[american institute of aeronautics and astronautics 7th aiaa/usaf/nasa/issmo symposium on...

Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.

A98-39702 AIAA-98-4701

A DESCRIPTION OF THE F/A-18E/F DESIGN AND DESIGN PROCESS

James A. Young*, Ronald D. Anderson", andRudolph N. Yurkovich*The Boeing Company

St. Louis, MO

Abstract

This paper describes the design and the designprocess used to develop the F/A-18E/F aircraft. It ispresented here to document the state-of-the art of thedesign process for development of a modern highperformance fighter aircraft. It is intended that thisinformation will provide a background for researchersdeveloping Multidisciplinary Design Optimization(MDO) processes for aircraft design. The designprocess itself was an advance for the F/A-18E/F inthat it marked the first application of the IntegratedProduct Development (IPD) design process to anEngineering Manufacturing Development (EMD)program at the McDonnell Douglas Corporation.Since the F/A-18E/F's flight test program is wellunder way, results are available by which to judge thesuccess of this design and the design process.Finally, some conclusions and recommendations foradditional work to improve the design process aremade.

Introduction

In 1990 the MDO Technical Committee (TC) wasformed as a technical committee of the AIAA. One ofthe tasks that this committee undertook was to definethe state-of-the-art as it existed at that time and theresults of this study were published as Reference 1.Since 1990 other documents have also presentedstate-of-the-art approaches with Reference 2 being anexcellent example. The references have done anexcellent job of documenting theoreticaldevelopments. However, the AIAA MDO TC felt thatmore was required to transfer the MDO message fromthe theoreticians to the aircraft designer and for thetheoreticians to have a better perspective on what isrequired to design a new aircraft. It was determinedthat a series of papers by industry documenting thecurrent design process as used on current designprograms would be an appropriate step in makingthis happen. This paper, which addresses theF/A-18E/F, is one of a series of papers in response tothat action item.

The F/A-18E/F, shown in Figure 1, represents thenext step in the evolution of the F/A-18 aircraft. Inaddition, its development represents a next step in theevolution of the aircraft design process. The E/F wasdesigned using the Integrated Product Team (IPT)approach and this represents a significant advancefrom the design process used in the development ofthe original aircraft. This paper presents adescription of the aircraft design as well as adescription of the design process.

Figure 1. F/A-18E/F Super HornetIn MDO an objective function subject to a set ofconstraints is defined and a mathematical process isused to minimize this objective function withoutviolating the constraints. Sensitivity derivatives areusually computed as part of the optimization process.Reference 3 provides a good description of themathematical process.

If the above definition of MDO is applied in a strictsense, then MDO was not used to design the F/A-18E/F. However, the F/A-18E/F was designed using aMultidisciplinary Design Process. Based on resultsobtained from the flight test program, the aircraft is avery successful one. Thus, the current design processmust also be regarded as successful.

The F/A-18E/F was designed to meet a specific set ofrequirements rather than by optimizing a specific

Director of Engineering, F/A-18 Program** Director, Phantom Works, Assoc. Fellow AIAA* Fellow, Assoc. Fellow AIAA

Copyright © 1998 by the American Instituteof Aeronautics and Astronautics, Inc.All rights reserved. 1

American Institute of Aeronautics and Astronautics


objective function. From the perspective of MDO,these requirements can be viewed as constraintswhich implies that the F/A-18E/F is a feasible design.

Description of the Aircraft

Aircraft Missions - The F/A-18E/F is a multi-mission aircraft designed for the US Navy. Theconcept of a multi-mission aircraft is significant forthe MDO process in that there are multiplerequirements that the aircraft must meet, and thiscomplicates the definition of an objective function.For a single mission aircraft, the formulation of theobjective function is a simpler task. The F/A-18E/Fwas designed to perform both air-to-ground and air-to-air missions. These missions were defined asrequirements and the goal was to develop a designthat satisfied them. A description of the MDO processas it applies to a multi-mission aircraft was initiallypresented in Reference 4.

Figure 2 illustrates the multi-mission concept startingwith maritime air superiority on the left andproceeding to all weather attack on the right. Thesemission extremes are significant in that historicallythey have been performed by dedicated aircraft. TheF-14D performs the air superiority mission and the A-6F performs the all weather attack mission. While theF/A-18C/D has some capability to perform thesemissions, it has not been optimized for them. For fleetdefense the F-14 with its Phoenix missile system issuperior to the F/A-18C/D. However, as a multi-mission aircraft, the F/A-18C/D still has significantcapability in this area. Similar arguments can be madefor the ground attack missions.

