The Long EZ Build: Strake leading edges

The strakes / fuel tanks were built with flat leading edges so that it would be easier to seal up the inside of the fuel tank. But this wouldn't make a really efficient leading edge for an airfoil. The plan here is to bond a bunch of foam to the leading edge... shape it like an airfoil.. and glass it.

First of all, this step was "forgotten" in the plans. CP#30 LPC#84 says: "We omitted to tell you to micro a urethane foam block (2 lbs/ft3 green) to the 'flat' leading edges of the strakes, see page 21-7, cure, then carve to match proper leading edge contour (ribs R23, R45 and the wing). Slurry the foam and lay up two plies of UND crossing each other at 45 degrees to the leading edge of the strakes. Lap this layup 1/2" onto the strakes top and bottom."

Simple right? Let's get started!

If you recall way back when I built the fuselage sidewalls (chapter 5), I embedded a conduit such that I could route wires from the leading edge of the strake to the avionics compartment. Well it was now time to install the conduit from the fuselage to the outboard of the strakes. I started by sanding/prepping the flat leading edges so that I could bond the 0.5" pex pipe conduit to it...

I marked where the conduit needed to line up with the one that was already embedded inside the fuselage wall...

I used a dremel to make a notch on the R23 and R45 tips so I could slide the conduit in...

I cut the inboard tip of the conduit so that it would be flush with the fuselage wall...

Roughed up the entire conduit so that the flox would bond well...

I then used duct tape to hold the conduit in place...

Used a clamp to hold the outboard end tight around the OD rib... the idea being that any wires that make their way into the OD/R45 compartment will be able to be fished through the strake leading edge and into the avionics compartment...

Happy with the conduit placement, I mixed up wet flox and dabbed it along the top.. making a nice fillet...

Here is a good shot showing the conduit that was already embedded in the fuselage wall... I made a hole to verify I could fish wires through... this will later get patched up... or I'll create a small "access cover"...

Repeated for the other side...

After it cured, I sanded everything again.. this time so that the foam blocks I was about to install would bond well to the surface...

Let's talk foam blocks. For the outboard fairing block, I purchased a styrofoam large cell block (8"x16"x25") from aircraft spruce... the idea was to cut this block in half to use for both sides...

Cut it in half... simple, right? Can't do it with the bandsaw... its too tall! So I came up with this contraption... I used a 2x1 to clamp it onto my workbench... and used the 2x1 and the table's edge as a saw guide...

Notice the table's edge acting as a guide...

This worked great! Taaa-daaaaa! Two peices!

Ok, but now I had to make it fit into the corner between the strake and wing root... so I traced a template and cut it out using cardboard...

Brought my template to the foam block and marked it...

Trying to test fit it... but that conduit is in the way!

So I marked the conduit location...

And milled out a notch using the dremel...

There we go!! That's what I'm talking about!!!

I then prepped the surface getting it ready to bond the block...

But how is it going to stay put while it cures? I prepped a come-along strap wrapped around the spar to do just that...

I also made a dummy rib out of cardboard to hold/protect my nav antenna cable...

I mixed up liquid pour foam, spread it all over the mating surfaces, and pressed the block in place... using the come-along strap to hold it there...

This worked great!

Of course, I had to repeat this for the other side as well..

Now to focus on the strake leading edge foam blocks... I purchased four styrofoam large cell blocks (4"x8"x16")... and used my bandsaw to cut each of these in half...

The result was eight (four for each side) blocks for the leading edge...

Since the conduit is in the way, I had to mill out a notch for each block...

Here I am test fitting each block... the conduit held the foam blocks in place

Looking very Lego like....

I trimmed the inboard most block flush with the fuselage wall...

And repeated for the other side...

Notice I also cut wedges to fill the gaps between the outboard fairing block and R1... and another wedge between R2/R3 where the strake kink is...

I mixed up small batches of liquid foam and installed one block at a time...

I knew I was going to have a mismatch between the foam blocks and the top of the strake...

So I mixed up more liquid foam and poured it along the transition...

With all the foam in place... it was time to shape a perfect airfoil leading edge.. I started with the big guns! It helps to have a NEW sanding belt (ask me how I know!)

I sawed off the heavy stuff... being careful not to let the saw touch the strake skin...

The key to shaping the foam blocks was understanding what to trim first. Step 1 was to establish the actual leading edge line. Notice the black "line".. this was a first go at identifying the leading edge (a line between the wing leading edge and the point where the strake kinks (the leading edge of R23). The second line is the line between the leading edge of R23 and the point where the inboard strake leading edge mates with the fuselage wall.

I used the saw to trim the leading edge... staying a bit more forward to be conservative...

I then placed a straight edge to the leading edge.. and sanded it perfectly straight...

The idea being to end up with a leading edge that would intersect exactly with the wing's leading edge...

Looks like the leading edge line was cut with a laser! This took a lot of patience... as it was a lot of: check with the straight edge.. sand a little... check with a straight edge... sand a little... until it was just right!

With the leading edge defined (from the top view), I now needed a way to mark the leading edge waterline. So I took out my leveling laser...

This isn't as fancy as it sounds... I literally taped a laser to the side of a box... placed the box on a stand... and leveled the box with shims...

I shimmed until the laser aligned with the wing's leading edge... and with the point on the fuselage wall where the strake will mate with the fuselage wall...

I then traced the laser with a marker... defining the strake leading edge waterline...

Taaaaa-daaaaaa! Strake leading edge defined!

It was now time to sand the airfoil shape. Mentally, I was picturing this as two tasks: one was to blend the top surface of the airfoil down to the leading edge waterline, and the second: blend the bottom surface of the airfoil up to the leading edge waterline...

I used the belt sander to remove the bulky stuff...

Then sanding by hand for fine tuning...

I used a surform for the top surface... slowly and patiently. There is a post I made earlier talking about the "strake fairing kink"... which talks about how the outboard fairing block has a "ramp" to it...

I noticed a low spot along the inboard top transition area... so I used more liquid foam on top and let it cure...

Over the period of a week or so... it finally took shape...

Beautiful transition between the strake and wing...

No low spots... perfectly blended...

Just about ready for glass!

I prepped/sanded all the areas that were going to get overlapped with glass...

Including the fuselage wall where it will get overlapped...

I taped wax paper on the wing's inboard section so that I could overlap the glass onto the wing and not worry about it bonding to the wing...

Bottom as well...

With everything set, it was go time! I painted plain epoxy on all overlapping surfaces (top, bottom and fuselage wall)... used dry micro to fill all voids and slurried the foam...

CP#30 LPC#84 says to use two plies UND crossing at 45. I rarely ever change the ply schedule... but I decided to go with one ply BID at 45... and a second ply of UND parallel with the leading edge. I just wanted a beefier leading edge.

Here is a shot showing the inboard leading edge.. with peel ply for a nice smooth transition..

The outboard end... overlapping onto the wing...

The fuel drain tap...

The next day, I marked the line where the strake butts up against the wing's inboard edge...

Using a dremel cutwheel and a hacksaw blade, I cut along the line.. being extremely cautious not to cut too deep and damage the nav antenna cable living underneath...