sub 5017 -------------------------------1122174710 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit 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) -------------------------------1122174710 Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable
Why do you need to balance a rudder to prevent flutter?  This is a= =20 very-often-heard question, even within an airplane design company, so don't=20= feel=20 bashful about asking.  Here's my version of the answer ...
 
The vertical fin deflects under load, just like a wing or horizontal=20 tail.  We are interested in dynamic effects here -- not the steady stat= e=20 loads that you'd get in a steady side slip.  Something applies a side l= oad=20 to the vertical fin (a gust or a rapid rudder input) and the fin bends towar= ds=20 one side.  If the center of mass of the rudder is behind the hinge line= ,=20 the rudder will lag behind the deflection of the fin.  This means that=20= the=20 rudder is deflecting in the direction to INCREASE the side load on the fin,=20= thus=20 amplifying the deflection.  The fin overshoots it's steady-state deflec= tion=20 (any moving thing will do that) and starts to move back towards the steady-s= tate=20 deflected position.  As it changes it's direction the rudder once again= =20 lags behind, thus providing a driving force to move the vertical fin farther= =20 than it otherwise would have gone.
 
If the center of mass is ahead of the hinge line, the nose of the rudde= r=20 will lag behind the fin deflection, thus the rudder applies an aerodynamic f= orce=20 which REDUCES the side load and therefore the tendency of the fin to=20 deflect.
 
The key here is not that we're trying to make the nose of the surface h= ang=20 downwards (well, we are, but gravity is not the issue), but rather than we'r= e=20 trying to move it's inertia (it's center of mass) to be forward of the hinge= =20 line, so deflections of the structure to which it's attached make the surfac= e=20 move to generate aerodynamic forces which DECREASE the load which made the=20 structure deflect in the first place.  It's the same for a wing/aileron= , a=20 stabilizer/elevator, or a fin/rudder.  We just use gravity to help us=20 measure where the center of mass is.  It doesn't matter whether the sur= face=20 is used horizontally (ailerons and elevators), vertically (rudders), or=20 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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