ACCURATE BIRD STRIKE SIMULATION METHODOLOGY FOR BA609 TILTROTOR

Cheng-Ho Tho Michael R. Smith ctho@bellhelicopter.textron.com mrsmith@bellhelicopter.textron.com Senior Engineering Specialist Chief, Structural Dynamics

Bell Helicopter Textron Inc., Fort Worth, Texas

ABSTRACT

Bird strike incidents are not uncommon and cause significant flight safety threats to aircraft safety. An aircraft must show compliance with continued safe flight and landing requirements following specified types of high-energy bird impact. The higher impact velocities for bird strikes and attendant potential increased structural weight that tiltrotor aircraft must survive underscores the need to develop more capable, validated analysis techniques. This paper presents the state-of-the-art bird strike simulation methodology developed at Bell Helicopter Textron Inc., based on the multi-material Arbitrary Lagrangian-Eulerian and Smooth Particle Hydrodynamics techniques. The constitutive parameters of the bird model are calibrated to correlate the hydrodynamic pressure using a benchmark problem with available test data. The validated bird models are sub-sequently applied to simulate the bird impact with the BA609 tiltrotor structures for the most load-critical test conditions. The analytical models are validated for multiple test cases with different design architectures, including the rotor spinner con-trols, cockpit nose, wing leading edge, and empennage. It is demonstrated that the developed analytical tool is capable of accurately predicting structural failure modes and deformation for aircraft subjected to the high-energy bird strike impacts.

1

NOMENCLATURE

& strain rate ij'& deviatoric strain rate dynamic viscosity density

0 initial density

nfail normal stress at failure ys static yield stress y dynamic yield stress fail shear stress at failure C reference effective strain rate 0C ~ 6C coefficients of polynomial EOS D diameter of the soft gelatin projectile E internal energy L length of the soft gelatin projectile P strain rate material parameter V impact velocity of the soft gelatin projectile

INTRODUCTION

Aeronautical structures always fly with the risk of impacting foreign objects such as birds, ice, runway debris, rubber, etc. Bird strike incidents, like the one shown in Fig. 1, are not uncommon and cause significant flight safety threats to flying aircraft. According to the Federal Aviation Administrations (FAA) National Wildlife Strike Database, threats to aviation safety due to wildlife impacts upon civil

Presented at the American Helicopter Society 64th Annual Forum, Montral, Canada, April 29 May 1, 2008. Copy-right 2008 by the American Helicopter Society Interna-tional, Inc. All rights reserved.

and military aircraft have killed more than 200 people and destroyed 186 aircraft (Ref. 1) since 1988, globally. During the 17-year period from 1990 to 2006, seven fatalities and 185 human injuries occurred in the U.S. due to bird impacts on aircraft. As shown in Fig. 2, the annual number of bird strikes quadrupled from 1,743 in 1990 to 7,089 in 2006. With an assumed 20% reporting rate, the annual cost of wildlife strikes to the U.S. civil aviation industry is estimated to be $603 million and 577,725 hours of downtime. The most common aircraft components struck by birds were reported as nose/radome, windshield, engine, wing/rotor, and fuselage. An aircraft must show compliance with continued safe flight and landing requirements following specified types of high-energy bird impact. Improving occupant safety by

U.S. DEPARTMENT OF DEFENSE

Fig. 1. Israeli Air Force UH-60 Blackhawk heli-copter, bird strike at 800 ft.

2

traditional means of hardening aircraft to withstand bird strikes can result in significant weight increase and cost. The higher impact velocities for bird strikes and attendant potential increased structural weight that tiltrotor aircraft must survive underscores the need to develop more capable, validated structural analysis techniques. In addition, experimental tests for bird strike development and certification are extremely expensive and time consuming. In order to reduce the number of costly prototype tests, a reliable analytical tool is necessary to accurately predict the structural responses/failures and to provide design guidance for aircraft subjected to the high-energy bird impact. The use of simulation provides the opportunity to cost-effectively evaluate numerous improved energy-absorbing structural design approaches that minimize structural weight and reduce the risk of not meeting civil or military rotorcraft bird strike design requirements. However, analysis techniques must be validated before they can be employed to accurately guide the design process. The objective of research by Bell Helicopter Textron Inc. has been to accurately predict the complex composite struc-tural responses, failures, and impact performances subjected to the bird strike dynamic loads to reduce development costs, avoid design turnback, and shorten cycle time while achieving a weight-efficient resistant design. Achieving this objective holds the potential to minimize and ultimately to eliminate costly bird strike testing by using simulation. Using both the Arbitrary Lagrangian-Eulerian (ALE) and Smooth Particle Hydrodynamics (SPH) techniques within the general-purpose nonlinear explicit finite element code, LS-Dyna (Ref. 2), the hydrodynamic pressure of the bird models is validated by first using a benchmark problem with available test data. The validated bird models are

