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Rib Lengths....

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  • Rib Lengths....

    Hi all, first question as an "Official Builder". I'm currently working on the wing rib form blocks. My plan was to make the form blocks such that the ribs would be approximately 1/16" (1.6mm) short at all locations to allow some play when when installing and fastening to the connection angles. After looking through a few sites and watching some of the videos on You Tube, I've notice that some builders have ribs cut so the vertical web of the rib essentially touches the spar, with some being notched around the spar doublers so the rib vertical web touches the spar. It's not clear on the plans if there is clearance or if the ribs are tight. Is having the rib vertical web make contact with the spar required?

    Thanks for the input.


  • #2
    I didn't want it tight as it could wear a defect into the spar/cap strip. I layed good masking tape between for assy and pulled carefully and cleanly right out with out any troubles after it's not needed. I think I only used one layer, but it may have been two. You can reuse them repeatedly so you're not having to remake (and pay for all that tape! )
    Last edited by marcusofcotton; 01-13-2022, 10:38 PM.


    • #3
      In the case of the patrol the spars are 032 and they are 32 inches apart.

      I put great effort into making the spar adjacent sides parallel on the mdf form. That edge will be flush with the spars someday. Maybe you could use some tape if you want during assembly.

      As to the distance (patrol) you could use 32- 0.032 inches as the distance. You will then have to clean up the edges with scotch brite which will make them a little smaller.

      When Bob draws a line he intends for you to interpret it as the center.

      I made my master 032 pattern as per the instructions:
      1) using a spray adhesive I stuck the mylar to a piece of 032 aluminum.

      2) Then I rough cut it to the black line. Then with great care I filed down the 032 master pattern to the center of the line. You can not file all the way to the inside or you would have no proof that it was the line.
      If someone else wants to use the outer edge that is fine --- just not the inner edge.

      So if bob says the spars are 32 inches part he means the center of the spars are 32 inches apart. - and then you have to account for the fact that that is a center to center distance.

      Rough cut mdf with a jig saw

      What I am recommending is that your mdf pattern has a width cut slightly less then 32 inche
      Use effort to make the rough mdf blank have parallel edges that are 32-032 with a table saw jig.

      You can not cut wood to a tenth of thousandths but you can try to make them parallel.

      Then take your accurately cut mdf (opposite sides) and line it up with the carefully made 032 pattern. Use the tooling holes to fix the pattern to the mdf blank

      It has to be sandwiched together with a spacer so that a router trim bit can follow the 032 metal master.

      sandwich.jpgi used the circular lightening holes as additional elevator bolt hold downs.

      I also use a slightly smaller undersize mdf piece to hold pattern tight with elevator bolts.

      Bottom layer:
      MDF that has two side cut on table saw.
      scrap undersize masonite to space trim bit bearing up a little.
      metal master
      mdf hold down plate Top layer.


      You have to use a jig to get the pattern very close to the line. Routers do not like to take large cuts. If you try to route away a lot of material the router will grab and yank out of your hands.
      Attached Files
      Last edited by sjt; 01-14-2022, 12:03 AM.
      Austin Tx


      • #4
        Making nose ribs or any of the mdf patterns I have at least have one clean flat MDF side that is referenced to Bob's lines on his mylar. Using elevator bolts I lock mdf and pattern together.

        Just as an aside I would put the tooling hole a little further away from the future spar. You do not want the hole in the aluminum rib used for tooling someday to be where you want to put a rivet for the rib spar angles.

        Last edited by sjt; 01-14-2022, 12:01 AM.
        Austin Tx


        • #5
          I cut my rib blanks right to the line on the Mylar and am doing what others here have suggested. And that is to trim the ribs to length upon assembly.
          Frank Forney
          Englewood CO

          EAA Chapter 301


          • Sir Newton
            Sir Newton commented
            Editing a comment
            This is exactly what I did. Set up a saw and trim each rib back according to rib position & cap strip thickness.

        • #6
          I made my mdf for with parallel edges on the sides adjacent to the spars. All ribs were the same length. I just put notches in.
          That leaves a little more meat for rivets.

          It was my experience that by leaving all the ribs full length (32-032) allowed me to sort the ribs by size and best fit .
          As has been discussed in other threads - a brake will not make a bend along a length of C-channel uniformly.
          Certain ribs will fit better at certain locations.

