Sat Sep 04 2021

Fitting popup tipup mechanism

When I had the mechanism fitted yesterday, I noticed that there was quite a bit of friction when cycling it up and down. So, today I spent a bunch of time identifying the causes of the friction and addressing them.

The primary causes of friction were:

  • The bellcrank stubs (one shown here) on which the guide bushings fit had some silver solder preventing that bushing from seating correctly. I used a file with a safe edge and a safe side (I ground away the teeth) to file it away.
  • These bellcrank arms were splayed slightly apart from the arms of it's partner. This resulted in the two bellcranks really sandwiching on the sides of the guide bushing when the bearings were installed. I tweaked the angle of these bellcrank arms to avoid that problem.
  • The cam slots required a bit more sanding to prevent the bearings from dragging on the top and bottom of the slot. The slots now allow the bearings to rotate freely all the way to the end of travel

I also decided to document some aspects of the design which were just fabricated in the shop. I may model these things up in SolidWorks for posterity.

Also took measurements of the final length of the torsion tube as well as the locations of the bellcrank and the two mechanisms.

Made note of what bolt sizes I need for the bearing mounting in the bellcranks. I had recently ordered AN3-7A bolts, but I want to have a washer on each side, so I'll need to order some slightly longer (AN3-8A) bolts.

The bolts attaching the rod end bearings to the forward end of the popup tipup arms are -10A size.

Documenting the bolt length here. I also need to capture the details for the AN4-13A (?) bolts with the anti-rotate tab welded on.

The down stop. I realize now that I'm writing this that the bolt should be AN3-12A. I forgot to update that after changing the quantities.

Same for the bolts which attach the mechanisms to the ribs. Should be AN3-12A.

I also documented the space between the top of the cam plate and the bottom of the rod end bearing. 3 1/8". I'm now thinking that these pushrods are slightly too short. I may need to order some longer AN bolts to remake these (yet again). Perhaps AN3-50A (5" long).

Those rod end bearings are MW-3M, BTW.

Decided to install the pop rivets to attach the forward ends of the ribs to the brackets already installed on the firewall. These are CR3213-4-1 Cherrymax rivets. They have (roughly) the same shear strength as an AN470AD4 rivets.

One rib done.

Because of the pop rivet tool interfereing with the rivets attaching the brackets to the firewall, I needed to use this little Avery pop rivet angle tool.

Because I had to make this little doubler to fix the figure 8 hole in the rib flange, and the doubler picks up the top rivet in this row, I needed a single Cherrymax rivet with a slightly longer grip range.

Cherrymax rivets installed at all locations.

Well, dangit...the top Cherrymax rivet on the left side row (actually the rightmost rib) didn't pull tight. You can see a little bit of the shank of the rivet between the gap between the rib and the bracket. I'm going to have to remove this rivet and reinstally.

The removal process for Cherrymax rivets is different than AN rivets. First step is to use a punch to drive the center mandrel down through the head of the rivet. This will shear the locking ring that goes around the mandrel. I'm using my bucking bar to backup the structure while using this little spring punch.

Once the locking ring is sheared, drive the center mandrel through, then pick out the locking ring. Tried to get a picture of the locking ring, but it fell into the airplane with all the rest of the drill shavings, so it's gone. Sorry.

From here on, it's pretty much the same as an AN rivet. Drill #30 through the head, use a punch to pop the head off, and some wire cutting pliers to pull the tail out.

Reinstalled another Cherrymax rivet. This time, no gap.