Locating spars in assy fixture

[geraldmorrissey]

I'm curious how builders have located the fwd and aft spar wing attach holes axis' with respect to one another. Coincident? Offset?. I've read all I can find on the subject but never saw a resolution. Stan from Texas, Chewie and others, what say you? Will call Bob if no response but my guess is he will say "coincident". I plan to use a table for the initial assembly and a vertical skinning fixture.
THX
Gerry
Patrol #30

[sjt]

I built on a table. Strictly speaking you could build in a vertical stand but I would claim that the attach holes will still be offset by 1/2 inch.

Here is my reasoning:

Using the line the Bob calls CL I place two blue squares that show the spars are at right angles to to this CL line.

The distance between the attach point at the Main spar is 3.13 inches.
The distance between the attach point at the Rear spar is 2.625 inches.

2.625+ 0.5 = 3.125

atan(.5/32)= 0.9 degrees down contribution to the wing incidence angle.

master.jpg


s1.jpg
My center ribs are cut using a jig. The jig was made with parallel sides that matched the main spar and rear spar parallel lines.
I even accounted for the distance being a tiny bit less then 32 inches because when Bob says that the spars are 32 inches apart he means the center of the aluminum 032 sheets are are 32 inches apart.
So I cut my mdf for to be parallel sides 32-0.16-.016 wide. I made the mdf form top and bottom surfaces using a table router that followed the master 032 template.
s2.jpg
I had to notch for cap strips. But the center rib trailing edge is (and must be ) perpendicular to rear spar. The other edge not shown is also perpendicular to the main spar.
You can see in the picture that I used some scrap 032 to line up the top spar flange to transition to the rear spar.

s3.jpg
Bob's template has parallel lines representing the main and rear spar. The main and rear spars are parallel.
The only degree of freedom is blocking up the rear spar to accomplish the smooth transition between rib and spar.

s4.jpg
By my calculations blocks of 1.34 inches are necessary to accomplish both goals of spars being parallel and having a smooth transistion.

s5.jpg
The cad drawing above shows what happens if you space the rear spar up by 1.34 inches.
I drew in what it would look like if you had a dowel concentric with the main spar and perpendicular to the main spar. The cad drill rod would not line up with the rear spar attach hole. It would be about 1/2 (patrol) higher then the rear attach hole.

At some point, the attach plate holes that originally were perpendicular to the face of the attach plate will have to be gouged out to fit the fuselage.
That is why the attach fittings should be drilled undersize because they will have to match drilled to the fuselage someday.


s6.jpg
Just as a check I got a real drill rod and put it squarely thru the main spar as described. This picture has my main spar on table. The rear spar is spaced up 1.34 inches. My table is level. If you put a level on the drill rod it would read level. You can see from the picture that the drill rod with the sharpened point is where I claim it would have to be. 1/2 inch


s7.jpg
Rear attach fitting is purple.


s11.jpg
s12.jpg
My wing was built on a table. The plumb bob confirms that the 1/2 inch offset will result.
I put the riveted wing in a vertical stand.
I used a water level to set the tip and root height.
The main spar root is level in all respects.
The main spar tip is level in all respects.
The plumb bob is attached to a string concentric with the main spar attach fitting.


As far as I can tell if you built the wing in a vertical stand
and your mounted the spars level and parallel,
and the center ribs had parallel sides,
and the ribs will be riveted perpendicular the spars,
and the transition from the center rib to the rear spar is to be smooth,

Then the rear spar will have to be slid over the 1/2 inch shown in the picture.
It is still parallel, just not concentric.

The plumb bob would just happen because you needed to have a smooth transition between rib and rear spar.



Bob wants the wing to be hung with a 2 degree up incidence. The 1/2 inch offset will result in a 0.9 degree down incidence.
I hope that the fuselage fittings are to be welded on with a 2.9 degree incidence, although I have not verified the fuselage angle for fittings will be 2.9 degrees.

[sjt]

I also drilled the attach holes undersize because I expect that they will have to be drilled out and an angle someday to mate with the attach fittings.


If you expand this image fully in the web browser you will not be able to see the edge of the green fuselage hole.

You can see something of the internal hole of the aluminum attach plate hole since it has to eventually be squared up with the fuselage fitting.

