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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
This book is the first of a series of volumes that cover the topic of aerospace actuators following a systems-based approach. This first volume provides general information on actuators and their reliability, and focuses on hydraulically supplied actuators. Emphasis is put on hydraulic power actuators as a technology that is used extensively for all aircraft, including newer aircraft. Currently, takeovers by major corporations of smaller companies in this field is threatening the expertise of aerospace hydraulics and has inevitably led to a loss of expertise. Further removal of hydraulics teaching in engineering degrees means there is a need to capitalize efforts in this field in order to move it forward as a means of providing safer, greener, cheaper and faster aerospace services. The topics covered in this set of books constitute a significant source of information for individuals and engineers seeking to learn more about aerospace hydraulics.
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Table of Contents
Cover
Title
Copyright
Introduction
Notations and Acronyms
1 General Considerations
1.1. Power transmission in aircraft
1.2. Primary and secondary power transmission functions for actuators
1.3. Hydraulic power actuation
2 Reliability
2.1. Risks and risk acceptance
2.2. Response to failure
2.3. Redundancy
2.4. Feared events and failure rates in actuation
2.5. Fundamentals of reliability calculation
2.6. Short glossary of technical terms pertaining to reliability
3 Hydraulic Fluid and its Conditioning
3.1. Needs and constraints
3.2. Fluid conditioning
3.3. Monitoring and maintaining the fluid in working conditions
3.4. Energy phenomena caused by the fluid
4 Hydromechanical Power Transformation
4.1. Hydromechanical power transformation
4.2. Functional perspective
4.3. Technological shortcomings
4.4. Pump driving
5 Power Metering in Hydraulics
5.1. Power metering principles
5.2. Power-on-Demand
5.3. Metering by hydraulic restriction
5.4. Impact of restriction configuration and properties on the metering function
5.5. Servovalves
6 Power Management in Hydraulics
6.1. Power distribution
6.2. Providing power
6.3. Protecting
6.4. Managing the load
7 Architectures and Geometric Integration of Hydraulically-supplied Actuators
7.1. Introduction
7.2. Arrangement of actuation functions
7.3. Architecture and routing of hydraulic power networks
7.4. Integration of components and equipment
7.5. Integration of actuators in the airframe
Bibliography
Index
End User License Agreement
List of Tables
1 General Considerations
Table 1.1.
Secondary power requirements for a large commercial aircraft [COM 05]
Table 1.2.
Examples of power needs
Table 1.3.
Power and energy variables
Table 1.4.
Types of actuators according to the nature of their signal and power interfaces
Table 1.5.
International and common units used in the field of hydraulics
Table 1.6.
Benefit of transporting energy in hydrostatic form
Table 1.7.
Evolution of the working pressure of hydraulic systems in aeronautics
2 Reliability
Table 2.1.
Risk matrix: risk acceptability as a function of its gravity and its frequency [FAA 14]. For a color version of the table, see www.iste.co.uk/mare/aerospace1.zip
Table 2.2.
Acceptability thresholds and consequences on passengers [FAA 14]
Table 2.3.
Examples of safety margins for hydraulic components [FAA 01]
Table 2.4.
Examples of response to failure
Table 2.5.
The main advantages and disadvantages of static redundancy
Table 2.6.
The main advantages and disadvantages of static redundancy
Table 2.7.
Orders of magnitude of failure rates for various actuation elements and failure modes
Table 2.8.
Example of reliability data analysis
Table 2.9.
Glossary of important technical terms relating to reliability
3 Hydraulic Fluid and its Conditioning
Table 3.1.
Typical variations of the phosphate-ester hydraulic fluid with pressure and temperature [SAE 08]
Table 3.2.
Requirements and generic solutions implemented to condition the fluid
Table 3.3.
Maximum number of solid particles tolerated in 100 ml of hydraulic fluid as a function of their size and contamination grade
5 Power Metering in Hydraulics
Table 5.1.
Power metering principles
Table 5.2.
Orders of magnitude for hydraulic restrictions
Table 5.3.
Timeline of the historic development of servovalves
Table 5.4.
Orders of magnitude for standard servovalves
Table 5.5.
