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updated: 09/05/2022
| In this chapter, we have to build the Centre section box spar which
joins the wing to the fuselage. The centre section box spar is the heart of the
wing structure, so accuracy is very important. Note that the top of the
spar is flat and that it sweeps aft and that the bottom has positive dihedral in
order for fuel to run inboard while in level flight. Whilst waiting for the glass & micro to dry in Chapter 13, I decided to make a start on making the Jig for the centre section box spar. I paid a visit to our local hardware supplier "Bunnings" and purchased a couple 16mm thick MDF boards as well as a 1kg tin of "3M Builders Filler" to stick everything down with. The first thing I did was to set up my worktable and prepped the surface, so that it is was flat and levelled in all directions. A straight reference line was drawn along the full length of the table to allow the set up of board "D". After "D" was secured to the worktable with bog at 90°, I then proceeded to set-up "Shelf A" first, before moving on to setting up "Shelf B" and then "Shelf C". I converted all imperial measurements to metric and used my vernier calliper to double check my pencil markings on each shelf, before finalising the set-up and securing with bog.
I added another two "Part E" support legs, instead of the single supports on each side of "A" as shown in Chapter 14, Page 6. This set up seems to provide better stability for "Shelf B & C". The top edges were angled to 8 1/2 degrees to get the correct slope relative to A. The cutting of the boards and setting up each section seemed to take me forever and I suppose it doesn't help that I hate working with wood. I find setting up the circular saw is a real pain in the "A@$%". The whole process to finish Step 1, from start to finish took me about 3 days, as I was double checking every dimension and making sure everything was square.
Step 2 – Assembly of Foam Parts I located the boxes in storage with all my foam and set aside 2 Sheets 1"X24"X48" 2 LB density Poly Foam (Wicks Parts No. F100-025) for cutting out the 6 side pieces required for CS2 & CS3. I also used 2 Sheets of 1/4" - H100, 6#, PVC Divinycell Foam (Wicks Parts No. F100-055) for the 6 pieces required for the CS1 bottom and CS4 top Sections. The internal bulkheads CS6 & CS7 and CS5 & CS8 were also cut at the same time. Once that was completed I then proceeded to cut to size as per Chpt 14, page 7 the 24 x 2024 T3 Aluminium LWA plates. The top, aft and bottom sides are then micro'd together using the jig to maintain the proper posititioning. The hardest part of this step is determining the location of the "top" and "bottom" sides of the spar. The aft is clearly defined in step one as "Caution! The spar is mounted aft face down in the jig, but all statements like “top”, “forward”, etc., relate to how it is mounted in your completed aircraft." Which side is the "top" and which is the "bottom"? The only reference I could find was in step 2 on page 2. In a parenthetical comment, CS2 is mentioned as being the "top" This step took longer than I had I thought it would, a lot of time was spent thinking about how to cut the cloth, making a paper cutting template and pre cutting all of the fiberglass in one go. The actual layup took me about 6 hours with some help. Due to the length of time to complete the layups I usedf the slow cure hardener MGS 287. All of the LWA1 Aluminium plates were floxed into place with the radius in the corner as shown on the section views pages 8 & 9.
Step 4 – Install the Forward Face In step 4, the open side of the spar is covered in 3 pieces of 1/4" PVC foam which makes up part CS4 (Centresection spar fwd face). The 3 pieces are first glassed with 1 ply BID on the inside face and then bonded with wet micro in place to complete the box. Once the epoxy holding the foam together has cured, the spar is removed from the jig and the jig was dismantled as it is no longer needed. The spar cap troughs are made by sanding the soft urethane foam to get the correct shape. Finally, two of the corners of the spar are removed.
At this point the Project was put on hold whilst we built another house and I started a new job, little did I know it would take me nearly 6 years before I would get started again on the Cozy, very disappointing as I had hoped to be up flying already? With the Spar setup in my new workshop, part of our 3 car garage, I then started with sanding and profiling the urethane foam to form the Top and Bottom Spar cap troughs. Using the Templates to ensure correct depth and taper.
Using a Saber saw I cut the 1/2" x 1" angle on both Top & Bottom edges full span along the FWD as shown in section views A-A, B-B, C-C AND D-D. In hind sight I think I would have followed advise from other builders and left this until the Spar Cap layups had been completed.
The outside hard points are added in this step. These 1/4" thick Aluminium hard points are the LWA4 pieces made back in step 1 and are placed directly over the inside hard points. Then a 4-layer shear web of unidirectional fiberglass is placed over the bottom spar cap trough, across the aft face and then over the top spar cap trough. I brought out my Glass cutting table and pre-cut the cloth for the 4 layups. The shear web layup, with two people, took close to 5-hours to complete with the full length peel plied. I cut the unidirectional fiberglass on a 45 degree bias for the shear web layup and since the UND is placed end-to-end, with no overlap, I did not square the ends for the centre joins, i.e. cut off the triangles. As each layup is alternating in direction I organised these on the table from layup 1 to 4 on plastic strips and rolled them up. I took each lay up to the Spar and rolled it out directly on top of the Spar. It worked reasonably well with a little bit of fiddling to get the 45 degree joins lined up. I used this method for all of the rest of the UNI shear web layups.