MariSSiSAir

Superior- CombatFighter

FighterEscort

CloseAir

Support

AirDefenseSuppres-

sion

Day/NightAttack

AllWeatherAttack

KNATFF/A-18A/B/C/D

A-6F/A-12

F/A-18E/F

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Figure 2. Hornet Spans the Mission SpecAs the F-14D's and the A-6F's are retired form thefleet, they will be replaced by F/A-18E/F's. Thus, theoriginal mission spectrum of the F/A-18C/D has beenexpanded even further for the F/A-18E/F as shown inFigure 2.

Each of these missions has a specific set ofrequirements that the aircraft must meet. An MDOapproach to meeting these requirements was not takenbecause MDO design techniques were not available at

the time the F/A-18E/F was designed. However, forfuture aircraft design this approach may offersignificant improvements if appropriate tools can bedeveloped.

History of the Configuration - The F/A-18E/F is aderivative of the F/A-18C/D aircraft, which wasoriginally derived from the Air Force lightweightfighter competition. Consequently, there is a greatdeal of history behind this configuration with thegeneral shape of the aircraft being defined by theoriginal YF-17. Figure 3 shows the planform view ofthese three aircraft and the heritage of the E/F aircraftis obvious. Table 1 summarizes some of the basicgeometry data. The original YF-17 had a wingspan of35 ft and a wing area of 350 sq. ft. For the F/A-18Athe corresponding numbers are 37.5 ft and 400 sq. ft.While the basic aerodynamic concept of the YF-17and the F/A-18A were essentially the same, theinterior of the F/A-18A was completely redesigned.Most of the required changes were a result oftransforming what was a light-weight fighter for theAir Force to a ship-board multi-role aircraft for theUS Navy

YF-17 F/A-18A F/A-18EGP81128003.CVS

Figure 3. Comparison of Aircraft Planforms

DimensionsSpan(without missiles)LengthHeightTail spanWheel trackWing area

WeightsEmpty

FighterconfigurationMaximum

YF-17

35.0ft

56.0ft14.5ft22.2ft06.9ft

350sq ft

17,000lbapprox.23,000lb

F/A-18A

37.6ft

56.0ft15.3ft21.6ft10.2ft

400sq ft

21,830lb

34,700lb

51 ,900lb

F/A-18 E

42.9ft

60.2ft15.8ft23.3ft10.45ft

SOOsq ft

30,600lb

47,900lb

66,000lb

Table 1. Comparison of Specifications



At the time that the F/A-18A was under goingpreliminary design, the lightweight fightercompetition was still ongoing. This provided aconstraint on the original F/A-18A that resulted in theF/A-18A still having essentially the same size andshape as the YF-17. For the MDO process, this issignificant because it implies a constraint that wouldnot be present if the F/A-18A were a totally newaircraft.

Similar observations can be made for the F/A-18Erelative to the F/A-18A. The F/A-18E configurationgrew relative to the F/A-18A configuration. The spanof the F/A-18E is 44.7 ft and the wing area is 500 sq.ft. However, the general shape of the aircraft has beenmaintained. A comparison of the F/A-18E SuperHornet to the original Hornet is shown in Figure 4. Inaddition to the Super Hornet being a larger aircraftwith a new inlet, changes in the Leading EdgeExtension (LEX) and the addition of a wing leading-edge snag are apparent.

&P81128004.CVS

Figure 4. Super Hornet Compared to OriginalHornet in Flight

Hornet 2000 Study - The evolution of this newconfiguration had its origins in the Hornet 2000study, Reference 5, which was conducted in 1988 bya joint team composed of the US Navy andMcDonnell Douglas. Over the life of the F/A-18A/Band F/A-18C/D aircraft many changes wereincorporated that resulted in an increase in weight andthe internal space being used for new and additionalavionics equipment. Because of this growth,reductions in range and other performance metricsoccurred. In addition, changes to meet the increasedthreat that the aircraft was to face were required. It

was anticipated that the capabilities of the threatwould continue to increase. Quoting from the Hornet2000 study;

"Major advances in threat capability have occurredsince the F/A-18 was designed in the mid 70s. Theoriginal design goal for the Hornet was to havesuperiority over FISHBED and FLOGGER class airthreats and to penetrate battlefields with SA-2, SA-3,SA-6, and SA-7 class surface-to-air threats. Thatthreat has changed rapidly in character and capability,primarily as a result of successful Soviet efforts intechnology transfer. The Soviets have demonstratedan ability to implement rapidly technologiesdeveloped domestically and acquired through legaland covert means. Through this aggressive programof modernization, the ability of the threat to confrontthe Carrier Battle Group has increased significantly."

Since 1988, a great deal has happened to change thenature of the threat. However, while the need to dealwith the Soviet threat may have diminished, newthreats have emerged. The need to deal with thesethreats formed a significant requirement for anadvanced Hornet.

