Development, Validation, and Application of the

DynafaceR© Helicopter/Ship Dynamic Interface

Simulation Software Package

Dr. Robert G. Langlois, Senior Dynamicist, rlangloi@mae.carleton.caMichael LaRosa, Aerospace Engineering Analyst, michael larosa@indaltech.comDr. Atef R. Tadros, Director of ASIST Engineering, atef tadros@indaltech.com

Indal Technologies Inc., Mississauga ON Canada

Keywords

Dynamic interface analysis, shipboard operation, he-licopter, ship

Abstract

The importance of safely operating helicopters frommoving ships in potentially-severe sea conditions iswidely recognized. In many cases, particularly onsmall military and coast guard vessels, helicopter op-erability is maximized by the use of systems that as-sist with shipboard helicopter recovery and on-deckhandling. Regardless of the level and type of air-craft securing provided, safe shipboard operation re-quires detailed understanding of the effects of shipmotion on the embarked aircraft. It is necessaryto determine the limits for safe operation in termsof operational and environmental conditions, the se-curing requirements if appropriate, the factors thatcompromise the safety of operation, and the long-term impact of shipboard loading on the design ofaircraft and securing equipment. To assist with theanalysis of the dynamic interface that exists betweenships and embarked aircraft, Indal Technologies Inc.(ITI), a developer of helicopter handling equipment,has developed, extensively validated, and applied theDynaface R© aircraft/ship dynamic interface analysissimulation software package. This paper outlines thedevelopment and basis of the mathematical model,validation activity, and the various ways in whichDynaface R© can be applied to enhance the analysisand ultimately the safety of shipboard aircraft opera-tion. Sample analysis applications are presented anddiscussed. It is shown that the simulation package issuitable for supporting engineering analysis and de-sign, simulation-based acquisition, operational plan-ning, and incident investigation.

Introduction

The importance of safely operating helicopters frommoving ships in potentially-severe sea conditions iswidely recognized. Critical applications exist rang-ing from medical evacuation of personnel from civil-ian vessels, to the role of the helicopter as the pri-mary weapons platform on many military ships. Inorder to fulfill diverse roles, shipboard helicoptersmust be operable in the greatest range of sea condi-tions possible. In many cases, particularly on smallmilitary and coast guard vessels, helicopter operabil-ity is maximized by the use of systems that assistwith shipboard helicopter recovery and on-deck han-dling. These systems vary in complexity rangingfrom manually-applied chain lashings to completelyautonomous systems that recover, secure, and tra-verse shipboard aircraft without strictly requiringany personnel on the ship deck. Regardless of thelevel and type of aircraft securing provided, safe ship-board operation requires detailed understanding ofthe effects of ship motion on the embarked aircraft.It is necessary to determine the limits for safe opera-tion in terms of operational and environmental con-ditions, the securing requirements if appropriate, thefactors that compromise the safety of operation, andthe long-term impact of shipboard loading on thedesign of aircraft and securing equipment. To assistwith the analysis of the dynamic interface that ex-ists between ships and embarked aircraft, Indal Tech-nologies Inc. (ITI), a developer of helicopter handlingequipment, has developed, extensively validated, andapplied the Dynaface R© aircraft/ship dynamic inter-face analysis simulation software package[1]. Thispaper outlines the development and basis of theDynaface R© mathematical model, validation activity,and the various ways in which the Dynaface R© soft-ware package can be applied to enhance the analysisand ultimately the safety of shipboard aircraft oper-

ation.The component programs that form the

Dynaface R© package are identified by solid blocksin Figure 1. Elements in dashed blocks are linear

