NASA Technical Memorandum 104322

Evaluation of High-Angle-ofAttack Handling Qualities for

the X-31A Using Standard

Evaluation Maneuvers

Patrick C. Stoliker and John T. Bosworth

NASA Dryden Flight Research Center

Edwards, California

1996

National Aeronautics and

Space Administration

Dryden Flight Research Center

Edwards, California 93523-0273

PIO

PST

RPC

SSLA

STEM

pilot-induced oscillation

poststall

roll performance classification

slow-speed line-abreast

standard evaluation maneuver

INTRODUCTION

Controlled flight at high angles of attack (AOAs) provides a modem fighter aircraft with the

ability to turn rapidly, providing enhanced nose-pointing capability. The ability to accurately point

the nose of the aircraft in a timely manner is the basis for handling qualities criteria and ratings.

With the exception of recent flight programs such as the F-16 Multi Axis Thrust Vectoring

(MATV), 1 F-18 High Angle of Attack Research Vehicle (HARV), 2 and X-29A, 3 an opportunity for

flight test evaluations at poststall (PST) angles of attack has not existed. The Handling Qualities

Military Standard (MIL-STD-1797) 4 provides a summary of criteria for handling qualities that

have been derived primarily for a more conventional flight regime. Simulation-based criteria 5,6

have been developed to specifically address flight in the PST regime. Additional criteria 7-9 have

also been developed to address handling qualities of modem augmented aircraft. Using the X-31A

linear simulation, analytic evaluations of the handling qualities at high AOAs were performed to

predict the characteristics of this aircraft.I°

Designed specifically for investigation of flight in the PST regime, the X-31A Enhanced Fighter Maneuverability (EFM) program evaluated the benefits of thrust vectoring in a close-in combat

(CIC) environment with emphasis on PST or flight at greater than 30 ° AOA. Following the completion of the original X-31A CIC objectives, a high-AOA handling qualities flight test program

was performed. Standard evaluation maneuvers 11 (STEMs) were used to assess longitudinal and

lateral gross acquisition and fine tracking at high AOAs. Pilot ratings and comments were collected

immediately following each maneuver. These data were analyzed and compared with existing handling qualities criteria.

The development and preparation for the high-AOA handling qualities flight testing, a summary of the flight test data, a comparison of the results with existing handling qualities criteria, and a

summary of lessons learned during the flight testing are covered in this paper.

AIRCRAFT DESCRIPTION

The X-31A airplane (fig. 1) is a single-seat fighter configuration with an empty weight of

approximately 12,000 Ibm that uses a single GE-F404-400 engine (General Electric, Lynn,

Massachusetts). Fuel capacity is approximately 4000 Ibm. Two aircraft were built by Rockwell International (Downey, California) and Daimler-Benz Aerospace (Germany). The wing planform is

2
EVALUATION OF HIGH-ANGLE-OF-ATTACK HANDLING QUALITIES FOR THE

X-31A USING STANDARD EVALUATION MANEUVERS

Patrick C. Stoliker and John T. Bosworth

NASA Dryden Flight Research Center

Edwards, CA

ABSTRACT

The X-31A aircraft gross-acquisition and fine-tracking handling qualities have been evaluated

using standard evaluation maneuvers developed by Wright Laboratory, Wright-Patterson Air

Force Base. The emphasis of the testing is in the angle-of-attack range between 30 ° and 70 ° . Longitudinal gross-acquisition handling qualities results show borderline Level 1/Level 2 performance. Lateral gross-acquisition testing results in Level 1/Level 2 ratings below 45 ° angle of

attack, degrading into Level 3 as angle of attack increases. The fine-tracking performance in both

longitudinal and lateral axes also receives Level 1 ratings near 30 ° angle of attack, with the ratings

tending towards Level 3 at angles of attack greater than 50 °. These ratings do not match the expectations from the extensive close-in combat testing where the X-31A aircraft demonstrated fair to

good handling qualities maneuvering for high angles of attack. This paper presents the results of

the high-angle-of-attack handling qualities flight testing of the X-31A aircraft. Discussion of the

preparation for the maneuvers, the pilot ratings, and selected pilot comments are included. Evaluation of the results is made in conjunction with existing Neal-Smith, bandwidth, Smith-Geddes,

and military specifications.

NOMENCLATURE

AOA

CHR

CIC

EFM

HARV

HUD

KIAS

LOES

MATV

MAX AB

angle of attack, deg

Cooper-Harper rating

close-in combat

Enhanced Fighter Maneuverability

High Angle of Attack Research Vehicle

head-up display

knots indicated air speed

lower-order equivalent systems

Multi Axis Thrust Vectoring

maximum afterburner
EC94-42478-1

Figure 1.X-31A airplanein poststallflight.

a doubledelta with an inboard leading-edge sweep of 56.6 ° and an outboard sweep of 45 °. The

wing area, span, and mean chord are 226.3 ft 2, 22.833 ft, and 12.35 fl respectively. Four trailingedge flaps on the wing can be deflected symmetrically for pitch control and differentially for roll

control. The leading-edge flap is scheduled to deflect as a function of AOA. The aircraft has an allmoving canard for pitch control and to meet the requirement for aerodynamic recovery from extreme AOAs. The vertical tail contains a rudder for directional control at AOAs less than 40 °. Pitch

and yaw moments can be generated by the three thrust-vector vanes. The inlet lip is moveable and

is deflected as a function of AOA. These control effectors were all integrated into a control

system 1°,12 that provided the capability for good control throughout the AOA range.

In the longitudinal axis, the control system uses load factor command to a maximum 30 ° AOA.

In the PST regime, from 30 ° to 70 ° AOA, deflections of the control stick command a specific

AOA. Three in. of aft stick commands 30 ° AOA; and full deflection, or 4.5 in., commands 70 °

AOA. This characteristic results in a stick sensitivity in AOA command of 33.3 deg/in of stick deflection. The nominal stick force is 5 lbf/in. The rate of change of AOA command was limited to

25 deg/sec. The longitudinal control system also includes an AOA command limiter that was set by

the pilot. The AOA limiter provided the capability for the pilot to set the limit for the AOA command in 5 ° increments from 30 ° to 70 ° AOA.

For the lateral--directional axes, deflection of the control stick commands velocity-vector roll

rate. The roll stick deflects 3 in. left and right. The maximum allowable roll rate is 240 deg/sec at

a low AOA. In PST, the velocity-vector roll rate is between 30 and 50 deg/sec, scheduled as a function of dynamic pressure and AOA. During envelope expansion, the pilots had difficulty using fulllateral stick when using full-aft pitch stick because of interference with their legs. To accommodate
this, thelateral-stickdeflection-to-roll commandgainwaschangedlinearly from 1to 2 between

30° and70° AOA. Thischangeresultsin full-roll ratecommandbeinggeneratedwith half-stick

deflectionat70° AOA. Therudderpedalscanbeusedto commandsideslipatlow AOAs,andtheir

commandauthorityisreducedto0° at an AOA greater than 40 °. The basic operation of the aircraft

is designed for "feet-on-the-floor" flying.

The primary source of information for the pilot was the head-up display (HUD) (fig. 2). This

display contained a conventional pitch ladder and heading display. Altitude and altitude rate were

displayed on the upper right, while airspeed and Mach number were shown on the upper left. On

the left side of the display were two tapes that showed the AOA and load factor. These data were

displayed digitally at the top of the tapes. The current AOA command limit was indicated by an arrow next to the AOA tape. The HUD also contained a 2-mrad fixed pipper, depressed 2 ° from the

waterline with an inner 20-mrad and outer 40-mrad reticle. Flight test instrumentation allowed inflight recording of the HUD.

Mach number

Airspeed _

Aircraft g _

+4.6 +3.8

Angl_o_ attack +9 -

+8-

+7-

Angle-of- +6 -

attack

limit---_ +5 -

 4_q/-g

+3-- J

+2--

(3£110_ +1 - /

_> +0-

_2_

_ g

F Rate of

350 000 010 -1200 climbldescent

I I I I I _"_6_ _ Altitude

Heeding Jl___l _ Sideslip angle

+5.0

s -VV- s

Aircraft reference

/

• /-- Pitch ladder

f "_ _ 40-mred outer reticle

___ 20-mrad inne_reticle

5 .... 5

%

/ V, \

[ _.,-- Bank angle

960376

Figure 2. Head-up display symbology.

AIRCRAFT SIMULATIONS

Three simulations were used in the preparation for and analysis of the flight test maneuvers: a

six-degrees-of-freedom, nonlinear simulation 13that incorporated flight hardware and a fixed-base

cockpit mock-up; a batch version of the six-degrees-of-freedom simulation; and linear simulations

of the longitudinal and lateral--directional axes.

The cockpit for the piloted simulation incorporated the pilot displays and controls. A 5 ft by 6.5

ft fiat screen projection provided the pilot a limited view out of the cockpit. The field of view for

4
thisscreenwasapproximately30deg laterally and 20 deg vertically. One feature of the simulation

was the capability to project a target aircraft that could be used for practicing the maneuvers. The

target aircraft trajectory could be "flown" and recorded to allow for training with a repeatable

maneuver.