In addition to recognizing the need for a new aircraft,the Hornet 2000 Study identified plannedimprovements for the F/A-18C/D aircraft through1995. These improvements were in three major areas:avionics, propulsion, and equipment. The avionicsupgrades were to improve the F/A-18 weapon systemcapabilities in the areas of situational awareness, airsuperiority, air-to-surface attack and survivability.The propulsion upgrade consisted of replacing thebaseline engines with the Enhanced PerformanceEngine (EPE). This engine offered significantperformance improvements at higher speeds andcould be incorporated without airframe changes. Theequipment growth consisted of installing an On-Board Oxygen Generating System (OBOGS)increasing the aircraft cooling capacity by an ECSupgrade, and adding a bay in the left hand LEX toallow installation of additional avionics.

In summary, because of the changing nature of thethreat and because the basic aircraft, even with theEPE, had just about reached the limits of itscapabilities, a new aircraft was required. The Hornet2000 Study produced a set of requirements and anaircraft configuration that addressed them. This studylooked beyond the 1990s to determine therequirements for the aircraft such that it couldcontinue to meet the threat. The goal of the study wasto identify high value upgrades and develop a phasedincorporation plan to ensure continued F/A-18survivability and effectiveness.



This new aircraft configuration, however, had aconstraint that required as much commonality aspossible with the original aircraft. Even with thisconstraint, early in the design process, severalalternative configurations were investigated and asample of these configurations is shown in Figure 5.While this study was specifically directed toupgrading the F/A-18, it is believed that it isrepresentative of the type of trade study that would beconducted by industry and therefore should berelevant to the development of MDO to the designprocess. Seven potential configurations to meet theUS Navy missions needs in 1995 and beyond wereinvestigated. These configurations spanned the rangefrom minimum changes through Block Upgrades tomajor concept changes that reflected canard-wingarrangements popular at the time. The configurationswere built from the same baseline and took advantageof planned upgrades. They also shared commonrequirements for an updated weapon system,survivability improvements and increased thrust.

• FY88 Baseline Plus- FY90 and FY92 Avionics

Upgrades- Weapon System Upgrade- Survivability Enhancement- Enhanced Performancey

Engine

Increased FuelGrowth II EngineActive Array RadarINEWS

GP81128005.CVS

Larger Wing WithIncreased Chordand Span

Fuselage PlugsLarger Wing WithIncreased Chordand Span

Figure 5. Configuration Options

Configuration I minimized the impact to the airframe.Weapon system updates were achieved within theexisting space/volume. Pilot situational awarenesswas improved and workload decreased by upgradingthe cockpit to display integrated weapon systeminformation. Advanced air-to-air missile capability

was provided along with the capability to carry air-to-air missiles on the out-board pylons.

The remaining configurations incorporated changes tothe airframe. Common elements include increasedfuel, new Growth II engines, and an electronicallyscanned active array radar.

Configuration II expanded mission flexibility withadditional internal fuel in a raised dorsal. Aconfiguration of this type was successfully used onthe A-4M. Performance improvements were achievedwith the higher thrust engines that required enlargedinlets. External stores carriage speeds were increasedwith a stiffened wing. Target detection range wasmore than doubled by adding the active array radar.Adding new electronic warfare equipment for passivemissile detection and laser warning enhancedsurvivability.

Configuration III incorporated the upgrades ofConfiguration II while replacing the stiffened wingwith an enlarged wing for enhanced carrier suitability,maneuverability, and mission performance.Additional growth space was also provided.Configuration IIIA enhanced the transonic/supersonicflight regime by utilizing a fuselage plug rather thanthe raised dorsal for increased fuel. ConfigurationIIIB optimized the wing area growth of ConfigurationIII with an increased wing span for improvements inmission radius and carrier suitability performance.Configuration IIIC combined the fuselage ofConfiguration IIIA and the wing of ConfigurationIIIB for enhanced transonic/supersonic flight andimproved mission and carrier suitability performance.

Configuration IV added fuselage plugs similar toConfigurations IIIA and IIIC. However, theaerodynamic configuration was completely new andwas targeted at potential co-development by the USNand an international customer. The wing was acranked arrow wing and the stabilator was replacedby a canard. The vertical tails were also of a newdesign. This configuration shared the fuselage and allof the internal components of Configurations IIIA andIIIC including one of the major cost contributors, itsavionics suite.

A detailed discussion of the features and benefits ofeach of these configurations is beyond the scope ofthis paper. Each presents new operational benefitsand, in general, as additional benefits are added so isadditional cost. For the future, MDO could be used todetermine which configuration best meets the newrequirements for an improved strike fighter. At thetime the study was conducted, MDO techniques to aidin this decision did not exist.



