beginner question --- on wing center ribs

[fairchild1934]

I feel like I should know this-- but i have to admit I dont. This is my first aircraft build. I am building wing ribs and aerolon ribs at the moment. My question is on the large center section ribs- between the two spars.

There is a note on the plans ( B model wing ) which seems to indicate that the ribs are a slightly different overall length depending where they are to be installed between root to tip.
it appears that this difference in length is due to the spar thickness being thicker toward the wing root where the spar has more built up layers and thinner as you go to the tip where the spar is thinner..

There is a particular length that shows on the plan that I can simply measure. My question is this----- what is the "usual" method of making the center ribs. Can you make them all at the maximum length and then trim the end individually as you fit them in at each station ?????? And a related question------ is there any good reason that the ends of the ribs need to be "up against" the face of the spar ? Since they are attached to the spars with 90 degree angles- it would seem that the attach angles would/could allow a small gap between the ends of the ribs and the face of the spars.........
And.... does anyone know the length measurement for the longest ones which should be out near the tip so I can double check myself ?

Been meaning to ask this for a while but have not gotten around to it------

Thanks loads to anyone who might happen to know. Im sure i could call Bob/Maria--- but im sure they are busy--- so I thought it better to ask here first :-)

Thanks ahead of time for any guidance !

Tim

[SpruceForest]

The wing spar webs are both .032, so the dimension for the center ribs given is the aft surface of the main spar to forward surface of the aft spar. Due to the varying thickness of the spar caps on the aft face of the main spar (plus the spacers, doublers, and wing mount plates), each rib location will have a combination of forward and aft trimming to be done to maintain web-to-web spacing. Also an option to do as SJT (Stan) does, and trim the minimum (notch) to clear the spar caps and allow the rip to butt up against the web where that applies. This thread covers some of that as well:

https://bearhawkforums.com/forum/bearhawk-patrol-plans-build/74975-notes-on-forming-patrol-wing-ribs-plus-side-bar-on-rib-fit-to-spars

My suggestion is to take the time to lay out the spar faces in either full or sub-scale, figure out the spacer/doubler/wing mount plate stacks, and do up something like what I did for the Patrol as attached below. Building up a physical model for reference is another way to visualize the job, as making a model pushes you thru the same processes as actual layout and construction. Whether you want to do the full cut trim or notching around the caps, you'll have a guide on what gets cut off the forward and aft of each rib.

Patrol Spar Spacers and Rib Trim Table.pdf

[fairchild1934]

Thanks SF----
mocking up might be hard now as I have not made the spars yet. Let me read through your two links and see if i can understand all that and I will get back to you... :-)
The spar construction with all the little spacers in it I find mikes my head hurt because its so hard to visualize on the plans. The spar is the only part on the whole plane that I think could benefit from a cad 3-d view. :-)

OK--- reading back through your "notes" --- I am reminded that i am using all most the exact same method as you are for making ribs.
I see on the 2-nd document-- at the bottom there is "note 3" -- that says the designer recommends trimming the ends. (of the ribs)
That was my thought too-- I just want to verify that I have enough length on my center ribs that there will be material to trim------
but my thought was that it did not appear that they needed to "push against" the spars.... (that is not a tight fit--- within reason.... since I dont see what advantage that would be..) because its the attach 90 's that are attaching them to the spar.

[SpruceForest]

The biggest reason to butt against the web or spacer/plate is having some extra real estate to work with re: rivet edge distance should you have to go to a larger size due to mistake. No structural gains because the rivets handle the load transfer (shear and any axial loading). If you get enough movement to allow fretting of the rib against web, you have other, bigger problems to worry about. FWIW, the attach angles can be a little scant on room for rivets, so always check when laying out your angle-to-rib rivet line.

[Frank]

A lot of clues can be found in photos and videos of factory wings. That's how I confirmed that a small gap between spars and ribs is typical (LSA). Logical sequence would be to initially cut the ribs to the spar web center-lines. Then construct and place the spars in correct orientation to each other. Then trim each end of the center ribs as necessary.

[fairchild1934]

Yes Frank -- that was what I was supposing----- I just want to make sure I am LONG ENOUGH before fitting.....

Spruce Forrest---- If I wanted to prevent fretting of the end of the rib against the spar web --- isnt that an argument FOR having a small clearance between the surfaces ? (maybe 1/16?inch)
Unless Im missing something ? ( which I often am--- )
tim

[SpruceForest]

Fretting occurs with movement between two or more components in contact. If you had fretting between your ribs and the web, I suspect that you might have movement of the angle bracket, spacers relative to the web, caps, etc. That would imply a bunch of smoked rivets, cracked angle plates, or other dire stuff. The idea of the angle bracket and rivets is to fix the components relative to each other, making the loads transfer both predictable and efficient once the skins are on and everything is riveted up. I don't see a fretting problem arising given the lack of callouts re: maintaining an offset or other solution, and generally fretting is more of something we might see on components which are not structurally fixed (e.g., tank skins with screws which might not be properly torqued, access panels on cowling fixed in place with a few latches and residual spring tension in panel versus frame.

Ultimately, call Bob if you are concerned. He's more familiar with the design and what actually happens with his airplanes over time than anyone else out there.

[sjt]

3.jpg 1.jpg image2.jpg
I can only speak in case of the patrol.

You should cut them all to the max length. In the case of the patrol the distance between the two spars is 32 inches. So in principal the ribs should be 32 -0.032/2 -0.032/2 .

There is a reason to leave them long.

When you cut the blanks out from a sheet , they very likely are cut at different angles to the grain of the 2024. What that means is that no matter how you try to hammer them on the same form block they will be a little different one to the next.

Also, when you make the C channel main spar there will be slight variations along the length.

For me, I took all of the right and all of the left and choose the best average fit up along the length.

Cutting a notch is a trivial thing


Flat table.
2x4 cut to 31.66 to space center of spars at 32".
Rear spar raised off of table with shims

4.jpg

5.jpg
When I made my mdf template I made very sure the sides were 31.66 apart ad parallel. All my blanks are 31.66
For reasons explained above they all do not bend the flange the same

[fairchild1934]

SJT----- NOWWWWW you are hitting the bulls eye of what I needed to know---- the proper length before any trim so you know there is extra to trim--- and you can make the fits as tight or loose as you like. Just dont want to come up short when the time comes to fit. ( I am also a patrol and i have not made spars yet. )
Thank you - that length is a golden bullet for me !
Just measured--- all my center ribs are 31.500 in length. That makes them .080 inch less than your on each end. How much over length were your ? That is how much did
you have to trim off -- approximately ?
I REALLY dont want to have to make another set -----