Aircraft are expected to undergo substantial technological changes over the next decades. Stringent fuel economy and CO2 emission standards are driving forces accelerating advanced engine and material designs. Counter-rotating open rotor (CROR) architectures, also known as propfans, are a promising path being explored to power next-generation commercial jets, as they could improve fuel consumption by 20-30 %. To achieve safe integration, the aircraft structure should tolerate certain failures from open-rotor engines. Against this backdrop, the EU-funded ELEMENT project focused on practical solutions and modelling activities to mitigate the risks of uncontained engine failures. “High-energy debris could impact the fuselage, causing large structural damage that could compromise aircraft and passengers’ safety,” notes project coordinator Jorge López-Puente.
Virtual models replicating real test conditions
Researchers validated the maturity level of different shielding configurations that help minimise the penalty weight and protect aircraft from different engine failures. Both physical and virtual impact tests of debris exiting the engine case at high speeds were performed on simple panels and full-scale representative aircraft structures. “Virtual tests could decrease the number and cost of experimental tests and ideally limit them to the exact number needed for aircraft certification. Computer simulations are currently widely used in static but not in dynamic conditions,” explains López-Puente. Virtual tests are key to more flexible product designs and could speed up the certification process of structures, while simultaneously meeting safety standards. Researchers put a great deal of effort into preparing the experimental set-up for coupon-level testing. A carbon fibre-reinforced polymer was used to produce the prism fragment representing the blades, whereas a steel sphere represented the metallic fragments. Different rig-setups and materials were developed and tested to integrate the rigid and flexible shielding structures. In total, the team conducted more than 300 test coupons using a pneumatic launcher. High-speed cameras recorded the absorbed energies and the debris’ residual velocities in the shielding structures.
Physical test impact analysis on aircraft structures
A first, researchers carried out physical experimental tests truly representative of the virtual models to assess the impact of uncontained engine debris. Importantly, these tests did not require engine destruction. “Large specimens representative of real aircraft structures allow significant reduction in experimental costs and increase the repeatability of our testing procedures which is necessary for validating integrated solutions. Ultimately, these developments could help establish standards for more affordable certification test campaigns,” he notes. “Experimental tests are usually performed on coupons of reduced dimensions to save costs. Studies that involve impacted structures representative of real-scale aircraft are either confidential or non-existent,” adds López-Puente. What’s more, current studies typically focus on metallic fragments that do not appreciably deform once they strike the aircraft or soft bodies such as birds that completely fracture once they collide with the fuselage. Studies involving fragments or fuselage sections made of carbon fibre-reinforced polymers are scarce. Their mechanical behaviour could deviate from those reported in the literature for other materials. Project experimental and theoretical results could greatly facilitate CROR engine integration into future aircraft. Potential benefits are not limited to the realm of aircraft but could also extend to other fields such as high-speed rail, focusing on how track ballasts affect the train structure.
ELEMENT, debris, shielding structure, uncontained engine failure, counter-rotating open rotor, virtual test, carbon fibre-reinforced polymer