The new engine, which was assumed forConfigurations II through IV, fostered a significantmultidisciplinary design integration activity. At thetime of the Hornet 2000 study, this new engine wasdesignated the F404 Growth II engine. The Growth IIengine was to be an upgraded version of the F404-GE-400 engine that would have significantperformance improvements throughout the flightenvelope. It was to provide approximately a 25percent increase in sea level, static installed thrust. Atup-and-away conditions the installed thrust increasewas estimated to be up to 40 percent over the currentengine. The improved performance was to beachieved through incorporation of engine componentsthat featured advanced aerodynamics and materials.The engine also featured increased engine airflow andhigher operating temperature capabilities without areduction in the current hot section life. While agrowth inlet was required for optimization of theGrowth II performance, the engines fit within thecurrent F/A-18 engine bay. The engine that isinstalled in the F/A-18E/F has been designated as theF414-GE-400 engine and is an advanced derivative ofthe Hornet's current F404 engine family.

Configurations I through IV were evaluated againstthe following set of criteria: carrier suitability, strikemission, fighter mission, maneuverability, fire controlsystem, survivability, growth potential, weaponsystem effectiveness and cost, both recurring andnon-recurring. This evaluation was summarized in astop-light format as shown in Figure 6 where G-green-indicates good, Y-yellow indicates marginal,and R-red indicated serious concern. It should benoted that if cost is considered, the conclusion as to

GP81128006.CVS

Figure 6. Hornet "2000" ConfigurationEvaluation vs. 2000 Threat

which configuration is optimum is difficult toformulate. Clearly, all of the configurations representsome degree of improvement, but at some cost. All ofthe new configurations cost more than the baselineand all require some investment. The cheapestsolution is to do nothing. On the other hand, asdescribed earlier in the discussion of today's threat, todo nothing would put the aircraft in a situation whereit would not be able to compete.

The Hornet 2000 Study identified four major studypaths, with seven configurations for the HornetUpgrade. The first path, Configuration I, wasattractive from a cost standpoint but had degradedaerodynamic performance and little remaining growthpotential. The second path, Configuration II hadimpressive weapon system improvement but sufferedfrom carrier suitability shortcomings. The third pathmade up of Configurations III, IIIA, IIIB, and IIIC,had significant performance, carrier suitability andweapon system improvements. The fourth path,Configuration IV, had Control Configured Vehicle(CCV) potential with the canard-cranked arrow wingarrangement.

The Hornet Upgrade Configurations IIIB and IIICoffered the greatest increase in weapon systemcapability, carrier suitability and performance. Theyincluded a larger wing, more fuel, growth engine, 10percent growth inlet, active antenna, upgraded crewstation, integrated CNI avionics and an integratedelectronic warfare system.

The final conclusion was that Configuration IIIC wasthe best path for upgrade since it was considered tohave the best carrier suitability performance.

This discussion of the process that led to what wasdetermined to be the best configuration providesvaluable insight into the design process for MDOcode developers. As stated elsewhere, an objectivefunction that could be used to determine the optimumconfiguration would prove very difficult to formulatein this case. In fact, typical parameters that have beensuggested as objective functions such as minimumweight or minimum cost were not the finaldiscriminators of the selected configuration. In thefinal analysis, the configuration that was selected wasthe one that best satisfied the requirements within theconstraint of retaining major F/A-18A configurationcharacteristics.

The F/A-18E/F Program - The F/A-18E/F program,which has its origins in the Hornet 2000 program, wasawarded to McDonnell Douglas on May 12, 1992.The cost of this program for the development phase



was $5.803 billion in 1992 dollars. This cost numbercan be regarded as another constraint on the design.

The F/A-18E/F rolled out on September 19, 1995 andits first flight was November 29, 1995. The aircraft asit appeared at roll-out is shown in Figure 7. Tenaircraft were built to support the flight and groundtest programs, seven flight test articles and threeground test articles. The flight test program began atNaval Air Warfare Center Patuxent River, Marylandon February 14, 1996 and is ongoing. However, it isestimated that the HMD portion of the program isnow 90 percent complete. As of January 31, 19981,463 flights representing 2,239.4 flight hours, hadbeen flown.

Figure 7. F/A-18 Super Hornet as it Appearedat the Roll-out Ceremony

While the Hornet 2000 study defined the basic shapeand size of the E/F, the details of the design still wereto be worked out.

Basic Changes - The primary changes developedduring the study and the subsequent refinements aresummarized here:

1) The area of the wing was increased by 25% to 500square feet. This change was made to increase therange and payload of the aircraft.

2) A snag in the leading edge of the wing wasincorporated. This design feature was part of theoriginal F/A-18A design but was removed due toexcessive loads on the leading edge flaps. It wasreintroduced here to improve carrier landing handlingqualities.

3) The LEX was enlarged and reshaped for betterhigh angle of attack performance. Initially the LEXwas basically an enlargement of the LEX used on the

C/D aircraft. However, during wind tunnel testing thehigh-angle-of-attack characteristics of the E/F aircraftwith that LEX were not as good as those of the C/Daircraft. The new LEX shape restored the excellenthigh-angle-of-attack characteristics that werepioneered on the F/A-18A aircraft.

