NASA Technical Reports Server (NTRS) 20060020751: Crash Certification by Analysis - Are We There Yet?

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by  NASA Technical Reports Server (NTRS)

Crash Certification by Analysis – Are We There Yet?

Karen E. Jackson and Edwin L. Fasanella Karen H. Lyle
[email protected], [email protected] [email protected]
US Army Research Laboratory Structural Dynamics Branch
Vehicle Technology Directorate NASA Langley Research Center
Hampton, VA Hampton, VA

Abstract

This paper addresses the issue of crash certification by analysis. This broad topic encompasses many ancillary issues includ-
ing model validation procedures, uncertainty in test data and analysis models, probabilistic techniques for test-analysis corre-
lation, verification of the mathematical formulation, and establishment of appropriate qualification requirements. This paper
will focus on certification requirements for crashworthiness of military helicopters; capabilities of the current analysis codes
used for crash modeling and simulation, including some examples of simulations from the literature to illustrate the current
approach to model validation; and future directions needed to achieve “crash certification by analysis.”

Introduction Historically, MIL-STD-1290A (AV) [1] defined the certi-
fication requirements for crash performance of military
The rationale for incorporating crashworthy design fea- helicopters by specifying a list of lateral, vertical, longitu-
tures into rotorcraft is to minimize the number of fatalities dinal, and combined velocity impact conditions that the
and serious injuries experienced by the crew and troops, airframe must withstand with minimal collapse and asso-
and to reduce the amount of structural damage to the air- ciated loss of volume (15% or less). The military stan-
frame and payload during a severe, but survivable, crash. dard encouraged the designer to demonstrate compliance
Ideally, the initial cost and weight associated with incor- with the requirements by using analytical methods. How-
porating crashworthy design features are offset by cost ever, the standard also states that: “Instrumented full-scale
savings associated with the reduced number of injuries crash test(s) are desirable to substantiate the capability of
and lower levels of aircraft damage. Crashworthy design the aircraft system to prevent fatalities and minimize inju-
of rotorcraft requires a systems approach in which various ries during crashes of the severity cited herein. If the sys-
subcomponents work together to absorb and dissipate the tem testing is not conducted, then analysis shall be re-
kinetic energy of impact. During a crash, the helicopter quired to show that the individual components and sub-
must limit the loads and decelerations that are transferred systems function together effectively to achieve the spe-
to the occupants to humanly tolerable levels, usually cific overall level of crashworthiness.” Thus, the military
through crushable landing gear, energy absorbing sub- standard established both full-scale crash testing and ana-
floors, and load-limiting seats. Structural collapse of fu- lytical modeling as tools to achieve crash certification.
selage frames and other structural components used to
support the overhead rotor and transmission mass must be The purpose of this paper is to discuss certification or
prevented, thereby providing a livable volume for the qualification of helicopter crashworthiness by analysis.
occupant. The seats, restraint systems, seat track, and The terms ‘certification’ and ‘qualification’ are used in-
floor provide a secure tie down in the crashworthy heli- terchangeably in the paper and they are defined as “con-
copter, thus preventing the occupant from becoming a firmation that a design requirement or military specifica-
projectile inside the fuselage during a crash. In addition, tion has been met or achieved.” The paper begins with a
head strike potential should be mitigated or eliminated brief review of crash safety requirements for military
through the use of pre-tensioned restraint systems and/or helicopters. Next, a summary of modeling and analysis
cockpit airbags. Occupant survivability also depends on procedures is provided. In order to assess the accuracy of
eliminating post-crash fire hazards and providing for crash simulations, examples are summarized in which the
emergency egress. The crashworthy helicopter must be codes are used to simulate full-scale aircraft crash tests.
capable of all these things while experiencing high transi- Finally, future directions needed to achieve “crash certifi-
tory, multi-directional decelerations associated with im- cation by analysis,” are discussed in two subsections: De-
pact onto different terrains. sign Requirements and Improved Analytical Methods.

