When he was commissioned to create a sculpture for the proposed new Terminal 2 building, the artist Richard Wilson had a simple idea: to capture the motion of a small stunt plane as it cartwheeled through the air. To embody its movement through space and time in a solid object made of timber, steel and aluminium and suspend it high above the concourse.

It was a good starting point. The new building – equivalent in size to Tate Modern’s Turbine Hall and designed to cope with the passage of 20 million travellers a year – is so vast that one could almost imagine a small stunt plane flying through it.

Terminal 2 is now open and running and Slipstream, the completed sculpture, is its ‘wow’ factor, a 76-metre-long solidified flightpath, swooping down from the ceiling in a dramatic twist, looping up in a roll towards departures, its sinuous forms seeming to defy gravity.

Richard Wilson explains the concept:

“… Imagine filling the void of Terminal 2 with clay. Now imagine a small stunt plane cutting through the mass of clay somersaulting, spiraling, twisting, climbing and gliding. Once out the other side, imagine the void left by the plane in the clay is filled with fast setting plaster.

When hard, imagine the hall is excavated of all clay to leave only the plaster form. Imagine what this solid but fluid shape of the movement of the plane through space could look like!”

There were no precedents for a project like this, and to turn this seemingly simple idea into reality required complex calculations and sophisticated engineering by the structural engineering team, Price & Myers Geometrics.

The initial problem was to devise a mathematical method to capture the precise ‘shape in space’ of the complex volume described by the motion of the aircraft. Starting from first principles, the architect, Ralph Parker, part of the Price & Myers team, devised a solution using film animation software to re-create the continuous trajectory of the stunt plane within a BIM model of the building. The animation consisted of 300 frames, like a ‘freeze-frame snapshot’ only in three dimensions. Using a series of scripts to process the large datasets involved, the trace of each vertex in the aircraft over the course of the 300 frames was formed into a spline describing its motion.

The splines were then joined in sequence to create a series of ruled, developable, ribbon surfaces, revealing the motion in its entirety. As Slipstream acquired shape and volume, the designers had to make decisions about its structure, how it would be fabricated and what materials would be used. Its huge scale – 76 metres in length – and complex shapes made it clear that an internal armature was needed to give it structural support.

Steel was the only suitable material for this. The designers had to formulate a three-way design process; to refine the plane’s flight-path, to understand the volumes that this created and to make sure that a structure of adequate strength could fit inside it. It took 48 versions to refine the Slipstream form to accommodate a suitable supporting structure. At the same time, the need for a fast production sequence meant that it had to be prefabricated of panel-based CNC (Computer Numerical Control) cut parts, a ‘flat-pack’ sculpture kit.

Timber – OSB (oriented strand board) panels, ply spars and ply stress skin – were the ideal materials for the main components of the kit; they could be cut into shapes to create the complex three dimensional shapes and they would provide the ideal base on which to fix the final aluminium sheet cladding. Drawing on software used in the aerospace industry, the Price & Myers design team developed the complex volumes into an integrated model of more than 30,000 pieces generated by parametric modeling together with complex custom scripts running many millions of operations.

Slipstream was split into 22 ‘cassettes’ which were manufactured offsite in the factory of Commercial Systems International (CSI) in Hull. The size of each cassette was dictated by what could be transported on a standard low-loader truck. Each of the 22 cassettes was assembled on a production line from individually CNC-labelled interlocking pieces. Each piece of OSB, ply and aluminium had a unique ID which was CNC-routed on to its surface.