I have had occasion elsewhere on this blog to talk about the replica Spitfire flying and engine controls that I developed with 3D printing, but here I will talk about how I went about building a replica cockpit section in order to further enhance the feel of flying a Spitfire.
Part 1 : Before starting, a few thoughts on preparing the project
My main constraints were budget, tooling and space. ie I didn’t have much money to spend, I don’t have rollers or presses to make frames or to shape sheet metal and I had to be able to build the cockpit in my single car garage, already full with all the sort of stuff one keeps in a garage. I therefore decided to build the cockpit using readily available 18mm plywood for the frames and 3mm thick MDF for the skin. By the way I use metric as although the original plans are in imperial, most of the hardware I use is metric, since I live in France.
Spitfire plans : a thorny subject. Several “publishers” sell scanned original Supermarine Aviation Works Ltd (SAW) engineering drawings in bundles per mark of Spitfire. Generally the first bundle includes Mks I and II, the second Mks IV-VIII and the third Mk IX and PRXIX. The principle is that the first bundle is fairly complete with drawings for just about every aspect of the early Spitfire. The latter bundles only include the drawings for parts that differed from the original set. To illustrate this point, the frames of the Spitfire in the Mk I did not really change much until the introduction of the low back Spitfire IX/XVI and onwards, so in the bundles for the later marks you only get drawings that show modified parts.
Each bundle cost me about 35 Euros, I got mine from Sicuro Publishing, but there are other suppliers. You need to be careful with theses bundles as they are incomplete, several are “general arrangement” drawings with no dimensions, others are warped and there are tons of fairly useless drawings of brackets fittings and bits that you won’t need. Some have only a single dimension quoted on the drawing, so I was able to measure other parts of the drawing and infer the measurement – however care is needed as some copies of the drawings are warped ! On some sheets the measurements are present but difficult to read so you sometimes need to look for similar drawings to find one where the dimensions are legible. I bought all three bundles because I wanted to make a Spitfire IX cockpit and it turned out to be a good strategy as I was able to find some missing details for Spitfire IX in the drawings for earlier marks.
When you look at drawings it is vital to look at the sheet reference data. For example there is a big difference for cockpit hood between Spitfire I, Spitfire pressurised or Spitfire PR XI. Such sheets can however be useful to get a general idea if you don’t find the exact one for the mark of Spitfire you want to replicate.
I also purchased Paul Monforton’s Spitfire Engineering e-book which covers the Spitfire IX/XVI. The principle of this book is that he measured 5 or six existing Spitfires and made them up into CAD drawings. While the book is very useful for overall exterior dimensions it does not give the interior profile of the fuselage frames, so if this important for you you need the SAW sheets. The book is however a mass of details about all exterior aspects of the Spitfire including the thickness and fitting of the skin panels even down to the number, position and type of rivets. It also has tons of colour photos of particular bits that are useful. I guess the book was really designed for producers of model kits who need a wealth of detail about the exterior or “visible” parts of the Spitfire. In the cockpit area there is some good detail of the instrument panel and some of the hand controls, but for example you will not find the exact length of the control column. The main value of Monforton’s book was to check exterior frame dimensions against the SAW drawings and to use Monforton data as a basis when SAW drawings were missing or illegible.
The Spitfire cockpit is about 85 cm wide at its widest point in the cockpit area, so bear this in mind if you want to bring your cockpit indoors, as a standard door width is 80 cm – ie it won’t fit !
I plan to take my cockpit around to airshows and events so I needed it to fit through standard doors, so rather than cheat on the width I cut the section into 80 cm chunks along it’s length.
Regarding width, the Spitfire frames were designed to take an aluminium sheet skin. The thickness of this sheet varied at different areas of the fuselage. In the cockpit area it is just under 1mm thick. Monfortons dimensions are exterior ie with the skin. If you are using 3mm ply you need to reduce SAW exterior frame dimensions by 2mm or reduce Monforton’s dimensions by 3mm.