Architecture of three aeronautical DDVs in service
6 Power Management in Hydraulics
Table 6.1.
Function/component matrix
Table 6.2.
Mass and power loss in tubes
Table 6.3.
Intrinsic strengths and weaknesses of different accumulator technologies
List of Illustrations
1 General Considerations
Figure 1.1.
Secondary power flows for an
Airbus A230 type single-aisle aircraft [LIS 08]. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 1.2.
Actuation needs on a commercial aircraft
Figure 1.3.
Swashplate actuation on an AS332 helicopter
Figure 1.4.
Power requirement for the actuator of an Airbus A320 aileron [MAR 09]
Figure 1.5.
Functions in the information and power chains [CND 02]
Figure 1.6.
Airbus A350 XWB aileron actuator incorporating position control electronics
Figure 1.7.
First applications of hydraulics to aerospace [THO 42]
Figure 1.8.
Operating principle of the autopilot in roll [DON 40]
Figure 1.9.
Jacottet-Leduc irreversible servo-control [JAC 54]
Figure 1.10.
Jacottet-Leduc and Samm hydraulic assistance cylinders (Alouette III)
Figure 1.11.
Typical hydraulic system architecture. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
2 Reliability
Figure 2.1.
Response of a system to the failure of one of its elements
Figure 2.2.
Force summation in flight control actuators
Figure 2.3.
The main types of redundancy
Figure 2.4.
Simple parallel redundancy
Figure 2.5.
Redundancy by averaging
Figure 2.6.
Majority voting
Figure 2.7.
Command–monitoring architecture
Figure 2.8.
Detection–correction
Figure 2.9.
Evolution of reliability variables as a function of service quantity. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 2.10.
Comparison of reliability depending on the type of element association. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
3 Hydraulic Fluid and its Conditioning
Figure 3.1.
Actual fluid and its pollution
Figure 3.2.
Example of fluid volume variation in the reservoir of a hydraulic circuit caused by geometric effects
Figure 3.3.
Example of pneumatic pressurization system for the hydraulic circuits of a commercial airplane [DIE 14]
Figure 3.4.
Example of a hydraulic architecture with a bootstrap reservoir
Figure 3.5.
Air pressurized reservoir of the Airbus A380 on the left, and bootstrap reservoir of the CASA C295 on the right
Figure 3.6.
In series filtration architecture
Figure 3.7.
Example of a hydraulic fluid temperature range as a function of altitude for a commercial airplane [BEH 13]
Figure 3.8.
Air/oil cooling on Airbus A380
Figure 3.9.
Simulation of hydraulic fluid temperatures in an Airbus A350 cruising at -80°C ambient with and without adjustable heater valves [BEH 13]
Figure 3.10.
Example of hydraulic architecture associated to the functions of monitoring and maintaining working condition for fluid conditioning
Figure 3.11.
Service panel of the green circuit of Airbus A350
4 Hydromechanical Power Transformation
Figure 4.1.
Hydromechanical transformers – parameters and symbols
Figure 4.2.
Evolution of fluid in the cavity of a positive-displacement machine operating in pump mode [IVA 01]. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 4.3.
Examples of positive-displacement hydromechanical transformers [CCP 05]
Figure 4.4.
Commutation effect
Figure 4.5.
Hydraulic pump driven by the engine via the accessory transmission gearbox
Figure 4.6.
Hydraulic pump driven by an electric motor (EDP and local hydraulic generation)
Figure 4.7.
Variable displacement power transfer unit (PTU) [VIC 98]
Figure 4.8.
Hydraulic pump driven by dynamic air (image on the right used with the permission of Triumph Thermal Systems, Maryland Inc.) [EDU 13]
5 Power Metering in Hydraulics
Figure 5.1.
Efficiency of a positive-displacement machine at partial load [EAT 00]
Figure 5.2.
Power metering by action on the pump mechanical drive
Figure 5.3.
Power metering by action on displacement at the source
Figure 5.4.
Example of dynamic response of a variable displacement, pressure-compensated pump (main power generation of a commercial aircraft)
Figure 5.5.
Power metering by action on displacement at destination
Figure 5.6.
Concepts of power metering by variable hydraulic resistance
Figure 5.7.