Before beginning the layup of the Top Spar cap a dam needs to be fitted to the aft face of the trough, so that the spar tape will not fall out of the trough as each of the 23 layers of spar tape are added to the Top trough. I built the dams out of 1/4 inch plywood, in 3 sections covered in box tape and used Wayne Hick's method of attaching the dams by using drywall screws. The screws held the dams solidly in place and tightly against the aft face of the spar. I used a ruler and a Venier to set the depth of the dam across the full length.
I rolled out the Spar tape onto the bench using my ladder turned upside down to support the roll, measured and cut each length before beginning the layup. I made each length 1" longer to ensure I wasn't short. Each layup was also numbered so that there was no confusion on how many layers had been laid up. I also marked the Spar cap trough with the length of each layup, so each layup was centred correctly. The quantities worked out OK and no extra tape was added. After the Top was completed I reused the 1/4" plywood strips for the bottom Spar cap, replacing the box tape to ensure it released OK. As the top (23) and bottom (17) spar cap layups take some time to get done, I used 287 MGS slow hardener and with the help of my wife, we managed to get these completed in about 5 hours for each side. Step 7 – Shoulder harness & outside layups I cut the shoulder harness hardpoints as per plans out of 1" spruce. I used a sharp blade to cut and remove the foam down to the inside glass layer to accept the wooden blocks. These were microed into place ensuring that they were level with the top of the foam.
I then precut the cloth for the 4-layer unidirectional fiberglass layup for the outside of both spar caps, using the same method I used for the shear web. I also moved the Spar off the table onto a Trestle to improve access to both sides and the bottom of the Spar and also to speed things up. The 2 us managed to get completed each side in about 4 hours. After trimming, the Spar was turned over and prepped for the fitting of the 4 X 1/4" thick Aluminium hard points. Step 8 – Hardpoint Reinforcement The LWA2 and LWA3 1/4" thick Aluminium hard points manufactured back in step 1 are now floxed into place as shown in the section views. After cure, flox corners were made, and lay up #7 consisting of 3 plies UND 3 in. wide centred over LWA2 and LWA3 was installed as shown in Fig. 16. The layup was lapped onto the top and bottom at the outboard hard point, and onto the top at the inboard hard point. I then added 1 ply of BID 5 in. wide at 45 degree orientation, centred over the UND strips, only on the aft face. The edges were peel plyed for a good transition and then left to cure.
Step 9 – Access for Wing attach & end layup Referring to page 14-5 for the dimensions for the 2 access holes, I drilled holes in the 4 corners and proceeded to cut out the holes on the FWD face of the Spar using my Fein. I then marked 2 lines 10mm's part around each access hole, using a Stanley Knife I carefully cut down to the inside glass and removed the foam. I then sanded a 45 degree angle from the cut line out to the 2nd 20mm line, also radiusing the corners to allow for a good transition for a glass to glass edge.
2 layers of UND was cut at 45 degrees and laid up in alternating direction across the full span of the spar lapping by 1" onto the bevelled edges as shown in Fig.18. I had some trouble getting the cloth to lay down properly in the 2 glass to glass areas for the access holes. I wanted a couple hours for the epoxy to start getting sticky and with the use of the hair dryer worked out the last remaining bubbles and managed to get a good finish. I Peel plied the full length.
The Plans call for the use of EZ Poxy 10/87 on the FWD face of the spar, which is supposed to be the best for fuel resistance and as I'm using MGS for my Cozy project and don't seem to be able to buy EZ Poxy 10/87 in Australia, I contacted Marc Zeitlin for some advise and he informed me that some Folks had built strakes from MGS, and with 100LL and todate hadn't had issues, to his knowledge. However he did state that MGS might be less resistant to chemicals found in other gasolines, so who knows what that might mean years downstream. He recommended getting EZ10/87 or finding a tank sealant, such as a liquid (not paste) Pro-Seal or the equivalent brushable Polysulfide sealant to seal the tank after all epoxy is applied. It would take some effort to ensure full coverage without access panels, however. He finished off with the following. "Will the MGS work? Sure. Would there be concern, long term, about chemical resistance in some manner? For me there would be some, yes". So I contacted my Local SAAA Chapter 24 Technical support person, Lindsay Danes and he basically said the same thing and recommended that I "Complete the
FWD face of the spar using the MGS (same resin as you’re doing for everything
else & which I reckon would be fuel resistant anyway, but have no supporting
evidence to point you to), peel ply it and then simply coat it with another
resin ( which I have faith in due to service experience). These would be
either Vantico LY3600 or the West system 105/205. Both these are ‘known to be
good’ in several projects which I know of… so have faith in them.
Alternative ( 2nd Choice) is to coat the face using Derakane Vinylester
resin like the Glasair boys use for their whole aircraft including tanks. (
it is also used to line chemical storage tanks). All above will be more
than adequate in my opinion and shouldn’t cause you any loss of sleep /
doubt".
"My Long EZ was only layed up and an additional coating brushed on ( admittedly with Safe-T-Poxy) as recommended by Rutan - this has stood the test of time. Main objective is to seal from pinholes therefore no leaks… nothing structural". Step 10 – Installing in The Fuselage I have decided as other builders have recommended to wait until the wings are completed and match drilled to the spar, before finsihing this section and permanently glassing the spar to the Firewall.
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