Flight Control System for Unstable Subscale Aircraft Design and Testing of a

108 Pages·2015·26.01 MB·English

Master’s Thesis in Aeronautical Engineering
LIU-IEI-TEK-A—15/02190—SE
Design and Testing of a
Flight Control System for
Unstable Subscale Aircraft
ALEJANDRO SOBRÓN RUEDA

MASTER’S THESIS IN AERONAUTICAL ENGINEERING
/
LIU-IEI-TEK-A—15 02190—SE
Design and Testing of a Flight Control System
for Unstable Subscale Aircraft
ALEJANDRO SOBRÓN RUEDA
Division of Fluid and Mechatronic Systems
Department of Management and Engineering
Linköping University
SE-581 83 Linköping
Sweden 2015
Design and Testing of a Flight Control System
for Unstable Subscale Aircraft
Master’s Thesis in Aeronautical Engineering
© ALEJANDRO SOBRÓN RUEDA, 2015.
Supervisors: Ph.D. David Lundström, IEI, Linköping University
Dipl.Ing. Ingo Staack, IEI, Linköping University
Examiner: Prof. Ph.D. Tomas Melin, IEI, Linköping University
Division of Fluid and Mechatronic Systems
Department of Management and Engineering
Linköping University
SE-581 83 Linköping
Sweden
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ISRN LIU-IEI-TEK-A—15/02190—SE
Cover: Artist’s impression of the Generic Future Fighter concept demonstrator,
modified with permission from original renders by Erik Gustavsson.
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Abstract
The primary objective of this thesis was to study, implement, and test low-cost elec-
tronic flight control systems (FCS) in remotely piloted subscale research aircraft with
relaxed static longitudinal stability. Even though this implementation was carried out
in small, simplified test-bed aircraft, it was designed with the aim of being installed
later in more complex demonstrator aircraft such as the Generic Future Fighter con-
cept demonstrator project. The recent boom of the unmanned aircraft market has led
to the appearance of numerous electronic FCS designed for small-scale vehicles and
even hobbyist-type model aircraft. Therefore, the purpose was not to develop a new
FCS from scratch, but rather to take advantage of the available technology and to ex-
aminetheperformanceofdifferentcommercialoff-the-shelf(COTS)low-costsystems
in statically unstable aircraft models. Two different systems were integrated, cali-
brated and tested: a simple, gyroscope-based, single-axis controller, and an advanced
flight controller with a complete suite of sensors, including a specifically manufac-
tured angle-of-attack transducer. A flight testing methodology and appropriate flight-
testdata analysistoolswere alsodeveloped. Thesatisfactory resultsarediscussed for
different flight control laws, and the controller tuning procedure is described. On the
otherhand,thedifferenttest-bedaircraftwereanalysedfromtheoreticalpointofview
byusingcommonaircraft-designmethodsandconventionalpreliminary-designtools.
The theoretical models were integrated into a flight dynamics simulator, which was
comparedwithflight-testdataobtainingareasonablequalitativecorrelation. Possible
FCSmodificationsarediscussedandsomefutureimplementationsareproposed,such
as the integration of the angle-of-attack in the control laws.
Keywords: aircraft design, systems integration, subscale flight testing, avionics, flight
control system, remotely piloted aircraft.
i

Acknowledgements
This thesis was only possible thanks to the support and contribution of many people.
First of all, I would like to thank my supervisors David Lundström and Ingo Staack
for giving me great freedom in selecting and exploring the research topic. Their ad-
vice and guidance have been essential to this work. I would also like to extend my
sincere gratitude to every person in the Fluid and Mechatronic Systems, and Machine
Designdivisionswhosufferedmynumerous,crypticquestions. Specialmentiontomy
examiner Professor Tomas Melin, to Professor Petter Krus for the approval of the re-
searchbudget,andtoDavidBeugerforhishelpwith3Dprinting. IamthankfultoOla
Härkegård from Saab Aeronautics for his expertise and counselling within flight con-
trol. I would not like to forget my skilled colleagues Aevan N.D., Sharath S.M.K., and
especiallyAthanasiosP.whoproofreadthisthesisandprovidedwithvaluablefeedback
that is much appreciated. Last but not least, thanks to Rudinë J. for her patience and
unconditional support.
This work is dedicated to every one who has inspired and encouraged me to keep
always learning over the years. This includes remarkable professors, talented col-
leagues, and above all, those who taught me the most important lessons in life: my
parents Emma and José María.
Alejandro Sobrón Rueda, Linköping, June 2015
iii

