Elevator Push / Pull Tube Modification
Modern builders usually have these cables making their upward turn around pulleys mounted under the pilot’s seat instead of using the wooden cross member as the fairlead.
Another option is to build a push / pull tube arrangement from the aft control stick to the walking-beam instead of cables and pulleys. I would like to take a moment and say that there is nothing wrong with the pulleys and cables arrangement. Lots of builders choose this method and it works great. The push / pull tube arrangement is just an option, not necessarily an “improvement”.
In the mid 90’s I was attending one of my first Brodhead reunions and was looking over a beautiful red and cream Pietenpol. The owner had a push / pull tube arrangement in the cockpit and I liked how it looked so my building partner, Dale Johnson, and I decided we would build something similar.
The connection between the front and the rear control sticks remains unchanged. From the rear control stick to the walking beam we installed 1/2 X .035 4130 tubing with rod end bearings on the tube ends for smooth actuation.
The distance from the control stick to the walking beam is relatively long for a 1/2” tube to travel unsupported when under compression. The tube also must change direction aft of the pilot’s seat to connect with the walking beam. To satisfy both of these needs we use a system of two push / pull tubes connected end-to-end and install a pivot assembly where the tubes meet. This pivot assembly is located just aft of the pilot’s seat where the tube direction changes and allows articulated movement between the two push / pull tubes.
The pivot assembly is not under a lot of stress so it can be built light. We used 3/8 X .035 tubing, supported laterally on bearings identical to the walking beam bearings. The bearings are mounted on the vertical wood frame members that also support the pilot’s seat back. A triangulated arm is welded to the lateral tube. The apex of this triangulated arm is the common attach point for both of the push / pull tubes.
The rod end bearings we used are aircraft rated Heim HME-4M. Female threaded collars are welded into the ends of the 1/2” tubing to attach the male-threaded rod-bearings. Lock nuts are installed at each rod end bearing. Use the larger AN970 washers on the outside of all rod ends during assembly. This is a safety consideration and will maintain control system integrity in case a ball pops out of a rod end.
A final consideration is control geometry. The plans show a distance of 6 ˝” from the point where the control stick mounts to the torque tube (its rotational axis) to the point where the elevator cables attach. The distance between the rotational axis of the walking beam and its cable attach points is 3 7/8”. This ratio of 1.7 / 1 must be maintained with the push / pull tube attach points if you want to retain the pitch control geometry as designed by Mr. Pietenpol. On NX18235 the push / pull tube is mounted 3 ˝” above the torque tube. This coincides with the mounting location of the tube connecting the forward and aft control stick. A single bolt can then be used to attach the connecting tube and the push / pull tube at the aft control stick. It also mounts the push / pull tube low enough on the stick to clear the pilots seat. The mounting point on the walking beam is (3.5 / 1.7 = 2.0) 2” below its rotational axis.
I purposely haven’t given dimensions of the pivot assembly or the push / pull tube lengths. This is because no two Pietenpols are built alike and you’ll have to build your assemblies to fit your own airframe.