T-45 Stability Augmented Steering System

T-45 Stability AugmentedSteering System

20 October 2005

“The Goshawk Learns Some Basic Manners”

T-45 Stability AugmentedSteering System

Ms. Christina StackProject Engineer

Many thanks to…LCDR Allen Blocker, USN

Lead Project Officer

Mr. Jim Reinsberg

Lead Boeing Engineer,

Designer and Developer of SASS

Mr. David Klyde

Systems Technology, INC

Mrs. Marge Draper-Donley

NAVAIR

Chapter Overview

• Background

• Problem

• Solution

• Testing

• Results

• Lessons Learned

T-45 Background Info

“In the beginning…”


…The Great Legislative Body declared,

“Thou shalt take a foreign, land-based jet and remake it in thy naval image.”


We needed just one or two minor changes…


SIMULATEDGUNNERY SYSTEM•HEADS UP DISPLAY•VCR

NEW NOSESTRUCTURE•NEW NLG•NOSE WHEEL STEERING•LAUNCH, HOLD BACK BARS

NEW MAIN LANDINGGEAR AND STRUCTURE•IMPROVED BRAKES @3K

F-405-RR-401ADOUR ENGINE•5845 LBS THRUST•EMI PROTECTION•BACK UP FUEL CONTROL

LEADING EDGE SLATTED WING

REDESIGNED GLASS COCKPIT

ON BOARD OXYGEN GENERATING SYSTEM

STANDARD ATTITUDE HEADING REFERENCE SYSTEM

TAIL HOOK

COMPOSITE STABILIZER

CENTRAL VENTRAL FIN

ADDITIONAL STRUCTUREFOR CATAPULT/ARRESTMENT

NEW EJECTION SEATS (NACES)

SMURF

ADD YAWDAMPER

6” FIN CAPEXTENSION

ADRS

APPROACH IDLE STOP


What could possibly go wrong?

Ground Handling History

“The Saga of the T-45 All-Terrain Vehicle”

JEEP

– Aug 1992: Digital full time NWS incorporated.

– May 1993: “Overly sensitive directional control characteristics during landing rollout.” – Part IK

This deficiency will persist until 2004.


Picture worth 1000 words…



From 1992 to 2000,

Averaged 2 accidents per year and

Almost 1 Class A mishap per year

Due to ground handling deficiencies

Understanding the Problem

“If only Dale Ernhardt, Jr. could fly”

But, in 1998…

Navy and Boeing seek help of Systems Technology, Inc (STI) and NASA

Or “That Other Space Agency,” depending on who you ask around Mojave, CA…



“They got skills!”

Insert your tire here.


Results:

• Tire models were in error; materials critical


Results:


Tires +


Results:


Tires +

Landing gear mechanics and geometry +


Results:


Tires +


Dynamic interactions =


Results:


Tires +


Dynamic interactions =

Aircraft response feels like an acceleration-command system and exhibits an “oversteer” condition


If you’re going to be in the ground handling business, you better get familiar with some terms and concepts:

Cornering Stiffness

Braking and Blown Tire Affects

Hydroplaning

Thermal Management

Understeer Gradient

Understanding the ProblemUndersteer Gradient

angle slip of degree per

load) icalforce/vert (side of slope

stiffness lateral Tire effective

""

“Static” Understeer Gradient (UG) is a tire property….• Positive = understeer, Negative = oversteer• Weak function of CG, ignores “artificial” stability. • Strong function of TIRES, “installation factors”, and aerodynamics (vertical load).• Cars are understeer for controllability; race cars are oversteer for agility• T45 is Oversteer

Slip Angle Velocity Vector

Heading 2

2

CG

)*(

)(

21

12*57.3 UG

Understanding the ProblemUndersteer vs. Oversteer

Note: Spin out and plow out occur when tires saturate (like “stall”) These are limit performance characteristics.

