real life adventures with unsteady aerodynamics by dr. atlee m. cunningham, jr. lockheed martin...

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Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort Worth, Texas Presented for “Aerodynamic and Fluid Dynamic Challenges in Flight Mechanics” Working Group Meeting 27 th AIAA Applied Aerodynamics Conference San Antonio, Texas, 22-25 June 2009 Copyright © 2009 by Lockheed Martin Corporation A08-23854013 Unsteady aero can be destructive Buffet Wake vortex encounter

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Page 1: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Real Life Adventures with Unsteady Aerodynamics

by Dr. Atlee M. Cunningham, Jr.

Lockheed Martin Senior Fellow

Lockheed Martin Aeronautics Company, Fort Worth, Texas

Presented for

“Aerodynamic and Fluid Dynamic Challenges in Flight Mechanics” Working Group Meeting

27th AIAA Applied Aerodynamics Conference

San Antonio, Texas, 22-25 June 2009

Copyright © 2009 by Lockheed Martin Corporation

A08-23854013

Unsteady aero can be destructive

Buffet

Wake vortex encounter

Page 2: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

The Influence of Unsteady Aerodynamics is Multi-Faceted

Design, development, and testing of today’s aircraft involves the close integration of many diverse technologies and systems which is expected to become more complex in the future. For example, these technologies cover Airframe – aerodynamics, structures, aeroelasticity,

aeroservoelasticity (ASE) Flight controls – active controls, g/AOA limiters, weapons

delivery Propulsion – thrust management/vectoring, auxiliary systems Stores/weapons – internal/external carriage, delivery systems Maintainability – logistics, spares, inspections, service life Pilot/crew – comfort, performance, limits, communications

Page 3: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

The Influence of Unsteady Aerodynamics is Multi-Faceted – More So for Fighter Aircraft

Fighter aircraft can be more complex than transport aircraft due to: Multi-role – air-to-air, air-to-ground, large stores inventory Highly transient maneuvers – offensive/defensive, weapons release Operations at edge of envelope – high AOA, transonic and buffeting

conditions Susceptibility to wake vortex encounters – air-to-air tail chasing

Fighter design envelopes contain many loads conditions that are not steady state: Rapid maneuvers – unsteady aerodynamics, changing control laws,

buffeting Edge of the envelope – non-linear, path dependency, extreme

conditions Complex systems interactions – fail-safe designs, redundancies

Page 4: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Real Life Adventures with Unsteady Aerodynamics Outline

Aircraft Buffeting and Limit Cycle Oscillations (LCO) Flight Experiences Wind Tunnel Testing Observations

High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations

Page 5: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Real Life Adventures with Unsteady Aerodynamics Outline

Aircraft Buffeting and Limit Cycle Oscillations (LCO) Flight Experiences Wind Tunnel Testing Observations

High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations

Page 6: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Differences Between Buffeting and LCO

Structural buffet response is driven by unsteady separated flows acting on the structure Unsteady flows unaffected by structural motion Forcing is wide band and affects many structural modes of

vibration

LCO is a nonlinear interaction between aerodynamics forces and structural response Similar to flutter but limited in response amplitude Nonlinear forces act to drive the LCO

Page 7: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Why are buffeting and LCO important

Buffeting sources Spoilers and deployed flaps during landing Wing mounted stores and protuberances Weapons and landing gear bays Leading edge separation and vortex breakdowns Shock induced separation Inlet spillage and thrust reversers

Buffeting problems Fatigue of TE controls, spoilers, etc. Fatigue of fins, antennae, twin vertical tails and other

downstream surfaces Severe vibrations of wires/cables/equipment/bulkheads in

open bays

Page 8: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Why are buffeting and LCO important (cont’d)

LCO sources Transonic speeds, freeplay, nonlinear damping Embedded shocks and induced flow separation Susceptible structural vibrations with low damping (sensitive

to various nonlinearities)

LCO problems False indications of flutter onset Pilot discomfort/distractions/limitations Weapons system limitations Control surface buzz