The batch version of the simulation was used primarily for the generation of linear state-space

models. Using these plant descriptions from the batch simulation, the linear simulation was used to

generate transfer functions for use in the handling qualities criteria. These transfer functions could

be used directly in the criteria evaluation or in the calculation of lower-order equivalent systems

(LOES) parameters. The aerodynamic models for the linear simulation were fourth order. The control system included sensor models, filters, and high-order actuator models.

HANDLING QUALITIES EVALUATION

During the X-31A flight testing, an informal handling qualities evaluation was conducted during the CIC testing and formal evaluation using STEMs. The CIC testing was performed to evaluate the effectiveness of PST maneuverability. 14From a predetermined set of starting conditions, the

X-31A airplane was flown against an adversary aircraft. Both pilots were free to maneuver as required to try to establish a tracking situation. In addition to the test pilots assigned to the program,

service pilots demonstrated the ability to become quickly familiar with the aircraft and to fly aggressively without any limitations on control stick inputs in the PST flight regime. In all of the CIC

engagements, the pilots flew the aircraft aggressively to try to "win" the simulated combat.

CIC testing is used as a comparison with the formal handling qualities testing because of the

demonstrated ability of the X-31A pilots to successfully accomplish gross acquisitions and perform fine tracking in a high-gain environment at high AOAs. During CIC evaluation, the X-31A

aircraft was generally able to outperform adversary aircraft by using PST maneuvers. Although no

handling qualities ratings were made during these tasks, the general consensus was that the X-31A

aircraft had good handling qualities (Level 1 or Level 2) in this flight regime, and no major handling qualities deficiencies were noted. Similar handling qualities were expected from the STEM

evaluations. A disadvantage of using CIC to evaluate handling qualities is that the AOA varies considerably and the handling qualities characteristics cannot be sorted out as a function of AOA.

A method for providing consistent techniques for flight-test handling quality evaluation has

been addressed by the definition of a set of STEMs. 11 These maneuvers can obtain evaluations at

a constant AOA that can then be compared to analysis. During a limited flight test evaluation, the

X-31A aircraft used four evaluation maneuvers: three STEMs, and a maneuver developed from

CIC testing. The flight test maneuvers were derived from STEM 10 (High-AOA Longitudinal

Gross Acquisition), STEM 3 (High-AOA Lateral Gross Acquisition) and STEM 2 (High-AOA

Tracking). The chase airplane for the X-31A aircraft, an F-18 aircraft, was used as the target airplane. Data were collected using a pilot rating sheet that was completed immediately following

each maneuver, postflight interviews, a review of in-flight video recordings made through the

HUD, and a comprehensive set of telemetered data. The techniques for performing these maneuvers were developed using experience gained from the F-18 HARV program. In order to emulate

the acquisition and tracking tasks that were performed during the CIC investigation using the

5
X-31A airplane,anadditionalevaluationmaneuverwas flown. This maneuverusedslow-speed

line-abreast(SSLA)initial conditionsandresultedin acquisitionandtrackingtasksat avarietyof

AOAs. The formal handlingqualitiestestingcovereda 5-monthperiodand usedfive different

pilots duringtheperformanceof 19flights.

Flight preparationinvolvedpracticein thesimulatorto establishguidelinesfor maneuvering

thetestandtargetaircraft.Theinitial startingpositions,targetmaneuver,andtiming weredefined

so that the gross-acquisitionor fine-trackingtasksoccurredat a specificAOA. To accurately

achieveconsistentinitial startingconditions,twooperationalgroundradarswererequiredbecause

theX-31A aircraftwasnotequippedwith aradar.Duringtesting,thepilots couldachieveconsistentspacingwithout thegroundradarsby comparingtherelativetargetsizewith theHUD reticle.

Pilot commentswererecordedonaquestionnaireimmediatelyfollowing eachmaneuver.The

completionof the questionnairerequireda pilot rating usingthe Cooper-Harperrating (CHR)

system15(fig. 3) andanevaluationof theconfidenceclass(fig. 4). Theconfidenceclassratingwas

usedto helpassesstheeffectivenessof themaneuversfor ratinghandlingqualities.Changingthe

Aircraft Demands on the pilot in selected Pilot I

rating I

Adequacy for selected

task or required operation*

l Yes

Is it

satisfactory without

Yes

Is

with a tolerable pilot

workload?

Yes

Is it

controllable?

characteristics task or required operation*

Excellent Pilot compensation not a factor

Highly desirable for desired performance

Good Pilot compensation not a factor

Negligible deficiencies for desired performance

Fair - some mildly Minimal pilot compensation

unpleasant deficiencies required for desired performance

Minor but annoying Desired performance requires

deficiencies moderate pilot compensation

Moderately objectionable Adequate performance requires

deficiencies considerable pilot compensation

Very objectionable but Adequate performance requires

tolerable deficiencies extensive pilot compensation

Adequate performance not

Major deficiencies attainable with maximum tolerable

pilot compensation. Controllability

not in question

Considerable pilot compensation

Major deficiencies is required to control

Major deficiencies Intense pilot compensation is

required to retain control

Deficiencies

warrant

improvement

Deficiencies _-4

require

Improvement

I I

Improvement I-_ Major deficiencies

mandatory ! ]

Control will be lost during some

portion of required operation

* Definition of required operation involves designation of flight

phase and/or subphases with accompanying conditions.

Figure 3. Cooper-Harper rating scale.

960377
initial conditions or additional practices improved the confidence class ratings. Pilot comments

were solicited regarding difficulty, predictability, aggressiveness effects, and control system effects. Following these comments, a pilot-induced oscillation (PIO) rating (fig. 5) and a second

CHR were recorded. For lateral gross acquisitions, a rating using the roll performance classification (RPC) 16 (fig. 6) was also solicited. The RPC was developed through simulation studies to address the open-loop nature of lateral gross acquisition. The RPC is intended to judge the initial rate

and rate onset and is not based on the ability to arrest the roll rate. The pilot comments were transcribed using the HUD video recordings that were available for every flight.

Each maneuver was also evaluated using the telemetered data. Linear models were calculated

for each maneuver based on the AOA, airspeed, altitude, and estimated fuel state. The linear

models were used to generate the parameters and frequency responses required for the handling

qualities criteria.

Classification Description

A

B

C

The pilot rating was assigned with a high degree of confidence.

The pilot rating was assigned with only a moderate degree of confidence

because of uncertainties introduced by moderate differences in

environmental conditions, or in aircraft configuration or state, or in task,

from what was desired.

The pilot rating was assigned with minimum confidence because of

important differences between the desired and actual environmental

conditions, aircraft configuration or state, or task, requiring considerable

pilot extrapolation.

960378

Figure 4. Classification of pilot confidence factor.

Numerical Description

rating

No tendency for pilot to induce undesirable motions.

Undesirable motions tend to occur when pilot initiates abrupt

maneuvers or attempts tight control. These motions can be prevented

or eliminated by pilot technique.

Undesirable motions easily Induced when pilot Initiates abrupt

maneuvers or attempts tight control. These motions can be prevented

or eliminated but only at sacrifice to task performance or through

considerable pilot attention and effort.

Oscillations tend to develop when pilot initiates abrupt maneuvers or

attempts tight control. Pilot must reduce gain or abandon task to recover.

Divergent oscillations tend to develop when pilot initiates abrupt

maneuvers or attempts tight control. Pilot must open loop by releasing

or freezing stick.

Disturbance or normal pilot control may cause divergent oscillation.

Pilot must open control loop by releasing or freezing stick.

Figure 5. Pilot-induced oscillation rating scale.

960379

7
Roll performance for

mission effectiveness

Enhancing -

tactically superior

Satisfactory -

mission requirements met

Improvements In

roll performance

None warranted

May be warranted,

but not required

Numerical

1

2

Unsatisfactory - Required 3

mission requirements not met

Unacceptable - Mandatory 4

tactically useless

960380

Figure 6. Roll performance classification.

Longitudinal Gross Acquisition

STEM 10 was used as the basis for the longitudinal gross-acquisition task. The X-31A airplane

started 3000 ft in trail of the target aircraft. At the initiation of this maneuver, the target aircraft entered a steady turn to the conditions indicated in Table 1. After predetermined time delays, the

X-31A pilot selected maximum afterburner (MAX AB), roiled the aircraft so that the target aircraft

was in the pitch plane, and then aggressively pulled to capture the target in the pitch plane within

the criteria (fig. 7). The pipper and reticles in the HUD provided a reference for evaluating the

gross-acquisition and fine-tracking tasks. The goal of the tasks was not to drive the pipper to the

target, but to acquire or track the target within the specified criteria in relation to the pipper. The

timings were selected so that the gross acquisition would occur at the desired AOA of either 30 °,

45 °, or 60 °. The AOA limiter was not used during this testing. Table 1 shows the maneuver timing

for each flight condition.

