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Atmospheric Flight Mechanics Conference and Exhibit14-17 August 2000Denver, Colorado

AIAA 2000-3913 CPResearch on the F/A-18E/F Using a22%-Dynamically-Scaled Drop Model

M. Croom, H. Kenney, and D. MurriNASA Langley Research CenterHampton, VA

and

K. Lawson Naval Air Systems CommandPatuxent River, MD

AIAA-2000-3913

RESEARCH ON THE F/A-18E/FUSING A 22%-DYNAMICALLY-SCALED DROP MODEL

Mr. Mark A. Croom, Ms. Holly M. Kenney, and Mr. Daniel G. MurriNASA Langley Research Center

and

Mr. Kenneth P. Lawson*

Naval Air Systems Command

* Member AIAACopyright © 2000 by the American Institute of Aeronautics And Astronautics, Inc. No copyright is asserted in the United States underTitle 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein forGovernmental Purposes. All other rights are reserved by the copyright owner.

ABSTRACT

Research on the F/A-18E/F configuration wasconducted using a 22%-dynamically-scaled drop modelto study flight dynamics in the subsonic regime.Several topics were investigated includinglongitudinal response, departure/spin resistance,developed spins and recoveries, and the falling leafmode. Comparisons to full-scale flight test resultswere made and show the drop model stronglycorrelates to the airplane even under very dynamicconditions. The capability to use the drop model toexpand on the information gained from full-scaleflight testing is also discussed. Finally, apreliminary analysis of an unusual inclined spinningmotion, dubbed the "cartwheel", is presented here forthe first time.

INTRODUCTION

Flight dynamic issues can significantly impactthe safety and effectiveness of high-performancevehicles such as the Navy's latest strike aircraft, theF/A–18E/F Super Hornet shown in figure 1.Dynamic aspects often dominate the aerodynamics andcan strongly influence the flight mechanics,particularly during the large-amplitude, high-ratemaneuvering that is required of this class of aircraft.The typical challenge to the designer is to maximizemaneuvering performance while maintaining goodlevels of departure resistance throughout the intendedoperational envelope and beyond (for margin).

A time-honored approach to achieve thisbalance between stability and maneuverability is togain a comprehensive understanding of the aircraftflight dynamic properties by subjecting theconfiguration to a battery of tests.

Figure 1.- The F/A–18E/F Super Hornet

In the subsonic flight regime these tests (see figure 2)typically include captive and free-flying wind-tunneltests (static force and moment, forced-oscillation, free-flight, free-spin, and rotary-balance, among others),outdoor drop model tests, numerical based analyses(e.g., piloted simulation studies), and culminate withfull-scale flight testing. All of these tests wereperformed during the F/A–18E/F development;however, this paper concentrates on the drop modeleffort.

Figure 2.- Ground-based test methods.

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Drop model and aircraft flight test resultscovering a range of flight conditions andconfiguration arrangements are presented to illustratesome of the fundamental subsonic flight dynamiccharacteristics of the F/A–18E/F. In so doing, asampling of the drop-model test technique capabilitiesis demonstrated.

Some of the drop model maneuvers wereperformed to compare to aircraft flight data; thesecomparisons are presented where available and showthat the model method accurately represents the full-scale aircraft. Other drop model maneuvers wereconducted to study areas outside of the scope of theaircraft flight test program that extended the depth ofinformation on the F/A–18E/F design and flightdynamics in general.

NOMENCLATURE

Symbols:AVΩ angle between velocity and rotation

vectors, degClp

roll damping coefficient

CL lift coefficient

Cm pitching moment coefficient

Cmqpitch damping coefficient

g acceleration due to gravityN model-to-airplane scale ratiop body-axis roll rate, deg/secq body-axis pitch rate, deg/secr body-axis yaw rate, deg/sect time, secV free-stream velocityα angle of attack, degβ sideslip, degδlat normalized lateral stick, +=right

δped normalized pedal, +=right

ρ air densityψ heading angle, deg

Abbreviations:CAS control augmentation systemFCS flight control systemLEF leading-edge flapLEX leading-edge extensionSRYP synchronous roll yaw parameterTEF trailing-edge flap

DESCRIPTION OF TEST METHODS

The test methods used to generate the primaryresults presented within this report were the dropmodel technique and flight tests of the actual aircraft.As such, this section will focus on those two testmethods even though the report contains limitedresults from other ground-based tests - specifically,captive wind-tunnel, spin tunnel, wind-tunnel free-flight, and simulation. Additional information aboutthese test methods can be found in thereferences.1,2,3,4 However, a general discussion of thenon-captive scale-model test methods is givenfollowing a description of the aircraft and the full-scale flight test program.

Aircraft Description

The F/A-18E/F is a high performance, twinengine, supersonic fighter/attack aircraft. Theairplane features twin canted vertical tails andmoderately swept variable camber wings with leadingedge extensions (LEX) extending from the wing rootforward on each side of the fuselage. Two GeneralElectric F414-GE-400 turbofan engines power theaircraft. The E-model is a single place aircraft; the F-model is a tandem two-place version of the E-model.

