onto the slats

I wanted to get back to the 3D printer this week, but my time was better spent in the shop working on the slats.  I did manage to get the 3D scanner to work, but more on those later.

Slats are aerodynamic lift assisting devices attached to the front edge of wings.  There are many types of slats and methods to accomplish the same aerodynamic principles.  Large commercial aircraft often have hydraulically activated slats that extend on command from the wing.  Some aircraft have slats that automatically deploy when the right conditions exist for it to be beneficial.  In both cases, these are overly complex to design and build and not very common in light aircraft.  Slats benefit STOL aircraft because in normal cruise, the profile of the wing acts the same as a wing without slats.  However, at higher angles of incidence, such as in climb or descent, the slat forces air from below to the top of the wing, increasing lift dramatically, allowing much slower stall speeds (and steeper climb/approaches typical of STOL aircraft). 

On Zenith STOL aircraft, the slats look/work like this:

I've learned the importance of gathering all my parts/materials before starting to build a section, so I started by laying out all the parts I have made so far for the slats.  There are four to build, 2 inboard and 2 outboard - just like the flaps.  I just noticed in the picture below I'm missing one of the slat doublers.... hmmm.  I'll have to double check my count.

With a good idea of what's needed and what I need to still make (skins), I had another look at the plans.

The slats are a fairly simple structure to make without too many parts - this keeps them very lightweight.  Like the flaps, accuracy is important so that the pick-ups match the attach points on the wings.  Also like the flaps, it took some sleuthing to deduce the "distance between slat supports" by flipping back and forth several times between the slats diagrams and the wing diagrams.  Not sure why Zenith couldn't just place the measurement on both pages! Each of the drawings have different points of measure.  If this was a match drilled hole kit, no issue but for a scratch builder it takes some figuring!

Next I started laying out the skins.  The width of the skin for a completed slat is deceiving as it curves on both the top and the bottom around the ribs.  As a result I was disappointed to find that I can't fit two slat skins on every sheet, it's about 20mm too wide.... argh!  I'll use the remaining metal on other parts but it would have been much quicker and nice to get two skins from each sheet.  While I had the rolls out, I got all four slat skins measured and cut to size.

Full size 4x12 foot sheets are cumbersom to work with, so I cleaned up the bench a bit in order to make room, which was long overdue anyhow.

Other parts I still needed to make were the slat support brackets for attaching to the wings.  These are traced from my card stock template onto 040 aluminum and cut out roughly on the bandsaw.  From there they get ground down to size and sanded smooth.

Here is a pic of the four skins rolled up and the slat support brackets all ready for their turn in assembly.

I put away the skins for now and started to formulate a plan to assembly the first inboard slat.  The general construction of each slat consists of a 025 slat doubler angle, 3 slat ribs and 2 slat support brackets.  The slat doubler is essentially the spar of the slat.

Each of the slat support brackets also has a doubler made from a bent piece of standard 025 "L" angle.  I decided to make up all eight at thee same time, 4 left/4 right.  It starts with a 120mm long piece of L angle. 

Measure out 50 mm and 80 mm in the centre of the bend where the relief holes will be.  Use a centre punch to make a dirll point: 

This notched piece of hardwood makes an excellent backer for drilling the holes so the part doesn't wander:

Use snips to make relief cuts to the edges of the holes:

Next was trying to decide how to make all the support brackets consistent with the ribs.  The plans call the back edge of the support bracket to be 140 mm from the nose of the rib and that the rear lower edge of the rib be 28 mm elevated.  So I figured the best way was to layout a rough sketch of the dimension lines on a board and trace out where a rib sits in relation to the bracket.  This should ensure consistency for each rib that needs a support bracket and doubler.  At the top right you seen the curved doubler (more on this later).

I struggled for a couple of minutes to figure out how I was going to place rivet holes from the inside of the rib, through the support bracket and into the doubler on the opposite side.  The I figured out if I used a right side doubler inside a left side rib (and vice versa) I could trace lines on the inside where the doubler would be approximately.  Now I know the holes I'm drilling will mate up with the doubler. 

I duplicated the same process for the opposite side slats.  In order to keep parts together with their mates, I put a alphanumeric mark on each set of parts.  Slat support "J" matches up with rib "J" and doubler "J"

here is a better look at a curved doubler as it would be oriented under the rib and slat support.  The doubler provides extra skin support around the slot that will be cut for the slat bracket.

Each inboard and outboard slat has 3 ribs.  The outers have attach brackets and support doublers, the middle rib is just a rib.  I'm pondering adding a doubler to the middle ribs to further support the skin.  They are real easy to make and attach and weigh nothing.

Here are the ribs for the inboard and outboard right side slats.  The one in the foreground shows how the doubler creates the sandwich of the rib and support bracket go together.  Again, I decided to complete all 8 at the same time as I had the layout and process readily available.

Next up I started figuring out how I was going to bend the skins.  Slat skins from the factory come pre-bent and it's important to be accurate here.  A few test strips of 016 measured out according to the plans, bent then adjusted and bent/tested again to correct errors had me in good shape.  I documented where the bend lines are compared to the plans once the skin is actually bent.

With the lines laid out, I made sure to mark each line with an bend order number, where the bend setback would be and what radius.

Clamping the long straight edge down to the bench makes it easy to scribe the long bend lines.

The underside edge of the skin needs a corner relief cut at each end to allow the tip insert to fit correctly.  Measured the required cut and corner drilled first to make a clean inside corner when cutting:

The first bend must be the middle one as the throat of the bender isn't deep enough to make the bend from the other end of the skin if I make the small (2nd/3rd) bends first.  After measuring several times to confirm the first bend (everything counts on the first bend being correct) I placed it in the bender and used a long piece of 025 doubler as a forming shoe.  It worked perfectly.

Placing the slat doubler inside the bend confirms the bend is correct and true.  I pulled the slat doubler out past the skin to show the match in this picture.

The second bend adds the up angle on the lower part of the slat.  Again the bend here needs to be exact - too narrow, the next flange will be too wide.  Too wide and the next flange will be too narrow.

More double and triple checking and the 2nd bend turned out perfect.

Here is the skin back in the bender getting ready for bend number 3.  This bender can bend aluminum sheet up to 025 easily, but is really designed for lighter/softer aluminum trim coil/flashing/soffit which is can be bent to sharp 90 degree corners.  To adapt to bending smoother radii required of aircraft aluminum, we insert a pre-bent strip of aluminum called a "shoe" to help form the bending sheet around the shoe to create the correct radius.  In this case, I use a "shoe" of 020 to bend the skin around, leaving a perfect 1/8" radius in the skin.

Careful measurment and planning leads to a perfect set of bends - very pleased how it turned out!

Next I needed to figure out how to lay out the rivet lines for the slat doubler that fits inside bend # 1 (the 90 degree corner).  I placed the doubler on the edge of the bench and slid the skin over top.  The goal here is to make the rivet lines line up with the centre of the slat doubler flanges.  So measure the middle of the flange.....

..... then slide the mark on the doubler to the skin edge and mark the skin.  

Do the same at both ends then connect the marks with the straight edge.  Voila, a perfect rivet line.

The first rivet is placed 30 mm in from the edge of the skin which is also the centre line of the inboard rib.  From there, the rivet spacing is 50 mm.  Rivet locations marked with black marks along the rivet line.  These continue across until meeting the location of the centre rib.  Then the same process starts 30 mm in from the opposite end on 50 mm spacing towards the middle.

Next I'll flip the skin around the do the same layout for the other side of the flange of the skin/doubler.

Overall a productive couple of days in the shop.  More coming soon!