Disclaimer

This blog is for entertainment purposes only, and is not meant to teach you how to build anything. The author is not responsible for any accident, injury, or loss that occurs as a result of reading this blog. Read this blog at your own risk.

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. 


Monday, September 16, 2013

The missing half inch...


Once I got to the point at which the main gear geometry had to be set, I ran into a snag with the fuselage stations (FS) numbering. 

I felt like I was missing something important, or it might have just been a mistake at the source, one that never got corrected by a CP plans change. 

Now, it is very possible that I am still overlooking something so, if you have already run into this  problem and figured it out, please feel free to share. I’m just one guy, building on his own with minimal support, so all help is welcome.

Here are the details. Looking at page 9-1, the back of the wing spar is referred to as FS125.5 (125.5 inches from station 0), furthermore on this same page, the main gear axles location is calculated as being 15” in front of it at FS110.5 (125.5 - 15 = 110.5).  


Construction manual page 9-1


Now, the back of the wing spar is butted to the front of the firewall, so the terms are interchangeable, as far as as the fuselage stations are concerned.

Looking at the back cover you can start to see a shift in fuselage stations, while the wheel axles are still at FS110.5, the firewall is now at FS125, hence the missing half inch. This would make the new distance from the axles to the firewall only 14.5”. 


Rear cover of the construction manual


There are a few more references to FS125 as the front face of the firewall. Here are four of them, one from the CPs, two from the construction manual, and one from the plans drawings.


CP#28 page 8


Construction manual page 7-2


Construction manual page 14-2


Page A5 of the plans drawing

It would seem that the overwhelming evidence is for the rear of the wing spar to be located at FS125.0. This also means that the wheel axles are supposed to be at FS 110.0, if the 15” spacing is to be observed.

This makes the most sense to me, since I was confused by the single 125.5 fuselage station reference. Although probably not a big problem, I wanted to put it out there in case someone else was scratching his head over it.

As much as some of us builders idolize Burt Rutan, we sometimes forget that he is human like the rest of us, and an occasional typo is bound to go unnoticed, even in a document like the Long EZ plans, with over 30 years of close scrutiny.



UPDATE #1

A few hours after blogging this, and weeks since trying to figure it out, it finally hit me... Burt is not human!

Though he could have made things a little easier to understand.

The key to unraveling the mystery was right in front of my eyes all along, and ALL of the previous numbers from the plans were, and are in-fact correct.

How is it possible?

Allow me to explain... looking back at the text in page 9-1 it says that the 15” are measured from the back of the wing spar at BL26.75 (26.75" left/right of the fuselage centerline).

So far so good, but my previous statement, that the back of the wing spar and the front of the firewall share the same fuselage station, is correct only up to BL23.0, because further outboard the wing spar angles backward, and my statement is no longer correct.

Let's take another look at page 14-2...


Mystery unveiled

As you can see, the back of the wing spar at BL26.75 is indeed at FS125.5, because it is past the wing spar bend, and it can be easily measured on the drawing itself, if you keep in mind the scaling factor.

The bottom line is, if you measure the gear position from BL26.75, the axles are 15” forward, while if you measure from the front of the firewall, the axles are only 14.5”. There was no missing 1/2" after all!

The weird thing is that the wing spar does not get built until chapter 14, while we are still working on chapter 9 here, so I’m not quite sure how one could have used the reference given, except by turning it into a measurement to be taken from the front of the firewall, a feature that actually is on the plane by chapter 9.

Anyway, “all’s well that ends well”, the mystery is solved once and for all, and hopefully I have beat this horse enough for one day.



UPDATE #2

Perhaps the modified drawing below might make it easier to understand how the 15" measurement is supposed to be taken...


Drawing modified to highlight where the measurement should be taken from