Page 9: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Real Life Adventures with Unsteady Aerodynamics Outline

Aircraft Buffeting and Limit Cycle Oscillations (LCO) Flight Experiences Wind Tunnel Testing Observations

High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations

Page 10: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-16 Ventral Fin Buffet

Early F-16 Ventral Fins Were Subject to Partial or Complete Loss During Flight Inlet Spillage Turbulence Was Primary

Buffeting Source Safety of Flight Issues Were Marginal Biggest Problems Were With

Maintenance and Spares

Requirements to Carry LANTIRN Pods Upstream of the Ventrals Were Imposed in the Early 1980’s First Flight With LANTIRN’S Resulted in

Loss of R.H. Ventral Fin Redesign of Ventrals and Supporting

Structure Was Accomplished but Was Not Good Enough

Later Structural Redesign Was Also Incorporated but Problems Still Existed With Cracking and Loss of Fins

A03-08521011

Page 11: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-16 Ventral Fin Buffet (Cont’d)

M=0.95, 10 Kft, Clean Aircraft With/Without LANTIRN Pods, 1-g Level Flight

LANTIRN Pods’ Wake Turbulence Almost as Severe as Inlet Lip Spillage During Throttle Chop

High-Amplitude Responses Exist Continuously Even at Lower Machs LANTIRN’S Somewhat Reduce Throttle Chop Effects

A03-08521012

Page 12: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

An Investigation Was Conducted in the Mid-1990’s To Redesign the Ventrals and Supporting Structure Tests Conducted in Fort Worth (Upgraded Block 40 F-16) and

The Netherlands (Early Block 15 F-16) Aerodynamics and Structural

Modifications Evaluated

Four Fin Configurations Tested Baseline Block 40 (BSLN) Block 40 With Stiffer Skins

(MMC Aluminum) (MMC) Block 40 With Stiffer Skins and an

Aerodynamic Nose Cap (MMC NC) Thicker Fin With an Airfoil

Section Shape (NACA)

The MMCNC Fin Has Been Adopted as the Only Spare Fin for All F-16’s Failure Rates Have Dropped Dramatically

F-16 Ventral Fin Buffet (Concluded)

A03-08521013

Page 13: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-111 TACT Wing Buffet

An F-111 Was Used as a Test Bed for Investigating the Potential Benefits of Transonic AirCraft Technology (TACT) Conducted by NASA, AFRL and General Dynamics Fitted With a Supercritical Wing With Variable Sweep Extensive Flight Test Program Highly Instrumented Wing for Buffet Research 1/6-Scale Wind Tunnel Model (Steel and Aluminum Wings,

Instrumented With Pressure Transducers in Same Locations as on the A/C)

Buffet Prediction Research Was Conducted by NASA Ames and General Dynamics Used Pressure Time Histories With Mode Deflections To

Obtain Generalized Force Time Histories Predictions Made With Equations of Motion and Aerodynamic

Damping Derived From the Wind Tunnel Model Response

A03-08521014

Page 14: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-111 TACT Wing Buffet (Cont’d)

Predicted and Measured RMSAccelerations Versus Angle of Attack

A03-08521015

Page 15: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-111 TACT Wing Buffet (Concluded)

Buffet Predictions for the Wing Bending Mode Agreed Well With Flight Test Data AOA Range From 7 Deg to 12 Deg, M=0.8 Wing Sweeps of 26 Deg and 35 Deg

Torsion Mode Predictions Were Mixed Good Agreement for Wing Sweep of 35 Deg Significant Under Prediction for 26 Deg Sweep

Similar Results From Earlier Buffet Research for the F-111 About the Same AOA, Wing Sweep and Mach Number

Ranges

Strong Suspicion That a Torsion Mode LCO Existed

A03-08521016

Page 16: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-111 TACT Wing LCO

A03-08521017

= 9.1°

= 10.0° = 11.1°

Page 17: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-111 TACT Wing LCO (Concluded)

