High Fidelity Crash Analysis of Ngctr-TD Composite Wing

The T-WING project is a research project aimed at designing, manufacturing, qualifying and testing the new wing of Leonardo Next Generation Civil Tilt-Rotor technical demonstrator (NGCTR-TD), as part of Clean Aviation Fast Rotorcraft activities. The methodology proposed in this paper encompasses the development of high-fidelity modelling and simulation procedures in support to virtual certification methods for crashworthiness requirements of tiltrotors. Finite Element Analysis (FEA) of an aircraft drop test is a complex and detailed process that aims to simulate the structural behaviour during an impact or drop event. This type of analysis is critical for assessing the safety of an aircraft in emergency landing situations. Wing crashworthiness requirement is specific for tilt rotors: during a survivable crash event, the wing design must ensure a pre-defined rupture with the purpose of alleviating the inertial load acting on the fuselage to preserve the occupants from injuries and fire, guaranteeing the escape paths. Thus, the highly integrated T-WING wing box concept has been designed with the specific feature of frangible sections near the wing-fuselage intersection. The activation of fracture of the external semi-wings in correspondence of frangible section is triggered by the achievement of a well-defined crash vertical load factor. The objective of the methodology is to simulate the crash effects on the whole wing, using explicit non-linear and time-dependent FE analysis, to verify the wing spanwise placement of the frangible sections, the failure mode, the loads acting at the fuselage links, and the acceleration transmitted to the structure. This work is focused on a standalone analysis of the wing plus a lumped scheme of the fuselage, and it is part of a wider activity which will comprise, in the crash simulation, the most relevant vehicle systems (e.g. fuselage model). Moreover, this numerical activity has been compared with experimental results obtained on a different but similar structure, in terms of global acceleration at wing centre of gravity. Good agreement in terms of acceleration has been found between numerical model and experimental relevant test.


Introduction
T-WING consortium has designed and manufactured the wing of the Leonardo Next Generation Civil Tiltrotor Technology Demonstrator (NGCTR-TD), shown in Figure 1, to take it to TRL6 through experimental flight ( [1], [2], [3], [4]).The final aim of the project is to qualify for flight the wing structure, shown in Figure 2, and the moveable surfaces.The development encompassed design, manufacturing and ground tests up to flight, in compliance with the technical specifications set by the Leonardo and Airworthiness rules ( [5], [6], [7], [8]).One of the hot topics in VTOL aircraft is crashworthiness (or impact resistance), that is, the ability of a structure to protect occupants during an impact.This topic, which is already critical in conventional aircraft ( [9], [10], [11]) is particularly complex in the case of VTOL aircraft, for two main reasons: 1. crash impacts are more significant in terms of potential damage compared to conventional aircraft due to hovering maneuver.2. there are no reference airworthiness rules up to date, but only adaptation from helicopter rules to tiltrotors.Regarding the second point, the foundation for a possible certification standard was laid as part of two specific studies by the Federal Aviation Administration (FAA) and NASA ( [14], [15]).These studies refer to a setup of principles to be adopted in design to preserve passengers in terms of loads to which they are undergone, and possible injuries related to the design of the aircraft itself, both pre-and post-crash.The identification of an event to which crashworthiness and, by implication, a passenger injury that could have been prevented and should not have happened, is, therefore, closely related to the concept of "survivability."For this reason, to guarantee the aircraft safety, it is necessary that the sequence of the phenomena is such as to ensure that the occupants of the aircraft survive.
This occurrence depends on three factors: 1. Impact forces transmitted to passengers. 2. Fuselage occupant volume.
The most relevant effect on the human body is caused by the action of acceleration, commonly measured in g's.The accelerations, or decelerations, that can occur in an emergency landing are of high intensity, namely greater than 10g for a time interval of less than 1 second.In such situations, knowledge of the limits of human tolerance to impact is necessary to predict the potential nonfatality of an accident.Human tolerance to impact accelerations is a function of the energy transferred to the body during the crash sequence or the mechanical work done by the impact forces.
The reduction of the fuselage volume also affects survivability; therefore, it must be ensured that this space is not compromised either due to structural collapse of the fuselage structures or intrusion of other aircraft structures placed inside or outside of the fuselage.
Deformed escaping paths comprise the vehicle escape increasing the risk of fire and toxicity.
If accelerations exceed prescribed limit, the fuselage internal space dramatically decrease, and the escape paths are highly deformed, then the survivability is significantly compromised.Therefore, to ensure maximum protection for crew members and passengers, certain structural design features must be adopted in the construction of an aircraft to enable energy absorption (see CS 29/25.561,[12]) The NGCTR-TD's wing is designed to guarantee the integrity of the fuselage and the safety of passengers under the inertial loads transmitted by the wing mass, in case of moderate crash (potentially survivable).A design feature is to control the failure of the wing during a crash to prevent fuselage collapse, as performed on the military application of Bell V-22 ( [13]).

Figure 3 -Frangible Section Concept
The phenomenon to be studied (see Figure 3) is the propagation of internal forces, of whole aircraft structures, due to ground impact reaction and the corresponding inertial forces developed by suspended masses above the fuselage.

