Wing Assembly - Plumb bob at wing / fuselage attachment bolt holes

[Lee S]

I've been struggling with this one for some time now and am looking for some help. Analysis paralysis has set in.

I have been using Eric Newton's manual as well as B-Spot's wonderful videos. Both use a plumb bob to align the front spar bolt hole to be directly above the rear spar bolt hole while positioned vertically in the jig. However, these two bolt holes are not the same distance from their respective spar centerlines (see photos). The distance between the bolt hole and the spar centerline is shown on the drawings to be 1 1/8" at the rear spar vs the 1 1/2" dimension on the front spar. It seems to me that the spar centerlines should be aligned vertically and be plumb while in the jig and not necessarily the 2 bolt holes. Does this even matter? It seems logical to me that the wing could be assembled in any attitude as long as it is straight and true and not twisted. Has anyone run into this and what did you do?

Thank you,
Lee S
#P289


IMG_6442.jpg IMG_6443.jpg IMG_6446.jpg

[Frank]

Same issue with the LSA and it's a fascinating unresolved puzzle. Bob has not weighed in on this as far as I know.

I determined that the front and rear mounting holes are not "co-linear" by simply placing front and rear mounting brackets over the mylar drawing. I don't see any way for those holes to be centered on each other.

When I put the center ribs and spars into the vertical jig, I made the spars level and plum in all directions and let the mounting holes fall where they will. My offset is about 5/8 inch.

This will be interesting down the road because "The Bearhawk LSA Book" describes mounting the wings to the fuselage using a 1/4 inch rod though front and rear steel fittings as part of an alignment method.

I'm going to save my question for Bob at that time unless otherwise resolved.

For kit builders, I wonder what their kit manuals have to say about mounting the wings.

[BTAZ]

I built an LSA from scratch (finished in early 2017) and it was a point of discussion on the forum back then. I built the wings vertically and set the spar holes to be co-linear (the LSA plans (at least back then) did not clearly show the offset from front/rear spar centerline c like it is shown in the Patrol plans posted). My thought process was "The print for the fuselage sets the spar attach brackets co-linear. One would expect that once the wings are mounted they should cleanly "hinge" on those points as if they can't, any up/down flex in the wing puts a binding force on the attach bolts/brackets (both when first setting the dihedral(which is removed during the match drill process for that exact dihedral angle only) and due to subsequent flight loads). Only way to avoid binding at the fuselage mounting brackets over a range of dihedral angles would be for everything to be colinear".

But I cannot say if I chose the correct path other than the wing assembly went OK and the flight characteristics of the resulting plane match Bob's published specs.

[geraldmorrissey]

Much has been written on this exact subject. Look back through Patrol scratch built archives, look for Stan from Austin, TX, he goes by SJT. He has done an in depth analysis on this.
Good Luck
Gerry

[Lee S]

Thanks for the tip, Gerry. Unfortunately, I did not find anything from Stan in the forum related to this, however I found a nice article that Wayne Massey wrote back in June 2007. There are a lot of moving parts here, and the fear is to end up with a helical shaped wing. To that end, I think I am going to assemble my wings in the horizontal position using a horizontal jig, similar to that in Mark Goldberg's factory tour video. It seems easier to ensure no twist if I assemble the wings horizontally on a flat, level table. I'll end up spending more time on building the jig, but that time is pale in comparison to scraping the wing and starting over because it turned out helical. I'd be really interested in seeing a drawing on Mark Goldberg's horizontal wing jig.

[geraldmorrissey]

Look for a string called "Locating spars in an assembly jig" originated by me on 11-29-2021 in the Patrol scratch built forum.

