Sunday, 16 August 2009

Making new rear d-windows

By June it was clear that there were a number of tasks that needed doing before I could go ahead and get all the welding done on the fuselage. One of these taks was to build new rear D-windows. This was important as a tab would have to be welded that would fix the window to the fuselage and tie the window into the stringers along the fuselage.

The old D-windows looked very similar to the Harer STC windows but the position was wrong, they were fixed too high so that the top of the window was higher than the door frame and the line of the door window was not carried through to the rear window. The old window frame was also very heavy so I also have the oppotunity to make thing lighter again.

I have no specific plans for constructing a set of windows so I based the shape of the new windows on those in the Harer STC but slightly smaller. The design was schetched out full size on stiff paper which was then taped to my work surface. The main difficulty was getting the correct angle between the rear door frame (to which the window will be attached) and the line of the door window so that the top of the two windows will line up correctly. I cannot guarantee that this is the same on all aircraft but it worked out to be 100 degrees for me.

The frame is made in two parts, the main part holds the window glazing and is attached to the door frame with machines screws and nuts, and the fusalage tubing with self tapping screws. A separate outer frame is then used to trap the glazing in.

The basic window frame was cut form 0.020" NS4 aluminium using a band saw. The inner curves being cut with a fly cutter and a similar process was used to cut out the outer frame. finishing was achieved using aluminium nibblers and lots of filing starting with a course file and gradually finer files until the final finish using fine grit paper.

The next step is to recess the main frame to take the 2mm Lexan glazing. This was done using Rob's metal rollers. Rob adapted the rollers so that they could take attachments for putting joggles into sheet metal. Manually adding the joggle to the inside of the main window frame resulted in a very pleasing finish although it introduces a warp across the sheet. The warp will be taken out when the edges of the window are bent to stiffen the frame.

At this point the Lexan was cut to size and fitted to the frame. Remember to drill the holes in the Lexan slightly oversize to allow for expansion and contraction with the weather etc. Self tapping screws are used to hold the outer frame and Lexan to the inner frame.

Finally, the frame is bent at the edges to add stiffness to the whole assembley and provide a surface to mate with the door frame. There is still one or two minor tasks to do with fixing the window to the door frame mainly to do with cutting slots around the door frame mounts so once this is done I'll post pictures of the windows in situ.

I've also sent the fuel tanks off to have new fittings welded in and have the tanks pressure tested and have sent all the tail feathers off to have some remedial welding done. At the time of writing, these are back so the next blog will probably show the results of the welding and the work that is progressing on the renovated tail feathers.