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Bonded Rear Window – 8 hrs

December 8, 2012 Leave a comment

Saturday Dec 8, 2012

I have decided to include a Trutrak Gemini PFD in my panel as a back-up instrument for altitude, airspeed, attitude and heading. It is a relatively low cost, compact instrument that gives me a completely independent source of primary flight data. But that means I need to route pitot and static air lines to the front of the fuselage. It was easy to add a Tee in the pitot line as it comes into the fuselage at the wing and route the green pitot line forward. The white static line comes from behind the baggage compartment so I need to run it under the seats. I added a Tee in the line as it goes to the Dynon ADAHRS. You can see it here along the main longeron. The new line is going down toward the bottom skin behind the baggage bulkhead.

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Looking lower down in the fuselage you can see that I drilled a hole in the lower rib and installed a snap bushing so the static line can pass over to the right side of the fuselage and enter the conduit under the copilot’s seat.

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When the line exits the conduit under the copilot it passes through another snap bushing and jogs back toward the centerline where one of the two standard snap bushings allows it to pass through the main spar to the center console. That gets it to the forward part of the cabin. I’ll worry about the routing from there to the Gemini later.

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After that I was basically satisfied that I had completed enough of the aft fuselage work that I could permanently install the rear window. It will be harder to get access to that area with the window in but my Sikaflex bonding supplies are ending their shelf life so I need to bond the window in soon or spend another $150 or so for fresh materials later. I decided to bond it now.

I spent some time finishing up the masking to protect the adjacent surfaces from the primer and to give me clean lines on the bonded fillets. I applied two layers of electrical tape for bonded edges; the first layer to be removed after priming and the second layer to be removed after forming fillets. My process here is a hybrid using Sikaflex along the top of the roll bar and using screws along the edge of the top skin. The screws give me comfort that the window is secure regardless of the strength of the bond and the Sikaflex gives me a nice finished look along the canopy seam with a black stripe that hides the roll bar. However to seal the window along the stop skin I am using Alex Plus caulk which was recommended on the EAA web site for sealing around antennas. I was originally going to use silicone but even minute traces of silicone can cause problems with paint adhesion later on. The siliconized caulk is rated to last 35 years and will not interfere with the adhesion of paint to the skin.

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In the photo below you can see the masking of the window itself. The masking along the far edge is for the Sikaflex. The masking around the remaining perimeter is for the black primer I am applying around the edge to give a nice finished look. The black will extend ¼ inch past the edge of the skin much like the black perimeter band you see on auto glass. This is just for aesthetics.

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Here it is after the activator and primer were applied. It is very easy to leave streaks in the primer that can be seen from the outside of the window so you have to be careful to coat it smoothly and check for visible defects with a bright light on the back side. I took my time checking and touching up to make sure I could not see flaws from the outside. From this point I have 24 hours to get this bonded in place before the primer “expires”.

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I allowed the primer to set up for about a hour (10 minutes is minimum according to Sika) then I placed the window on the fuselage about 2 inches forward of the final position. This gave me access to apply a thin layer of Alex Plus caulk to the edge that mates to the top skin. The caulk goes on white but cures clear so it won’t be obvious. Then I just slid the window into place and clecoed it to the skin with the reinforcement doubler strips on the inside. Going around the perimeter I replaced the clecos with the screws and washers but I used free-running nuts to just snug up the window while the caulk cures. I don’t want to squeeze all the caulk out but I do want a nice thin layer.

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With the rear window attached by 30 screws (15 on each side) to the top skin, there was a gap between the window and the roll bar of about 0.25 inch across the top, decreasing to zero along the sides to where the lower forward corners touched the roll bar. This is how I had shimmed it when I drilled the holes for the screws so the bond gap was basically locked it. The only variability was due to the flexibility of the Plexiglas so to get the front and rear canopy surfaces as flush as I could I put a strap around the fuselage and used a few wood shims in places where the rear window was slightly higher than the front canopy. This was really not necessary but I was just trying to tweak it as well as could. I closed the front canopy to check the fit before applying the bond material.

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When I was satisfied with the fit I injected a bead of Sikaflex into the gap between the rear window and the roll bar trying to get about ¼ to ½ inch penetration. I did not want to push in too much for fear of oozing out the back side.  Once I got the gap filled I used popsicle sticks and my finger to smooth out the bead and make a fillet. Here is a close up after pulling off the masking tape.

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Here is a wider view. Now I just have to let this set up overnight then I will inject a bead of Sikaflex on the aft side of the roll bar along the window gap.

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Categories: Rear Window

Canopy Prep – 2 hrs

October 21, 2012 Leave a comment

Sunday Oct 21, 2012

Today I spent some time planning out the build process for the near future. It is quite a dilemma trying to determine the best order to do things in this phase. I decided that I needed to adjust the clearance between the front and rear canopy sections. It is currently about .020 or less but I know that more gap is needed to prevent the two pieces from binding when the temperatures go up. The plans say .032 is the recommended gap and others on the forum have said it may need more. So I set about to adjust it to about .032 for now. I can always increase it later if necessary.

It took about 5 or 6 iterations of sanding and checking to get the gap right. Here is a photo looking down the seam across the top of the canopy. I think it looks really cool.

My plan is to install the Dynon ADAHRS in the aft fuselage and then SikaFlex the rear window. The ADAHRS will be easier to install without the rear window in place.