Power metering for a single or double effect hydraulic load
Figure 5.8.
Different configurations of center for hydraulic valves
Figure 5.9.
Power distribution and metering
Figure 5.10.
Flow control valves
Figure 5.11.
Four-way proportional valve, viewed as a Wheatstone bridge
Figure 5.12.
Example of an aerospace hydraulic proportional valve design
Figure 5.13.
Series distribution
Figure 5.14.
Parallel distribution
Figure 5.15.
Static hydraulic characteristic curve (flow vs. differential pressure) of a hydraulic restriction
Figure 5.16.
Two concepts of variable orifice construction
Figure 5.17.
Generic geometric characteristic curves of variable orifices
Figure 5.18.
Geometrical characteristic curves of the channels of a rudder actuator servovalve
Figure 5.19.
Power characteristic curves of a critical, closed-center, symmetrical valve supplied at constant pressure [MAR 02b]
Figure 5.20.
Leakage flow of a rudder actuator valve [ATT 08]
Figure 5.21.
Static hydraulic characteristic curve of a rudder actuator servovalve [ATT 08]
Figure 5.22.
Different types of controls for hydraulic valves
Figure 5.23.
Redundant power metering valves with linear cylindrical spool (left) and rotary spool (right)
Figure 5.24.
Generic architecture of a servovalve
Figure 5.25.
Signal view of different types of servovalves
Figure 5.26.
Cross-section of the proportional SABCA pilot valve and its implantation on the thrust vector control actuator of launcher Ariane 5
Figure 5.27.
Concepts of hydraulic pilot stages. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 5.28.
Static hydraulic characteristic curves of pressure stages [TCH 79]: left: double flapper-nozzle; right: jet-pipe
Figure 5.29.
Schematic of a servovalve with flapper-nozzle or jet-pipe pilot stage
9
. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 5.30.
Influence of the control current magnitude on the frequency response of a rudder actuator servovalve [ATT 08]
Figure 5.31.
Direct drive valves in service
Figure 5.32.
Moog linear-motor direct drive valve: left: operating principle and components [SCH 93]; right: examples of DDV-equipped actuators (image
©
Moog).
For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
6 Power Management in Hydraulics
Figure 6.1.
Concepts of isolation from supply and depressurization
Figure 6.2.
Concepts of isolation from supply and return lines
Figure 6.3.
Concepts of source merging
Figure 6.4.
Concepts of flow sharing
Figure 6.5.
Accumulator symbols
Figure 6.6.
Adjusting the pressure level (simple and explicit symbols)
Figure 6.7.
Pressure relief
Figure 6.8.
Different protection concepts against overload
Figure 6.9.
Concepts for preventing desorption and cavitation
Figure 6.10.
Protection against fluid over-consumption
Figure 6.11.
Hydraulic fuse (braking system of Boeing B737),(© Chris Brady)
Figure 6.12.
Concepts for ensuring irreversibility
Figure 6.13.
Concepts to release the load (also called load de-clutching)
Figure 6.14.
Shimmy damper
Figure 6.15.
Snubbing at end-stop
7 Architectures and Geometric Integration of Hydraulically-supplied Actuators
Figure 7.1.
Layout of flight control actuation onboard Airbus A330-200 (
©
Airbus). For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 7.2.
Architecture of hydraulic power generation onboard Airbus A320 (
©
Airbus)
Figure 7.3.
Example of hydraulic circuit routing ensuring spatial segregation Left: AW 609 tilt rotor [FEN 05] Right: Airbus A330 (
©
Airbus). For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 7.4.
Interfaces for integration of components
Figure 7.5.
Example of manifold integration for hydraulic power generation and distribution [FAA 12]. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 7.6.
Example of manifold integration for an active-standby actuator. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 7.7.
Integration of functions in the actuator housing
Figure 7.8.
Local electro-hydraulic generation system (LEHGS) of Airbus A380
Figure 7.9.
Main rotor hydraulic subassembly (MHRS) of Bell 429
Figure 7.10.
Aft strut fairing module of Boeing B787 (side views © Parker)
Figure 7.11.
Two concepts of hydro-mechanical actuators
1
. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
Figure 7.12.
Moving-rod hydro-mechanical actuator
Figure 7.13.