[jaredyates]

https://bearhawkforums.com/forum/bearhawk-patrol-plans-build/70351-locating-spars-in-assy-fixture

[Frank]

(This was bugging me so I took a chance and sent Bob an email)

Frank,
Thats true.
Bob

On Mon, 1 Dec 2025 17:32:36 -0700, Frank Forney <fforney@indra.com> wrote:

Hi Bob,
Before I give you a call can you look at this composite drawing I put together?
To me, it shows that the front and rear wing attach holes are not “co-linear”.
I’d like to confirm that is the case, or not.
Thanks,
Frank Forney
Englewood CO

[sjt]

This has to be the case for the patrol - probably other models have something similar
shim.jpg

[sjt]

https://bearhawkforums.com/forum/bearhawk-general-discussion-news/87472-trimming-final-rib-length

If the spars are parallel, AND IF YOU WANT THE REAR SPAR TO HAVE A SMOOTH TRANSISTION THEN the rear spar -on a patrol- has to be raised off table by 1.34"

I put the math on a different post some time ago.

It turns out that the holes are not co-linear by 1/2 inch (patrol)

The visual picture of what is going on is seen by the two squares. Each square proves the spar line is vertical to the table - and they are in fact parallel.



1.jpg




2.jpg



I tried to illustrate with this mock up. I had a length of drill rod that I sharpened to a point. The drill rod is co-linear with main spar attach hole.
There is no way that it would also be co linear with rear spar hole. It would be off by about 1/2 inch.



The table is level . I put a high precision machinist level on top of drill rod. The bubble is in the center.
At a much later date I took a picture when I placed it in stand to skin wing. The plumb bob shows the 1/2 inch .

My wing was riveted on horizontal table. It can not change when hung vertically.


3.jpg

[sjt]

Back then I was advocating against ever pre cutting the center ribs.

The case I was making is that the spar lines on the master template are parallel. And with great purpose I made sure that my mdf form maintained the parallel edges

4.jpg 20.jpg

A table saw can be used to make a MDF form block that has two parallel edges exactly the correct distance.
14.jpgp.s. There is a skull and bones on the tape since this is my master tape, and I do not want my son to use it as a hammer.



I then have parallel edges that I actually use during assembly stage.

Those tiny notches for cap strips are made only when a specific rib is selected for a specific location.

Notice the top aluminum for a smooth transition.

You can also make out the wood blocks (1.34" for patrol)

The spar is perpendicular to table. The center rib edges are parallel to rear spar in this picture
.
5.jpg 21.jpg


This picture illustrates where attach hole ends up. I have both spars perpendicular to table
The main spar rests on table. The center rib has parallel edges. You have no choice on the main spar as to how to locate the center rib at the main spar.
The main spar center rib has a smooth transition.

The above picture is the rear spar. You have one degree of freedom. The center spar can not be rotated to any angle. The center rib height was fixed when you mate it to
main spar. This means that the rear spar can be slid up or down. And you choose the up or down such that there is a smooth transition from center rib to rear spar flange.

That is why the rear spar is spaced up 1.34 inches with wood blocks.


The Main spar is supported by table. It is perpendicular to table. The center rib front side is parallel to main spar.


Again, wait to the last moment to make those tiny notches. I do not recommend pre-cutting center spars.


I made sure my mdf template had parallel edges. In the final assembly, those edges are helpful.

If you pre cut center ribs you loose that reference edge.

6.jpg

[sjt]

This is not part of the construction process but I went and found the original mdf form to show what I was trying to achieve.
7.jpg 8.jpg


9.jpg

[sjt]

This is where the final co-linearity of the attach holes comes from.

This is not mine. This is the tool used before the factory started doing it before shipment.


To that end it is better to make the hole in the attach plates a little under size.

10.png 11.png




Just as an aside, I think this is one of the new improvements made by Mark on his kits. He has the wings and fuselage fittings drilled at the factory.

The older kits required a tool to help make the attach holes. The tool is not that complicated and looks like what you would expect to make a collinear hole thru
fuselage and 2024 attach fittings.

[sjt]

I would recommend making the 2024 attach fittings hole under size. When the wing is finally placed next to fuselage it will have to be match drilled.
The holes in the fuselage attach fittings are concentric with a 3.1 degree up angle.
The holes in the 2024 wing are not concentric and are 0.9 degrees down angle (compared to cord line)

They will be after match drilling though.


12.jpg

[sjt]

EAA used to have free solidworks. I made up this 3d model.


The fuslage attach holes are angled up - As I recall when I did the math it was something like 3 degrees.
13.jpg