Commercial aircraft hydraulic systems : Shanghai Jiao Tong University Press aerospace series

267 Pages·2015·21.97 MB·English

by  Liu

The development of the book was sponsored by
Shanghai Jiaotong University Press
Commercial Aircraft
Hydraulic Systems
Shanghai Jiao Tong University Press
Aerospace Series
Shaoping Wang
Department of Mechatronic Engineering
Beihang University, China
Mileta Tomovic
Batten College of Engineering and Technology
Old Dominion University, USA
Hong Liu
AVIC
The first Aircraft Institute
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Foreword
In general, the flight control system is the critical system of an aircraft. The
aircraft hydraulic actuation system and its power supply system are very
important, related systems that directly influence aircraft flight performance
andflightsafety.Overthepastseveraldecades,aircraftsystemdesignfocused
predominantly on the design principle itself without considering the related
system effects. The hydraulic power supply system provides high-pressure
fluid to the actuation system; therefore, its characteristics and performance
could influence the actuation system performance. On the other hand, the
actuation system utilizes hydraulic power to drive the surfaces, the perfor-
manceofwhichnotonlydependsonthedisplacementcontrolstrategybutalso
on the power supply performance. This book focuses on the aircraft flight
control system, including the interface between the hydraulic power supply
system and actuation system, and it provides the corresponding design prin-
ciple and presents the latest research advances used in aircraft design.
The aircraft hydraulic system evolved with the flight control system.
Early flight control systems were purely mechanical systems in which the
pilot controlled the aircraft surfaces through mechanical lines and movable
hinge mechanisms. With the increase in aircraft velocity, the hinge moments
and required actuation forces increased significantly to the point at which
pilots had difficulty manipulating control surfaces. The hydraulic booster
appeared to give extra power to drive the surfaces. With the increasing
expansion of flight range and duration of flight, it became necessary to
develop and implement an automatic control system to improve the flight
performance and avoid pilot fatigue. Then, the electrically signaled (also
known as fly-by-wire (FBW)), hydraulic powered actuator emerged to drive
the aircraft control surfaces. Introduction of the FBW system greatly
improved aircraft flight performance. However, the use of many electrical
devices along with the flutter influence of the hydraulic servo actuation
system led to a reliability problem. This resulted in wide implementation of
redundancy technology to ensure high reliability of the FBW system.
Increasingthenumber ofredundantchannels will potentiallyincreasedegree
of fault. To achieve high reliability and maintainability, a monitoring and
fault diagnosis device is integrated in the redundant hydraulic power supply
system and redundant actuation system.
Modern aircraft design strives to increase the fuel economy and reduction
inenvironmentalimpacts;therefore,thehigh-pressurehydraulicpowersupply
ix
x Foreword
system,variable-pressurehydraulicsystem,andincreasinglyelectricalsystem
are emerging to achieve the requirements of green flight.
Thisbookconsistsoffourchapters.Chapter1presentsanoverviewofthe
developmentofthehydraulicsystemforflightcontrolalongwiththeinterface
between the flight control system and the hydraulic system. The chapter also
introduces different types of actuation systems and provides the requirements
of the flight control system for specification and design of the required hy-
draulic system. Chapter 2 introduces the basic structure of aircraft hydraulic
power supply systems, provides the design principle of the main hydraulic
components, and provides some typical hydraulic system constructions in
currentcommercialaircraft.Chapter3introducesthereliabilitydesignmethod
ofelectricalandmechanicalcomponentsinthehydraulicsystem.Thechapter
provides comprehensive reliability evaluation based on reliability, maintain-
ability, and testability and gives the reliability evaluation of the aircraft
hydraulic power supply and actuation system. Chapter 4 introduces new
technologies used in modern aircraft, including the high-pressure hydraulic
power supply system, variable-pressure hydraulic power supply system, and
new types of hydraulic actuators.