Oversteer:Right nose wheel steering (into the skid) cannot prevent “spin out” - “Icey”. (UG < 0.0)

Understeer:Left nose wheel steering (into the turn)cannot prevent “plow out” (UG > 0.0)

THIS is oversteer.

Picture courtesy of Wally Pankratz Racing Photoshttp://www.starite.com/racing/wally_photos.htm


What can be done?

Tier 1•Tires•Yaw Rate feedback to NWS

Tier 3

• LEAD-LAG*• Roll Stiffness• Ergonomics

Tier 2

• NWS Freeplay• NWS Rate• NWS Servo

Major Improvement Minor ImprovementProvisional

Improvement

The Solution

The Stability Augmented Steering System

Full-time yaw-rate feedback to the NWS.

• Attempts to nullify yaw rates using NWS based on set gains that vary with airspeed

K = gain constant; R = yaw rate; δNWS = NWS commanded

KR = δNWS

• 4 pilot-selectable gains were used for flight test

SASS

SASS

Power SASS Control Law SelectorSwitch “CLAWS”

SASSLooking aft at rear cockpit

Aft, left bulkhead

SASS unit

New Metrics

“Teaching an old dog new tricks”

• Heading Angle Bandwidth (HABW)– Based on longitudinal flying qualities specifications

• Runway Offset Capture and Hold (ROCH)– Traditional FQ parameter capture and tracking

task

• ROCH with Braking (ROCHB)– Increases difficulty of ROCH and more

operationally representative

New Metrics

• HABW– Defined as highest frequency at which you have less

than 45° phase lag between rudder input and aircraft yaw response

– Measured using rudder pedal frequency sweeps at discrete groundspeeds, described in rad/sec.

– Demonstrates a pseudo-track of understeer gradient and PIO ratings

Key: Wider is better!

New Metrics

• 4 Selectable SASS Gains (rad/sec HABW*) – SASS-0: 1.0 - baseline aircraft– SASS-1: 2.0– SASS-2: 2.5– SASS-3 (Initial Testing): Starts as Baseline Aircraft – SASS-3 (Follow-On): 2.0 below 50kts; 2.5 above

70kts.

Perform Rudder Sweeps for each gain at discrete speeds to measure HABW.

New Metrics

• ROCH– 50 KGS, 75, and 100 KIAS discrete points

New Metrics

Capture

Hold

Intercept Angle = 5-7 o at 50kts,

4-6 o at 75kts,

3-5 o at 100kts

Adequate Overshoot +/- 5 ft

Desired Overshoot

+/- 2 ft

50ft offset

New Metrics

• ROCHBIntercept Angle = 3-5 deg at 100kts

When angle established, symmetrical braking at 0.15Nx until 50KGS

while performing centerline control task.

New Metrics

Capture

Hold

Adequate Overshoot +/- 5 ft

Desired Overshoot

+/- 2 ft

50ft offset

Test Planning

“Are you sure this isn’t dangerous?”

Test Planning

• Phase 1– Concept proof and Gain selection – Metric and Modeling Validation– CV Suitability

• Phase 2– Hybrid Gain evaluation– Production Unit Verification

• Phase 3– Crosswind and wet runway– Operational Evaluation using Instructor Pilots

Test Planning

• Crosswind and Wet Runway Tasks

– Centerline Maintenance– Upwind / Downwind Captures– Max braking test points.

