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<DIV><FONT face=Arial size=2>Re-read the Ackerman page, and these posts.
Sounds like "right" on a pull-pull setup is clevis attache points ON the hinge
line, and no fore/aft offsets on the servo arm side.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Dean P. - Can you provide and "preview" insights on
the Exc. 90 review, as I have one that is awaiting testing shortly. With
Ackerman, but not the "bad" kind.</FONT></DIV>
<DIV><BR>Bob Pastorello<BR>NSRCA 199 AMA 46373<BR><A
href="mailto:rcaerobob@cox.net">rcaerobob@cox.net</A><BR><A
href="http://www.rcaerobats.net">www.rcaerobats.net</A></DIV>
<DIV> </DIV>
<DIV> </DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=tkeithb@comcast.net href="mailto:tkeithb@comcast.net">Keith Black</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=discussion@nsrca.org
href="mailto:discussion@nsrca.org">discussion@nsrca.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Monday, November 29, 2004 9:25
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Elev. Pull pull
Question</DIV>
<DIV><BR></DIV>
<DIV><FONT face=Arial size=2>According to the Ackerman principle having the
clevis hole exactly on the hinge would result in a the cables keeping the same
tension through the entire throw range. Behind the hinge would cause the cable
opposite the pulling cable to create slack. In front of the hole will cause
disastrous effects! </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Personally I'd like the idea of having the same
tension through the entire throw range, however, for safety sake I set the
hole just slightly behind the hinge (1/64 to 1/32) because it's difficult to
see when it's EXACTLY over the hinge. This results in me having a VERY minor
loosening of the opposite cable.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>So what's the "disastrous effect"?
</FONT><FONT face=Arial size=2>One of the local new pattern guys spent a year
building his first 2M plane and misunderstood the issue of having the control
horn on the hinge line. He thought the *post* of the control horn was supposed
to be on the hinge. He devised an interesting system to get his control post
exactly on the hinge which put the clevis hole around 1/8" in *front* of the
elevator hinge line. On the first flight when in a downline the elevator
fluttered and snapped the stab (though it didn't fall off). The plane was
landed and about two months later he came back with a repaired stab and tried
it again. Again in a downline the stab snapped. After this second occurrence I
took a look at his setup and noticed the clevis hole being in front of the
hinge line. After thinking about it for a bit I realized what had happened.
When the hole is in front of the hinge it will create Reverse Ackerman. This
means that as the pulling cable tries to move the control surface to one side
the opposite side gets tighter. At high airspeeds as the pressure pushes
the rudder one direction the opposite side will tighten and pull the rudder
back to the other side, this in turn will tighten the first side and pull the
rudder that direction. What he'd created was an Ackerman induced
flutter.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Sadly after the stab was again repaired and the
hinge problem corrected within the next couple of flights the elevator servo
came loose during a flight and the plane was lost. It's believe that the
flutter from the earlier flights loosened the elevator servo. </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Therefore, even though I would like the hole
exactly on the hinge line, I give myself a slight safety margin.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Keith Black</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV>----- Original Message ----- </DIV></BLOCKQUOTE></DIV></BODY></HTML>