When the attach plate is match drilled to fuselage it will be possible to put a bolt thru.


The rear fuselage attach fitting and the main fuselage fitting are not colinear when the part is first welded onto fuselage. They cant be.
The rear fitting is lower then the main fitting. You would not be able to look thru all four holes of the 4130 fittings.

If I had go guess, The drilling out process would allow you to run a straight drill rod thru all holes and attach plates.

In principle this will allow the wing to pivot up and down to set the dihedral angle with the strut.





s15.jpg

[sjt]

w1.jpg

Water level made with food dye and a large syringe forced into a block of plastic.


w2.jpg
Tip and Root are made level on the main spar

w3.jpg
Support for the tip is blocked up to match root level.

w4.jpg
Bolt thru attach hole to support the plumb bob. The plumb bob is just a check for me because wing is already riveted by the time it gets to vertical stand.

One other thing I did was make my angle bracket with an array of holes. The angle bracket is match drilled with the tip angle bracket. Using one of the holes that are pre drilled I put some fishing line thru. I selected the hole that would give me a flush string to the main spar along the length. This probably does not matter because the tip and root are level to the bracket. Also, the tip and root are level via the water level. The only other angle at the tip is arbitrary.

[sjt]

Building on a table:
I drew in pencil on my table a large rectangle.
The spars are 32 inches apart on a patrol but I probably drew those line a little different to account for the cap strips 32 3/16 x 172 3/4 rectangle

Using some math and a string I could locate the 4 corners of the rectangle.
I also made some "squares" out of wood that could be used to extend the line from the table up to the capstrips.


a1.jpg


There are pencil marks on the table. A blue square fixture is parallel and coincident with the perpendicular pencil marks.
The rear attach holes are indexed to the pencil mark on table.
It is just possible to see the intersection of two pencil lines. I drew a square mark in ink to signify the importance of that intersection.
The rear spar has 1/16 capstrip. My silver square is 1/16 thick. The exact corner of the blue sqaure is on the table reference mark.
The square carries the mark on the table up to the rear attach point. The 1/16 silver square accounts for the rear spar cap strip thickness.



a2.jpg

Here is the main spar. Another blue square is indexed to the pencil line that can not be seen in drawing.
I show the attach hole indexed to the edge of the square.
At this point the attach holes are in the same plane even though they are not coincident.




a3.jpg
I made a bunch of wood squares that will extend the pencil line that can be seen in the picture up to the 1/16 capstrip.
As I said, I probably did not use 32 inches as the distance between parallel lines.
I would have accounted for cap strips.


a4.jpg

Pencil lines on table are used to line up spars. In this picture at the base of the main spar a pencil line can be seen. It appears to be 1/4 inch in front of spar. The pencil line is extended up to the two 1/8 capstrips along the length of the main spar. At the wing tip the line is 1/4 in front of main spar (at tip without capstrips)

I guess I would have made the parallel lines along the spar 32inches + 1/4 -1/16 = 32 3/16 apart.
32 3/16 would allow a square to touch table at pencil mark. One edge of the square is on table. The other edge of square is on capstrip. The corner of the square is on the pencil mark.

What I wanted was a pencil mark 1/4 in front of main spar 032 c-channel. And, I wanted a pencil mark 1/16 in front of Rear spar 032 c-channel.

a5.jpg

The center ribs are all of the correct length. I never cut out notches for cap strips until assembly time. The spacing between spars are tight (and square) against ribs


By the time I took this picture, I had removed many of the wood squares that had been used to extend pencil line up to the spar. It is
still possible to see a wood square on rear spar capstrips, and a wood square on main spar capstrips.

[sjt]

From the drawing on a patrol:
the station "N" and "P" are 38.5 inches apart.
The drawing says that the drop between "N " and "P" is 1.75 inches

From what I can tell the fuselage attach fittings are inclined 2.6 degrees.
The 1/2 inch offset above is a negative 0.9 degrees.

There is a discrepency 2.6 - 0.9 = 1.7 degrees up NOT 2 degrees up.

I am not sure if I understand To achieve a wing spec of 2 degrees up I would have to weld the rear attach fitting down 2 inches not 1.75.?????