Moving-rod, active/active tandem, hydro-mechanical main rotor actuator of the Tiger helicopter
Figure 7.14.
Various types of actuator/airframe integration
Figure 7.15.
Benefits of the reaction/kick link design on airframe mechanical loading
Figure 7.16.
Rack and pinion kinematics for landing gear steering (top view)
Figure 7.17.
Push–pull kinematics with torque summing for landing gear steering (top view)
Figure 7.18.
Nose landing gear steering actuator: left: Boeing B787, right: Airbus A350
Guide
Cover
Table of Contents
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Series Editor
Jean-Paul Bourrières
Aerospace Actuators 1
Needs, Reliability and Hydraulic Power Solutions
Jean-Charles Maré
First published 2016 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd
27-37 St George’s Road
London SW19 4EU
UK
www.iste.co.uk
John Wiley & Sons, Inc.
111 River Street
Hoboken, NJ 07030
USA
www.wiley.com
© ISTE Ltd 2016
The rights of Jean-Charles Maré to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.
Library of Congress Control Number: 2016936925
British Library Cataloguing-in-Publication Data
A CIP record for this book is available from the British Library
ISBN 978-1-84821-941-0
Introduction
This book is the first of three volumes that cover the topic of aerospace actuators following a system-based approach. The first volume provides general information on actuators and their reliability, and focuses on hydraulically supplied actuators. The second volume addresses more electrical actuators (electro-hydraulic, electro-hydrostatic and electromechanical) and their signal flows (Signal-by-Wire) as well as their power flows (Power-by-Wire). The third volume illustrates the concepts introduced in the two previous volumes by showcasing various examples of applications of actuation in aerospace (flight controls, landing gear and engines) as well as different types of aircraft (commercial, military or business aircraft, helicopters, convertibles and space launchers).
In order to successfully develop a system perspective in this book series, a top-down approach (from the requirements to the solution) has to be carefully combined with a bottom-up approach (from the technological maturity to the solution). The main guiding idea is, therefore, to focus on requirements and architecture (functional then conceptual) while revealing restrictions imposed by technology on concept implementation, in particular, with regard to functions and phenomena induced by technological choices. Indeed, in practice, solutions are designed according to the functional aspects that have to be implemented and combined. Subsequently, performance is assessed while taking into consideration technological shortcomings1 such as magnetic hysteresis, winding inductance, mechanical clearance or even hydraulic resistance of pipes. Architectural aspects are generally poorly documented in bibliographies compared to design and technological aspects. However, this does not mean that design and technology are of little importance. Quite the contrary, they too are essential aspects. Indeed, it very often turns out that, as of a given date, technology becomes the limiting factor for the industrial relevance of a project in terms of performance, reliability and cost. Similarly, the accuracy of mathematical models and the computing power they require often restrict safety margin improvement and design optimization.
Design, modeling and technological aspects are generally well documented in bibliographies but the architectural feature, both functional and conceptual, is usually rarely addressed. The difficulty, therefore, lies in formalizing architectures and concepts. The use of mathematical models and technology description is only resorted to when they are necessary for comprehension and technology selection in this volume. The “architecture”- oriented system approach highlights the required functionalities and interdependencies between system components. This approach shows more promise for improving global performance than the “component”-oriented approach. This is because a globally optimal system is hardly ever obtained by simply combining components that are each individually optimal. In commercial aeronautics, developing such an approach is made even more difficult by the organization of design offices: they are often divided up according to aircraft partitioning, and, therefore, often according to “trades”. This partitioning was standardized for the first time by the Air Transport Association more than 50 years ago [A4A 14]. Nevertheless, a cross-cutting view has been increasingly encouraged by setting up plateaus, by promoting system expert positions and by the emergence of initial training in systems engineering covering both power and signal aspects.
In this first volume, emphasis is put on hydraulic power actuators. This choice can come as a surprise in this day and age where “more” electrical solutions and even “all” electrical solutions are almost always put forward. However, this choice is justified by several important considerations:
– Hydraulic technology is used extensively for actuating purposes on all aircraft, including the newer ones. The lifespan of a commercial aircraft is typically about 30 years and its marketing life also frequently extends beyond 30 years. New aircraft models of the 2010s should therefore typically still be flying in 2070.