Lateral Gross Acquisition

Lateral gross acquisitions were flown using STEM 3 as a baseline. For these maneuvers, the

target aircraft established a steady turn at specified conditions, and the pilot of the X-31A aircraft

maneuvered the aircraft to the target AOA (30 °, 45 °, or 60 °) at maximum afterburner. Depending

on how rapidly the pilot applied aft stick, the aircraft could be at 1 g or an elevated load factor at

the desired AOA. When the target aircraft was at a prespecifled angle away from the nose of the

X-31A aircraft, the X-31A aircraft was maneuvered aggressively using only lateral stick to acquire

the target in the roll plane within the criteria (fig. 8). Table 2 shows the initial conditions for these

maneuvers. To assist the pilot in remaining at the targeted AOA and to try to constrain the maneuver to the lateral axis, the AOA command limiter was set to the desired value.

8
4

Table 1. Task descriptions for longitudinal gross acquisition.

Flight condition

Maneuver

timing Maneuver description

30 ° AOA, Mach 0.45 T=0

T + 4 sec

T + 4 sec

Target begin maneuver: MAX AB, constant

20 ° AOA turn, maintain 200 KIAS.

X-31A advance throttle to MAX AB.

X-31A roll in plane with target, perform

rapid pull to 30 ° AOA.

30°AOA, Mach 0.60 T=0

T + 4 sec

T + 5 sec

Target begin maneuver: MAX AB, constant

20 ° AOA turn, maintain 200 KIAS.

X-31A advance throttle to MAX AB.

X-31A roll in plane with target, perform

rapid pull to 30 ° AOA.

45°AOA, Mach 0.50 T=0

T + 5 sec

T + 7 sec

Target begin maneuver: MAX AB, constant

25 ° AOA turn, maintain 170-180 KIAS.

X-31A advance throttle to MAX AB

X-31A roll in plane with target, perform

rapid pull to 45 ° AOA.

60°AOA, Mach 0.50 T=0

T + 5 sec

T + 8 sec

Target begin maneuver: MAX AB, constant

25 ° AOA turn, maintain 170-180 KIAS.

X-31A advance throttle to MAX AB.

X-31A roll in plane with target, perform

rapid pull to 30 ° AOA.

Desired:

Adequate:

Aggressively acquire target within 25* or 40** mrad longitudinally of

pipper with no overshoot and within a desirable time to accomplish

the task.

Aggressively acquire the target within 25* or 40** mrad longitudinally

of pipper with no more than 1 overshoot and within an adequate time to

accomplish the task.

* Criterion for 30°and 60 ° AOAs ** Criterion for 45 ° AOA 960381

Figure 7. Performance criteria for longitudinal gross acquisition.
Desired: Aggressively acquire target within 25* or 40** mrad laterally of pipper with

no overshoot and within a desirable time to accomplish the task.

Adequate: Aggressively acquire the target within 25* or 40** mrad laterally of pipper

with no more than 1 overshoot and within an adequate time to accomplish

the task.

* Criterion for 30 ° AOA ** Criterion for 450 and 600 AOAs

Figure 8. Performance criteria for lateral gross acquisition.

960382

Table 2. Task descriptions for lateral gross acquisition.

Angle of

Attack Test Condition Test Description

30 ° 170 KIAS

X-31A 1500 ft Echelon

and behind F- 18

AOA limit = 30 °

F-18 (target): Roll and pull to 170 KIAS/30 ° AOA,

adjust power/attitude to maintain conditions.

X-31A: MAX AB, pull to 30 ° AOA. When target is

30 ° off nose, acquire target laterally.

45 ° 170 KIAS

X-31A 1500 ft Echelon

and behind F- 18

AOA limit = 45 °

F-18 (target): Roll and pull to 170 KIAS/30 ° AOA,

adjust power/attitude to maintain conditions.

X-31A: MAX AB, pull to 45 ° AOA. When target is

300-45 ° off nose, acquire target laterally.

60 ° 170 KIAS

X-31A 1500 ft Echelon

and behind F- 18

AOA limit = 60 °

F-18 (target): Roll and pull to 170 KIAS/30 ° AOA,

adjust power/attitude to maintain conditions.

X-31A: MAX AB, pull to 60 ° AOA. When target is

300-45 ° off nose, acquire target laterally.

Fine-Tracking Evaluation

The fine-tracking evaluation consisted of two phases. Phase 1 testing was performed at AOAs

of 10% 15, ° and 20 ° to establish a reference point for comparison with other conventional AOA

evaluations and testing in the PST regime. During phase 1, fine tracking was performed only in the

longitudinal axis. Phase 2 testing, based on STEM 2, evaluated fine tracking at AOAs of 30 °, 45, °

and 60 ° for the longitudinal and lateral axes. The AOA command limiter was not used in finetracking evaluations.

Initial testing in Phase 2 concentrated on longitudinal fine-tracking evaluations while the maneuver setup was refined. Because only one axis was being evaluated at a time, the maneuver had

to be set up with the target approximately in the reticle so that maneuvering could be performed

only in the axis being evaluated. After an acceptable set of starting conditions was developed, the

same setup was used for the longitudinal and lateral tracking tests at each AOA. The X-31A pilot

10
wouldpracticethemaneuvertoensurethatthesetupwouldresultinthedesired AOA and then perform the maneuver twice. First, a longitudinal fine-tracking task was performed and pilot ratings

were given. Then a second maneuver was performed where lateral tracking and ratings would be

done. Table 3 shows the maneuver sequence and figure 9 shows the criteria. To test the ability to

make precise longitudinal changes in track point, the maneuver description called for the pilot to

move the pipper from nose to tail. Similarly, the lateral tracking task required the movement of the

pipper from wing tip to wing tip.

Table 3: Task descriptions for fine tracking.

Angle of

Attack Test Condition Test Description

10 ° 0.80 Mach number

X-31A 1500 ft behind F-18

F-18 (target): Roll and pull to 3 g, adjust power/

attitude to maintain conditions.

X-31A: Roll and pull to 10 ° AOA for longitudinal

tracking.

(Repeat with target at 1.8 g and initial Mach number

of 0.60.)

15 ° 0.75 Mach number

X-31A 1500 ft behind F-18

F-18 (target): Roll and pull to 3.5 g, adjust power/

attitude to maintain conditions.

X-31A: Roll and pull to 15 ° AOA for longitudinal

tracking.

(Repeat with target at 2.1 g and initial Mach number

of 0.55.)

15 ° 0.70 Mach number

X-31A 1500 ft behind F-18

F-18 (target): Roll and pull to 4.0 g, adjust power/

attitude to maintain conditions.

X-31A: Roll and pull to 20 ° AOA for longitudinal

tracking.

(Repeat with target at 2.4 g and initial Mach number

of 0.50.)

30 ° 180 KIAS

X-31A 1500 ft behind F-18

F-18 (target): Roll and pull to 180 KIhS/25 ° AOA,

adjust power/attitude to maintain conditions.

X-31A: MAX AB, at 20 ° angle off, roll and pull to

30 ° AOA for tracking.

45 ° 180 KIAS

X-31A 1500 ft behind F-18

F-18 (target): Roll and pull to 160 KIAS/30 ° AOA,

adjust power/attitude to maintain conditions.

X-31A: MAX AB, at 30 ° angle off, roll and pull to

45 ° AOA for tracking.

60 ° 180 KIAS

X-31A 1500 ft behind F-18

F-18 (target): Roll and pull to 170 KIAS/30 ° AOA,

adjust power/attitude to maintain conditions.

X-31A: MAX AB, at 45 ° angle off, roll and pull to

60 ° AOA for tracking.

11
Desired:

Adequate:

Pipper within +/- 5 mrad band for 50 percent of task and within +/- 25 mrad

for the remainder of the task; no objectionable PIO.

Pipper within +/- 5 mrad band for 10 percent of task and within +/- 25 mrad

for the remainder of the task; no objectionable PIG.

Figure 9. Performance criteria for fine-tracking tasks.

_0_3

Combined Maneuvers

Pilots consistently commented on the difference between the types of maneuvers used in the

handling qualities evaluations and the maneuvering performed during CIC. To address the perceived handling qualities differences between CIC and STEMs, a combined maneuver was evaluated during one flight. For this maneuver, the starting conditions were those of the SSLA setup

from the CIC flight tests. The X-31A and F-18 aircraft started side by side at the same speed and

altitudem215 knots indicated airspeed (KIAS) and 25,000 ft--separated by 1500 ft. For the handling qualities evaluation, the maneuvering began on the call of the X-31A pilot. The aircraft

initially turned towards each other with the X-31A aircraft going over the target aircraft. Then the

F-18 aircraft performed a single heading reversal and maintained a steady turn at 30 ° AOA and

170 KIAS. The X-31A aircraft maneuvered as required to acquire and track the target. Multiple

acquisitions were achieved by lagging off of the target aircraft and then maneuvering aggressively

to reacquire the target. Figure 10 shows the rating criteria.

