New tire design for Concorde

Michelin is developing the NZG (Near Zero Growth) tire to meet stringent new safety criteria necessary for use on the Concorde.

An aircraft tire being developed by Michelin meets stringent new safety criteria necessary for operating the Anglo-French designed and built Concorde, said the company. The tire, called NZG (Near Zero Growth), has been designed to withstand low- and high-speed foreign-object damage and very high stress levels. The Mach 2 Concorde airliner fleet was withdrawn from service in July 2000 after an accident last year when one crashed shortly after takeoff from Paris. Since the accident, there has been considerable concern about the possible effects of tire disintegration caused by the impact of foreign objects.

Michelin has been developing the new NZG tire since 1999. After the Concorde crash, the European Aeronautic Defence and Space Co. (EADS—formed by the merger of Aerospatiale Matra, CASA, and DaimlerChrysler Aerospace) contacted tire manufacturers across the world inquiring about new techniques that would result in tires with improved foreign-object damage (FOD) resistance. Michelin responded with details of its NZG technology. In January this year, EADS witnessed tests and as a result, it asked Michelin to develop its new tire technology for the Concorde. The NZG tire is said to be more damage resistant and lighter than conventional tires. At a media briefing shortly before the Paris Air Show, Michelin publicly announced the existence of NZG and its potential for contributing to the Concorde's possible return to service. Michelin has not released full details of NZG but said in Paris: "The NZG technology proposed for the Concorde's main wheel tires is based on the use of new 'high modulus' materials that reduce structural deformation. By limiting carcass growth when the tire is inflated (less than 3% compared to 6% for a nylon-reinforced radial tire), the tread rubber works under less tension and is much less vulnerable to shocks and damage. Moreover, as the structure is more abrasion-resistant, the tire has a longer service life." Compared to a Michelin bias-ply tire designed for the Concorde, the NZG has about half the number of layers, which brings a weight reduction of about 20%.

The NZG has been designed to withstand low- and high-speed foreign-object damage and very high stress levels.

A key requirement for the Concorde's new tires concerns the mass of tire debris projected in the event of tire destruction. To demonstrate NZG's ability to meet criteria, a "wide range" of laboratory tests were completed. These included being subjected to deliberate damage and endurance under extreme conditions. This work was carried out at Michelin's Test Center and at the Centre d'Essair Aeronautique (CEAT).

The first Michelin NZG tire was sent to Dunlop Aviation Braking Systems (makers of the Concorde's wheels and carbon brakes) to verify the tire fitted perfectly onto the main wheel and to perform the necessary tests for approval of its use on the wheel. An acceleration/maximum energy-braking test was performed on a CEAT dynamometer to prove that the Michelin NZG tire was compatible with the carbon brake.

By January this year, major test work began at Michelin's Test Center in Spain. The tire was required to run over a 30-mm steel blade without deflating or projecting debris. The tire, inflated to 1600 kPa, was mounted on a weighted trailer exerting a load of 22.9 tonnes per tire, a mass corresponding to that of the Concorde.

The Michelin bias tire construction consists of six bead wires, 18 casing plies, two crown protection plies at 45°, and a tread pattern with five to six grooves.

A bias tire was used for comparative test results. At 10 km/h, the tread and several structural layers of the bias tire were slashed, but it remained inflated and no debris was projected. However, a similar test at 20 km/h saw the tire burst with projection of debris. When the NZG underwent the 20 km/h test, only the tread was slashed. Further testing followed at Michelin's test rig at Clermont-Ferrand. The NZG tire then went to CEAT for the specific qualification test (TSS 5.3) for the Concorde application, which involved a series of takeoff and landing cycles under wide-ranging conditions of load and pressure together with tests simulating particular conditions, including doubling of load, rejected or completed takeoff, and takeoff with under-inflated tires.

In March, a blade test was carried out at the high speeds defined in the specification (324 and 382 km/h). Again, it was a comparative test. The bias tire burst while the NZG tire "remained fully functional."

The new NZG tire construction is made up of two bead wires, four casing plies, seven crown plies, one crown protection ply, and a tread pattern with four grooves.

Next came a test to measure residual strength after damage, the damaged tire to be subjected to three complete cycles (taxi, takeoff, landing, taxi): the first two under normal conditions, the third under sustained load conditions. A bias tire that underwent the 10-km/h test was used. It remained inflated, but was too damaged to be tested in the three-cycle test, said Michelin. The NZG radial tire damaged at 20, 324, and 384 km/h completed the three cycles without pressure loss or debris projection. The external appearance of the tire was not modified by the test. The tire damaged at 20 km/h lost its tread at the end of the second cycle without pressure loss.

Testing also included running a tire over a cylindrical rod of 19-mm diameter with a height of 30 mm. The damage test involving the rod was performed at only 20 km/h, but proved to be "the most severe condition," according to the company. Only new and worn NZG tires were tested. In both cases, the NZG tire ran over the obstacle without pressure loss or projection of debris. The three residual strength assessment cycles were then completed.

Michelin said that the largest piece of tire debris shed in tests was 1 kg, with nothing else heavier than 0.4 kg. According to the company, the new tire can withstand being operated with 40% under-inflation at the Concorde's takeoff speed of more than 400 km/h.

- Stuart Birch