The F/A-18E/F flight control system is adigital, quadruplex, fly-by-wire, full authority controlaugmentation system (CAS). All control lawcomputations are performed by four digital computersworking in parallel. It includes a built-in redundancymanagement system that detects and isolates controlsystem component failures. Due to control systemcomponent redundancy, multiple like failures arerequired before degradation in flying qualities orstability occurs. On the F/A-18E/F aircraft, nobackup control modes are implemented where controlof the actuators revert from CAS closed loop controlto an alternate control function (e.g. no heavymechanical backup). Instead, CAS operation alwayscontinues following failures to provide potentiallydegraded, but acceptable flying qualities.

Longitudinal control uses symmetric deflectionof the stabilators, leading- and trailing-edge flaps,ailerons, LEX spoilers, and toe-in of the rudders.Lateral-directional control uses differential deflectionof the stabilators, ailerons, leading- and trailing-edgeflaps, and symmetric rudder deflection. In the takeoffand landing configuration, there is no differentialdeflection of the flaps.

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Full-Scale F/A-18E/F Flight Test Program

The full-scale flight test effort represented thelargest and most aggressive high angle of attackprogram ever undertaken for the development of aproduction fighter.5 Its scope included two distinctlydifferent models (the single place E-model and two-place F-model), store loadings ranging from cleanwing to a full complement of external 480-gallon fueltanks and heavy bombs (symmetrically andasymmetrically loaded), and fully developed spins to120 deg/sec. The test team used a phased approach,which systematically increased the team’s knowledgeof the airplane such that each increment in risk wasled by an increase in the team’s confidence in theairplane and supporting models.

Phase 1 testing examined the departuresusceptibility by the application of single axis inputsat various flight conditions. Phase 1 also provided theupdates to the simulation aerodynamic model thatproved crucial to the safe execution of later phases.

Phase 2 provided the first look at upright andinverted spins, and sustained out of control flightmodes. Spins deliberately preceded aggressivemaneuvering (Phase 3) so that any unpredicteddepartures would be expected to place the test teamback into familiar terrain.

Phase 3 opened the envelope for aggressivemaneuvering, and included the full spectrum of multi-axis aggravated maneuvers, spins to high yaw rates(120 deg/sec), and zero-airspeed tail-slides.

Initially these phases were strictly followedwith no wing stores on a specially equipped single-place (E-model) airplane.† The sequence was thenrepeated in each of the desired symmetric externalstore loadings, followed by limited scope tests of thetwo-place (F-model) airplane.

Finally, asymmetric stores testing wascompleted in phases 4 and 5. Phase 4 consisted ofsingle and multi-axis aggravated inputs and uprightspins and inverted spins for asymmetric loadings upto 14,000 ft-lbs. Phase 5 consisted only of singleaxis inputs across a limited angle of attack range(≤25°) for weight asymmetries up to 28,000 ft-lbs.Over the course of three years, a total of 221 flights,and 378 flights hours were devoted to high-angle-of-attack maneuvering, departure resistance, and spintesting.6

† Modifications included a spin recovery parachute andemergency power accommodations.

Dynamically-Scaled Model Testing

The three non-captive dynamic model testtechniques (spin, free flight, and drop model as shownin figure 2), have been instrumental in understandingsome of the complex types of behavior of fighteraircraft at extreme angles of attack. They aresubsonic techniques and, in combination, cover awide range of high-angle-of-attack behavior as shownin figure 3. These techniques complement each otherto improve the overall understanding of the spin andrecovery characteristics. For example, drop modeltests are used to augment the results obtained by spintunnel testing. In this regime, drop model testpoints, which are typically limited in number, serveto spot check the more statistically significant spintunnel results. Additionally, the influence of modelscale (affecting issues from manufacturing tolerancesto Reynolds number effects) is potentially exposedowing to the diverse scale factors between the 3.6%-scale spin-tunnel model and the 22%-scale dropmodel.

Drop model(post-stall dynamics)

Angle of attack

Yawrate

Spin Tunnel(developed spin)

Wind-tunnelfree-flight

(stall/departure)

Figure 3.- Dynamic model test techniques.

The scale models used in all of the non-captivetests must be dynamically scaled to accuratelyrepresent the full-scale aircraft flight dynamics. Thisnot only requires geometric scaling, but also requiresmaintaining the equality of the scale model and full-scale airplane ratios of inertial effects to gravitationaleffects.7,8 Therefore, the units of length are scaledfrom geometric ratios; units of mass are scaled basedon equal relative density factors; and time is scaledbased on equal Froude numbers. Table I provides thefactors needed to determine the dynamically scaledmodel properties. The scaling factors must beproperly applied throughout all areas affecting theflight dynamics. For example, time scaling sets therequirements for actuator response time, flight control

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computer frame rate, filter performance, and any othertime related quantity‡.

Table I.- Scale Factors for Dynamic Models.

Model values are obtained by multiplying airplanevalues by the following scale factors where N is themodel-to-airplane scale ratio, σr is the ratio of air densityat the airplane altitude to that at the model altitude, l is arepresentative unit of length, m is the vehicle mass, andg is the acceleration due to gravity.

Quantity Scale FactorLinear dimension NRelative density (m/ρl3) 1Froude number (V2/ l g) 1Weight N3/σr

Moment of inertia N5/σr

Linear velocity N0.5

Linear acceleration 1Angular velocity N-0.5

Time N0.5

Drop Model Test Technique Description

Figure 3 shows that a significant void ofinformation exists between the pre-stall and stalldeparture results produced by the free-flight tests andthe fully developed spin behavior observed in the spintunnel tests. The radio-controlled drop modeltechnique was originally developed in the 1950's tofill this void, providing information on the post-stalland spin-entry motions of airplanes.