subsequently applied to simulate the bird impact with the BA609 tiltrotor (Fig. 3) structure for the most load-critical test conditions. The analytical models are validated for mul-tiple test cases with different structural design approaches. In early 2007, the BA609 tiltrotor flight test program was temporarily halted in Italy, since the original design of the spinner and rotor controls failed the bird strike tests. A modified bird strike-resistant spinner design was needed quickly, to avoid potential long-term program delay. The bird strike simulation methodology developed at Bell was successfully employed to guide the spinner interim design that provided needed protection of critical internal control components by satisfying the bird strike requirements. Using the test data from several bird strike tests, multiple analytical correlation cases were performed. The correlation cases showed excellent agreement between the measured and predicted bird strike damage to the rotor spinner and controls, cockpit nose, wing leading edge, and empennage. The correlation study established a high degree of confi-dence in the analytical capability in predicting the dynamic responses and structural failures subjected to high-energy bird strike impacts. The analytical results of the modified rotor spinner design, along with the multiple correlation cases, were provided to the Italian airworthiness authority, the National Agency for Civil Aviation (Ente Nazionale per lAviazione Civile, or ENAC). As a direct result of this ef-fort, the full envelope flight clearance was granted by ENAC in 2007 without the need for costly and lengthy bird strike tests. This paper summarizes the bird strike simulation methodol-ogy developed at Bell based on the multi-material ALE and SPH techniques within LS-Dyna. The multiple correlation cases for the cockpit nose, wing leading edge, and empen-nage using similar techniques to establish a high-degree of analytical confidence are also presented.

1000

2000

3000

4000

5000

6000

7000

800019

90

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

Year

Num

ber o

f Bird

Str

ikes

to C

ivil

Airc

raft

1,743

7,089

Greg Ochocki/Lake City Nature photos

Fig. 2. Number of birds strikes to U.S. civil air-craft (1990 2006). Source: Ref. 1.

BA609BA609

Fig. 3. BA609 tiltrotor.

3

TECHNICAL APPROACH

Technical Challenges

Bird strike simulation is very complex and imposes a lot of numerical challenges since it involves transient, highly nonlinear dynamics (both geometry and material), con-tact/coupling, failure modes, and numerous other complexi-ties. The technical challenges of bird strike simulation in-clude, but are not limited to: 1) level of difficulty for bird material characterization; 2) numerical instability due to extremely high deformation and disintegration of the bird during and after the impact; 3) complex composite failure modes (such as delamination and debonding); and 4) post-failure material degradation characteristics. Unlike the implicit technique, the explicit integration tech-nique used in the impact analysis is conditionally stable, requiring the critical time step to meet the Courant criterion, which is ultimately determined by the smallest element size in the finite element model. Using the traditional Lagran-gian approach for bird modeling, the element size tends to become very small and distorted. As a result, the element quality (such as aspect ratio or warpage angle) deteriorates due to the extremely high material deformation of the bird model. This ultimately results in prohibited computational time and often produces an unstable numerical solution. Complicated states of stress are usually involved during the high-energy bird strike impacts. Depending on the impact conditions, the stress of the bird will likely exceed the yield strength in the constitutive model, causing the bird model to behave as an elastic-plastic hydrodynamic flow. Unlike the elastic-plastic structure whose deformation is related to strain in the stress calculations (displacement gradient), the fluid-like bird model is modeled as a function of the strain rate (velocity gradient). Two methods capable of analyzing elasticplastic hydrodynamic flow are employed for the bird modeling in this paper: (1) Arbitrary Lagrangian-Eulerian (ALE) and (2) Smooth Particle Hydrodynamics (SPH). Analytical Techniques

