Saturday, 12 May 2012

De Havilland Mosquito

This documentary on weapons design reminded me of the real genius of the de Havilland Mosquito. This wasn't its speed, though it could outpace almost every other aircraft of the time. It wasn't the versatility it offered, though it did everything from photo-reconnaissance to night-fighter missions. Nor its manoeuvrability, which allowed it to fly low and fast under the radar and avoid anti-aircraft guns.

The real genius was the consideration of the design and manufacture process together, and it was only because it was designed with these considerations in mind that it was possible at all.

In 1938 the reigning RAF philosophy was for bombers to be heavy and carry gun turrets for protection. De Havilland's suggestion was radically different, make the bomber smaller, lighter and faster. You wouldn't need defensive turrets because you could outrun the enemy, speed would be your shield. This also meant you could make the design much simpler, easy to produce and reduce the number of crew needed for each aircraft.

With the RAF's opinion set against fast bombers, de Havilland needed something else to set the aircraft apart, so it wasn't competing directly with heavy bombers for manufacturing facilities which where very limited and reserved for a small number of aircraft types.

The answer was to build the mosquito out of 'old' technology; wood. This was more plentiful and any woodworker, of which at the time there were many, could build one, not relying on the scarcer supplies of aluminium and metalworkers. Also the work could be copied world wide in other parts of the British Empire without the need for heavy machine tools.

Without turrets, armour plate and multiple crew stations the design was very simple. The body was made out of a sandwich structure, very light balsa wood layer between birch, so strong that apart from a few areas it needed now additional reinforcement. The body was constructed in two halves which could be easily pre-stuffed with equipment and screwed together. This was assembled on a mould, which gave the correct shape and supported the structure during curing.



The wing was equally simply produced, one piece with spars running from end to end, saving structural complexity; so weight and assembly time. The connection between body and wing, simple again. Four bolts connect the wing in a recess in the fuselage.

The results of this simplicity and ease of production spoke for themselves, over seven and a half thousand mosquitos were built.

The latest generation of carbon fibre civil aircraft are using practices seen on the mosquito; and it's not coincidence. Wood, like carbon fibre, is a fibrous composite material. It can be shaped the then glued hard (today done at high temperatures in an autoclave) and encourages you to create large components that are made in one go, glued together at the same time. Unlike metals it can't be poured into a mould or forged.

The same techniques are being used again to meet the challenge of high-rate production for aircraft. Boeing 787s and Airbus A350XWBs currently under construction are being assembled in ways that would be very familiar to a mosquito builder. Large sections with systems pre-stuffed while easily accessible are shipped to the assembly line and the carbon fibre woven over mandrels (much like moulds) with stringers and spars embedded within them, glued and cured together.


While the main move to composites is for it's structural benefits, lower weight and higher stiffness,  I hope that as designs evolve we can be as clever as the mosquito designers. Using the materials we have access to in a way which really plays to their strengths and see the design end to end, not just as the finished product. Otherwise we might end up with more Tiger I tanks; clever but too expensive to be produced in useful numbers.