With the need to provide combat aircraft withthe capacity to aggressively maneuver at and beyondstall, the drop model technique has been furtherdeveloped to permit the investigation of the relatedflight dynamics issues while the test configuration isundergoing large amplitude, high rate maneuvers overthe entire low-speed flight regime and is not solelyfocused on spin entry. For example, past drop modelprograms of other aircraft configurations haveexplored a variety of low-speed phenomena includinghighly dynamic motions such as divergent wing-rock,aggressive tumbling, and roll departures as well asmore docile areas such as pitch control enhancement, ‡ This is not a recent discovery nor is it inconsequential.Achieving scale time often has been the most demandingfactor in subsystem selections; for instance, everyupgrade to the drop model flight control computerhardware since the 1970's has been driven by scaledthroughput requirements.

parameter identification, and control law refinementsfor accelerated and unaccelerated stabilityimprovements.9,10,11 As mentioned above, the high-angle-of-attack flight regime was not an afterthoughtfor the F/A–18E/F program; indeed, all three of thedynamic model methods were conducted for thisairplane including drop model.

The overall operation of the drop-modeltechnique involves dropping an unpowered,dynamically-scaled model from a helicopter, flying itthrough a series of maneuvers by remote control fromthe ground, and recovering it with a flight terminationparachute.

Figure 4.- 22% F/A-18E drop model.

Weighing from 880 to 1100 pounds(depending on the airplane store/fuel loading conditionof interest), the 22%-scale F/A-18E/F model (picturedin figure 4) is dropped from altitudes of up to 15,000feet (model scale) and is tracked by a manuallyoperated ground-based tracker on which video cameras,a radar ranging system, and telemetry antennas aremounted.§ In addition, a camera is mounted in thecanopy of the model to provide an onboard-pilot'sview.

§ The equivalent airplane altitude depends on the modelscale factor, the model-to-airplane mass ratio, and the airdensity: 15,000-ft model is about 35,000-ft full-scaleequivalent.

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The pilot sits at a control station and monitorsdata displays and the video images while providingthree-axis control through a customized cockpit asshown in figure 5. The overall control loop consistsof downlinked flight data, a digital computer onwhich the control laws are implemented, and acommand uplink. Once the pilot has executed theintended profile, the onboard flight terminationparachute is deployed to decelerate the model prior tosplashing down into the ocean.

Figure 5.- Drop model cockpit.

The drop model carries an extensivecomplement of flight instrumentation to provide real-time state feedback information to the flight controlcomputer, to drive real-time data displays for theground-based pilot, and for post-flight data analysis.Duplex flight sensors measure angle of attack,sideslip, airspeed, 3-axis angular rates and linearaccelerations, and 3-axis attitude. Simplex flightinstruments include pitot and static pressures, radarrange, pilot's-eye-view video, and control surfaceposition transducers. Various system-monitoringparameters and all of the flight sensor signals aredownlinked to a ground-based data logging systemand, along with pilot comments, video imagery fromthe ground, and digital recordings of ground-generatedsignals (such as pilot commands and mode selections)form the collection of data obtained during a flight.

OVERVIEW OF AIRPLANE FLIGHT TESTRESULTS

For the reader's benefit, a brief overview of theairplane falling leaf and spin recovery flight testresults are presented here. More comprehensivediscussions of the airplane flight test program and itsresults can be found in the references.5,6

Falling Leaf Out of Control Flight Mode

The elimination of the falling leaf mode fromthe Super Hornet is just one of the design team’sgreat triumphs. Although the mode naturally existsin the bare, unaugmented airframe, the E/F flightcontrol laws successfully inhibit any undesirablemotion. Despite extensive development flight testingand early operational evaluation, no sustained fallingleaf has been observed in any loading, or at anypermissible center of gravity location unlessoperating with the feedbacks disabled (CAS-off).

Spin Recovery

The scope of the spin program includedupright and inverted spins to 120 deg/sec, clean,symmetric and asymmetric external store loadings,FCS failure states, the full center of gravity range,misapplied recovery controls, and both E- and F-models.

As with the falling leaf testing, spins wereinitiated with the feedbacks disabled (CAS-off).Upright and inverted modes were identified. Whilethe upright modes were more stable and repeatable,inadvertent entries from CAS-on departures favoredinverted modes.

As predicted, recoveries in Auto SpinRecovery Mode were generally prompt with theapplication of full lateral stick with the yaw rate forupright spins, and against the yaw rate for inverted.Releasing the controls completed the recovery as theairplane restored CAS-on operation with the yaw rateunwinding through 15 deg/sec.

RESULTS

The results from the drop model test programare grouped by phenomena: longitudinal controlcharacteristics; spins (including several subtopics); anunusual attitude high rotation rate mode dubbed“cartwheel”; and a synchronized rolling and yawingmotion often referred to as a falling leaf. Resultsfrom the other scale model and the aircraft tests arediscussed along with these results where appropriate.

Airplane and drop model test results are oftenbest conveyed on the printed page in the form oftime-history plots of various flight parameters.Many of the quantities measured during drop modeltests (including time itself) need to be scaled usingthe proper factors as discussed above. All model

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quantities presented in this paper have been scaled torepresent full-scale equivalent values unless explicitlyindicated otherwise. Note that the lateral-directionalpilot inputs (lateral stick and pedal) are normalizedsuch that full throw equates to ±1 unit which is notthe usual convention for the airplane but allows forco-plotting.