We thank all of the committee members of a large aircraft flight control
serieseditorialboardandalloftheeditorsofShanghaiJiaotongPressfortheir
helpandassistanceinsuccessfullycompletingthisbook.Theauthorsarealso
gratefultoMsHongLiu,MrZhenshuiLi,andMrYisongTian,whoreviewed
thebookoutlineandcontributedtothewritingofthisbook.Weareindebtedto
their comments. We should also mention that some of the general theory and
structure composition were drawn from related references in this book;
therefore,wewouldliketoexpressourgratitudetotheirauthorsforproviding
outstandingcontributionsintherelatedfields.Finally,wehopethatthereaders
will find the material presented in this book to be beneficial to their work.
Shaoping Wang
Mileta Tomovic
Hong Liu
July 2015
Preface
Aircraftdesigncoversvariousdisciplines,domains,andapplications.Different
viewpoints have different related knowledge. The aircraft flight control series
focus on the fields that are related to the aircraft flight control system and
providethedesignprinciple,correspondingtechnology,andsomeprofessional
techniques.
Commercial Aircraft Hydraulic Systems aims to provide the practical
knowledgeofaircraftrequirementsforthehydraulicpowersupplysystemand
hydraulic actuation system; give the typical system structure and design prin-
ciple; introduce some technology that can guarantee the system reliability,
maintainability,andsafety;anddiscusstechnologiesusedincurrentaircraft.The
intentionistoprovideasourceofrelevantinformationthatwillbeofinterestand
benefittoallofthosepeopleworkinginthisarea.
xi
Chapter 1
Requirements for the
Hydraulic System of a Flight
Control System
Chapter Outline
1.1 TheDevelopmentofthe 1.3 ActuationSystems 13
HydraulicSystemRelated 1.4 RequirementoftheFCS
totheFlightControlSystem 1 totheHydraulicSystem 33
1.2 TheInterfacebetweenthe 1.5 Conclusions 50
FCSandHydraulicSystem 8 References 51
1.1 THE DEVELOPMENT OF THE HYDRAULIC SYSTEM
RELATED TO THE FLIGHT CONTROL SYSTEM [1]
The flight control system (FCS) is a mechanical/electrical system that trans-
mits the control signal and drives the surface to realize the scheduled flight
according to the pilot’s command. FCSs include components required to
transmit flight control commands from the pilot or other sources to the
appropriate actuators, generating forces and torques. Flight control needs to
realize the control of aircraft flight path, altitude, airspeed, aerodynamic
configuration,ride,andstructuralmodes.BecausetheperformanceoftheFCS
directlyinfluencesaircraftperformanceandreliability,itcanbeconsideredas
one of the most important systems in an aircraft.
A conventional fixed-wing aircraft control system, shown in Figure 1.1,
consists of cockpit controls, connecting linkages, control surfaces, and the
necessary operating mechanisms to control an aircraft’s movement. The
cockpitcontrolsincludethecontrolcolumnandrudderpedal.Theconnecting
linkage includes a pushepull control rod system and cable/pulley system.
Flight control surfaces include the elevators, ailerons, and rudder. Flight
control includes the longitudinal, lateral-directional, lift, drag, and variable
geometry control system.
Since the first heavier-than-air aircraft was born, it is the pilot who drives
the corresponding surfaces through the mechanical system to control the
aircraft, which is called the manual flight control system (MFCS) without
CommercialAircraftHydraulicSystems.http://dx.doi.org/10.1016/B978-0-12-419972-9.00001-2
Copyright©2016ShanghaiJiaoTongUniversityPress.PublishedbyElsevierInc.Allrightsreserved. 1
2 CommercialAircraftHydraulicSystems
FIGURE1.1 StructureoftheinitialFCS.
power. A very early aircraft used a system of wing warping in which no
conventionallyhingedcontrolsurfaceswereusedonthewing.AMFCSusesa
collection of mechanical parts such as pushrods, tension cables, pulleys,
counterweights,andsometimeschainstodirectlytransmittheforcesappliedat
the cockpit controls to the control surfaces. Figure 1.1 shows the aircraft’s
purely mechanical manipulating system, in which a steel cable or rod is used
to drive the surfaces. If the pilot wants to move the flaps on a plane, then he
would pull the control column, which would physically pull the flaps in the
direction that the pilot desired. In this period, the designer focuses on the
friction,clearance,andelasticdeformationofthetransmissionsystemsoasto
achieve good performance.