– The maturation of more or all electrical solutions assessed in the laboratory can take more than a decade before reaching a stage satisfactory enough that the new technology can be implemented on aircraft. For instance, the electro-hydrostatic actuators that were implemented on Airbus A380 in 2007 had begun being developed in the mid-1980s.
– It is important to think in terms of requirements and performance rather than restrict choices to a given technological solution. In this regard, a more or all electrical solution should not be an end in itself; it should only be a means of providing safer, greener, cheaper and faster
2
services.
The progressive or complete removal of hydraulics teaching in engineering degrees contributes to a loss of initial skills for engineers. These two established facts advocate for a capitalization effort of knowledge that is prone to disappear.
Following the emphasis put on the system approach, requirements and architectures, this book develops an approach that is complementary to other existing publications on the topic. These constitute a significant source of information. Consequently, the following books should be recommended to broaden the scope of this series:
– generally speaking, for all aircraft systems, [CRA 99, DAN 15, MOI 01, ROS 00, SAU 09, WIL 01] and [WIL 08];
– for all aerospace actuating purposes, [RAY 93] and [SCH 98];
– for aerospace hydraulics, [GRE 85, JEP 85] and [NEE 91];
– for general hydraulics, [BLA 60, FAI 81, FAY 91, GUI 92, MAR 80, MER 67] and [VIE 80].
The present volume consists of seven chapters. Chapter 1 provides an overview of aerospace actuators with an emphasis on requirements and applications. Chapter 2 addresses reliability, that can heavily impact architecture choices in very early development stages. The following chapters focus on actuators supplied by hydraulic power sources. These chapters successively review the energy carrier function, the hydraulic fluid as well as its conditioning, power conversion, control and management; and finally actuators and hydraulic systems integration.
1
Depending on the application, it is possible for the same physical phenomenon in a technological component to either be harnessed to perform a specific function or to appear as a technological defect that threatens functional performance.
2
Concerning commercial aeronautics, it seems that the fast aspect is no longer one of the main goals because given the current state of technology, this aspect heavily impacts the other three (safety, environmental friendliness and cost).
Notations and Acronyms
Notations
Symbols
a
Weibull parameter
–
A
Availability
–
b
Weibull parameter
–
B
Bulk modulus
Pa
c
Speed of sound
m/s
C
Torque/Control port
N m/-
C
h
Hydraulic capacitance
m
3
/Pa
C
p
Specific heat at constant pressure
J/kg/°C
C
q
Flow coefficient
–
d
Diameter
m
E
Energy
J
F
Force
N
f
Friction factor
–
f(u)
Density of probability
–
F(u)
Probability of failure
–
g
Gravitational constant
m/s
2
h(u)
Failure rate
depends
I
Electrical current
A
I
h
Hydraulic inertia
kg/m
4
J
Moment of inertia
kg m
2
k
Gain
depends
K
Mechanical stiffness
N/m
l
Length
m
L
Inductance
H
M
Mass
kg
n
Number of elements
–
N
Number of items in service
–
p
Lead
m
P
Power
W
Pressure
Pa
Q
Volume flow rate
m
3
/s
R(u)
Probability of success (or, in short, reliability)
–
R
Electrical resistance
Ω
s
Orifice cross-sectional area
m
2
S
Area
m
2
t
Time
s
u
Service quantity
depends
U
Electrical voltage
V
v
Linear velocity
m/s
V
Volume
m
3
V
0
Displacement
m
3
/rad
x
Position
m
y
Position/opening
m
z
Vertical position
m
Δ
Difference
–
ε
Control error
depends
η
Efficiency
–
λ
Constant failure rate
/FH
μ
Dynamic viscosity
Pl
θ
Angular position
rad
Θ
Temperature
°C
ρ
Specific density
kg/m
3
τ
Time constant
s
ω
Angular velocity
rad/s
ω
Angular frequency
rad/s
ξ
Pressure drop coefficient
–
Subscripts
0
Initial, reference, no-load
a
Apparent
b
By-pass
c
Load/kinetic
C
Coulomb
d
Valve/metering/drain/difference
DC
