Pre-Drilled Main Spar and Rear Spar Assembly

by Frank Justice

Alignment and Assembly Notes:

  1. Building the main spars is not particularly difficult or prone to error since almost all of the holes are already drilled and the proper size. One function of the directions given here is to have you operating on the lightest piece possible, since a complete assembly is very hard to handle. There are a number of rivets which you may have to cut so now is the time to locate a good cutter. It is important after cutting rivets to grind the ends flat if necessary, then put the normal bevel back on them or it will be almost impossible to align and assemble the spar pieces properly.
  2. The holes in the predrilled spar parts may be off by as much as 1/8". This is a minor irritation more than anything else and does not cause any problem at this stage. Make all your measurements for locations from the root end of the spar rather than from any existing hole.
  3. Read the first few pages of the wing section of the construction manual concerning the spar; do not skip the part that pertains to making your own spar pieces. There is information there you need concerning drilling in the heavy flange strips, countersinking required for the rivets in the inboard five spreader bars, priming, and driving heavy rivets.
  4. Disassemble the spar by removing the screws and drilling out the rivets. All of the parts are already identified except the main spar web and the spar web doubler (the large Alclad channel pieces); it is easy to determine which one is which later.
  5. RV-4 and -3 builders- the basic spar dimensions are the same as for the RV-6, but the locations of the inner few ribs are different. When the -6 was designed they used the same wing design but just shortened the skin area since the fuselage was wider. They moved the inboard-most ribs so that there are five instead of three closely-spaced ones.
Factory Identification of Components
All of the flange strips and spreader bars are engraved with the wing serial number, an R or L to indicate which wing, and some position codes. Flange strips are engraved on the angle cut end (inboard or root end) with one dot indicating the lower edge of the upper strip and two dots indicating the upper edge of the lower strip. Spreader bars are marked with their position, #1 being the innermost and #15 being the outermost. There is a good picture of this in the construction manual; hang up this page in your work area for easy reference.
Measurement Reference Point
Note that the inboard end of each spar web and flange strip is cut at an angle; this gives the wing its dihedral, so the longer edge is always down. The flanges on the main spar web point aft and the flanges on the smaller spar web doubler point forward.

Most of the measurements on the drawings for the spar use the inboard edge of spar before the angle cut is made as a reference. In the predrilled spar that point no longer exists on the main spar web since the flange and a small part of the web is trimmed off. Unless otherwise indicated, the lower inboard corner may be used as a reference without trying to make correction for the missing material.

Flange Strip Taper
You may taper the sixteen heavy and four thinner flange strips as described in the manual and drawing 13 to make the spar lighter. The longest flange strips are thinner and tapering is not as helpful so most builders do not do it. A table saw with a hollow-ground planer or thin-kerf carbide-tipped blade with as many teeth as possible works best, giving a cut that requires little cleanup. Use a taper jig (available from Sears or woodworking tool outlets) or make one. The flange strip must be held very securely to a guide of some sort or the cut will be very rough and go off track quickly. Set the blade so it is just barely high enough to cut all the way through; this will give the least chatter and kickback as the cut is made. Feed slowly, no more than 1/4 inch per second. If the strip begins to jerk or chatter, slow down and/or clamp it better or the blade may suddenly grab it and chew it up.
Spar Web Lightening Holes and Jigging Tab
  1. Make the lightening holes in the spar webs. Some of the ones in the main web are needed for easy access to wing rib bolts later. The location of the holes and their size is not critical; just don't let them get within 1/4" of any spreader bar holes. See drawing #12.
  2. Cut off the flanges and just a little bit of the web of the main spar channel one inch back from the tip edge. This leaves a handle for attaching the finished spar to the wing jig. The end of the remaining flange will be used as a reference point for some measurements later.
Jig Alignment Holes
  1. Drill 1/16" holes through the main spar web as follows:
    a. 7" from the tip end of the tab that was left for jigging and 4 29/32" from the top outside edge of the flange.

    b. 99" from the tip end of the jigging tab and 4 29/32" from the top outside edge of the flange. (This number is for an RV-6. On an RV-4 or -3 just make sure the hole is where it is away from any ribs.)

