Forties star returned to the skies... with a little TWI magic

Case Study

Two Cambridgeshire neighbours, the Aircraft Restoration Company and TWI, joined forces recently in a combined effort to breathe new life into a second world war Spitfire - with a damaged canopy frame.

Now owned by Rolls Royce the aircraft is the subject of a major re-fit at ARC's headquarters in the grounds of the Imperial War Museum at Duxford. The   manufacturer was Supermarine Aviation and it was built in 1944 at the company’s Castle Bromwich base, after which it served one year of active service.

The component earmarked for TWI’s attention was a badly cracked hoop-shaped magnesium casting designed to support the cockpit glass. It was repaired using a welding process which had not been invented when the aircraft was manufactured, namely alternating current tungsten inert gas using an AZ92 magnesium 2.4mm diameter electrode consumable.

‘At first we thought it was aluminium’ says ARC’s Roger King. ‘But from my background in motor racing I reckoned it was magnesium. At the same time as we handed over the problem to TWI an early drawing was found confirming that this material was magnesium. Wartime materials specifications were so woolly, but TWI was able to identify an appropriate modern day consumable for the repair.’

The significance and importance of the canopy frame’s integrity and airtightness was soon to unfold. 

Unlike most of the marque this Spitfire was unarmed and featured a pressurised cabin and an appropriately modified rear bulkhead. The reason being, it was designed specifically for photo reconnaissance above 40000 feet. As Roger King explains “The design philosophy was simply; if you go high enough, and fast enough, they won’t catch you.”

The aircraft’s construction is largely aluminium, steel and rubber. Titanium was in its  infancy in 1940s aviation manufacture and is absent from the Spitfire’s design. This example, a Mk XIX, is powered by a Rolls Royce Griffon, a 36 litre V12 engine, somewhat bigger than its 27 litre cousin, the Merlin.

The Spitfire is one of the very early aircraft to embrace stressed skin design and is of largely riveted construction. Welding is limited to minor parts with little or no structural role.  

‘There were specialist photo reconnaissance versions in several marks of Spitfire’ explains King. ‘This one has an operational ceiling of around 42 000 feet and a top speed of about 450 mph.’

Interestingly the cockpit is not structurally enhanced to accommodate pressurisation. There is also a hole in the aircraft belly where a very large format camera would have been installed. Some Spitfires also had side pointing cameras. Sometimes the aircraft would be flown at very low level through sensitive enemy installations and pictures would be taken horizontally or while banking. The Spitfire also features one of the earliest engineered applications of composites, namely Tufnol, a cotton matrix with a phenolic resin binder which makes a very stiff material.

The Spitfire last flew some months ago when it arrived at Duxford. It is hoped to be completed during the 2012 display season.