Detection–correction
e
Elastic
em
Electromagnetic
f
Leakage
g
Gravity
h
Hydraulic
hc
Hydro-kinetic
hm
Hydro-mechanical
hs
Hydro-static
l
Laminar or linear
m
Mechanical/mean
M
Maximum
n
Rated
o
Orifice
p
Position/pilot
P
Supply/parallel
q
Quadratic/thermal
r
Reflected/response/feedback
R
Return
s
Structure
S
Series
t
Turbulent
u
Controlled
v
Volumetric
VM
Majority voting
x
Position
∝
Asymptotic
Superscripts
*
Setpoint
¯
Per mass unit
Acronyms
ACMP
Alternative Current Motor Pump
ADP
Air Driven Pump
CSMG
Constant Speed Motor Generator
EBHA
Electric Backup Hydraulic Actuator
EBMA
Electric Backup Mechanical Actuator
ECAM
Electronic Centralized Aircraft Monitor
EDP
Engine Driven Pump
EHA
Electro-Hydrostatic Actuator
EMA
Electro-Mechanical Actuator
EMP
Electro-Mechanical Pump
FbW
Fly-by-Wire
FH
Flight Hour
HMA
Hydro-Mechanical Actuator
HSA
Hydraulic Servo Actuator
HSMU
Hydraulic System Monitoring Unit
LEHGS
Local Electric Hydraulic Generation System
MEPU
Monofuel Emergency Power Unit
MLA
Maneuver Load Alleviation
MRHS
Main Rotor Hydraulic Subassembly
MTBF
Mean Time Between Failure
MTOW
Maximum Take-Off Weight
MTTF
Mean Time to Failure
MTTR
Mean Time to Repair
PbW
Power-by-Wire
PCU
Power Control Unit
PoD
Power-on-Demand
PTU
Power Transfer Unit
PWM
Pulse Width Modulation
SbW
Signal-by-Wire
SPGG
Solid Propellant Gas Generator
RAT
Ram Air Turbine
THS
Trim Horizontal Stabilizer
TVC
Thrust Vector Control
1General Considerations
1.1. Power transmission in aircraft
1.1.1.Needs and requirements for secondary power and power flows
On an aircraft, a distinction is made between primary power, which is used to ensure lift and airborne movement, and secondary power, which is used to power systems (flight controls, avionics, landing gear, air conditioning, etc.). Although much less significant than primary power, secondary power is nevertheless non negligible, as shown in Table 1.1.
Table 1.1.Secondary power requirements for a large commercial aircraft [COM 05]
Actuation (flight controls and landing gear)
Instantaneous power: 50–350 kW
Cabin lighting
15 kW permanently
Galley
120–140 kW intermittently (warming oven)
90 kW permanently (cooling)
In-flight entertainment
50–60 kW permanently
Cockpit avionics
16 kW
Cabin air conditioning
190–300 kW
Power is generally conveyed from sources to users by redundant networks in electrical, hydraulic and pneumatic form. For a typical 300 seat aircraft, these networks are estimated to respectively transmit a power of 230 kVA, 230 kW and 1.2 MW.
Figure 1.1 illustrates the complexity of secondary power networks for a single-aisle aircraft of the Airbus A230 type [LIS 09]. On this diagram, power flows from power generators situated on the inner ring, through distribution networks located around the intermediate ring, to power users gathered on the third ring. The outer ring depicts the surrounding air which is considered here as being equivalent to a thermal power source. Power flows are depicted by colored arrows whose colors indicate the nature of the power involved.
Figure 1.1.Secondary power flows for anAirbus A230 type single-aisle aircraft [LIS 08]. For a color version of the figure, see www.iste.co.uk/mare/aerospace1.zip
1.1.2.Actuation functions
A function can be defined as the act of transforming matter, energy or data in time, shape or space [MEI 98]. In practice, the perspective from which a function is viewed depends on the engineering task at hand. For instance, for the purpose of power scaling, the actuation function can be viewed as the transformation of power received at the source into power transmitted to the load; this transformation takes place both in shape (e.g. hydraulics toward translational mechanics) and in space (aspect of power transmission from point A to point B). In contrast, when designing flight controls, the actuation function is considered as the act of converting a signal (e.g. an electrical command for positioning a load) into another signal (current position of the load).
