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Wednesday, September 25, 2013

Main landing gear - part 3

The straight tower of Pisa (12.8 hrs)
After carefully leveling the fuselage upside down on the big table, a set of plywood tabs needed to be made, and installed on the gear attachment tubes. 

The gear bow would then be lowered in place, its geometry checked, adjusted, and locked in temporarily, using Bondo to attach the plywood tabs to the gear bow.

Building the real fiberglass tabs happens later, on the table, after removing the gear bow from the fuselage. This is a major layup, not so much for its surface area, but for its thickness, measuring in at 72 plies.

I used leftover plywood from the firewall building days to make 4 tabs.

Four plywood blanks stapled together

Trimmed to size

Four tabs completed

Sharp edge chipped off just handling them.

Leftover instrument panel foam became spacers for the tabs, once they were mounted onto the steel tubes.

Improvised foam spacer

Spacers were helpful in setting, and maintaining the initial tabs geometry.

The 25 lbs gear bow was difficult to handle by myself to say the least, and I managed to further chip the sharp plywood tabs edges.

More chipping of the tab sharp edge

Trailing edge tab to gear bow contact

Further tab damage

Because it was very top heavy, I ended up shimming it with more leftover foam pieces, so that I could just let go of it long enough to take a good look at it, and try to figure out how to start taking measurements.

Foam spacer, and progressive tab damage.
Foam used to shim and hold the gear bow

Foam supporting the heavy gear bow

Gear bow finally standing on its own

I quickly learned that there was no chance I could ever hope to take accurate measurements working solo, under these conditions. What I needed was a semipermanent structure to use as a reference system, and it would have to be very straight. 

So I did what I usually do when I need straight lumber, head on over to Lowes and spend a few hours tearing through their premium pallets of 2x4s.

Loot in hand I scrambled back home, and settled in for a night of carpentry.

Honda Element easily handling six 8 footers 

Since I spent a considerable amount of time making sure my big table was leveled, I decided to use it as a base on which to add a plumb tower-like structure.

I used lag bolts as pivot points at the bottom of the vertical beams, with big double washers between 2x4s aiding rotation, and shims further up to keep my level happy.

Big table "improvement"

Behold the tower!

Ruler added to the longitudinal member 
This fuselage is not moving anytime soon!

I spent more time making sure the fuselage was also plumb, and square, and transferred centerline markings to the tower.

Triple checking every vertical and horizontal surfaces

Plum-bob on centerline, firewall flush with vertical 2x4s, and side supporting arms... levelled.

Transposing the fuselage centerline to "the tower"

Centerline marked

With the centerline determined, it was easy to identify and mark the now infamous BL 26.75, on either side of the centerline.

Finding BL 26.75 from the centerline marking

Marking BL 26.75 on the rear crossmember

Marking BL 26.75 on the front crossmember

Problem number 2 was identifying WL -22.0, aka the vertical reference for the wheel axles (more on why this is not correct in the next post). Knowing that the top of the rear gear bracket is at WL 12.35, I first ran a tape measure up 12.35” (fuselage is upside down) to identify WL 0.0.

Top of the extrusion at WL 12.35

Measuring from the gear extrusion

Measuring 12.35" up (down really) to WL 0.0

To confirm this measurement, I put an aluminum extrusion in contact with the top of the longerons (bottom in the picture), and measured down (up in the picture) to the previously found WL 0.0. Since the top of the longerons are at WL 23.0, I expected to read 23.0” on the metal scale, and that is indeed what I found.

Fuselage side

Double checking WL 0.0

WL 0.0 confirmed 23" up (down really) from the top of the longerons

This finally gave me a solid foundation from which to start positioning the gear bow, and I simply measured down (up in the picture) 22.0”.

WL -22.0 conveniently aligned with the top of the crossmember

If you are wondering why WL -22.0 ended up at the very end of the gear bow, you'd be raising a very valid issue. There would seem to be nearly 1.25" of fiberglass missing there! 

This is not what I was expecting to find out!

But, since this post was about the raising the tower, I'll pick up this discussion in the next one. 

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