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November 15, 2008

AUX Fuel Tanks

Wing Skinning

Trimming Skins

From what I’ve seen, most folks are assembling the aileron and flaps in vertical jigs. I decided to use the flat table instead. Eric Newton used the table top also. The table must be flat! I used Eric’s simulated skin method to locate the two end nose ribs first and drilled and clecoed them. Then I strung up the rest of the nose ribs like a shish-ka-bob on the uncut aileron balance weight tube (no lead yet) and placed each next to their respective attach angles which were already clecoed to the spar. Using the balance tube keeps all the nose ribs aligned very well. By sliding neighboring ribs out of the way I could clamp the nose rib to the attach angle then use the 12” bit to match drill each nose rib through the predrilled attach angle.

John’s low-tech hint of the day:

Bags of shot make great stabilizers/clamps/holders/third-hands. Each bag is 25 pounds and conforms to parts you are trying to stabilize. Here the bags are keeping the spar flat to the table and also keeping the spar from sliding around while I match drill the nose ribs.
 

Modified Hinge Area:

The early pioneer builders have said that the space between the nose ribs is marginal for applying fasteners in the area. Some suggested spacing these ribs a bit further apart. I moved each nose rib in the hinge area 1/4” away from the hinge centerline. Do NOT move the hinge centerline! This required the hinge .063 doubler to be increased in width to capture the rivets coming through them. I also increased the width of the hinge gussets to properly cover the back ribs which are kept in line with the nose ribs. Not a difficult mod, but you must remember that this mod changes many of the hinge area distances that are specified on Plans Sheet 8 (the Aileron and Flap spars). I penciled in my new specs on the plans sheet so I wouldn’t make calculations from the wrong spec in the future.

This was my setup for drilling the back ribs to the spar. (This is the flap, but I used the same procedure for the ailerons.) Things to consider in this picture are:

1. I used four jig boards to hold the spar vertically at the correct angle (the angle of the bottom flange- 1.5 degrees for the flaps and 1 degree for the ailerons) relative to the table top. The boards are clamped to the table and the spars are clamped to the angled face of the board. The boards were aligned horizontally to keep the spar straight.

2. The two end ribs were attached according to plans dimensions and used as a reference for the remaining ribs. The staight-edge is an Al I-beam from a sliding glass door frame. It is straight and relatively stiff and makes a great straight-edge tool.

3. Shot bags hold the straight-edge stable and hold the ribs at the proper forward-aft position.

To drill a rib I would place it onto the attach angle and slide it back to bump into the straight-edge. Then lay a shot bag over the straight-edge and over the tail end of the rib. This would positively position the bottom of the rib upon the table top. Clamp the rib to the attach angle and match drill through the predrilled attach angle with a 12” bit and cleco as you go to hold things steady.

There really is not much “slop” to work with if you use the router method to cut the ribs. They are so uniform that they seem to just drop in place correctly. So uniform in fact that you will have to trim the forward end of those ribs that abut the hinge doubler plates.

Time to put the nose skin on the skeleton.

Do Y’all remember the scene in the movie Jeremiah Johnson where the old trapper brings Jeremiah a bear to skin? The old man runs through the front door with a live Griz right behind him and as he exits through the back door he shouts to Jeremiah who is standing flatfooted in the cabin “Skin that’un Prilgrim and I’ll bring you anuther’un!”. I felt like Jeremiah when I started this part. I needed 5 hands and 10 tools. Just keep a cool head and measure everything thrice and it all comes together.

1. Mark the centerline of each nose rib flange, top and bottom all the way back through the spar flanges.

2. Instead of transferring measurements from the spar to the skin, I did this: Set and clamp the skin on top of the spar flange and transfer the centerline marks directly to the skin at the tip bend. No measurement or transfer errors this way! Now use a square to extend the centerline marks to the top and bottom aft ends of the skin. In the picture to the left imagine the skin lifted straight up, rotated 90 degrees counterclockwise, and the skin bend placed directly on top of the spar bottom flange.

3. Mark a straight line to define the first row of holes in the skin and predrill these holes. Now you can see the centerline mark on the ribs through the holes to ensure correct alignment of the ribs.

4. The rest is the same procedure outlined in the Bearhawk newsletter describing the procedure for skinning the wings. I drilled and clecoed each row (inboard to outboard direction) but stopped before drilling the spar flange.

5. At this point the spar is held straight.

After the skin bottom was drilled and clecoed, I flipped the aileron over to drill the top of the skin.

1. With the tail-end of the back ribs on the table there should be no twist in the spar. I used shot bags to keep the aileron from moving around on the table.

2. The I-beam straight-edge was used to bend the skin down over the nose ribs and clamp to the spar flange. This worked great, as there was no tendency to bend a crease into the skin since the pressure was distributed evenly along the whole length of the skin.

3. Drill and cleco each row starting at the nose tip and working back to the spar.

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