Bird strike analysis of new composite inlet for tilt rotor aircraft

Bird strikes are an important phenomenon to consider when designing and servicing aircraft structures. Most major bird strike incidents result in aircraft propulsion damage. Because an engine is the sole thrust-providing system of an aircraft, the effect of bird strikes on engine inlets and systems must be investigated and mitigated to the maximum extent. Especially in the case of (vertical take-off and landing) VTOL aircraft, such as an aircraft with tilting rotors, this effect is critical from the point of view of the operation, from the point of view of flight mechanics and the overall control of the aircraft. This work aims to propose the proof of resistance of a new composite air inlet for a new tilting rotor aircraft, which is experimentally verified and supported by numerical simulations performed on flat and simple curved test panels. The new, very effective method was used to calibrate the composite material model, which is further used in the following numerical simulations.


Introduction
Foreign object damage (FOD) such as birds, hail, debris etc. are important phenomena that must be taken into consideration when designing aircraft.The critical parts of aircraft or helicopters are the windshield, nose, leading edges, rotor blades, fan blades, engine cowlings and inlets [1].The windshield is mainly important for the safety of the crew and the control of the aircraft.The nose of civil aircraft usually consists of a sub-assembly with a cover, under which the meteorological radar is placed.This is essential for safe passage through areas with storm clouds and water precipitation.The proper function and intact integrity of the leading edge are crucial to flight performance as transmit most of the the resulting aerodynamic lift forces.The moving and static parts of the engines ensure the propulsion of the aircraft.Engine failure has a major influence on the manoeuvrability and climbability of the aircraft because aerodynamic lift forces increase with the square of speed.Airworthiness certification regulations require that all forward-facing aircraft components should be proven to withstand bird strikes to a certain level before they can be employed in an aircraft [2].A bird impact test provides a direct method for determining bird strike resistance, however, the design of aircraft structures typically involves many iterations from design to manufacturing to testing and back, requiring that many bird impact tests be conducted especially in composite structure design.These empirical verifications, which cause damage to expensive prototypes or part of structure [3], [4], [5] and the biological hazard of using real birds, can be costly and time-consuming.Furthermore, experimental data from these tests are often narrowly focused, constituting a barrier to their direct use in refining structural design.Owing to these shortcomings, several numerical methods based on CEL (Coupled Eulerian-Lagrangian) [6] or SPH (Smoothed Particle Hydrodynamics) [7], [8], [9] have been developed.They give the option to simulate bird strikes to reduce the number of intermediate tests required and subsequently shorten the duration of the component design phase.Recall that many of these bird models are verified only on isotropic material or impact on the rigid wall [10], [11].The principal objective of the present work is to provide a more universal experimental procedure, applicable e.g., to other types of materials or production technologies.Validated numerical simulation will allow the final prototype design, and its manufacturing, to be optimal and satisfy the certification requirements [19].The testing program was established to assist in the selection of composite material and optimized lay-up from the point of view of energy absorption from bird impacts on the composite inlet for a new generation of tilt-rotor aircraft.The flat and simple curved test specimens, which were used in the high-speed impact resistance verification tests, were designed also to provide confirmation of the performance of the selected composite material and to assist in the finite element modelling of the global structure [13].Figure 1 shows the proposed building block diagram (BBD) for demonstrating the durability of a VTOL aircraft composite air inlet.

Figure 1. Building block diagram (BBD) for demonstrating the durability of a VTOL aircraft composite air inlet
The flat and curved panels shape was derived from the preliminary design of the final part, bird mass and impact speed range.The flat test samples represented the monolithic and sandwich parts of the real structure (Flat region, figure 2).The simple curved test specimens represented a monolithic real geometry complex structure design (Curved region, figure 2).

Figure 2. Analysis of bird strike on the critical parts of the inlet
for the design of replacement test panels.

Curved region
Flat region

Numerical simulation
A bird strike is a high-velocity impact in which materials with a big difference in properties (a bird is a soft impacting material compared to the material of the aircraft structure), contact each other.They result in nonlinear material behaviour, high strain rates, and large deformations.Finite Element (FE) software such as ABAQUS can predict the loads and deformations of both the bird projectile and the complex aircraft component being impacted, within acceptable levels of accuracy.During highvelocity impacts, bird tissue behaves like a fluid [3].

FE model of bird projectile
The material of the projectile simulating the replacement of a real bird can be described by the socalled "elastic-plastic-hydrodynamic" material model.The hydrodynamic material model is defined in ABAQUS FE software [12] by a tabulated equation of state using Hugoniot curves for water-like homogenized bird materials [13,14].The projectile nodes representing the bird from the ABAQUS finite element (FE) model were charged with an initial velocity, and a combination of tensile failure and shear failure criteria was used.The geometry of the projectile (bird) was idealized as a 60 mm long cylinder with two hemispheric ends.The projectile form used in the simulation reflects the general shape of the bird used in the physical tests.The geometry of the bird model was meshed by 10,770, C3D8R 8-node linear brick elements with conversion to SPH elements (Smoothed Particle Hydrodynamics) [10].The density of the bird material in the FE model for the defined volume was established to reflect the weight of the birds used in the physical tests.

