Integrated Control with Structural Feedback to Enable Lightweight Aircraft

National Aeronautics and Space Administration

Integrated Control with Structural Feedback to

Enable Lightweight Aircraft

Brian R. Taylor Aerospace Engineer NASA Dryden Flight Research Center 2011 Technical Conference NASA Fundamental Aeronautics Program Subsonic Fixed Wing Project Cleveland, OH, March 15 - 17, 2011 Fundamental Aeronautics Program www.nasa.gov Subsonic Fixed Wing Project

Agenda

• System Level Metrics • Benefits of Active Structural Control • Active Structural Control Research Areas • Integrated Control for the Prevention of Critical Loads – Problem Statement – Control System Architecture – Simulation Studies • Approach • Results and Discussion • Future Research Fundamental Aeronautics Program Subsonic Fixed Wing Project

NASA Subsonic Transport System Level Metrics

…. technology for dramatically improving noise, emissions, & performance

N+1 (2015)*** N+2 (2020)*** N+3 (2025)*** Technology Benefits Technology Benefits Technology Benefits CORNERS OF THE Relative to a Relative to a TRADE SPACE Single Aisle Reference Large Twin Aisle Reference Configuration Configuration Noise - 32 dB - 42 dB - 71 dB (cum below Stage 4) LTO NOx Emissions -60% -75% better than -75% (below CAEP 6) Performance -33%** -50%** better than -70% Aircraft Fuel Burn Performance -33% -50% exploit metroplex* concepts Field Length *** Technology Readiness Level for key technologies = 4-6 ** Additional gains may be possible through operational improvements * Concepts that enable optimal use of runways at multiple airports within the metropolitan areas

SFW Approach

- Conduct Discipline-based Foundational Research

- Investigate Advanced Multi-Discipline Based Concepts and Technologies

- Reduce Uncertainty in Multi-Disciplinary Design and Analysis Tools and Processes

- Enable Major Changes in Engine Cycle/Airframe Configurations

Fundamental Aeronautics Program Subsonic Fixed Wing Project

Benefits of Active Structural Control

• Reduction in aircraft fuel burn due to reduced structural weight

– Reduced structural margins • Gust loads • Aeroelastic instabilities • Gust loads and aeroelastic instabilities inherent in high lift-to-drag,

high aspect ratio, low sweep wings (N+3 aircraft)

• One study has shown a 25% reduction in airframe weight through

the use of active flutter suppression

– “System Benefits of Active Flutter Suppression for a SensorCraft-Type Vehicle”, Nicolai, Hunten, Zink, and Flick Takeoff Gross Weight 96,000 lb Empty Weight 36,332 lb Wing Area 2445 ft Aspect Ratio 14 Fundamental Aeronautics Program Subsonic Fixed Wing Project

Research Areas

Materials and Actuation – Conformable mold-line – Effector development Sensors

Actively Controlled, Lightweight

– Fiber optics

Structure

– Stagnation point sensors Improved fuel burn, noise, and performance Control Laws – Unstable rigid body dynamics – Unsteady aeroelastic effects (limit cycle oscillation, flutter, dynamic gust load response) – Reduction of peak loads – Wing shape optimization Test, Evaluation, and System Analysis – Flight validated models and simulations – Multi-disciplinary design guidelines – Uncertainty analysis and propagation Fundamental Aeronautics Program Subsonic Fixed Wing Project

Fiber Optic Sensor Development

• Objective: enable a high density of • Approach: capture data from ground real-time measurements of deflection, and flight tests on aircraft and simple rotation, bending moment, torsion, and test articles to investigate fundamental mode shape principles and draw comparisons to other sensor technologies – Validate method and equations – Determine accuracy of measurements – Determine optimal fiber placement Ground Test of Global Observer wing Fundamental Aeronautics Program Subsonic Fixed Wing Project