ALE Approach. In the ALE technique, the fluid-like *MAT_NULL (*MAT_9) material model is employed to model the bird and surrounding air shown in Fig. 4. This type of material does not have yield strength. It calcu-lates the pressure from a specified equation-of-state (EOS) related to the density ratio and internal energy. The poly-nomial EOS entity (*EOS_LINEAR_POLYNOMIAL), which defines the pressure P in the following, is used in modeling the bird and surrounding air in this paper:

( )2 3 20 1 2 3 4 5 6P C C C C C C C E = + + + + + + (1)

0

1 = (2)

where C0 ~ C6 = coefficients of polynomial equation E = internal energy = density

0 = initial density

The polynomial coefficients (cubic order) used for the bird strike analysis are

21 0 302.98 ksiC V= = (3)

2 1(2 1) 908.95 ksiC k C= = (4)

3 ( 1)(3 1) 1514.92 ksiC k k= = (5)

0 4 5 6 0C C C C= = = = (6)

Note that the initial bird density is assumed to be about 95% of water density, i.e., 8.89 10-5 lbf/in3 (950 kg/m3). The speed of sound in water, V, is 4,865 ft/s (1,483 m/s). The air is modeled by using Gamma Law of equation-of-state as follows:

4 5 0.4C C= = (7)

and

0 1 2 3 6 0C C C C C= = = = = (8)

The viscous (deviatoric) stress is computed as

Fig. 4. ALE bird modeling.

4

'ij ij = & (9) where is the dynamic viscosity (fluid resistance to shear or flow) and 'ij& is the deviatoric strain rate. SPH Approach. Bird modeling using the Smooth Particle Hydrodynamics (SPH) method in LS-Dyna is illustrated in Fig. 5. SPH is one of the meshless methods first introduced by Lucy in 1977 (Ref. 3). It is a Lagrangian numerical tech-nique used to solve the fluid equations of motion. Specifi-cally, it uses an interpolation kernel of compact support to represent any field quantity in terms of its values as a set of disordered particles. The material is discretized, and proper-ties of these clouds of nodes are associated with the center of the particles. A chosen interpolation kernel determines the amount of effect that a known point value contributes to the point of interest. The unique characteristic of SPH is that it does not require element meshing and thus avoids mesh tan-gling. It is generally considered to be much more accurate than the conventional Lagrangian finite element method in solving problems with large material distortions such as crash simulations, high velocity impacts, compressible fluid dynamic problems, etc. Without the mesh grid, the required derivatives of a problem can be numerically calculated with-out the attendant errors of element distortions. In this paper, the elastic-plastic hydrodynamic model *MAT_ELASTIC_PLASTIC_HYDRO (*MAT_10) is em-ployed for the SPH bird constitutive modeling. The EOS formulation and parameters are the same as in the ALE technique. In addition, a rectangular deactivation zone is defined to release the bird particles which no longer interact with the impacting structure. The dimensions of the rectan-gular deactivation zone are selected to be 8 8 22 inches in this paper. The deactivation zone in the SPH technique tends to effectively reduce the computational time in search-ing for the associated fluid particles.

BIRD MODEL VALIDATION

In this paper, the bird is treated as a soft gelatin projectile whose geometry is modeled as a cylinder with two hemi-spherical ends, as shown in Figs. 4 and 5. The ratio of the length to the diameter of the bird is selected to be 2:1. The weight of the bird is prescribed depending on the test condi-tions (4.0 lb for airplane mode and 2.2 lb for VTOL mode). The length of the bird, L, can be determined readily based upon the assumed density. Namely, L can be calculated as 4.438 inch (11.27 cm) and 2.43 inch (6.17 cm) for the air-plane mode and VTOL mode, respectively. To validate the constitutive parameters of the bird models, a benchmark problem that simulates the soft gelatin bird im-pacting on a rigid steel plate is constructed in LS-Dyna. Figure 6 shows the impact of a 4-lb bird with a rigid plate at a velocity of 225.5 kn (116 m/s) using both ALE and SPH techniques. Unlike the SPH bird, which produced spherical deformation during the transient dynamic contact with the rigid plate, it is observed that the ALE bird deformation tends to be influenced by the mesh sensitivity of the cubic surrounding air, which is discretized by one-point ALE solid elements. Nevertheless, both ALE and SPH techniques pro-duced comparable size of deformation at each of the time steps. Figure 7 shows the test/analysis correlation for the normalized pressure at the center of the rigid plate against the normalized time history. The normalized pressure (Pn) and time (tn) responses are expressed as

2102

nPPV= (10)