4) The wing thickness-to-chord (t/c) ratio wasincreased. The C/D aircraft has a t/c of 5 percent atthe wing root and a linear reduction from there to 3.5percent at the wing fold. It is constant, 3.5 percent,from the fold to the tip. The E/F has a t/c at the wingroot of 6.2 percent, tapering to 5.5 percent at the wingfold and further tapering to 4.3 percent at the wingtip. The increased t/c provides an increase in torsionalstiffness with no increase in structural weight. It alsoallowed increased fuel carriage in the wing. However,the penalty is an increase in supersonic drag. Theincrease in torsional stiffness completely eliminateslimit cycle oscillations when the aircraft is carryingexternal stores as has been verified by the flight testprogram.

5) A third wing station was added. This significantlyenhanced self escort capability and gave the aircraftadditional load carrying capability of 2,300 pounds.These new wing stations can be used for either air-to-air or air-to-ground weapons.

6) The inlets were enlarged for the increased airflowrequired by the F-414 engines and reshaped forimproved radar signature. This reshaped inlet isclearly visible in Figure 8.

Figure 8. F/A-18E/F Super Hornet ReshapedEngine Inlet

There are additional changes below the skin. Theseinclude substantially new structure, new mechanicalsystems, and modified cockpit displays. Theavionics, however, are ninety percent commonbetween the two aircraft. The reasons behind thesedesign changes can be related to the designrequirements described in the next section.



Description of the Design ProcessIntegrated Product Development (IPD) - Duringthe 1980s McDonnell Douglas ran several pilotprograms to test what was then an innovative conceptfor aircraft design called Integrated ProductDevelopment and this process played a significantrole in the design of the F/A-18E/F. IPD is theprocess of defining, designing, developing,producing, and supporting a product, using amultidiscipline team approach. Note that theindividuals on the team need not be multidisciplinebut rather that the team has the required disciplines toperform its job. IPD, also known as concurrentengineering, pertains to the concept, analysis, anddesign stages of a product and provides the basis forbringing the optimum new product or productversion to production in the shortest time. The wordoptimum as used here may not imply the same thingas one would obtain from a formal mathematicalprocess. IPD encompasses the product life-cycle frominitial concept through production and support. IPDalso includes Integrated Product Definition plusproduct upgrades and process improvement for thelife of the product. Integrated Product Definition is asubset of Integrated Product Development.IPD requires a shift from serial to concurrent processstructures. Traditionally, each discipline completedits tasks and passed the results on to the nextdiscipline resulting in a sequential, or serialized,development process which generated reworkbecause the delivered item did not fulfill the downstream customer's requirements, was incomplete, orwas changed after release. Several iterations may berequired to get the product delivered, corrected, andcompleted. The processes involved in the definitionof a product have serial tasks. The IPD processstrives to take the serial processes and perform asmany of them concurrently as possible. Concurrentperformance of sequential tasks requires redesign ofthose tasks to accommodate the new processes.The IPD approach to product development has sixdefinition phases that are shown in Figure 9. The firstfour phases are referred to as configuration synthesisand the last two are referred to as product/processdevelopment. At the end of configuration synthesis aconceptual layout of the aircraft is available and atthe end of the product development phase, the buildto / buy to packages are defined. For the F/A-18E/F,the Hornet 2000 study corresponds to theconfiguration synthesis portion of the IPD process.

High Level Requirements Definition

Product/Process nunDevelopment

GP81128009.CVS

Figure 9. The Six Phases of IntegratedProduct Development

The six phases of product definition are executedduring the DoD Acquisition Phases as shown inFigure 10. Each acquisition phase will satisfy certainmilestone requirements before contracts are let forsubsequent phases. Configuration synthesis,consisting of high level requirements, initial concepts,and configuration baseline definition phases, isexecuted during the concept exploration anddevelopment acquisition phase. The conceptuallayout definition phase of configuration synthesis willoccur during the Demonstration and Validation(DEM/VAL) acquisition phase. The assembly layoutand build-to and support-to-package definitionphases, for product and process development, areaccomplished during the EMD acquisition phase.

Program Phases

Phase 0Concept Exploration

o"5w?

£D

Phase 2Engineering andManufacturing

Development Phase

IPD Phases

h Le

vel R

eqs

f

1

1

<u

a

nfigu

ratio

n B

0o

0

i.

o1

<

P

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coco

PhasesProduction

Build Evaluation/Production Phases

T3

£

-Pro

duct

ion

a.