Longitudinal Response

The F/A-18E/F must possess significantlevels of pitch control throughout its large trimangle-of-attack range to meet the high demands placedon it as an air superiority fighter.12 One particulararea of interest is its ability to generate nose-downrecoveries from anywhere within this range and to doso with sufficient authority to overcome opposingmoments generated by inertia coupling or othereffects. Several of the aforementioned test methodsprovide information relevant to this topic includingstatic tests, free-flight tests, and drop model tests.Meeting pitch control margin requirements is acomplex endeavor due to factors that must beaddressed such as thrust effects and entry conditionsand is beyond the scope of this study. However, thebasic longitudinal response characteristics can beassessed with the drop model.

Results from static low-speed aerodynamictests of a 15%-scale model of the F/A-18Econfiguration in the 30- by 60-Foot Wind Tunnelillustrate some of the fundamental longitudinalcharacteristics of the configuration. Untrimmed liftand pitching moment coefficient data are shown infigure 6 for symmetric horizontal tail deflectionsacross the entire deflection range of –24° to +20° withfixed LEF/TEF deflections of 34°/4° for an aft centerof gravity location of 29.4% mean aerodynamic cord.As mentioned above, these data indicate that theconfiguration can be trimmed across a large angle-of-attack range that encompasses the angle of attack formaximum lift. Also, the angle of attack forminimum nose-down control power occurs near thesame point. These static test results were used todirect studies of similar phenomena using free-flightand drop model tests.

+20+100-10-20-24

δh, deg

0Cm

0

-10 0 10 20 30 40 50 60 70 80 90

CL

α, degFigure 6.- Effect of horizontal tail deflection on static

longitudinal characteristics.

Maneuvers were performed during the free-flight and drop model tests where the respectivemodels were stabilized near maximum lift and aforward pitch stick input was rapidly applied. Theresponses through maximum pitch acceleration andpitch rate were measured and indicated that good levelsof pitch authority exist.

Similar maneuvers were performed with theairplane as shown in figure 7 where airplane and dropmodel time-histories are co-plotted. These data showthat the drop model results correlate well with thosefrom the airplane.

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0 1 2 3 4 5 6

time, sec

α

q

Drop ModelAirplane

Figure 7.- Longitudinal response characteristics fordrop model and airplane.

Spins

Extensive studies of the spin characteristics ofthe F/A-18E/F involved spin tunnel, full-scaleaircraft, and drop model testing - each of which hascontributed uniquely and corroboratively to the overallspin investigation. The free-spin tests conducted inthe Langley 20-Foot Vertical Spin Tunnel with a1/28th-scale model identified upright and inverted spinmodes for the F/A-18E/F.13 Those tests encompasseda wide variety of mass properties (both symmetric andasymmetric) that spanned the operational envelope forboth the E- and F-models. During each particularspin test point, the control surfaces either remainfixed or were moved from one preset position to oneother preset condition (i.e., these were open-loop testswith no state feedbacks). For the symmetricallyloaded configuration the highest rate upright spin wasa fast (spin rate = 200 deg/sec), flat (α from 78° to90°) mode occurring with fixed pro-spin controls.This rate exceeded the spin rotation limit placed onthe airplane during the flight-testing phase. As such,certain aspects of follow-on spin testing would berelegated to the drop model as discussed below.

Spin recovery properties were also determinedin the spin tunnel tests for a similarly wide range ofconditions and indicated that the configuration hassatisfactory recovery characteristics using aerodynamiccontrols. For example, recovery from theaforementioned spin mode was 33/4 turns usingailerons into and rudder against the rotation direction.Spin tunnel tests also identified the salient features ofthe emergency spin recovery parachute systemimplemented on the full-scale high-angle-of-attack

test aircraft. A complete account of the spin tunnelinvestigation is given in reference 13.

The remainder of this section relates to thedrop model and full-scale spin tests. Except duringthe spin resistance tests, all upright and invertedlimited-rate spin tests of the drop model and airplanewere conducted using the manually activated spinmode to generate the large control surface deflectionsneeded to drive the vehicles into spins. The controland mode sequencing was similar for the upright andinverted cases with the obvious differences. In both,the longitudinal stick was applied followed by spinmode selection, pedal deflection, and, after a delay, thelateral stick was ramped in while the longitudinalstick was relaxed. Upright spins were initiated from afull aft stick condition and were generated using crosscontrols (lateral stick direction opposite to the pedaldirection); inverted spins started from forward stickand required coordinated controls. The recoverymethod for both the upright and inverted spins was todisengage the spin switch, neutralize pedal, andreverse the lateral stick (i.e., lateral stick into thedirection of the spin arrow).

The discussions of the spin tests are organizedinto five subsections: the first two address theincipient spin properties whereas the final threeinclude the developed spin and recovery aspects.

