ICAS 2000 CONGRESS

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Abstract

An overview of future structures technology formilitary air vehicles is given. The overview isprefaced by a discussion of the factorsinfluencing structures research today. The keyto meeting affordability and rapid technologyinsertion is identified as simulation-basedprototyping, including certification by analysis.Other enabling structures technologycandidates for future systems are discussed,including multi-functional structures, extremeenvironment structures, affordable compositestructures and active flow control for structuresapplications.

1 Introduction: Factors InfluencingStructures Technology Research

The design emphasis for military air vehicleshas changed in recent years from a primaryfocus on aircraft performance, lethality, andsurvivability to the current focus of affordability(see Fig. 1). While affordability is now thecenter of attention, requirements still exist toretain the previously emphasized attributes.

Systems requirements for military aircraftwill also change dramatically in the twenty-firstcentury. 1The United States and many alliesshare the vision of global reach and forceprojection requirements brought on by theregional conflicts that are predicted for the 1 Chief, Structures Division, Air Vehicles2 Chief Scientist, Air Vehicles; Fellow, RoyalAeronautical Society

This paper is declared work of the U.S. Government andis not subject to copyright protection in the United States

foreseeable future. To meet those threats,tomorrow’s air vehicles may look significantlydifferent from today’s fighter, bomber,reconnaissance and support aircraft [1].

This combination of changes in systemsand design requirements will give rise to newdesign processes for air vehicles and some ofthe most pronounced changes will be instructures technology. Novel structurestechnologies will be considered early in thedesign phase, as will sister technologies fromaerodynamics, propulsion, flight control,materials and subsystems [2]. Life cycle costwill be an overriding design variable. Airframestructures will be called on to answer manysystem requirements and the enablingtechnologies will be certification by analysis,active flow control, multifunctional structures,simulation-based prototyping, affordablecomposite structures, and extreme environmentstructures

2 Certification of Structures by Analysis

The first implementation of certification ofstructures by analysis may be in structuralsustainment because small, near term payoffsare expected there first. The need for morestructural qualification tests is expected toincrease as maintainers turn to materialsubstitution and design replacement for minorand major subcomponents to solve chronicaging aircraft problems. Certification byanalysis, however, has the potential for muchwider applications, and is considered in themilitary to be an integral part of simulationbased acquisition research.

AIRFRAME STRUCTURES TECHNOLOGY FORFUTURE SYSTEMS

Joseph M. Manter1 and Donald B. Paul2

Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, USA

Keywords: Structures Technology, Certification by Analysis, Active Flow Control,Multifunctional Structures, Simulation-Based Prototyping, Affordable Composite Structures,

Extreme Environment Structures

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For new aircraft, full-scale airframe static,durability and damage tolerance tests areexpensive procedures required by the military.These tests are becoming ever more expensivedue to increased labor costs, more complexdesigns, and more sophisticated test equipmentcosts required to reduce program risk. Thesefactors will continue to accelerate certificationcosts as will new design concepts such asaeroelastic tailoring, extreme environmentstructures, multi-functional structures andrelying on probabilistic design requirements fortransient loads.

Increased computational power will enablestructural qualification simulations to usesophisticated finite element codes to modelstructural response and computational fluiddynamics codes to better model static anddynamic aerodynamic inputs [2]. Additionally,codes could simulate thermal, acoustic andflight control inputs for a more faithfulrepresentation of these design developmenttests.

While absolute replacement of thesetraditional tests is probably tens of years awayand even longer in terms of customeracceptance, incremental payoffs will be seenmuch sooner. These payoffs will most likely beseen in the reduction of the building block tests,from coupon to component, required today.

