Pushrods and pull-pull setup for rudder and elevator

Rudder will be connected to servo with a pull-pull system, operated by two kevlar leads or similar. The leads will cross inside the fuselage.
Elevator will be actuated by MK elev coupler. There will also be a pull-pull system between the coupler and servo. From the coupler to elevator halves movement is driven by 4 mm carbon fibre pushrods. Pushrods will have ball links 2 mm thread.
Horns on control surfaces will be of 4 mm CF-balsa-CF sandwich that a friend of mine has produced.

MK elev coupler

Click to see a larger picture Click to see a larger picture Click to see a larger picture Click to see a larger picture  

MK coupler

and a cardboard
pattern for
pull-pull arm

Coupler
inside fuselage

Elev pushrod
dry fitted in place

  

 

Some progress:

Click to see a larger picture Elev coupler has got a pull-pull arm. This is required as I use kevlar wires instead of push rod to reduce weight and
possible vibration problems. Important: MK coupler arms are NOT allways perfectly ALIGNED. I had a 5-degree
misalignment! That had to be corrected by applying a little CA on shafts and carefully grinding them to correct form.
Not a big job but should not be necessary for a quality brand like MK.
Click to see a larger picture Same from different angle. The additional double arm is of 2 mm PE plate. Plate is fixed on MK coupler with
two bolts secured with CA.
Click to see a larger picture See additional servo arm on rudder servo. The arm is needed to bring wires further from pivot point. This reduces
force on wire and helps prevent slack. Note that ball links reside under the servo arm to reduce load on servo
bearings. This is by far better solution than an additional support on servo axis as it is pratically impossible to put
three bearings accurately on same axis. Additional support causes more harm than help.
Click to see a larger picture This is CF-balsa-CF plate. That is what rudder, elev and aileron horns are made from.
Very light, stiff and strong.
Click to see a larger picture Cut out horns. The bigger one is for rudder, weight 3 grams, same as a pair of elev horns.
For elevator the holes for ball links will be drilled later after glueing horns in control surfaces.
It is important to have the angles identical to get exactly the same amount of deflection.
I made a jig from cardboard to accomplish that.
Click to see a larger picture The leading edge of rudder partially removed. A slot was cut for horn. The tail of horn is embedded inside.
Be sure that the location is correct. I made a fake rudder from 10x20 balsa spar and glued a ply horn on it.
With this tool I set up the wire line and found the exact location for the horn in rudder.
Click to see a larger picture From another angle.
Click to see a larger picture The horn was glued to rudder with Cascol Pu glue. The horn was cleaned with solvent and mildly ground before
glueing. You can see how the glue expands when it has just a little moisture from a spray bottle.
Remember to protect the horn with tape against expanding glue.
Click to see a larger picture Cut out for elevator horn. Same technique as rudder.
Click to see a larger picture Elevator push rods. They are made from 2 mm stainless steel and 4 mm carbon rod. Glued together with
Super Epoxy (TM). If you suspect the strenght of this the steel rod is stronger than model airplane grade 3 mm or
4-40 as the steel is quenched and tempered, tensile strenght 1980 N/mm2. Original use is bicycle spokes.
Glue joint is tested with 30 kg weight, no problem. Weight for two rods with ball links 10 grams together.
Click to see a larger picture The push rods assembled
Click to see a larger picture The control setup of tail, in other words, rudder pull-pull setup and elevator push rod setup.