Elev. Pull pull Question

[Keith Black]

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 ----- 
  From: Bob Pastorello 
  To: discussion at nsrca.org 
  Sent: Monday, November 29, 2004 5:19 PM
  Subject: Re: Elev. Pull pull Question


  If we align the clevis attach point with the hinge line, doesn't that "undo" the desired Ackerman effect that we need? (Thanks Keith)
      I agree with stuff over the hinge line in principle, but would appreciate any informed technical explanation (go, Dean!!), as I like to understand the "why" of stuff.

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


    ----- Original Message ----- 
    From: Dean Pappas 
    To: discussion at nsrca.org 
    Sent: Monday, November 29, 2004 10:42 AM
    Subject: RE: Elev. Pull pull Question


    Hello Bob,
    Go buy some DuBro # 867 control horns. They have almost 3/4" of overhang between the bolt and the little hole for the clevis, in the little screw-on pieces.
    Use those pieces, but set the bolt and nut aside, and see if the little holes line up with the center of the hinges.
    If not, then drill new holes in the black plastic bits so that they do line up: one straight line through both holes and the center of the hinge.
    Now when you get the horns all the same length, it will be much easier to get the throws matched in both directions.
    This is absolutely necessary for matching two elevator halves properly, but is also a good idea for rudders.
    The problem with the stock setup is that the cables slacken at the ends, and tighten at neutral.
    While this might sound like a good thing, it is awful: the servo has to fight the tightening cables as the surface comes closer to neutral,
    then as it gets there, the surface may try to "spring" to the opposite direction.
    The servo and gear train will prematurely beat themselves up chasing this problem.
    You want the cable tension to be almost constant throughout the throw.
    There will be pictures of this when I get the Excelleron review out the door.
    One of the previous posts pointed to a website that showed the proper geometry.
    DuBro and Rocket City both make parts that are useful, for this problem.

    Check DuBro.com and put #867 into their search engine, you can see the lonng overhang part. It's either 5/8" or 3/4" from bolt to clevis hole.

    Later,

    Dean Pappas 
    Sr. Design Engineer 
    Kodeos Communications 
    111 Corporate Blvd. 
    South Plainfield, N.J. 07080 
    (908) 222-7817 phone 
    (908) 222-2392 fax 
    d.pappas at kodeos.com 

      -----Original Message-----
      From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On Behalf Of Bob Pastorello
      Sent: Friday, November 26, 2004 8:58 AM
      To: NSRCA
      Subject: Elev. Pull pull Question


      May seem obvious, but I could use some help here....setting up my Excelleron, centers are okay, cable tensions are pretty close, centering is good.  Up elevator is fine, on Down elevator, ONE elevator moves farther, but they track together except at the endpoint, as close as I can tell.

      Anyone have suggestions on how I'd fine tune this out...??
      Hope everyone had a great T-day!!

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

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