Wing Rib Preparation

by Frank Justice

Marking the Rib Stations

  1. Hang up drawing #10 in the work area where you can see it easily. You will be using it for most of this process.
  2. Separate the ribs by type. Using drawing 10 as a guide mark station numbers 1 through 14 starting at the root end, and the wing side (right, left), on each main rib. If you have the ribs that re factory-notched for the spar flange strips, make sure the deepest notches get the lowest numbers. Mark the leading edge ribs that line up with a main rib as 14 through 9 and mark the inboard one that doesn't as A. Mark each tank rib 6 through 1. Note that not all ribs on a side face the same way. Since you will be cleaning and possibly priming these later, one of those sets of numbers and letters that you tap with a hammer is a great time saver. Be sure to make any such marks as lightly as possible and not near any edges where stresses are concentrated. Lacking this you can put small round notches in the edge of the flanges that go against the spar using a rat-tail (round) file Put one mark for left and two for right, then farther down put one mark for the inboard rib, two for the next, etc.
  3. Debur the edges of the flanges.
  4. Bend all of the flanges to where they are approximately perpendicular to the face of the ribs.
Making a Fluting and Rivet Location Guide
This section describes another fixture you can make that you don't have to have but will save a little time and prevent misalignment; then it will save the people you loan it to (or yourself when you build your second RV) about two hours of time and worry. You can also do the same things but on your benchtop if you prefer.
  1. At one end of a 12" x 48" x 1/2" or larger work board draw the outline of one of the W-409 leading edge ribs, including the tooling holes. Draw a line through the tooling hole marks extending to the far edge of the board. Using a square remark the rear edge line exactly perpendicular to the tooling hole line. Lay a main rib of the same orientation (right, left) over the line so that its tooling holes lay directly over the line and its forward flange lies right at the perpendicular line. Draw the outline of the main rib and its tooling holes.
  2. Precisely locate and drill 3/16" holes at the tooling hole locations and centered on the line between them. Cut the heads off of AN470AD6-12 rivets and drive them into the four holes, leaving about 1/4" protruding.
  3. Mark the locations on the outline where the leading edge and main skin edges will meet at the spar; these are 1" back from the forward end of the main rib (from drawing 21) rather than right at the ends of the ribs. Then mark the locations for the rivets on the outline as follows using SK-46, 48, 49, 50, and 51 as a guide. See also drawing 21 to get another perspective. The location of the first main rib rivets is 1/4" rearward of the skin line mark. On the top side the rearmost rivet is about 7/16" past the rear of the rib and on the bottom side about 3/8" inside the rear (this is where the rear spar flanges are). The first rivets at the trailing edge of the leading edge skin are actually on the main rib; the second ones are 1/2" forward of the edge of the leading edge rib except on the three inboard ribs where they are 3/4" forward of the edge. Lay a rib on the pattern to place the location of the forward-most rivet on the leading edge ribs; note that the W-409 rib noses are slightly different from the W-408 noses, but it may be possible to get the same pattern to work for both.
  4. You must decide at this point whether you want to take advantage of the 2" spacing allowed for pop rivets in the wing walk area and if so whether you want them in the top or bottom skin. See the construction manual for the different skinning techniques. It is possible to do these skins with all driven rivets with only a little extra effort, and these instructions define that technique. If you definitely want to avoid pop rivets, or if there is any doubt about what you want to do, use 1 1/4" spacing instead of 2". Now mark the rest of the rivets between the end ones.
  5. Put V-shaped marks halfway between each of the rivet marks to indicate flute points.
  6. One at a time place those ribs that go on the pattern with their flange side up and flute to make them lie flat.
  7. Place a pair of ribs of the opposite orientation onto the tooling hole pins flange side up and mark their outline. Lay one of the fluted (or marked) ribs upside down on this pattern and use it to mark these fluting locations. Then use this pattern to flute the rest of the ribs.
  8. On another work board (or the same one if large enough) draw an outline of a T-603 tank rib and a T-404 tank rib. SK-47 is not very useful here except to show the spacing. On the outlines mark the ends of the long flanges and put a mark 5/16" in from these to indicate the first rivets. Then mark the rest of the rivet locations. On the T-403 ribs (the heavier end ones) the rivets are 3/4" apart. On the T-404 ribs the rivets are 1" apart. Flute these ribs.
