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

[Buff Miller]

Keith,

I've been following this thread for the past few days with great interest.  The discussion has been very informative and interesting, but I have to throw in a concept that has yet to be mentioned.  I suspect everyone has been "to close to the forest to see the trees" thinking control horns and geometry.  I've been using a system for the past ten years that I developed from an article Gene Rogers, former District 6 VP, covered in the K-Factor back in the early 1990's.

This system does not use control horns for the pull-pull setup on the rudder.  It uses two equal diameter pulleys.  One on the rudder the other on the servo.  I make the pulleys out of ply, three circles sandwiched together.  The two outer circles of 1/32" ply are roughly 2 - 2 1/2" in diameter.  The inner circle is of 1/16" ply about 1/8" - 3/16" smaller in diameter.  I bevel the edges of the two outer circles so that when I glue the three circles concentrically together they form a "V" pulley.  Also, dremel two notches in the "V" for the cable ends to pass out and back into the "V" much like a Hanger 9 aluminum servo wheel to enable anchoring the cable to the pulley.  Drill a small hole for a tiny wood screw, no larger than a #2.  On the servo pulley the middle of the cable can duck out of the "V" pass under the washered wood screw and back into the "V" on the other side of the screw.  The loose ends of the single cable will pass under and around the wood screw of the rudder pulley.

Install the rudder pulley in a slot cut in the rudder equal to the thickness of the pulley, 1/8", and a depth of the radius of the pulley.  Make certain that the center of the pulley is centered right at the edge of the bevel or hinge line of the rudder.  This keeps all of the geometry intact. Epoxy it in place after MonoKoting the rudder but prior to hinging the rudder to the fuse.  Nearly one of the last steps in the construction of your plane.  In preparation for the pulley cut a similar 1/8" notch, also equal or slightly larger than the radius of the pulley in the fuselage.  I epoxy a balsa block internally in this area if it is an fiberglass fuse to beef up this area.

The servo pulley is simply bolted with two 2-56 socket head bolts to a round servo wheel.  All tightening of the cables are done at the rudder pulley outside of the fuse.

The main advantage of this system is that the servo is able to maintain constant torque on the rudder through out the entire deflection of the rudder without any cable slacking or binding issue.  Plus you couldn't get any other system much cheaper.  You can also increase the torque of the servo, at the price of reducing the throw of the rudder, by making the servo pulley smaller in diameter than the rudder pulley.  Or, increase the throw of the rudder, at the price of reducing the torque of the servo, by making the servo pulley larger than the rudder pulley.  If the rudder is positioned quite far back in relation to the elevator, such as on a CAP 232, the rudder could in theory be made to rotate nearly 150 degrees or more assuming you double bevel the rudder and fuse at acute angles on the hinge line.

Just another idea to consider.  It almost took longer to write this explanation than it takes to make such a system.  Enjoy.

Buff Miller
  ----- 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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