Design and Testing of Wing Leading Edge of a Light Transport Aircraft

Design and Testing of Wing Leading Edge of a Light Transport Aircraft Stanley C. Salem, Kotresh M. Gaddikeri, MNN Gowda, Ramesh Sundaram Advanced Composites Division, National Aerospace Laboratories, Bangalore, Karnataka, 560 017 J Dhayanidhi, B Nagarajappa, S Sathiyanaarayan, Satish Chandra Structural Technologies Division, National Aerospace Laboratories, Bangalore, Karnataka, 560 017 Anuradha Nayak, Vijay Petley, S Ramachandra SIMA, Gas Turbine Research Establishment, Bangalore, Karnataka, 560 017 The design of wing leading edges are primarily based on certification requirements as defined by the regulatory for bird strikes. A typical wing leading edge comprises of skin, rib and baffle plate. The idea of a baffle plate is that it will prevent the bird from impacting the spar, in case of penetration of the skin. The selection of right materials, configuration and fasteners play an important role in containing the damage due to bird strike. The selection of the skin thickness was done based on the RAE empirical formula and through limited bird strike tests on plain leading edge specimens. The role of baffle and selection of its thickness were based on bird strike FE analysis. The selection of fasteners was done based on a rivet pull through tests on an aluminum plate. The selection of rib spacing was based on a parametric study through bird strike FE analysis. The design, analysis and fabrication of leading edge specimens was carried out at NAL and the bird impact tests were conducted at the Gas Turbine Research Establishment, Bangalore. An optimum design configuration of the leading edge is arrived through these studies which meets the regulatory requirements. V p = Penetration velocity in knots Nomenclature τ = Flat panel / curved panel / leading-edge skin thickness in mm W = Bird weight in Kg α = Sweep angle / impact angle in degrees r = Leading edge nose radius in mm Introduction Bird Strike is a major threat to aircraft safety and can cause significant structural damages to aircraft; the economic loss has been estimated to be more than $1 billion per year to the aviation industry worldwide [1]. Many external components like leading edge surface of wing and empennage, engines etc. are susceptible to being hit with birds, particularly during takeoff and landing. A tremendous amount of energy is imparted to the

structure and ensuing damage/ deformation of the structure should not propagate uncontrollably, which could impair control devices/ flight critical instruments etc. housed internally or damage the fuel tank. There are specific regulatory requirements which have to be met for bird strike so as to ensure safety. In this direction, NAL and Gas Turbine Research Establishment GTRE, Bangalore initiated a collaborative program on the design of leading edge component. This paper discusses the design methodology, parametric studies of design parameters using numerical simulation and validation of design by limited testing for wing leading edge of a transport aircraft. Certification Requirements for Bird Strike Bird strike requirements as per the Damage Tolerance clause of FAR Section 25.571 states that The airplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of impact with a 4-pound bird when the velocity of the airplane relative to the bird along the airplane s flight path is equal to V C at sea level or 0.85V C at 8000 ft, whichever is more critical; The damaged structure must be able to withstand the static loads (considered as ultimate loads) which are reasonably expected to occur on the flight. In the approach to design of wing leading edge, the objective criteria was that baffle plate deflection for the civil aircraft under consideration should not be more than 25 mm to prevent any damage to the control rods inside the nose box which connects to the ailerons. Design Philosophy Design of leading edge primarily hinges around the energy absorbing capability of the structure for bird strike. This can be achieved through a combination of materials and structural configuration. The selection of right materials, rib spacing and fasteners for the leading edge play an important role in containing the damage due to bird strike. This has led to usage of varieties of leading edges for different aircrafts. Metals, generally aluminium, are well suited for such applications due to their ability to undergo plastic deformation which increases the energy absorption capability. Aluminium alloy of 2024 T3 was selected as the skin material as it has high failure strain compared to other aluminium alloys. The wing leading edge under consideration houses the control rods for aileron actuation. These rods are mounted on a series of brackets which in-turn are mounted on the web of front spar. In order to protect these control rods from getting damaged in case of bird hit, a baffle plate was used in the design as an additional barrier. Location of baffle plate was fixed based on the constraint from control rod location. The primary

