Side panel brackets

Flight tests at work being quite demanding lately, I can barely work on the Pitts. Last week-end, I had a few hours to work on the aluminium brackets that snap on the top longerons to attach the side panels.

They originally had nyloc nutplates. I decided to change them to all metal nutplates for multiple reasons:

- The rivet holes on the brackets were not dimpled, therefore the rivets were chafing with the aluminium panels

- Nyloc may not be reused if the nylon doesn't provide enough friction (AC43-13)

- All metal nutplates are smaller and lighter

Took me a few hours to drill the old rivets, dimple all rivet holes, dimple all new nutplates and squeeze all the new rivets in place. I used a hand rivet squeezer from my friend Scott both for doing the dimples and rivet squeezing. My friend Marc also helped in replacing the nutplates on the rear brackets. Thanks Scott and Marc!

Here is the new vs old nutplate comparison:

I may have to partially tap the nutplate threads as they are pretty tight. I'll see once I will try to reinstall the side panels. Here is the result once snapped on the fuse:

Will stick some anti-chafing tape on the brackets before reinstalling the panels. I'm thinking of replacing  nutplates also on the vertical braces (right of the above picture), but the access may be challenging for dimpling and riveting. 

Mission for next work session at the hangar: install the main fuel tank.

Landing Gear Installation

Today, a great milstone was achieved on my rebuild: I reinstalled the landing gear with its new attachment points design.

As explained in this post, a major change in the way the gear is attached to the fuse was required. I got the last part I needed last week: the butterfly plate. Big thanks to my friend Scott for cutting the part with his router. I also thank my brother-in-law Jean, who anodised the part in his shop!

So here's the assembly.

Orange: butterfly plate made of 0.065" thick 2024-T3 aluminium (grey anodised)
Green: radius plates made of 7075-T6 aluminium (black anodised)
Blue: Backing plates made of 303 SS

I lifted the tail high enough so that the bottom longerons are parallel to the ground, easing up the installation of the gear:

Once all fasteners properly torqued:

Details of the stack:

Next step is to prep the engine before reinstalling on the airframe. I like where this is going  : )

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.

Finalized the torque tube

Once the torque tube welding was done and the bushings cut to final size, I removed a little paint affected by heat, primed and painted the repaired area:

I was anxious to reinstall everything in the airplane to see how the rod-end fitted now that the idler axis was perpendicular to the torque tube. Deception. The realignment fixed maybe 50% of the problem, but still there was contact between the idler and a rod-end when trying to deflect the stick laterally...

I therefore decided to bite the bullet (again) and build a new, wider idler to provide more clearance to the rod-end. 

Thanks to my friend Scott for helping me to build a second set of parts. Next step is the welding...

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!

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!

The research

The day after The Finding, I was flying to Wichita for business and I remember all sorts of questions were going through my mind. How am I going to fix this? How long I'm going to be grounded? But mainly: Why is my fuselage cracked?? I left home that morning with the firm intention of coming back two weeks later with answers to my questions.

Here's a little summary of what I found by talking to Pitts gurus in Wichita and on forums.

•  There is an old, well known problem with spring gears mounted on metal tube fuselages: if the attachment points are rigid in torsion, the flexing of the spring gear induce torsional loads on the longerons which will fail with fatigue, one day or another. The Grove Aircraft company in California has a website that illustrate this well:

• One way to prevent these torsional loads to be transferred to the fuselage structure is to install radius plates instead of flat plates in between which the gear is clamped. This allows some flexing along the X axis. As the gear flexes, the line of contact will follow the radius and therefore only forces will be transferred to the fuse, not moments.

 

I found the following references regarding the installation of radius plates on different aircraft:

Grove Aircraft Radius Plates

Christen Eagle Service Letter No. 364

I also learned that the Pitts Model 12 and the Steen AeroLab S1 are designed with radius plates.

• The heavy duty installation is to create a hinge like the Extra 300. However this is something quite difficult to retrofit on an already existing fuselage:

Source: NickyP on biplaneforum.com

• With a spring gear installation, the fuselage needs additional strength compared to the original bungee gear fuselage. The diagonal member that go from the back of the gear to the top engine mount attachment point is not part of the original Pitts plans and is specific to the spring gear installation. In addition to that, Steen adds a sleeve over the forward portion of the bottom longeron to provide additional strength at the gear location. See Biplane Forum threads here and here.

• A spring gear on a Pitts typically requires the installation of a "butterfly plate" i.e. an aluminium sheet that is bolted to the center of the gear as well as on the gear attachment points. This plate is to prevent lateral movements of the gear (along the Y axis) relative to the fuselage. See biplane forum thread here.

Now, what do I have on my airplane? The diagonal member is there; it's the one cracked! For the radius plates, I did not know until I came back from my trip. But even if I didn't, how come the crack appeared near the top longeron? I had to come back home to answer these questions...

The finding

I anticipate many posts on this topic since it's going to take quite some time for me to fix.
 
Recently I was working on a little upgrade of my fuel system. This involved removing a panel I never removed before, under which I noticed a crack on the fuselage structure. Sheesh. I immediately called my Pitts mechanics mentor and he confirmed what I feared: having to tear the airplane down. DoubleSheesh.

Here are progressively frightening pictures (not for the faint-hearted aviation geeks like me)

I made this lovely finding one day before leaving for a 2 week business trip, not really knowing what to do. I took this opportunity to make some research on my spare time abroad. Next post will be on the results of this research and on the different advices I got from many people I contacted during this trip. One thing to notice on the last picture is the grey paint mark on the purge valve cable sleeve. I'll explain my theory on why this happen also in future posts...