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French facilities test flight safety and security

Just as a car manufacturer needs to test structural damage through impact, scientists at Onera in France are combining experimentation and digital simulation to refine their knowledge of structural damage and failure mechanisms in aeronautics. But don’t expect to see any crash test dummies!

Tags: Air, Galileo
It’s a long way up…and down -  crash test tower at Onera, France

The Office National d’Études et de Recherches Aérospatiales (Onera) has developed its own version of a crash testing runway in the form of a vertical crash tower that can accurately and scientifically test intermediate structures or structural subcomponents. Combined with its B20 catapult bench – a big-scale free flight laboratory – research on aircraft modelling and flight mechanics can take place at limited cost and in a controlled environment.

“[ONERA provides] industrialists with data and models for improving transport performance and safety while lowering manufacturing and operational cost,” according to Actuscience, a monthly magazine of French science and technology news.

The B20 free-flight laboratory, which replaces the B5 and B10 labs, started operation in late 2002. The B20 lab is part of the European Awiator technology platform, an EU-funded project which stands for Aircraft Wing with Advanced Technology Operation. It catapults instrumented models in free flight to a distance of up to 50 metres, during which time the models may be subjected to real-life interference, such as wind bursts during flight. This lab has proven to be one of the milestones in pan-European research into turbulence and wake vortices, playing a pivotal role in pan-European research into turbulence systems.

Springing to action

The crash tower’s design is relatively simple. It is supported by four concrete pillars which are around 15 metres high and reinforced by wire netting. Heavy weights are dropped from the tower and guided down a complicated system of ‘guide-rails’ which are mounted along the pillars through a series of engine-driven horizontal jacks. Just like the legend that Galileo tested his new theory of projectile motion by throwing weights from the top of the Leaning Tower of Pisa, Onera is scientifically testing out theories from a height.

Each time a different weight or structure is tested, the track gauge running along the tower can be adjusted to the different sized trolleys carrying the weights, which are specified according to the energy required to crush the tested structure – i.e. various aircraft components, landing gear, passenger equipment, etc.

The crash area is separate from the rest of the laboratory and tower. Actuscience describe its function: “The tested structure is set up on an 80 tonne seismic weight resting on shock absorbers. The system filters any mechanical waves from the outside environment. The crash tower was designed to drop weights of up to one tonne in free fall. The dropped weight can also be propelled by using elastic springs for initial speed, hence increasing the available impact energy. Ground impact occurs at a speed to the order of 20 metres per second, i.e. the equivalent of available impact energy of roughly 100,000 joules. Cells installed on a dynamometer table are used to measure the reaction stress applied to the structure. Other sensors are also used: strain gages, speed indicators, contactless optical motion sensors and fast cinematography equipment for analysing 3D distortions.”

Validating flight 

The B20 catapult system and lab has other functions as well. In addition to studying flight mechanics and models, it also demonstrates and validates the performance of new designs, such as advanced wing technology (in the Awiator platform) and new flight control design and laws, as well as for developing methods to more accurately measure aerodynamics.

This unique catapult launch technique helps study flight conditions – such as vertical or lateral gusts of wind and flight control steering – and other factors which are difficult to duplicate in a wind tunnel, such as remote wakes, flight through vortex and landing. “For the development of a big-capacity transport plane, the study of remote wakes is extremely relevant. The experimental method offers the added feature of enabling the observation and analysis of the wakes emitted in the ‘terrestrial referential’, unlike wind-tunnel experiments. The access to a remote field is possible here, the major advantage of this facility compared to wind tunnels where observations are restricted to a few spans,” notes Actuscience.

B20’s bench height is a real advantage, continues Actuscience, “[allowing for] the down speed of the vortex trail, the observation may potentially cover a distance of roughly 200 spans downstream before the effect of the ground has an impact on wake dynamics. This distance is the equivalent of 8.5 nautical miles behind an A380, hence fully covering the current separation distances between planes readying for landing”.

A turbulent experience

Simulating flight under turbulent conditions is near impossible using the traditional wind-tunnel testing approach. However, B20 can be adapted to simulate winds and gusts of varying profile and intensity in the flight space, making it an ideal modelling tool for real-life conditions.

Onera says it is equally well adapted to the study of ground impact and plane behaviour during landing. The B20 lab takes into account the relationship between the way an aircraft moves and its aerodynamics which can be affected by ground factors. With this technique, researchers are able to accurately simulate the ‘dynamic effects’ from the angle of the landing trajectory.

Source: Onera, Actuscience