Fiber Optic Sensor Development

• Results: data captured in flight and ground tests – Good results for bending deflection of simple test articles and a full scale UAV • Remaining Work: progression to more complicated structures, validate loads measurements, increased sample rate, increased resolution, and multi-core fiber • Significance: enable structural weight reduction – Structural feedback to control system (shape and loads) • Maneuver and gust load alleviation Swept Plate Undergoing Test • Active control of aeroelastic instabilities • Structural health monitoring Fundamental Aeronautics Program Subsonic Fixed Wing Project

Aerodynamic Force Sensor Development

• Objective: enable real-time measurement of external forces acting on the aircraft in the presence of flow separation through direct measurement of stagnation point – Model generation – Parameter identification – Stagnation point control • Loads, aeroelastic instabilities • Approach: progression from open- loop observability and validation to Aerostructures Test Wing closed loop control of stagnation point location Fundamental Aeronautics Program Subsonic Fixed Wing Project

Aerodynamic Force Sensor Development

• Results: validation of stagnation • Significance: enables

point to sectional forces through structural weight reduction, wind tunnel tests and open-loop optimization of sectional forces flight test (observability) to reduce drag, and precise lift control during takeoff and landing

• Remaining Work: close the loop

– Direct measurement of around stagnation point sensors to external forces without the check controllability associated structural lag – Indirect measurement of Reference 1 drag – Provides an observable for ] (/deg) -1 the state of separated flow around stall conditions – Detection and diagnosis of shock/boundary-layer interaction and separation- Lift Coefficient [10 induced load fluctuations Angle of Attack (deg) Fundamental Aeronautics Program Subsonic Fixed Wing Project

Integrated Control for the Prevention of

Critical Loads

NASA DFRC NASA ARC Brian Taylor Susan Frost John Burken Khanh Trinh Christine Jutte Fundamental Aeronautics Program Subsonic Fixed Wing Project

Problem Statement

Aileron Position Roll Moment Achieved Roll Moment Achieved Structural Limit Exceeded Structural Loads Within Limits • Use active structural feedback to determine the optimal control surface positions to: – Meet commanded rigid body moments – Minimize structural loads – Ensure that structural loads do not exceed limits • Optimal control allocation with structural feedback is a potential solution Fundamental Aeronautics Program Subsonic Fixed Wing Project

Optimal Control Allocation

• Separate the regulation task from the control allocation task

• Mixed optimization to solve multiple objectives simultaneously

Control System System r y v u Control System Control Law Actuators Allocator Dynamics x Conventional Control Law with Optimal Control Allocation (ref. 2) Fundamental Aeronautics Program Subsonic Fixed Wing Project

Control Allocation Objectives

• Meet commanded rigid body moments (v) – Solve for surface positions ( u) such that v = Bu • Need both structural feedback and predictive portion similar to the rigid body moments and the control effectiveness (B) matrix – Measured load (M) and incremental load due to incremental control surface deflection (T) L = M + Tu • Minimize structural loads • Ensure that structural loads do not exceed limits – Treated like control surface saturation • Cost function:

J = Bu − v + γ M + Tu

Subject to:

u ≤ u ≤ u

M + Tu ≤ L

( )

min max max Fundamental Aeronautics Program Subsonic Fixed Wing Project

Control Allocation Diagram

Control System System r y v u Control System Control Law Actuators Allocator Dynamics M x Structural Feedback Control Law with Optimal Control Allocation Incremental Load

• Feedback of measured load to control

T =

allocator

Incremental Deflection

• Inclusion of the T matrix

– Computed from FEM code Incremental Moment – Lookup table B = Incremental Deflection – Parameter estimation Fundamental Aeronautics Program Subsonic Fixed Wing Project

Simulation Study Approach

• Modified Generic Transport • Roll maneuver

Model (GTM) simulation – Load control on and off with load

limits set low enough to require – 6 ailerons, 4 elevators, 2 rudders change in allocation – FEM of wing, horizontal tail, and • Same rigid body motion with vertical tail reduction in load at critical – “Critical points” at inboard edge of point control surfaces and roots – Distributed lift force over entire wing/tail and point forces due to control surfaces Applied Point Forces Right Wing Critical Points Fundamental Aeronautics Program Subsonic Fixed Wing Project