3 Active Flow Control

Active flow control is the local predictable,schedulable, or on-demand altering of the localflow around an air vehicle to gain a desiredoutcome. Active flow control, requiring theexpenditure of energy [3], can be affected by theuse of micro-electromechanical motors, zero-mass flow synthetic jets or fluidic injection fromengine air bleed or some other mass flowsource. Early applications of active flowcontrol will be vehicle flight control, inletdistortion control, control of boundary layertransition or separation, the mixing of primaryand secondary exhaust flows or the control ofthe local flow within open air vehicle cavities.

Destructive acoustic levels prevalent withinopen weapons bays result in substantial damage

to internal structures and internally carriedweapons and subsystems. Today’s modern airvehicles configured with weapons bays oftenuse passive devices to attenuate that acousticenergy and alter the airflow to enable a safeseparation of stores. These passive devices,however, are optimized for a unique storeconfiguration and flight condition and theireffectiveness is compromised at off-designconditions. A more robust design capable ofeffective operation for a wide variety of storeconfigurations and flight conditions will useactive flow control coupled with a closed loopcontrol system (see Fig. 2 ).

A closed loop active flow control designhas been demonstrated in a wind tunnel test andshown effective in reducing the acoustic levelswithin open weapons bays. The test simulatedboth baseline and closed loop control of theweapons bay for various Mach numbers, massflows and flow injection frequencies. The testdemonstrated the feasibility of the closed loopsystem, reducing some tones by 20 decibels anda noticeable reduction in the overall soundpressure level [4]. More applied and advanceddemonstration research will enable air vehiclesof the future to operate weapons bays withexisting and novel stores over larger operatingenvelopes with greatly reduced acousticenvironments.

4 Multifunctional Structures

Multifunctional structures include concepts thatextend airframe functionality to perform tasksbeyond load reaction. Potential benefits exist toincrease survivability, lethality, aerodynamicefficiency and thermal efficiency. At the sametime, this new technology may also reducemanufacturing cost, while maintaining orimproving reliability, supportability andrepairability. Candidate technologies include:

• Conformal load bearing antennas• Integral heat exchangers• Flight control actuators• Embedded sensors• Embedded power systems

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AIRFRAME STRUCTURES TECHNOLOGY FOR FUTURE SYSTEMS

Multifunctional structures may containactuators and sensors that will allow them toalter actuators’ mechanical state (position orvelocity) and or mechanical characteristics(stiffness or damping). Benefits of suchstructures include aeroelastic control, loadalleviation, and elimination of detrimentaldynamic oscillations at reduced structuralweight while simultaneously achieving astructural integrity equivalent to present safetyrequirements. Such designs (see fig. 3) willopen the vehicle design space to significantlyreduce takeoff gross weight [5].

Multifunctional structures are in an earlystage of development, but already benefits canbe foreseen in reduced life cycle cost andreduced direct operating cost throughimprovements in both performance andmaintainability. Active/adaptive structures,structure health monitoring andstructure/avionics integration are three areaspresently being pursued. There is a potential toreduce inspections on both new and repairedairframes thereby reducing maintenance costs.

Eventually, multifunctional structures areexpected to develop to the point where they canfacilitate on-demand and in-situ monitoring ofstructural health. Once they become reliableenough, costly airframe tear down inspectionsneed only be performed when there is a faultindication.

A multifunctional airframe that integratesantenna functions into the loadbearing structureoffers many significant benefits. Elimination ofstructural cutouts will save weight and lowermanufacturing costs. The larger inherentplatform area can enhance current or enableadditional antenna functionality. The cleanerdesign will reduce radar cross section and airvehicle drag. Finally, the elimination of bladeantennae will reduce the damage susceptibilityinherent in blade antennae [6].

5 Simulation Based Prototyping

Solid modeling coupled with feature-baseddesign software and advanced visualizationtechnology is already enabling the designer tochange design variables and evaluate the effect

of these changes on the response characteristicsof the structure, in real time. It has becomecommonplace to display stress contours,deformations and vibration mode shapes incomputerized color graphical depictions, forhighlighting critical areas.