With the increase of size, weight, and flight speed of aircraft, it became
increasingly difficult for a pilot to move the control surfaces against the
aerodynamic forces. The aircraft designers recognized that the additional
power sources are necessary to assist the pilot in controlling the aircraft.
The hydraulic booster, shown in Figure 1.2(a), appeared at the end of the
1940s, dividing the control surface forces between the pilot and the
boosting mechanism. The hydraulic booster utilizes the hydraulic power
with high pressure to drive the aircraft surfaces according to the pilot’s
command. As an auxiliary component, the hydraulic booster can increase
the force exerted on the aircraft surface instead of the pilot directly
changing the rotary or flaps. As the earliest hydraulic component that is
FIGURE1.2 EvolutionoftheaircraftFCS.(a)Mechanicalmanipulatingsystemwithbooster,
(b) irreversible booster control system, (c) reversible booster control system, (d) stability
augmentationcontrolsystem,and(e)FBWsystems[2].
4 CommercialAircraftHydraulicSystems
related to the aircraft FCS, the hydraulic booster changed the surface
maneuver from mechanical power to hydraulic power and resisted the hinge
moment of surfaces without the direct connection between the control rod
and surfaces. There are two kinds of hydraulic booster: reversible booster
and irreversible booster. In the case of the irreversible booster control
system shown in Figure 1.2(b), there is no direct connection between the
control rod and the surface. The pilot controls the hydraulic booster to
change the control surface without feeling of the flight state. The advan-
tages of hydraulically powered control surfaces are that (aerodynamic load
on the control surfaces) drag is reduced and control surface effectiveness is
increased. Therefore, the reversible booster control system emerged through
installing the sensing device to provide the artificial force feeling to the
pilot, shown in Figure 1.2(c). The reversible booster control system
includes the spring, damper, and additional weight to provide the feedback
(feeling) so that a pilot could not pull too hard or too suddenly and damage
the aircraft. In this kind of aircraft, the characteristics of booster (maximum
output force, distance, and velocity) should satisfy the flight control
performance.
In general, the center of gravity is designed forward of center of lift for
positive stability. Modern fly-by-wire (FBW) aircraft is designed with a
relaxedstabilitydesignprinciple.Thiskindofdesignrequiressmallersurfaces
and forces, low trim loads, reduced aerodynamic airframe stability, and more
control loop augmentation. This kind of aircraft operates with augmentation
under subsonic speed. When the aircraft operates at supersonic speed, the
aircraft focus moves backward, and the longitudinal static stability torque
rapidly increases. At this time, it needs enough manipulating torque to meet
the requirements of aircraft maneuverability. However, the supersonic area in
the tail blocks the disturbance propagation forward, and the elevator control
effectiveness is greatly reduced. Hence, it is necessary to add signals from
stability augmentation systems and the autopilot to the basic manual control
circuit. As we know, a good aircraft should have good stability and good
maneuverability. The unstable aircraft is not easy to control. Because the
supersonic aircraft’s flight envelopeexpands, its aerodynamics are difficult to
meettherequirementsatlow-altitude/low-speedandhigh-altitude/high-speed.
In the high-altitude supersonic flight, the aircraft longitudinal static stability
dramatically increases whereas its inherent damping reduces, then the short
periodic oscillation in the longitudinal and transverse direction appear that
greatly influences the aircraft maneuverability. To maintain stability of the
supersonic aircraft, it is necessary to install the stability augmentation system
shown in Figure 1.2(d). Because the stability augmentation system can keep
theaircraft stableeveninstatic instabilitydesign,theautomaticflightcontrol
system (AFCS) appeared. The AFCS consists of electrical, mechanical, and