Karen H. Lyle

Simulation of Aircraft Landing Gears with a Nonlinear Dynamic Finite Element Code

Recent advances in computational speed have made aircraft and spacecraft crash simulations using an explicit, nonlinear, transient-dynamic,e niteelement analysiscodemore feasible.This paperdescribes thedevelopment of a simplelanding-gearmodel, which accurately simulatestheenergy absorbed by thegearwithoutaddingsubstantial complexity to the model. For a crash model the landing gear response is approximated with a spring where the force applied to the fuselage is computed in a user-written subroutine. Helicopter crash simulations using this approach are compared with previously acquired experimental data from a full-scale crash test of a composite helicopter.

Application of probability methods to assess airframe crash modeling uncertainty

Full-scale aircraft crash simulations performed with nonlinear, transient dynamic, finite element codes can incorporate structural complexities such as geometrically accurate models, human occupant models, and advanced material models to include nonlinear stress-strain behaviors and material failure. Validation of these crash simulations is difficult due to a lack of sufficient information to adequately determine the uncertainty in the experimental data and the appropriateness of modeling assumptions. This paper evaluates probabilistic approaches to quantify the effects of finite element modeling assumptions on the predicted responses. The application of probabilistic analysis using finite element simulations of a fuselage vertical drop is the focus of this paper. The results indicate that probabilistic methods show promise for future applications.

Finite Element Simulation of a Full-Scale Crash Test of a Composite Helicopter

A finite element model of the Sikorsky Advanced Composite Airframe Program (ACAP) helicopter was developed using the nonlinear, explicit transient dynamic code, MSC.Dytran. Analytical predictions were correlated with experimental data obtained from a full-scale crash test of the Sikorsky ACAP helicopter flight test article that was conducted at the Impact Dynamics Research Facility of NASA Langley Research Center. The helicopter was impacted at 38-ft/s vertical and 32.5-ft/s forward velocity with an attitude of 6.25° pitch (nose up) and 3.5°-left roll. The objective of the crash simulation was to evaluate the capabilities of a commercially available transient dynamic code in predicting the response of a composite airframe subjected to impact loading. The model was developed from an existing MSC.Nastran modal-vibration model of the helicopter. Considerable modifications were made in converting the original modal-vibration model to a model for crash simulation. Following conversion of the model, a two-stage modeling approach was used to generate analytical predictions. Due to the relatively long pulse duration, a rigid structural model containing a fairly complex landing gear model was executed from initial contact through landing gear stroke. Prior to fuselage contact, the nodal displacements and velocities were output to a file. Then, a flexible structural model was executed with the nodal displacements and velocities used as initial conditions. This paper describes the development of the finite element crash model, the two-stage modeling approach, and the correlation of the analytical predictions with the experimental data from the full-scale crash test of the ACAP helicopter. ________________________ Presented at the American Helicopter Society 56 Annual Forum, Virginia Beach, Virginia, May 2-4, 2000. This paper is a work of the U. S. Government and is therefore in the public domain. Introduction An important aspect of crashworthiness research is the demonstration and validation of analytical/computational tools for accurate simulation of airframe structural response to crash impacts. The “validation of numerical simulations” was identified as one of five key technology shortfalls during the Workshop on Computational Methods for Crashworthiness [1], which was held at NASA Langley Research Center in 1992. Crash simulation codes can be used during the airframe design phase to certify seats and aircraft to dynamic crash loads, to predict seat and occupant response to impact with the probability of injury, and to evaluate numerous crash scenarios not economically feasible with full-scale crash testing. The US Army has been active in supporting the development and utilization of crash modeling and simulation codes for many decades. More than 25 years ago, the US Army sponsored the initial development of a kinematic crash analysis code, KRASH [2], by the Lockheed-California Company. Kinematic codes employ a semiempirical modeling approach using lumpedmasses, beams, and nonlinear springs to represent the airframe structure. These codes rely heavily on test data for definition of spring properties to characterize the crushing behavior of the subfloor and other structural components. Good correlation between the analytical and experimental data is usually obtained for global parameters, such as engine or landing gear response. However, these codes would be unable to predict localized responses, e.g. the stress level in an airframe component at a particular time during a crash event. Currently, a new generation of crash analysis codes have been developed that will accurately simulate the nonlinear, transient dynamic response of airframe structures. These finite element codes, such as LS-DYNA [3], MSC.Dytran [4], and PAM-CRASH [5], use an explicit solver that eliminates the need to repetitively decompose large global stiffness matrices as is required for implicit codes. Explicit codes require an extremely small time step, typically less than a microsecond, whose duration is controlled by the smallest element in the model. Thus, impact simulations having a pulse duration on the order of 30-40 milliseconds can require several CPU hours to solve on an engineering workstation. Consequently, efficient beam, shell, and solid elements are needed to achieve quick run times for very large models. The new codes are capable of modeling nonlinear geometric behavior including large structural deformations. In addition, these codes are very effective in modeling materials, such as metals, that deform plastically and that have well known failure mechanisms. However, the use of light weight, high strength composite materials for aircraft construction brings with it difficulties in modeling material response and failure behavior. Structural composite materials can exhibit a wide range of material responses from linear elastic to completely nonlinear anisotropic behavior, depending on the individual fiber and matrix properties and laminate stacking sequence. Also, laminated composite materials exhibit a wide variety of failure modes including matrix cracking, fiber failure, and delamination that can occur singly or in combination. These failure modes can change depending on the type and rate of loading. In general, the initial failure event in a single ply of a composite laminate does not produce catastrophic failure. Consequently, the capability to model the progressive failure of composite materials, from initial damage to ultimate failure, is needed. With the increased application of composite materials in the construction of advanced aircraft and rotorcraft, it is important to build confidence in the computational capabilities of these codes through analytical/experimental validation. In 1996, the US Army initiated a fourphase research program to evaluate the capabilities of commercial crash simulation codes for modeling the impact response of a composite helicopter. As part of this program, a finite element crash model of the Sikorsky Advanced Composite Airframe Program (ACAP) helicopter [6, 7] was developed. In this effort, an existing MSC.Nastran [8] modal-vibration model of the helicopter was converted to an MSC.Dytran model for the crash simulation. In 1999, a full-scale crash test of an ACAP helicopter was performed at the Impact Dynamics Research Facility (IDRF) [9] of NASA Langley Research Center to generate experimental data for correlation with the simulation. The present paper will describe: (1) the development of the helicopter crash model, (2) modifications made to the crash model to better represent the test article, (3) a summary of the experimental results from the full-scale crash test of the ACAP helicopter, and (4) the validation of the crash simulation through analytical/experimental correlation. Finite Element Model Development Description of MSC.Dytran Finite Element Code The commercial code, MSC.Dytran, was used to perform the crash simulation of the ACAP helicopter. MSC.Dytran is a three-dimensional finite element code for simulating highly nonlinear transient response of solids, structures, and fluids. The MSC.Patran [10] preand postprocessing code was used with the MSC.Dytran “Preference” to build the finite element crash model and to post-process the results. Description of the Sikorsky Modal-Vibration Model An MSC.Nastran model of the ACAP helicopter that was originally developed for correlation with modal vibration data [11] was obtained from Sikorsky Aircraft. The model, shown in Figure 1, had approximately 5000 nodes, 9,500 elements, 219 material models including many different composite materials, and over 700 different property cards. The elements included 5,453 shell elements; 1,956 beam elements; 1,770 rod elements; and 372 concentrated masses. Because this model was originally used for modal analysis, extensive modifications were required to convert it for a crash analysis. Figure 1. Sikorsky Modal-vibration model of the ACAP helicopter. Conversion of the modal-vibration model to a

