ASTM E-17 Seminar – December 6, 2006 – Page 1

ROUGH RUNWAY ANALYSIS

Aircraft Dynamic ResponseJack Hagelin

Boeing Technical FellowASTM E-17 SeminarDecember 5, 2006

2

Types of Runway Roughness Impact on Aircraft

Three Types of Runway Roughness Impact on Aircraft:

1) Limit Loads – Single Discrete Bump Events

2) Fatigue Loads – Continuous Long Wavelength Bumps

3) Truck Pivot Joint – Continuous Short Wavelength Bumps

Each type imposes a different runway roughness criteria.

Current standards address mainly first two types.

Third type is not directly addressed in current standards.

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Limit Load Dynamic Taxi Analysis

Limit Load Critical Conditions Constant speed taxi Accelerated Takeoff Decelerated Landing Rollout Ride Comfort

Runway profile used as direct input Goal is to use roughest runway reasonably expected in normal operation (FAR 25.235). In absence of other data, AC 25.491-1 provides SF28R profile data. Analyze both forward and reverse directions, and multiple tracks if available. Discrete bump conditions also analyzed (Boeing single bump and FAR double bumps) Runway crown effect for airplanes with more than two main gears

Important input parameters: Airplane weight, cg, pitch inertia, speed, aerodynamic forces, and thrust Landing gear location (wheelbase), number of gears, shock strut air curve, tire stiffness, brake

force Tail aerodynamic force affects nose gear load Airplane on-the-ground natural frequencies (usually from 0.5 to 1.5 Hz)

Important output parameters: Landing gear vertical loads, airplane cg acceleration, forward fuselage, and engine loads

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Bump Height vs. Length Diagram FAA Certification Runway (AC 25.491-1) – SFO 28R

3

2

1

5

Limit Load Dynamic Taxi AnalysisResults – San Francisco (SFO) Runway 28R

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Bump Height vs. Length Diagram Sample Recently Measured CIS Rough Runway

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Sample Rough Runway 200m section with critical bump

Smooth line is curve fit used to flatten profile

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3D Power Spectral Density (PSD) Plot Sample CIS Rough Runway

1 2 3

Airplane Speed of 60 knots

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Limit Load Dynamic Taxi AnalysisSample CIS Rough Runway

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Bump Height vs. Length Diagrams Russian Certification Runways (A, B, C, D)

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3D PSD Diagrams Russian Certification Runways (A, B, C, D)

1 2 3

12

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Airport

CG

Ver

tica

l Lo

ad f

acto

r (g

's)

Limit Load Dynamic Taxi AnalysisCG Vertical Acceleration Comparison

Limit Load Factor = 1.7g per AC 25.491-1

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Fatigue Dynamic Analysis

Fatigue Critical Conditions Preflight taxi Takeoff roll Landing rollout Post-flight taxi

Runway profile used as direct input ‘Typical’ runway/taxiway profiles used to match in-service statistical data

Important input parameters: Airplane weight, cg, pitch inertia, speed, aerodynamic forces, and thrust Landing gear wheelbase, number of gears, shock strut air curve, tire stiffness Airplane on-the-ground natural frequencies (usually from 0.5 to 1.5 Hz)

Important output parameters: Landing gear vertical loads Airplane cg acceleration Engine loads Fuselage, wing, and horizontal tail incremental loads

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Sample Taxiway which may be rough for Fatigue

Critical Taxi Speed = 24 knots for 1 Hz airplane

response

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Main Gear Pivot Joint Analysis

Current runway standards do not adequately address this issue

On rough runways, the bogie pitch mode exhibits low damped resonant response at 10 to 20 Hz range due to short wavelength roughness (~2 to 7 m)

Important parameters for pivot joint analysis:

Pivot pin diameter

Friction coefficient in joint

Bearing pressure due to post load

Bearing surface velocity

PV (Pressure * Velocity)

Heat energy generated in the joint

Time duration of heating

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Pivot Joint Heat Calculations

dtFRdtVFFdsE

AFR|PV

||||

/|

Friction Energy =

Pressure-Velocity =

Pivot Pin

= friction coefficient at bushing surface

F / 2 F / 2

F / 2 F / 2 Inner Cylinder

Bogie Beam

R

FA

F = Post Load

FF

Bogie Pivot Joint

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Short Wavelength Runway Profile AnalysisPower Spectral Density (Overall Runway)

• Shows bump height intensity versus bump wavelength

Acceptable

Threshold

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Runway Profile Analysis3D Relative Power Spectral Density

Acc

epta

ble

Ro

ug

hR

un

way

Dis

tan

ce (

m)

Rel

ativ

e R

un

way

Hei

gh

t P

SD

(d

B)

Bump Wavelength (m)

2 7

1500m

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Runway Profile Analysis RMS of PSD Sorted by Airport

0

0.5

1

1.5

2

2.5

3

3.5

4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Runway

RM

S o

f P

ow

er S

pec

tral

Den

sity

[m

m]

Potential Heat Damage

Probable Heat Damage

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Pivot Joint Dynamic Takeoff AnalysisResults – Typical International Runway

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Pivot Joint Dynamic Takeoff AnalysisResults – Borderline Runway

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Pivot Joint Dynamic Takeoff AnalysisResults – Rough Runway

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Dynamic Analysis Results –Takeoff RollPivot Joint Heat per Unit Bearing Area (Normalized) vs. Runway

0

0.5

1

1.5

2

2.5

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Runway

Piv

ot

Join

t H

eat

(No

rmal

ized

)

Potential Heat Damage

Probable Heat Damage

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Dynamic AnalysisRunway RMS vs. Pivot Joint Heat

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5 3

Pivot Joint Heat (Normalized)

RM

S o

f P

ow

er S

pec

tral

Den

sity

[m

m]

Potential Heat Damage

Probable Heat Damage

Acceptable

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Dynamic AnalysisCG Vertical Load Factor vs. Pivot Joint Heat

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

0 0.5 1 1.5 2 2.5 3

Nornalized Pivot Joint Heat

CG

Ver

tica

l Lo

ad F

acto

r (g

's)

Correlation of CG Acceleration with Pivot Joint Heat

is not strong.

(Due to widely different response

frequencies)

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Summary

Three types of airplane response to runway roughness require three different acceptance criteria.

Airplane dynamic analysis is necessary to evaluate effects of rough runways on aircraft design, however, runway profile analysis can be used to determine acceptable roughness levels.

For limit loads, single bump height vs. length chart is good criteria. (This criteria is not adequate for fatigue or pivot joint).

For fatigue loads, a PSD criteria may be used in the 10 to 75 m range.

For pivot joint, PSD approach works well (2 to 7 m range).

RMS value in this range is a good measure of pivot joint impact.

Work is in progress with FAA to establish short wavelength runway roughness standard to address pivot joint friction.