2n LV

t tVtD

= = (11)

where 0 = density of the soft gelatin projectile V = impact velocity of the soft gelatin projectile L = 2D = length of the soft gelatin projectile Note that the pressure time response can be categorized into three major stages: (1) shock wave stage; (2) pressure re-lease stage; and (3) steady-state flow pressure stage. While the steady-state flow pressure stage is considered to be more critical for bird impact events, the pressure response using the ALE technique correlates reasonably well with the test conducted by Wilbeck (Ref. 4). The SPH technique tends to over-predict the steady-state flow pressure, while both the ALE and SPH techniques produce very similar shock wave and release pressure. The pressure release and steady-state flow pressure stages correlate favorably with test data in both techniques. It is likely that there was an insufficient Fig. 5. SPH bird modeling.

5

data sample rate to produce the desired fidelity for the shock wave peak pressure in the test.

MULTIPLE CASES FOR TEST CORRELATION

The bird models validated by the benchmark problem are adopted to perform multiple analytical correlation cases on the BA609 tiltrotor using LS-Dyna. The four test cases for different design configurations include the rotor spinner, cockpit nose, wing leading edge, and empennage.

1. Rotor Spinner and Control

The rotor spinner assembly provides the swashplate drive load and aerodynamic fairing for the rotor hub and controls. Following a high-energy bird impact, the spinner assembly must demonstrate that there is no substantial damage to the rotor controls that would prevent continued safe flight and landing (Ref. 5) of the aircraft. Test Conditions. Table 1 depicts the bird strike test condi-tions according to the FAAs Issue Paper G-1 (Ref. 5) that

Fig. 6. Comparison for ALE and SPH birds impacting on a rigid plate.

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Normalized Time

Nor

mal

ized

Pre

ssur

e

Test ALE SPH

A B C A: Shock wave stageB: Pressure release stageC: Steady-state flow pressure stage

Test ALESPH

Fig. 7. Pressure correlation for ALE and SPH techniques.

6

identifies the BA609 Certification Basis derived from appli-cable Parts 25 and 29 requirements of Title 14 Code of Fed-eral Regulations (14 CFR). Specifically, the requirement identified as TR.631 states:

(a) The aircraft must be capable of continued safe flight and landing during which likely structural damage or system failure occurs as a result of (1) In airplane mode, impact with a 4-pound bird

when the velocity of the aircraft relative to the bird along the aircrafts flight path is equal to Vc at sea level or 0.85Vc at 8,000 ft, whichever is more critical;

(2) In VTOL/conversion mode, impact with a 2.2-pound bird at Vcon or VH (whichever is less) at altitudes up to 8,000 ft.

(b) Compliance must be shown by tests or by analysis based on tests carried out on sufficiently represen-tative structures of similar design.

For the BA609 tiltrotor in the airplane mode (i.e., 0 pylon angle), the bird must be 4.0 lb (1.8 kg) with an impact veloc-ity of 240 kn (123.4 m/s), oriented parallel to the airplanes flight path. The most vulnerable components for impact in airplane mode are the spinner cone and upper spinner spoke. The impact in VTOL/conversion mode (i.e., 60 pylon angle selected as critical) requires a 2.2 lb (1.0 kg) bird at 144 kn (74.1 m/s) to achieve the maximum velocity component nor-mal to the centerline of the proprotor mast, which is the most critical loading direction for the rotor cyclic links and other rotor controls components. Note that the total kinetic energy of the airplane mode test condition is approximately five times higher than the VTOL/conversion mode. Bird Strike Test. Seven bird strike tests were conducted at the Southwest Research Institute (SwRI) of San Antonio, Texas. The tests included three airplane mode and four VTOL/conversion mode shots. The bird strike testing was conducted using a 5.5 inch barrel with compressed gas which fired fresh chicken carcasses that were prepared in accordance with procedures defined by ASTM F330-89, at specific target locations on the spinner and rotor controls assemblies (Fig. 8). The test article consisted of the rotor

control assembly, the spinner assembly, the de-ice distribu-tors, and other supporting aircraft components, to provide either geometric or structural representations of the aircraft. In order to insure the bird impacted the desired target loca-tions, the test article was held stationary, and the speed of each bird impact was adjusted to account for the rotational speed. Because the controls are loaded in compression in most flight regimes, the test article had a steady collective load applied to the controls. A high speed photography sys-tem was used to record the bird impact. Finite Element Modeling. Figures 9 and 10 show the LS-Dyna bird strike finite element model of the rotor spinner in airplane 0 pylon angle mode. The geometry of the bird is represented as a cylinder with two hemispherical ends. Both bird and surrounding air are represented by the 1-point ALE multi-material solid element in LS-Dyna. Both the spinner cone and side panels are made of the carbon/epoxy fabric material represented by Belytschko-Tsay shell elements, with user-defined integration rules to calculate the

