Idler 2.0 part III

Just a quick post today to close the loop with the idler saga. I fitted the KP4 bearings with the spacer in between. At first, when pressing the bearings in, they did not rotate freely due to residual load on the spacer. I took a screwdriver to play with the spacer radially and after a few cycles, the bearings rolled well. Here is the final product:

In position once all fasteners propely torqued:

I also fixed the new trim tab to the elevator for the first time today. Next step is to paint it and connect the trim tab wire!

Idler 2.0 Part II

Finally had the chance to fit the new idler in the airplane with all the elevator control system connected from the sitck to the elevator. I wanted to do that before painting the idler in case i had trimming to do first.

On the first try, there was still a little restriction when deflecting the stick to the RH stop. I suspected the washers I added on each side of the rod end connected to the aft hole of the idler to be interfering with the bearings. I grinded the washers to allow more clearance and it helped, but didn't cure completely the problem. I used a belt grinder to remove material from the washers:

After a thorough inspection of the rod end clearances throughout the range of motion of the stick, I haven't found any fouling condition. I was puzzled. This meant that I was reaching the limit in twist of the rod ends.

I took a step back and realized that the stick had more travel on the RH side than the LH side... In fact, I had only one washer on the RH stop bolt vs two on the other side. Before changing anything, I measured the angle on each side and confirmed the RH travel was 24° vs 20° on the LH side. The travel recommended in the assembly manual is +/- 20° so I decided to add a washer on the RH stop bolt and get back to symmetrical travel. I was happy to find that the restriction was gone after that.

I removed the idler, cleaned, primed and painted. Next step is to fit new bearings and reinstall in the fuse!

Idler 2.0 part I

Quick post today: pick up my new, thicker idler today. The internal distance between faces went from 7/16 (first idler) to 17/32. This is going to allow rod end clearance adjustment using a standard washer on one side and a light washer on the other side.

If you wonder what the bushing on my cutting board is for, it's a spacer that goes between the two KP4 bearings so that they don't take axial load when torquing the axle bolt.

Idler + torque tube assembly

Happy with my new idler, I tried a first fit in the torque tube and realized it was not aligned with its longitudinal axis. The idler had a non negligible nose left angle. I knew the idler was straight i.e. its angle was due to the torque tube tower bushings being misaligned. Here's a sketch to better visualize the problem. The black line is the perfect axis which is perpendicular to the longitudinal axis of the airplane. The blue line is to show how the actual axis of my torque tube was (exagerated):

 

In order to determine if the misalignment was excessive or not, I tried to connect the whole elevator linkages together with the torque tube + idler. It turned out I had very limited aileron movement due rod end bearings being at the end of their ±10° twisting range. This problem is well explained in a thread on Biplane Forum here. 

After my efforts to build a new idler from scratch, finding out it didn't fit wasn't pleasant. However, I understood a little better what happened to the idler. My #1 theory for the twisted idler is now that the twist was done from day 0, in a bench vice. I suspect the builder originally had the same problem as above and decided to twist the idler instead of cutting the torque tube bushings and weld them again.

When I realized that, I decided to bite the bullet and fix the torque tube. In order to have the new bushings at 90° from the torque tube axis, I built a jig with blocks of wood aligned on a board. To align everything up, I used a piece of extruded aluminium of the same width as the OD of the torque tube, riveted with pop rivets:

Next step was to cut the old bushings and grind them down to the torque tube tower surfaces. After doing that I enlarged the LH bushing hole to the OD of the bushing. Here's what it looked like in the jig:

By doing that, it was clearly visible that the original bushings were not 90° from the torque tube axis:

After enlarging the RH side to allow the tube to reach the RH wood block, I finally had a welding jig ready. I removed some paint and gave it to my friend Mark so that he welds both sides in place. Here's the result:

Then I cut the tube to final bushing lengths:

I now have my torque tube tower axis at the right angle! Next step will be to prime and paint then reinstall in the airplane. Stay tuned!