• Pilot Training

Test PlanningRisk Mitigation

Sim Fam Flight

TP w/in NATOPS limits

Rudder Sweeps X

ROCHs FAM 1

ROCHBs FAM 1

Crosswind Testing FAM 2

Crosswind Testing outside current NATOPS limit

X

Wet Runway Testing FAM 2

Wet Runway Testing outside current NATOPS limit

X

• Build-up– Increasing vs. Decreasing Airspeed

• Worst control region from 60-80 knots• 50 knot points during high-speed taxi• 75 and 100 knot points during roll-and-go

– Maneuvers Normal “navigational” inputs

Rudder Sweeps ROCHs

ROCHBsOperational

Evaluations


• Runway Excursions (End vs. Side)– KIO Distance Criteria established

• Off the End– 50knots to rotation then back to stop– Verbal “KIO” call over radio at marker

• Nominal roll-and-go at each speed prior to test points for testable runway familiarization

• Off the Side– 30 deg hazard pattern– Based on runway surveys


Test Planning

= E-28

= 1000’

Lens

200-300’

5’ Mound

24

1300

’

Ditch

200’L x 6’D Pond

100’

100’

6’ Deep

6’ Deep

2’ Deep

6

14

32

*

* Radar Tower300’ from either runway

Center Field

1500

’

6-10’ Ditch w

/ mound

6-10

’ D

itch

w/

mou

nd

MK 7Site

CatSite

Tree line

Lens

300’

Con

trol

B

ox

• Thermal Management (Hot Brakes)– Brake vs. Tire Design

• Fuse plug designed to melt at 350° F to prevent explosive pressure release.

– Thermal profile• Transfers from brakes to tires and axle• Approximately 20 min to max temp

– KIO Temperature Criteria• 150° F with axle temp instrumentation• 120° F using handheld pyrometers


• Thermal Management (Hot Brakes)– Test Point management

• High-speed taxi vs. roll-and-go

– Brake cooling• “Penalty laps”

• Cooling fans

• Personnel safety issues


• Tire Health Monitoring– Tread Wear Inspections

• 10 passes

• Max Brake test points


Results and Conclusions

“And the winner is…”

Abandon task

Modify task

Abandon taskModify task

BrakingBraking

Field pressure, 100kts, ROCH-B : • Mode 0: HQR 6, PIO 4

“Felt very “loose” and unpredictable … resulted in a series of overshoots 5 ft… To prevent undamped oscillations from developing required reducing pedal input rates to less than 2 Hz…”

Ped

alY

aw R

ate

Nx

Airs

pee

dH

eading

Occasional large inputs out of phase and “undamped”

Flight Test Data AnalysisBaseline ROCH-B, Field Pressure

Braking Braking

Field pressure, 100kts, ROCH-B : • Mode 2: HQR 2, PIO 1

“…. Capture was not difficult, requiring a single 1½ inch pedal input to set heading, and a small pedal inputs to maintain. Excellent damping qualities allowed sharp or smooth pedal inputs to easily maintain centerline…”

Ped

alY

aw R

ate

Nx

Airs

pee

d

Heading

“… not difficult… ”

Flight Test Data AnalysisSASS-2 ROCH-B, Field Pressure

ROCHField Pressure, 100KIAS

HQR PIO

• SASS has almost no undesirable motions

Increasing HABW Increasing HABW

Baseline

SASS

Baseline

SASS

• SASS level 1

ROCHBField Pressure

1

2

3

HQR PIO

Increasing HABW Increasing HABW

Baseline

SASS

Baseline

SASS

• SASS has almost no undesirable motions

• SASS level 1

40 50 60 70 80 90 100 110 120

Airspeed (knots)

0

0.5

1

1.5

2

2.5

3

Hea

din

g A

ttit

ud

e B

and

wid

th (

rad

/sec

)Year/Aircraft/Tire Pressure/Maneuver

95/T03/Field/Sweeps00/T03/Field/Sweeps00/T03/Field/ROCH2-01/T02/Field/ROCH2-01/T02/350 psi-Nose/Sweeps

2-01/T02/350 psi-Nose/ROCH4-01/T02/350 psi-Nose/ROCH99/F/A-18 C/D/Standard/Sweeps99-00/F/A-18 E/F/Standard/ROCH02/T05-SASS-0/Field/Sweeps

