THE LH LEADING EDGE

 

The outboard leading edge assembly is much like the fuel tank.  You use the same assembly fixture (below right) and the ribs are similar.  Unlike the rest of the ribs, however, the inboard rib has no rivet holes in it.  There is a splice strap that goes between the skin and this rib that connects the fuel tank to the leading edge.  This “Matched Hole” assembly process has few limitations but this is one of them.  To get holes in three curved thicknesses of material to line up when they are pre-drilled would be a neat trick but not practical expense wise.  Anyway, back to the assembly process.  The photo at left below shows 5 ribs undergoing the fluting process.

Fluting 5 of the 6 outboard leading edge ribs Leading edge in the assembly fixture

The photo at left below shows the splice strap clamped to the fuel tank.  At right below, the strap is drilled to match the holes in the fuel tank.   These holes will ultimately be filled with #8 counter-sunk screws so they had to be dimpled after they were drilled to size.  Some care has to be taken so that the strap is perfectly square to the fuel tank.  If not, the possibility exists that it won’t attach to the outboard leading edge properly.

Splice strap clamped in place Splice strap drilled to match fuel tank holes

Below left is a shot of the leading edge cleco’d in place.  The leading edge went on and off of the spar so many times, I wanted to put a zipper on it.  The reason was that I kept checking the splice strap as I drilled the holes into it.  After the splice strap was drilled to match the leading edge skin and the tank, I placed the inboard rib near it and marked the approximate location of the rivet holes on it.  I then fluted the rib and placed it in position on the spar (lower right).  When I was sure that it was aligned properly, I drilled the fastener holes from the splice strap into it. 

Fit checking the outboard leading edge Inboard rib of outboard leading edge

The next step in the leading edge assembly was to removed the protective coating from around the rivet holes and dimple the rivet holes.  The photo below left shows me at work with the “C” frame dimpling tool on the leading edge.  (I did not take any photos of the protective coating removal.)  As usual, I dimpled the ribs with a pneumatic squeeze (not shown).  Next, the spar needed to have the fastener holes counter-sunk where the leading edge rivets to it.  The plans suggest to dimple a scrap piece of aluminum and use it for a tool to set up the counter-sink stop.  This is an excellent idea and I used it.   

Dimpling the left outboard leading edge Counter-sinking the spar for leading edge rivets

After all the fit, drill and counter-sink details were complete, it was time to etch, alodine (photo at left below) and prime (photo at right below) the leading edge parts.  Notice the rectangular hole at one end of the skin.  This is for the landing light and will be covered in a different section.  The photo at left below shows  Where is the sixth?  Also, where is the sixth rib in the first photo in this section?  The answer is that one rib had to be installed with along with the outboard aft rib assembly because they share the same rivets.  See the “Ribs and Spar” section for details.

 

Etched and alodined leading edge parts Primed leading edge skin

The next step in this process was to put the assembly into the assembly fixture (below left) and rivet it together (below right).  This process was straight forward and I easily accomplished it by myself.  The skin and ribs were thinner than the fuel tank skins and I had a little trouble with rivets bending.  I switched to rivets ½ size shorter than the plans called for and the problem with bending disappeared.  I noticed that the plans call for rivets that are too long in several places.  If a rivet bends over instead of developing a symmetrical shop head, the problem is almost always that it is too long.  Shooting rivets into dimpled holes seems to make the problem worse.

Left outboard leading edge in assembly fixture Riveting the left outboard leading edge

The photo at left below shows the splice strap after it was riveted to the leading edge and the inboard rib.  It has all of the nutplates that will allow the tank to be attached to it with counter-sunk screws cleco’d in place awaiting rivets.  The attach holes for the nutplates are also flush and the plans have you countersink these holes.  If you countersink these holes you will enlarge them because the skin is not thick enough for a countersink and the cutting edges of the countersink will go completely through the material.  The solution is to dimple instead of countersink.  That means that the nutplate must also be dimpled (below right).  The example is not a nutplate for the tank attach but you get the idea.

Tank attach nutplates ready for rivets Dimpling a nutplate

If you are wondering how I attempted to keep the wing from twisting as I installed the leading edge, it was simple.  I leveled the spar in two planes, inboard to outboard and top to bottom, when I placed it on the assembly fixture initially.  In the two photos below you can see the level on the wing spar at the inboard end and the outboard end (arrows).  This levels the spar top to bottom.  I previously leveled the spar inboard to outboard (not shown).

Level at inboard end Level at outboard end

I then attached the fuel tank with all of its fasteners including the brackets that attach it to the spar (not shown).  I put the levels in the same spot as above to make sure that the spar was still level (not shown).  I cleco’d the outboard leading edge assembly to the spar and checked for level again (not shown).  I didn’t take many pictures as I was leveling because my concentration level was too high.  Next, I put a plumb bob at each end of the wing.  I attached them to the front of the leading edge of the tank and outboard wing with tape (photos below left and right).  

Plumb bob at outboard end Plumb bob at inboard end

I then simply measured from the string that holds each plum bob to the spar with a 6 inch scale.  The goal was to keep the measurement at each end as close to each other as possible.  See the two photos immediately below.  My personal tolerance, don’t forget I was inventing the procedure as I went along, was .100” (1/10”).  I am happy to report that I was able to keep the twist to less than ½ of that.  AS I riveted the leading edge to the spar, I periodically checked these measurements.  I didn’t have to adjust anything but I did keep checking.

Measurement at inboard end Measurement at outboard end

Ouch!!!  What the heck happened.  The photo below left was taken as I was riveting the leading edge to the spar.  The rivets that go into the spar were out in the open so I set up a pneumatic squeeze to set them.  When I got to this rivet, something on the other side grabbed my squeeze and I damaged the skin.  The culprit was one of the nuts that hold that tie-down bracket to the spar.  The photo at right below shows these nuts and their proximity to the leading edge rivet holes.  Damn, I should have looked in all the bays before riveting but it was going so smooth.  I drilled the rivet out, straightened the skin, and re-shot the rivet.   I know I will be putting filler in here prior to paint.   

Damaged skin at rivet The culprit

Here is a picture of the left wing with the leading edge attached, below left.  The rivets that hold the leading edge ribs to the spar have not been shot at this point but all of the skin rivets are installed.  The picture at right shows the outboard lower wing skin cleco’d in place.  There was no particular reason that I cleco’d this skin in place.  I just wanted to see what it would look like.  (Actually, I cleco’d all of the skins on at this point but I did not take a photo.) 

Outboard leading edge skin attached Outboard lower wing skin cleco’d in place

The photo at left below shows two “boogered” up rivet heads in the rib to spar attach.  I actually shot the rivets right after I finished riveting the outboard leading edge skin to the spar but I took the photo 2 weeks later just before I drilled them out.  The installation of these rivets was a problem for two reasons.  I did not have a bucking bar that fit inside the leading edge that I could hold easily and the rivets were too close to the ribs.  I solved the problem by grinding the offset rivet set I used so that it could lay on the spar and still allow the rivet to be shot.  As for the bucking bar, I purchased another one.  See photo at right below.

Two “boogered” up rivet heads Modified rivet set and new bucking bar

The picture below left shows me re-installing the rivets.  At right below is a picture of the rivets after I re-shot some of them.  I did not have bad rivets in every hole initially because  I quit riveting right after the picture of the two “boogered” up rivet heads was taken. 

Installing rivets in leading edge ribs to spar Rivet heads shot with proper tools.

The outboard leading edge assembly is now finished so we move forward to the wing skins.