Contents
List of Figures vii
List of Tables ix
Nomenclature xi
1 Introduction 1
1.1 Objectives and Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Background 5
2.1 Subscale Flight Testing in Aircraft Design . . . . . . . . . . . . . . . . . 5
2.1.1 Applications to Flight Control . . . . . . . . . . . . . . . . . . . . 7
2.1.2 Some Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.3 Subscale Flight Testing at Linköping University . . . . . . . . . 9
2.2 Unmanned Aircraft Systems Technology . . . . . . . . . . . . . . . . . . 10
2.2.1 Generic Radio Control Systems for Aircraft Models. . . . . . . 12
2.3 Flight Control Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3 Theoretical Models 15
3.1 Mass and Inertia Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 Aerodynamic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.1 Vortex Lattice Method . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.2 Neutral Point Estimation . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.3 Parasite Drag Estimation . . . . . . . . . . . . . . . . . . . . . . . 23
3.2.4 Propulsion System Effects . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.5 Some Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . 25
3.3 Mechatronic System Model . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.3.1 Control Surface Actuator . . . . . . . . . . . . . . . . . . . . . . . 27
3.3.2 Pitch Rate Gyroscopic Stability Augmentation System . . . . . 28
3.3.3 Pitch Attitude Control System with AVCS Gyroscope. . . . . . 29
3.3.4 Advanced Pitch Attitude Control Augmentation System . . . . 30
3.3.5 Advanced Pitch Rate Control Augmentation System . . . . . . 31
3.3.6 Proposed Angle-Of-Attack Control Augmentation System. . . 31
3.4 Flight Dynamics Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.4.1 Definition of Coordinate Frames and Aircraft Variables . . . . 33
v
Contents
3.4.2 Forces and Moments . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.4.3 Linearised Small-Disturbance Longitudinal Motion . . . . . . . 36
3.4.4 Assembly of the Complete Flight Dynamics Model . . . . . . . 40
3.5 Study of the Aircraft Dynamics . . . . . . . . . . . . . . . . . . . . . . . . 41
4 Flight Control System Integration 45
4.1 Basic Stability Augmentation System . . . . . . . . . . . . . . . . . . . . 45
4.2 Advanced Flight Control System . . . . . . . . . . . . . . . . . . . . . . . 46
4.3 Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.3.1 Embedded Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.3.2 Airspeed Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.3.3 Angle-of-Attack Sensor. . . . . . . . . . . . . . . . . . . . . . . . . 52
4.3.4 Flight Data Recording . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.4 Flight Control Laws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5 Experimental Evaluation 59
5.1 Test-Bed Aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2 Flight Test Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2.1 Flight Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.2.2 Post-flight Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3 Validation of the Estimated Neutral Point . . . . . . . . . . . . . . . . . 64
5.4 Evaluation of the Gyroscope-Based Flight Control System . . . . . . . 65
5.5 Evaluation of the Advanced Flight Control System . . . . . . . . . . . 67
5.5.1 Angle-Of-Attack in the Attitude Control Loop . . . . . . . . . . 71
5.6 Relaxed Stability Limits for Manual Control . . . . . . . . . . . . . . . . 74
5.7 Correlation with the Flight Dynamics Simulator . . . . . . . . . . . . . 75
6 Discussion and Conclusions 77
6.1 Systems Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6.2 Flight Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
6.3 Instrumentation and Data Analysis . . . . . . . . . . . . . . . . . . . . . 79
6.4 Some Suggestions for Flight Testing Unstable Models. . . . . . . . . . 80
Bibliography 82
A Appendix A: Characteristics of the Test-Bed Aircraft I
vi