Gross acquisition

Fine tracking

Desired:

Adequate:

Desired:

Adequate:

Aggressively acquire target within 25 mrad of pipper

with no overshoot and within a desirable time to

accomplish the task.

Aggressively acquire the target within 25 mrad of pipper

with no more than I overshoot end within an adequate

time to accomplish the task.

Pipper within +1- 5 mrad band for 50 percent of task and

within +/-25 mrad for the remainder of the task; no

objectionable PIO.

Pipper within +1-5 mrsd band for 10 percent of task and

within +1- 25 mrad for the remainder of the task; no

objectionable PIG.

960384

Figure 10. Performance criteria for the combined maneuvers.

HANDLING QUALITIES RESULTS

Handling qualities testing was done during 19 flights over a 5-month period in 1994.

Five pilots participated in the testing, using both X-31A aircraft. When acquiring the pilot

12
commentsat thecompletionof eachmaneuver,a CHRwassolicitedbeforeandafterthedetailed

comments.HavingthepilotrepeattheCHRattheendof thequestionnaireallowedareassessment

of theratinginlight of themoredetailedcommentsanddiscussion.Thesecondratinggivenis used

asthereferencefor thisreport.Thefirst andsecondCHR weregenerallythesame.

LongitudinalGrossAcquisition

Longitudinalgross-acquisitiontaskswereflown onfive flightsby threepilots.Theinitial timingsforthesemaneuverswerebasedonthepilotedsimulation.Becauseof thelimited field of view

provided by the projection television display in the simulator, transferring this simulation experience to flight was difficult. A total of 49 gross-acquisition tasks were performed with 28 receiving

pilot ratings. Twenty tasks were practices and one task was an unsuccessful gross acquisition. Eleven of the practice maneuvers occurred on the first flight. Results from this first flight were used to

refine the maneuver timing, and consequently, each of the other pilots typically required only one

practice at each target AOA. The goal was to collect data at 30 °, 45 °, and 60 °, with the actual AOA

for acquisition falling between 22 ° and 65 ° .

It became apparent after testing started that horizontal bands located in relationship to the pipper as specified by the performance criteria (25 or 40 mrads) rather than a circular reticle would

have provided the pilot with the appropriate reference for the task. Review of the HUD data and telemetry data showed that, during acquisition, if the target was entering the HUD field of view on

either side of the reticle, a lateral input to bring the target within the reticle often occurred.

Figure 11 shows the CHRs plotted as a function of AOA. These data show a trend for CHRs increasing from "2" to "4" as AOA increased from 20 ° to 65 °. The one CHR of "5" was the result of

a very large overshoot during capture. For these maneuvers, the pilots developed a technique to put

in a nearly full-aft stick initial input and then leading the AOA capture with forward stick. As a

compensation technique, Pilot B noted, "I'm starting to get a feeling for when I need to lead the

pitch rate to get the capture task." Figure 12 shows this phenomenon where maximum pitch rate

during the maneuver is plotted as a function of AOA. At the higher AOAs, the pilots would hold

aft stick longer, allowing a larger buildup of pitch rate prior to the countering control movement.

A confidence class rating of "A" was given for all but one of the maneuvers, meaning that the

pilots' ratings were assigned with a high degree of confidence. The PIO ratings for 18 of the

28 tasks were "1," indicating that the pilots observed no undesirable motions. The remaining tasks

received a PIO rating of "2," indicating undesirable motions that did not compromise task performance. These data indicate that the X-31A aircraft would have Level 1 performance at less than

40 ° AOA. The trend would be for borderline Level 1/Level 2 at AOAs greater than 40 °. These ratings matched the expectations from the CIC testing.

In conjunction with these pilot ratings, a number of pilot comments add insight into the data.

During the testing where the target AOA was 30 °, Pilot A reported, "Thirty is the critical point. It' s

better [for the evaluation] to be above 30; below 30 is too easy." For the PST AOAs, the pilots

13
consistentlynotedthatthestickforcesweretooheavyandthatthestickmotionwastoolarge.For

theacquisitionsat45oand60°,thepilotsnotedalateraldisturbancethatcomplicatedthetask.This

disturbancewasnotedduringenvelopeexpansionandwasattributedto asymmetricforebodyvortex coresthatchangedasafunctionof AOA.

Cooper-Harper

rating

10--

9--

8--

7--

6--

5--

4--

3--

2--

1--

Pilot

O A

[] B

Oc

Level 3

[]

O

Level2 _000 0

Level1 0 _ _' 0

O0 __0

t I t I I I I

0 10 20 30 40 50 60 70

Angle of attack, deg

960385

Figure 11. Cooper-Harper ratings as a function of angle of attack for longitudinal gross acquisition.

40 -- Pilot

O A

35-- [] B

Oc

30--

25--

15--

10--

5--

I

10

Maximum

pitch rate, 20

deg/sec

[] []

8

O•

O

O

[] O []

0

O0

oO o

0

0 o

I I I I I I

0 20 30 40 50 60 70

Angle of attack, deg

960386

Figure 12. Maximum pitch rate as a function of angle of attack for longitudinal gross acquisition.

14
Figure13showsanexampletimehistoryfor grossacquisitionat45° AOA. To show pitch-stick

movement, a comparison of AOA command with AOA response and pitch-rate response are

shown. Nearly full-aft stick is used to initiate the maneuver, followed by a number of stick inputs

on the order of one-half inch. These small stick displacements result in a rate-limited AOA command. An inspection of the trailing-edge flaps and thrust-vector vanes also showed periods of rate

limiting. None of the pilot comments indicated that rate limiting in either the command path or in

the control surface response affected the handling qualities.

Lateral Gross Acquisition

The lateral gross-acquisition task was performed by four pilots during five flights. Nineteen of

the total 49 acquisitions received pilot ratings (fig. 14). The remaining 30 maneuvers were practices. Two of the pilot ratings are not included in the summary of data because the AOA varied from

60 ° to 35 ° during attempted gross acquisitions at 60 ° AOA. The large number of practices required

for this task shows the increased difficulty over the longitudinal gross acquisitions. Unlike the longitudinal acquisitions, where the task was primarily confined to one axis after the X-31A aircraft

was banked into the correct plane, the lateral acquisitions required motion in multiple axes. First,

the aircraft is performing velocity-vector rolls that result in a significant coning motion at high

AOA. This motion is further complicated by the fact that the velocity vector settles during the maneuver. During extended maneuvers, the velocity vector is almost straight down, allowing the "helicopter gun attack." It should be noted that Pilot E had two sorties on one day and required the

same level of practice maneuvering in both flights. This pilot had also practiced similar maneuvers

in a domed simulation, which increased familiarity with the task being performed.

During the initial flight practices, the acquisition was not occurring at the desired AOA with the

target in the HUD field of view. Adjustments were made in the distance the X-31A aircraft was

trailing the target aircraft, the lateral displacement from the target aircraft, and the offset angle after

the target began maneuvering before the X-31A pilot initiated acquisition. Typical difficulties with

the performance of these maneuvers were loss of sight of the target aircraft by the pilot under the

nose of the X-31A aircraft, causing termination of the maneuver for safety concerns, and acquisition of the target above or below the HUD field of view as a result of improper initial lateral offset.

Figure 14 shows a comparison of CHRs with AOA, revealing a degradation in handling qualities as AOA is increased. The cases near 30 ° AOA generally fall into the Level 1 category. At 45 °,

the pilot ratings are consistent with Level 2 handling qualities. At 60 o, the trend is for Level 3 handling qualities. For this task, the majority of the maneuvers (11 of 17) were given a confidence

class rating of "B," which shows only a moderate degree of confidence in the ratings. All of the

data at AOAs greater than 50 ° were rated confidence class "B." Based on pilot comments, this

rating can be attributed to the difficulties with adjusting the initial conditions to account for the

multiple-axis maneuvers required of the X-31A aircraft. The general trend for increased CHRs

with increasing AOA is present in the ratings regardless of the confidence class rating.

15
i I

A ............... .4 ............... L, ........... d ............. .L ............... L ..............

q_ I I I I

t I I I

I I 4

i i

Pitch-stick

deflection, 2 ............... i ............... i .......

i i

in ! !

1 : ....... _ ....

,, ,,

0 _ ........ .L .............

i i i i

I I I I I

," ', , , ,. l

i i i i

i t i E i

-- ._ ........... u .............. J ......... J- ............... _- ..............

4o Angle-o,-e.eck/ /i \ W '_ ,'_'\ ",

ii command _ / / _Angle of 'I i \ %\

Angle of , , / / ; attack i I \ \

k,

I

i

I I I I I

Pitch rate,

deg/sec

40

30

20

10

-10

i

i

i

i

p i t

i i i

i i

i i

............... _ ............... r .................... T ............... _ ..............