A Step Change in Pitching Moment With the Onset of Shock-Induced Trailing Edge Separation Was Key to Driving a Torsion Mode LCO Step Increase in Nose-Down Pitching Moment With Increasing AOA Aerodynamic Lag Produces an Unstable Hysteresis Loop Aerodynamic and Structural Damping Counteract the Unstable

Loop

A Simple One DOF Math Model for the Torsion Mode With a Nonlinear Step Change in Generalized Force Produced an LCO Time History Solution to the One DOF Equation of Motion

A03-08521018

Page 18: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-16 Wing LCO

F-16 Wing LCO Was Reported in the Early 1980’s for Certain Air-to-Air Wing Store Combinations During Wind-Up-Turns AOA in the 5 Deg to 7 Deg Range, M=0.90 to 0.96 Conditions Corresponded to Onset of Shock-Induced Tailing Edge

Separation (Similar to F-111 TACT)

An Investigation Was Funded by the USAF to Investigate the Phenomenon Cooperative Program Between General Dynamics and the National

Aerospace Laboratory in The Netherlands Extensive Wind Tunnel Tests at Transonic Speed With About 100 High

Response Pressure Transducers on the Wing Oscillating Wing Panel Analytical Studies To Develop Prediction Methodology “NONLINAE” Was the Result

A03-08521019

Page 19: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

F-16 Wing LCO (Concluded)

Predictions of Wing LCO for the Early F-16 Version Agreed Well With Flight Test Data Strong Sensitivity to Structural Damping Suggested Probable Source of

A/C to A/C Variations in LCO Levels Minimum of 2 Critical Modes to Reproduce LCO

Problem Was Significantly Reduced With Subsequent Structural Upgrades Stiffer Wing Was Developed for Block 40 F-16s To Accommodate Higher

Gross Weights

A03-08521020

Page 20: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Real Life Adventures with Unsteady Aerodynamics Outline

Aircraft Buffeting and Limit Cycle Oscillations (LCO) Flight Experiences Wind Tunnel Testing Observations

High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations

Page 21: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Model Geometry and Instrumentation/Flow-Visualization Locations

Force/Pressure Model Geometry and Instrumentation Layout

Flow-Visualization Light Sheet Positions

108.65

76°

AA

5

Section A-A

0.73

3 C

roo

t

820.

7Pitching Axis

Bearings

Shaft6

7Wing

Balance

4 3 2 1

62.3 7.0

Hydraulic Actuator

Wind Tunnel Sidewall

TurntableAccelerometersPressure Section

A05-15010003

Conf. 5 Conf. 2 Conf. 1

9

8

7

RotationAxis

14

1

2

3

4

5

6

10

11

12

13(T.E.)

Sheet Angle From Vertical = 4.7 deg

Page 22: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Flow-Visualization Technique andImage Orientation

Set-Up for Spanwise Sheet Visualization Sheet Rotated 90° for Chordwise Sheet

Use Side Camera Pulsed Laser

Model Relationship With Spanwise Sheet at “Position 9”

Image Reversed (Negative) for Improved Quality

High Speed Video CameraOptical System (OS) (on Travel Mechanism)

Water Input

Camera

HST Side View

ModelFlow

Mirror Slat With Observation Widows

XZ

HSVC OS

Laser Light Sheet

Pitching Axis

Side Camera

FlowHST Cross-Section

Pressure Row

4 3 2 1

Trailing Edge

Wing Leading

Edge

Targets

Strake

A05-15010004

Page 23: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Steady Pressure Distributions on the Clean Wing

• M = 0.90, AOA = 6.45 deg to 11.39 deg

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

ALPHA = 6.45 deg ALPHA = 8.39 deg

ALPHA = 10.39 deg

ALPHA = 11.39 deg

Attached Attached, Pre-SITES

SITES Leading-Edge Separation

A05-15010006

Page 24: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Pulsed Laser Flow-Visualization, Transition From Attached to Leading-Edge Separation