Brief Review of Crash Safety Requirements
_______________________________________
Presented at the American Helicopter Society 62nd An- Military helicopter design requirements for crashworthi-
nual Forum, Phoenix, AZ, May 9-11, 2006. This paper is ness were developed by close examination of U.S. Army
a work of the US Government and is, therefore, in the accident data during two different studies. The first study
public domain. reviewed all accident data for rotary- and light fixed-wing
aircraft that occurred between July 1960 and June 1965. various velocity change criteria using analytical methods.
During a second study, attack and cargo helicopter acci- However, the MIL-STD-1290A (AV) does not specify
dents that occurred from January 1971 through December which analytical methods or codes should be used to per-
1976 were studied. The data from these studies were ex- form these analyses. Separate military specifications for
amined to determine the velocity changes in the vertical landing gear, seats, fuel tanks, and other subsystems are
and longitudinal directions at impact for survivable also cited in MIL-STD-1290A (AV).
crashes. The results of the study showed that the 95th
percentile of all survivable accidents occurred at less than Appendix A in MIL-STD-1290A (AV) lists the testing
42 ft/s vertical and 50 ft/s longitudinal impact velocities. requirements for various aircraft components and subsys-
Analysis of the data also showed similarity between ro- tems including the fuel system, crew and troop seats, litter
tary- and light fixed-wing aircraft. Consequently, the supports, landing gear, and flammability tests. The Gen-
same criteria were used to qualify both types of aircraft. eral Requirements section of the military standard states
In general, there were insufficient data to determine a that, “The component and subsystem tests described in
lateral velocity distribution. Thus, a representative value Appendix A are mandatory. Instrumented full-scale crash
of 30 ft/s lateral velocity was selected. Later, reductions test(s) are desirable to substantiate the capability of the
were made in the longitudinal velocity change and in the aircraft system to prevent fatalities and minimize injuries
attitude angles based on economic factors and operational during crashes of the severity cited herein. If the system
constraints. However, the vertical and lateral velocity testing is not conducted, then analysis shall be required to
requirements were unchanged. show that the individual components and subsystems
function together effectively to achieve the specific over-
The findings from the two studies mentioned previously all level of crashworthiness.”
and many additional investigations into the safety and
survivability of Army helicopters led to the development Table1. Crash Impact Design Conditions, with Landing
of the Crash Survival Design Guide in 1965, which was a Gear Extended
compilation of a series of reports on accident analyses,
full-scale crash test data, proposed design criteria, and Condition Impact Direction Velocity Object/
prototype crashworthy systems. Since then, the Guide has No. (Aircraft Axes) Change Surface
been updated and expanded several times to encompass (ft/s) Impacted
the increasing knowledge gained from continuing re- 1 Longitudinal 20 Rigid
search in rotorcraft crashworthiness. Today, the latest (cockpit) vertical
edition of the Aircraft Crash Survival Design Guide [2], 2 Longitudinal 40 barrier
published in 1989, consists of five volumes. (cabin)
3 Vertical* 42
Information contained in the Crash Survival Design 4 Lateral, Type I 25 Rigid
Guide was used in the development of Military Standard horizontal
MIL-STD-1290A (AV) for Light Fixed and Rotary-Wing 5 Lateral, Type II 30 surface
Aircraft Crash Resistance [1], which establishes minimum
crash resistance criteria for implementation in the initial 6 Combined high
stages of aircraft system design. The initial release of the angle* 42
standard was in January 1974, and it was subsequently Vertical 27
revised in September 1988. In the late 1980’s, an Aero- Longitudinal
nautical Design Standard ADS-36 for Rotary Wing Air- 7 Combined low
craft Crash Resistance [3] was developed specifically for angle 14 Plowed
qualifying the U.S. Army’s new reconnaissance and at- Vertical 100 soil
tack helicopter, the RAH-66, which eventually became Longitudinal
the Comanche helicopter. The ADS-36 contains similar
information as the MIL-STD-1290A (AV); however, *For the case of retracted landing gear, the seat and air-
some of the criteria were modified so that the RAH-66 frame combination shall have a vertical crash impact de-
would be designed with crash resistance equivalent to the sign velocity change capability of at least 26 ft/s.
existing UH-60 Black Hawk. For the discussions in this
paper, the crash requirements in the military standard will Several additional comments should be made regarding
be described. crash criteria. More recent studies of military helicopter
accidents [4-6] show clearly that the “one size fits all”
MIL-STD-1290A (AV) contains seven different specifi- approach to crash design criteria is not appropriate. A
cations for aircraft crashworthiness, as listed in Table 1. recommendation from Reference 4 is that “design stan-
The crash performance requirements for each of these dards should be tailored to class or type of aircraft in or-
impact conditions are summarized in Table 2. To meet der to minimize cost and maximize the crash protection
the performance requirements, the designer is requested to offered by each type of helicopter.” A similar recom-
demonstrate the capabilities of the aircraft to withstand mendation is stated in Reference 5, “cost/benefit trade
studies should be conducted…to establish the optimum It is important to note that there are no requirements for
required design crash velocity.” In addition, Reference 6 full-scale crash testing of civil rotorcraft, similar to MIL-
recommends a “variable crash resistance design criteria STD-1290A (AV) or ADS-36. Instead, there are seat
for military helicopters as a function of their size, main requirements described in the Federal Aviation Regula-
rotor blade disk loading, rotor blade inertia, autorotative tions, Parts 27 and 29, for normal and transport civil ro-
rate of descent, and mission type.” Data contained in torcraft (see References 8 and 9). These criteria are
References 4 through 7 indicate that more recent accident summarized in Reference 10, and comparisons of military
studies show that a significant change has occurred in the and civil requirements are presented in References 11 and
kinematics of military helicopter crashes. As new and 12. In general, the findings show that civil helicopters
improved crashworthy design features have been intro- typically crash at consistently lower velocities than do
duced over time, the data show that survivable helicopter military helicopters. Consequently, applying the military
accidents are occurring at higher impact velocities. Con- standard to civil helicopters is not appropriate.
sequently, since crash design requirements are based on
accident statistics, it is important to update the statistical Finally, MIL-STD-1290A (AV) was canceled by the US
data on a regular basis. Likewise, the design require- Department of Defense in the mid-1990’s to give defense
ments should be re-evaluated periodically to ensure their contractors more freedom to develop creative and innova-
validity and relevance. tive designs without the constraints provided by the mili-
tary standards.

