Control horns, is it just me or... ?

[Verne Koester]

Keith

The biggest problem is with the rudder where you'll likely be using a pull-pull setup. I resorted to the big Sullivan horns on my Smaragd to get the pivot point right on the hingeline. If you don't with a  pull-pull setup, you'll have differential throw from one side to the other which will show up as one cable getting tighter (on the pulling side) while the other cable goes slack. One way around that is to offset the attachment on the servo horn exactly the same distance that the pivot point is offset from the hingeline. For example, if the pivot point on your rudder horns is 3/32 rearward of the hingline, you would offset the attachment points on the servo horn, rearward, by 3/32. This really does work, but requires a round servo horn since you won't have any material left going 3/32 rearward on the arm-type servo horns. Even with the round type, the constant radius of the round horn means that your new attachment points are going to be closer to the middle of the wheel than at their normal point meaning less throw. I've done this with Futaba's and was able to get enough throw using their extra-large wheel that comes with the servo. JR's largest wheel is a bit smaller and I couldn't get enough throw out of it with the offset.

The other solution is to double bevel the hingeline at the rudder post. You bevel the rudder LE as always but only half as much which effectively gets your rudder horn closer to the hingeline. You also bevel the tailpost on your fin the same amount. You'll end up with all the throw you'd want and then some while moving the hingeline rearward to help accommodate the "not long enough" horns. This is what I'm going to do on my next Smaragd so I can use those pretty anodized horns I got from Lamar! Having said all this, I wish we had hardware available that eliminated the need for all this. I don't worry about this on the ailerons and elevator since offsetting the servo horn is a simple matter of rotating the horn until the geometry creates equal throw. I let my radio take care of any tweaking once I get the geometry within a half degree or so. I usually end up changing it in the trimming process anyway. Hope this helps.

Verne
  ----- Original Message ----- 
  From: Keith Black 
  To: discussion at nsrca.org 
  Sent: Tuesday, September 02, 2003 9:57 PM
  Subject: Control horns, is it just me or... ?


  When it comes to control horns for pattern planes it seems to me that some of the most commonly used choices have a major flaw.

  On my last plane I used the MK Aluminum Horns on all surfaces except for the rudder where I used the IM pull-pull horns. Both of these are nice horns except they have a major flaw when using a pull-pull setup. The flaw is that the little black plastic arm that screws onto the horn and connects to the clevis is so short that it doesn't extend out over the hinge line. I know that with pull-pull systems it isn't *absolutely* necessary for the connection to be exactly above the hinge, but the further back it is the more slack there will be in the cable that is not pulling.  In my opinion with these horns the connection point is too far away from the hinge unless the horn is virtually mounted on the control surface bevel (especially when it comes to rudder). This can easily be seen in these "instructions" for installing the MK horns http://www.centralhobbies.com/buildingtips/mkhorns/mkhornsinst.html. Notice that in this picture there surface isn't beveled yet and the horn is almost on the edge.

  On my new plane I wanted to avoid this problem so I did some searching to find a better solution. To begin with I looked for a longer arm to connect to the MK horn, one that would extend out to the hinge line. I found two options from Sullivan but both require too big of a horn thread (6-32 and 8-32). My search then led me to two other choices that seem to be good alternatives. The first is the ZNLine aluminum control horns, but these are apparently only distributed by ZNLine (http://www.znline.com/produits.php?langue=english&cle_menus=1025879382&cle_data=1025901639) which means an international order. The other solution I've found is by RC Model Enterprises (http://home.att.net/~rc-enterprises/rcme_014.htm) but these control horns seem very pricey!

  If anyone could point me to a longer 3mm connecting arm that will fit on the MK and IM horns I'd be happy (BTW, I've used the MK BB connectors on the MK horn and they still aren't really long enough).

  Is it just me, or have others experienced this dilemma as well?

  Thanks,
  Keith Black

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