Figure 1: Structure of component programs in theDynaface software package

frequency-domain programs that generate responseamplitude operators (RAOs) characterizing shipmotions that form the basic input to all forms ofdynamic interface analysis for moderate to severesea conditions. The other blocks in the first columnin Figure 1 post-process the output data fromSHIPMO[2] and SMP[3] into a form appropriatefor use with the simulation package. The secondcolumn of programs generates time series and statis-tical data describing the continuous and peak shipmotions at various shipboard locations as well as themagnitudes of equivalent acceleration parametersthat provide effective indications of the severityof flight deck conditions as they affect shipboardhelicopter operation[4]. The Dynaface R© simulationprogram at the centre of the diagram forms the coreof the package and consists of a 15-degree-of-freedommathematical model and time-domain solution ofthe response of the helicopter to ship motions.Program blocks to the right of the central blockprovide statistical, animation, and other analyticaltreatments of the generalized displacement and forcedata that results from the Dynaface R© simulationprogram.

Subsequent sections of this paper describe thedevelopment, validation, and application of theDynaface R© package.

DynafaceR© Development

Governing Dynamics

Figure 2 shows a typical embarked helicopter securedto the deck by a rapid securing device (RSD). The

RSD is part of an ITI Aircraft/Ship Integrated Se-cure and Traverse (ASIST) system that secures thehelicopter from a helicopter-mounted probe as shownin Figure 3[5]. The objectives of on-deck dynamicinterface simulation are to mathematically representthe in-service aircraft and ship system with sufficientfidelity to gain insight into the dynamic interface be-haviour yet also maximize simulation speed such thatvery large numbers of simulation cases can readily beinvestigated within the scope of a single dynamic in-terface study.

Figure 2: Image of a typical shipboard helicoptersecuring condition

Figure 3: Image of the ITI ASIST securing system

Dynaface R© includes a special-purpose 15-degree-of-freedom mathematical model of the aircraft/shipsystem. The degrees of freedom comprise threetranslations and three rotations for the ship, threetranslations and three rotations for the aircraft body,and one prismatic or revolute degree of freedomper suspension station depending on the suspensiontype. Forces acting on the aircraft portion of thesystem include deck reaction forces, securing forces,aerodynamic forces, inertial forces, and gravitationalforces. A total of seven primary coordinate systems

are used to derive the equations of motion: an iner-tial frame, a ship frame, an aircraft frame, a rotor tippath plane frame, and wheel frames corresponding toeach suspension station (maritime aircraft normallyhave at least one steerable or castorable wheel). Allsuspension, external, and securing forces are mod-elled, analytically or empirically, depending on thequality and availability of data, and the resultingequipollent forces and moments are evaluated andapplied through Newton-Euler equations. While thesimulation is special-purpose to promote solution ef-ficiency, it includes sufficient generality such that alarge variety of aircraft and virtually all ships canreadily be modelled. The simulation currently con-tains cantilever and leading/trailing arm suspensionmodels having up to two wheels each that can beattached to the fuselage in either nose-wheel or tail-wheel configurations, up to two main rotors, and alarge variety of securing devices. The model includesdetailed representation of the oleo stiffness, damping,and friction characteristics; induced rotor forces; anda nonlinear tire model that supports complex tire be-haviour including lift-off and touch-down, rolling dueto suspension travel, brake slippage, and sliding.

Computationally, speed is maximized by remov-ing physically impossible discontinuities from modelcharacteristics, carefully controlling coupling be-tween model degrees of freedom, and carefullymatching the numerical integration with the equa-tion structure. These considerations have led to asimulation that meets the objectives of accuracy andspeed.

Suspension Systems

Two widely used suspension station configurationsimplemented in the model are cantilever and lead-ing/trailing arm suspensions shown schematically inFigures 4 and 5 respectively. Each adds one degree offreedom (linear or angular) to the model per station.The governing equations are developed in the air-craft frame with suspension stations treated as forceproducing devices.

Investigation has demonstrated that while the air-craft body and suspension stations are kinemati-cally coupled, mass coupling is very weak. Conse-quently, treating suspension stations as force pro-ducing elements rather than tightly coupling therespective equations reduces the solution complex-ity and high frequency content and correspondinglyimproves speed performance without affecting accu-racy.