Pulsed Laser Sheet at 80% Span Oriented Streamwise 9 Nano-Sec Pulse Duration

A05-15010007

Attached Flow Normal Shock Small Separation Bubble

at Foot of Shock

SITES “LAMBDA” Shock Larger Region of Shock-

Induced Separation

Leading-Edge Separation

ALPHA = 10.0 DEG ALPHA = 10.5 DEG ALPHA = 11.0 DEGLeading Edge Trailing Edge Leading Edge Trailing Edge Leading Edge Trailing Edge

Page 25: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Spanwise Location of Separation for Tip Missile/Launcher Configuration

M = 0.9, AOA = 6 deg to 10.5 deg, Spanwise Sheet Positions 11, 12, 13

Attached Flow Up to 8.0 deg

Transition to SITES at 8.0 deg, Continues up to 9.5 deg Not Full Chordwise Separation

Leading-Edge Separation Transition at 10.5 deg Full Chordwise Separation

A05-15010008

Page 26: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Comparison of Clean Wing and Tip Missile Configurations, AOA = 8.5 Deg

M = 0.9, Spanwise Sheet Positions 11, 12, 13

A05-15010010

AOA = 8.5°

Page 27: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Comparison of Clean Wing and Tip Missile Configurations, AOA = 10.5 Deg

Mach = 0.9, Spanwise Sheet Positions 11, 12, 13

A05-15010011

AOA = 10.5°

Page 28: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Natural Unsteadiness for Clean Wing and Tip Missile Configurations

M = 0.9, Sheet Positions 12 and 13 - 640 Frames/sec! Transition From SITES to Leading-Edge Separation

A05-15010012

Clean Wing, Sheet Position 12, AOA = 11 Deg

Wing With Tip Missile, Sheet Position 13, AOA = 9.51 Deg

Frame = 871 Frame = 873 Frame = 875 Frame = 877

Frame = 879 Frame = 880 Frame = 887 Frame = 888

Page 29: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Oscillatory Pitch Motion for the Tip Missile Configuration, AOA = 8.0 Deg

M = 0.9, Amplitude = ±0.5 Deg, Frequency = 40 Hz

AOARange

A05-15010013

MaximumAOA = 8.5°

MinimumAOA = 7.5°

Page 30: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Oscillatory Pitch Motion for the Tip Missile Configuration, AOA = 9.0 Deg

A05-15010014

M = 0.9, Amplitude = ±0.5 Deg, Frequency = 40 Hz

AOARange

MaximumAOA = 9.5°

MinimumAOA = 8.5°

Page 31: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Oscillatory Pitch Motion for the Tip Missile Configuration, AOA = 10.0 Deg

M = 0.9, Amplitude = ±0.5 Deg, Frequency = 40 Hz

A05-15010015

MaximumAOA = 10.5°

MinimumAOA = 9.5°

AOARange

Page 32: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Real Life Adventures with Unsteady Aerodynamics Outline

Aircraft Buffeting and Limit Cycle Oscillations (LCO) Flight Experiences Wind Tunnel Testing Observations

High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations

Page 33: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

High Rate Maneuvers and Other Transients

Path dependency is very important

Flow separation induced time lags are significant Lag for re-attaching flow is higher than separating flow “Static” flex-to-rigid ratios are different for attached or separated

flows Conventional ASE analysis tools do not account for these effects

System induced time lags and false state conditions can be destabilizing Data acquisition, processing and transferring to commands

depends on sample rates and sensors Controller/actuator lags are more significant Structural vibration modes can introduce false state conditions

During rapid maneuvers, the aircraft kinematic state (accels, rates, etc. may not be indicative of the aircraft loads state

Page 34: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

High Rate Maneuvers and Other Transients

Store ejection and wake vortex encounters are very sensitive to the current aircraft conditions Rapid control law changes due to store downloads cannot be

assessed with current ASE methods and may be critical if active flutter suppression is used

Aircraft response to wake vortex encounters is also affected by pilot/control system commands and how the wake is entered

During rapid maneuvers, the aircraft kinematic state (accels, rates, etc. may not be indicative of the aircraft loads state