02/T05-SASS-1/Field/Sweeps02/T05-SASS-2/Field/Sweeps02/T05-SASS-0/Carrier/Sweeps02/T05-SASS-1/Carrier/Sweeps02/T05-SASS-2/Carrier/Sweeps

T-45 Field

T-45 w/ CarrierPressure Nose Tires

F/A-18 C/D

SASS-0 Field

F/A-18 E/FSASS-1 Field

SASS-2 Field

SASS-0 Carrier

SASS-1 Carrier

SASS-2 Carrier

F18 E/F

F18 C/D

T45 w/ carrier pressure nose

T45

HABW Results

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

1.800

40 60 80 100 120

Knots Indicated Airspeed

Yaw

Rate

Feed

back G

ain

Mode 0

Mode 1

Mode 2

Mode 3

“The Winner”SASS-3

• SASS is a definite improvement over the baseline airplane– “No SASS is Silly”

• Ground handling issues are no longer crosswind limiting factor for the aircraft.

Conclusions: The Good

• SASS not designed to counter ground handling problems encountered as a result of blown tires.

Conclusions: The Bad

• Students still need to be good pilots:– Instructors will still be able to see incorrect

student inputs, however…– Incorrect student inputs will be tempered

by SASS

Conclusions: The Ugly

Lessons-Learned

“If I only knew then what I know now…”

Reminder “Off the Shelf” is not necessarily “Off the Shelf”

• Over 15 years later, we are still making improvements to an off the shelf system.

• This small sub-system still had growing pains during the installation.

It’s Great! Lets Release It?

• Still need full checkouts:– System Faults due to installation when

used from aft cockpit– Need to capture aft cockpit comments as

well as forward cockpit comments

GH Metrics

• Stability Factor/Understeer Gradient – steady state parameter that defines oversteer/understeer tendencies of a given configuration (not easily applied to augmented configurations)

• Heading Attitude Bandwidth – controlled element frequency domain parameters that define ability of the pilot to attain stable, closed-loop control

• Overshoot Ratio – time domain measure that defines of the amount of attitude overshoot generated from a step input

• Capture Time – time domain measure that defines the time required to capture an attitude within a given tolerance

• Pilot-Vehicle System Parameters – Frequency domain measures that characterize the closed-loop pilot-vehicle response

SASS Components

AIRCRAFT CHANGE:• Add new SASS hardware between the rudder pedal and the Steering Control

Electronic Set (SCES):

Rudder Pedal Position LVDT

Steering ControlElectronic Set (SCES)

Current connection between Rudder Pedal

Position LVDT transducer and the SCES Electronics

will be re-directed through the NWS Yaw

Rate Feedback Controller.

SASS

Mu vs. Slip angle

HighResolution

Data-1.5

-1

-0.5

0

0.5

1

1.5

-20 -15 -10 -5 0 5 10 15 20

Nose Tire Sideforce vs Slip Angle450 lb load

Field

Carrier

Slip Angle

MU

Braking

MU

Main Tire Sideforce vs Braking5 deg slip at 3500 lb load

• 350psi Nose Tire Is “Better”• Moderate Braking Reduces Stability• New tires?

HQR Scale

PIO ScaleNO TENDENCY FOR PILOT TO INDUCE UNDESIRABLE MOTIONS.

UNDESIRABLE MOTIONS TEND TO OCCUR. Motions can be prevented or eliminated by pilot technique.

UNDESIRABLE MOTIONS EASILY INDUCED. Motions can be prevented or eliminated but only by sacrificing task performance or with considerable pilot attention and effort.

OSCILLATIONS TEND TO DEVELOP Pilot must reduce gain or abandon task to recover.

DIVERGENT OSCILLATIONS TEND TO DEVELOP. Pilot must open loop by releasing or freezing the stick.

DISTURBANCE OR NORMAL PILOT CONTROL MAY CAUSE DIVERGENT OSCILLATION. Pilot must open control loop by releasing or freezing the stick.