The dominant passive suspension element is a gasoleo that generates stiffness, damping, and frictional

Figure 4: Schematic representation of cantilever sus-pension

Figure 5: Schematic representation of a trailing armsuspension

forces in response to relative displacements, veloci-ties, and transmitted forces and moments across theelement. Oleo stiffness is modelled using the idealgas law for the primary compression region and astiff linear spring for extension. A continuous anddifferentiable transition between the two regions isachieved using a cubic polynomial. This is illustratedschematically in Figure 6 though the extent of thetransition region is very much exaggerated for clarity.Complex oleos may involve additional stiffness stagesthat are appended to the oleo stiffness characteristic.The nonsymmetrical damper design and possible in-clusion of pressure relief valves necessitates that amultistage damping model be used with a dampingforce in each region described as a nonlinear functionof velocity for velocity-dependent hydraulic dampersor displacement in the case of oleos containing me-tering pins. The transition velocities between regionsvary with time because the pressure relief valve ac-tuation may depend on the total transmitted force.A modified friction model[6] is used to evaluate theoleo friction force

Ff = (q̇/|q̇|) Ff max (1 − exp (−α|q̇|)) (1)

where α is the decay rate of the modified frictionmodel, q is the suspension station configuration co-ordinate, and Ff max is the maximum possible oleofriction force comprised of oleo seal and normal forcecontributions

Ff max = Ff seal + µFN (2)

Leading/trailing arm suspensions include additionalfriction resulting from angular motion through atleast three joints, each introducing friction that isrelated to the joint reaction force. Suspension testresults indicate that in some cases frictional con-tributions to the total suspension force are similarin magnitude to the stiffness contribution. Conse-quently, suspension friction must be modelled veryaccurately to achieve dynamic results that are rep-resentative of the in-service aircraft.

Figure 6: Schematic representation of the oleo stiff-ness model

Tire forces are calculated assuming vertical com-pression and tire design-dependent stiffness in thelongitudinal and lateral directions[7] and a multi-stage cubic stiffness in the vertical direction. Linearviscous damping is assumed in all three componentdirections. However, additional complicating factorsexist in evaluating tire forces. These relate to the tirecontact condition. First, when the tire loses contactwith the deck, the tire ‘contact’ point tracks the pro-jected touchdown point. In this way, residual tire de-formation is released when a tire lifts off and does notexist initially upon touchdown. This is illustratedschematically in Figure 7. Second, tire sliding occurswhen the resultant of the longitudinal and lateralforces exceeds the instantaneous allowable frictionalforce. Third, suspension kinematics in the case ofleading/trailing arm suspensions couple the tire con-tact point to suspension compression. Fourth, under

severe securing conditions, the wheel brake slip lim-its can be reached leading to brake slippage and tirerolling. The inter-relationships between these phe-nomena motivate the need for a sophisticated tiremodel specifically designed for the dynamic interfaceproblem.

Figure 7: Schematic representation of tire behaviourduring intermittent tire contact

Aerodynamics

Aerodynamic forces acting on the aircraft result fromaerodynamic drag and rotor induced forces and mo-ments. Aerodynamic drag is calculated based on theequivalent frontal and side areas of the aircraft fuse-lage and the relative wind speed. The rotor thrustis modelled using a constant thrust value during thedescent phase of the touchdown transient followed bydecaying rotor thrust as the pilot reduces the rotorcollective to its minimum. This optional decreasingthrust can be triggered by the first wheel contactwith the deck. In the case of maritime aircraft, evenwith the rotor at its minimum collective, ship motiongenerates an angle of attack of the rotor disc relativeto the apparent wind. This effect is highlighted bythe flow visualization presented in Figure 8. Test-ing has demonstrated that the rotor-induced thrustcan reach 30% of the aircraft weight for the casewhere the rotor collective is at its minimum. Conse-quently, potentially large rotor forces and momentscan be developed. These are evaluated continuouslythroughout the simulation based on helicopter man-ufacturer rotor data and the instantaneous wind con-ditions and angle of attack. Testing in this area hasbeen performed at the National Research Council ofCanada Institute for Aerospace Research in collabo-ration with ITI[8]. Further experimentation is cur-rently underway. The complexity of air flow over andaround ships is a growing field attracting consider-able research effort[9].