I i i J

i i i

i ' '

2 4 6 8 10 12

Time, sec

960387

Figure 13. Time history from longitudinal gross acquisition.

16
10 --

9--

8--

7--

6--

Cooper-Harper

rating 5 --

4 --

3

2

1

Pilot

[] B

AD

eS

Level 3

Level 2

Level 1

•

mm

O • •

m

I I I I I I I

0 10 20 30 40 50 60 70

Angle of attack, deg 960388

Figure 14. Cooper-Harper ratings as a function of angle of attack for lateral gross acquisition.

This task did not emulate the lateral acquisitions performed during CIC testing. During simulated combat, the X-31A aircraft typically maneuvered within the turn radius of the target aircraft.

The X-31A velocity vector was nearly straight down, resulting in a "helicopter gun attack." The

CIC results did not indicate a tendency for Level 3 handling qualities at the higher AOAs. While

this task did identify handling qualities deficiencies, it is not clear that the STEM task is representative of the maneuvering pilots may be required to perform in the PST flight regime. For the

STEM, the pilot had to aggressively initiate the maneuver with full-lateral stick; while in CIC testing, the pilot input was proportional to the change in nose-pointing angle required.

The PIO ratings tended to increase as a function of AOA. Three cases had a rating of "2"; undesirable motions were present but did not affect task performance. An additional three cases had

a rating of "3," indicating that undesirable motions did compromise task performance. One case

was given a rating of "2-3," also falling into the category of undesirable motions. Two cases

showed nondivergent oscillations and received a PIO rating of "4." Two cases did not receive a

PIO rating, and six cases had a rating of "1."

Figure 15 shows stability-axis roll rate for each maneuver plotted as a function of AOA and

shows that, for the PST range, that rate was relatively constant at approximately 40 deg/sec. The

peak rate occurred for a maneuver at 25 ° AOA, and in general, the higher roll rates were the result

of the pilot using roll stick before achieving the desired AOA while the aircraft was still pitching

up. Nine maneuvers received an RPC rating of "2," or satisfactory. Eight cases received ratings of

"2.5," which falls between the satisfactory and unsatisfactory levels. One maneuver received a

RPC rating of "1," which equates to enhancing or tactically superior. This maneuver had the second highest stability-axis roll rate. The pilot commented, "I would say it' s just fine tactically. I got

17
_ound asfastasI wantedto." Althoughtheonsetratewasgood,thepilot wasunableto accurately

arresttheroll rate,resultingin a CHRof "8" andaPIO ratingof "4." Addressingtheundesirable

motions,thepilot stated,"Lots of them.Many overshoots;borderlinePIO at theend."Thepilot

alsonotedthatthetaskwas"very difficult."

Maximum

roll rate,

deg/sec

70

6O

50

4O

30

20

10

-- Pilot

[] B

__O C

A D

• E

I

[]

A

O

• 8

O

I I I I t I I

0 10 20 30 40 50 60 70

Angle of attack, deg

960389

Figure 15. Stability-axis roll rate as a function of angle of attack for lateral gross acquisition.

Figure 16 shows time histories. It can be seen that the pilot used full-stick displacement three

times during the maneuver with a peak stability axis roll-rate command of 40 deg/sec. Although the

stability-axis roll rate was high for this flight condition, the pilot had difficulty using it effectively

for the aggressive gross-acquisition task. These data indicate that using the RPC to assess only the

roll onset does not necessarily equate to good gross-acquisition performance.

During the performance of this task, the pilots regularly used full-roll stick displacement,

regardless of the AOA. For the high AOAs, this displacement would be more than required to get

maximum roll command because of the modification in the relationship between stick deflection

and full-roll rate command discussed above. One reason for this excess displacement would be that

no feedback to the pilot exists when full-roll command is generated. No pilot comments were directed towards any effects caused by the limiter that result when the pilot stick deflection is larger

than actually required for full-roll command. Examination of the time histories do not indicate any

particular effects from the control inputs. Analogous to the longitudinal task, rather than a circular

reticle, vertical bands at the specified distance from the pipper would have provided the pilot with

a more appropriate reference for the task.

18
Angle

of

attack,

deg

Lateral

stick

displacement,

in.

60

50

40

30

20

10

i i i

................ i................ ::/_-, ....... ,_?-i ..............

1 r, f ', _i- _, ;AN.

................. _.............. _:__,,:, ,----\;_:\-\ ............

Angle-of'apc_I//: ! ' _'i _. \

command_ ] _, i ', _. \ i ......... _ ...........-_-I---_..........

................',...... 7 _ Angle of ! _.

......... i.......... ._17..................................... "............

'_ ..... /J

."3. _'T _-__ S__ _- ....................................................... _. .................

I I I I

-2

t

r I

' i

................ _i ................ i:.................... i ................

i

I I I T

Stabilityaxis

roll rate,

deg/sec

20

-20

-4O

, i

i i

' _J', I

.................................................. .%-_,............................

iR°ll'rate ' / \ _ i

: command _/! \_ --1

,, \/, \ _ IX,_.:

........... -D'----k_--t:._

! ivlt U \'t; " -

.............:,3'_', ......./:_. .....v_.........................

rt

_------," .... ,_-'-# ..... 4.................................

I |/ ',

i *

i i

I

0 5 10 15 20 25

Time, sec 960390

Figure 16. Time histories for lateral gross acquisition.

Fine-Tracking Evaluation

Fine tracking was evaluated during eight flights by three pilots. In Phase 1 testing at low AOAs,

17 tracking tasks were performed and 9 maneuvers rated. Of the eight practices, six were required

19
in thefirst flight. DuringPhase2testingathigh AOAs,45trackingtaskswereperformed.A total

of 19practiceswererequired,and 16longitudinaland 10lateralfine-trackingtaskswererated.

Duringthefirst flight, sixpracticeswererequiredto getinitial conditionsthatallowedonescorable

task.Forthenext3flights,efficiencyimproved,with 9practicesrequiredto get13maneuversthat

couldberated.AsthetargetAOA increased,twoor threepracticeswererequiredto achievethedesiredaircraftpositioningatthetargetAOA. Thelast2 flights requiredonly 1practicefor 13scorablemaneuvers.

Onefactorthat affectedthepilot ratingswasthe amountof time spenttracking.Theoriginal

flight cardscalledfor 4 secof tracking.However,thepilots oftenspent20secor moreperforming

thetrackingtask,resultingin significantvariationsin flight condition(particularlyAOA). In one

casewheretheintendedAOA was30° but thetrackingoccurredbetween30° and23°, thepilot

commented,"Thereweretwo distinctiveairplanes.WhenI wasattheinitial AOA around30°, it

wasquiteabit harderto trackthanwhenI settledin. My ratingwill beassociatedwith theinitial

valuesof thetracking."Notall of thepilotswereasconciseinidentifyingtheAOA rangefor their

rating,andtheengineershadto identify theAOA.

LongitudinalFineTracking

Initial difficulties with fine trackingresultedfrom theinitial conditionsof the aircraft. The

spacingof 3000ft usedduringthelongitudinalgrossacquisitionswasreducedto 1500ft, but the

maneuvertiming usedfor grossacquisitionwasnot changed.Thischangeresultedin theX-31A

aircraft going a considerabledistancedownrangewhile the targetwasmaneuvering.When the

X-31A airplanewasmaneuvered,it wasoutsidetheturnof theF-18airplane.Suggestionsfrom the

pilot in thecontrolroomtobasethemaneuverontherelativeanglebetweentheaircraftallowedthe

onescorablemaneuverinthefirst flight. Duringthesubsequentflights,thestarttimefortheX-31A

maneuverwasbasedontheoff-boresightangleandresultedin morerepeatabletasks.

Figure 17 showsCHRsplotted as a function of AOA and showsanincreasein rating (or

decreasein handlingqualities)asAOAs increases.ForAOA lessthan30°, theratingsareconsistently "3" or less,indicatingLevel 1 handlingqualities.Between30° and50° AOA, theratings

rangedbetween"3" and"7." ThehighestratingsareatthehighestAOAs.ThisrangewouldberatedLevel 2 with twoLevel 3 ratingsnear50° AOA.

All of theratingswerein confidenceclass"A" forAOAs lessthan30°. ForthePSTratings,ten

werein confidenceclass"A" andsix wererated"B." Theseratingsreflecta highdegreeof confidencefor mostof theratings.All thepilots notedthatthetrackingtaskusedfor thehandlingqualities evaluationwasdifferentfrom thetypeof trackingthatwasdoneduringtheCIC evaluations.

Onepilot summarizedit by saying,"The trackingwe're trying to dohereiskind of dynamic-pitch

tracking andnot the kind of trackingwetypically did duringtheend game,which tendedto be

morein matchingyawrates."PIOratingsalsotendedto increasewith AOA for thistask.Theratings rangedbetween"2" and"4," indicatingundesirablemotionsandoscillationsthroughoutthe

PSTrange.

