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Monday, January 30, 2017

Ch 10 - Canard - Part 9

Locating the inner elevator hinges (7.0 hrs)

It might seem a bit early to start worrying about elevator hinge locations since I haven't skinned the top of the canard yet, however I need to bury a number of hard foam inserts into the canard where the hinges will go, so I have to make sure I know fairly well where that will be.

Normally, the hinge locations would come straight from the plans, but my longer and wider than specs nose created a few additional challenges, especially when you factor in my choice of Cozy Girls offset torque tubes. 

GU canard hinge location

Basically, I will be using the original plans for generic guidance, the Roncz plans for more in-depth directions, and the Cozy Girls instructions for more specific assistance. These last ones often refer to the Cozy manual which I don’t have.

Roncz canard hinge location

Saying that elevator installation geometry gives me headaches would be a gross understatement, and at this stage I am not even sure I understand it all myself. Thankfully my buddy Wade has already assembled his elevators to the canard, and made a short video of how it is all supposed to go together. I will link here with his permission.

Wade's elevator mounting video

Thanks again Wade.

In the video you might have noticed that Wade used only six hinges to mount his elevators, four of them are externals, while the two inner ones end up inside the fuselage. His outer hinges match the Roncz plans’ locations, while the inner ones were moved due to his slightly larger nose. Mine will have to be moved as well. Unfortunately for me his fuselage is slightly narrower than mine, so I won't be able to replicate his hinge measurements, and I will have of figure out their locations myself.

Because the original torque tubes rotate about an offset axis, they need a small slot (a bigger hole really) cut in the fuselage to be able to move. This historically leaked cold air into the fuselage during the winter. The Cozy Girls offset torque tubes eliminate the need for the slot, which allows for a tighter seal with the nose structure, minimizing cold air infiltration in an area of the plane that is not usually heated.

I will concede that the change in hinge location due to the non-standard nose is further complicated by my choice of offset torque tubes, but the reward of warmer feet in the winter appealed to me.

The original torque tube (in back) would have required a slot in the fuselage to be cut for clearance.

Checking how these offsets fit on the fuselage of my Long EZ (remember these were designed for a Cozy), it was clear that they could be mounted in a number of ways.

Offset torque tube centered on fuselage. Note the the wide gap.

Offset torque tube biased inward to bring the elevator closer to the fuselage

Talking it over with Wade, I decided to bring the offsets as much inboard as possible, in order to bring the elevators closer to the fuselage. I figured I could leave about 0.100” (2.5 mm) of play to account for finishing the skin, and then still have a little gap. 

All I needed to do at this point was to measure the distance between the slots built into the offsets, as this would represent twice the BL of the hinges. The problem was that handling two torque tubes, two thickness gauges, and a measuring tape long enough to take multiple measurements required at least six hands.

At least four hands short as usual, I decided to use a PVC pipe scrap leftover from the anodizing bubbler build, and two yet-to-be 3D printed adapters to hold the offsets in place. I could then test at will, shortening the pipe as needed, until satisfied with the results, then I would bring everything to the bench where more precise measurements could be taken.

The idea had a great potential to improve accuracy, and simplify my measuring task, however a little CAD work was necessary.

I started by redrawing the offset torque tubes, then modeling inward from there.

Drawing up the part gave me a much better understanding of this complex piece. The Cozy Girls rock!

The left side of this adapter fits the torque tube, the right side fits the PVC pipe.

Adapters and torque tubes

Adapters and PVC pipe

This is what I was working to achieve

With the adapter files transferred to an SD card, I fired up the 3D printer, and let it rip.

The easiest way to print this is standing on edge

This way of printing leaves smooth sides already the proper size

Snug fit
Once this assembly is properly fitted to the fuselage, this will be the distance I need to measure.

Unfortunately, the low-density infill adapter proved too weak to carry any weight, and failed before I got the chance to position the assembly on the fuselage and take measurements.

Anatomy of failure

So, I maximized the infill on the second print, which unfortunately lasted only moments longer.

Adhesion between contiguous ABS layers proved insufficient

The shear force acting on a plane parallel to the layer deposition plane, made the adapters unsuitable for their intended purpose. 

While I could have printed the adapter horizontally in the 3D printer, this would have made it much rougher, and thus harder to fit on the tubes. Instead, I decided to redesign it slightly, and insert a 0.25" (6.3 mm) steel pin into them after printing.

A different kind of "composite"

Because this would be an interference fit, I added a small hole through the bottom for air pressure to escape as the pins were forced in.

Broken dreams and "empty" promises

I ended up reaming the center hole 0.001" (0.025 mm) undersize

A few gentle taps with a plastic mallet persuaded the pin

Steel pin bottomed out

The changes to the adapter worked out well. Although this process took longer to bear fruit than I had expectedprecisely measuring the hinge distances became a total non issue.

Finally back in business

After a few PVC pipe shortening sessions on the belt sander, I reached a happy distance.

Using gauge blocks (on both sides) to quantify elevator clearance

The distance between the slots turned out to be 20.0" (50.8 cm) at the outer edges, and 19.6" (49.8 cm) at the inner edges, so I chose 19.8" (50.3 cm) as slot centerline to slot centerline distance, which gave me a hinge BL of 9.9" (25.1 cm).

So much work for such a tiny number.

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