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 31.969 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 31.969 apart ad parallel. All my blanks are 31.969
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 -----

[sjt]

I am not sure why the i wrote down 2x4 was 31.66

The ribs should be 32 -0.032/2 -0.032/2 . =31.968 perhaps a bit shorter

[sjt]

In truth Bob told me to leave them all long and ignore the capstrips notches to start with.
So all of my ribs start at parallel 31.968.(patrol)

That is why I started out with them full size. To save money I cut rib blanks at arbitrary angles on a single 12 foot sheet. These means that some ribs have a different angle relative to the grain of the 2024.

b.jpg

I can not be sure but when I bent them on an mdf form with a a plastic hammer there was some variation in distances measured across the flange. It also turns out that if you use a 10 foot brake to make the spar there will be some variation across the length of the main spar.

I found that some ribs gave a better fit up at some locations. I sorted all of the ribs form the most wide to the least wide and matched them up with the right and left wing C channel spars as to the best overall fit.

At a latter point I had to sand out a tiny notch for capstrips once the rib for that location was chosen. n.jpg s.jpg






I made sure that the ribs had parallel sides. In the picture, you can see some bungy cord that holds the main and rear spar -which are perpendicular to the table.







The ribs themselves provide the correct spacing during the assembly of the main and rear spars.



This picture is very informative.
You can see why the parallel edges match up with the spars that are held perpendicular to the table.



You only ever had one degree of freedom.

You have to move the rear spar vertically off of the table to provide the smooth transistion from rear spar to center rib.

There is a piece of scrap aluminum clamped in such a way that the transistion is smooth.


For me, I made some wood blocks on a table saw that were 1.34 inches to achieve the correct rear spar height.

The 1.34 inch is a calculated number it is just nice to see that it is in agreement with what must also be true.


r.jpg

[SpruceForest]

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. :-)

No need to make a mock up full size - you can make up a 1/6th scale or 1/8th scale model of the main spar out of some 1/32" ply or balsa or even scrap hardwood for a web (leave the flanges off the mockup - not needed), plus balsa or solid wood strips for the caps, spacers, doublers, etc. Some folks will bemoan time spent on anything but sheet metal and welding, but a few hours spent to be able to understand what the plans actually call for, as well as have an aid for double-checking space stack, etc. ready at hand when cutting metal saves time and material in the long run. I did an 8x increase in fore-and-aft measurement on the caps, plates, spacers, and doublers (but not the web), which makes understanding the why's behind the differing station stacks easier. Also a good tool to mark out lightening hole and aileron/flap drive locations and spacer stacks. Short of having a CAD model with views available, physical models are the next best thing and handy to have in the shop to refresh memory on a task delayed or not already crystal clear. As I've noted, at some point the factory will likely have to move everything to CAD if only to make down-stream mod work easier.... but until then, quick scaled or approximated mock-ups can help.

These shots are for the Patrol, but you get the point. If I had been thinking ahead, I'd have done the inner and outer spacer layers and the I/O caps in maple and mahogany (yes - that is Bigleaf mahogany from an orphaned guitar side... needs must and all that... but just about anything else works, including balsa and spruce from the hobby shop).

IMG_2558.jpg
Forward she of left main spar... makes the reason for the stacked 1/8" spacers versus the 1/4" spacer quite clear.

IMG_2559.jpg
Same thing on the spar cap stack... the outer forward upper caps go to the face of the outer wing mount plate... again, this all becomes quite clear in a good 3D drawing or with a physical model.

IMG_2560.jpg
Wondering about the strut attach strap spacers? Pretty evident how that stack works.
IMG_2564.jpg

Easier to visualize the aft side cap stack and overlap of the outer aft wing mount plate, plus where caps begin and end relative to wing stations.

A coat of spray shellac after the model is done keeps fine point Sharpie from bleeding into the raw wood. Station numbers are pretty much arbitrary, and keep in mind both center and nose ribs get trimmed. Also - ANY notch in ANY of the materials we use creates a stress riser, so radius those notches and understand when you can and cannot get away with notching (if Bob indicates a stack, prob a good idea to honor his engineering). You can add station/measurements for lightening holes, etc., but I have a spread sheet for that stuff that stays with the plans for traceability.

This was a few hours of actual labor on the model to cut, dimension, and assemble things, plus a day or work to understand all the cap locations, spacers, etc. It forced me to resolve the types of confusion on stuff that typically shows up here as head-scratchers. Resolving any ambiguity in Bob's plans before cutting metal helps with blood pressure control and overall motivation. Just saying...

[sjt]

b.jpg
For all of the locations a few very small notches were required.

The rib position at the splice does require an additional cut.

I like to use lathe bits as spacers.

4.jpg e.jpg

[sjt]

I could not build without a table saw.
In the pictures above you can see the slide attachment I made.

I can make parallel and right angle cuts in mdf and aluminum.

Because of my preoccupation with parallel edges on the center rib forms (actually all of the forms) I used the table saw jig to first make mdf blanks with the 31.968 (yes i know how stupid it is to carry 3 decimal digits)


I then got my 032 template that has parallel edges marking the spars over top the mdf blank with parallel edges. This allows for me to use a router to create the airfoil edge of the mdf form block
a.jpg
r.jpgThe picture shows my master template being lined up on top of mdf blank.
The lines on mdf drawing will match the edges of the mdf that was already made with parallel edges.
I then have to hold template down against blank and cut the other two edges with a router


ff.jpg f.jpg

All of my form blocks used parallel edges first, then I cut the airfoil side with a router.
You could line up all of my form blocks (nose,center,false rib, aerlon flap)and recreate the shape of the mdf template.


t.jpg


Below is an unrealistic picture of what it looks like to have a teenager wear safety equipment.
I placed a heavy piece of steel to hold down the spar so he did not cut his fingers off.

I myself once did something stupid and cut into my thumb on a table saw.


tt.jpg

[rodsmith]

Love that your son is involved with the build!

[sjt]

My son does not believe in safety equipment, no shoes.
f.jpg

[fairchild1934]

when you try that with metalworking-- it does not last long------

I might have to wait and see if my center ribs will reach or not. In the mean time i can make another cutting jig so my next ones will be longer.