Table 2. Performance Requirements for Crash Design Conditions

Impact Direction Performance Requirement
Longitudinal - Cockpit The designer shall demonstrate analytically that the basic airframe is capable of impacting lon-
(∆V ) gitudinally into a rigid vertical barrier at a constant velocity of 20 ft/s without crushing the pilot
x
or copilot stations to an extent that would either preclude evacuation of the aircraft or preclude
a livable volume for the aircraft occupants. For this impact, the engines, transmission, and rotor
system shall remain intact.
Longitudinal - Cabin The basic airframe’s capability to impact longitudinally into a rigid vertical barrier or wall at a
(∆V ) constant velocity of 40 ft/s without reducing the length of the passenger/troop compartment by
x
more than 15% shall be demonstrated analytically. Any consequent inward buckling of walls,
floor, and/or roof shall not be hazardous to the occupants and/or restrict their evacuation.
Vertical (∆V) The designer shall analytically demonstrate the capability of the aircraft system, with ro-
z
tor/wing lift equal to structural design gross weight (SDGW) and with landing gear extended, to
withstand vertical impacts of 42 ft/s on a rigid horizontal surface without (1) reducing the
height of the cockpit and troop compartment by no more than 15%, and (2) allowing the occu-
pants to experience injurious accelerative loading. For the case of retracted landing gear, the
designer shall analytically demonstrate the capability of the aircraft to withstand impacts of 26
ft/s on a rigid horizontal surface with the same requirements as for the gear extended case. The
capabilities, with gear up or down, are required for all aircraft orientation (attitudes) upon im-
pact in +15° to -5° pitch and ±10° roll.
Lateral (∆V ) The designer shall analytically demonstrate the capability of the aircraft to withstand lateral
y
impacts of 25 ft/s for Type I and 30 ft/s for Type II aircraft without reducing the width of the
occupied areas by more that 15%. Precautions should be taken to minimize the chances that the
occupants, including their extremities, could become trapped between the structure and any
impacting surfaces following failure of doors, canopies, or hatches.
Combined The designer shall analytically demonstrate the capability of the aircraft with 1 SDGW ro-
tor/wing lift and with landing gear extended to withstand the following combined impacts with-
out a reduction of the cockpit or cabin compartments that would seriously injure the occupants:
(1) a combined impact onto a rigid horizontal surface with vertical and longitudinal velocity
changes of 42 and 27 ft/s, and (2) a combined impact onto plowed soil with vertical and longi-
tudinal velocity changes of 14 and 100 ft/s, respectively.