FE model
One of NGCTR-TD requirement states that because of a survivable crash event, any cabin occupants must be protected from equipment mounted externally above the cabin.The NGCTR-TD has a high wing configuration, it means that the inertial loads due to wing mass (e.g.structures, internal systems and nacelles masses) directly affect the occupant survivability.The design approach was to trigger a failure of external portion of semi-wings (left and right) in suitable sections to alleviate the forces on the fuselage during the crash event.It is therefore crucial to set up high fidelity models and methodologies to investigate how the wing breaks under such crash conditions.The wing finite element model (FEM) encompasses the wing, the moveable surfaces, the nacelle primary structures and the attachments with the fuselage according to the design by using 3-dimensional elements (shell or solid).The masses of internal systems, fuel, bladders, engines, transmissions, rotors, and any other component hosted by the wing have been simulated by using lumped mass elements.Full fuel mass condition corresponding to 12196 lb (structures plus systems and fuel mass) has been considered.The analysis was performed by using LSDYNA MPP single precision R12.0 software on Cluster HPE Pro-Liant DL560 Gen10 with N°64 parallel processors.Starting from the wing model, a rigid plate that simulates fuselage's lift beams attachments was realized.The wing links are connected to the plate by spherical joints.During the drop test the rigid plate impacts on a deformable honeycomb absorber that simulates fuselages stiffness contribution.It is described in detail in paragraph 3. The boundary conditions are: a) initial velocity of 7 m/s on the entire wing (that is equivalent to a drop height of 3 m) b) the gravity effect.The crash analysis of the wing structure allows to catch the failure section and modes, the wing-fuselage link loads and the vertical acceleration at Center of Gravity and all along the wing.The pre-process steps are: 1. Detailed modelling of the wing, wing-fuselage links and absorber compatibly with LS-Dyna.

Numerical test configuration
The wing drop test data and test configuration have been supplied by Leonardo only as a reference, since the tested wing is similar but different.The test boundary conditions were the following: 3m free falling, impact on a deformable honeycomb structure, that simulates fuselages stiffness contribution, therefore the numerical simulation of the drop test was set on these boundary conditions.The numerical model comprises moveable surfaces, fuel and nacelle masses.

ABSORBER NUMERICAL CHARACTERIZATION
The numerical model of the absorber assembly was characterized upon honeycomb mechanical 3D characteristics, in order to simulate in the wing crash test the whole honeycomb subsystem with a concentrated non-linear spring 1D elements.The actual absorber assembly is made of four different honeycomb blocks stacked two by two and separated by aluminium sheets (figure 6).As shown in figure 7, the honeycomb blocks are simulated with 1D spring and damper elements with an experimental Force-Displacement curve instead of 3D elements to reduce computational time.The graph shown in figure 8 depicts the compression behaviour (force-displacement characteristics) of the springs which simulate the honeycomb assy under a vertical drop case.The behaviour of spring was built based on art.[17]:   The mean value has been found to be in accordance with experimental data.The simulation is able to predict the values of absorber crushing (honeycomb), mean collapsing force, and mean acceleration registered on rigid plate (see figure 8) with wing lumped mass hypothesis.

WING DROP TEST SIMULATION
Once having modelled the honeycomb assy, the subsequent step has been to couple the rigid plate with the detailed FEM of the wing in order infer the frangible sections: position, failure modes, failed components.Results of the analyses show (figure 10) that the wing breaks outboard the wing-fuselage intersection.

SIMULATION OUTPUT DATA
In addition to wing's frangible section detection, the acceleration and the forces acting on the wing during the crash event are reported.In figure 13 wing's global acceleration path is shown compared to experimental data value.In terms of loads acting on the fuselage, the analysis is able to report the time dependent value of wingfuselage links forces (figure 14).These values will be used to assess fuselage strength under crash loads.

CONCLUSIONS
The present work consisted in the setup of the modelling and simulation methods to perform the impact of the NCGTR-TD wing on a deformable structure.The high-fidelity models and analyses were used to predict the structural behaviour of the complex structure during crash.The numerical model has given a good agreement with an experimental wing drop test performed on a similar structure.The next step will consist of integration of the wing in the whole NGCTR-TD FE model to assess the crashworthiness requirements.

Figure 8 -
Figure 8 -Force -displacement characteristicsWith this scheme inputed in the FEM, an analysis was launched by considering the total wing mass lumped in its Center of Gravity and an initial velocity of 7 m/s (corresponding to a drop height of 3 m of the experimental test).

Figure 9
Figure9depicts the time wise value of the acceleration picked at wing Center of Gravity plus its mean value.The mean value has been found to be in accordance with experimental data.The simulation is able to predict the values of absorber crushing (honeycomb), mean collapsing force, and mean acceleration registered on rigid plate (see figure8) with wing lumped mass hypothesis.

Figure 10 -
Figure 10 -Failed wing In figure 11 a focus on the internal wing bay where failure occurs in a sharp fashion and it involves stringers, spars and lower panel.

Figure 12 -
Figure 12 -Top and Bottom view of damage on composite parts

Figure 14 -
Figure 14 -Wing-fuselage links time wise forces