Table 1. FAA Bird Strike Test Conditions

Test Condition Bird

Weight Impact Speed 14 CFR

Airplane 4.0 lb

(1.8 kg) 240 knot

(123.4 m/s) TR.631

VTOL/conversion 2.2 lb

(1.0 kg) 144 knot

(74.1 m/s) 29.631

Fig. 8. Test fixture for rotor and spinner controls

assembly.

Fig. 9. LS-Dyna finite element model of rotor

spinner.

7

constitutive constants through the shell thickness. In addi-tion, the bulk viscosity control entity (*CONTROL_BULK_ VISCOSITY) is activated and the coefficients (q1, q2) are defined to smear the discontinuities of the shock wave generated by the bird strike impacts. The material model MAT_ENHANCED_COMPOSITE_ DAMAGE (*MAT_54) is employed for the anisotropic car-bon/epoxy fabric materials used in the spinner cone and side panels. This material model is only valid for thin shell ele-ments. It is important to activate the lamination shell theory when using this material model so that the assumption of a uniform constant shear strain through the thickness of the shell can be corrected. For sandwich shells where the outer layers are much stiffer than the inner layers, the structural response will tend to be too stiff unless the lamination theory is activated in the calculation. Various types of composite failures can be defined by using either Chang-Chang (*MAT_54) or Tsai-Wu (*MAT_55) failure criteria. In this paper, Hashin and Chang-Chang fail-ure criteria were employed for the fiber failure and matrix failure, respectively. The failure criteria are described in the following as: Hashin criterion for fiber failure:

Tensile fiber: 2 2

11 12 1tX S

+ (12)

Compressive fiber: 12

11

cX

(13)

Chang-Chang criterion for matrix failure:

Tensile matrix: 12

12

2

22

+

SYt

(14)

Compressive matrix:

1122

21222

2222

+

+

SYSY

S cc (15)

The metallic components of the controls such as the upper spokes, lower spokes, cyclic link, and collective head are all represented by Belytschko-Tsay shell elements, using the elastic-plastic (*MAT 24) material model with plastic strain failure criterion, as shown in Fig. 10. In addition, the mate-rial strain rate effect is taken into account using the Cowper-Symonds formulation, which calculates a dynamic yield stress by scaling the static yield stress:

1

1P

y ys C = + + &

(16)

where y is the dynamic yield stress, ys is the static yield stress, & is the strain rate, C is the reference effective strain rate for which the yield stress doubles, and P is the strain rate material parameter to be determined from the dynamic tensile tests. In this paper, C and P for the aluminum con-trols components are selected as 5 and 6500 /s, respectively. Based on numerical studies, the strain-rate effect of alumi-num is much less significant than steel. Test Correlation. The finite element model is validated by correlating against the bird strike tests conducted at SwRI. The most load-critical test condition was selected as the cor-relation case to compare the primary as well as the secon-dary structural impact responses, including composite failure mode, failure size, failure location, and elastic/plastic de-formation. The schematic of the bird location is illustrated in Fig. 11. Figure 12 shows the dynamic impact of bird strike test and simulation correlation for airplane mode shot-targeting at the cyclic lever. The impact speed is 240 kn with a 4-lb bird. The composite failures (size and location) of the spinner cone correlate favorably with the correspond-ing test, as shown in Fig. 13. Both analysis and test show that the spinner cone is severely damaged for this car-bon/epoxy design configuration. 2. Cockpit Nose Cone

Bird strike tests and corresponding simulation analyses were conducted on the BA609 nose cone, which is made of

Fig. 10. LS-Dyna model of rotor spinner internal

components.

