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To:  lml@lancaironline.net
Date: Sun, 26 Feb 2012 16:42:35 -0500
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From: Charles Brown <browncc1@verizon.net>
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Subject: Re: [LML] Re: 360 rudder balance
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References: <list-5414034@logan.com>


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Grayhawk,

Control surface balancing has nothing whatsoever to do with gravity or =
with surfaces hanging level when mounted.  Gravity hinge moments are (a) =
small  (b) steady   (c)  in the case of ailerons and rudder, invisible =
to the pilot  (d) in the case of elevators, trivial at most and can be =
taken out with the trim tab as desired.

Control surface balancing is about making sure that if the surface =
starts to flap up and down at very high speeds and dynamic pressure, =
when vortex shedding frequencies start to match natural frequencies of =
the structure, it doesn't through inertial coupling excite a =
corresponding motion in the fixed surface it's mounted to (wing, =
vertical stabilizer, horizontal stabilizer) and drive the whole =
collective structure unstable.  The best way to prevent inertical =
coupling is to have the CG of the moveable surface exactly at the =
hingeline -- neither above, below, ahead or behind.  That's possible in =
a centerline hinge, not possible in a hinge that's on the top surface.  =
The best compromise for a top-surface hinge is to have the CG directly =
below the hinge.

CB

On Feb 24, 2012, at 3:36 PM, Sky2high@aol.com wrote:

Gents,
=20
Chris is right.  In other words:
=20
Interesting.  Think about a balance beam scale used by that Justice =
lady.  First, if the weights were above the pivot point, the =
scale/system would be unstable.  Notice that the weights hang much below =
the pivot making the system stable and, should it be perfect, the beam =
would stay in whatever position if the weights were equal.  But, such =
scales have a very slight built in bias to only be in balance when the =
beam is level.
=20
...

Grayhawk=

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<html><head></head><body style=3D"word-wrap: break-word; =
-webkit-nbsp-mode: space; -webkit-line-break: after-white-space; =
"><div>Grayhawk,</div><div><br></div><div>Control surface balancing has =
nothing whatsoever to do with gravity or with surfaces hanging level =
when mounted. &nbsp;Gravity hinge moments are (a) small &nbsp;(b) steady =
&nbsp; (c) &nbsp;in the case of ailerons and rudder, invisible to the =
pilot &nbsp;(d) in the case of elevators, trivial at most and can be =
taken out with the trim tab as desired.</div><div><br></div><div>Control =
surface balancing is about making sure that if the surface starts to =
flap up and down at very high speeds and dynamic pressure, when vortex =
shedding frequencies start to match natural frequencies of the =
structure, it doesn't through inertial coupling excite a corresponding =
motion in the fixed surface it's mounted to (wing, vertical stabilizer, =
horizontal stabilizer) and drive the whole collective structure =
unstable. &nbsp;The best way to prevent inertical coupling is to have =
the CG of the moveable surface exactly at the hingeline -- neither =
above, below, ahead or behind. &nbsp;That's possible in a centerline =
hinge, not possible in a hinge that's on the top surface. &nbsp;The best =
compromise for a top-surface hinge is to have the CG directly below the =
hinge.</div><div><br></div><div>CB</div><br><div><div>On Feb 24, 2012, =
at 3:36 PM, <a href=3D"mailto:Sky2high@aol.com">Sky2high@aol.com</a> =
wrote:</div><br class=3D"Apple-interchange-newline">

<meta content=3D"text/html; charset=3DUTF-8" http-equiv=3D"Content-Type">
<meta name=3D"GENERATOR" content=3D"MSHTML 9.00.8112.16441">
<div style=3D"FONT-FAMILY: Arial; COLOR: #000000; FONT-SIZE: 10pt" =
id=3D"role_body" bottommargin=3D"7" leftmargin=3D"7" rightmargin=3D"7" =
topmargin=3D"7"><font id=3D"role_document" color=3D"#000000" size=3D"2" =
face=3D"Arial">
<div>Gents,</div>
<div>&nbsp;</div>
<div>Chris is right.&nbsp; In other words:</div>
<div>&nbsp;</div>
<div>Interesting.&nbsp; Think about a balance beam scale used by that =
Justice=20
lady.&nbsp; First, if the weights&nbsp;were above the pivot point, the=20=

scale/system would be unstable.&nbsp; Notice that the weights hang much =
below=20
the pivot making the system stable and, should it be perfect, the beam =
would=20
stay in whatever position if the weights were equal.&nbsp; But, such =
scales have=20
a very slight built in bias to only&nbsp;be in balance when the beam is=20=

level.</div>
<div>&nbsp;</div>
=
<div>...</div><div><br></div><div>Grayhawk</div></font></div></div></body>=
</html>=

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