The initial instructionsfor thefine-trackingtaskscalledfor nose-to-tailtracking.Becauseof

the unique geometriesthat could resultduring the high-AOA maneuvering,the tracking tasks

20
requiredbothlateralandlongitudinalstickinputsto performthenose-to-tailtrackingbecausethe

maneuverplaneof theX-31A airplanewouldnotcorrespondwiththeplaneof symmetryof thetarget aircraft.This instructionwasmodifiedto statethattrackingwasnot necessarilyfrom noseto

tail, but shoulduseonly pitch stickinputsandusetheappropriateaircraftfeaturesasareference.

Evenwith themodifiedinstructions,the pilots would oftenusediagonalstick inputsduring the

trackingtasks.

Cooper-Harper

rating

10 -- Pilot

O A

g--_ C

Ao

8 _

7 -- Level 3

6 --

5--

4 -- Level 2

Level 1

3--

O

2--

1 --

I

0 10

O0

0

0 O0

00o

A

I I I I I I

20 30 40 50 60 70

Angle of attack, deg 960391

Figure 17. Cooper-Harper ratings as a function of angle of attack for longitudinal fine tracking.

Longitudinal Fine-Tracking Handling Qualities Criteria

The X-31A data were evaluated using the Neal-Smith, bandwidth, and Smith-Geddes criteria

to assess the applicability at high AOAs. These criteria are all based on the pitch stick-to-pitch attitude transfer function. An analytic study l° had shown that other criteria based on LOES were not

applicable to high-AOA flight. Transfer functions were generated using the linear models based on

the mass properties and flight conditions (Mach number, altitude, and AOA) associated with the

pilot ratings. The transfer functions were used in the criterion assessment and correlated with the

pilot ratings. With the exception of a few data points, the linear analysis results correlate with handling qualities ratings obtained in flight. The low-AOA data and the data with CHRs of "3" tend to

fall into the Level 1 regions for all of the criteria. The data with the higher CHRs seem to fall in

clusters, and for all the criteria, these clusters move away from the Level 1 regions.

Figure 18 shows X-31A data plotted using the Neal-Smith criterion. 7 The Neal-Smith criterion

uses a simple compensator model to close the loop of the pitch stick-to-pitch attitude transfer function. The magnitude of the resonant peak in the resulting closed-loop transfer function is compared

21
with thephaseangleof thecompensation.Thehigh-AOAdataindicatethatlessleadcompensation

canbeallowed.With two exceptions,datawith Level 1ratingsrequiredlessthan15° of leadcompensation.A clusterof datanear40° of leadcompensationwith adjacentCHRsof "5" and"7" exists that may indicatethe proximity of the Level 3 boundary.Figure 14 showsthe existing

boundariesassolidlines, andboundariesindicatedby theX-31A PSTdataareshownasdashed

lines.Additionaldataarerequiredto determineif theseboundariesarevalid.

Resonance

peak,

dB

15

10

-- Neal/Smith [bandwidth = 3 rad/sec, _ Criterion

time delay = 0.3 sec] ----- X-31 data

CHR

\ [] 3

\ 4

Level 3

-- A _-'_'-._ A s

v \\ _ e 6

\\\\ _ • 7

Level 2 <_> \\\

-- Ax\

40 60 80

Lead compensation, deg

i

a_ Level [3o1

o I

- 20 0 20

960392

Figure 18. Comparison of X-31A data with the Neal-Smith criterion.

For the bandwidth criterion 9 (fig. 19), all of the data show an estimated equivalent time delay

of approximately 0.04 sec. This criterion has correlated handling qualities with the estimated

equivalent time delay and bandwidth frequency calculated from the pitch stick-to-pitch attitude

transfer function. A reduction in bandwidth exists that is consistent with an increase in AOA and

CHR. The X-31A data indicate that the Level 1 boundaries are reasonable. Several data points exist

with a bandwidth of approximately 3 rad/sec that have CHRs of "5" and "7," indicating that it

might be appropriate to move the Level 3 boundary to this bandwidth as shown by the dashed line.

When compared with the Smith-Geddes criterion s (fig. 20), the X-31A data indicate that the

slopes of average CHR as a function of phase angle at the bandwidth frequency need to be steepened. The criterion calculates the bandwidth frequency based on the slope of the gain relationship

from the pitch attitude-to-pitch stick transfer function. In general, the tolerance bands for the average CHR would be valid for most of the data points with a pilot CHR of "3" or "4." An altemate

relationship between average CHR and phase angle at the bandwidth frequency is presented as a

dashed line.

22
Equivalent

time delay,

sec

.07

.06 --

.05 m

.04 --

.03 m

.02 m

.01 --

Level 3

MIL-STD bandwidth requirements - Category A

Level 2

I I \

5 10

Bandwidth, rad/sec

Criterion

------ X-31 data

CHR

O 2

[] 3

<> 4

L_ s

• 6

• 7

I

15

960393

Figure 19. Comparison of X-31A data with the bandwidth criterion.

Average

Cooper-Harper

rating

10 E

9B

8 ...... _\ \

7. "\, _ \, :\"

__ CHR "\' _ \'\ \ ,

o= ", \ ,\

ZXZX5 ""

3-- H []

uriterJon "----.........---._

Criterion bounds "_._.._ ....

2 .... X-31 data O

, I I I I I ] I

-220 -200 -180 -160 -140 -120 -100

Phase angle at bandwidth frequency, deg

960394

Figure 20. Comparison of X-31A data with the Smith-Geddes criterion.

The one data point that is anomalous for all three criteria is the 30 ° AOA tracking case that

received a CHR of "6." The confidence class rating was "A," indicating a high degree of confidence in the rating. In addition, the PIO rating of "3" indicated that undesirable motions affected

23
thepilot's ability to performthetask.Thepilot did attributesomeof the difficulty to aggressiveness, commenting, "The more aggressive you are, the more you oscillate." Another pilot performing a similar maneuver gave a better CHR of "4," but also commented, "If you are aggressive, you

get undesired motions." Other than pilot technique, one difference noted between the two tasks was

that the task that received the degraded rating was performed at a higher airspeed. An analytic

investigation of handling qualities 1° did show a degradation in predicted handling qualities during

PST flight as airspeed increased with a constant AOA.

Lateral Fine-Tracking

Figure 21 shows CHRs plotted as a function of AOA. As with the longitudinal tracking data,

some scatter in the ratings exists near 30 ° AOA, but the trend is toward higher CHRs as AOA increases. The three data points at an AOA at or greater than 40 ° had confidence class ratings of "B."

The lower AOA data received a confidence class rating of"A." The PIO ratings are consistent with

the other tasks in that an increase in undesirable motions as AOA increased existed, with oscillations being reported at the highest AOAs.

Cooper-Harper

rating

10 m

o

9-- <>

A

8--

7--

6--

5--

4--

3--

2--

1--

Pilot

A

C

Level 3 O

O

Level 2 O O 1_

O

Level 1 O_

A

I I I I I I I

0 10 20 30 40 50 60 70

Angle of attack, deg

960395

Figure 21. Cooper-Harper ratings as a function of angle of attack for lateral fine tracking.

A consistent pilot comment was, "The more aggressive you are, the harder the time you have

tracking." As well as the impact of aggressiveness on the task performance, the pilots also commented that the task frequently required diagonal stick inputs as opposed to pure lateral stick motions. Lateral tracking initially required wing tip-to-wing tip tracking. The tracking task was

redefined to use only lateral stick inputs, but the pilots continued to use diagonal inputs.

24
LateralFine-TrackingHandlingQualitiesCriteria

UsingLOESderivedfrom thelinearmodels,dutchroll frequency,dutchroll damping,therollmodetimeconstant,andtheequivalentime delaywerecalculatedandcomparedwith thecriteria

from MIL-STD-1797(fig. 22and23).ThesecriteriapredictLevel 1handlingqualitiesthroughout

theAOA range,whichisnotconsistentwith thehandlingqualitiesratings.Thesedataindicatethat

dutchroll frequencyanddampingandtheroll-modetime constantarenot the factorsaffecting

high-AOA handlingqualities.

1.2 w

1.0 m

Equivalent .8 --

dutch roll

damping, .6

rad/sec .4

•2 --

0

©

Level 1

Level 2

I I I I I I

CHR

02

I-]3

Z_s

Qs

• 7

I

Equivalent

dutch roll

frequency,

rad/sec

10

8

6

0 10

-- Comparison of dutch roll

mode with MIL-STD criteria

_

©

-- Level 1

t Level2 1..... T 1

20 30 40

Angle of attack, deg

<} • Z_

T i---I

50 60

960396

Figure 22. Comparison of X-31A data with MIL-STD-1797 dutch roll frequency and damping

criteria.

The lateral fine-tracking ratings were also compared with the Smith-Geddes criterion 8 (fig. 24).

Although a limited amount of data exists, there appears to be general agreement with this criterion.