8

fiberglass honeycomb sandwich design configuration, to further validate the simulation methodology. Test Condition. The test conditions simulate an impact of a 4-lb bird with the aircraft at a cruise speed (Vc) of 240 kn. This is equivalent to a kinetic energy of 10,206 ft-lb. The pass/fail criteria require that no portion, including fluid from the bird, may enter the cockpit; particles may not spall off inside the windshield or structure that could injure the crew. Finite Element Modeling. Figure 14 shows the LS-Dyna bird strike finite element model in airplane mode (i.e., 240 kn impact speed with a 4-lb bird) using the ALE technique. The idealized bird is modeled using the same techniques, including the constitutive model, geometry, and calibrated parameters as described in the previous section. The inner and outer skins of the nose cone are made of Al 2024-T42 face sheets, while the interior of the panel is made of fiber-glass flex-core honeycomb.

The aluminum face sheet is modeled with 4-node Be-lytschko-Tsay shell elements and an elastic-plastic (*MAT 24) material model representation, with defined true stress-strain curve, as shown in Fig. 15. The failure criterion of the face sheet is based upon the plas-tic failure strain threshold, failp., and is selected to be at 15% for Al 2024-T42 for this model. In this approach, the ele-ment is deleted and excluded from the finite element compu-tation once the plastic strain exceeds the failure threshold. The honeycomb core is modeled using 3-layer constant stress solid elements with the honeycomb material model (*MAT 126). Figure 16 illustrates the typical mechanical property of the honeycomb core. In the figure, E is the Youngs modulus,

1tE is the first tangent modulus,

2tE is

the second tangent modulus, f is the foam crushing strength, and d is the densification strain. This combined shell and solid modeling technique for the composite sand-wich design configuration allows the failure details of the face sheet and honeycomb core to be evaluated separately, as shown in Fig. 17.

Fig. 11. Schematic of bird location for airplane

cyclic lever shot.

Fig. 12. Rotor spinner bird strike correlation.

Fig. 13. Rotor spinner bird strike post-test

deformation.

Fig. 14. Nose cone LS-Dyna model.

9

The shell and solid elements share common nodes and are bonded by a tie-contact interface with the prescribed stress-based failure criterion in LS-Dyna:

2 2

1n

nfailfail

+ (17)

where nfail = normal stress at failure fail = shear stress at failure In addition, this technique allows one to simulate the debonding phenomenon for the sandwich core. The only

drawback of this technique is that it is computationally more expensive, due to the increased number of elements required for detailed modeling of the sandwich core using shell and solid elements. The other viable technique for modeling the composite sandwich core is to represent the structure using shell ele-ments only, with an effective smeared property for the face sheet and honeycomb core. Even though this approach is much simpler in terms of modeling effort and is more com-putationally efficient due to a lesser number of elements, it usually results in much stiffer structural responses. Further, the failures of the face sheet and honeycomb cannot be evaluated in as great of detail. The element is only deleted after both the face sheet and honeycomb fail. Test Correlation. Figure 18 shows the airplane mode bird strike test correlation for the BA609 nose cone honeycomb sandwich design configuration. This is an airplane mode shot condition (i.e., 240 kn and 4.0 lb bird) and is one of the most load-critical shots. The deflection on the honeycomb panel caused by the bird impact matches closely between test and analysis. There was little rebound of the bird on the impact location. Subsequently, the bird tripped near the bulkhead, bending the forward flange of the bulkhead and damaging the panel. Figure 19 shows the post-test damage location, secondary impact fracture, and failure mode of the honeycomb panel for both test and analysis. The damage location, failure size, and failure modes correlate exception-ally well with the test.

Fig. 16. Typical Mechanical Property of Honey-

comb.

Tensile True Stress-Strain Curve forAl 2024-T42 Clad Plate (t

10

Fig. 17. Modeling approach for composite honeycomb sandwich.

Fig. 18. Bird strike simulation correlation for nose cone honeycomb configuration (top side view).

Fig. 19. Bird strike structural damage correlation

for nose cone honeycomb configuration (bottom side view).