A Summary of DOD-Sponsored Research Performed at NASA Langley's Impact Dynamics Research Facility

The Impact Dynamics Research Facility (IDRF) is a 240-ft.-high gantry structure located at NASA Langley Research Center in Hampton, Virginia. The IDRF was originally built in the early 1960s for use as a Lunar Landing Research Facility. As such, the facility was configured to simulate the reduced gravitational environment of the Moon, allowing the Apollo astronauts to practice lunar landings under realistic conditions. In 1985, the IDRF was designated a National Historic Landmark based on its significant contributions to the Apollo Moon Landing Program. In the early 1970s the facility was converted into its current configuration as a full-scale crash test facility for light aircraft and rotorcraft. Since that time, the IDRF has been used to perform a wide variety of impact tests on full-scale aircraft, airframe components, and space vehicles in support of the General Aviation (GA) aircraft industry, the U.S. Department of Defense (DOD), the rotorcraft industry, and the NASA Space program. The objectives of this paper are twofold: to describe the IDRF facility and its unique capabilities for conducting structural impact testing, and to summarize the impact tests performed at the IDRF in support of the DOD. These tests cover a time period of roughly 2 1/2 decades, beginning in 1975 with the full-scale crash test of a CH-47 Chinook helicopter, and ending in 1999 with the external fuel system qualification test of a UH-60 Black Hawk helicopter. NASA officially closed the IDRF in September 2003; consequently, it is important to document the past contributions made in improved human survivability and impact tolerance through DOD-sponsored research performed at the IDRF.

APPLICATION OF PROBABILISTIC ANALYSIS TO AIRCRAFT IMPACT DYNAMICS

Full-scale aircraft crash simulations performed with nonlinear, transient dynamic, finite element codes can incorporate structural complexities such as: geometrically accurate models; human occupant models; and advanced material models to include nonlinear stressstrain behaviors, laminated composites, and material failure. Validation of these crash simulations is difficult due to a lack of sufficient information to adequately determine the uncertainty in the experimental data and the appropriateness of modeling assumptions. This paper evaluates probabilistic approaches to quantify the uncertainty in the simulated responses. Several criteria are used to determine that a response surface method is the most appropriate probabilistic approach. The work is extended to compare optimization results with and without probabilistic constraints.