Simulation Study Results

Outboard Aileron Bending Moment

Fundamental Aeronautics Program Subsonic Fixed Wing Project

Simulation Study Results

Roll Performance

Fundamental Aeronautics Program Subsonic Fixed Wing Project

Simulation Study Discussion

• Roll moment generation moved from outboard aileron surfaces to inboard surfaces – L1 norm optimal control allocation used for study – L2 norm would more evenly distribute demand – All control surfaces that generate roll moment available (split elevator and rudder) • Roll performance maintained without violating load limits • Can be used to: – Enable optimal control allocation on existing aircraft • Control surface commands constrained not to exceed existing load limits – Enable lighter weight structure on future aircraft • Reduce future load limits while still meeting performance demands • Extension to algorithm for gust load alleviation Fundamental Aeronautics Program Subsonic Fixed Wing Project

Next Steps

• Study T matrix assumptions and accuracy necessary for flight • Extension of algorithm for gust load alleviation – Gust load alleviation literature survey – Incremental load per incremental gust • “G” matrix – Need measurement of gust – Addition of gust loads into optimal allocator objectives • Improved sensor fidelity – Delays inherent in each sensor • Hardware test of algorithm and feedback Fundamental Aeronautics Program Subsonic Fixed Wing Project

Future Research

• Characterize input and response of structure in flight – Stagnation point sensors, fiber optics, strain gages, accelerometers, photogrammetry • Create and release validated models and simulations • Active load control, gust load alleviation, and flutter suppression NASA F-18 FAST – Dedicated surfaces – Demands spread across all surfaces • Passive aeroelastic tailoring • Develop analysis techniques • Validate design and optimization tools using flight data Multi-Utility Technology Testbed Fundamental Aeronautics Program Subsonic Fixed Wing Project

Your Title Here

Nomenclature

Symbol Definition r Commanded rates v Commanded moments u Commanded surface positions y Aircraft response x Observed aircraft states M Measured load L Predicted future load T Incremental load per incremental surface deflection B Incremental moment per incremental surface deflection G Incremental load per incremental gust Weighting matrix

γ

u Maximum surface position max u Minimum surface position min L Load limit limit J Cost function Fundamental Aeronautics Program Subsonic Fixed Wing Project

References

1. A. Mangalam et al, “Aerodynamic and Structural Measurement of the Aerostructures Test Wing for Flutter Testing,” Presentation at NASA Dryden Flight Research Center, Edwards, CA. October 2010.

2. O. Harkegard, “Backstepping and Control Allocation with Applications to Flight Control,” Linkoping Studies in Science and Technology.

Dissertations No. 820. 2003.

Integrated Control for the Prevention of Critical Loads Publications

1. S. Frost et al, “A Framework for Optimal Control Allocation with Structural Load Constraints,” AIAA Atmospheric Flight Mechanics Conference, Toronto, Canada. August 2010

Acknowledgements

John Bakalayar (DFRC) Marty Brenner (DFRC) Christine Jutte (DFRC) Susan Frost (ARC) Arun Mangalam (Tao Systems) Fundamental Aeronautics Program Subsonic Fixed Wing Project

Backup Slides

• Elevator outboard

Fundamental Aeronautics Program Subsonic Fixed Wing Project

Backup Slides

• Elevator inboard

Fundamental Aeronautics Program Subsonic Fixed Wing Project

Backup Slides

• Aileron outboard

Fundamental Aeronautics Program Subsonic Fixed Wing Project

Backup Slides

• Aileron middle

Fundamental Aeronautics Program Subsonic Fixed Wing Project

Backup Slides

• Aileron inboard

Fundamental Aeronautics Program Subsonic Fixed Wing Project

Backup Slides

• Rudder upper and lower

Fundamental Aeronautics Program Subsonic Fixed Wing Project