The Air Force Automated StructuralOptimization System (ASTROS) is an exampleof a tool that has been developed for facilitatingIntegrated Product Design. ASTROS provides amechanism for effective communication acrossdifferent disciplines, including aerodynamics,flight controls and electrodynamics. In thefuture, there will be a system of design toolsthat will facilitate virtual prototyping and enablesimulation of advanced technologies andconfigurations before physical flight.

Alternative configurations can be exploredrelative to their ease of manufacture and cost.With the capability to “immerse” the designer,the pilot, or the maintainer in the design,customer familiarity with the product can beginbefore it is produced [7]. Concepts currentlyunder study in the Air Force ResearchLaboratory’s Multi-Disciplinary Center areillustrated in Fig. 4.

By linking these advanced, high fidelityengineering models to battlefield and campaignanalysis simulations, the true payoffs ofstructures and other technologies can beevaluated in a virtual wartime scenario (see Fig.5).

6 Affordable Composite Structures

Advanced composite structures can facilitate ahigh degree of subsystem integration, as theirmechanical, thermal and electromagneticproperties are tailorable and amenable toembedded sensors, actuators and subsystems.

Composites are becoming cost effective insubelement areas where they have not been inthe past. Rapid progress in textile subelementsfabricated by Resin Film Infusion (RFI) andResin Transfer Molding (RTM) of braided andwoven preforms is being made. Fuselage framesand cut out reinforcements are near termapplications. Since textile composites provideintegral reinforcement in the “Z” direction,

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significant improvements in intralaminar andinterlaminar strength can be obtained to reactand transfer out-of-plane space loads. Theseadvantages should offset the disadvantages oflower stiffness and compression strength forbraided and woven composites vs. laminates.

The potential payoff of applying textiletechnology to sandwich structures will allowelimination of the skin-to-core adhesive bondproblems. Since there is an integral linkbetween upper and lower face sheets there is nodebonding. Impact damage is minimized sincethrough the thickness fiber serve to blockdelamination growth and thereby localizedamage.

Another approach to improving the out-of-plane properties of laminated compositestructure is Z-pinning technology. Z-pinning hasbeen developed by the Aztex Corporation incooperation with AFRL, United States Navy AirWar Center and the airframe companies.

The technology introduces reinforcing pinsthrough the thickness. The pins provideincreases in pull-off loads and offer amechanical interlocking capability to inhibitcrack propagation and provide a fail-safelinkage if a crack initiates.

The USAF and US Navy are supportingprograms to characterize the structural and costbenefits of this technology. Specific focus hasbeen directed at understanding how thetechnology can be used to modify failure criteriain composites, enhance ballistic survivability,and reduce cost through the elimination offasteners.

Non-autoclave processes such as Electron-Beam curing offer great potential for loweringcomposite structure processing costs. Cationicepoxy resin composite parts can be non-autoclave cured in minutes. In the electron beamprocess, the electron’s kinetic energy isdeposited directly in the material volume ratherthan by surface heating and thermal diffusion.Both aircraft and space vehicle structures arebeing produced by this method of processing.Examples are a 14-inch diameter 3-foot longintegral fuel tank for the US Army’s Longfogtactical missile by Oak Ridge NationalLaboratory and an Aerospatiale filament wound

rocket motor case. The latter program did notemploy resins or processing techniques requiredto produce structures with the properties andquality required of airframes. It did demonstratethe feasibility to produce structures limited insize only by the facilities required for shieldingE-beams. At its present stage in development,E-beam processing technology is far frommature and has to be characterized as high riskbut with high potential to reduce processingcosts.

Composite airframe applications willcontinue to grow at the steady pace of the past.A major increase in the use of composites willbe in the automobile industry and civilinfrastructure. Because of the magnitude ofthese applications, lower cost compositematerials will become available for aerospaceuse. Also, the emphasis on lower cost airframeswill encourage use of innovative design thatexploits low cost manufacturing techniques andup front system arrangements that allowoptimum load paths. The combination of thesetwo developments will fundamentally shift theairframe cost vs. weight curve [7].