Spin resistance. The spin tunnel resultsindicated that the F/A-18E/F spin modes are fullyrecoverable using full-authority control deflections.Accordingly, the aircraft control system designersimplemented a sophisticated spin mode logic thatprovides full authority command capability from thecommand stick and rudder pedals and serves as a pilot-directed spin recovery scheme (as opposed to a totallyautonomous spin prevention scheme).14 This modeis normally armed automatically so that inadvertentencroachments into the incipient spin regime can bepositively and rapidly arrested by the pilot. Note thatto engage anti-spin controls, the pilot must deflectthe lateral stick in the anti-spin direction; otherwise,the control laws remain in CAS mode. However, thespin mode can be manually entered to performintentional spin maneuvers (discussed later). TheCAS mode provides departure/spin resistance by,among other means, reducing excessive controldeflections at high angles of attack. The effectivenessof the scheme is demonstrated by the drop model dataof figure 8 where the pilot applied and sustained full

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pro-spin (cross) controls while remaining in CASmode from a full aft stick initial condition. The yaw

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Figure 8.- Spin resistance in CAS mode for dropmodel.

rate smoothly accelerated reaching a very manageablemaximum of about 30 deg/sec with only slightoscillations in sideslip: the model showed nopropensity to depart. In fact, the maneuver is amoderate-angle-of-attack coordinated roll that was veryquickly terminated when the pilot neutralizedcontrols. Thus, the CAS mode provides high levelsof departure/spin resistance while maintaining a gooddegree of maneuverability even in the presence of pro-departure pilot inputs. In the next sections, themanually activated spin mode was used to generategreater amplitude control deflections so that thedeveloped spin properties could be investigated.

Sustaining the spin. Throughout the courseof spin testing, both the drop model and the airplanehad difficulty sustaining a spinning conditionfollowing what appeared to be reasonable spin entrymaneuvers. The typical progression is shown infigure 9 where both drop model and airplane data aregiven.

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Figure 9.- Self-recovering spin from spin mode fordrop model and airplane.

After application of pro-spin controls, the motionprogressively became oscillatory (notably in angle ofattack, roll rate, and pitch rate) as the yaw ratedeveloped. The lateral oscillations grew until theexcursions reached sufficient amplitude, usually in thedirection of the lateral stick (i.e., left roll in a nose-right spin attempt), to cause the angle of attack tokinematically decrease. Once at low angle of attackwith large rolling surface deflections, the motionsbecame roll dominated negating the yawing motionand producing a self-recovery from the spin. Note,too, that this considerably complex and dynamiccondition is well represented by the drop model.

The difficulty to routinely achieve a sustainedspin was most prevalent early in the testing sequencesand became less so as more experience was gained.For the airplane, the use of split throttles augmentedthe aerodynamic yaw acceleration that, of course, wasnot available to the unpowered drop model. Instead,the drop model relied on sustaining a highly stabilizedinitial condition through the use of pilot selectablepitch and roll dampers. Once these methodologies

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were developed, spins in the manually activated spinmode were routinely achievable.

Simulation-based spin predictions. A veryextensive mathematical aerodynamic simulation ofthe F/A-18E/F containing approximately one milliondata points has been assembled using informationfrom numerous static and dynamic wind-tunnel testsand analytical methods.15 Nonetheless, the ability toaccurately measure and, more importantly, toconstruct with confidence the proper math modelformulations capable of representing even thefundamental dynamics (much less the subtle ones) inthe highly complex non-linear high-angle-of-attackregime is often beyond the capacity of some of thecurrent modeling methods.16 Due to theseuncertainties, it is prudent to perform a flight-to-simulation validation effort if possible.

Drop model flight data was used to assess thesimulation math model in the spin regime. Fiveoptional math model constructs of the rotationalcharacteristics are included as a part of the simulationsince research to determine the most accurate way tomodel rotational characteristics is ongoing.Simulation-generated data for the five options werecompared to the drop model spin data. Of the five,the option that best matched the flight data is shownin figure 10 as "Simulation Option A." The overallspin rate and average angle of attack agrees well withthe drop model data. However, the simulationproduced smoother motions than the flight dataindicate. The requisite pitch and roll oscillationsneeded to transform an emerging spin into a roll-dominated self-recovering motion are generally notpredicted by any of the options. However, addingsimple degradations in the linear pitch damping(Cmq ) and roll damping (Clp

) terms improved the

match as indicated by the “Simulation Option A +Mods” trace. These comparisons illustrate theprocess where flight data (from drop model or aircraft)can be used to validate or refine an aerodynamicdatabase in terms of either selecting modelingapproaches or creating incremental modifications.

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Figure 10.- Comparison of simulation modificationsto drop model spin.

Limited-rate spins. The drop model techniqueis uniquely suited among the scaled model methods toperform spin entry maneuvers (recall that research inthe Spin Tunnel begins with the developed spin).Accordingly, the drop model can be used toinvestigate spin maneuvers that fall short of reachingsteady-state conditions. Since the airplane waslimited to yaw rates below most of the predictedsteady-state spin rates, the drop model is also themodel method of choice to conduct airplane-to-scale-model correlations.

Limited-rate upright spin maneuvers from theairplane and the drop model, one of which is shownin figure 11, indicate that the vehicle is well behavedin the spin regime. Yaw rate increases smoothly inresponse to the command. Once at full lateral stick,the motions become oscillatory, but the yaw ratecontinues to increase. After about 2 turns the call torecover was made at a peak yaw rate of about 100deg/sec. A rapid recovery was achieved in less thanone turn.

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Note that the airplane and drop model are inexcellent agreement. The yaw rate buildup, the peakrate achieved, the recovery properties, and even theunsteady nature including the frequency of oscillationand to a high degree the amplitude of oscillation inangle of attack, yaw rate, and sideslip are all wellrepresented by the drop model.

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Figure 11.- Upright limited-rate spin for drop modeland airplane.