  2. Cleco the doubler to the spar and drill through these holes into it.
  3. See SK-29 in the construction manual.
Making the Spreader Angles and Tiedown Attach Bars
  1. Note to RV-4 builders- Another builder tells me there is a spreader bar in the -4 that does not exist in the -6, and it is a little confusing, It is located at station 56 1/4". It is 4 11/16" long, and butts up against the spar flanges.
  2. Cut ten pieces of 3/4 x3/4 x .062 (1/16") angle 7' long. Cut off the corners at both ends of one side of each. Mark eight of them for the five #30 holes in the middle (on the side without the corners cut off) as shown in drawing 13 section C-C.
  3. Clamp the longest flange strips (the thin ones) and the second longest flange strips in position on the main spar webs, using a few rivets taped in to insure proper alignment. Clamp four of the angles with the five holes marked on them to the opposite side of the spar web at approximately the locations shown on drawing 13 for the outer four. These numbers are 78.125 (78 1/8), 89.2 (89 3/16), 98.5 (98 1/2), and 108.5 (108 1/2), measured from the root end of the spar on the longer side. The exact location of the angles should be such that holes drilled into the angles guided by the nearest flange strip holes will go through the center of the inside face of the angles. Note that the part of the each angle that points away (the upper leg of the "L") is closer to the wing root except the one at the 78.125" position.
  4. Drill the five #30 holes in each angle and through the spar web. Drill #12 holes in the ends of the angles using the holes in the flange strips as a guide.
  5. Clamp the fifth piece of angle to the spar web at approximately position 67.75 (67 3/4), with the outward-pointing part of the angle toward the wing root. Drill #12 through the angle using the flange strips as a guide.
  6. Round one corner of the tiedown bar (see drawing #20) at the threaded hole end so that it will fit onto the inside corner of the spar web. Drill the three bolt holes #12 in the bar as shown, taking care to insure that the holes are exactly perpendicular to the body of the bar. The one hole for the bolt that holds the tiedown bar to the spreader angle will have to be slightly off the center of the bar; this is so that the head of that bolt when inserted into the angle and bar will clear the other side of the angle.
  7. Clamp the bar to the spar web and the second angle from the root (see drawing 13). Drill the two #12 holes through the spar web and the flange strips using the bar as a guide. Remove the bar and the spreader angle still clamped together from the spar web and drill #12 through the angle using the bar as a guide.
  8. Mark the position of the threaded hole in the tiedown bar onto the flange of the spar web. Drill this hole #12 to check its position; then enlarge it to 1/2".
  9. Repeat this procedure for the other spar.
Finishing the Parts
  1. Deburr every hole on both sides so there is a noticeable chamfer; this makes it easier to align and disassemble parts later without making burrs on the rivets.
  2. Clean and prime everything. Before the primer dries use a solvent and completely clean the primer out of the four largest holes at the end of each flange strip and spar web.
Spreader Angle Assembly to Spar Web
  1. Assemble each tiedown bar to its spreader angle with an AN3-10A bolt, two flat washers, and a locknut. Grind down one side of one of the washers and the head of the bolt as necessary to keep them from gouging the spreader angle.
  2. Put the four outermost spreader angles (including the one with the tiedown bar) in place on each main spar web. Attach each to the spar web with five AN470AD4-5 rivets.
Main Spar Web and Doubler Assembly
  1. This procedure assembles and rivets the main spar web and the spar web doubler together with the fifteen spreader bars between. The spreader bars must be secured in place to prevent them from shifting once the riveting is started; otherwise it will be very time-consuming to get them back in place.
  2. Lay the spar doubler on a table flange side down. Place the fifteen spreader bars on it with the identification marking side up. Bar # 1 goes closest to the end that is cut at an angle. Us a little contact cement under each (first making sure it doesn't eat your primer) and drop a couple of rivets in for alignment.
  3. Cut 40 (20 per side) of the AN426AD4-12 flat head rivets shorter by 1/16" to make them into -11 size. Cut 100 (50 per side) of the AN470AD4-12 universal head rivets shorter by 1/16" to make them into -11 size. This is necessary because these rivets are not made in the -11 size, and they are very likely to bend over in the longer length. You may have to make some of the flat head rivets out of the AN426AD4-22 rivets; make sure that you have at least six of the long ones left.