Modeling and Analysis Procedures use of analytical methods. First, crash testing of full-scale
aircraft, especially prototype aircraft, is relatively expen-
As noted earlier, the military standard for crash resistance, sive. Also, due to the limited availability and high cost of
MIL-STD-1290A (AV), clearly states the intent for the test articles, it is generally not feasible to perform re-
designer to demonstrate compliance with the various ve- peated tests or a large number of tests for different impact
locity change requirements through analytical methods. conditions. Secondly, the timeframe for the initial publi-
There were likely two main reasons for encouraging the cation of the military standard in the mid 1970’s corre-
sponded with the initial release of KRASH, a kinematic subfloor/tub section or the crushable stage in a landing
lumped-spring-mass crash analysis code [13-14]. During gear. Component crush test data are used to define the
this same time period, a new code, DYNA3D [15], was load-deflection response of the springs.
being developed at Lawrence Livermore National Labs
under sponsorship by the Department of Energy. MSC.Dytran
DYNA3D was an explicit transient dynamic finite ele- MSC.Dytran [16] is a three-dimensional, explicit finite
ment code capable of simulating high-speed impacts. element code capable of analyzing high-speed problems
Later, the public domain version of DYNA3D was ob- involving large deformation of structures and solids. The
tained by commercial vendors who made modifications original DYNA3D command structure was modified such
and now market commercial versions such as that the command structure of MSC.Dytran is similar to
MSC.Dytran [16] and LS-DYNA [17], to name a couple NASTRAN [18]. MSC.Dytran has the capability to
of the many spin-offs. To assess the current state-of-the- model non-uniform gas dynamics and fluid-structure in-
art in computational methods for crash analysis, it is im- teractions. It does this by coupling a Lagrangian proces-
portant to understand the capabilities of each of the com- sor for structural modeling with an Eulerian processor for
monly used codes. Thus, brief descriptions of KRASH, modeling the gas dynamics. In addition, MSC.Dytran
MSC.Dytran, and LS-DYNA are provided in the follow- offers contact elements to handle sliding and frictional
ing subsections. contact of structural elements. The code has been com-
mercially available since 1992 and has been applied to
KRASH several problems related to high-speed impact such as
The Lockheed-California Company developed KRASH airbag analysis, ballistics, blast vulnerability, blast con-
[13-14] under initial sponsorship by the U.S. Army in tainment, ship collision, bird strike, and helicopter crash-
1974. The FAA later supported further development of worthiness.
the code. The most recent public domain version of the
program to be released was KRASH85. The KRASH LS-DYNA
program predicts the response of vehicles to multidirec- LS-DYNA [17] is a general-purpose finite element code
tional crash environments. Structural models are devel- for analyzing the large deformation dynamic response of
oped from massless interconnecting beam elements, con- structures including structures coupled with fluids. The
centrated rigid body masses, and spring elements. The main solution methodology is based on explicit time inte-
beams represent the stiffness characteristics of the struc- gration. An implicit solver is also available. A wide va-
ture between the masses. Plastic deformation is ac- riety of contact definitions are available including self-
counted for through stiffness reductions. The masses can contact, surface-to-surface contact, and node-to-surface
translate and rotate under the influence of external forces contact. Spatial discretization is achieved by the use of
including gravity, aerodynamic, and impact forces; as eight-node solid elements, two-node beam elements,
well as the constraint provided by internal element forces. three- and four-node shell elements, truss elements, mem-
Impact forces are introduced into the model through brane elements, discrete elements, and rigid bodies. LS-
nonlinear external springs attached to the masses. Spring DYNA currently contains over one hundred constitutive
stiffness must be input as a force-deflection table that may models and ten equations-of-state to cover a wide range
be determined through component testing or independent of material behavior. Fluid-structure interaction problems
finite element analysis prior to conducting the KRASH are simulated using Arbitrary Euler-Lagrange (ALE) cou-
analysis. pling. Recently, a Smooth Particle Hydrodynamic (SPH)
method was added to provide additional “hydrocode”
A commercial version of KRASH is available from Dy- capabilities. LS-DYNA is operational on a large number
namic Response Incorporated (DRI-KRASH). DRI- of mainframes, workstations, and PC’s, and can be exe-
KRASH contains several upgrades to the KRASH85 ver- cuted using shared memory processors (SMP), or with
sion including (1) a water impact algorithm, (2) a landing multiple parallel processors (MPP).
gear module, and (3) a severity index for predicting head
injury. In addition, the DRI-KRASH code contains an Illustrative Examples of Test and Analysis Correlation
improved pre-processor for model generation and a post-
processor for data reduction. In order to assess the fidelity and accuracy of analytical
codes, examples are summarized in which the codes de-
In general, KRASH models are relatively easy to put to- scribed previously are used to simulate a full-scale crash
gether, though considerable engineering judgment is re- test. In general, rigorous mathematical methods have not
quired to define the beam stiffness properties. The mod- been used to correlate crash test data and analytical pre-
els are relatively small, consisting of only a few beam dictions, even though a great deal of research is being
elements, masses, and springs, and, consequently, they performed to develop these methods, both in the time and
execute quickly on a personal computer. KRASH simula- frequency domain. In general, analysts typically use
tions rely heavily on experimental data as input to define global parameters to evaluate the level of correlation,
spring stiffness properties. For example, spring elements such as overall deformation and timeline of events, com-
might be used to represent the crushing response of the parison of acceleration and velocity results, and prediction
of discrete structural failures. These illustrative examples of the vertical acceleration response of the main transmis-
highlight typical simulations performed by the authors sion mass is shown in Figure 2.
and others. They are provided to demonstrate the applica-
tion and complexity of the various approaches, as well as
some of the limitations in the predictive capability.

