Planes, brains and panels of steel

It's not surprising that architect Feilden Clegg Bradley took its inspiration for the pavilion from the historic planes rather than the mundane hangar building that is the typical home of historic aircraft.

"We wanted to get away from a metal box," says architect James Risebero of Feilden Clegg Bradley. The design team has certainly done that. The semi-circular building resembles an oversized aircraft fuselage and even has riveted stainless steel panels to mimic the facade of modern aeroplanes. The interior, meanwhile, has a tensile fabric backdrop for the planes. These semi-translucent panels are a visual reference to vintage planes such as the Sopwith Camel, which was created by stretching fabric over a wooden frame.

Suspending the planes from the ceiling was just one of the technical challenges for the project team (see "How's it hanging", page 7). The team also wanted to flood the exhibition space with daylight without degrading some of the most fragile exhibits with UV rays. The specification of the fabric was tricky too. It had to offer 40% translucency but also be resistant to dirt where it was within reach of sticky fingers – fabric meshes required to diffuse light are also ideal for trapping dirt. And the exterior stainless steel panels presented specification challenges as the thin skin can ripple if not handled with care.

The barrel-shaped form of the building offers Feilden Clegg Bradley several advantages, says Risebero. "It's very simple and easy to read and an efficient way of enclosing space. It offered the maximum amount of space for the money we had. It's also a very stable structure from which to hang planes," he says. The structure of the pavilion comprises 13 steel arch trusses at 6 m centres, which sit atop 4 m high concrete fins. The barrel vault has an internal height of 14 m and a span of 26 m. The pavilion also houses mezzanine boxes and walkways, which provide smaller exhibition and audiovisual spaces as well as a cafe and toilets.

A central rooflight fitted with polycarbonate is one of the main sources of light in the pavilion. The others are the glazed ends of the pavilion and a low strip of glazing running along the west elevation. The rooflight has a UV-protective coating but the architect was still keen to avoid sunlight falling directly onto the fragile exhibits below. Feilden Clegg Bradley came up with a solution incorporated in a galvanised steel maintenance walkway. It has incorporated blades into the steel mesh walkway, which are angled to prevent direct sunlight from ever reaching the exhibits below.

Feilden Clegg Bradley relied on the specialist knowledge of structural fabric company Base Structures to specify the tensile fabric. The company took the architects' initial drawings and calculated the geometry required to realise the architect's designs. It requires a specialist like Base to incorporate double curvature geometry into the fabric – this is essential as it offers the fabric strength and ensures rigidity.

The tensile panels are hung between the columns and are fixed from steelwork at the apex of the arch to a horizontal cable that is fixed to the bottom end of the fabric. The cable can be tensioned once in position to reach the desired level of tautness in the fabric.

The architect wanted to specify one fabric for the interior but Base pointed out during the design process that a different finish would be required where the fabric is within reach of visitors. "The architect wanted a mesh fabric over the bulk of the interior, but we were concerned about the first floor walkway," says Mark Smith, director at Base. "If people touch it with sticky fingers it will make it uncleanable." The solution was to add a lacquer to the PVC-coated polyester fabric. The polyvinylidene flouride lacquer fills the holes in the mesh, and gives the fabric durability as well as making it easy to clean.

This seemingly simple process had a knock-on effect on the specification. The lacquered fabric stresses more than uncoated PVC fibre glass fabric so Base had to take this into account when cutting the panels. Welding the lacquered panel to the unlacquered panel is also tricky. Before the edges can be welded using microwaves, the lacquer must be ground off. Base is one of the few UK companies that can offer this service.

Between the fabric and the stainless steel panels are floodlights. These are hidden from view by the fabric but throw a soft diffused light onto the exhibits. There is also a sprinkler system in this space and above that is the built up roof, which provides the roof's waterproofing. This consists of insulation sandwiched between two metal decks. The decks rest on purlins suspended between the arch trusses.

Above the deck are spacer bars, a stiffening sheet and finally the stainless steel metal panels. As these panels only act as a rainscreen it means that the drainage is concealed and the building has a smooth, curved surface – like a plane. To keep the shape free of clutter, Feilden Clegg Bradley used "gargoyles" to throw rainwater clear of the facade into a gravel filled drainage system on the ground.

The installation of the panels was tricky. The architect specified a 1 mm thick stainless steel panel, which the main contractor reduced to 0.6 mm to save money. This meant that the installer had to be careful not to dimple the steel when riveting it in place (see "The secrets of stainless steel", below). To prevent excessive glare from the stainless steel, the architect specified a textured finish. This not only diffuses the reflection but also imbues the panel with more strength.

The finished building is a splendid contrast to the older sheds at Hendon, and Feilden Clegg Bradley have obviously enjoyed the exercise. It has been chosen to design another RAF museum at Cosford. This is a much bigger project, and will feature one of the biggest tensile structures in the country (see "Cold war at Cosford", below).

Following the successful collaboration at RAF Hendon, Feilden Clegg and Bradley will once again be working with Base Structures to realise the design and installation of the fabric panels. The completion of the next production from those magnificent men and their welding machines is expected in 2006.