A polished Eurofighter Typhoon canopy was completed by Aerospace Composite Technologies.

One of the key design changes in the Typhoon canopy is that transparencies now play more of a structural role in absorbing high aerodynamic loading, said ACT, a GKN Westland Aerospace Company. It added that production canopies will have low-observable coatings applied, using proprietary manufacturing technology and processes developed by ACT, which screen electronic emissions generated in the cockpit. The Eurofighter transparencies have established a "significant change" in the standard methods of design and manufacture of military canopies, utilizing the computer-generated skin data to develop the canopy design, tooling, and inspection criteria without the need to build a single physical master model. The canopy for the single-seat Typhoon is only marginally shorter than the two-seater, at 2.6 m.

The two-seat Eurofighter Typhoon has a 2.7-m long canopy.

GKN Westland Aerospace has released details of the process it uses to manufacture the Typhoon canopies. Billets of modified as-cast acrylic measuring some 2 m square by 50 mm are heated in an oven before being placed in a bi-axial stretching machine. The material is then clamped around its edges and stretched to a finished size of approximately 3 m. The process modifies the material's properties to make it more resilient. Both sides of the resultant acrylic sheet are then ground and polished to obtain an accurate optical finish. The sheet is then inspected for any optical distortion and any internal material inclusions such as voids or fibers. Next, the sheets are prepared for forming, a proprietary process developed by ACT. The prepared sheet is formed into the three-dimensional shape of the Eurofighter canopy using specialized tooling also developed by ACT. Following an initial edge profile, the canopy is inspected for optical clarity using an optical collimator. The equipment uses a number of lasers projected through the canopy onto photo-optic cells that measure its optical properties for both monocular and binocular deviation – typically over 800,000 recorded points across the critical optical zones of the front windshield and rear canopies. The company said that the optical collimator, installed last year, performs this process in hours instead of the days needed when using previous conventional methods.

The single-seat Eurofighter Typhoon features a marginally shorter canopy measuring 2.6 m long.

The canopy is then moved to a 5-axis, twin-table CNC (computer numerically controlled) machine, which performs detailed machining and edge profiling operations. On completion, the canopy is moved to a coordinate measuring machine (CMM) to inspect the machined hole locations and correct edge profile/tongue thickness, ultimately qualifying the product's interchangeability. From there the canopy enters the polishing bay where it is also given a precoating clean. It enters a clean room for the crucial stages in which ACT's proprietary, low-observable coating system is applied. To maintain an ultra-clean, dust-free environment, ACT has created a clean room within the clean room, housing a coating vacuum chamber and other facilities crucial to maintain the canopy's high optical performance. Once coated, the canopy undergoes final inspection.

Stuart Birch

Aerospace Engineering June 2000