The limited-rate inverted spin is similarly wellbehaved as shown in figure 12. Note that the aircrafttime history has been spilt so that the entrycommands and the recovery commands aresynchronized between the drop model and airplane data(the spin condition was held for a longer period duringthe drop model maneuver). As in the upright case,once full pro-spin controls were applied and yaw ratehad developed, the angle of attack coupled and themotions became more oscillatory. The yaw rate wasallowed to reach a peak of about 90 deg/sec whenrecovery controls were applied. The spin was quicklyarrested in approximately one turn.

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Figure 12.- Inverted limited-rate spin for drop modeland airplane.

As in the upright case, the drop model invertedspin properties are in excellent correlation with theairplane. Additionally, since the pro-spin controlswere held for a longer time, the drop model resultsindicate that the spin remains benign while the pilotsustains pro-spin inputs for approximately 5 turns(verses 2 turns as obtained during the airplane test).

The very high level of agreement between thedrop model and the airplane validates the drop modeltechnique. With its viability established, the dropmodel can be used with confidence to off-load oraugment the airplane's flight testing efforts. To alimited extent, both of these aspects wereaccomplished during the F/A-18E/F drop modelprogram and are discussed next.

Beyond basic limited-rate spins. For varyingreasons, certain maneuvers were not performed duringthe full-scale flight test program. For instance, sincethe spin characteristics for the E- and F-models wereexpected to have certain similarities, it was prudent toavoid unnecessary safety risks incurred by extensivelytesting an unmodified F-model aircraft (recall an E-model was equipped with special flight test

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emergency recovery systems). As such, spin testpoints for the F-model were limited to uprightattitudes to avoid prolonged exposure to extremenegative angles of attack conditions where the enginesare more susceptible to stall. However, to furtherincrease confidence that F-model spin recoverycharacteristics are similar to the E-model, the dropmodel was used to explore the inverted spin.

The drop model data of figure 13 confirmstrong similarities exist in the inverted spincharacteristics between the E- and F-models. Notethat the F-model maneuver is split in order tosynchronize the entry and recovery portions of themaneuver. Very similar yaw accelerations, peakrates, and yaw decelerations are evident. In bothcases, positive recovery was quickly realized withinone turn after application of recovery controls.

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Figure 13.- Inverted limited-rate spin for E and F dropmodel.

The drop model was also used to augment thefull-scale vehicle testing by exploring conditionsbeyond the limits imposed on the airplane, both interms of lateral weight asymmetry and rotation rate.

As mentioned above, the phase 4 airplaneflight testing included spins with lateral weight

asymmetries up to 14,000 ft-lbs. and phase 5expanded the asymmetry to 28,000 ft-lbs. butrestricted the angle of attack; hence, no spins wereconducted with this loading. Drop model tests wereused to examine the spin conditions at the 28,000 ft-lbs. asymmetric loading out to a spin rate of 90deg/sec in the more severe direction of heavy wingout. Tests were performed for the E-model (uprightspins) and the F-model (inverted spins). The datashowed that with the asymmetric store loadings, bothmodels were more prone to enter the spin, yetrecoveries occurred within 3 turns. These resultsindicate that no unexpected accelerations were presentfor the extreme store loadings tested.

High rate spins tests were performed with thedrop model to observe the nature of the spin as itevolves towards steady state beyond the imposedairplane flight-test spin rate limit of 120 deg/sec.During these tests an unusual and extreme rotationmode (cartwheel) was uncovered and is the topic ofthe next section. However, note that while themotion is extreme, it does not occur suddenly, andrecoveries were always achieved. As such, theinterest is more academic since a pilot would haveample opportunity to affect recovery well before thecartwheel onset.

Cartwheel

High rate spin tests with the drop model wereperformed in the same basic manner as the limited-rate spin studies where the spin mode was engagedmanually to allow for large control surfacedeflections. The data of figure 14 show the effect ofsustaining pro-spin controls for approximately 45seconds. As with the limited-rate spins, the yaw ratesmoothly increased as the pilot applied pro-spinlateral stick. At full lateral stick, the motionsbecame somewhat oscillatory, yet yaw rate continuedto increase and began to level out at 115 deg/sec.Sustaining the pro-spin controls for an additionalthree turns resulted in an increase in yaw rate and theonset of the cartwheel motion. As the cartwheelmotion evolved, the yaw rate began to level off near180 deg/sec, but the maneuver was terminated prior toreaching an equilibrium condition.

The cartwheel is an inclined spinning motioncharacterized by the rotation vector becoming normal(or nearly so) to the velocity vector. The anglebetween these vectors, given by AVΩ, increased to a

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maximum average value of about 60° during thecartwheel maneuver as shown in figure 14.

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Figure 14.- Drop model cartwheel.

Rigorously, a pure cartwheel motion occurs when thevectors are truly orthogonal (AVΩ = 90°)**, and the

** Note that large values of AVΩ may merely indicate thepresence of a pitching motion as is evident at thebeginning of the time history. A pure pitch motion alsoproduces an AVΩ of 90°.

motion is dominated by body-axis yaw rate. Then,the angle of attack time history would be a series ofsteps†† alternating between 0 and 180° while sideslipwould sinusoidally vary with an amplitude of ±90°.Superimposed on this motion is a slow precession ofthe rotation vector, observed in this test to have aperiod on the order of 20 seconds. Regarding thepilot's g-environment, the cartwheel producesloadings similar to any high-rate spin: inclining thespin axis realigns the Earth's 1-g field with respect tothe pilot, but the rotational contributions dominate asis the case of a fast un-inclined spin.