11

3. Wing Leading Edge

Bird strike tests and corresponding correlation were per-formed for the BA609 wing leading edge structure. The LS-Dyna finite element model is shown in Fig. 20. In this de-sign configuration, the local/global modeling technique is employed to model the wing leading edge to reduce the computational cost. In the local/global modeling technique, a finer mesh is used for the anticipated bird impact zone only. For the non-impact zone, the structure is meshed with coarser elements. The transitional elements between the finer impact zones and coarser non-impact zones are con-nected by applying a tie-break contact interface in LS-Dyna to allow the impact load paths to be transmitted correctly. Since the wing leading edge structure is long, the required fidelity of the element size to accurately predict the failure modes will result in extensive computational time. The lo-cal/global technique described herein significantly reduced the computational time due to fewer number of elements. Both the ALE and SPH bird modeling techniques are em-ployed to simulate the bird strike impacts with the wing leading edge structures. Figure 21 shows the comparison of the structural responses using both ALE and SPH tech-niques. Both techniques exhibit very similar responses in terms of bird deflection and splitting. 4. Empennage

Bird strike correlation on the BA609 empennage structure was performed. Figure 22 shows the bird impact locations on the leading edge of both the vertical stabilizer and hori-zontal stabilizer. Figure 23 shows the test/analysis correla-tion for the airplane mode test condition at the leading edge of the horizontal stabilizer (labeled as Location 3 in Fig. 22) using the SPH technique. Both test and analysis indicated that the leading edge on the horizontal stabilizer successfully

Fig. 21. BA609 wing leading edge bird strike com-parison (ALE vs. SPH).

Fig. 20. Local modeling technique for BA609 wing leading edge bird strike.

Fig. 22. BA609 Empennage Bird Strike.

Fig. 23. BA609 horizontal stabilizer bird strike cor-

relation.

12

split the bird. Figure 24 shows the test/analysis correlation for the airplane mode shot at the center of the vertical stabi-lizer (labeled as Location 5 in Fig. 22) using the ALE tech-nique. Both test and analysis demonstrated that the bird did not penetrate into the vertical stabilizer. Figure 25 compares the permanent plastic deformation revealing favorable corre-lation in terms of the structural responses.

CONCLUSIONS

The bird strike simulation methodology developed at Bell using both ALE and SPH techniques in LS-Dyna has been demonstrated to be a viable alternative to costly and time-consuming tests. The constitutive bird models have been validated using multiple analytical correlation cases and test conditions of different design configurations for the BA609 tiltrotor. In each design configuration, the most load-critical test conditions were selected to evaluate the structural fail-ures of the BA609 bird strike test specimen subjected to the bird strike dynamic loads. The correlation cases showed excellent agreement between the measured and predicted bird strike damage to the rotor spinner controls, cockpit nose, wing leading edge, and empennage. Specifically, the analysis achieved favorable post-test correlation in terms of structural deformation, composite/metal failure modes, and failure location, as well as the secondary impact fracture. The correlation study established a high degree of confi-dence in the analytical capability, in predicting the dynamic responses and structural failures subjected to the high-energy bird strike impacts. The validated analytical tool has been successfully used as a design tool to produce useful load mechanism information to guide the interim design for the BA609 rotor spinner control.

ACKNOWLEDGEMENTS

This project was funded by the Center for Rotorcraft Innova-tion (CRI) and the National Rotorcraft Technology Center (NRTC), Aviation and Missile Research, Development and Engineering Center under Technology Investment Agree-ment W911W6-06-2-0002, entitled National Rotorcraft Technology Center Research Program. The authors would like to acknowledge that this research and development was accomplished with the support and guidance of the NRTC and CRI. The views and conclusions contained in this document are those of the authors and should not be inter-preted as representing the official policies, either expressed or implied, of the Aviation and Missile Research, Develop-ment and Engineering Center or the U.S. Government. Fig-ure 1 is an image from the U.S. Department of Defense. The photograph in Fig. 2 is copyright by Mr. Greg Ochocki, Lake City Nature Photos and reproduced by permission.

REFERENCES

1. Cleary, E., et al., Wildlife Strikes to Civil Aircraft in the United States 1990-2006, Federal Aviation Ad-ministration National Wildlife Strike Database, No. 13, July 2007.

2. LS-Dyna Keyword Users Manual, Version 971, Livermore Software Technology Corporation, May, 2007.

3. Lucy, L.B., A Numerical Approach to the Testing of the Fission Hypothesis, Astronomical Journal, Vol. 82, 1977, pp.1013-20.

4. Wilbeck, J.S., Impact Behavior of Low Strength Pro-jectile, Air Force Materials Laboratory, Technical Re-port AFML-TR-77-134, 1977.

5. Federal Aviation Administration, Issue Paper G-1, FR Doc. E7-19053, September, 2007.

Fig. 24. BA609 vertical fin bird strike correlation. Fig. 25. BA609 empennage bird strike correlation.