KRASH Simulation of the Sikorsky ACAP Helicopter
Since KRASH was developed under sponsorship by the
U.S. Army and was initially released at approximately the
same time as the military standard for crash resistance, it
is appropriate to assume that the U.S. Army would en-
courage the use of this code by defense contractors in
developing new aircraft and would accept KRASH simu-
lation results as proof of crash performance. An example
will be illustrated. In the late 1970’s, the U.S. Army ini-
(a) KRASH model.
tiated the Advanced Composite Airframe Program

(ACAP) [19-23]. The purpose of the ACAP was to dem-
onstrate the potential of advanced composite materials to
save weight and cost in airframe structures while achiev-
ing systems compatibility and meeting Army require-
ments for vulnerability reduction, reliability, maintain-
ability, and crash resistance. In 1981, the US Army
awarded separate contracts to Bell Helicopter Textron and
Sikorsky Aircraft Corporation to develop, manufacture,
and test helicopters constructed primarily of advanced
composite materials. Each company manufactured three

airframes that were tested under a variety of static and
(b) Photograph of the 1987 drop test.
dynamic conditions to demonstrate compliance with the

program objectives. Crash tests of the Bell and Sikorsky Figure 1. Sikorsky ACAP model and test article.
ACAP static test articles were conducted in 1987 to dem-
onstrate their impact performance and to verify compli-
ance with crash requirements [21], [23].

The Sikorsky-developed KRASH model of their ACAP
helicopter is shown in Figure 1(a), along with a photo-
graph of the test article, shown in Figure 1(b). The model
was developed to aid in the early design process [20]. It
consisted of 53 discrete masses, 23 spring elements, 75
beam elements, and 44 nodes. The spring elements were
used to represent the tires and landing gear stages. The
impact test was performed at 38 ft/s vertical velocity, with
an impact attitude of 10° pitch and 10° roll. The test con-
ditions did not match the MIL-STD-1290A (AV) re-
quirements; however, the vertical velocity selected for the
test matched the vertical velocity requirement in ADS-36.

The correlation between drop test data and the pre-test
KRASH simulation is presented in three categories in Figure 2. Comparison of the vertical acceleration response
Reference [21]. First, a timeline comparison is discussed of the rotor transmission mass.
that shows the timing of key events, such as initial right
landing gear contact, left gear contact, nose gear contact, This example is just one of many that could be illustrated
initiation of honeycomb stroking in the gears, and fuse- in which KRASH predictions showed excellent correla-
lage contact. The correlation of the timing of these events tion with test data, especially in predicting the responses
is very good. Next, comparisons of force and displace- of landing gear, engines, and rotor transmission masses.
ment responses of the landing gear are shown. These Part of the explanation for the high level of agreement is
correlations are also excellent. Finally, the acceleration the fact that experimental data are used in defining the
responses of the rotorcraft transmission mass, engines, properties of spring elements that make up the model. In
and occupants are presented. The test-analysis correlation the case of the Sikorsky ACAP, the landing gear proper-
ties had been determined from a prior drop test of the
landing gear alone [22]. Also, the model was developed sponse. Analytical predictions of the sequence of events,
such that the inertial properties of the test article, i.e. total structural damage, subfloor crushing, and the acceleration
weight and center-of-gravity location, were matched ex- responses of the airframe and large mass items such as the
actly. Given these factors, one would expect that the re- engines and rotor transmission were correlated with the
sponse of the high mass items such as the engines and experimental data to validate the crash simulation.
rotor transmission mass would be well predicted. Once a
model, such as the Sikorsky ACAP helicopter model, is Side and three-quarter views of the helicopter model and
validated, it can be used to predict the response of the impact surface are shown in Figure 3. The final helicop-
airframe to a variety of other impact conditions. The ter model consisted of 4,000 nodes and 7,000 elements,
FAA and Cranfield Impact Centre have been cataloging including 3,000 beam and rod elements, 3,000 quadrilat-
existing KRASH models of different aircraft for use in the eral shell elements, and 1,000 triangular shell elements.
Aircraft Accident Investigation Tool (AAIT) [24]. Crash The impact surface was represented using 250 solid ele-
investigators use AAIT to simulate and reconstruct air- ments. Thirty-four different material property cards were
craft crashes as part of the accident investigation. The used to represent a variety of composite laminates. A
AAIT software package integrates the KRASH structural master-surface to slave-node contact was defined between
analysis code with the SOMTA [25] occupant simulation the structural model and the impact surface.
code into a single environment. AAIT output includes
prediction of overall aircraft motion, sequence of break- To perform the simulation, a two-stage modeling ap-
up, contact marks, forces on occupants, etc. proach was employed in which a rigid structural model of
the helicopter was executed during deformation of the
MSC.Dytran Model of the Sikorsky ACAP Helicopter landing gear. At 0.045 seconds before fuselage contact,
Next, an MSC.Dytran simulation of a full-scale crash test the x-, y-, and z-locations of all grid points and the corre-
of a composite prototype helicopter, the Sikorsky ACAP, sponding nodal velocities in the rigid model were output
is highlighted [26-32]. In 1997, a full-scale crash test of to a file. These initial conditions were then input as the
the Sikorsky ACAP residual flight test article was con- starting point of the flexible model simulation. This
ducted specifically to generate experimental data for cor- rigid-to-flexible approach was used to significantly de-
relation with the MSC.Dytran simulation. An existing crease the CPU time required to complete the simulation,
modal-vibration model of the Sikorsky ACAP helicopter and because the rigid model made the introduction of the
was obtained and converted into a model suitable for the pitch angular velocity easier. The modeling approach is
impact simulations. An external user-defined subroutine described more fully in Reference 28.
was developed to represent the complex landing gear re-

y
x

(a) Side view.