As mentioned before, even though the motionis extreme, it does not come on abruptly: severalunmistakable spin turns precede the yaw accelerationassociated with the cartwheel affording ampleopportunity to recover. Furthermore, the 180 deg/secrotation was arrested within 3.5 turns using standardrecovery controls (neutral pedal and lateral stick in thedirection of the yaw rate).

This unusual cartwheel motion was alsoobserved during the free-spin tests in the Spin Tunneland during simulation studies. However, for differentreasons, the results from these techniques weredifficult to trust without further corroboration fromthe drop model. In the Spin Tunnel, the motion wasdifficult to observe for very long since the radius ofthe precession path quickly grew larger than thetunnel test section. Furthermore, launch dynamicscould not be completely ruled out: since the model ishand-launched with pre-rotation, it is possible toinadvertently incline the spin axis to the velocityvector. In the simulation, the rates and motionamplitudes were so large that the validity of theaerodynamic math model was suspect. This wasparticularly so in light of the math model's inabilityto represent the oscillatory nature of the normal spinas previously discussed. However, with theaforementioned degradations to the linear pitch androll damping terms in place, the simulation is ingood agreement with the drop model flight data asshown in figure 15. The simulation represents theoverall levels of yaw rate and even the oscillations inthe motion. For the simulation run, the controlswere sustained until a steady-state condition wasreached. The simulation prediction indicates a peakyaw rate of about 280 deg/sec develops before themotion stabilizes at nearly 200 deg/sec. Once at the

†† Sensor dynamics, signal conditioning, and the likemay alter the measured angle of attack waveform.

13American Institute of Aeronautics and Astronautics

steady condition, the angle-of-attack waveform isindicative of a nearly pure cartwheel motion.

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Figure 15.- Cartwheel for drop model and simulation.

The simulation was used to continue to studythe cartwheel phenomenon. Like a spinning top, themotion is dependent on complex rigid-body dynamicsthat include precession and nutation effects, and themotion was found to be highly dependent on theinertia and mass properties. Preliminary results fromthe simulation indicate the cartwheel phenomenon isprevalent only over a relatively small range of masscharacteristics.

The cartwheel mode, though of little realconsequence operationally to the F/A-18E/F, mayhave general implications more applicable to otherconfigurations or even other classes of vehicles. Assuch, research in this area needs to continue. Thishighly dynamic motion requires the use of a varietyof ground-based tools in order to gain confidence inthe analysis methods and the concomitant findings.The drop model can provide the flight data that theother techniques can use to validate against.

Falling Leaf

Another large amplitude dynamic motion, onethat is relevant to the F/A-18, is an out of controlmotion known as the falling leaf mode. This modecan pose a significant safety-of-flight hazard. It ischaracterized by in-phase rolling and yawing motionsand can, in severe cases where the resulting nose-upinertia coupling is high, prohibit recovery to low-angle-of-attack flight. Research17 into thisphenomenon has produced a predictive measure, thesynchronous roll yaw parameter (SRYP), that onlyrequires static and dynamic stability derivatives andthe vehicle inertias; an evaluation of SRYP indicatesthe un-augmented F/A-18E/F is susceptible to thefalling leaf mode. Note that SRYP is the value ofthe predicted ratio of the peak yaw rate to peak rollrate and will be discussed below. As mentionedabove, the control law design team successfullyeliminated the falling leaf mode from the augmentedairplane.

During the airplane flight tests, falling leafmaneuvers were achieved, but only while using thespin mode to manually override the CAS. Even atthat, the maneuvers were fairly difficult to establishand, like the spin, required a certain pilotingtechnique. The results from those maneuvers can beused to validate the falling leaf analysis method’speak rate ratio predictions for the flight conditionsobtained. To evaluate the prediction tool beyondthose conditions, it is desirable to use a flightvehicle, like the drop model, to collect data at extrememass conditions. First, however, the suitability ofusing an unmanned vehicle to conduct falling leafresearch needed to be determined.

Tests were conducted with the F/A-18E/F dropmodel at a condition where lessons learned from thefull-scale flight test could be employed. It was foundthat using a pilot-enacted computer-generatedsinusoidal stick command that mimicked the pilotingtechnique used during the full-scale tests produced thebest results in the drop model tests. The maneuver,shown in figure 16, was initiated at a high-angle-of-attack wings-level condition.

An asymmetric two-cycle sinusoidal signalwas superimposed on the lateral stick to excite themotion. Following the input wave train, the modelunderwent 3.5 cycles of the falling leaf motion. Theratio of the peak yaw rate to peak roll rate was 0.35:this is in excellent agreement with the airplane testresult of 0.34 for the condition tested. The falling

14American Institute of Aeronautics and Astronautics

leaf motion was quickly damped when the CAS modewas re-engaged - a finding consistently noted duringthe full-scale flight tests.

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For this test condition, several findings areapparent: the theory and the drop model validateagainst the airplane, and the drop model is capable ofperforming this type of research. Whether the modelmethod can independently uncover this motion(without, for example, relying on a previouslydetermined potentially vehicle-specific pilotingtechnique) is still to be determined. However, thedrop model is well suited for these studies since itinherently possesses the complex aerodynamicsassociated with this large amplitude phenomenon.