Power requirements for actuation are numerous and diverse. They essentially concern the following.
– Primary flight controls
The purpose of primary flight controls is control of aircraft trajectory. On a conventional aircraft, as the one pictured in Figure 1.2, they take the form of control surfaces responsible for controlling the three rotational degrees of freedom: the ailerons for roll, the rudder for yaw and the elevator for pitch.
Figure 1.2.Actuation needs on a commercial aircraft
As for helicopters, they offer four degrees of freedom. Helicopter flights are controlled by acting:
– on the swashplate,
Figure 1.3
. The swashplate translation with respect to the rotor axis makes it possible to collectively act on the pitch of the blades in order to act on the intensity of the lift vector generated by the main rotor. Tilting the swashplate about the two axes perpendicular to the rotor axis causes the pitch to vary cyclically (pitch of the main rotor blades during one rotor revolution). In turn, this allows the rotor lift vector to be tilted about the roll or the pitch axis;
– on the collective pitch of tail rotor blades for yaw control.
Figure 1.3.Swashplate actuation on an AS332 helicopter
Convertible aircraft such as Boeing V22 or Agusta-Westland AW609 also put to use the nacelle tilt about the pitch axis.
– on launchers (as well as on fighter aircrafts with thrust vector control feature), the thrust force generated by the booster or the jet engine is steered about the yaw and pitch axis in order to direct the thrust vector according to the desired trajectory;
– mechanically signaled flight controls also rely on actuators to superimpose on the pilot’s setpoint, the demands of the autopilot as well as stability and control augmentation commands.
– Secondary flight controls
Secondary flight controls make it possible to modify the aerodynamic configuration during particular flight phases. On conventional aircraft, slats and flaps increase the chord and curvature of the wings. This is done to increase the lift of wings at low speeds and therefore decrease the takeoff or landing speeds. Airbrakes (also called spoilers) reduce aircraft speed by increasing aerodynamic drag. Trim tabs, for instance the trimmable horizontal stabilizer, ensure the global equilibrium of the aircraft during the given rectilinear flight phases (e.g. climb, cruise or approach) so that primary flight controls operate around their neutral position on average.
– Landing gears
These require numerous actuation functions:
– for raising or lowering landing gear by sequencing the opening or closing of doors, extending or retracting the gear and locking it in a raised or lowered position;
– for steering the wheels in order to ensure steerability on the ground during taxiing;
– for wheel braking in order to dissipate as heat part of the kinetic energy associated with the horizontal speed of the aircraft (in addition to airbrakes and thrust reversers during landing). Left/right differential braking can also contribute to improve steerability on the ground;
– also worth mentioning, landing gear struts are autonomous hydropneumatic components. Upon touchdown, they absorb the kinetic energy associated with the vertical speed component of the aircraft with respect to the ground.
– Engines
Engines also rely on actuators to steer inlet guide vanes on the turbine stator, to deploy or stow thrust reversers, to operate maintenance panels, to modify the geometry of air intakes or nozzles, or even to control propeller blade pitch.
– Utilities
Other actuators are also used, for example, to operate cargo doors (and passenger doors on new large aircraft such as Airbus A380 and Boeing 787), rotor brakes for helicopters, winches, weapon systems (aiming guns, raising or lowering the arresting hook, etc.) among other things1.
1.1.3.Actuation needs and constraints
The solutions implemented for actuation in aeronautics and space have to meet numerous requirements and comply with the following strict constraints.
– Type of mission
On the flight timescale, the need for actuation can be seen as continuous, such as, for example, the need for primary flight controls. However, this need can also be considered transient, meaning that it only exists during a minor portion of the mission. For example, this is the case for the landing gear steering function or for secondary flight controls. Lastly, the need for actuation is said to be impulsive when it only appears for a very short amount of time. An example of this is landing gear unlocking.
– Controls
The vast majority of actuators are closed-loop position controlled (e.g. for flight control surfaces or for steering nose landing gear). Although it is less frequent, they can also be closed-loop speed controlled (e.g. to drive back-up electric generator hydraulically) or even closed-loop force or pressure controlled (e.g. for braking). Additionally, some controls can be of the on/off type, such as, for example, landing steering locks.
