Trimming final rib length

[woodtic]

Question: It seems some people cut their ribs to proper length prior to forming and others final trim to different cap strip thickness on assembly. For those that trimmed on final assembly, what method did you use? Did you reprime the cut? Also, when match drilling on assembly, after deburring, did you reprime the holes prior to riveting? Thanks!

[sjt]

I did not prime new holes. I heard of a guy working on a vans RV that was priming his rivets. I do not think he ever finished project.

I would recommend not trimming.

The process of making rib blanks usually entails cutting them from a sheet of 2024 at arbitrary angles. 2024 has a grain structure. The end result is that even if you use the same technique for bending a rib, the ribs will not be identical.

The next factor is the spar. Bending the spar using a brake will also not achieve a perfectly consistent width.

If you had forgone cutting them short you can still mix and match and place ribs at the average best fit location for along the spar.

rib.jpg

trimming is wrong idea. I recommend just sanding a notch to go around the capstrip.
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There are edge to hole distance limits. Doing it as shown above will limit the chances of having a rivet hole to close to the edge
I did not prime the notch. I guess I could go back and still spray that area even after assembly.


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Another advantage of not trimming can be had during the assembly of the wing.

I have claimed that the flange distance is different from one rib to the next. However if you made the mdf form (patrol 32"-0.032") with care the two parallel edges are indeed still parallel. This allows for the rib itself to be used during the assembly process.

From the picture you can see that the rear spar is held off of the table by carefully made wood blocks. The only way the rib can have parallel edges match the parallel edges of the two spars is if the rear spar is raised off of the table.

It would bother me if I had true edges that I messed up. The loss of the opportunity to use it during the assembly would be unfortunate.

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For the patrol the distance - by my calculation would be 1.34 inches.


When transferred to the vertical jig , this is what should be expected :


shim.jpg
I built on a table. When wing is put in a vertical jig this is of interest in verifying the rear spar is in the correct alignment
in the direction of the spar. You would not want to skin the structure and have it askew. The first section of skin attached will lock main and rear
spar in place.

d.jpg
g.jpgWater level for end to end level before skinning.

[Frank]

It seems to me that the full length tip ribs will be the first determinant of rib lengths: it is not adjustable. The jig holes (nose, center and back) all need to align with those of the tip rib.

I'm not at this stage yet but I will certainly consider sjt's tips regarding sorting for best fit and notching.

[sjt]

The tip rib is the last thing to go on. It is done using the alignment technique for the other ribs:
STRING.jpg

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FOR THE PATROL ONLY:

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[Frank]

Thanks for the photos and tips Stan. Yes I will really have to study before I begin assembling. Currently I am still building up the spars for the right wing. I think I am right that the nose rib lengths are set by the tip rib. But we shall see. I am following btaz's photos and commentary to a large degree. For his second wing skeleton assembly he decided to rivet the nose and back ribs to their respective spars before setting them up in the vertical jig. I will really have to think that one through too!
Fro the Bearhawk LSA Book:
IMG_1067.png

[sjt]

I am all in favor of any method that keeps the spars parallel. That is one reason I do not recommend pre cutting them - assuming the mdf template had parallel edges to begin with.

Screenshot from 2024-01-09 21-55-36.png
Your picture above is not to scale but it does illustrate that the attach hole in the main spar attach is not concentric with the attach hole in the rear spar. You can not drop a plumb bob thru the main spar and expect it to be concentric with the rear attach hole.

plumb.jpg
It should look like the above picture. When using the table method of building it is achieved by 1.34 inch wood spacers on rear spar.
When I placed my wing in a vertical jig I machined a black plastic 1 inch insert with a 1/2 radius to confirm my plumb bob tip is 1/2 inch away from rear attach hole.
In the case of the patrol it will be 1/2 inch away from concentric.

Below is my template from Bob drawing glued to a 032 backer. The spar lines are on drawing are lined up with the spars on the table with the rear spar blocked up 1.34"

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[sjt]

As an aside,

I made more mdf forms than I can count. But, all them used as trim router bit and the master 032 template.

mdf.jpg

The above picture would be typical of how I started making the center mdf form. With a table saw and a jig it is possible to cut perfectly parallel edges 32"-032" inches in width. Then I would pin the template to the the mdf form and use a table router with a trim bit to make the airfoil profile.

When making the nose rib template, I started with a perfect straight edge. Then I lined up the master template using the spar line and pined it to mdf. A table router with a trim bit followed the profile.

I made a mdf form for the full size tip. The tooling holes by definition had to line up with all of the others.

In my case I added the tip rib last. A string thru all of the nose rib tooling holes had to be concentric thru the tip rib tooling hole. The tip rib also needed a 1.34 inch block at rear spar.

[Rob]

I guess I am confused. Maybe when I start assembling my Patrol wing it will make more sense. As I always understood it, the wing attach bolts on the front and rear spar should line up in a straight line. In as you could but a straight rod through the attach holes. The same as your front and rear landing gear legs are in a straight line. Are you saying this isn’t true? The wing attach fittings are not in a straight line?
Thanks Rob

[sjt]

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)


temp12.jpg


plumb.jpg
drawing 1 on plan set has a 2 degree up cord line

[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.
r.jpg

[Frank]

This is not terribly precise, but overlaying parts from different drawings illustrates the wing attach holes at the spar ends are not in line with each other. Screen Shot 2024-01-10 at 9.10.00 PM.png

[Rob]

I asked Bob about this and he said to line up attach holes inline with plumb bob as in this video.

https://youtu.be/EP_j1t2Cp0s?si=bwOnKqrydXxYz-zy

[sjt]

I would not want to lead anyone on the wrong path, This method did seem to work for me.

I did not build the wing on vertical jig. When building on a table, you will encounter what Frank's picture shows. If the spars are perpendicular to the table, Then, the rear spar will have to be lifted off the table by some amount or the wing profile will not match the rear spar web flange. The question is how much.

I had math reasons for using a wood block measuring 1.34" to space up the rear spar. The profile on the flanges seemed to work out for me

mdf.jpg
1.jpg
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The above picture is the rear spar. It was spaced up 1.34" on table during table assembly.
The center rib is tangent to web flange and has an exact fit to spar.
If I was to consider using a plumb bob at this stage, it would have been shifted over 1/2"





3.jpg
I was paranoid about perpendicular and level
4.jpg
Both spars are level in every direction, I used a water level to control level across its length.


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I choose 1.34" for math reasons. But It seemed to be the right amount to make the center ribs tangent and perpendicular. to web
flange.

I used edge clamps (top of flange) to follow profile. I do not see how it could have been off by 1/2 inch.

[sjt]

When building on a table I used a level in place of a plumb bob. I used a length of drill rod I sharpened to a point held thru the Main
spar attach point. The table is level. The drill rod is level. I did not expect the holes to be coaxial.

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[Rob]

Ok, thanks for the clarification. I am not saying your wrong or misleading anyone. I’m just trying to wrap my head around what you are doing, understand, and learn, so I do the right thing building my wings. Your plumb bob picture off the bolt head was throwing me. Understand raising the rear spar building on a table. I kind of figured all along if spars straight up and down on table, and ribs up tight and square to spar, wing level, then those attach holes are pretty much going to set themselves.

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