  9. Make sure that there is no twist in the ribs, especially the leading edge and tank ribs. Finish making the flanges perpendicular to the face of the ribs. If necessary re-bend the flanges that touch the main spar so they line up with the line perpendicular to the tooling hole line, then recheck the straightness.
  10. Make lightening holes in the leading edge and main ribs if they did not already have them. Do not do the tank ribs.
  11. Straighten the flanges on the two W-625 short ribs.
Rivet Holes for Mating with Spars
Make hole-marking templates as follows:
  1. Cut a strip of thin piece of aluminum scrap about 1" wide and 7- 19/32" long. Make a line of five small holes in the strip (whatever size suits your marking method) 1" apart and 5/16" from one edge of the strip, with the third hole in the center.
  2. Cut another 1" wide strip 3-3/4" long. Make a line of four holes 1" apart as before, but with the first hole 1/4" from one end of the strip. On both sides of this strip write "rib bottom" and draw an arrow from the note toward the end of the strip where the hole is 1/4" from the end.
  3. Mark the holes on the main-spar edge flanges of all the main ribs by holding the rib web and the longer template down on the work-board and centering the template on the flange. They will be spaced right and meet the minimum edge-distance (1/4").
  4. Set aside the ten main ribs that go in the wing walk area (the five lowest station numbers on each side). Find the next four ribs on each side (numbered 6 thru 9) and use the shorter template to mark the outer two holes and the one middle one closer to the indented flange. Mark all four holes on the rear of the remaining main ribs. Note that the hole that is 1/4" from the end of the flange is at the corner where the rib has an indentation to clear the rear spar flange; this is the "bottom" of the wing.
  5. Drill #40 all the marked holes in the main ribs.
  6. Place a leading edge rib (not the tank ribs) with its corresponding numbered main rib on the rivets in the workboard (the work board pins will normally make the skin edge flanges line up well enough to make a fairly smooth curve). Drill #40 through the main rib holes into the leading edge rib. Repeat for all of the leading edge rib/main rib pairs. The innermost leading edge ribs, numbered A, have no corresponding main rib; use any of the main ribs as a template for those two.
Aileron Bellcrank and Reinforcing Ring Clearance Cutouts
  1. On both the main ribs for station 73.5 make the 3/4" -wide cutout between the two lightening holes where the aileron bellcrank goes. See drawing 19. The location and dimensions of this cutout are not critical at all.
  2. Do not do this step if you have the predrilled wing skins: Cut out the flange and about 1/16" into the web part on the lower edge of the rib to allow room for the bottom skin access plate reinforcing ring; the cutout starts 6 3/4" from the leading edge of the rib and ends 8 5/8" from the leading edge. See drawing 21.
Tank Rib Cutouts
  1. There are two ways to make the tanks; standard and inverted-flight. The inverted-flight version is necessary if you plan to do aerobatics involving extended inverted attitudes or negative G-forces. For simple rolls and loops the inverted-flight version is not required. The following directions are for the standard tanks which are easier to build. The inverted-flight version has a flap on the large hole at the bottom of the inboard-most inside rib to keep the fuel from flowing toward the wingtip during maneuvers, a flop tube fuel pickup located near the leading edge instead of a fixed tube located on the access plate, and a different placement of the fuel level sensor to keep it out of the way of the flop tube fuel pickup. These changes are shown mainly in the photographs in the construction manual; the flop tube is shown on drawing 21..
  2. Make the five holes in each of the eight T-404 (thinner material) tank ribs as shown at the top of drawing 18. Make the cutouts on the thicker inside end ribs (station 22.0) as follows: Drill a 7/16" hole as shown for the vent line elbow (location not critical). Take out all of the larger lightening hole, including the reinforcing ring part.
Cutouts for Wiring Conduit
Now is the best time to plan for a conduit through the wings that will allow you to push wiring through for lights or antenna cables. CPVC pipe (the tan stuff for hot water) 1/2" diameter from a building supply store works well; it will hold two RG58 antenna cables and some wire. Van's sells flexible conduit for this purpose which is slightly lighter and flexible; hence it cannot chafe on the ribs when the wing flexes but it may be harder to pull wires through. Thin-wall aluminum tubing could also be used. To make the conduit have a straight run drill holes in all the main ribs about 2 1/4" back from the spar and 1 1/2" down from the topskin flange to accommodate the tubing. Some builders prefer to put their conduit in front of the main spar to avoid having to route all of the wiring over or through the spar inside the fuselage; this will require a sharp bend at the tank-leading edge junction, not condusive to easy pulling of wires.