Securing Systems

The dynamic interface simulation model provides ca-pability to simulate a wide variety of existing andproposed passive and active securing devices that

Figure 8: Flow visualization of air flow over a typicalfrigate flight deck

may operate independently or in combination. Pas-sive securing systems are those in which one or morestructural members fitted to the helicopter and fixedto the ship react the helicopter loads, restraining thehelicopter from excessive movement and transferringthe loads into the ship’s structure. Securing is lim-ited only by the strength of the securing member(s)and the supporting structure. Examples of systemsfalling into this category are the ITI Recover As-sist Secure and Traverse (RAST) system, the ITIASIST system, landing gear securing devices, andnon-pretensioned lashing cables. Active securing sys-tems are those in which a mechanical/hydraulic de-vice, fitted to the helicopter and attached to the ship,continuously applies a force in an effort to createsufficient friction to prevent tire sliding. Securing islimited by the magnitude of the force, the extensionof the securing element(s) under load, the landinggear capacity, the tire deflection limits, and the deckcoefficient of friction. Examples include a varietyof hydraulically-controlled pretensioned link systemsand tensioned lashing cables.

Solution Strategy

The objective of the computer simulation is to prop-agate the dynamic solution for the aircraft motionsand securing forces forward in time. The general ap-proach that is adopted for this purpose is describedin the following procedure.

1. Evaluate the prescribed ship motion (displace-ments and velocities) at the current simulationtime.

2. Evaluate internal forces developed by suspen-sion stations and securing devices and expressthem as equipollent forces and moments basedon the prescribed ship motion and aircraft statevector known from the previous time step.

3. Evaluate externally applied forces resultingfrom gravity, aerodynamic drag, and rotor forcesand express them as equipollent forces and mo-ments.

4. Evaluate the time derivative of the system statevector. The derivatives of velocities are deter-mined using Newton’s law for aircraft linear ve-locities, Euler’s equation for aircraft angular ve-locities, and the suspension station governingdynamic equations for the relative wheel ve-locities. The time derivatives of the configu-ration coordinates are simply set equal to thecorresponding velocities known from the previ-ous time step.

5. Numerically integrate the composite derivativevector using an adaptive time step integrationalgorithm to yield the displacements and veloc-ities at the next time step.

This procedure is repeated for the duration of thesimulation or until user-specified limits are exceeded.Limits that are checked include relative aircraft an-gular motions, axial oleo forces, vertical tire forces,and lateral tire deflections.

The solution can also be run with subsets of the15 available degrees of freedom to facilitate special-ized types of analysis such as validating suspensionmodels and simulating helicopters with gagged oleos.

The formulation is implemented in ITI’s pro-prietary simulation software package Dynaface R©.Based on descriptions of the ship, aircraft, and op-erating environment, the model predicts general-ized displacements and generalized forces as a func-tion of time. These generalized output values in-clude aircraft relative angular displacements, secur-ing forces, landing gear reaction forces, suspensionforces, tire deflections, induced aerodynamic forcesand moments, and animation data.