CONCLUSIONS

Drop model tests were performed as part of acomprehensive investigation into the subsonic flightdynamics of the F/A-18E/F. The tests wereconducted to provide information to the aircraftdevelopment program and follow-on full-scale flighttests in areas deemed beyond their scope (for safety orschedule reasons). In the course of pursing this goal,drop model data were collected that illustrate theability of a dynamically-scaled drop model to predict

the flight dynamic characteristics of the full-scaleairplane. Furthermore, conducting drop model testsafforded an opportunity to expand knowledge in thefield of low-speed high-angle-of-attack flightdynamics. The key findings from the drop modeltests are summarized below.

1. The current results show that the drop model testtechnique can be used to accurately predict high-angle-of-attack flight dynamic characteristics.The F/A–18E/F drop model data exhibitedexcellent correlation with full-scale airplaneresults, even during highly dynamic maneuversincluding spins and falling leaf maneuvers wherecomplex flowfields dominate the aerodynamics.

2. Test data collected by the drop model predictexcellent departure resistance and spin recoverycharacteristics during maneuvers beyond thescope of the full-scale flight test program. Theareas investigated included upright and inverteddeparture / spin characteristics with and withoutextreme asymmetric weight loadings.

3. An unusual inclined spin mode, dubbed the"cartwheel", was discovered during the dropmodel testing while at extreme spin conditions.This characteristic is not considered to be anoperational concern for the full-scale vehicle,since prolonged pro-spin inputs were required toexpose this spin mode. However, the cartwheelis of interest from a flight mechanics standpointand future aircraft programs may want to evaluatethe possible existence of this unusualcharacteristic.

REFERENCES

1 Chambers, J.R.: "Use of Dynamically ScaledModels for Studies of the High-Angle-of-AttackBehavior of Airplanes," Presented at the InternationalSymposium on Scale Modeling, Tokyo, Japan, July1988.

2 Snow, W. L.; Childers, B. A.; Jones, S. B.; andFremaux, C. M.: Recent Experiences withImplementing a Video Based Six Degree of FreedomMeasurement System for Airplane Models in a 20-Foot Diameter Vertical Spin Tunnel, Proceedings ofthe SPIE Videometrics Conference, Volume 1820,1992, pp. 158 - 180.

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3 Murri, Daniel G.; Grafton, Sue B.; and Hoffler,Keith D.: "Wind-Tunnel Investigation and Free-Flight Evaluation of a Model of the F-15 STOL andManeuver Technology Demonstrator." NASATP–3003, August 1990.

4 Cunningham, K.; Kenney, P.S.; Leslie, R.A.;Geyer, D.W.; Madden, M.M.; and Glaab, P.C.:"Simulation of a F/A-18E/F Drop Model Using theLaSRS++ Framework," AIAA-98-4160, 1998.

5 Madenwald, Niewoehner and Hoy, “F/A-18 E/FHigh AOA Flight Test Development Program,”SETP 1998 Report to the Aerospace Profession:Proceedings, September, 1998, pg. 128-142.

6 Heller, M.; Niewoehner, R.J.; and Lawson, K.P.:"High Angle of Attack Control Law Developmentand Testing for the F/A-18E/F Super Hornet,"AIAA–99-4051, August 1999.

7 Scherberg, M.; and Rhode, R.V.: "MassDistribution and Performance of Free Flight Models,"NACA TN 268, 1927.

8 Wolowicz, C.H.; Bowman, J.S.; and Gilbert,W.P.: "Similitude Requirements and ScalingRelationship as Applied to Model Testing," NASATP-1435, August 1979.

9 Croom, M.A.; et al.: "Dynamic Model Testing ofthe X-31 Configuration For High-Angle-of-AttackResearch," AIAA 93-3674 CP, August 1993.

10 Croom, M.A.; et al.: "High-Alpha FlightDynamics Research on the X-29 Configuration UsingDynamic Model Test Techniques," SAE-881420,October 1988.

11 Villeta, Jose R.: "Lateral-Directional Static andDynamic Stability Analysis at High Angles of Attackfor the X-31 Configuration," M.S. Thesis, GeorgeWashington University, August 1992.

12 Ogburn, M.E.; et al.: "High-Angle-of-Attack Nose-Down Pitch Control Requirements for Relaxed StaticStability Combat Aircraft," NASA CP-3149, October1990.

13 Fremaux, C.M.; et al.: "Spin-Tunnel Investigationof 1/28-Scale Models of the McDonnell DouglasF–18E and F–18F Airplanes," NASA TM SX-4678,August 1995.

14 DeMand, R.P.: "F/A-18E/F Flight ControlSystem Design Report, Volume II, Control LawOperation and Mechanization, Revision F," TheBoeing Company MDC 95A0037, March 1998.

15 Dzenitis, E.G.; Evans, C.L.; Marks, B.A.; andMiller, G.A.: "F/A-18E/F Stability and ControlAnalysis/Data Report," McDonnell Aircraft andMissile Systems MDC 97A0092, December 1995.

16 Brandon, J.M.; and Foster, J.V.: "Recent DynamicMeasurements and Considerations for AerodynamicModeling of Fighter Airplane Configurations," AIAA98-4447, August 1998.

17 Foster, John V.: “Investigation of theSusceptibility of Fighter Airplanes to the Out-of-Control Falling Leaf Mode,” High-Angle-of-AttackTechnology Conference, NASA Langley ResearchCenter, VA. September 1996.