(b) Three-quarter view.

Figure 3. MSC.Dytran model of the Sikorsky ACAP helicopter.
A full-scale crash test of the Sikorsky ACAP flight test ment model of the airframe. Problems arose in trying to
article was conducted in June 1999 [27]. For the crash align the gears and to mitigate the large spike in force that
test, the aircraft was outfitted with two crew and two was input into the finite element model at impact.
troop seats and four instrumented anthropomorphic dum-
mies. The measured test conditions were 38-ft/s vertical
and 32.5-ft/s horizontal velocity onto a rigid impact sur- Acceleration, g
face with 6.25° nose-up pitch and 3.5° left-down roll atti- 150
tude. A pitching angular velocity of 9.6°/s (increasing
nose-up) at impact was measured from film analysis. Test
100
Approximately 120 channels of data from the airframe, Analysis
seats, and dummy occupants were collected at 10,000
samples per second. Pre- and post-test photographs are 50
shown in Figure 4.
0
-50
-100
0 0.05 0.1 0.15
Time, s

(a) Pilot floor.
(a) Pre-test photograph.
Acceleration, g
100
80
60 Test
Analysis

40
(b) Post-test photograph.
20
Figure 4. Pre- and post-test photographs of the Sikorsky
ACAP residual flight-test hardware.
0

The test-analysis correlation of the full-scale crash test of -20
the Sikorsky ACAP helicopter consisted of a comparison
-40
of a timeline of major events, a comparison of predicted
0 0.05 0.1 0.15
and observed structural damage, and acceleration and
Time, s

velocity time history comparisons at nine different loca-
(b) Right troop floor.
tions on the airframe. In general, reasonably good agree-
ment was obtained. The simulation predicted the location Acceleration, g
and amount of maximum crushing in the subfloor and
50
failure of one of the beams used to support the rotor
transmission overhead mass. The simulation predicted 40 Test
the timing of major events within ±0.007 seconds. How- Analysis
30
ever, because a two-stage modeling approach was used in
which the structural model was simulated using rigid ma-
20
terial properties for the first 0.045 s, the failure of the tail,
seen in Figure 4(b), was not well predicted. Comparisons 10
of selected test and analysis filtered acceleration re-
0
sponses are plotted in Figure 5. These results show rea-
sonable correlation, though the magnitude and/or timing
-10
of the peak accelerations may not agree exactly.
-20
0 0.05 0.1 0.15
The modeling of the Sikorsky ACAP full-scale crash test
Time, s
illustrated several important issues regarding the success-
(c) Right engine.
ful application of nonlinear, explicit finite element codes.

The first issue involved modeling of the landing gear.
Figure 5. Comparisons of selected vertical accelera-
Considerable time was spent attempting to couple a
tion responses for the ACAP crash test.
mechanistic model of the landing gear to the finite ele-
A two-stage modeling approach was implemented for this
simulation that worked well. However, improved meth-
ods are needed to allow coupling of a mechanistic model
of a landing gear with a full FEM of an aircraft or
helicopter. Another issue involved modeling of
composite material failure. Fairly simplistic failure
models, such as maximum stress and/or maximum strain
were used. Recently, the capabilities for modeling
composite material failure using explicit transient
dynamic codes have greatly improved. Finally, the
transition from a modal-vibration NASTRAN model to an
explicit nonlinear transient dynamic model was not easily
accomplished and required several months’ effort. Many
of the material property definitions did not translate. In
the modal-vibration model, all inertial properties were
simulated using concentrated masses. Consequently, new (a) Pre-test photo.
material cards had to be defined for the MSC.Dytran
simulation including material density. Since the mesh of
the original modal-vibration model was too detailed in
some regions and too coarse in other regions,
considerable rediscretization was required. In general, the
process of converting one type of model, e.g. a structural
loads or modal-vibration model, into a crash simulation
model is not an easy or straightforward task.

LS-DYNA Fluid-Structure Interaction Simulation
In this section, an LS-DYNA simulation of a vertical drop
test of a composite fuselage section performed onto water (b) Photograph taken just after impact.
is highlighted. Understanding multi-terrain impacts and
Figure 6. Photographs of the composite fuselage section
how to model them is important because helicopters
and drop test.
rarely crash onto hard prepared surfaces.

