Real Life Adventures with Unsteady Aerodynamics by Dr. Atlee M. Cunningham, Jr. Lockheed Martin Senior Fellow Lockheed Martin Aeronautics Company, Fort.
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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
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
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
Real Life Adventures with Unsteady Aerodynamics Outline
High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations
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
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
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
Real Life Adventures with Unsteady Aerodynamics Outline
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
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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)
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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
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F-111 TACT Wing Buffet (Cont’d)
Predicted and Measured RMSAccelerations Versus Angle of Attack
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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
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F-111 TACT Wing LCO
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= 9.1°
= 10.0° = 11.1°
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
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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
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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
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Real Life Adventures with Unsteady Aerodynamics Outline
High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations
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
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
Real Life Adventures with Unsteady Aerodynamics Outline
High Rate Maneuvers and Other Transients Flight Experiences Wind Tunnel Testing Observations
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
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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
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g
Roll Rate (deg/s)
10 20 30 40
8º
LEF=7º
9º
10º
7º
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
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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
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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
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Real Life Adventures with Unsteady Aerodynamics Outline
Shocks and Vortices above Separated Flows Vortex Bursting, Outboard Wing Panel Still Lifting
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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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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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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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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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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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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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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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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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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.