£D)in"to.*;_c

O)

E

8Q

mIT

°GP81128010.CVS

Figure 10. IPD Phases Related to the MajorDoD Acquisition Phases

The F/A-18E/F program hierarchical team structurefollowed the Work Breakdown Structure (WBS),segmenting the work into discrete elements forestimating and budget allocation, tracking, andperformance as shown in Figure 11. Budgets wereallocated to each product center and team, making iteasier for the team leader to manage the assigned



WBS/ProductTeam Level

L Level 1

h Le

1

Weapon System

l_ t[ -J -̂ I1 1 <•Air

Vehicle

vein

1

Airframe.

-1-System

Test andEvaluation

SystemEngineering/

ProjectManagement

1 ———— |

IntegratedLogisticsSupport

rL A, rU A

1 1 1

Propulsion Avionics Technology

1. Level III

| Strui

t

lures | | Hydraulic |S \

| Secondary Power | | Flight Control | | Fuel System |

1

Armament/WeaponDelivery

Level IV-̂ 1 AJ A, /LAT

1 1 I S

| Wing | [Forward Fuselage] | Horizontal Tail | | Vertical Tail [

^ LevelV ^ ^ "* *•

Product Teams Align With WBSGP81128012.CVS

Figure 11. Work Breakdown Structure asImplemented on the F/A-18E/F

work and maintain control of budget and schedule.Each level could then be assigned the responsibility,authority, and accountability for their product.

The E/F program was managed under the CostSchedule Control System or C/SCS. This systemworks with the WBS defined above along with adetailed schedule and cost for each task. Metrics inthe form of a Cost Performance Index (CPI) and aSchedule Performance Index (SPI) are two of thetools that were used to ensure that the E/F Programremained on schedule and within budget. These twoindices provide the following information. The CPI isa measure of the work accomplished versus what itcost to accomplish it. This is an indication of the costefficiency with which work has been accomplished.The SPI is a measure of the work accomplishedversus what was scheduled to be accomplished. Thisis a measure of the schedule efficiency with which thework has been accomplished. These indices, along,with others were applied to the tasks defined throughthe WBS. Results were reported to the programmanagers so that they always knew where they stoodrelative to cost and schedule. It should be noted thatthe E/F program has basically remained on cost andon schedule since contract award in 1992.

Design Requirements - While the F/A-18C/D hasperformed well and demonstrated that the concept ofa multi-mission aircraft is valid, usage also showedseveral areas where the aircraft could be improved.

During the advanced design process, a number ofrequirements were investigated using standard tradestudies and a final set of requirements was formulatedand an enlarged aircraft that met these requirementswas defined. These requirements were formulatedrelative to the C/D aircraft. In addition to therequirements defined below, if a requirement werenot specifically identified, it was implicitly assumedthat the E/F would be as good as or better than theC/D aircraft.

These requirements covered five areas whereincreased capability was desired. These were:

1) Increased Bring Back - The maximum weight ofordnance and fuel with which the aircraft can land onthe carrier has been increased from 5,500 Ib to9,000 Ib.

2) Increased Payload - The aircraft store stationshave been increased from 9 to 11 and can be used foreither air-to-air or air-to-ground weapons.

3) Increased Range - The maximum range of theaircraft has been extended up to 40 percent dependingon the mission.

4) Increased Survivability - The ability to avoiddamage from hostile forces was improved by up to 8times depending on the threat.

5) Growth - Space for new hardware as well aselectrical power and cooling capability have beenincreased by up to 65 percent.

These requirements were quantified and in effectbecame constraints that the design had to satisfy. Inaddition to the requirements described above a set ofTechnical Performance Measurements (TPMs) weredefined which were allocated as appropriate to theIPD teams and were tracked for the aircraft. TheseTPMs were: weight empty, reliability,maintainability, survivability, signature, average unitairframe cost, growth in terms of internal volume,electrical power, and cooling, and built-in test whichwas tracked as false alarm rate and fault detection andfault isolation. In addition each team hadrequirements for cost, schedule, and risk.

Each of the TPMs was tracked in terms of its currentvalue relative to a design-to value and a specificationvalue. Figure 12 shows this tracking process as afunction of time for empty weight. The chart showsthat as of May 98 the actual weight was 666 Ibs.above the design-to weight. However, this weight wasover 384 Ibs. below the spec value. Thus, whileweight was not being minimized as an objectivefunction, its value was being closely tracked to ensurethat its upper limit was not exceeded. In addition the

8American Institute of Aeronautics and Astronautics


III!!! T&E AllowanceEMD Margin

-" Design-To..........29,514lbHUH Out of Tolerance

Variance....... .......... 666 Ib

Current Status BasisDesign-To.. ....0%Estimated.. ....2%Calculated. ..23%

Total.......... 100%

31.0

JASOND J FMAMJ J ASONDJ FMAMJ JASONDJ FMA1996 | 1997 | 1998 | 1999

GP81128013.CVS

Figure 12. F/A-18E/F Empty Weightweight was being kept below the spec value inanticipation that changes might be required as a resultof EMD testing. Similar tracking was carried out foralloftheTPMs.