requirement in the design was to limit the maximum baffle deflection to 25 mm to prevent any damage to the aileron control rods residing in the nose box. The velocity of bird strike was estimated as 115m/s based on the FAA regulations for the aircraft under consideration. The initial selection of the skin thickness of 1.6mm was based on the RAE formula [2]. The empirical formula derived by RAE for a Leading edge made in aluminium is as follows. V p 1 3 2 3 2 1 [ r + 30 + 1000) ] = 98τW cos α exp 1234( r It was considered important to understand the behavior of nose skin for penetration in order to arrive at an optimum design. The formula developed by RAE was used to arrive at the penetration velocity. The RAE formula has been validated earlier through analysis with commercially available explicit FE codes like PAMCRASH using various bird material models [3, 4 and 5]. The criterion of resistance was taken as nonpenetration of the leading edge skin by the bird. Based on the RAE formulae, the penetration velocity was found to be approximately 119m/s for thickness of 1.6mm, W = 1.8 kg, α = 0 deg, r = 21 mm. Limited tests were carried out on a plain wing leading edge configuration with different thickness as shown in Fig 1. The bird strike velocity in the test was within ±4m/s of the desired velocity. The test also showed a fair agreement with the RAE formulae. 1.6 mm Aluminum 20224-T3 plain edge impacted 2mm Aluminum 2024-T3 leading edge impacted @ @ 120m/s. 112 m/s. Fig. 1 Bird impact test on plain aluminum leading edge specimen Selection of Rib Spacing based on parametric studies using numerical simulation Nose ribs should be spaced so as to control the zone of plastic deformation and restrict the load transferred to the supporting spar. Larger rib spacing would mean that zone of plastic deformation will be larger; however lesser loads would be transferred to the supporting spar, whereas, shorter rib spacing would lead to large loads being transferred to the supporting spar and also in certain cases could result in tearing of the skin at the rib skin connections. The rib spacing was selected based on a parametric studies carried out using PAMCRASH, an

explicit impact FE code, for a structural combination with varying rib spacing (300, 450 and 600 mm) and nose skin thickness (1.2, 1.4, 1.6, 1.8 and 2.0 mm). The CFC ribs with 1.5mm thickness and aluminium baffle plate of 2mm thickness were chosen for the parametric study. In the case of closer rib spacing, the bird impact led to rupture of the skin in the rib-skin attachment region due to the increased stiffness as shown in Fig.2a. Nose skin tore along the rib web as it transferred the impact loads to CFC ribs, thereby rupturing the skin. Moreover, the energy absorption by aluminium nose skin was limited owing to the tearing of the nose skin. In the case of larger spacing of the ribs, the energy was dissipated through excessive plastic deformation of the leading edge skin as shown in Fig. 2b. Optimum rib spacing was arrived and the baffle plate deformation was limited to 25mm. a) Leading edge with Closer Rib spacing b) Leading edge with Larger Rib spacing Fig. 2 Typical leading edge deformation with plastic strain contour to arrive at the optimised rib spacing for bird strike Selection of Rivets between baffle plate and nose skin The deformation pattern of nose skin due to bird impact revealed that the rivets connecting the baffle plate and nose skin would be subjected to tensile forces (Fig. 2b). Since the tensile strength of the rivets is limited, experimental studies on rivet pull through were carried out to facilitate the selection. The tests were performed based on the procedures defined in ASTM D 7332 (Procedure B). Special fixture was fabricated to test Aluminium 2024 T3 plates of size 68mm x 68mm x 2mm fastened using a single rivet. Monel rivets (blind rivets) and aluminium rivets (bucking rivets) of 3.2mm and 4mm diameter were evaluated separately. The tests were performed using an UTM machine adopting controlled displacement at a rate of 0.5 mm/min. The pull through characteristics showed a similar pull through strength capability for plate fastened through 4mm Monel and Aluminium rivets (Fig 3).

a) Schematic Diagram of Pull Through Fixture which was fabricated[6] b) Pull through testing of Aluminium plate fastened through rivets c) Pull through Characteristics of Aluminium plate fastened through 4mm Aluminium rivets and Monel rivets Fig. 3 Pull Through testing of Aluminium Plate and its Characteristics Bird strike testing on Wing Leading Edge component The design of leading edge segment shown in Fig. 4 was frozen based on the studies carried out through numerical simulation and rivet pull through tests. This segment was used for bird strike testing for validating the design. Three bird strike tests were carried out on nose segments with different design options so as to arrive at an optimum design. Aluminum nose skins of 1.6 and 2 mm were evaluated with different types of rivets. Aluminum baffle plate of 2mm thickness and CFRP ribs with 1.5mm thickness were used for all the tests. Test cases are classified based on the nose skin thickness and rivets connecting it to the baffle plate. Ribs Baffle Plate LE Nose Skin Fig. 4 Leading Edge test specimen with skin, baffle and two side ribs Case 1) Leading Edge segment with 2mm Aluminum nose skin and Monel blind rivets of 4mm diameter Nose segment was impacted with a 4lb bird with a velocity of 114m/s. There was no penetration of the bird and the deflection of aluminum leading edge skin due to plastic deformation was controlled by the baffle. The permanent deformation observed in the baffle was 23mm. CFRP Ribs showed significant deformation by bending inwards. Monel rivets successfully resisted the pull through load experienced due to the outward buckling of the skin during the impact. Fig 5 shows the deformed shape of the test specimen and FE simulation subjected to bird strike and the similarity in deformation pattern validates the numerical simulation carried out.

