Evaluation of Material Alternatives for Rotorcraft Canopies under Drone versus Bird Strike

Kurzfassung

The conference contribution concerns the behaviour of plastics windshields adhesively bonded to rotorcraft canopies under impact. As glazing materials polycarbonate and acrylic glass were considered. The load cases, drone strike versus bird strike, were taken for the evaluation. Motivation for this work is the discussion of bird strike criteria of light vs. large rotorcraft along EASA CS-27 and CS-29, respectively, and the preceded ARAC sponsored by FAA. Furthermore, bird strike databases like the FAA's Wildlife Strike Database indicate increasing bird strike rates in the last one to two decades. Lately, EASA has published a bird strike requirement in CS-27.631 for windshields on 17.12.2021. For the given work, the load case was defined along the bird strike requirements by EASA CS-29 for large VTOL. For the considered canopies a bird weight of 1.0 kg (2.2 lb) at impact velocities between 77.2 m/s (150 kts) and 113.2 m/s (220 kts) was applied. The drone strike load case was established with identical kinetic energy as for bird strike. The applied drone or commercial unmanned aerial system (UAS) was a quadcopter with a weight of approx. 1.2 kg (2.7 lbs). Due to its weight higher than the bird weight, the impact velocity range was reduced here to 69.7 m/s (135.5 kts), and 102.2 m/s (198.8 kts), respectively. The numerical evaluation of the transparency material alternatives was calibrated by coupon and element tests with consideration of strain-rate and temperature dependency as well as the determination of failure strain. The windshield and canopy modelling used thick shells only. The adhesives modelling was based on a cohesive zone modelling which was calibrated by Mode I and III testing data. The applied artificial bird and the simulated bird model were validated against real bird data. Similarly, the UAS results from test and simulation were set into relation with the soft body measurement. All tests used an identical load measurement system or a numerical representation of it. While, the numerical bird was modelled by smoothed particle hydrodynamics (SPH) based using a uniform particle distribution, the UAS applied a thick shell and solid modelling with a calibration based on earlier component impact tests. The key element for the validation of the numerical approach was an instrumented fixture for a flat canopy demonstrator. The design consists of functionally coated aerospace grade polycarbonate which was bonded by a polyurethane adhesive to an aerospace CFRP fabric - epoxy prepreg canopy frame. The instrumented setup allowed a force measurement at each corner of the transparency plus deformation and full-field strain measurement. Damage pattern and possible recommendations were discussed.