FE model of test specimens
The fabric material model is based on the assumption, that the resulting mechanical properties are determined by the principle of superposition of warp and weft layers.The response of the fabric material in terms of elasticity and strength is the result of the optimization process described in [13].
Layup contains n layers in terms of fabric, while the FE model has 2n layers.Each layer corresponding to the warp orientation is supplemented by a layer of weft turned 90 degrees counterclockwise [14], this is shown in the figure 3  Warp and weft elastic properties may be calculated by using Mori-Tanaka method [15,16] or may be obtained as the result of the Abaqus Micromechanics-plugin, which is using representative volume element (RVE) method [21,22].Both methods result in the same values.Mechanical properties of the components and composite are pointed in table 1 and table 2. The novelty consists in the application of the assumption of the superposition of the warp and weft properties and its implementation in the optimization chain, which was tested in the past only for unidirectional composites [13].Table 3 shows a match of the trained ABAQUS FE model to the real material and its deviance from the technical datasheet of the fabric-reinforced composite.

Experimental verification
The physical bird impact tests were performed at the Czech Aerospace Research Centre (VZLU) according to airworthiness requirements [2].In the experimental verification, real bird projectiles (freshly killed chicken) were used according to the rules of airworthiness [20].The weight of the bird projectile was 1 kg [2].The projectile was accelerated by compressed air through the smooth borehole of a gun barrel up to the required velocity according to specifications.Figure 4 shows the air gun test facilities.Test-rig attachments for the bird-strike test of flat and simple curved specimens are shown in figure 5 and figure 6.The reaction forces during the bird strike of curved panels were measured by load cells (see figure 8).Three load cells of KISTLER 9105A type were used in the test rig.The signal from load cells was stored by using BMC Messsysteme GmbH data acquisition system completed with Vishay 2230 type amplifier.The measurement frequency was 10 kHz with 50,000 sample/s for load cells.
A technique of dynamic displacement measurement by using a laser triangulation sensor MICRO-EPSILON optoNCDT during the impact process was used for quantitative analysis and more precise verification of the data obtained from the numerical model (figure 7) [13].The measurement range of the laser sensors was 300 mm (centre of test specimens) and 40 mm (test rig frame).The actual bird impact point was then determined via the high-speed camera record and visual inspection.The measurement data acquisition rate is adjustable over a range from 1.5 to 49.1 kHz and was set at 20 kHz during testing.The resolution of the sensor is 0.3 m.

Comparison between test a simulation
All tested specimens were manufactured from the same composite material reinforced by fabric with layup [0°, 45°, 90°, -45°]S.The same method of FE mesh creation, considering anisotropic elasticity, strength and Hashin-Damage evolution [18] was used.The shell elements (S4R, S3R) [12] 7th with the reduced formulation were used.Impact simulations were performed by using the ABAQUS explicit solver.

Flat test specimen
The model of the flat panel specimen is shown figure 9 a).The composite plate was fixed in the clamp and hit by the impactor at an angle of 42°.The displacement was measured at the checkpoint "C", which is shown in the figure 9 b).The reference point is considered to be located at the centre and the bottom side of the composite panel.The results comparison between the experiment and calculation from the trained numerical model is shown in table 4.  10 were fixed and directly hit, while the force was calculated.

Conclusions
It can be seen that a more complex load condition leads to a slightly higher error in the simulation results, which is in order of percent units.Basic technological tests, such as tension and bending, based on which the model was trained, cannot, for example, affect the impact of the sharp bones of the real bird.The application of measuring systems for high-speed loading, such as laser distance meters or force meters for measuring reaction forces, will enable quantitative assessment and verification of numerical models.The obtained results from the experimental measurement and the subsequent NDT analysis will enable the refinement of the material model for simulations on the real structure.

Figure 3 .
Figure 3. Model of the fabric

Figure 4 .
Figure 4. Air gun VZLÚ for high-speed impact tests

Figure 5 .
Figure 5. VZLU test-rig attachment for the bird-strike tests of flat test specimens.

Figure 6 .
Figure 6.VZLU test-rig attachment for the bird-strike tests of curved test specimens.

Figure 7 .
Figure 7. Displacement measurement during the bird strike test.

Figure 8 .
Figure 8. Measurement of reaction force during bird strike on curved panels by load cells.

Figure 9 .
Figure 9. Impact on the flat FE panel

Table 2 .
Strength properties of the components

Table 3 .
Deviance of the fabric model parameters from the datasheet values

Table 1 .
Elastic properties of the components 7th International Conference of Engineering Against Failure Journal of Physics: Conference Series 2692 (2024) 012051

Table 4 .
Magnitude of Displacement -Maximum

Table 5 .
The results of the magnitude of force compared with the experiment are shown in table5.Magnitude of reaction force -Maximum 7