Pilot Attempts to Enter

Control Loop

Causes Divergent

Oscillations

Pilot Initiated Abrupt

Maneuvers or Tight Control

Causes Oscillations

Do Undesireable

Motions tend to Occur?

Is Task

Performance Compromised?

Divergent

1

2

3

4

5

6

No

Yes

No

Yes

No

Yes

No

Yes

No

Yes

• Not task driven like HQR• Field pressure T45 = ?

• Not task driven like HQR• Field pressure T45 = ?

Summary of Issues• Multiple "Triggers”

• Aggressive corrections• Inadvertent braking/rudder inputs• Side force in cockpit• Insufficient brake pedal feedback• Crosswinds• Blown tire handling qualities

• Control problem amplified by "Sustainers" • landing gear dynamics• brake sensitivity and feel• roll/yaw coupling• lateral acceleration cues• [Reversible] Rudder pedal mechanical characteristics

• Described as:• Crosswind lifts upwind wing• Velocity vector loosely coupled to aircraft nose (feels like it’s on ice) • Roll/yaw coupling “lean” out of turn during aggressive maneuvering• PIO - pilot induces undesirable motion via coupling with side force

Understeer Gradient

Neutral steer

Main tire cornering power Nose tire cornering power

Und

erst

eer

grad

ient

FordThunderbird

Neutral steer

T45

Understeer

Oversteer

HABW ResultsGross Wt; Tire Pressure; Airspeed; Gain selection

FUEL

Main Tire Pressure

Nose Tire Pressure

BASELINE HABW=2.0 HABW=2.5

SASS Gain SchedulesTable 1

SASS GAIN SCHEDULE Switch

Position IAS (kts)

KR R*1 Switch Position

IAS (kts)

KR R*

<Baseline> <HABW=2.5> 0 0 0.00 - 2 0 0.96 12.55 0 50 0.00 - 2 50 0.96 12.55 0 60 0.00 - 2 60 0.96 12.55 0 70 0.00 - 2 70 0.99 12.15 0 80 0.00 - 2 80 1.03 11.64 0 90 0.00 - 2 90 1.12 10.70 0 100 0.00 - 2 100 1.23 9.72 0 110 0.00 - 2 110 1.33 9.04 0 120 0.00 - 2 120 1.53 7.83 0 200 0.00 - 2 200 1.53 7.83

<HABW=2.0> <HYBRID> 1 0 0.72 16.67 3 0 0.722 16.67 1 50 0.72 16.67 3 50 0.722 16.67 1 60 0.72 16.67 3 60 0.852 14.05 1 70 0.73 16.44 3 70 0.99 12.15 1 80 0.75 16.11 3 80 1.03 11.64 1 90 0.77 15.69 3 90 1.12 10.70 1 100 0.78 15.38 3 100 1.23 9.72 1 110 0.78 15.48 3 110 1.33 9.04 1 120 0.77 15.58 3 120 1.53 7.83 1 200 0.77 15.58 3 200 1.53 7.83

Note: (1) R*= Yaw rate where feedback = 12 deg NWS (2) Transition are of Hybrid from SASS-2 to SASS-1

SASS Gain SchedulesTable 2

T-45C TEST ENVELOPE

Parameter

Test Limits Current NATOPS LimitsCheckout Crosswind

sWet

RunwayGear A/S Limit 200 KIAS 200 KIAS 200 KIAS 200 KIAS

Runways No Wet Runway

No Wet Runway

Wet Runway

All Runways

Cross-Winds 15 kts(1) 30 kts(1) 20 kts(1) 20 kts(1, 2)

Tire Limitation

176 KGS 176 KGS 176 KGS 176 KGS

Note: (1) Tower/Field anemometer will be used for data purposes. (2) NATOPS crosswind limits for wet runway is 15 knots

T-45 Background Info2.5” Pedal deflection =12° NWS30° Rudder

T-45 Stability Augmented Steering System

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