Some caution must be used when applying the results of linear analysis to the lateraldirectional high-AOA tracking tasks. Several nonlinear effects are evident in the data. The flight

condition changes rapidly during the task from a high-speed, high-AOA condition to a low-speed,

reduced-AOA condition. The maximum roll-rate command is scheduled as a function of airspeed

so that the pilot experiences a reduced command authority as the airspeed decreases. The rate limit

for the stability-axis roll-rate command was reached several times during the fine-tracking tasks.

The high workload demand on the thrust-vectoring system resulted in rate limiting of the thrustvector paddles. Also, at high AOAs, moving the pipper from wing tip to wing tip required a combined lateral and longitudinal stick input.

25
-- CHR Level 2

O 2

.12

.10

.08

Equivalent

time delay, .06

sec

.04

.02

0

1.2

1.0

.8

Equivalent

roll-mode .6

time constant,

sec .4

.2

B

m

m

Comparison of time delay

with MIL-STD criteria

I 6

Comparison of roll-mode time

constant with MIL-STD criteria

Level 1

Level 2

[]

<>

Zx

I

Level 1

0 10

oI

20 30 40 50 60

Angle of attack, deg

960397

Figure 23. Comparison of X-31A data with MIL-STD- 1797 roll mode time constant and time delay

criteria.

Average

Cooper-Harper

rating

R0,D

6 -o2 ',,\ Z_,

03 \ \\,

A s ,\ ..... z_e6

• 7 " ".----.

Criterion _ "_" "-- 2

Criterion bounds

1 I I I I I I I

-220 -200 -180 -160 -140 - 120 -100

Phase angle at bandwidth frequency, deg

960398

<>

Figure 24. Comparison of X-31A data with Smith-Geddes criteria for the lateral axis.

Even a full six-degrees-of-freedom nonlinear simulation did not entirely reproduce the

dynamics observed during some of the lateral fine-tracking tasks. Figure 25 shows some of the excursions in yaw rate and sideslip angle that were not duplicated with the nonlinear simulation.

26
These excursions approximately correlate with target overshoots where the target wanders outside

the 20-mrad reticle and may be related to asymmetric forebody vortex cores. To accurately predict

handling qualities requires an analytic model that includes all of the dynamics, so the effect of these

vortices should be included.

Roll rate,

deg/sec

10

0

-5

-10

Flight test

------ Simulation

i

z_-_ _ ............... -4-.................... t............................... _......

i i i

: , ,,

i _1 i .

I I

i :

t i/_ : A i

i _ !Z_ __'_at_....... _'............ __1. ............................................... ,_......

°k _ /._ ....._-- i J \ .-,,. !

I',./ V \ --\ // \ t / / ; 1 : ,.,

- .5 ............................... 4-_J- _12 ..................

Yaw rate,

deg/sec

Sideslip

angle,

deg

1.0

.5

0

--.5

- 1.0

0

"_' _ i \/ \ / i i

.............................. -i ............................. I................. i

I I I

5 10 15

Time, sec 960399

Figure 25. Comparison of flight and simulation data for a lateral fine-tracking case.

27
CombinedManeuvers

The combinedmaneuverwas flown four timesduring oneflight by one pilot. Two of the

maneuverswereusedfor practice.CommentsandCHRratingsof "3" and"4" weregivenonthe

othertwo maneuvers.Nodistinctionexistedin theratingsforlateralor longitudinaltasks,butfine

trackingandgrossacquisitionwereratedseparately.In summary,theratingsweregivenwith high

confidenceandwereborderlineLevel 1/Level2 for bothtrackingandacquisition.No undesirable

motionswerepresentduringthegrossacquisition,andthemotionsdid notaffectthetaskduring

fine tracking.Followingtheflight, thepilot reported,"The SSLA setupwasanexcellentstarting

conditionto evaluatehandlingqualitiesin thePSTregime."

Figure26showstimehistorydatafrom thesecondratedmaneuver,whichspanned60sec.The

AOA rangedbetween30° and70°.Pitch-sticksensitivitycanbeseenin theAOA commandwhere

15° excursionsinthecommandattheAOA commandratelimit of 25deg/seccanbeseen.Full-roll

stickwasusedearlyin themaneuver,andapproximately66percentofthestickdeflectionwasused

in alateracquisition.Peakvelocity-vectoroll ratesof nearly40deg/secwereobserved.Thefourth

traceshowsthetiming for thethreegrossacquisitionsperformedandtheperiodsof tracking.This

maneuverwasinitiatedat analtitudeof 25,000ft andwascompletedatanaltitudeof 14,000ft.

As with theothertasks,thepilot commentedthatthestickforceswere"too heavy"andthemotionswere "too large."Becausethismaneuverintentionallyuseddiagonalstick inputs,thepilot

wasableto commentonstickharmony,"The stickmovementis muchtoohigh; andyouhavethe

nonharmonybetweenthepitch stick,whichis sosensitive,andtheroll stick,whichis not sosensitive."

Becausethepilotratingscovermaneuversthatspanalargeflight envelopeandencompasstwo

axesof control,comparingthemwithanalyticresultsisdifficult. Thepilot liked thismaneuverbetter andfelt it wasmorerepresentativeof thetypeof flying doneduringtheCICinvestigation.The

maneuveralsoresultedin theLevel 1/Level2 ratingsthatwereexpected.Additional testingis required,but thistypeof maneuvermayprovideabettermeansof evaluatingthePSThandlingqualities, but like CIC, it is of limited value for analysisor designbecauseof the varying flight

conditions.

LESSONSLEARNEDFORHIGH-ANGLE-OF-ATTACK HANDLING QUALITIESTESTING

Whenflying anewtask,backupcardsshouldbepreparedfor anestablishedtaskin theevent

thefirst taskis notworkingout. Duringthefirst PSTfine-trackingflight, it wasquickly apparent

to thepilots in the airplaneandonthegroundthat thetestasdesignedwouldnot resultin anacceptablefine-trackingtask.Almostanentireflight wasusedto getonedatapoint.Testingof alternateflight cardswouldhavecollectedadditionaldata,andgroundreviewwouldhaveadjustedthe

testsetupfor theacquisitionof PSTfine-trackingdata.

28
4 --

If :: :: _J,-. i ...... , ..... ]

0 ---- "_..................... I.......... -_'_ .... ;_ .... ._ ,-- ....... _.-.... _-'-T .... ,-p .........

I ",.,_. "_._:_ ,' ............ l

'[....... _:.. 'i- ...... -'-...............i...... i.......]

Stick

deflection,

in.

Angle of

attack,

deg

0t

60 ......

40 ]-- --//_/- .........

20 [._I ...............

: I \

,_,:: ...... -,_:- ...................:-__ I-', ......

.... ___ _:- i, --_-_

............._,_;:-2;.-;.'L_ ...............................................

command

0

Stability- - 10

axis

roll rate,

deg - 20

- 3O

- 4O

I1! ................................ ,- &-__ "-___

....::_vi_________,.. _pv/_'r -- '..[,l__,___'?___ ...._.i,:_,a " ....... I............ _--_-H _.........

....... Iii_---9'---- _ ......................... .,,--- '............

......... L .................... I_ --_-'R:lJ-ret:n d

Indication

of task

: T

i i

Gross : Gross Gross

................ acquisition .......................... acquisition ............. acquisition ......

........................ I

iiiiiiiiiiiiiiiiiiiii .... ..... ...........

0 20 40 60

Time, sec 960400

Figure 26. Time history from a combined maneuver.

Some of the pilots thought a domed simulation would have helped them better prepare for the

tasks. But it is interesting to note that during the lateral gross acquisitions, the one pilot who had

performed the maneuvers in a domed simulation required the same amount of in-flight practice as

the other pilots.

29
Careshouldbetakenin taskdefinition.For thefine-trackingtasks,thepilots wereaskedto do

separate longitudinal and lateral tracking tasks. Even with instructions that the inputs should be

limited to pitch or roll inputs, the pilots continued to use diagonal stick motions to perform the

more classical tracking tasks of nose-to-tail and wing tip-to-wing tip. The task definition should

also include a reasonable time limit for the performance of the task. One of the reasons 4 sec was

initially chosen was to try to minimize variation in flight condition during the performance of the

task. This time limit was not enforced during the testing and resulted in a tracking task that lasted

20-30 sec with large AOA variations.

Modifications to the HUD could have provided the pilots with the proper cues for the tasks. For

longitudinal gross acquisition, horizontal bars at 25 and 40 mrads would have provided the proper

reference for the task that was being rated. Similarly, vertical bars could have been used for lateral

gross acquisition in place of the circular reticles. This display might reduce the tendency of the pilot to try to place the pipper on the target.

CONCLUSIONS

The Standard Evaluation Maneuvers (STEMs) provided repeatable tasks that could be compared with analytic linear and nonlinear simulation results. With suitable initial conditions and

practice, gross acquisition and fine tracking could be performed at the desired angle of attack

(AOA). Pilot comments indicated that these maneuvers were not consistent with the types of maneuvering performed during the close-in combat (CIC) evaluations. This testing identified problems that may not be significant in actual tasks. Further testing is needed to resolve these

differences.