Post-test examination of the subfloor region revealed ex-
The U.S. Navy is concerned with the crash safety of its
tensive damage to the outer skin. The center portion of all
helicopter fleet due to the large number of crashes that
five foam blocks that were designed for energy absorption
occur on water. As a result, the Navy has sponsored re-
for a rigid surface impact showed no sign of crushing, and
search programs to improve the crash safety of rotorcraft
there was also very little debonding of the face sheets
during water impacts. In general, the structural response
from the foam. A post-test view of the bottom of the sec-
and load transfer mechanisms for water and soft soil im-
tion taken from the front is shown in Figure 7, in which
pacts are much different than those on hard surfaces.
the five foam blocks in the subfloor can be distinguished.
Also, very little research exists on the crashworthy re-
The unsupported areas of the outer skin between the foam
sponse of helicopters impacting into water as opposed to
blocks showed the most damage. No damage was ob-
hard surfaces. The MIL-STD-1290A (AV) does not con-
served to the floor and upper fuselage cabin region.
tain any specifications for water impact. However, the

Navy does have water-ditching requirements. References
A number of different LS-DYNA models were created to
33 through 38 describe a few recent research studies in-
represent the air and water fluid regions. Both rectangular
volving simulations of water and soft soil impact. An
and cylindrical Euler meshes were created, and the size of
example of one of these studies, documented in Reference
the mesh was varied from constant 3 and 6-inch cubic
35, is summarized below.
meshes to a refined 1-in. mesh with a gradient. Also, a

smooth particle hydrodynamic (SPH) model of the water
A 25-ft/s vertical drop test of a 5-ft. diameter, 5-ft. long
was developed in LS-DYNA. Although the SPH model
composite fuselage section [40] was conducted into a 3.5-
gave good results, it was somewhat slow in execution. A
ft. deep, 15-ft. diameter pool of water. The empty fuse-
description of the various water models and their resultant
lage section weighed 208 pounds. The fuselage section,
predictions can be found in Reference 35. The LS-DYNA
outfitted with instrumentation, seat rails, and ten 100-lb.
coupled fluid structural algorithm (ALE/Euler) has the
lead masses attached to the floor, weighed approximately
capability to allow the bottom skin of the fuselage, which
1,200 pounds. A photograph of the composite fuselage
is the coupling surface, to fail. Models with and without
section is shown in Figure 6(a). A frame from a video
failure of the bottom fuselage skin were created and com-
taken during the water impact test is shown in Figure
pared.
6(b).
flow through the areas formed by the deleted failed ele-
ments. However, partially due to the coarse elements in
the bottom skin, the failure was more severe in the model
than observed in the test.

Figure 7. Post-test photograph of the bottom of the fuse-
lage showing damage. The four dark regions are areas
between the foam blocks.

(a) Gradient mesh.
In order to study the failure of the bottom skin, an LS-
DYNA model was created with a refined 1-in fluid mesh
directly under the section, which became coarser further
from the section, as shown in Figure 8(a). The failure
strain was set on the material card for the bottom fiber-
glass skin to allow the elements to fail after a given strain
is reached. As failed elements were deleted, holes formed
in the bottom surface that allowed the water to flow
through the failed skin. The failure of the bottom skin is
shown in Figure 8(b) for 0.01 seconds after impact. The
figure shows the bottom skin of the fuselage viewed from
an angle from above. In this case, the failure strain was
set to 2 percent, which is a practical value for an angle-ply
fiberglass laminate. The results show that the outer skin
between the foam blocks fails catastrophically allowing (b) Lower skin failures.
the water to flow through as shown in the right side of the
Figure 8. Front view of a slice of the gradient-mesh.
figure. Although the failure is dramatic, the initial peak

accelerations were only reduced by a small amount from
Future Directions
the original model without failure as shown in Figure 9.

Note that both the analysis and test data were filtered with
The future directions needed to achieve crash certification
a low-pass filter. Since the run times for these models are
by analysis are discussed in this section and they are di-
long, the model with failure was only executed long
vided into two subsections: 1. Crash Qualification Re-
enough to capture the fundamental pulse; i.e., 0.04 sec-
quirements and 2. Improved Analytical Methods.
onds. The amount of damage predicted by this simulation

