Main landing gear - part 4

The other “missing half inch”(6.6 hrs)

  

The back cover of the plans is the only place where the Water Line (WL) for the main gear axles appears. The drawing shows that the axles should be located at Fuselage Station (FS) 110.5, and WL-22 (X and Z directions if you like). 

Main gear axles location

Talking to other builders I realized that few were aware of this WL requirement, certainly due to its obscure location, so it appears that many builders placed the square aluminum tabs holding the axles at the end of the gear legs, like this drawing shows, and “called it a day”.

Square aluminum tab mounted on the gear bow

The point here is that there must be a lot of planes that have been flying just fine for decades without a precise vertical positioning of the axles, and this leads me to believe that it might not be as critical a requirement as I previously thought.

Nevertheless, in the last blog post I was running short of gear bow height, so I needed to understand why.

Just to be sure I didn’t purchase the wrong item, I called Featherlite (makers of my bow) to compare measurements. Their gear turned out to be 90.3” (2.3 m) long, same as mine.

Measuring the bow size

Bow length in inches

Then it hit me! 

If you remember, in Ch. 5 - Landing Gear mounts - Part 1, LPC#45 modified the gear extrusions by moving the 3/8" (0.95 cm) holes in all four extrusions up 0.4" (1 cm), thereby moving the entire main gear up 0.4". 

So, the vertical reference for the axles shown in the back cover as WL-22 is incorrect. The proper main gear axles vertical reference should in fact be WL-21.6, to reflect the fact that the whole gear was raised due to LPC#45.

Adjusted main gear axles location

Armed with this new realization, a more appropriate geometry for the gear axles would be as in the next picture.

New Water Line still falls short

As you can see, even though I recovered 0.4” using the new WL-21.6, I am still coming up short with the placement of the gear axles. 

Half of the now reduced 0.8” (2 cm) height difference, between the actual and the desired axle location, was of my own doing, since the 2” (5 cm) wider fuselage allowed the gear bow to drop 0.4” (1 cm) further than it would have on a standard width fuselage.

Amount of gear height loss due to fuselage widening

All of the previous pictures were taken while I was still erroneously measuring the 15” (38 cm) spar-to-axle distance from the front of the firewall (something I discussed 2 posts ago).  Measuring from the back of the wing spar at BL26.75 forced me to rotate the gear backward, thus recovering 0.2” in height. 

To compensate for the heavier IO-320 engine, I rotated the gear bow even further back, moving the gear axles rearward another 0.5” (1.3 cm) to FS 211.0. This way I was able to recover an additional 0.2” (0.5 cm). My good friend Ary ran some quick calculations on it, and as expected there seemed to be no real issues there.

Finally, using foam blocks as shims while trying to adjust the gear height, I ended up lengthening the gear bow attachment tabs up to a maximum of 0.3” (0.76 cm), roughly 20% for the longest one. This is not something I measured ahead of time, but it sort of happened automatically, since the sharp tips of the plywood tabs were chipped beyond recognition, and mostly gone by this time, and the small gaps got filled with Bondo. 

If you add it all up, It would seem that I am still missing 0.1” (0.25 cm), but I decided to leave that one be, since trying to take precise measurements in this area is very difficult, and hardly repeatable. Once the gear tabs are glassed, I’ll mount the gear bow back on the fuselage, and check to see exactly where my axles end up. Not a very scientific method, I know, but I should be pretty close at this point, and I’m getting tired of all of these “treasure hunts” you have to embark on, in order to replicate the plans' dimensions.

Main gear axles location at WL-21.6 restored

Of all my modifications, lengthening the gear tabs is not one I really wanted to make, because there are strong forces acting on them during takeoff and landing, and the longer they are, the more leverage these forces will have to flex the tabs.

Luckily, since the gap outboard of the tabs is supposed to be filled with a flox spacer (a later modification designed to prevent the main gear bow from sliding on the tubes), I had enough room to easily increase the tabs’ thickness by 50%, in order to compensate for the taller tabs, while at the same time reducing the need for a thick flox spacer.

My gear tabs will now be 108 plies of fiberglass, compared to the original 72 plies. 

Main gear cross section with modified layup schedule

With the geometry decided upon, it was time to lock it in by “Bondoing” the plywood tabs to the fiberglass bow.

Right rear tab attached to the bow with Bondo

Left rear tab attached to the bow with Bondo

Front tabs attached to the bow with Bondo

Another look at the rear tabs

Once dried, I removed the entire bow from the fuselage.

Tabs and bow removed together from fuselage

Next, I glued foam scraps a to act as reference marks, and scribed plus marked a corresponding mark on the tubes, before removing them.

Left tube and foam reference marked

Right tube and foam reference marked

Right tube removed to prepare for glassing

Left tube removed and foam reference firmly in place

Lastly, I took some time to sand the Bondo smooth... 

Bondo sanded down

Wooden tab and Bondo smoothed

Fiberglass will lay flat over the smooth transition

... and taped over them in preparation for glassing.

Duct tape will allow the wooden tab to be removed after glassing

More duct tape covered the remaining tabs

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. 

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