If the strict definition of MDO is used, MDO was notused to design the F/A-18E/F. However amultidiscipline process that produced a design thatsatisfies all of the constraints was used. As anexample, the technology disciplines of aerodynamics,flight control flying qualities, structural loads anddynamics, and materials and structural developmentwere linked through a common database and analysistools as shown in Figure 13. Each of these disciplinesis driven by a specific set of requirements and each isRequirements

Teams

AerodynamicsRight

Control FlyingQualities

StructuralLoads andDynamics

Materialsand StructuralDevelopment

Common Database and Analysis Tools

• MissionPerformance

• Carrier SuitPerform

•WeaponSeparationRequirements

• Flying QualCriteria

• Control Laws• FCC Hardware/Software

• Design Loads

Environment

StabilityRequirements

• DesignAllowables

• CompositeAllowables

• PPV/FullScale TestRequirements

Interactions Produce Balanced Requirements JGP81128014.CVS

Figure 13. Airframe Technology KeyProducts and Requirements

responsible for a given set of products that takentogether define the airplane.

For example, the structural loads and dynamics groupis responsible for design loads, the dynamicenvironment, and aeroelastic stability. In order toaccomplish this each discipline must communicatewith the other disciplines. One tool used toaccomplish this was the use of a common database.

Taken as a whole the interactions among thesedisciplines produce a balanced set of requirements.

The interactions among the disciplines can be viewedfrom the standpoint of common tools as well ascommon data. An example of this is shown in Figure14. In this case a tool referred to as MODSDF whichis a six degree-of-freedom simulation code is beingused to determine critical design loads. For this toolto work, input is required from several sources. Theseinputs can be in the form of criteria, such as MilSpecs or data such as mass properties. One of theingredients is past experience.

GP81128015.CTS

Figure 14. Flight Technology RequirementsDevelopment

An example of how this process can be used toimprove the design is shown in Figure 15. In this casethe trailing edge flap was used as a maneuver loadalleviation device and its effectiveness wasdetermined using the MODSDF code. As the aircraftpulls load factor the trailing edge flap is scheduleddown by the flight control system as a function ofload factor. The result is a modification of the liftdistribution with less lift on the outer panel of thewing and more on the inner panel. This reduces wingbending moment, which results in a reduction in wingweight. This process is an example of amultidisciplinary approach to design that produces abetter aircraft than would be possible if eachdiscipline simply worked alone.



Before

X(increased Trailing-Edge-Flap Deflections Reduce}

JWing-Fold and Wing-Root Bending MomentsGP81128016.CVS

Figure 15. Load Alleviation of Wing-Fold andWing-Root

One final point about the design process needs to bemade and that is the importance of the aerodynamicdatabase. Figure 14 shows that two of the drivers forthe MODSDF code are the aerodynamic wind tunneltests and the loads wind tunnel tests. The generationof this data is one of the key ingredients in the designprocess. During the period from the start of HMD in1992 to first flight in 1996, approximately 18,000hours of wind tunnel occupancy time wasaccumulated with more than half of this being used byaerodynamics. In addition to the MODSDFsimulation tool, pilot in the loop simulation is alsoextremely important and over 1000 hours of pilot inthe loop simulation were accumulated by first flight.Both of these simulation tools require a detailedaerodynamic database.

Results

The F/A-18E/F has completed the majority (90%) ofits flight test program and the results to date havebeen outstanding. The program is on schedule, oncost, and the aircraft is below the specificationweight. The aircraft has met all of its requirementsand will provide the Navy with an aircraft that willmeet its needs well into the 21st century. While theseresults validate the design process that was used forthe F/A-18E/F aircraft, it is always possible toimprove. What follows is a discussion of a series ofquestions from the session organizers concerning

what is needed in the MDO process. This discussionis based on the experience from F/A-18E/F programand other experience of the authors.

Barriers, Obstacles, etc. - The major obstacle toMDO is the inability to analytically determine thedesign variables and their sensitivities. Meaningfuldesign does not occur until the wind tunnel data basehas been determined. While Computational FluidDynamics (CFD) may ultimately replace the windtunnel, until this happens the aerodynamic modelcannot be coupled with the other disciplines.Organizational barriers can exist. However, the F/A-18E/F program showed that the transition to an IPDorganization is possible. Surely, the transition to anMDO based organization is possible once the benefitsare demonstrated.

Design Problem and Design Goal - The goal is todesign an aircraft that satisfies the requirements. AnMDO code should aid in making the design feasibleas rapidly as possible. Once a feasible design hasbeen found, the next most important thing is todetermine the robustness of the design.

State of Software Integration Tools - Tools such asdatabase management, simulation, distributedcomputing, etc. have all contributed to the integrationof the design process. The F/A-18E/F uses a commondatabase for aerodynamics, control dynamics, loads,and structures.

