Thursday, September 6, 2012

Starting with the frame

The Wirraway is constructed around a welded chrome-molybdenum steel allow tube framework, so that’s where I need to start as well. Here is what the steel tube framework looks like:

The dimensions for each of the tubular members (length, outer diameter and gauge) are listed in the Wirraway Overhaul and Repair manual, so this will be the data I use to create the virtual framework. The fuselage frame is divided into four sections:
  • Main frame
  • Aft fuselage frame
  • Engine mount (not shown in the picture above), and
  • Aft fuselage monocoque

Three of these sections are constructed from welded tubes and the fourth is an aluminium sheet monocoque construction. First I will start on the main frame. The fuselage reference line (F.R.L.) or centre-line is 14¾ inches below the top cross-member in this front face, and the thrust line is another 4 inches below the F.R.L. The side frames are 30 inches apart (from the centre of the tubes), and the main frame is 106 ½ inches long from the centre-line of the front verticals to the centre-line of the rear verticals.
The diagonal bracing tubes down each side of the main frame are not symmetrical, a fact missed by many scale modellers. On the starboard (right) side of the main frame, the 4 tubes next to the rear seat are angled outwards, to allow the rear seat to pivot so it can face forwards or backwards. In the centre of the main frame, there is a strong roll-over brace to protect the crew in case the aircraft overturns. The starboard side frame is also missing one upper diagonal bracing tube at the very front, to allow for the installation and removal of the magazine for .303 rounds for the front guns. And at the bottom of the main frame below the rear seat there is a heavy structure supporting 2 plates for mounting the seat pivot bearing. 
The first step in constructing the frame is to create all the tubes, and the result looks like this:

The next step will be to start adding all the fillets, brackets and attachments which are also welded to this basic structure…