Elev. Pull pull Question

[Bob Pastorello]

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.

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.

Bob Pastorello
NSRCA 199  AMA 46373
rcaerobob at cox.net
www.rcaerobats.net


  ----- Original Message ----- 
  From: Keith Black 
  To: discussion at nsrca.org 
  Sent: Monday, November 29, 2004 9:25 PM
  Subject: Re: Elev. Pull pull Question


  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!  

  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.

  So what's the "disastrous effect"?  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.

  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. 

  Therefore, even though I would like the hole exactly on the hinge line, I give myself a slight safety margin.

  Keith Black

  ----- Original Message ----- 
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