Figure 9: Sample animation frame indicating one ofseveral optional graphical post-processing options

DynafaceR© Validation

Development of the Dynaface R© simulation has em-phasized the importance of accurately predictingthe interface parameters between an embarked he-licopter and ship. To achieve this, the simulationmodel input data consists of a combination of theo-retical aircraft design data, experimentally-measuredsuspension data, empirical tire data, and externally-calculated or measured rotor data. In all cases, theinformation source was selected based on optimiz-ing the quality of the input data. Therefore, theinput data source to some extent influences the rela-tive importance of verification and validation at thesimulation component level. However, during devel-opment, each component of the Dynaface R© simula-tion was both verified and validated at the compo-nent level and once assembled into the full simula-tion. Validation activity comprised a combinationof comparisons with analytical solutions, compar-isons with other simulation results, comparisons withjig suspension drop test data, and comparisons withboth land-based and sea trial full vehicle experimen-tal data.

The most complex element in the complete sim-ulation is the landing gear model. For this reason,extensive validation activity focussed on this elementof the model. During landing gear design and tun-ing, designers routinely conduct an extensive experi-mental drop test program whereby a large volume ofdata is collected. The experiment involves mountingthe full-scale landing gear in a device that allows thelanding gear to translate downward from its fully-extended condition with a predetermined initial sinkspeed. The landing gear then experiences the touch-down transient and settles to its static condition.This type of experiment is performed for a range ofinitial sink speeds and a range of helicopter weights.The ground reaction force and suspension compres-sion are two variables that are usually measured withrespect to time in these tests. This data providesan excellent opportunity for validating the landinggear and tire elements of the simulation. The dataalso provides a means for validating the landing gearmodels of specific aircraft prior to using them for dy-namic interface analysis. Figure 10 shows a sampledrop test validation result for a cantilever main land-ing gear suspension, where the simulated and mea-sured ground force is plotted versus time. Drop testvalidation of this type has been performed for a largenumber of aircraft having both leading/trailing armand cantilever type suspensions and has shown ex-cellent agreement between measured and predictedresponses similar to what is seen in Figure 10.

Figure 10: Comparison between experimental andsimulated drop test data for a typical cantilever mainlanding gear suspension

The most direct, and arguably most comprehen-sive, validation of the complete simulation resultsfrom comparing the simulated aircraft response withthe measured aircraft response during an actual seatrial. Figure 11 shows such a comparison for amedium-sized tail dragger helicopter operating ona typical frigate in severe sea conditions. The plotcompares the simulated and measured relative rollangle between the aircraft and ship in response tomeasured ship motion. The comparison shows thatthe simulation captures the behaviour of the actualshipboard aircraft though some differences do exist.This does not reflect a limitation of the simulationbut rather highlights the difficulty associated withattempting to perform detailed validation using ex-isting data collected in a relatively uncontrolled en-vironment. Two main difficulties exist with mostavailable sea trial data. First, ship and helicoptermeasurements are rarely perfectly synchronized; andsecond, the exact aircraft configuration and prevail-ing environmental conditions are often not available.The response of an aircraft depends significantly onthe inertial properties of the aircraft, its orientationon the ship, and the exact wind conditions (as a func-tion of time) to which it is subjected. As this datawas not available for the sample case presented inFigure 11, nominal aircraft parameters and a steadywind were assumed thereby likely accounting for thedifferences observed in Figure 11. An unrecordedwind gust could easily account for the differences be-tween the measured and simulated values in the areaof 37630 seconds. While sea trial data is representa-tive of actual operating conditions, in the dynamicinterface analysis application, unless very carefully-conducted dedicated sea trial experiments are per-formed, the most rigorous validation data is obtained

from experiments conducted in carefully-controlledenvironments.

Figure 11: Comparison between measured and sim-ulated sea trial data for a typical medium-sized taildragger helicopter operating on a typical frigate insevere sea conditions

DynafaceR© Application

Dynaface R© was developed as a versatile analysistool. Since it’s initial development, significant expe-rience has been gained with its use. This section de-scribes the process of individual helicopter input filedevelopment from data typically provided by aircraftmanufacturers. It also describes the process involvedin performing several important types of dynamic in-terface analysis for which the Dynaface R© package iswell suited.