Page 35: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Real Life Adventures with Unsteady Aerodynamics Outline

Aircraft Buffeting and Limit Cycle Oscillations (LCO) Flight Experiences Wind Tunnel Testing Observations

High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations

Page 36: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

High Rate Transients

Flow separation State change is very quick

Lag for flow separating is much less than for reattachment

Leading/trailing edge and shock induced separation are affected

Max lift overshoot on pitch-up can occur

Control law changes Data processing and structural

dynamics can induce lagging Actuator response is more

lagging

Time lags in flow state transitions and control law changes are problematic

A08-23854019

1.5

1.

0.5

0.

-0.5-10. 0. 10 20. 30. 40.

CNCN

DEG DEG

2.

50.

1.5

1.

0.5

0.

-0.5

-10. 0. 10 20. 30. 40.

DEG

2.

50.

CNCN

1.5

1.

0.5

0.

-0.5

-10. 0. 10 20. 30. 40.

DEG DEG

2.

50.

1.CNCN

Page 37: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

High Rate Transients

High-g roll maneuver anomaly RWD roll initiated during high-g

symmetric maneuver Right wing tip flow probably

separated Sudden re-attachment on right

wingtip reduced roll rate and increased g’s

Attributed to LE flap change from 7° to 10° during maneuver

Time lags in flow state transitions and control law changes are problematic

A08-23854004

g

Roll Rate (deg/s)

10 20 30 40

LEF=7º

10º

Page 38: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

High-g Maneuvers

Spanwise bending moments are less if the wing tip is separated Shock induced separation is most

common Flex-to-rigid ratios are higher

Wing washout at high-g’s and transonic speeds can eliminate shock induced separation Nose-down twist from wash-out weakens

tip shocks Verified by CFD solutions for a wind

tunnel model investigation

CFD based aeroelastic solution for an F-16 in a high-g maneuver demonstrated this effect Large wing tip deflections of over 20 in. Weakened wing tip shocks Correlated well with flight test data

Static aeroelasticity can be highly non-linear where wing tip flow separation is present

CFD based aeroelastic solution for an F-16 high-g maneuver

A08-23854020

Page 39: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Wake Vortex Encounters

Loss of Airbus A300, AA Flt 587, 17 Nov 2001 A300 followed 747 on climb-out Loss attributed to pilot over-reacting to

severe wake turbulence

Uncommanded double roll on approach to DFW Occurred at about 5k ft in landing pattern Rapid double roll in about 1 second No post encounter rolling

Losses of F-16 ventral tail tips Occurs during air-to-air combat training

exercises (tail chasing) Four incidents since 1980’s Attributed to wing tip vortex from lead

aircraft Pilot unaware of loss

These can range from annoying bumps to structural damage to loss of aircraft

A08-23854012

S-80 encounter in DFWLanding path ~ 5K FT

Left wing uplift, right wing downRWD roll back to neutral

Left wing down, right wing uplift LWD roll

Left wing upliftRWD roll

A08-23854013

Page 40: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Real Life Adventures with Unsteady Aerodynamics Outline

Aircraft Buffeting and Limit Cycle Oscillations (LCO) Flight Experiences Wind Tunnel Testing Observations

High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations

Page 41: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Model Geometry and Instrumentation/Flow-Visualization Locations

Force/Pressure Model Geometry and Instrumentation Layout

Flow-Visualization Light Sheet Positions

108.65

76°

AA

5

Section A-A

0.73

3 C

roo

t

820.

7Pitching Axis

Bearings

Shaft6

7Wing

Balance

4 3 2 1

62.3 7.0

Hydraulic Actuator

Wind Tunnel Sidewall

TurntableAccelerometersPressure Section

A05-15010003

Conf. 5 Conf. 2 Conf. 1

9

8

7

RotationAxis

14

1

2

3

4

5

6

10

11

12

13(T.E.)