was more severe than observed.
Crash Qualification Requirements
It is highly recommended that the U.S. military establish a
Acceleration responses are shown in Figure 9 at two loca- new military standard for crash resistance of rotary wing
tions for LS-DYNA simulations with and without failure aircraft. This standard should focus on a systems ap-
of the bottom skin. The acceleration pulses with failure proach to crashworthiness [6], [41] in which subcompo-
of the bottom skin drop off too quickly after the initial nents are utilized to lower the acceleration levels transmit-
peak due to the excessive failure. Since the actual strain- ted to the occupants to a humanly tolerable level during a
to-failure data for the angle-ply laminate was not avail- helicopter crash. These subcomponents include the land-
able, the objective of specifying a failure strain was to ing gear, the subfloor, and the seats. In addition, the
determine the effect of failure on the simulation. Also, cabin structure must be designed with the proper stiffness
note that when the failure strain criterion was met, the and strength to maintain a livable volume for the occu-
elements were deleted. Other failure options available in pants, to prevent failure of the seat attachments, and to
LS-DYNA such as “constrained tied nodes failure” were retain the high mass items such as the rotor transmission
not evaluated, but may reduce the severity of the damage. and engine. Interior hazards such as the flight control
A finer mesh would be another option; however, a mesh- stick and instrument panel should be minimized by pro-
density study of the bottom skin was not performed. viding adequate restraint systems and/or using cockpit
When failure strains were applied to this model, water did airbags. Finally, post-crash hazards should be mitigated,
allowing efficient occupant egress. These items were In addition, crash qualification criteria should be written
included in MIL-STD-1290A (AV) and should be in- requiring multi-terrain impact testing and simulation. The
cluded in any new proposed standards. However, upon accident data indicate that 80% of all helicopter crashes
reviewing MIL-STD-1290A (AV), it is apparent that the occur on multi-terrain including soil and water [33]. Con-
performance requirements are written to specify limits on sequently, the importance of multi-terrain cannot be ig-
reduction in cabin volume. There are no specifications nored, especially when research studies have shown that
for human tolerance to impact loading. If a new qualifi- helicopters, designed for crash resistance onto hard sur-
cation standard is written, it should include performance faces, do not perform well during multi-terrain impacts
requirements that ensure occupant survivability, such as [34-35, 37-39]. Also, landing gear are completely inef-
limits on Head Injury Criteria (less than 1000), lumbar fective as energy absorbers during water impact. The
loading (less than 1,500 lbs.), and Dynamic Response requirement for multi-terrain crash resistance would in-
Index of 20 or less based upon seat pan vertical accelera- crease the need for novel energy absorbing structural de-
tion responses, and comparison of occupant acceleration sign concepts. In addition, external devices such as air
responses with the whole body acceleration tolerance bag systems could be used to augment the crash perform-
curves developed by Eiband. A complete assessment of ance of helicopters during water or soft soil impacts.
these occupant injury criteria is described in Reference
32. To be effective and minimize cost and weight, a variable
Acceleration, g design requirement must be written based on helicopter
type, mission, weight, and autorotative rate of descent [6].
40
Front left inboard (ch10) The military standard defines one set of design criteria for
LS-DYNA, no fail
all military helicopters. However, accident data show that
30 LS-DYNA, 2 percent failure strain
it is easier for medium- to large-size helicopters to meet
the requirement than it is for small-size helicopters.
20
There is also the issue of weight penalty associated with
crash design features. The “one-size-fits-all” approach
10 penalizes the small-size helicopter because a larger per-
centage of its gross weight is dedicated for crash resis-
0 tance. Consequently, a variable design approach is re-
quired that takes into account a helicopter’s type, size,
-10 mission, and autorotative rate of descent.

Finally, the issue of testing versus analysis must be ad-
-20
0 0.02 0.04 0.06 0.08 dressed. It is the opinion of the authors that analytical
Time, s methods are not entirely sufficient to qualify or certify

rotorcraft for crash resistance at this time. Therefore, it is
(a) Front left inboard.
recommended that at least one full-scale crash test be per-

Acceleration, g formed for qualification purposes. The test should be
40 conducted with combined vertical and forward velocity
Rear left inboard (ch18) components and should be performed onto an impact sur-
LS-DYNA, no fail
face that would be most representative of the type of crash
30 LS-DYNA, 2 percent failure strain
the helicopter would likely experience in the field, based
on its mission. The crash test would serve to qualify the
20 helicopter and would provide test data for validation of a
three-dimensional finite element simulation. Conse-
10 quently, the test article should be outfitted with seats,
dummies, and restraint systems, and any other system that
is intended for occupant protection. The helicopter
0
should be instrumented to accomplish both tasks, i.e. cer-
tification for crashworthiness and test-analysis correla-
-10
tion. Once the model is validated for the test condition, a
probabilistic analysis should be executed to determine
-20 other possible impact conditions or surfaces where the
0 0.02 0.04 0.06 0.08
aircraft might be vulnerable, i.e., the validated model
Time, s should be utilized to perform a complete vulnerability/risk

assessment of the design.
(b) Rear left inboard.

Figure 9. Filtered experimental acceleration responses Improved Analytical Methods
compared with LS-DYNA predicted inboard accelerations Attendees of the Workshop on Computational Methods
with and without failure of the bottom fiberglass skin. for Crashworthiness [42], held September 2-3, 1992, at