Cutouts to Clear Spar Flange Strips
A notch must be cut out of some of the ribs to clear the main spar flange strips. This is already done in more recent kits. Make the notch clear the strips by about 1/16" but it is not necessary to follow the contour where a tapered strip is on top of a full width strip. Be sure to radius the inside corner. Be careful in deciding which ribs to notch and by how much; hold each rib up to its proper location on the correct side of the spar, and don't get confused by the leading edge rib that doesn?t have a corresponding main rib.
Making Reinforcing Angles
  1. Cut 24 pieces of 3/4" x 3/4" x .062 (1/16)" angle 7 5/8" long. Cut two more pieces 7 1/2"; mark these "first leading edge rib". Along the centerline of one side of each drill five holes #30 per drawing 20; the holes are 1 3/4" apart and the end holes are 5/16" from the end. These are for the main and leading edge ribs where they are notched to clear the flange strips.
  2. Take four of the angles and mark them as 11 left, 11 right, 12 left, and 12 right. Clamp each to the leading edge rib of the same designation in the location shown for them in drawing 10 (rib stations 93.5 and 103.5). The side with the holes goes against the rib. Mark through the holes in the rib flange onto the angle. Remove the angles and drill 1/4" at these marks. (These are clearance holes for the blind rivets that are used to hold these ribs to the spar)
Making Bellcrank Support Angles
  1. If you have bought the predrilled wing skins, do not do this step; it is not necessary to cut the main rib since a reinforcing ring is not needed: Cut two pieces 2 1/2" long. In one side drill three holes #30 per drawing 21, 7/8" inches apart. These are to reinforce the main rib where the flange is cut out for the bellcrank access hole reinforcing ring. Note that drawing 19 shows these as 2 1/2" long whereas drawing 19a (the one that shows the newer bellcrank support gussets) shows them as 2" long. Either will work.
  2. Cut two pieces 6 15/16" long. In one side drill four holes #30 per drawing 19, 5/16" and 1 1/4" from each end. These hold one end of the bellcrank support angles.
  3. Cut four pieces of 3/4" x 3/4"x .125 (1/8)" angle 9 1/2" long. In one side of each drill holes #30 1/4" and 1 1/4" from each end. On the other side of two of the angles drill #30 at a point 6 7/16" from one end and 5/16" from the edge. Clamp each of these angles to the one of the remaining two angles, against the side that is not drilled. Drill through both angles with a 1/4" drill using the #30 hole as a guide. See drawing 19. You can also taper these angles as shown in the photographs in the construction manual to lose a few ounces of weight.
Deburr, Clean, and Prime
  1. Since the rivet holes in the ribs will be enlarged later there is no need to fully deburr them now. Deburr the other holes and edges.
  2. Carefully round off the noses of the leading edge and tank ribs, especially at the notches. The skin will be pulled tight here and any defect will show in the finished wing.
  3. Prime everything except the tank ribs. Put an especially heavy coat on the noses of the leading edge ribs.
Building the Aileron Bellcrank Bracket on Rib 73.5
  1. Place the three previously prepared angles on main rib #9 as shown in drawing #19 and drill through the holes into the rib. Drill through the rearmost holes of the horizontal angles into the vertical angle.
  2. Trim the rib web as necessary to give access to the back side of the rivets that hold the two horizontal angles to the vertical angle. Rivet this assembly together.
  3. Place the short piece of angle that reinforces where the rib flange was cut out to clear the access plate reinforcing ring. The angle should be 7/16" in from the skin line so it will clear the nutplate that will be on the access plate reinforcing ring. Drill through its three holes into the rib and rivet it on.