Sheet Angle From Vertical = 4.7 deg

Page 42: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Mach Effects on Steady Normal Forceand Pitching Moment

M = 0.225, 0.6, and 0.9

1.2

0.8

0.4

CN

0.12

0.08

0.04

CM

0

00 10 20 30 40 50

0.225 8.0

0.60 8.0

0.90 8.0/15.0

M Re x 10-6

A01- 01100015

Page 43: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Rapid High AOA Maneuvers with Sideslip for Low Speed Full Span Straked Wing Model

Symmetric pitch maneuvers can produce max lift overshoots Buffet on pitch-up is low Buffet on pitch-down is higher and more

persistent

Asymmetric pitch maneuvers can affect roll moments Dynamic distortion through unstable

regions is very sensitive to pitch rate Effects are very path dependent

Example – pitching model with sideslip Large AOA pitching motions at -5 deg

sideslip Unstable roll moments between about 16

deg to 38 deg AOA for steady flow Motions limited to minimum flow state

changes distort the steady characteristics

Motions that cross several flow state changes completely change the character

Rapid post stall maneuver airloads are highly path dependent

A08-23854018

ClCl

DEG DEG

0.02

0.01

0.

- 0.01

- 0.02

- 0.03

- 0.04

- 0.05

- 0.06-10. 0. 10 20. 30. 40. 50.

DEG DEG

0.02

0.01

0.

- 0.01

- 0.02

- 0.03

- 0.04

- 0.05

- 0.06-10. 0. 10 20. 30. 40. 50.

A08-23854018

ClCl

DEG DEG

0.02

0.01

0.

- 0.01

- 0.02

- 0.03

- 0.04

- 0.05

- 0.06-10. 0. 10 20. 30. 40. 50.

DEG DEG

0.02

0.01

0.

- 0.01

- 0.02

- 0.03

- 0.04

- 0.05

- 0.06-10. 0. 10 20. 30. 40. 50.

Low pitch rate

High pitch rate

Ro

llin

g M

om

en

tR

oll

ing

Mo

me

nt

Page 44: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Attached Transonic Flow, AOA ≈ 9 Deg

Outer Panel Forward and Aft Normal Shocks Beginning of Strake Vortex Flow

A01-01100003

Page 45: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Leading-Edge Separation of Outboard Panel, AOA ≈ 11.5 Deg

Outboard of Section 1 Continued Development of Strake Vortex

A01- 01100005

Page 46: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Transition to Shock-Induced, Trailing-Edge Separation (SITES), AOA ≈ 10.5 Deg

Pulsed Laser Sheet at 80% Span Oriented Streamwise 9 Nano-Sec Pulse Duration

Attached Flow Normal Shock Small Separation Bubble

at Foot of Shock

SITES “LAMBDA” Shock Larger Region of Shock-

Induced Separation

A01- 01100004

Page 47: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Leading-Edge Separated Flows atAOA = 11.0 Deg and 22.0 Deg

Pulsed Laser Sheet at 80% Span Oriented Streamwise 9 Nano-Sec Pulse Duration

Initial Occurrence of Leading- Edge Separation

Supersonic Flow Above Separation Zone

Separation Shear Layer Structure Visible

Fully Developed Leading-Edge Separation

Supersonic Flow More Pronounced Above Separation

Shear Layer Structure More Distinct (Shocklets and Finger Vortices)

A01- 01100007

Page 48: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Transonic Vortex Flow, AOA ≈ 12 Deg to 19 Deg

Strake Vortex Dominant Shock Observed in Vortex Core

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Page 49: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Shocklet/Finger Vortex Flow,AOA ≈ 19 Deg to 27 Deg

Strake Vortex Bursting Multiple Shocks and Vortex Pairs Existing Above Separated Flows

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Page 50: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Transition to Turbulent Separation BoundaryFlow, AOA ≈ 27 Deg

Progressive Stalling Vortex Bursting, Inboard Wing Panel Lifting

Shocks and Vortices above Separated Flows Vortex Bursting, Outboard Wing Panel Still Lifting

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Page 51: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Progressive Stall Development, AOA >27 Deg