7 Extreme Environment Structures

One of the most demanding new structuresresearch areas is extreme environmentstructures. The extreme environment relative tomilitary applications is the combined regime ofhigh acoustic and thermal loads (see Fig. 6) inconjunction with the mechanical loadsexperienced by a vehicle in flight.Unfortunately, the combined environmentloading is usually several times more severethan the summation of each individual load [8].Even today’s most common extremeenvironment situation, the closely coupledpropulsion/ airframe designs characterized byexhaust washed structures, requires research toreduce maintenance costs. Tomorrow’sextreme environment encounters also will beexperienced by air vehicles operating in thehypersonic speed regime and space accessvehicles capable of aircraft-like operations.

To meet these challenges, both the militaryand civilian communities are vigorously

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AIRFRAME STRUCTURES TECHNOLOGY FOR FUTURE SYSTEMS

pursuing research in next generation thermalprotection systems, high temperature structuresand integrated environmental control systems[9]. Some promising techniques aremechanically attached blanket thermalprotection systems for initial application to theleeward side of a transatmospheric vehicle. Thelong-term solution to reducing mass fractionsfor space access vehicles involves reduction onthe reliance of parasitic thermal protectionsystems and exploitation of high temperaturematerials [8]. Basic research is underway tocharacterize high temperature ceramic matrixcomposite design criteria [10].

Because analytical models today areinadequate to define the acoustic loads and itsinteraction with the structures, the Air ForceResearch Laboratory (AFRL) will continue torely on experimental facilities to develop newstructural design concepts and the analyticaltools to mitigate the reliance on theexperimental facilities.

A cornerstone of tomorrow’s research inextreme environment structures will be AFRL’snew $17.5 million Consolidated AerospaceStructures Research Laboratory (CASRL).Scheduled to begin construction in November of2000, the CASRL will merge and improve thecapabilities of AFRL’s Combined EnvironmentAcoustic Chamber and its Structural MechanicsFacility to yield a world class facility requiredto meet USAF research needs for its spaceaccess and future strike vehicles (see Fig. 7).Slated for completion in fall 2002, the CASRLwill be able to simulate, for a 10-foot by 10-footarticle, a combined environment of a 174 dBoverall sound pressure level (for a 50-500 hertzflat spectrum) acoustic load, a 70 BTU/ft2-secthermal load and a representative mechanicalload.

8 Summary

Research in airframe structures technology willbe vital in meeting the military’s determinedemphasis on affordability. New visions ofglobal reach and force protection have alreadyresulted in changes in the research direction ofthe United States military and many of its allies.

The key to meeting these challenges affordablyis simulation-based prototyping, includingcertification by analysis. Other enablingstructures technology breakthroughs will comefrom multi-functional structures, extremeenvironment structures, affordable compositestructures and the exploitation of active flowcontrol.

9 References

[1] USAF Scientific Advisory Board (SAB) Panel, NewWorld Vistas. Air and Space Power for the 21stCentury, Aircraft & Propulsion and MaterialsVolumes, SAB Summer Study, 1995.

[2] Sensburg, Otto, Draft Terms of Reference forQualification of Structures by Analysis, NATO RTAApplied Vehicle Technology Panel Meeting, May2000.

[3] Gad-el-Hak, Mohamed, Applied Mechanics Reviews,Vol. 49, pp 365-379, 1996.

[4] Shaw, L.L., and Northcraft, S. Closed Loop ActiveControl for Cavity Acoustics. AIAA-99-1902,Proceedings of 5th AIAA/CEAS AeroacousticsConference and Exhibits, Bellvue, Washington, May1999.