I also need to mount the ANL current limiter I bought from B&C Specialty. This is basically a high current (60 A) fuse. But I also need to mount an ammeter shunt in this area and I’m not sure exactly where to place each one so for now I bent one end of a copper bar and drilled a hole for the starter contactor terminal but I will wait to locate the currently limiter holes until I get the shunt which comes in the engine sensor kit from Dynon. I will order that tomorrow. Here is the copper bar yet to be cut to length with the current limiter base just stuck against the firewall but not drilled.

Machined Countersinks in Rear Window – 1.0 hr

July 5, 2012 Leave a comment

Thursday July 5, 2012

Tonight I used the countersink cage to machine countersinks for #6 screws in the rear window holes along the edge of the top skin. Then I used the 5/32 plexiglass drill bit to enlarge all the holes. This is so there will be a little extra gap between the screws threads and the plexiglass.

Then I set up the DRDT-2 and dimpled all the holes in the top skin for those #6 screws.

Four Mini-Projects – 8 hrs

June 9, 2012 Leave a comment

Saturday June 9, 2012

I think I finally decided what to do with the canopy release mechanism. I’m going to make a short handle that will extend just aft of the subpanel so I can release the pins when the canopy is tilted up. That will be useful during the remainder of the build phase and when I finally paint the airplane. It obviously won’t serve as an emergency canopy ejector but that’s OK by me. Seems like almost nobody installs this with the handle poking through the instrument panel.

So this morning I installed the crank arm on the mechanism. I aligned it so the arm was fully aft when the pins were retracted and drilled a hole large enough for a #8 screw. While I was at it I made some final modifications to make the mechanism work smoothly, trimming a little here and there to eliminate rubbing.

My friend Ron came by this morning and we worked on the doubler strips for the rear window screws. We unclecoed the top skin and laid it out on the bench and used it as a template to make strips out of .025 thick aluminum scrap left over from the wing walk doubler. Since there are 15 holes on each side of the fuselage I made three strips on each side that each cover 5 holes. It was helpful to have an extra set of hands to trim and match drill these pieces.

Another item on my list was to make bumper stops for the canopy frame to keep it from shifting too far forward under the influence of the lift struts. A very good set of instructs can be found at this site: http://rvbuildershotline.com/articles/2008/frame_brace/ so I will only summarize

Following those instructions I used a scrap piece of angle which I drilled to the longeron just ahead of the subpanel. You can see my rough-out sketch on the angle for the final shape.

I drilled a1/4 inch hole through the subpanel and the angle for a 1/4 inch carriage bolt that is 2-1/2 inches long. Then I cleaned up the shape of the bracket with the band saw, belt sander, and scotchbrite wheel.

Here is a view from the aft side showing the head of the carriage bolt which is dome shaped. I also made a mirror image copy of this for the right side. Now I just need to prime the brackets and rivet them on. I’ll set the fore/aft position of the stops later when I put the canopy frame back on.

Next up on the “to do” list is to install the canopy frame reinforcement kit. This requires a homemade tool to bend the edges of the lightening holes in the reinforcement plates. I used some hardwood scrap and made the tool you see below. The first one broke because the grain of the wood was running in the high stress direciont, so I made a second copy with the grain running parallel with the slot. That worked fine.

I removed the rough edges on the parts and created the raised inner lip on the lightening holes as you can see in this photo. It was easier than I expected, working in small steps to gradually increase the angle of the lip to about 20 degrees. This photo shows two of the three plates.

Made “Big Cut” #3 and Drilled Aft Window – 6 hrs

June 2, 2012 Leave a comment

Saturday, June 2, 2012

So after some consideration I decided that I would use the traditional drill and screw method to attach the rear window to the aft top skin. My reason is because I could not find a way to get the skin flush to the window with a SikaFlex bond without going extremely thin on the bond gap which I considered to be imprudent based on the technical specs from Sika. A bond of 1/8 inch would leave a step at the window/skin interface which would be aerodynamically ugly. So I decided I would drill holes for screws along the skin perimeter but bond across the top of the roll bar. So I marked the skin for the screw holes. The drawing said to use a spacing of approximately 2.5 inches. When I measured it out I came up with a spacing of 2.48 inches which made the pattern fit well.

After marking the holes I drilled them first with a #40 drill, then enlarged them to #30. I had ordered a 1/8 inch and a 5/32 plexiglass drill bits from A/C Spruce and they arrived on Friday so I was good to go. I carefully clamped the rear window in place making sure that the forward edge was on or forward of the centerline of the roll bar (yes, that took a while to check and recheck) then I drilled starting at the top two holes working my way down alternating between the right and left sides. I ran the drill at high speed and applied light pressure to allow the drill to gently cut through the plexiglass and I had no problems. Hallelujah!

Then I removed the window and deburred the edges of the holes with my hole deburring bit. Now that I had a firm hole pattern I laid out a line 3/8 inch outboard of the hole centers for the final edge of the rear window. This is what I call “Big Cut” #3. The first “Big Cut” was splitting the canopy. The second was rough cutting the rear window with 1.5 inch of excess material. And this is #3. The cut went well (with all the plastic dust all over me) and I sanded the final edge with 60 grit and 120 grit before refitting the window.

I could not resist putting both canopy pieces on the fuselage to see how it fits. The rear window looks good but I left a little excess material overhanging the roll bar centerline so I could do a final trim after drilling. That is my next step.