Installing the Bede BD-4C Floor

How I spent my winter vacation: Since I had the week between Christmas and New Years off work, it gave me a long stretch of time to attack the large job of installing the floor in my Bede BD-4C. I thought it would be simple but, in the end, I was very glad that I had so much time because the job turned out to be much more involved than I anticipated. From December 23 through January 2, I spent about 48 hours on the floor and ancillary tasks.

Before I could install the floor, I had to remove several things from the plane which were in the way. Since BedeCorp partially assembled my fuselage, they installed the landing gear so that the plane could be rolled onto a flatbed trailer for shipping to me. The main landing gear attaches to a gear box which sits on top of the floor. The nose gear attaches to several brackets which are also positioned on top of the floor. Finally, I had to remove a couple of gussets from the rear of the cabin area.

The metal for the floor, 48″ x 72″ and just 0.020″ thick, came rolled up in a box 15″ on a side. The first actual “floor task” was to trim sheet down to 46″ x 62″. I got an air shear from Harbor Freight and it made quick, easy work of the cutting.  The slow part was measuring, re-measuring, and re-re-measuring to be sure that I was not mis-cutting this large piece of metal. Had I goofed, the replacement would have been costly and the shipping more so. Fortunately, all went well and the floor fit just fine on the first try.

To insert the floor into the base of the fuselage, Candy and I gently bent it in half and slipped it through a gap near the rear of the cabin area. Think of a process somewhat like slipping a letter into a mail slot and somewhat like putting your foot into a sock and you will have the right idea.

Candy had a brilliant idea to keep the floor from sagging as we worked with it. We took a 2×4 and used my spring-loaded stool to press the board up against the bottom of the fuselage. It made a perfect support for the middle of the floor.

With the floor in the fuselage, I drew a line 1″ in from the edge, all the way around. By drilling rivet holes through this line, the rivets would be in the middle of the 2″x2″ angle of the fuselage on which the floor rests. I then used the rivet spacer tool to evenly space the rivets 2″ apart, all the way around the cabin area.

I bolted the gear box back into the fuselage. I drew rectangles on the floor for holes for the gear legs, using the holes in the gear box as templates. David and I also drilled rivet holes on 1″ spacing through the bottom of the gear box and the floor.

I had to do a little side work, installing nut plates in the angle bracket at the front of the cabin floor, so that the nose gear bracket could be bolted to the angle bracket.

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I  then bolted the angle bracket to the fuselage and the nose gear brackets to the angle bracket. This locked the angle bracket in place so that I could accurately drill the rivet and bolt holes through the floor and the angle bracket. I also drilled bolt holes through the floor to match those in a couple of nose gear brackets.

With all of the holes drilled, Candy and I removed the skin from the fuselage. All of the rivets would be countersunk (flush mounted), so in addition to the usual deburring work, I needed to dimple the skin and countersink the thicker aluminum pieces.

Dimpling is easy. Start with a dimple die set, one concave and one convex with a pilot pin sticking out of it. The pin centers the convex die over the hole. Whack the die with a hammer and poof! you get a neat dimple in the sheet metal, perfectly smooth and perfectly sized for the rivet.

Countersinking the metal is easy, too. The tool looks much like a fat drill bit, or a wood countersinking bit, but it has a smooth pilot pin sticking out of the middle. The pilot pin gets inserted into the rivet hole and keeps the countersink bit centered over the hole. The countersink bit gets screwed into a micro-stop which gets adjusted to exactly the desired depth. Pop the micro-stop, with countersink bit, into a drill and hit each rivet hole.

All of the rivets were to be installed from the bottom of the plane so the “bump” would be inside the fuselage and the outside skin of the Bede BD-4C would be smooth. Around the outside edges, the floor sits on top of the fuselage so, the rivets (being shot upward) would go first through the 0.063″ thick angle and then through the 0.020″ thick floor. The metal of the angles is thick enough that it could be countersunk.

Rivet holes for the gear box and the angle bracket at the front of the cabin area needed different treatment, though, since both the gear box and the angle bracket sit on top of the floor. This means that the rivets will penetrate first the thin floor and then the thicker material of the box and bracket. Since the 0.020″ floor is too thin to countersink, it gets dimpled. The metal on top of the floor needs to be countersunk to accept the dimple.

Countersinking all of the holes in the bottom of the fuselage and the gear box was easy but I was covered in aluminum shavings, since I had to do it all on my back underneath the plane. I will probably be picking bits of aluminum out of my beard for months!

Once all of the holes were deburred, countersunk or dimpled, it was time to see if the floor would fit. We slid it back into the fuselage and was I ever relieved to find that it did, indeed, fit. I had missed a couple of bolt holes but quickly drilled them. I also needed to slightly adjust the size of the large holes for the gear legs but, again, this was a quick fix.

David did yeoman’s duty with the rivet puller underneath the plane, easily installing 3/4 of the rivets. The floor, once riveted to the fuselage and gear box, is beautifully smooth and a delightful end to a long week’s work.