Progressive Stalling Strake Vortex Bursting

Progressive Stalling Wing Panel Completely Stalled Vortex Bursting Seen on Strake

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Page 52: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Unsteady Normal Force and PitchingMoment at M = 0.9

Pitch Oscillation From = 7 deg to 37 deg at 3.8 Hz

Lagging of Flow Transitions on Both Pitch-Up and Pitch-Down

1.2

0.8

0.4

CN

0.04

CM

0

0

0.08

0.12

SITES and Wing Tip Separation

“Conventional” Vortex Breakdown and Stalling

M = 0.9

0 10 20 30 40 50

Dynamic, Pitch-Up

Dynamic, Pitch-Down

Static

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Page 53: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Unsteady Flow for Oscillatory PitchingBetween 7.2 Deg and 37.7 Deg at 3.8 Hz

Pitch-Up/Pitch-Down Effects at 11.5 Deg

Pitch-Up Shows Delay of Outboard Panel Lift Breakdown

Pitch-Down Produces Delay of Outboard Flow Re-Establishment

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Page 54: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Unsteady Flow for Oscillatory PitchingBetween 7.2 Deg and 37.7 Deg at 3.8 Hz (Cont’d)

Pitch-Up/Pitch-Down Effects at 15 Deg to 18 Deg

Pitch-Up Shows Delay of Inboard Flow Breakdown Movement

Pitch-Down Shows Delay of Outboard Flow Re-Establishment

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Page 55: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Unsteady Flow for Oscillatory PitchingBetween 7.2 Deg and 37.7 Deg at 3.8 Hz (Cont’d)

Pitch-Up/Pitch-Down Effects at 27 Deg

Pitch-Up Shows Delay of Outboard Wing Stalling

Pitch-Down Shows Persistence of Outboard Wing Stalling but Rapid Development of the Strake Vortex

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Page 56: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Concluding Remarks – Why are unsteady aerodynamics important – Buffeting

Buffeting problems

Fatigue of T.E. controls, spoilers, etc.

Fatigue of fins, antennae, twin vertical tails and other downstream surfaces

Severe vibrations of wires/cables/hydraulic lines as well as equipment/bulkheads/weapons in open bays

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Page 57: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Concluding Remarks – Why are unsteady aerodynamics important – LCO

LCO problems

False indications of flutter onset

Pilot discomfort/distractions/limitations

Weapons systems limitations

Control surface buzz

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Page 58: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Concluding Remarks – Why are unsteady aerodynamics important – Transient conditions

High rate maneuvers and other transient problems

Severe aircraft buffet

High loads overshoot and excursions beyond static loads

Max loads that occur under transient conditions

Wing drop and stall flutter

Uncontrollable flow transitions

Wake vortex encounters

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Page 59: Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort

Lecture Data Sources

Wind Tunnel Database Summarized in This Paper Included in the RTO Database Verification and Validation Data for Computational Unsteady

Aerodynamics RTO-TR-26, AC/323 (AVT) TP/19, NATO, October 2000 Cunningham, A.M. and Geurts, E.G.M., “Transonic Pressure, Force and

Flow Visualization Measurements on a Pitching Straked Delta Wing at High Alpha,” Paper No. 7, NATO/RTO AVT-072/073, May 2001.

Cunningham, A.M., “Buzz, Buffet and LCO on Military Aircraft – The Aeroelastician’s Nightmares,” Presented at CEAS/AIAA/NVvL IFASD, Amsterdam, The Netherlands, 4-6 June 2003.

Cunningham, A.M. and Geurts, E.G.M., “Flow Visualization Investigation of Transonic Limit Cycle Oscillation Conditions for a Fighter-Type Wing with Tip Stores,” Paper No. 28, NATO/RTO AVT-123, April 2005.

Cunningham, A.M. and Holman, R.J., “Time Domain Aeroelastic Solutions – A Critical Need for Future Analytical Methods’ Developments,” Paper No. 12, NATO/RTO AVT-154, May 2008.

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