Wing Jig Setup
  1. Set up the wing jig as described in the construction manual. Use 108" (for an RV-6) as the spacing between the inside surfaces of the uprights if possible to allow best work access without getting excessive sag in the spar. The spacing between the inside edges of the crossarms should be no less than 105" (for an RV-6). The skins will extend 1/2" past the main spar flanges at the outboard end, so the crossarm to which the spar jigging tab is clamped must be notched so you will have the full tab to clamp onto. The upper crossarms need to extend at least one foot from the uprights and the top side should be about four feet from the floor.
  2. Note to RV-4 and -3 builders- since there is more skin on the -4 and the -3 (narrower fuselage) the jig posts and crossarms need to be farther apart. 117 inches for the distance between the uprights and 114 for the spacing between the crossarm edges will work. Actually, these same numbers will work for an RV-6 since the spar is very rigid at the inboard end.
  3. Some builders prefer to build both wings at the same time to avoid having to figure out everything and making the same mistakes twice. This is not as important when using these directions. There should be at least four feet between the two spars to allow plenty of working room. At least one builder put long cross-arms on one set of posts to do both wings. He used a spacing of 24 inches between the spars but recommends using at least 30 inches.
Drilling the Rib Attach Rivet Holes in the Main Spar
The following procedure assumes that the holes in the spar are not in the exact locations where the drawings say they should be, as is sometimes the case with the pre-drilled spars. If you ordered the predrilled wing skins this will not be a problem. It adjusts the rib position as necessary to center the location of the bolt holes in the rib reinforcing angles. If the spar holes are not precisely located then you will not be able to predrill the skins according to the dimensions in the construction manual sketches. None of the ribs needs to be precisely located to the drawing dimensions except where the ends of the pre-cut outboard skins lie; the inboard edges of the leading edge and outboard main skins (two-piece) must completely cover the rib flange at their inboard edge and still overhang the outermost rib by enough to attach the wingtip (one-half inch in plans, 5/8 is better). On the leading edge side of the spar (the side with the doubler) measure from the outboard end of the spar flange (not the jigging tab) and mark a point 54 1/2" in toward the root. The end of the flange is where the outside edge of the outermost ribs will be located later. The mark should be 3/8" past an empty bolt hole. If it is off more than !/8", either trim some more from the spar flange or place a new mark on the spar as the outer rib edge location, whichever is appropriate. This new location will be used as the reference for placing the rest of the wing components. Do the same on the trailing edge side of the spar at a point 62 1/2" in for the main skins joint rib.
  1. Set the main spar on a table, trailing edge (large flange side) up. Using drawing 10 mark the locations of the main rib webs on the spar. Use the outboard end of the spar flange (or the mark made in the previous step), 126.0", as your reference and the location of the web of rib #14. Since the drawing dimensions are all to the inboard side of the rib, add 5/8" for those ribs that have their flanges pointing toward the root (#14, 4, 3, 2, and 1). Also mark which direction the flanges go for each one.
  2. Place the main spar in the jig with the large flange down and roughly level it.
  3. At the outboard end of the spar put the #14 main rib in place (with its web lined up with the end of the spar flanges or the mark and its flanges toward the root, then move it inboard about 1/8" to give a little more room to attach the tip to the skin later). Clamp some fairly stiff aluminum strips to the rib flanges to make them align themselves exactly with the spar flanges; the skin surface will not come out smooth if you don't. Drill #40 through the rib flange holes into the spar. Cleco the leading edge rib on also.
  4. Do the next rib (#13) in the same way, with its web lined up with your marks on the spar, flanges toward the outboard end.
  5. The third leading edge rib in (#12) gets a reinforcing angle, and there are two bolts attaching the ribs to the spar in addition to the rivets through the flanges. To insure that the rib gets placed so the bolt hole is in a good location on the angle, do the following steps. On the leading edge side mark the location of the centers of the bolt holes onto the spar flanges so you can see them with the angles in place. Place the angle over the holes with the hole centers in the middle of the flat area and the upright side toward the root. Shift the angle if necessary to make it perpendicular to the spar flange. Make sure that the bolt holes in the angle will not be so close to the upright side that a nut or washer on the bolt will dig into the angle corner. Mark on the spar along the edge of the angle. Mark the angle through one of the bolt holes in the spar and drill this hole #12 in a drill press. Put the angle back on the spar with a bolt through the hole and a clamp on the other end, making sure the angle is perpendicular to the spar flange using a square. Carefully drill the other bolt hole in the angle through the spar. If you are worried about enlarging the bolt hole in the spar you can find a piece of tubing in a hobby store that will fit snugly inside the bolt hole and over a smaller drill for making a pilot hole. Put a bolt in this hole.
