Aerospace industry moves to carbon fibre wings

The superior strength to weight ratio of carbon fibre reinforced polymers has made the material a popular choice for aerospace applications, as engineers look to make more efficient – and therefore lighter – aircraft structures. But, while the material offers improvements in some key properties, others are not always desirable.

The mechanical properties of composites can be tailored and while many think of it as a very stiff material, it can in fact be made to flex by quite a significant amount. This has become vital in the aviation industry, which have designed advanced wing systems that rely on the ability to flex, quite significantly, to gain aerodynamic performance.
Indeed Airbus, Boeing and Bombardier have all been – or are in the process of – developing carbon fibre wings, and this has led to tests to verify the required strength and flexibility has been achieved.

Test procedure
Testing the strength and flexibility of wings has been an important test for the development and certification of aircraft large and small since almost the beginning of aviation.

However, where sandbags were once placed on the wing to represent the aerodynamic forces likely to induce deflection, the latest generation of carbon fibre wings require significantly more load to satisfy test requirements.

Indeed, when Airbus recently completed its ultimate load wing test for the carbon fibre wing of its advanced Airbus A350 XWB, the forces induced were massive.

The test was carried out on a static test airframe to assess if the A350's wings are able to withstand the 'ultimate load' expected during service, by applying 1.5 times that of what the aircraft would ever likely encounter during operations.

During the test the composite wings displayed a truly awesome amount of flex, notably more than predecessor wings made primarily of aluminium alloys.

The custom made static test rig used hydraulic jacks and loading lines to push and pull the wings upwards until it reached 1.5 times its expected maximum in-service load. This saw the wingtips deflect by more than 5m. Notably, however, Airbus took the decision not to test the wings to failure.

The strains induced into the wings and the airframe were measured and monitored in real-time using more than 10,000 measurement channels.

The wings were fitted with thousands of strain gauges and other instruments to record and analyse the performance of the composite material.

The huge volume of data recorded was then analysed and correlated against computer models and simulation analysis of the structure that have been used as the basis for the design of the airframe and wings to date.

Certification of the A350 is scheduled for the third quarter of this year ahead of delivery and entry in to service by the fourth quarter.

This has followed in the footsteps of Boeing, which has been the first to market with an aircraft that uses an advanced carbon fibre composite airframe and wings, with its 787 Dreamliner.
Boeing too completed one of the more spectacular tests during the 787's development, also completing a wing bending test.

During the test, the wings were flexed even further than the A350, to more than 7.5m, which also equated to 1.5 times the most extreme load the aircraft wings were ever likely to encounter.

Each second of the two hour plus test saw thousands of data points collecting information about the carbon fibre material that makes up the structure of the wing. And, like Airbus, Boeing also opted not to take the wing to failure during this test.

However, in 2008 Boeing carried out destructive testing of an isolated 15m section of a 787 wing that resulted in catastrophic fracture. The wing box, pictured right, is a cantilevered beam that carries the wings to the fuselage and supports the leading- and trailing-edge devices, control surfaces, engines and landing gear. The upper and lower surface panels and the spars of the wing are made entirely of the same composite material being used on the fuselage.

Mark Jenks, vice president of 787 development at Boeing, says: "Successful completion of the wing box destruction test marked a major step forward in highlighting the innovation on the 787, in addition to determining the strength of the structure, the test helped us verify the analytical methods we have used to calculate the loads the structure will have to carry."

And, the most recent OEM to go through the testing process is Bombardier Aerospace, which is producing composite wings in its Northern Ireland facility for its forthcoming CSeries. It recently teamed up with control system specialists Moog to produce a test rig able to undertake a full scale structural test on its advanced carbon fibre wing.

The test rig is to be used to carry out fatigue testing of the composite structure and will go on to form part of the CSeries certification test programme. The system uses an Active Load Abort System that relies on a Moog Machine Controller. This independent multi-channel safety system will be able to synchronously unload the structure over a common time period, while maintaining the desired load distribution, in the event of a primary loading system failure.

The system is needed to protect the wing during the test by preventing undesirable and unrepresentative loads being applied. The test system is currently being commissioned at Bombardier's facilities in Belfast, prior to the test getting under way later this year.

Justin Cunningham

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