Care is needed when using SAW drawings for frames, because Spitfire frames are double sided and spaced apart, so aft of frame 8 the forward part of frames will naturally be slightly larger than the rear part. Be careful to identify the forward and rear drawings and those for port and starboard as some features are not symmetrical about the centre line.
Next you need to decide what length you want to have. If you want to have the rudder pedals as per the original you need to start at frame 7 and if you want to have the rear cockpit window and room for the hood to slide back you need to go to frame 12,5 ie mid way between frames 12 and 13 = 1,9 m. For you to get an idea, frame 8 is the frame on which the instrument panel is fixed and frame 11 is the frame to which the pilot’s seat is fixed.
Height. What needs to be taken into consideration is how the cockpit is going to sit on the floor and how you are going to get into it. The full cockpit section is an oval egg shape so if you go down this route you need to think about how to construct the cockpit along some sort of datum reference. Classic Spitfire construction is in a jig of known dimensions where the frames can be precisely positioned prior to assembly of the fuselage intercostals and skinning. The real Spitfire construction also uses L shaped longerons which are on the lower left and right of the oval.
I chose to use a flat base and the fuselage datum longeron as my reference points. This meant cutting all the frames at the same depth below the datum longeron centre-line (c/l). I chose this lower point so as to allow the control column pitch axis brackets to be fixed at the correct height. The Spitfire does not have a floor and in the real aircraft the inside of the bottom skin is visible, the pilot rests his feet on a transverse bar at frame 9 and the seat floats in midair connected by an adjustable mechanism to frame 11. The control column is fixed in a well in front of the F9 transverse bar. For realism I wanted to reflect this and to obtain the correct fore and aft throw of the column. My false flat bottom sits at about WL-6 ie 24” below fuselage datum longeron c/l. Having a flat base obviates the need for some sort of U shaped supports for the full oval shaped section and it allowed me to fit castor wheels to facilitate moving.
I used the false floor and the fuselage datum longeron as ways to space the frames and to begin assembly. The port and starboard datum longeron is not that simple of an affair as for one it is of trapezoid section and second it tapers towards the rear. I started with 40x40mm section wooden bars which I then planed to try and approach the correct tapered angle. My single 18mm thick frames are placed on the frame centre lines, but because they are less thick than the originals there is more space between each frame and this needs to be taken into account for the fitting and positioning of some controls.
I must admit that I did start by building F11 with two sheets of 3mm plywood with spacers to give the correct frame depth. But I abandoned this method for this project as it more than doubles the work for cutting out the frames, for fitting flanges on the exterior profile and for filling the gap on the interior profile. Plus I wasn’t sure that all this would be strong enough to support the pilot sitting in a seat suspended on the frame. On second thoughts I could have used a compression strut between the seat base and the false floor to alleviate the load on the frame. I still have the partially completed frame so I may end up fitting it one day.
Fixing the skin to the frames. In the real Spitfire the “Alclad” aluminum skin was pop riveted with a mixture of standard and flush rivets. In the cockpit area mostly flush riveting was used, so with wooden frames I elected to go for countersunk wood screws.
Finally you need to decide if you are going to use IR tracking or VR goggles as this dictates whether you need to cut away the instrument panel and maybe the cockpit skin to fit a video screen or not. I plan to start with IR tracking and evolve towards VR, so my cockpit will do both. I have a cut away instrument panel and will have an insert with the complete panel and skin for VR. Obviously there is a third option – to build an instrument panel with working gauges and to project the sky on a curved screen – I elected to not go down this route for budgetary reasons.
There can be some debate about the need for a cockpit when using VR. The only thing I can say is that a cockpit allows the replica controls to be positioned exactly as they should be, such that what you see in VR is exactly what you get by touch. Secondly the fact that you sit in an enclosed space means that you physically feel the confines with your hands, shoulders and head, which to my mind vastly improves immersion. If with VR you are not going anywhere the cockpit doesn’t need to be pretty – but half the fun of building is to make it look right even if only you are going to use it. I guess this is a hangover from model building.
In part 2, I will explain how I drafted the plans to full size and how I integrated the controls for the engine the flying surfaces and the various buttons, switches, dials and gauges.
79vRAF 