Model Development

In using the simulation, the aircraft and ship config-urations, environmental conditions, and simulationcontrol parameters are specified in a detailed set ofinput files. The simulation uses this information todescribe the physical system. Ship motion, whichis the dominant excitation for the aircraft/ship sys-tem is either input as experimentally measured seatrial data or developed from linear frequency-domainresponse amplitude operators (RAOs). The simula-tion then generates the time-varying prescribed shipmotion and propagates a time-domain solution bynumerically integrating the governing Newton-Eulerequations of motion for the system. An exhaus-tive set of optional results; including aircraft rel-ative angular displacements, securing forces, land-ing gear reaction forces, suspension forces, tire de-flections, induced aerodynamic forces and moments,and animation data; are saved in a selected subsetof 23 available output files. Simulation results are

post-processed by a suite of utility programs or ani-mated using either two- or three-dimensional anima-tion software tools.

Securing Analysis

Securing analyses are typically performed to identifyhelicopter securing requirements as a function of he-licopter configuration, ship operating conditions interms of ship heading and speed, and environmentalconditions including sea state, wind speed, and winddirection. Typical helicopter parameters that canvary throughout an analysis include: aircraft weight,rotor status, brake status, nose/tail wheel orienta-tion, aircraft alignment with respect to the ship’scentreline, aircraft location on the ship such as onthe flight deck or in the hangar, and type of securingsystem. Also, the type of helicopter embarked oper-ation can vary; possibilities include: free-deck analy-sis, on-deck securing, aligning for hot refuelling andrearmament, straightening, traversing, and hangar-ing. Consideration of all permutations can lead to avast number of simulation runs for which Dynaface R©

is well suited due to the speed at which it is able toperform the simulation runs. Figure 12 shows a typ-ical polar plot of how securing force requirementsvary with significant wave height and ship heading.

Figure 12: Sample result indicating the variation ofpeak securing loads with ship heading and significantwave height (4 m for inner trace, 5 m for middletrace, and 6 m for outer trace)

Upon selecting the helicopter configurations, shipoperating conditions, and environmental conditionsthat will vary throughout the analysis, Dynaface R©

is typically run for short time periods (40 seconds)around the occurrence of a peak ship motion event.As indicated earlier, the user is able to select from 23

output files including: landing gear forces, securingforces, suspension forces, deflections, and velocities,tire forces and deflections, aircraft displacements, ve-locities, and accelerations, and aerodynamic forcesand moments. Depending on the type of secur-ing analysis being performed, the complete set ofDynaface R© output files need not be outputted. Fora typical securing analysis, the critical data to beextracted from a simulation runs are the securingloads, landing gear loads, aircraft relative displace-ments, and change in tire contact position to identifywhether the aircraft has slid or slewed. A statisticalanalysis can be performed to identify the maximum,minimum, and frequency of occurrence of each pa-rameter. These loads and displacements can then becompared against specified design limits and used todevelop helicopter operating envelopes as a functionof ship heading and speed. Figure 13 shows a typi-cal helicopter operating envelope indicating the shipheading and speed combinations where the aircraftwithout a securing system has slid on-deck.

Figure 13: Ship headings where an unsecured heavyaircraft does not satisfy a securing definition in seascharacterized by both 4 metre (right) and 6 metre(left) significant wave heights

Dynaface R© can be used early in the design cy-cle to address helicopter/ship compatibility issuessuch as identifying whether a securing system is re-quired. If a securing system is required based onthe expected theatre of operation, simulations can beconducted using Dynaface R© to compare the relativeperformance of various helicopter securing and han-dling systems. Alternatively, for a given helicopterand ship combination, Dynaface R© could be used tosafely expand current operations. For example, mosthelicopter/ship operations are limited to sea state 5

conditions. Dynaface R© can quickly identify specificship heading and speed combinations that will notexceed helicopter limits in higher sea states there-fore expanding current capabilities.

