9.2.3 Lower Strake Alignment

These first four photos show my water level at work. I had to mark the location for the leading edge of each strake, which actually ends up on the doors, using the aft end of the main spar for reference.  Measuring 76 inches from that point is easy but getting it at the right location vertically isn’t. I used the water level to find those locations. Once I completed marking both sides, I double and triple checked to make sure everything is correct. I measured from the left leading edge mark to the right leading edge mark, and from the left leading edge mark to the lower spar location on each side as well as from the right leading edge mark to the lower spar locations. See the photos below. It looks like everything came out perfectly.

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Double and triple checking critical points for installation of the strakes.

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The bottom of the main spar is the reference point for the leading edge of the strake.

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Final check is the leading edge point for the opposite strake. Everything lines up! Whew!

I mentioned in the page notes that the fuselage does not sit “level” when on the main wheels. I used my fuselage dolly and one of the scissors jacks to make it level! The jack is sitting right underneath the engine firewall bulkhead which fully supports the entire fuselage so there’s no chance of damaging anything. With the wings on this thing it is starting to get heavy!

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The fuselage is not naturally level when sitting on its wheels so I need to jack it up to make it level for installation of the strakes.

Once the fuselage is level I then need to re-level the incidence of each wing. What actually is happening here is that I’m twisting the wing aft to fore to get the incidence correct. This is necessary for getting the alignment of the strake correct. The wing is going to naturally twist a bit while flying, which is what it’s supposed to do, but since the strake holds the fuel tanks, it can not twist. So, we align the wings for level, install the strakes, and then let everything come to their natural resting place.

This photo doesn’t show it, but once I got the wing incidence correct, I used a ring clamp purchased from McMaster-Carr to lock the bottle jack piston into place. (You need to match the ring diameter to the piston - in this case, 40 mm. McMaster has them in all sizes.) These jacks have a tendancy to bleed off pressure after awhile.

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Once the fuselage is re-leveled, I need to level the wing incidence. Because the bottle jack can lose pressure, once this is set I placed a clamp around the piston to lock it in place.

I know, this looks a bit precarious, but that’s ok. With the main wheels off the ground the airplane wants to teeter back and forth, so I’m using some old scrap foam and saw horses to hold the wings in place. This isn't critical - just there for stability.

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I needed something to hold the lower strake in position while trying to get it to fit. Here’s the contraption I came up with using a bunch of scrap lumber. It ain’t pretty but it works perfectly. I use some wedges to help align each end into place.

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My contraption for holding the lower strake into position for trimming and fitting!

Before trimming the strake, I marked all of the areas that either needed cutting, trimming or sanding. Measure twice and cut once! This is not something that I want to mess up. Honestly, it’s hard enough trimming these things in three dimensions to get them to fit just right. No matter how careful I was, I still ended up making some minor adjustments on most of these. 

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Here's the initial markup for prepping the inside of one of the strake halves. All four halves (two for each side) are prepped the same way.

Here’s the left lower strake after trimming it. There’s a layup that needs to be applied to the lower skin portion shown below. That layup extends two inches onto the non-trimmed part. I had an interesting problem when I first tried to do that layup. After it cured, it literally peeled right off! A totally failed layup! That never happened to me before. I still haven’t figured out what happened (although I have some theories) but my second attempt at doing the layup worked perfectly. Very weird!

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It took forever, but here is the left lower strake trimmed to fit.

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Here's a closeup shot of the beveled edge.

Once each strake half (there’s four of them - two lower and two upper) is trimmed, it’s time to do the strengthening layups from 2 inches above the beveled area, over the bevel, and then about 5 inches or so onto the flat area where foam used to be. Because these strakes have compound curves, it’s hard to get them to lay flat for the layup. I clamped one end, then used some of my rock weights to hold the rest of it down. (Those are heavy rocks too!) I also double-stick taped the edge  to the table to make it as flat as possible. all told, it worked fairly well!

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This photo shows the strengthening layup at the area where the foam and upper layer of fiberglass was cut away. The beveled edge, shown earlier, helps the fiberglass to transition smoothly between the upper and lower section.

This photo shows the lower left strake sitting on my contraption. It’s not perfectly lined up here - I’m showing this just for reference. 

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This photo shows the strake sitting on my contraption. It is not in its final position but gives you an idea of how it gets placed.

Before attaching the strake, I need to thoroughly sand the side of the fuselage where the layups go, both inside and outside. It seems a shame, really, since the fuselage, out of the factory, has such a nice, smooth finish to it. That finish is hard to get off too! Oh well.

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Here you can see that the fuselage is sanded in preparation for the fiberglass layups which hold the strake in place.

When I attach the strake for real I need supports to align it against the fuselage. That what these wood blocks are for. If you look carefully, you can also see some thin, black Sharpie lines close to the blocks. That’s where the edge of the strake aligns to the fuselage. The next photo shows how I support the aft end of the strake during installation.

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Small wood blocks are hot-glued into place to help support the strake before doing the internal layups.

The aft end of the strake, where I removed the foam, actually gets epoxied to the lower main spar as well as the wing. (Eventually a thin cut is made to separate the strake from the wing.) I needed to come up with some way to compress that aft end as tightly as possible to the spar and this is what I came up with. I’m using one of the old metal I-beams used originally for aligning the wing trailing edges, along with a few scissors jacks for compression. I haven’t glued anything down yet, but my initial testing tells me that this will work just fine.

At this point there isn’t a spare saw horse in the hangar. I’m using something like 14 of them!

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For epoxying the lip of the lower strake to the lower wing, I need some way to compress that lip so that it's as tight as possible. Three scissors jacks and one of the old I-beams used to help make the wings solves that problem!

This is as far as I’ve gotten for the moment. More to come. I’ve only trimmed and prepared the left lower strake. I’m now working on the right lower piece. Before those can be installed there’s a bunch of other work that needs to be finished. All of that work is coming up a bit later, so stay tuned!

ui© John Trautschold 2018