  6. Clamp the leading edge rib to the spar and angle, making sure the rib flange lines up evenly with the spar flange on both sides. Drill #40 the rivet holes onto the spar through the rib. Make sure the nose of this rib is lined up with the noses of the other leading edge ribs, and then drill #30 through the angle into the rib. Remove the angle and cleco the main rib in place. Drill #12 through the spar into the main rib.
  7. Repeat for ribs #11 and 10, making sure you use the angles previously marked for those ribs.
  8. Do the same for rib #9, but note that the reinforcing angle goes on the opposite side of the leading edge rib. For this reason there are no bolt holes to drill in the corresponding main rib. Use the angle marked "first leading edge rib".
  9. Drill the leading edge rib A (at station 71.5, the one with no corresponding main rib) and its angle to the spar as above.
  10. Position each of the rest of the main ribs on the spar one at a time, aligning the reinforcing angle with the bolt holes as before, and drill the bolt and rivet holes as before. Pay close attention to the orientation of each rib and angle on it as shown in drawing #10 (angle on opposite side from flanges, and flanges toward the wingtip except #4 and lower).
  11. Lay the rear spar next to the main spar with the outboard ends lined up. Mark the final position of the main rib webs onto the front side of the rear spar.
  12. Disassemble and debur.
  13. Rivet the angles to the ribs with the shop heads on the side where the rib flanges are to make later operations easier, except where it might dig into the lightening hole reinforcing ring.
  14. Cleco the leading edge ribs with angles to the spar. Clamp the angles to the flange strips and drill the angles #12 through the spar.
  15. Remove the leading edge ribs, turn the spar over, and do the same for the main ribs, including the ones which have no angles but still get bolt holes (#10 thru 12). This will be easier to do if the spar is placed at least temporarily on the jig.
  16. Place a main and leading edge rib together to the spar; drop bolts in the holes and then put two clecos in; enlarge the rivet holes to #30. Repeat for the rest of the pairs, removing each as you finish. Also do the inboard leading edge rib.
  17. Put the rest of the main ribs on the spar one at a time aligned with bolts. Enlarge the rivet holes to #30.
  18. Drill the innermost two leading edge ribs of the left wing only for the pitot tube passage as shown in drawing #20. The holes are 7/16" in diameter, 1 1/2" from the top side of the rib, and through the reinforcing angle as close as possible to the spar attach flange as possible with just enough room to put a bushing through the hole.
Mounting the Rear Spar
  1. Cleco on the leading edge and root main ribs and clamp the rear spar to them using the marks placed on the spar previously. Clamp the rear spar to the jig.
  2. On the root rib drill #30 through each flange strip into the rib flange. Drill one hole between these two through the doubler plate, the rear spar web, and on through the rib flange.
  3. Drill #30 through the four holes in the outboard rib into the rear spar; continue on toward the root by drilling through the rib flange holes into the spar.
  4. At the wing-walk area ribs drill through the flange strips into the rib flanges. At the second rib drill one hole between these two through the doubler plate, the rear spar web, and on through the rib flange. At the third, fourth, and fifth ribs drill one hole between the two through the rear spar web into the rib flange.
  5. Remove the rear spar from the ribs and jig. Deburr all the holes. Reinstall the rear spar, and drop bolts in the main spar-to-rib holes for those ribs that use them.
  6. Countersink all the rear spar holes for the outboard rib. This is so rivets there will not be in the way of the outboard aileron hinge bracket. (See drawings 16 and 21)
Final Wing Jig Alignment
  1. Place supports under the rear spar at two points to keep the main spar from bowing. Carefully level the main spar in both directions while adjusting the supports to keep it straight.
  2. Run a piece of thread through each of the two alignment holes in the main spar and the rear spar; tape the end of the thread at the main spar and tie a 10-32 nut to the other end just below the rear spar. Clamp the rear spar to the jig so that both threads hang through the center of the rear spar alignment holes.
  3. Clamp both spars securely to the jig.
wingribs.doc 11/24/94 Frank Justice

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