          You might still have to joggle, but I liked the choice that was available by not having prematurely cut them short.




          I cut a bunch of 2x4 spacers accurately with a miter saw. I then used some rubber cords to hold tight against 2x4.

          In the case of the patrol, the 2x4 were cut as 32-0.016-0.16 .
          I wanted the center of the spars to be 32 inches apart.

          The picture also shows that I spaced the rear spar up 1.34 inches and made a bunch of wood right angles to hold spars perpendicular to table.

          I just realized that my yellow tape measure is visible in the picture. It has a skull and crossbone drawn on it. It is my master tape measure that is only used
          on this project. It may be wrong but It is the reference that everything uses.

          The skull is necessary to distinguish it from other tape measures in the drawer. My boys are trained not to use it. They are free to hammer nails with the other tape measures -just not my airplane tape measure.

          Attached Files
          Last edited by sjt; 01-15-2022, 10:51 AM.
          Austin Tx


          • Frank
            Frank commented
            Editing a comment
            That's fascinating Stan. Lot to learn from that set-up. Thanks for posting! ​​​​​​​​​​​​​​

          • Frank
            Frank commented
            Editing a comment
            Not anywhere close to wing assembly yet, but I have this notion that the full-length tip ribs will play a role in determining rib lengths...? If the jig pin locations are used for alignment, then it's the tip ribs which are fixed, and the others made to match...? Just thinking out loud, so to speak.

        • #7
          The tooling hole in the nose rib is useful, but the spars are located with grid lines on the table.

          John Snap (n3uw) talked about building something he called torsion tables. I got carried away with the concept:
          Also, about this time my son wrecked the car and I got a free source of labor.
          Also - before the pandemic - wood was not a million dollars per foot.

          I got him to weld up about a hundred of what airplane people call nut plates. These nut plates are made with 1/2 inch nuts.


          Back on the subject of alignment, It was done using Pythagorean theorem - had to look up spelling.

          You can see pencil lines on the table below. Two of the reference lines are parallel and 172 3/4 apart. (patrol).
          The spars centers are 32 inches apart, but the other orthogonal lines are different from 32 inches to account for the capstrips.

          On the front surface of the rear spar is a 1/16 capstrip. On the front of the Main spar is a total of 1/4 inches of capstrips.
          So I made the other reference lines 32 3/16 apart.


          This the main spar at the attach hole. I used a blue angle plate and a 1/4 lathe bit to extend the pencil line on table up to hole.


          Below is a picture of the rear spar attach hole. The blue angle plate is on both pencil reference edges. The silver square is 1/16 thick. The 1/16 thickness accounts for the rear spar's 1/16 capstrips - which puts attach hole back off of pencil line by 1/16. The silver square also extends up the pencil line making both main and rear spar attach holes coplanar -(not coincident)


          The rear spar is spaced up 1.34 inches with wood blocks.


          I often use lathe bits and 3/16 square keystock as measurement tools. The lathe bits are also very helpful when welding because they can take the heat.


          Below is the main spar. The wood squares hold the spar at right angles to table.
          At this location the pencil line is extended up to the two 1/8 capstrips (1/4).

          You can also see a 1/4 lathe bit. I need that lathe bit at the wing tip because there are no capstrips at the wingtip.
          I set the lathe bit on the pencil line to make the 032 c-channel main spar offset from the pencil line by the correct amount.


          In the picture below you can see scrap aluminum used to create a smooth transition of the spar and rib.
          Because everything is done with a pencil line, there is only one degree of freedom left.

          The spars are mounted parallel to each other and square to a level table using a rectangular grid drawn in pencil.
          The spars are further more held apart with 2x4 lumber that can be made very accurately 32-0.032 long with square ends.
          The spars are held together against 2x4 with bungee.
          The rear spar height is set at 1.34 (patrol) off of table.
          The main spar is on the table.
          The center ribs from the day they were made had parallel edges the correct distance apart. (see first post about paranoia in making mdf form with parallel sides using a table saw with a jig slide.)

          That leaves just the vertical height which is satisfied with that scrap aluminum.

          I did use a string but only for the nose ribs. If you look closely, you can see a string going thru the center of
          the nose rib tooling hole. I also used a piece of scrap aluminum to create a smooth transition at spar.

          More blocks of wood are used to keep the nose ribs from being splayed.

          Last edited by sjt; Yesterday, 02:59 PM.
          Austin Tx