a. Test b. Simulation Fig. 5 Final Deformed shape of the Leading edge specimen under bird impact at 114m/s (Case 1) Case 2) Leading Edge segment with 1.6mm Aluminum nose skin and Monel blind rivets of 4mm diameter Nose segment was impacted with a 4lb bird with a velocity of 112m/s. There was no penetration of the bird and the plastic deformation observed in this case was larger than case 1, which was beyond 25mm. CFRP Ribs caved in more than in case 1. Monel rivets successfully resisted the pull through load. However, skin tearing was observed nearer to the centre of rivet line pointing to a larger skin thickness requirement for the nose skin. Fig 6 shows the deformed shape of the test specimen and FE simulation subjected to bird strike for case 2 a. Test b. Simulation Fig. 6 Final Deformed shape of the Leading edge specimen under bird impact at 112m/s (Case 2) Case 3) Leading Edge segment with 2mm Aluminum nose skin, Aluminum rivets of 4mm diameter and Profile modified CFRP rib Nose segment was impacted with a 4lb bird with a velocity of 118m/s. There was no penetration of the bird and the plastic deformation observed in aluminum nose skin was higher compared to case 1. However, this was attributed the profile modification of flanges of CFRP ribs which resulted in the ribs caving in more than case 1. 4mm diameter Aluminum rivets successfully resisted the pull through load just like Monel rivets and were

considered as an alternate design option. Fig 7 shows the deformed shape of the test specimen and FE simulation subjected to bird strike for case 3. a. Test b. Simulation Fig. 7 Final Deformed shape of the Leading edge specimen under bird impact at 118m/s (Case 3) Conclusion The methodology followed in the design of a typical light transport aircraft leading edge was discussed. The parametric study using numerical solution showed that there is an optimum rib spacing that is required for controlling the deformation and the tearing of the skin. Rivet selection was made based on the pull through test on Monel and aluminium rivets of 3.2mm and 4mm diameter. Alternate designs were tested for 4lb bird impact and numerical simulations of these tests were carried out. Leading Edge segment assembly with the aluminum skin, aluminum baffle and CFRP rib configuration resisted the bird impact at velocities of 112 118m/s without penetration of the bird and the final design was arrived based on the test results. Correlation of deformation patterns between test and numerical solution validated the FE model. Acknowledgments The support received from the Engineering Service Division for fabrication of the test specimen is acknowledged. The motivation provided by Mr. H.N. Sudheendra, Head, ACD and the Director, NAL is greatly acknowledged. References [1] John R Allan, The costs of Bird Strikes and Bird Strike Prevention, Human Conflicts with Wildlife: Economic Considerations. USDA National Wildlife Research Center Symposia, 2000.

[2] I. I. McNaughten, The design of leading-edge and intake wall structure to resist bird impact, Royal Aircraft Establishment, Technical report 72056, (1972) [3] J Dhayanidhi, K Karthikeyan and S Chandra, Bird strike impact and penetration studies using various FEA based bird models, International conference CAE 2007, IITM Mechanical Engineering, Chennai, Dec 13-15, 699-707, ISBN 978-81-904235-3-3, 2007 [4] L. C. Ubels, A. F. Johnson, J. P. Gallard, and M. Sunaric,, Design and testing of a composite bird strike resistant leading edge, The SAMPE Europe Conference and Exhibition, Paris,France,1-3 April 2003. [5] M. A. McCarthy, J. R. Xiao, C. T. McCarthy, A. Kamoulakos, J. Ramos, J. P. Gallard and V. Melito, Modelling of Bird Strike on an Aircraft Wing Leading Edge Made from Fibre Metal Laminates Part 2: Modelling of Impact with SPH Bird Model, Applied Composite Materials. 11 pp. 317-340, 2004 [6] MIL-HDBK-17-1F, Volume 1 of 5, 17 JUNE 2002