– Power and dynamics
Summed up in Table 1.2 are examples of power and dynamics needs as a function of the type of aircraft.
Table 1.2.Examples of power needs
Actuation function
Typical range
Aileron Airbus A320
Nose landing gear steering Airbus A320
Tiltrotor BoeingV22 OspreyMode conversion
Thrust vector control Ariane V
Stroke (mm) (degree)
20–700
44
±75
1143
±160
Speed (mm/s) (degree/s)
20–500 10–90
90 no-load
20
97
972 no-load
Force (kN) (Nm)
20–350
44
7000
80
347
Bandwidth (Hz)
1–20
≈ 1
≈ 1.5
3.2
7.9
It is important to keep in mind that the power consumption of actuation functions indicated here corresponds to the worst case scenario. In reality, under normal circumstances, actuation functions are operated well below these extreme values. Figure 1.4 illustrates this statement for the actuator of an Airbus A320 aileron. It can clearly be seen that during the course of a typical mission, only −80 to 20% of the available force and ±15% of the available speed are used (except checklist).
Figure 1.4.Power requirement for the actuator of an Airbus A320 aileron [MAR 09]
– Environment
Actuators are exposed to harsh climate conditions (pressure, temperature and humidity) as well as harsh electromagnetic (interference and lightning) and vibration environments. Every time they fly, actuators undergo a pressure and temperature cycle. For instance, at the cruising altitude of a jet (10,360 m or 34,000 ft), absolute pressure is only as high as 250 mbar and the temperature drops to −52.3°C (for a standard atmosphere [ICA 93]). Regarding hydraulics, the major constraint faced is maintaining the fluid temperature in its normal operating range. For example, actuators must be functional between −60°C and +100°C and they must be operational, meaning achieving full performance, between −40°C and +70°C.
– Lifespan
The lifespan of aircraft typically varies from 5,000 flight hours for fighter jets to more than 100,000 flight hours for new commercial aircraft (10,000 h for a NH90 helicopter, 48,000 h for the first Airbus A320 aircraft, 140,000 h for an Airbus A380). This lifespan generally corresponds to operating over the course of 30–40 years.
– Reliability
The acceptable probability of a failure depends on the criticality of the function to be performed (see Chapter 2). Since actuators often contribute to critical functions, tolerated failure rates are extremely low. For example, for primary flight controls, one catastrophic event is tolerated per 1 billion flight hours in commercial aeronautics. This major constraint heavily impacts the architecture of actuation systems. These systems therefore most often have to be redundant in order to respond to failure as required.
– Maturity
Maturity is a strong sales argument that directly impacts operational readiness. Concerning new commercial programs, the objective is to reach 99% on-time departures or with delays due to technical difficulties not exceeding 15 min.
– Topology
On aircraft, several dozen actuators are generally implemented and can be located dozens of meters away from their power source. Weight, position and performance of the power delivery network are therefore heavily impacted by the spatial layout of hydraulic systems. On an Airbus A380, there is, for example, more than 40 flight control actuators [MAR 04], some of them located more than 60 m away from the hydraulic power generator.
1.2. Primary and secondary power transmission functions for actuators
Generally speaking, the main functions associated with power transformation and metering2 are clearly identifiable (see Chapters 4 and 5). Conversely, when dealing with actuation solutions, other significant functions are often neglected even though these functions turn out to be the most difficult to master in practice. It is therefore essential to pay close attention to:
– reversibility, a concept which makes it possible for a passive actuator, for example, to let itself be driven by an active actuator through the load they share;
– protection against excess static and dynamic force, which restricts mechanical stress/strain on control surfaces or on the airframe, for example, during sudden gusts of wind;
– cooling or heating in order to maintain actuator temperature within its normal operating or functional range;
– damping, to dissipate energy and avoid resonance. For instance, to avoid shimmy
3
of the nose landing gear steering;
– Dissipation of the energy to be absorbed when reaching the end-stop. For example, this is important for thrust reversers;
– force equalization, which is intended to ensure that actuators equally share the responsibility of driving a single load without force-fighting. For example, for the three active actuators of a single rudder;