MDO Simulation for non-linear Loads - Thesignificant challenge here is the generation of thenon-linear aerodynamic database. Once this data baseis generated, the simulation and the control lawdesign can proceed. Once these elements are in place,loads calculations can proceed.

Barriers to the Use of Disciplinary Analysis inMDO - While several issues were identified, fidelityof the models is the most significant. It makes nosense to optimize a design based on low fidelity data.

Loosely Coupled versus Tightly CoupledApproach - There is no inherent reason why a tightlycoupled approach could not be used. However, it isdifficult to see how a tightly coupled approach couldcontain all of the constraints that are present in theloosely coupled one that makes use of all currentdetailed design tools. A tightly coupled code run byan expert could serve as a check on the more detailedloosely coupled approach. However, this could alsocreate conflict if the two methods don't agree.

Use of Sensitivity Derivatives - The use ofsensitivity derivatives will become wide spread onlyafter the design community becomes familiar with



them. At present the concept of dollars-per-pound iswell under stood by all designers but it is not clearthat all sensitivity derivatives in general are in thiscategory. On the other hand, a trade study where twovariables are compared directly can usually beunderstood by anyone.

Automatic Differentiation - The trend in industry istoward off-the-shelf software when possible.Extending this to automatic differentiation mightimply that the software vendors should assume thelead here.

Single Most Important Obstacle to MDO - Theaerodynamic model matures first and the othermodels depend on this one. An accurate aerodynamicmodel is based on wind tunnel data that may notproduce the sensitivity derivatives needed for MDO.

Use of MDO Based on Decomposition - SinceMDO tools as such did not play a major role in theF/A-18E/F design, there is no reason to single out anyparticular method. However, before any method willbe used in a production aircraft design environment,it will have to prove itself.

Top MD Development - Rapid CFD for air loads.

Summary/Conclusions

The design process for a modern high performanceaircraft is a complex process that involves theintegration of analyses, tests, databases, and finallythe people who make the process happen. For theF/A-18E/F aircraft these ingredients have cometogether to produce a superior product. While theprocess did not make use of mathematicaloptimization in a formal sense, the final product doesindeed satisfy all of the design requirements thatwould be represented in the form of constraints in theMDO process. In fact, since the F/A-18E/F is amulti-role aircraft, the formulation of a singleobjective function would be difficult if notimpossible. The following observations can be made:

1. Formal MDO was not used as part of the F/A-18E/F design process.

2. For a multi-mission aircraft, the formulation of anobjective function is difficult if not impossible todefine.

3. The aircraft is designed by its requirements. This isanother way of saying that the aircraft is designed tomeet a set of constraints.

4. The design process involves more than a couplingof mathematical tools. The people who operate thesetools are an essential ingredient.

5. The IPD design process contributed to the successof the F/A-18E/F program.

6. The design process is serial in that an aerodynamicdatabase is required to design the flight controlsystem. Both the aerodynamic database and the flightcontrol system are required to define loads. Loads arerequired to define structure. Flex-to-rigid ratios aredefined after the structure is sized. These ratios areused to correct the aerodynamic database. And thewhole process is iterated. All of this can be done oncethe moldline of the aircraft is defined.

7. The aerodynamic database is the key. Thisdatabase is very non-linear. For the F/A-18E/F, theaerodynamic database was established by wind tunneltesting. In the future CFD may have the capability togenerate this database.

8. For a multi-mission aircraft, MDO tools thatrapidly generate a feasible design, one that satisfiesthe requirements, would be valuable. Once the designis feasible, these tools should allow for rapid "whatif studies. The manufacturer and his customer shouldmake the ultimate decision for what is best to meetthe requirements.

References

1. "Current State of the Art on MultidisciplinaryDesign Optimization," An AIAA White Paper,Approved by the AIAA Technical ActivitiesCommittee, September 1991.

2. Sobieszczanski-Sobieski, J. And Haftka, R. T.,"Multidisciplinary Aerospace Design Optimization:Survey of Recent Developments," AIAA Paper No.96-0711, January 1996.

3. Venkayya, V. B., "Introduction: HistoricalPerspective and Future Directions," published inStructural Optimization: Status and Promise, editedby Manohar P. Kamat, Progress In Astronautics andAeronautics, published by the AIAA, 1993.

4. Yurkovich, R. N., "MDO from the Perspective of aFighter Aircraft Manufacturer," published in"Multidisciplinary Aircraft Design," Proceedings ofIndustry-University Workshop, Virginia PolytechnicInstitute and State University, compiled by R. T.Haftka, et. al., pp. 49-80, May 1993.

5. Anon. "Hornet for 2000 Final Report," TheMcDonnell Douglas Corporation MDC ReportB0833, 29 February 1988.


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