Supermarine Seaplane Update - Putting the Craft in Aircraft
The Supermarine Spitfire, Saviour of the Battle of Britain, did not just suddenly appear from the pen of the designer Reginald Mitchell. It evolved through a series of racing seaplanes, racing mainly for the Schneider Trophy race in the 1920's and early 1930's.
It started with the S1, then S2, S3, S4, S5 (our aircraft) the S6 and S6B. So you see, the spitfire was not a one off wonder, it took seven aircraft, a lot of heartache, hard work, and engineering skills to develop the technology and techniques that made the Spitfire possible.
We are building a Supermarine S5 flying reproduction, the aircraft that won the first of three races by the Supermarine racing seaplanes, by the rules that meant they could retain the Schneider trophy for good.
For us, things did not get off to a good start, the hangar that the build was to start in burnt down with all the wood, plans and materials - so the search for more wood began and plans were re-printed.
Needing to keep the project alive and with nowhere to start the build, one of the team members, who at that time was re-furbishing his house, knocked through two rooms, set up a workbench and the first phase, making the wings, began.
In the building process the spars and the ribs were first but wood had to be sourced, and it is a very specific wood. The whole aircraft is made from aircraft grade Sitka Spruce, which surprisingly is a soft wood, with a large stiffness to weight ratio and very straight grained making it ideal for aircraft construction. But unlike 80 or 90 years ago where aircraft grade Spruce was easy to come by, we had the additional challenge of having to find it.
There are many considerations when selecting the wood: the grain has to be straight, there can be no knots and it needs to the right weight/density. Preferably 3.8 to 4.2 kg/m3. (The density can be checked by floating a uniformly calibrated stick of Spruce in de-ionised water.)
With covid upon us at this time, we had to use what we could find in the UK, wood lost in the fire came from the Swindon Aircraft Timber Company, but with no supplies available and long waiting time for shipment from ‘Aircraft Spruce’ of the USA, we ended up sourcing our material from Stones in Devon who supply Sitka Spruce for building high quality boats and spars. Of fifty spruce planks we inspected we found four that we could use.
These were cut to the sizes we required to start the spar booms and the ribs. Unfortunately, our woes did not end there, we made 14 spar booms to end up with eight good spars. The reason for this was once the wood was cut and planed down to the sizes we required, some undesirable qualities came to light such as small knots and/or the grain not being straight enough. (If interested in suitable wood in aircraft structures, look up the documents ANC-18 and ANC-19, which gives a host of information on the subject.)
There are two spars for each wing, the forward and rear spar and each spar consists of two spar booms joined both sides by a plywood web.
Inside the spar are blocks that take the loads of the ribs, the aileron hinges, the compression tubes and the wing-root fittings that attach the wings to the fuselage.
Compression tubes support the flying wires without which, believe it or not, the wings are literally free to flap up and down. These flying wires gives the airframe its rigidity and strength for flight.
At the same time, nearly every room in the house was taken up with making the wing ribs. A jig has to be made first and then the wood placed into this jig and glued together. In all 32 ribs were produced.
With every table in the house taken up with rib jigs, it was amazing how everyone in the family started to understand and discuss the terminology surrounding these ribs (and sometimes some slightly derogatory terminology!) at the breakfast table.
Luckily once the ribs and the wing spars were completed, a hangar became available at Bodmin airfield and the whole project was then moved there.
A workbench was set up in the hanger and the ribs joined to the wingspars. To avoid warps being built into the wing, which would affect its flying characteristics, this has to be done very accurately. The wings are built vertically (see picture) and the ribs lined up by 'eyeball' and checked with a laser which marks the top of the ribs to show they are in line.
Once the ribs and the spars are joined, we now have a recognisable wing.
The wing tips, that beautiful elliptical shape so famous on the spitfire, originated on the S5 project. The wingtip consists of two ellipses joined at the wing tip. These were traced out onto a building board, using nails, string and a pen, just as they did nearly a hundred years ago.
Blocks were then screwed to the board around our drawing of the ellipse and six pieces of one eighth inch spruce wide by about 8 ft were stuck or laminated together and then pushed into our wingtip jig to form the elliptical wing shape (see picture). This was then scarfed onto our wing.
(A scarf joint is when two members are joined end to end in woodworking or metalworking using a tapered joint that makes two members appear as though they're one continuous piece.)
So now we had the bare completed wing, without the plywood skin covering or the metalwork fitted.
The next job was the ailerons, these are the surfaces that make the aircraft roll in flight. These are really like a small wing. They have a spar and ribs to make the airfoil section and are again ply covered like the wings. These exert the force on the wings by deflecting the airflow up or down to roll the aircraft. As one aileron moves up, the other moves down. The ailerons are hinged to the rear spar and are operated by a series of bell cranks and pushrods.
We decided to skin the bottom of the wing with 1.5mm ply to make the wing more rigid for transport to London where the metalwork was to be installed: the aileron hinges & pushrods, the compression struts and wing root fittings.
The ply for skinning the wings comes in sheets of 4ft x 4ft, which means every piece of ply has to be scarfed to each other. It is not possible to just stick one edge of ply to the next as it would not have the required strength. A 'slope' or gradient needs to be sanded on each edge of the ply to spread the load of the joint, this gradient is 1 in 15, and is a skilled job. All ply joints on the aircraft will have these scarfed joints.
When the wings and ailerons were returned from the metalwork department, the topside of the wings were covered in ply. When this ply is stuck to the airframe, it stops all twisting and movement of the wing and makes it rigid, and so the wing has to be very accurately laid out on the workbench and checked for straightness. Again, all the plywood joints will need to be scarfed for strength.
So now we have two wings with all the metal fittings installed and two operable ailerons. This is where we hope we have not forgotten anything being the professionals that we are!
They are a thing of beauty, these wings, and I hope Reginal Mitchell is happy with what we have emulated.
We are about to start covering the wings and ailerons in a material called Ceconite. This is a modern take on the original, which was covered in Medapolin, a lightweight Irish linen, which probably had a life of ten years or less. Ceconite is stronger, lighter, and longer lasting than Medapolin.
It is an absolute privilege to work on this aeroplane and it will be the only flying replica in the world. If you believe, as we do, that this is a worthwhile project we would appreciate your support.
We have a Facebook page and web site where you can follow the build:
If you have any questions or have any information which you think might be of help, we will be glad to hear from you.
Next, the floats!
The Supermarine Seaplane Team
+44 (0)7500 332 060