  4. Insert these rivets into the doubler and through the spreader bars; secure with riveting tape. Insert three AN470AD4-22 flat head rivets into this assembly at the fifth spreader bar.
  5. Place the main spar web flange down on the table. Place the doubler on it and align it so its rivets go through the holes in the main spar web. Use tape to secure the two webs together at the ends.
  6. Put the two longest and second longest flange strips in place between the main spar and the doubler. Tape about ten big rivets in place to align the main spar with the doubler and the flange strips.
  7. Drive all of the rivets for the spreader bars except the long ones at the fifth bar. Place the 3/4"x 3/4" bar on the rivets at the fifth bar and drive these rivets. If the shop head of these rivets is not flush with or lower than the surface of the bar, grind them down flush.
  8. Remove the tape and the big rivets from the assembly.
Adding the Flange Strips
  1. Lay the spar web assembly on the table with the main spar web down.
  2. Insert a close tolerance NAS1306-30 bolt into the two end holes of the assembly, head side up, and tape in place. Turn the spar assembly over. Mark each hole where a bolt will be used rather than a rivet in the assembly. Use drawing #12, noting that no rivets are used in the area at and inboard from the fifth spreader bar, and that the pattern is slightly different from top to bottom just outboard of there.
  3. Turn the spar assembly over and lay the other flange strips in place over the bolts. Put #10 machine screws at the locations prevously marked where rivets are not to be used. Put washers and nuts loosely on them to hold the flange strips in place. You could also use the bolts supplied with the kit, but if you also use the locknuts you should pick the proper size and add washers so the locking threads are not engaged. It is much easier to just buy about 40 screws and nuts.
  4. Put in the rest of the close tolerance bolts. Put rivets into the assembly at about every fifth or sixth hole to get everything to line up.
  5. Install an AN3-10A and an AN3-12A bolt with two washers each and a locknut to hold the tiedown bar to the spar web.
  6. Tape the last spreader angle at the 67.75-inch position in place on the flange strip; it will be at the third hole from the end of that strip.
  7. Make sure that the spar is flat on a surface with no bow. Verify that the close tolerance bolts are not binding up. Verify that a rivet will go into every rivet hole. It will probably be necessary to clean the primer out of some of the holes with a #12 drill; do this carefully so that you do not remove excess metal just because the pieces are not lined up correctly. In some cases the spar may not have been drilled accurately and some holes will line up while others won't. With the rivets in place clean out every hole so that rivets will go all the way in with little or no force; otherwise you might find later that the close tolerance bolts have become locked in their holes due to shifting of inaccurately-drilled components. Disassemble and clean off the shavings. Reassemble with the bolts and the alignment rivets. Put washers and nuts on the close tolerance bolts and tighten. Tighten the nuts on the screws (or bolts) at the bolt holes.
Final Riveting
  1. In a few locations none of the rivets supplied are even close to the proper length so longer ones will have to be cut down. In two areas for a total of 48 rivets per spar the ones supplied are just a little bit longer than what would be considered safe to drive. Several builders have driven these successfully with an Avery tool without cutting them. To do this it will be necessary to brace up the spar so it is stable and level or bent rivets will result. The best way to do this is to attach wooden blocks to both ends of the spar that hold it at a height where the heads of the rivets will rest gently on the lower die in the Avery tool.
  2. Insert the proper length of rivets from the front side of the spar into all the holes except the ones with bolts and the ones inboard of the fifth spreader bar. Tighten all the bolts. Drive all the rivets.
  3. Clean off any riveting tape and tape residure. Remove the screws and bolts. Put a coat of primer on the edges of the spar to keep moisture out from between the flange strips and webs.
Numbering the Rib Locations
Using drawing #10 and the empty bolt holes in the spar as a guide locate and mark each rib location to avoid mistakes later. On the trailing edge side (larger flanges) mark the outboard end as #14 and go down to #1, aking sure you do not number the location of the leading edge rib that does not have a corresponding main rib. Turn the spar over and mark these locations #14 through #9 and the lonesome leading edge rib location as #A.