The pilot-assigned ratings for gross acquisition and fine tracking for both the longitudinal and

lateral axes were dependent on AOA. More undesirable motions and then oscillations existed as

AOA increased.

The longitudinal gross-acquisition task was well-defined and provided an easily repeatable

task. The pilot ratings and comments indicated a high degree of confidence. These ratings reflected

the expectations from CIC testing with the aircraft having Level 1 or Level 2 handling qualities.

The lateral gross-acquisition task was one of the most difficult. The task required a significant

amount of flight time to adjust the starting conditions to achieve the desired AOA with the target

aircraft in the head-up display field of view for the X-31A airplane. The pilot proficiency for this

task did not improve as significantly as it did for the other acquisition and tracking tasks. The pilot

comments and ratings indicated a degradation in handling qualities as AOA increased, with

Level 3 handling qualities at an AOA near 60 °. The pilot comments noted that this type of acquisition was not similar to the acquisitions performed during CIC testing. The degradation in handling qualities was not expected from the CIC testing where the general assessment would have

been Level I/Level 2 handling qualities.

For the longitudinal fine-tracking task, consistent trends existed in regard to the Neal-Smith,

bandwidth, and Smith-Geddes criteria. The maneuvers that received Level 1 ratings in flight were

30
ratedLevel 1 by the criteria.The Levels2 and3 datafrom flight tendedto produceconsistent

resultswhencomparedwiththelinearmodelsandindicatedpotentialmodificationsforthecriteria.

TheX-31A handlingqualitiesratingsshowedadegradationwith AOA thatwasnot observed during the CIC testing.

Lateral fine tracking showed a degradation to Level 3 handling qualities as AOA increased.

The X-31A program provided only a limited amount of data that could be compared with the existing criteria. The data showed good general agreement with the Smith-Geddes criterion. These

data did not provide sufficient information to offer modifications to the existing criteria that predicted Level 1 or borderline Level 1/Level 2.

For both lateral and longitudinal fine tracking, the effect of the velocity-vector settling during

the maneuver had a significant impact. Future use of this STEM may require modifications to allow

a more stabilized starting condition for the fine-tracking tasks. During the X-31A testing, fully separating the lateral and longitudinal tasks was not possible. The pilots generally used diagonal stick

inputs regardless of the axes being evaluated.

For control stick harmony, the majority of the comments were noted during the fine-tracking

tasks. In these cases, the pilots were using diagonal stick inputs to perform the wing tip-to-wing

tip and nose-to-tail tracking. The one other task that elicited a comment on control stick harmony

was the combined maneuver. The pilot commented on the disparity in motion for longitudinal and

lateral stick displacements (large for roll and small for pitch). Although the control implementation

resulted in a limiter for large roll-stick deflection, no particular comments were given by the pilots.

The limited testing with the combined maneuver was commented upon favorably by the pilots,

but these ratings are not amenable to comparison with analytic results because of the rapidly varying flight conditions. This type of maneuver may be useful for providing an overall evaluation of

aircraft performance in the post-stall flight regime and should be considered as an additional

STEM. Like the CIC results, these data are of limited value for analysis because of the varying

flight conditions.

Dryden Flight Research Center

National Aeronautics and Space Administration

Edwards, California, June 5, 1996

REFERENCES

1Sweeney, Joseph E. and Gerzanics, Michael A., "F-16 MATV Envelope Expansion: Testing

For Controllable High AOA Maneuvering," Society of Experimental Test Pilots Thirty-Eighth

Symposium Proceedings, Sept. 1994, pp. 285-295.

2Wichman, Keith D., "High Alpha Handling Qualities Flight Research on the NASA F/A-18

High Alpha Research Vehicle," High-Angle-of-Attack Technology Conference, NASA Langley

Research Center, Virginia, Sept. 17-19, 1996.

31
3Webster,Fredrick R. and Purifoy, Dana, X-29 High Angle-of-Attack Flying Qualities,

AF-FTC-TR-91-15, Jul. 1991.

4U°S. Department of Defense, Flying Qualities of Piloted Vehicles, MIL-STD-1797,

Mar. 1987.

5Krekeler, Gregory C., Jr., Wilson, David J., and Riley, David R., "High Angle of Attack Flying Qualities Criteria," AIAA-90-0213, Aug. 1990.

6Wilson, David J., Riley, David R., and Citurs, Kevin D., Flying Qualities Criteria for 60 °

Angle of Attack, NASA CR-4535, vol. I, Dec. 1993.

7Neal, T. Peter and Smith, Rogers E., An In-flight Investigation to Develop Control System

Design Criteria for Fighter Airplanes, AFFDL-TR-70-74, vol. I, Dec. 1970.

8Smith, Ralph H., "The Smith-Geddes Criteria," SAEAerospace, Control & Guidance Symposium, Reno, Nevada, Mar. 1993.

9Hoh, Roger H., Mitchell, David G., and Hodgkinson, John, "Bandwidth --A Criterion for

Highly Augmented Airplanes," AGARD CP-333, Jun. 1982, pp. 9-1-9-11.

l°Stoliker, P.C., Simulation Prediction of High-Angle-of-Attack Handling Qualities for the

X-31A, NASA TM-4758, 1996.

llCord, Thomas J., Leggett, David B., Wilson, David J., Riley, David R., and Citurs, Kevin

D., "Flying Qualities Evaluation Maneuvers," AGARD CP-548, Mar. 1994, pp. 18-1-18-8.

12Beh, H. and Hofinger, G., "X-31A Control Law Design," AGARD CP-548, Mar. 1994,

pp. 13-1-13-9.

13Noflin, Ken A., Flight Simulation Software at NASA Dryden Flight Research Center, NASA

TM-104315, Oct. 1995.

lnEubanks, D., GUtter, R., and Lee, B., "X-31 CIC Flight Test Results," Four Power Senior

Nation Representative Full Envelope Agility Workshop, Eglin AFB, Florida, Mar. 1995.

15Cooper, George E. and Harper, Robert P., Jr., The Use of Pilot Rating in the Evaluation of

Aircraft Handling Qualities, NASA TN-D5153, Apr. 1969.

16Foster, John V., Ross, Holly M., and Ashley, Patrick A., "Investigation of High-Alpha

Lateral-Directional Control Power Requirements for High-Performance Aircraft," AIAA93-3647, Aug. 1993.

32

Form Approved REPORT DOCUMENTATION PAGE OMBNo.0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORTTYPE AND DATES COVERED

September 1996 Technical Memorandum

4.TITI.E AND SUBTITLE 5. FUNDING NUMBERS

Evaluation of High-Angle-of-Attack Handling Qualities for the X-31A Using

Standard Evaluation Maneuvers

6, AUTHOR(S)

Patrick C. Stoliker and John T. Bosworth

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Dryden Flight Research Center

P.O. Box 273

Edwards, California 93523-0273

9. SPONSORING/MONOTORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-505-68-30

8. PERFORMING ORGANIZATION

REPORT NUMBER

H-2128

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM-104322

11. SUPPLEMENTARY NOTES

Presented at the High-Angle-of-Attack Technology Conference, NASA Langley Research Center, Hampton, Virginia,

Sept. 17-19, 1996. Also presented at the AGARD Flight Vehicle Integration Panel Symposium on Advances in Flight

Testing, Lisbon, Portugal, Sept. 23-26, 1996.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified--Unlimited

Subject Category 08

13. ABSTRACT (Maximum 200 words)

The X-31A aircraft gross-acquisition and fine-tracking handling qualities have been evaluated using

standard evaluation maneuvers developed by Wright Laboratory, Wright-Patterson Air Force Base.

The emphasis of the testing is in the angle-of-attack range between 30 ° and 70 ° . Longitudinal grossacquisition handling qualities results show borderline Level 1/Level 2 performance. Lateral grossacquisition testing results in Level 1/Level 2 ratings below 45 ° angle of attack, degrading into Level 3

as angle of attack increases. The fine-tracking performance in both longitudinal and lateral axes also

receives Level 1 ratings near 30 ° angle of attack, with the ratings tending towards Level 3 at angles of

attack greater than 50 ° . These ratings do not match the expectations from the extensive close-in combat testing where the X-31A aircraft demonstrated fair to good handling qualities maneuvering for

high angles of attack. This paper presents the results of the high-angle-of-attack handling qualities

flight testing of the X-31A aircraft. Discussion of the preparation for the maneuvers, the pilot ratings,

and selected pilot comments are included. Evaluation of the results is made in conjunction with existing Neal-Smith, bandwidth, Smith-Geddes, and military specifications.

14. SUBJECTTERMS

Flight controls; Handling qualities; High angle of attack; Pilot-induced oscillation;

Standard evaluation maneuvers; X-31A aircraft

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OF REPORT OFTHIS PAGE OF ABSTRACT

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36

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