Fatigue Analysis

The continuous nature of shipboard helicopter load-ing, variability of loading conditions, potential mag-nitude of securing forces, and anticipated number ofload cycles often motivates detailed fatigue analysisof securing system elements and aircraft structureto which the aircraft portion of a securing systemis mounted and through which shipboard securingloads are transmitted.

The first essential step in any any fatigue method-ology is quantification of the dynamic loading actingon the securing device (if present), the landing gear,and consequently on the aircraft structure. The dy-namic loading is dependent on three main factors:aircraft and securing system design in terms of geo-metrical, inertial, and stiffness parameters; sea con-ditions; and operational factors such as ship head-ing and speed relative to the principal sea direction.Detailed nonlinear transient dynamic simulation ofthe aircraft response to environmental conditions isideal for this step in the overall process as it providesa means of exploring the full parameter space priorto detailed design of the securing and aircraft sys-tems. Extended periods of time-domain data is gen-erated by Dynaface R© to ensure statistically repre-sentative loading and this data is subsequently post-processed using rainflow counting of load cycles lead-ing to fatigue spectra[10]. Due to the potentially-large amount of simulation involved in consideringall permutations of aircraft configurations; phases ofon-deck operation including securing, manoeuvring,traversing, and hangaring; and ship operating condi-tions; it is often possible to base fatigue analysis on areduced set of representative operational cases withassociated probabilities of occurrence. Figure 14shows a typical fatigue loading result indicating thenumber of loading cycles per hour of operation as afunction of ship heading in a particular sea state.

Once the dynamic loading has been establishedusing dynamic interface simulation, the remainderof the analysis can be performed analytically, experi-mentally, or using a combination of both approaches.As an example, a methodology combining dynamicinterface analysis, detailed structural analysis andvalidation, and full-scale static and fatigue testingwas developed and implemented to structurally sub-stantiate the installation of the ITI RAST securingsystem in an existing Kaman SH-2G(A) Super Sea-

Figure 14: Sample result indicating securing deviceradial loading per hour of operation in sea state 5conditions

sprite helicopter[11].

Clearance Analysis

Shipboard aircraft are typically fitted with equip-ment and instrumentation ranging from cargo hooksto sonar domes and an expansive set of antennae.As the result of ship motion and the correspondingreaction of the aircraft suspension, relative motiondevelops between the aircraft and ship. Static andquasi-static analyses are insufficient to address theseissues and do not take into account the completekinematics of the both the aircraft, in terms of land-ing gear suspension and tires, and the ship. It is con-sequently important to investigate clearance issuesbetween aircraft-mounted and ship-mounted equip-ment through the use of transient dynamic analysisto avoid any potential interferences.

Throughout the life expectancy of a helicopter,upgrades are typically made to expand and mod-ernize the helicopter’s capabilities and thus may in-clude changes to aircraft components such as radardomes, landing gears, etc. These changes may cre-ate clearance problems between the aircraft and ex-isting ship-mounted equipment. Dynaface R© is ableto identify any clearance problems that may arise byallowing the user to specify critical aircraft locationsand corresponding ship locations as points of inter-est. Throughout the simulation, Dynaface R© outputsthe relative position of these points in each compo-nent direction. If interferences do exist, appropriateaction can be taken to rectify the problem early inthe design cycle.

Another common application is to ensure that

while an aircraft is being traversed through thehangar door and into the hangar that the relativemotion does not allow contact between the aircraftand doorframe. This is done by specifying severalpoints on the helicopter where the potential for con-tact exists. Dynaface R© is then run and the resultsanalyzed to determine if aircraft contact with thehangar is made. This is most useful when a newhelicopter is being considered to operate from an ex-isting ship where decisions need to be made as tothe necessity of upgrading the hangar. Alternatively,operating procedures can be changed to increase thelevel of aircraft securing to minimize the relative mo-tions.