[5] Pendleton, E., Bessette, D., Field, P., Miller, G., andGriffin, K. Active Aeroelastic Wing Flight ResearchProgram: Technical Program & Model AnalytricalDevelopment. Paper 98-1972, Proceedings of the 39th

AIAA Structures, Structural Dynamics, and MaterialsConference, Long Beach, CA, 20-23 April 1998.

[6] Lockyer, A. J., Design and Development of aConformal Load-Bearing Smart-Skin Antenna:Overview of the AFRL Smart Skin StructuresTechnology Demonstration (S3TD), SPIE SmartStructures and Materials, Vol. 3674, Paper Number3674-46, “Industrial and Commercial Applications ofSmart Structures Technologies,” pp. 410-424,Newport Beach, CA, March 1-4, 1999.

[7] Paul, D., et. al., The Evolution of U.S. MilitaryAircraft Structures Technology, submitted to AIAAJournal of Aircraft, Oct 1999.

[8] Paul, D., et. al., Structures Technology for FutureAerospace Systems, Paper AIAA 98-1869, 39th SDMAIAA, Long Beach, California, April 1998.

[9] Pratt, D.M., Brown, J., and Hallinan, K. P.,Thermocapillary Effects on the Stability of a Heated,Curved Meniscus, Journal of Heat Transfer, Vol. 120,pp. 220-226, 1998.

[10] Wolfe, H., Camden, M., Byrd, L., Paul, D., Simmons,L., Kim, R., Failure Criteria Development forDynamic High-Cycle Fatigue of Ceramic MatrixComposites, AIAA Journal of Aircraft, Volume 37,Number 2, Pages 319-324, March-April 2000.

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CO

ST

PERFORMANCE

FULLYINTEGRATEDSUBSYSTEMS

DESIGNCRITERIA &METHODS

RANGE

AGILITY

LOW OBSERVABLES

SPEED

LETHALITYUNITIZED

STRUCTURALCONCEPTS

SUPPORTABILITY

VULNERABILITY AFFORDABILITY

Fig. 1 A dominate feature of future military air vehicles will be affordability.

PASSIVE ACTIVE LEADING EDGE RAMP LEADING EDGE PULSED FLUIDIC INJECTION

NO FEEDBACK TONES

Fig. 2 Active flow control will significantly reduce acoustic levels within weapons bays.

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Min TOGWaspect ratio = 5.0

t/c = 0.035

Min TOGWaspect ratio = 3.0

t/c = 0.040

Fig. 3 An active aeroelastic wing will open the air vehicle design space.

Fig. 4 Advanced concepts studied in development of simulation-based research anddevelopment

Joined-Wing Concept Single-Stage Concept

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0

500

1000

1500

2000

2500

3000

3500

165 170 175 180 185

SPL(dB)

Tem

p(°F

)

Spaceplane

AFRL’s CEAC(4ftx6ft)

AFRL’s New CASRL(10ftx10ft)

Exhaust Washed Structure(4ftx8ft)

Design Space

Fig. 6 Combined extreme environments for exhaust washed structures and space accessconfigurations.

Fig. 5 The hierarchy of simulations in simulation-based prototyping (courtesy ofASC/ENM.)

Metrics:Measures of Outcome (MOOs)Measures of Effectiveness (MOEs)Measures of Performance (MOPs)

IncreasingResolution

IncreasingScope

MOPs

MOEs

MOPs

MOEs

MOO CAMPAIGN CAMPAIGN

ENGINEERING LEVELENGINEERING LEVEL

ONE ON ONEONE ON ONE

MISSION MISSION

AIR TO AIR COMBATAIR TO AIR COMBAT

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Consolidated Aerospace Structures Research Lab

New Construction

Simultaneously Validate: • Sound (174 dB)• Temp (3000°F)• Mechanical Load• Size: 10ft x 10ft

Building 65

NEW COMBINED ENVIRONMENT CAPABILITY

Fig. 7 AFRL’s new Consolidated Aerospace Structures Research Laboratory willsimulate combined extreme acoustic, thermal and mechanical loads.