Idler - Final result

Small update to show the final result with the idler.

Mounting it all together:

Final comparison:

My next objective is to mount to fuselage back on its gear. Stay tuned!

lower wing to fuse bushings + idler

Quick update for today. I mixed some gray epoxy paint and painted my new idler:

I used the remaning paint to paint the lower wing bushing that I needed to replace. This closes the loop on a long head scratching exercise (see post here).

Next step is to install new KP4 bearings in my new idler and begin the reassembly of the pitch control system!

Idled due to an idler

Haven't posted much lately, but I have worked on the Pitts quite a bit. There's a finding that slowed down my reassembly; here's the story:

I was almost going to start to put the airplane back together, but when I looked at the front end of my torque tube, I just couldn't keep it like this. Do you notice what's wrong on this picture? (except from the red dye penetrant residue and the burned paint)

It is subtle on the picture, but if you look closely at the very front end of the torque tube, you will see that the bolt that goes though the front bearing doesn't protrude enough to be compliant with the airplane Bible a.k.a. AC 43-13.

So I decided to bite the bullet and remove the torque tube assembly in order to replace that bolt with a longer one. While doing that, I took the opportunity to inspect each component of the assembly and I was shocked to find the forward idler like this:

As you can see, the part is twisted, so was my facial expression. One thing for sure is that this part was not going to fly again. I compared my part with the original Pitts drawing similar to this one:

The first obvious observation is that the original part uses a plain bushing instead of bearings. After some discussions with my friend Peter, I realized that on the Pitts S1-T, this part is also made with bearings which actually help in minimizing slop in the flight controls.

The second obvious difference is that the original part does not have any lightening hole. I guess the original builder of my idler liked swiss cheese; he made the part just like it. I don't especially like the idea of having lightening holes in a dynamic member of my elevator control system. Looking closely at the second picture, you may see that the top center hole as well as the tiny hole on the right are both elongated. When a part is stressed in torsion, internal shear loads tend to propagate at 45° (see explanation here). The holes being along a 45° line from the part's axis, the part was much weaker in torsion than originally designed for. And it twisted. I was happy to find this out at 0 ft and 0 kt. 

A third difference exists, but this one is not obvious on the pictures: the total height of the part is 3/16" smaller than on the drawings. I have not found the reason why the builder chose to build it like this; perhaps to save weight. When I realized that, I had to make sure that the geometry actually made sense. There is a famous sentence that is often used among the Pitts community: "Never try to second guess Curtis Pitts". 

I made measurements of the torque tube tower as well as the current idler and modelized it to have a clear view on the geometry. Here's what it gave:

Then I modelized the original Curtis Pitts assembly:

My initial fear was that the torque tube tower height was per drawing, but not the idler. However, the builder actually built the torque tube tower also shorter than the drawings, but perhaps not short enough according to the drawings above. The problem is that when the idler is fully aft (full nose down input) the shorter idler pushes the rod ends with more misalignment relative to the torque tube axis than the original design. When combining this misalignment with a roll input, the two pushrods can end up with a lateral misalignment as well. This can be a contributor to the torsional load on the idler.

Bottomline, both the shorter idler and the lightening holes were acting towards exceeding the part's limit loads in torsion.

No way I was going to but back a twisted critical part on the airplane. I had to replace it, which meant making a new one from scratch since it was not per original drawings. I decided to make it a little longer in order to have a better alignment with the torque tube axis while keeping sufficient clearance with the bottom of the torque tube:

In the awesomely well equipped shop of my friend Scott, we built the parts with 4130 steel:

The logical next step was to call back Mark, the welder I hired to repair my fuselage. Again, he did an excellent job. Here's old vs new:

Today I primed my new idler:

Next step is to paint and finally begin the reassembly! Oh I almost forgot to mention I made two new access holes in the fabric to provide better access to the aft idler as well (yes, the fabric shows its age on this one... )

Lots of work to install the reinforcement rings, but this will facilitate maintenance in this hard to reach area.

Thanks for reading!