Rear Spar

  1. The rear spar is fairly simple, and most of the work is shown on drawing # 14. Do not drill any holes other than those listed in this section. Dimensions and alignment are not critical for this assembly. Ignore the flap brace, aileron mount, and rib attach references in the drawings; these will be handled later.
  2. Mark off the cut lines for the four aileron hinge point doublers (2 each W-607E and W-607F) on the piece marked W-607 E?. Mark the hole locations on the flange side as shown in the drawing, making sure the holes are not so close to the bend that you will have trouble later with riveting. Make the hole patterns mirror images of each other for right and left. Cut the pieces apart.
  3. Tape the two upper flange strips together and the two lower flange strips together (W-607B and C); align the large ends and the smooth sides. Mark all the rivet hole positions except the ones where the ribs will attach; mark the cutoff point at the small end. Note that the lower strip goes all the way in to the root edge of the W-607D doubler but the upper strip stops even with the root end of the W-607A spar channel.
  4. Mark the outline for two of the W-607D doublers on the piece of 1/8" (.125) thick aluminum and cut them out. Tape them together. Use the diagram provided by Van's with the wing kit that shows the proper way to cut items from plate stock so there will be enough.
  5. Nest the two rear spar pieces together so that the flanges match, then lay one just above the other and trim off one inch of the flanges on the left end of one and the right end of the other. This leaves the tab at the tip end that is used to jig the wing.
  1. Drill #30 through the flange strip pairs using the marks on the flange strips as a guide.
  2. Clamp the flange strip pairs into position on the large doublers (W-607D), making sure that the outside edges of the flange strips are 3 3/4" apart at the other end.. Drill through the flange strips into the doublers, taking care not to enlarge the holes in the flange strips. Mark the fuselage skin clearance notch in the W-607D doubler (revision 10 of drawing). Unclamp and separate all the pieces, then cut out the notch and deburr them.
  3. Clamp a set of flange strips with a doubler onto the root end (opposite where you cut off the flanges) of each rear spar, making sure that they are on the side where the flange is over-bent down toward them. Keep the edge of the flange strip out of the bend of the flange to avoid stresses after riveting. Drop a few rivets through the doubler and flange strips to make sure the previously drilled holes stay aligned. Drill through the flange strips and doubler into the spars. Unclamp.
  4. Clamp the other doublers to the spars and drill through the marked holes #30.
  5. Drill the two jig alignment holes 1/8" diameter, 2.335" (2 11/32) from the outside top edge of the spar (the edge where the flange is overbent) at 7'' and 99" from the end of the tab left at the tip end for jigging. To verify these positions, lay the rear spar against the main spar and see that they match lengthwise when the tip ends of the spars are aligned..
  6. Smooth all edges, deburr all holes, polish out scratches. Clean and prime.
  1. Rivet the middle and tip doublers to the spar with AN470AD4-5 rivets in every hole except the ones along the bottom edge of the spar (where the flange is on the side away from the doubler); these are left open for later attachment of the aileron gap fairing. Rivet the flange strips to the spar with AN470AD4-6 rivets, and the flange strips and large doubler to the spar with AN470AD4-8 rivets. Put in at least the inboard 18 rivets with the factory heads on the leading edge side of the spar; this will make it possible to take the first four ribs in and out as part of the final riveting of the skins to the wing.
  2. Mark the outline of the rectangular aileron pushrod access holes on the wider doublers (W-607E) as described in drawing #21 except change the 3/4" dimension to 1 3/8" and the 5/8" dimension to 1/2". This will make the hole large enough so that it does not have to be enlarged later when fitting the aileron. The outboard edge of this cutout should be 47" from the end of the flange (not the end of the mounting tab) at the outboard end of the spar. Cut out these holes.
spar.doc 10/14/94 Frank Justice

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