Parameter Optimization

A wide variety of sensitivity analysis and optimiza-tion is possible using the simulation package. Op-portunities range from selecting the ideal placementof securing elements on the aircraft to consideringhow maritime aircraft can be better designed to bemore compatible with shipboard operation from anon-deck securing perspective.

An example of the type of detailed sensitivity anal-ysis that is possible is described in References [12]and [13]. In those studies, potentially-importanthelicopter geometrical and inertial parameters wereconsidered for several aircraft to determine how sen-sitive helicopter securing requirements were to eachparameter and to investigate how the sensitivityof securing requirements to helicopter design varieswith aircraft size. A sample result is shown in Fig-ure 15.

Figure 15: Sample result indicating how the aircraftrelative roll angle sensitivity varies with aircraft de-sign parameters and aircraft gross weight

As indicated previously, the simulation is flexibleand can be used to support sensitivity and optimiza-

tion analyses tailored to the specific objectives andrequirements of the user.

Incident Investigation

While emphasis is most often placed on analysis toprevent potential accidents involving shipboard air-craft, accidents do occasionally occur and the result-ing consequences can be severe including the loss ofhelicopter and ship personnel, loss of the helicopter,and damage to the ship. Accurate dynamic interfaceanalysis capability is essential for incident investiga-tion as there may be multiple factors that contributeto an incident. The Dynaface R© package has been ap-plied and is well-suited for this purpose as part of theoverall investigation.

Over the past few years, there have been severalincidents involving helicopters embarked on ships inboth military and civilian/commercial contexts. Al-though the exact conditions may not be known atthe time of an incident, Dynaface R© requires a min-imal amount of information to be able to reproducethe incident with some level of confidence. The typesof information surrounding the incident that shouldbe easily available and input to Dynaface R© include:the approximate aircraft weight, rotor status, brakestatus, aircraft on-deck alignment, aircraft locationon the ship, ship heading and speed, approximatewind speed and direction, and method of securing.The more difficult task is to identify the ship motionat the time of the incident. Dynaface R© is able to in-put experimental ship motion if measurements weretaken and recorded, or alternatively sinusoidal mo-tion could be used based on the observations of theship’s six degrees of motion made by witnesses. Oncethe incident is reproduced, the cause can quicklybe determined. For example, a helicopter rollovermay be caused by the combination of several fac-tors such as no securing system, the ship at an un-favourable heading, the rotor turning, the aircraftlightly loaded, and unfavourable wind speed and di-rection. Another incident may involve a helicoptercontacting the hangar door during traversing. Oncethe situation surrounding the incident has been iden-tified, appropriate action can be taken to prevent theincident from recurring. This may include restric-tions on ship heading and speed or the use of a morefavourable securing and handling system.

Conclusion

This paper has presented the mathematical frame-work and modelling approaches used in the

Dynaface R© simulation package; an overview of val-idation activity; and sample dynamic interface ap-plications. The objective was to comprehensivelyintroduce Dynaface R© and indicate its range of ap-plicability.

The simulation has been used by ITI and otherinternational organizations to model a variety of ex-isting maritime aircraft ranging in size from verysmall unmanned air vehicles to large helicopters.Specific aircraft modelled include various configu-rations of the Sea King, Sea Hawk, H-92, SuperPuma/Cougar, NH-90, Dauphin, Super Seasprite,Lynx, EH-101, Bell 212, and Bombardier CL-327.These aircraft have been operated on a variety ofships ranging from lively cutters, to frigates, to largerstable platforms. Extensive experience with themodel over the past fifteen years has demonstratedthat Dynaface R© provides a valuable analysis capa-bility that can be directly applied to support engi-neering analysis and design, simulation-based acqui-sition, operational planning, and incident investiga-tion. Ultimately, experience is demonstrating thatthe simulation is fulfilling its intended purpose ofpromoting the safety of shipboard helicopter oper-ation.

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